Wastewater Management: From Ancient Greece to Modern Times and Future

Abstract

Current wastewater management paradigms favor centralized solutions, as taught in traditional engineering schools, which imply high capital costs, long-range water transfer, long and disruptive construction and highly trained operators. On the other hand, small decentralized systems are seldom considered even though they require lower capital costs, less disruptive infrastructure construction and allow for the maintenance of a closer, more sustainable water cycle. This manuscript starts with an extensive review of the long history of wastewater systems, from the Greek antiquity to the modern era. The use of natural and physical systems in history and their evolution into modern technology is also analyzed. Finally, future trends are considered with emphasis on technological adaptation and sustainability of decentralized systems, with a view that lessons that can be learned from history and past practices. The manuscript aims to provide a critical overview of water and wastewater management in view of the oncoming challenges of this sector.

1. Prolegomena

The history is studied, not to learn what happened at some point in the past, but to understand the present and to trace the future.

Andreas N. Angelakis

Sanitation (wastewater collection and treatment) and safe water supplies are essential services in human settlements, large or small, with the dual purpose of protecting public health and the natural environment; substantially reducing hazards related to anthropic discharges to either; and providing a life-carrying resource to humans, animals and crops. Sanitation-related problems still persist in some of the world’s regions to this day, and are targeted by international programs, such as the UN’s Sustainable Development Goals, to achieve appropriate solutions. In 2020, 46% of the global population still lacked safely managed sanitation, and 26% were deprived of safe drinking water with nearly 1000 children dying each day due to otherwise preventable water and sanitation-related diseases. Goal no. 6 of the SDGs addresses “Clean Water and Sanitation for All” but, despite all efforts, it is estimated that 129 countries are still not on track to achieve this target by 2030 [1]. At present, according to the IMCC [2], the global water supply situation is far from optimal: water scarcity affects more than 40% of the world population and is projected to rise. Still, more than 80 % of wastewater resulting from human activities is discharged into rivers or sea without any or with inadequate pollution removal, making downstream waters unsuitable for many uses, including drinking and irrigation (approximately 70% of all water abstracted from rivers, lakes and aquifers is used for that purpose).

Sanitation is among the basic foundations for the successful, continuing and sustainable development of any society: early evidence of cesspits and Imhoff-type basins was found in Mesopotamian, Indus Valley and Egyptian settlements [3]; however, in many respects, ancient Greeks must be considered the forerunners of modern sanitation systems. Historical findings in cities on Crete and other Aegean Islands testify the presence of sophisticated sewage systems since the early Minoan civilization, considered Europe’s first advanced society, flourishing from ca. 2700 to 1450 BC [4]. Evidence of later sewerage systems was also found on Delos Island (dated as early as 400 BC) and in ancient Pella (around 300 BC) in Northern Greece, known as the historical capital of the ancient Macedonian kingdom. At the same times, less sophisticated open air networks for wastewater and stormwater conveyance present in several other Greek cities were indicated as causing the frequent occurrence of water-borne diseases, such as cholera, the plague, etc. [5].

Water and wastewater distribution and collection systems based on large and imposing infrastructures are among the oldest (e.g., Roman aqueducts and the Cloaca Maxima in Rome) notable engineering endeavors of advanced civilizations, and the infrastructure-intensive approach is still the most widely applied sanitation paradigm in urban environments of developed countries [6] even though large capital cities, such as London, surprisingly did not adopt this approach until the second half of the 1800s [7]. Centralized water management systems have evolved from those earlier examples, collecting wastewater in large pipe networks and transporting it for long distances to dedicated treatment plants. Long-range water supplies are also common nowadays as in Roman times due to the disconnection between available water sources and the strong demand in many contemporary large cities.

Small (decentralized) wastewater systems, on the other hand, are intended to treat and dispose (or, often, reuse) wastewater from small and low-density communities, buildings and dwellings [8]. In ancient times, although water supplies might have required substantial effort (e.g., the construction of long-range aqueducts), wastewater disposal and treatment were still not energy issues since they relied mostly on natural processes close to the source; the Water–Energy Nexus was not an issue then, although people were well aware of the other component, the Water–Food Nexus. In this respect, ancient “centralized” systems could be considered quite similar to present day decentralized ones as far as technological solutions are concerned. Centralized water supply and wastewater treatment systems are defined as large-scale facilities serving extended areas of metropolitan or regional interest. On the other hand, decentralized treatment systems are those employing smaller capacity plants, maintaining close proximity to the wastewater source, serving a limited area and avoiding long-range transfer, which is costly both in capital (sewer construction) and operation (energy for pumping). In these situations, the main technical challenge is the adequate choice of a treatment and/or disposal [9], aiming at the possible reuse of treated effluents (reclaimed water) for beneficial purposes.

Currently available technology allows for the fulfillment of variable reuse objectives at various scales and degrees by an appropriate choice of treatment [10]. The main difference between modern decentralized and centralized systems lies in infrastructure intensity: in the centralized system, which requires the construction of large wastewater treatment plants (WWTPs) and extended conveyance pipes, the investments related to the latter may represent up to 70% of the total capital cost of the system and could be hardly affordable in developing countries [11,12]. Life Cycle Assessment (LCA)-based studies show that the construction of large sewers alone implies a heavier environmental impact than the construction and operation of WWTPs [13]. This aspect is seldom considered nowadays when selecting options for wastewater management. Another substantial difference between modern centralized and decentralized systems is the energy intensity required for operation: centralized systems are associated with high energy consumption, mainly for long-range water and wastewater transfer, although wastewater purification itself may benefit from some economies of scale and from the benefits of widely tested technology [14].

The evolution and development of intensive wastewater treatment technology in the late 1800s and early 1900s is still limiting even nowadays as the diffusion of decentralized wastewater management in modern cities is due to the persistence of established heavily infrastructural approach paradigms. The development of megacities has rendered this traditional concept (the use of a centralized WWTP for all wastewater) untenable. Centralized WWTPs limit reuse applications by design, since potential (re)users often end up being far away from the discharge point, and energy-intensive pumping would be needed. Decentralization, as a consequence, is emerging as an inevitable necessity. After a long period of little consideration, decentralized treatment solutions are now advocated as a more sustainable approach due to population growth, the development and spread of megacities, energy issues and the rapid development of new process technologies. It is common opinion among water sector researchers that many more opportunities for local water reuse will come with decentralization.

This paper promotes a new vision of decentralized wastewater management and highlights its future role in meeting sustainable needs and goals as the world population, sanitation and sustainable water supply requirements continue to grow. The implementation of decentralized wastewater management is analyzed according to a timeline describing the evolution of sanitation practices and treatment technologies from prehistorical times to the current age. As noted by Lofrano and Brown, “there is no more reliable source of customs and behavior of a society than its waste products” [3].

More specifically, the paper is organized as follows: Section 1 is the prolegomena, i.e., an introduction to the theme and elements of the review, and is followed by Section 2, which explains the distinct histories of wastewater management from the prehistoric era to the medieval times in a chronological view, including descriptions of various types of technologies and practices used. Section 3 describes the wastewater status in the Early to mid-Modern times, Section 4 describes wastewater treatment in contemporary times and Section 5 describes wastewater treatment by decentralized small systems (e. g. soil, natural and biological). Finally, Section 6 is the epilogue that includes conclusive remarks and highlights.

2. Wastewater Management until the Middle Ages (ca. 3200 BC–1400 AD)

2.1. Prehistoric Times (ca. 3200–1100 BC)

The era of wastewater management started when humankind abandoned the ancestral hunter–gatherer lifestyle to adopt agricultural practices and establish permanent settlements, which were located based on the availability of reliable water sources. Until the onset of the first stationary civilizations, the disposal of excreta was managed by digging holes in the ground and covering them after use. These prescriptions (Mosaic Law of Sanitation) are reported in the Bible (Deuteronomy, 23) [3]. With permanent human settlements, the first observable public health and ecological impacts could be perceived. Although evidence of basic sanitation was discovered in early settlements of Mesopotamia and the Indus Valley, the first sanitation systems resembling those of today appeared in ancient Greece. The existence of drains and sewers in Minoan palaces was documented by Evans [15] and MacDonald and Driessen [16,17], who provided detailed drawings of the original structures. In Knossos Palace (Crete Island), a large stone culvert (79 × 38 cm) coated with mortar was discovered (Figure 1a) [18]. Its size allowed access for cleaning, and the structure was provided with vent holes at regular intervals for aeration, indicating sophisticated design [19]. From the palace, wastewater flowed to the Kairatos river east of Knossos and then into the Aegean Sea. Sophisticated drainage systems existed in other sites, such as in Phaistos (Figure 1b) and Zakros Palaces (Figure 1c). As at Knossos, the main channels were built of stone, and the lower section of terracotta with hydraulic gradients were designed to allow optimal flow even in critical conditions [20].

The drainage networks of Phaistos and Zakros Palaces extended to agricultural land downhill for irrigation and to the sea for disposal, respectively. Evidence indicates that these systems were fully functional even in times of intense rainfall [20,21].

During the Mycenaean era, permanent well-organized human settlements in southern mainland Greece favored the development of early wastewater management technology. In Mycenae, the major center of this civilization, a large open sewer parallel to the outer wall of the citadel was part of the central sewerage system leading outside the defensive wall down the steep hillside (Figure 2) for wastewater disposal to agricultural fields [18].

The most advanced Minoan era sewage system was that of Agia Triada villa, which still triggered the admiration of visitors after the millennia. The Italian writer Angelo Mosso [21], visiting the site in the early 20th century, noticed that the system still functioned perfectly during heavy rain, and recorded: “…all the sewers were still working! It was very interesting for me to see the water in drainages, and sewers so big that a man could enter. I doubt if there are other examples of ancient sewerages working after four thousand years”. The American H. F. Gray [22] adds: “Perhaps it may be allowed to doubt whether modern drainage and sewerage systems will operate at least one thousand years”.

Land treatment, i.e., wastewater spreading to the soil also has a long history, as evident by the layout of sewers in Minoan cities since 2600 BC [4]. Evidence of wastewater disposal to agricultural land goes back to civilizations in Crete, Sparti, Athens and Cyprus approximately 4000 years ago [23,24], an approach that could be considered an early acknowledgement of the Water–Food Nexus and is much investigated at present. It is interesting to note that although direct wastewater treatment by direct land application is not allowed in developed countries any longer, the interest in the practice of “fertigation” (irrigation with nutrient-rich treated effluents) is still high in view of the latest “Circular Economy” approaches [25].

Urban sewerage and water supply technology were developed in connection with the establishment of centrally administered settlements in several parts of Greece. Advanced internal systems, with wastewater channeled in vertical pipes inside the buildings’ walls or under the floor and drainage established at a level higher than the ground floor to protect houses from moisture have been found in palaces and town houses, such as in Polichni of Lemnos, where a main underground sewer leads storm- and wastewater out of the city walls [26], and in Akrotiri Thera, reconstructed after a major earthquake in the middle of the 2nd Millennium BC [27,28]. In this respect, calamities such as earthquakes, widespread fires or epidemics often prompted radical developments in urban sanitation throughout history.

Evidence of private bathtubs connected to drainage was discovered in Pylos (mainland Greece), dating from the Late Bronze Age. In Dimini (Mycenaean Iolcos), drained bathtubs were found located in dedicated rooms, implying an advanced knowledge of water management and high living standards of the inhabitants. In the same area, settlements dating to the Neolithic period (end of IV millennium BC) and to the Mycenaean period (ca. 14–13th century BC) that had well-constructed “megaroid” houses were discovered with well-preserved sewer drains [18]. By contrast, in prehistoric Macedonia, urban water supply and drainage networks did not exist, as local settlements had not reached a proper urban development status yet. Well-organized settlements with central administration, in fact, appeared in Northern Greece starting only in 7th century BC and developed during 6th century BC [18].

2.2. Historical Times (ca. 1100 BC–476 AD)

2.2.1. Classical and Hellenistic Periods

Technological progress in Greece was accompanied by an improved understanding of water and wastewater management. Around 600 BC, philosophers developed the first known scientific views on natural hydrological and meteorological phenomena. Later, during the Hellenistic period, significant developments were achieved in hydraulics that, along with progress in mathematics, allowed for the invention of advanced devices such as the Archimedean water screw that are still in use today [29].

Although the importance of sanitation was already understood from the early Archaic period, Alcmaeon of Croton (ca. 470 BC) was the first physician to state that the quality of water may influence people’s health. The Hippocratic treatise: “Airs, Waters, and Places” examined the effects of climate and the environment on human health, linking every disease to natural causes [30]. The importance of water for public health was thus recognized for first time, and the first well-organized baths, toilets and sewerage systems appeared [23]. This concept had apparently been forgotten in subsequent centuries, only to be “rediscovered” by London physician John Snow, who linked contaminated water to the causes of the Broad Street cholera outbreak that killed over 600 people in 1854.

Lavatories of this period consisted of wooden or stone-made benches with defecation keyhole-shaped openings supported by cantilever stone blocks laid over a ditch where flushing water was running [31]. It should be noted that the same system can be seen in many Roman thermae and public baths, and that this technology remained basically unchanged until the “invention” of the flush toilet in 1596 (adopted on a large scale only three centuries later) by Sir John Harington, godson of Queen Elizabeth I [32].

In ancient Athens, stormwater, human wastes and other effluents were all delivered to a collection basin outside town, from which flow was conveyed through brick-lined conduits to irrigate and fertilize fruit orchards and fields. An epidemic around 430–426 BC might have been the driving force for the further improvement of the city sewerage system during the IV century. Domestic sewers were either carefully constructed with stone walls and tiles or flat slab covers or were simply made of inverted roof tiles with the presence of manholes for cleaning and maintenance [33]. The remains of sewers in Hellenistic Athens are shown in Figure 3. Additionally, in classical Olynthus, terracotta pipes were used to evacuate domestic sewage to public drains placed between rows of houses [34,35].

Kassope, flourishing in 3rd century BC in the northwest of Greece, is considered one of the best examples of a city built according to the Hippodamian grid layout, later adopted by the Romans and in recent times (17th century) by William Penn, the founder of modern Philadelphia (PA, USA), and soon after imitated by new cites of the former British colonies. Kassope’s waste- and stormwater system layout also remarkably reflects this scheme (Figure 4). The bottom of the drains was unlined, allowing water to come in contact with the soil. In addition to simpler construction and lower cost, this particular feature resulted in water infiltrating the ground, reducing the flow rate and recharging the aquifer, a practice that nowadays is promoted under LID (Low Impact Development) schemes [36].

A significant feature of sewers during the Hellenic antiquity relates to open theatres’ drainage systems. Rainwater management of these sites was incorporated into their design since the earliest years of their appearance: the theatre plan resembles and acts as a runoff catchment, and therefore rainwater drainage management is essential. In stone theatres, drainage was an integral feature of the structure, while in wooden buildings, as in the Dionysus theatre in Athens, it was added later. Representative examples of drainage in the ancient theatres at Knossos and Phaistos in Minoan Crete, Dionysus in Athens, Arcadian in Orchomenos, Ephesus in Turkey and in Delos Island, including their hydraulic features, are provided by [37]. Rainwater harvesting and reuse was a common feature in ancient theatres to sustainably manage stormwater, and in some cases it is still operational today (Figure 5a). Collected water was stored in cisterns, such as the arched cistern (Figure 5b) discovered in Delos Island [38], or was distributed to supply nearby workshops, as in the Dionysus theatre of Athens [37]. These practices are also being reintroduced in current times, even though in less imposing circumstances [36].

2.2.2. Roman Period

Romans were exceptional civil engineers and perfected drainage and sewerage systems for the disposal of surplus water and to prevent flooding, both in cities and in the countryside. Due to the high population density in cities, sewage and rainwater were conveyed in subterranean combined drainage systems collecting both storm runoff and wastewater.

Roman engineers developed water supplies (Figure 6a), sewerage systems (Figure 6b) and other impressive sanitary infrastructure, such as public baths and toilets (Figure 6c), which are still visible in ancient Roman urban centers, where they played a major role in the period’s lifestyle [39]. At least five public lavatories were found in Dion, Thessaloniki and Philippi. They all had a rectangular ground plan and were usually embedded into large buildings, such as baths, thermae and palestrae. In Dion, public lavatories are also found outside the walls joining the Sanctuary of Demeter, built around 2nd century AD. The ditch under the defecation bench ensured continuous flushing through a constant water flow through an underground stone conduit from a source in the Asclepius sanctuary [40].

Stone sewer mains in Roman Macedonian cities ran along the central axis of the rectangular street grid; they were rectangular in section and made of recycled materials such as walls of stone and mortar or drainbeds of stone slabs [39]. In the Roman cities of Veria, Dion and Philippi in Northern Greece, mains had no upper lining but were covered with slitted road stone slabs to enable runoff inflow from the street. The sewers of Philippi had impressive dimensions, ranging from 0.55 m to 1.00 m in width and from 0.90 m to 1.70 m in height. Somewhat smaller were sewers in Roman Veria. A main sewer covered by a stone-built apsis was discovered in Thessaloniki (Figure 7a). In Thassos, massive slabs of road surface pavement were set onto the rims of underground sewer channels, functioning simultaneously as covers (Figure 7b) [41].

2.3. Medieval Times (ca. 476–1400 AD)

The decline of the ancient world influenced not only established technological achievements but also the transmission of relevant craftmanship. The practice of waste- and stormwater management were affected by new morals and social customs introduced by Christianity [18]. Roman advances in sanitation were forgotten; only a few cities preserved the Roman sewerage systems, which were soon absorbed by the urban sprawl. Cesspits became the main sanitation structures; in that era, it was common practice to throw excreta from the windows onto the streets. Due to poor sanitation, pests thrived and epidemics broke out, wiping out 25% of the European population. Remains suggest that existing lavatories still remained of common use, despite structural obsolescence and the promotion of privacy by the Christian religion [18]; individual toilets and bathing chambers were incorporated in pre-existing Roman baths, which originally had only common bathing pools [42]. The private nature of lavatories resulted in size reduction and positioning closer or next to the main rooms. In the East, on the other hand, the vivid tradition of the ancient world kept earlier habits alive: in the early Byzantine era (ca. 476–1453 AD), common lavatories accommodating multiple simultaneous users were still common, mainly in monasteries [43]. Despite that, waste- and stormwater management was generally neglected at the community level and was mostly considered a matter to be dealt with on an individual basis.

During these times, urban sanitation management progressed only in Arab cities, with separation of the three types of water: rain, considered essential for life; greywater, originating from domestic activities; and wastewater. The Arab culture valued rainwater as a divine endowment, so it was carefully stored in cisterns for its conservation and subsequent use. Domestic greywater was removed through underground or surface drains, and wastewater was sent to cesspits. It is interesting to remark that recent urban water management paradigms are now advocating similar source separation solutions [44]. Current approaches, however, value not only the water resource itself, but also its embedded content.

3. Wastewater Management in Early and Mid-Modern Times (ca. 1400–1900 AD)

The Renaissance period (14–17th century) did not foster the same advances in urban sanitation as it did in the arts. Progress in hydraulics was applied to supply water collection and distribution but did not affect sanitation much. Cities kept growing, and the filth and odor in nearly all European cities was unbearable. The city of Paris was a great paradox of the times: while beautiful fountains, ponds and canals were created in the gardens of Versailles, the city reached the highest levels of filth in its history. Open air defecation was common in many neighborhoods, and throwing excreta onto the streets was the most frequent disposal practice. Existing sewers consisted of open ditches, discharging directly into the nearest river or watercourse. The situation in London was not different. The close relationship between filth and disease, already clear in ancient Greek times, did not became clear again until the mid-nineteenth century when sanitation underwent radical changes, prompted by the great fire of Hamburg in 1842, which destroyed one quarter of the city. Since reconstruction was necessary, it was accompanied by a new sewage system using seawater for flushing and the vertical drains of connected buildings for airing. This system, financed by local businessmen, later inspired major European and U.S. cities.

One of the earliest documented wastewater management techniques of this era is land application, which combined disposal with agricultural use. This process known as ‘sewage farms’ was first developed in Bunzlau (today Boleslawiec, in modern Poland) in 1531 and later in Edinburgh (Scotland) in 1650, and it was not dissimilar to that used by ancient Greeks. In both cases, wastewater was used for beneficial crop production [45,46]. Initially, this was accomplished by transport with buckets and horse-drawn carriages, but as the population grew, sanitary sewers were built, with pipes and pumps, to transport sewage beyond the city boundaries to grasslands, into which sewage was infiltrated. In 1800, London did not rely at all on sewers to remove excrement, since these existed merely to convey surface runoff to prevent flooding. Respectable households disposed of human waste in cesspools, which were brick-lined underground chambers directly beneath the seat of a privy or connected to it by a short drain. Once filled up, these were emptied with buckets, and the removed matter was carted away to be sold to farmers as manure. The term ‘night soil’, still in use today, was used because cesspool emptying was an activity restricted to the hours of darkness.

With the rapid growth of cities, ‘sewage farms’ were viewed favorably as an appropriate solution to the disposal of large volumes of wastewater. Large farms were established in rapidly growing cities of Europe and the USA at the end of the 18th century and in Australia at the end of the 19th century, some of which are still in use (

Table 1. Selected early land treatments and reuse systems (adapted from [45,46,47,48,49,50]).

Location	Date Started	Type of Land-Based System	Area (1000 ha)	Flow (m3/d)
Europe
Bunzlau (modern Boleslawiec), Poland	1531	Sewage farm
Edinburgh, UK	1650	Sewage farm
Croydon-Beddington, UK	1860	Sewage farm	0.25	17.4
Paris, France	1869	Irrigation	0.64	30.3
Leamington, UK	1870	Sewage farm	0.16	3.4
Berlin, Germany	1874	Sewage farm	2.7	N/A
Augusta, ME, USA	1876	Irrigation
Milan, Italy	1881	Irrigation	3.5
Wroclaw, Poland	1882	Sewage farm	0.80	10.6
Braunschweig, Germany	1896	Sewage farm	4.4	60.0
USA
Calumet City, MI	1888	Irrigation	0.005
South Framingham, MA	1889	Irrigation
Woodland, CA	1889	Irrigation	0.07	15.5
Boulder, CO	1890	Irrigation
Fresno, CA	1891	Irrigation	1.60	10.6
San Antonio, TX	1895	Irrigation	1.60	75.7
Vineland, NJ	1901	Rapid infiltration system	0.0026	3405.9
Ely, NV	1908	Irrigation	0.16	6.1
Lubbock, TX	1915	Irrigation
Others
Tula (Mezquital) Valley a, Mexico	1896	Irrigation	90.00
Melbourne, Australia	1897	Irrigation	4.16	189.3

^a The initial irrigated area was less than 2000 ha but currently is 90,000 ha after reaching a maximum of 110,000 ha in 1995.

) [45,46].

At the beginning of the 20th century, sewage farms in France reached their highest usage: farms in Gennevilliers (900 ha), Achères (Achères plain, 1400 ha), Pierrelaye (2010 ha) and Triel (950 ha) were fed by wastewater supplied by the Colombes pumping station in Paris [44,46,47]. The practice was gradually dismissed because as cities continued to expand, farmland became scarcer and more distant, increasing sewage transfer costs. In addition, at some point the fact that sewage was contaminated with infectious pathogens and sometimes with industrial waste, and therefore not always suitable for crop fertilization, became apparent; thus, sewage treatment plants began to replace sewage farms. Modern versions of sewage farms consist of land applications of treated WWTP effluents (i.e., reclaimed water): as an example, in Milan (Italy), Nosedo WWTP > 1 million p.e. (population equivalent) irrigates over 100 km² of agricultural land with treated sewage that originated from the southeast portion of the city.

Due to the increase in fecal residual matter (sludge) disposal costs, sewage land disposal seems to be attractive again. Processed anaerobic digestate is used as a replacement of cost-intensive fertilization practices in current wastewater-centered Circular Economy schemes [51], and sewage sludge is still occasionally spread on farmland as a fertilizer despite concerns over its safety due to microbiological (pathogens such as E coli and salmonella viruses) and pollutant content (including persistent organic pollutants such as antibiotic residuals, heavy metals and microplastics) [52]. As late as 2020, 27,500 t of municipal sewage sludge were shipped from the Netherlands, where sludge spreading on farmland is banned, to the UK for land application disposal, although not without some local public protest [53].

4. Wastewater Management in Contemporary Times (1900 AD–Present)

With the industrial revolution, new possibilities opened up in the area of community water supply and wastewater management. Former technology relied heavily on gravity as the driving force for supplying and eliminating water to and out of a city. With the development of modern pumping machines, sometimes driven by water itself (e.g., hydropower, steam engines), it was suddenly possible to easily deliver water uphill from deep underground aquifers or long distances, and to secure a water supply in high rise buildings [54]. The industrial revolution, however, introduced a new problem: widespread chemical pollution. While progress had been made in the treatment of organic matter pollution, industrial discharges started polluting waterways and aquifers with products whose harmfulness was only discovered later due to the observation of their effects and to make progress in monitoring techniques [55]; heavy metals, hydrocarbons, new chemicals, pesticides, contaminants of emerging concern (CECs), etc. required a change of pace in wastewater management, which is still ongoing.

One of the earliest modern-era WWTPs in Europe was the sedimentation treatment plant in Bubeneč (Prague) built in 1900–1906 as a part of the new Prague sewerage system, designed for 700.000 inhabitants. The efficiency of the wastewater purification process was about 40% for organic matter, and sludge was sold (after drying) as a fertilizer. The first large-scale biological wastewater treatment facility in the U.S., the Jones Island Sewage Treatment Plant, was built in Milwaukee in 1926. Rather than landfilling excess biosolids, sludge was processed into an industrial quality fertilizer under the brand name “Milorganite” (acronym for Milwaukee Organic Nitrogen). This innovative product became very popular during the 1930s, before inorganic urea became commercially available at a low cost after WWII. Enormous technical progress has been achieved since, although the basic technical and scientific foundations still hold. Currently, phosphorus recovery from wastewater is becoming a desirable process due to its scarcity [56], and the development of sludge-derived materials for agricultural and other uses is a very promising area of R&D [57].

With increasing requirements for WWTP effluent discharges, energy efficiency and CO₂ emissions [58] and nutrient recovery [59], many challenges are still present in urban water and wastewater management. Wastewater is often handled separately from rainwater runoff [60] since separate sewer systems secure the collection of more uniform flows and pollution loads, and even domestic wastewater streams may be subject to segregation according to pollution content (black water from toilets and kitchens, greywater from laundry, showers and hand basins) [44]. In order to improve resources recovery (especially nutrients), even more extreme segregation approaches have been proposed (i.e., in-house separation of feces and urines) [61,62]. Due to the constantly increasing water demand, wastewater reuse and seawater desalination are increasingly common; WWTPs technology could secure effluents of such a quality that they may be reused for various purposes, even for direct potable use, without adverse consequences on public health and the environment [10,63]. The observed presence of ‘emerging contaminants’, i.e., chemicals without a regulatory status whose impact on the environment and human health are still poorly understood, has been amply reported in wastewater and aquatic environments. The search for effective and efficient technologies to remove them is the subject of intensive research [64].

At the beginning of the XXIst century, most EU member States reached compliance with the Water Framework Directive (WFD) and the UN’s SDGs. In Greece, today, more than 95% of the population is connected to wastewater collection systems, and more than 80% to WWTPs providing secondary biological treatment, with 83% providing biological nitrogen removal, 57% providing additional biological phosphorus removal and 93% providing effluent disinfection. Most larger WWTPs (>5000 p.e. capacity) have been completed, and those for agglomerations of a lower capacity are under construction. Investments in infrastructural assets are significant, with more than 26,000 km of sewer networks and almost 350 WWTPs built [65]. Urban rainwater is collected through combined sewers (about 10%) or separate surface drainage (75%) or directed to surface waters or soil infiltration (15%) [66].

For the last two centuries, technology has been easily transferred to almost any area of the world, and thus we may no longer talk about technological achievements of “different civilizations”. We all share a common scientific and technological civilization. The challenges lying ahead are, however, many: from the water supply point of view, the diminishing quantity and quality of primary sources require the increased exploitation of secondary ones, such as nonpristine, brackish and seawater, to fulfill the demand. The detection of emerging pollutants of anthropic origin also requires attention, both on the supply and the treatment sides. Many of these pollutants cannot be treated with “conventional” technologies; hence, they might escape traditional WWTPs and propagate in the environment [67]. Treatment intensification requires increased energy inputs, impacting the Water–Energy–Food Nexus.

Centralization vs. Decentralization: An Ongoing Debate

Centralized systems, although being the current most common paradigm in developed countries, are often not appropriate, especially in developing and low-income countries or rural areas with a low population density [68,69,70]. Decentralized systems imply lower investment, operation and maintenance costs compared to traditional schemes since they limit the extension of sewer construction and introduce low-impact treatment solutions [71,72,73,74]. Conventional pollutant removal processes could be carried out with lower inputs of energy and/or chemicals [75,76], and opportunities for local reuse would be enhanced [11]. Simple design and construction would allow for easy replications in underdeveloped areas using local resources and materials [77]. Even in highly urbanized areas, decentralization would limit the extension of conveyance networks and could exploit the same technologies of centralized systems in compact, remotely controlled facilities.

In decentralized systems, the treatment and disposal or reuse of the effluent is close to the source of generation, and this results in small and simple conveyance networks. The sewage volume is low, characterized by significant fluctuations. The size of the network allows for a more flexible choice of conveyance methods, such as pressurized or vacuum sewers, as alternatives to the more common gravity sewer. In decentralized systems, the conveyance infrastructure cost is lower even though the specific treatment cost (per unit pollutant removal) may be higher depending on the targets and selected process technology specific efficiency [67].

Remote areas require distributed wastewater management, and substantial progress has been made in developing such systems [78]. Decentralized systems, including the Natural Wastewater Treatment System (NWTS) and other alternative technologies, often present a viable economic solution to effectively cleaning and possibly reusing treated effluents for irrigation purposes [79]. The decentralized treatment of segregated sewage in developed countries’ urban setting can enhance local reuse in buildings and neighborhoods [80]. Energy-efficient decentralized wastewater management can play a major role as smart cities will develop in the coming years [81].

5. Evolution of Wastewater Treatment Technologies

The two main objectives of wastewater management systems are as follows: first, to protect and promote human health by breaking the cycle of disease; second, to provide water quality and ecosystems protection by avoiding the negative effects of excessive pollutants discharge into the environment. These objectives hold their validity regardless of which approach (centralized or decentralized) is implemented. In general, almost all current treatment technologies could theoretically be applied into decentralized settings, since progress in automation, sensors, remote monitoring and control technology make it easy to adapt almost any technology to a myriad of situations; however, not all of these possibilities would constitute sensible choices. The selection of the most appropriate technology among a set of available alternatives at a particular location is not a trivial task. Interconnected factors, such as capital costs, operation and maintenance costs, land requirement and sustainability are involved in the decision-making process. Often, choices are made by “technological inertia”, following established paradigms and design habits. Scenario-based multiple-attribute decision-making methodologies have been developed and applied to the selection of wastewater treatment alternatives [82]. The advantages of decentralization are several, as it could effectively and efficiently treat domestic sewage to protect health and the environment and could support the local water supply since wastewater treated in decentralized systems is more likely to remain in the local watershed, making it easier for communities to implement local reuse schemes [69].

The section that follows is an overview of the evolution of applicable technologies suitable to decentralized systems, with a commentary on their pros and cons.

5.1. Earth/Soil Systems

The simplest form of WWTP consisted historically of simple underground septic tanks (cesspools), where suspended solids settled while achieving some degree of stabilization. In hot climates, septic tanks can remove up to 50% of the organic load of ‘normal strength’ sewage, but usually achieve little in the way of pathogen reduction.

As an alternative, by letting wastewater infiltrate in the soil, treatment is achieved through natural physical, chemical and biological processes, which occur at the soil surface and below. Hydraulic and organic applied loads must be compatible with the physicochemical properties of local soils and of the underlying geology. The risk of groundwater contamination must be carefully evaluated. In the Minoan era, about 4500 years ago, significant progress in wastewater applications to agricultural land was achieved [4]. Wastewater holding and infiltration basins are still preserved in many cities of that era, and the practice continued with sewage farms for some time. As a result of stricter environmental legislation, existing infiltration/percolation systems of untreated sewage are being dismissed in most developed countries to avoid excessive groundwater pollution, which could result in severe drinking water resources impairment. An early house septic tank and soil application WWTP is shown in Figure 8.

5.2. Natural Wastewater Treatment Systems (NWTSs)

NWTS are those in which pollution abatement is carried out by natural means and processes occurring in the soil–waste–plant environment, such as physical, chemical, biological processes or combinations thereof [31].

NWTSs perform multiobjective contaminant targeting processes, removing turbidity and suspended solids, biodegradable bulk organic matter and trace organic compounds, microorganisms and nutrients [79,83]. Aquatic plant systems include natural and artificial (constructed) wetlands and floating plant ponds. Constructed wetlands are expressly designed for the treatment or polishing of polluted discharges: they consist of flooded areas, usually of a shallow depth (< 0.6 m), in which plants such as Cyperaceae (mainly of the genus Carex spp.), reeds (genus Phragmites, mainly P. communis), worms (genus Scirpus spp.) and grass (such as genus Typha spp.) grow [31]. Vegetation provides the basic substrate for bacterial growth, aids in the filtration and adsorption of contaminants, transfers oxygen into the water volume, and reduces algal growth due to shading. Pretreated (settled) liquid waste is applied at a low hydraulic rate, and purification occurs as the flow proceeds through the stems and roots of the existing vegetation [84]. There are different types of NWTSs, both soil based and aquatic, each with different constraints, operating conditions and design criteria [85,86,87].

Another common class of natural systems are stabilization ponds, which are anaerobic, facultative (combining aerobic and anaerobic processes) and purely aerobic. The obvious advantage of pond systems is their simplicity, while their long retention time favors pathogens abatement. Ponds could also produce secondary economic benefits, as aerobic ones may provide a good environment for growing fish, such as tilapia, which could be an advantage in developing countries. Effluent from these ponds may have a fairly high algal concentration, so it could also be suited for irrigation since algae contain nutrients and can minimize the level of nitrates entering the groundwater.

5.3. Engineered Biological Systems

Biological treatment is usually divided into aerobic and anaerobic processes. “Aerobic” refers to a process in which oxygen is present, while “anaerobic” describes a biological process in which oxygen is absent. Scientists have been able to control and refine both aerobic and anaerobic biological processes to achieve the optimal removal of organic substances from wastewater.

In spite of their obvious benefits, which include low energy inputs and extremely simple technology, NWTSs have several shortcomings: for instance, they demand large areal footprints (for example, constructed wetlands may require up to 12 m²/p.e. in continental climates) [69], and their efficiency is strongly dependent on the external environment and seasonal variations [76]. This makes them not ideal in situations where land availability is an issue. For these and other reasons, more compact wastewater treatment technologies have been developed.

Aerobic biological processes involve the insufflation of oxygen in wastewater in order for microorganisms to carry out organic degradation. These systems, e.g., the activated sludge (AS) processes, require a much smaller area than natural systems but more expensive infrastructures and command a much higher energy footprint. They usually provide good quality effluents, which can easily meet effluent discharge standards directly. At the simplest implementation level, they require only semiskilled personnel, which makes them a good technological option for implementation in low-income and developing countries.

Trickling filters (TF) are an aerobic technology that employs attached biomass to remove organic matter from wastewater. TF development occurred at about the same time as the AS process, as the first appearance of TFs for treating organic wastewater dates back to the late 1890s in Scotland. The first municipal TF system was inaugurated in 1908 in Reading (PA, USA) just a few years before the first AS system in Milwaukee (WI, USA) [88]. Biomass grows on a fixed bed of filling material, such as stones, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic or plastic media (characterized by a high void/fill ratio), while wastewater is distributed from the top, flowing downwards against a countercurrent of natural air flow, which provides oxygen to the biomass [89]. Wastewater treatment with TF is among the oldest and better characterized biological treatment technologies, requiring little energy and operational efforts, and has often been used in small treatment facilities.

AS systems require the separation of biological solids from the liquid phase, which is achieved in large clarifiers whose operation could constitute a major process bottleneck in various situations [90]. Rotating Biological Contactors (RBCs) were introduced in the 1970s as an alternative to the AS process. They consist of a partially immersed solid medium (plates, discs) that encourages microbial growth into a static biofilm while it is rotated on a horizontal shaft through the wastewater. The rotation leads to bulk fluid mixing, liquor convection through the biofilm and biomass separation by shear. By uncoupling the mean cell residence time from the hydraulic residence time (HRT), this allows for higher organic loadings and resistance to toxic shocks than suspended culture systems. RBCs are used for wastewater treatment when the facility footprint, maintenance needs, energy or start-up costs are limiting factors, and they can facilitate the development of decentralized water treatment networks [91].

The moving bed biofilm reactor (MBBR) was developed in Norway in the late 1980s to address the needs of small sewage treatment plants (i.e., less than 2000 p.e.) and overcome some limitations of the AS systems. MBBR technology can be used to easily upgrade existing AS WWTPs by increasing the loading capacity with little additional costs (biocarriers and effluent sieve), and this led to the commercial success of the technology, with more than 1200 installations in 50 countries. The compact footprint and high carbon and nitrogen removal performance make MBBR a valuable option for either small decentralized facilities or upgrades of existing centralized facilities [92].

Another technological modification aimed at overcoming the need for final solids to be separated from effluents occurred in the introduction of filtration media. Initially, sand or textiles (TF) were used. Sand filtration is an ideal treatment technology for developing countries and rural communities due to its low cost and ease of operation and maintenance [93]. Sand filtration was first used in the early 1800s as the sole drinking water treatment: in 1829, the first prototype sand filter bed entered operation at the Chelsea Waterworks Company to purify River Thames’ polluted water for supply distribution to London residents. As a result, water-borne diseases such as cholera and typhoid became less common, and this meant that London could further expand as a leading industrial city and global economic hub [94]. Sand filters are still used in waterworks worldwide as a first step in potable water treatment.

Sand filters may be used as a polishing step in basic wastewater treatment after solids in raw wastewater have been separated in a septic tank. Wastewater is applied over a bed of coarse sand (0.3 to 1.5 mm), percolates through this layer, and after some recirculation loops is sent to a disinfection tank. In addition to the sieving effect, organics are also removed by an active biological film developing within the sand volume. Wastewater treated by sand filtration is usually colorless and odorless; sand filters can provide a good effluent with a 95% removal of organic load and solids and may provide significant nitrogen removal. Sand filters are relatively easy to operate and do not require the use of chemicals but may need frequent backwashing to avoid clogging the pore volume. Pumice is an aggressive filtering alternative to sand (and other media) for the treatment of municipal and industrial wastewater. Its low specific gravity and high porosity make it ideal for these applications, with several advantages over other media such as expanded clay, anthracite, sand and sintered PFA [95]. Comparison tests between deep bed sand and pumice filters for treating water found pumice to be superior in turbidity removal performance and head loss reduction.

TFs were originally adopted in the United States as decentralized wastewater management systems. Their efficiency combined with their low cost and simplicity of operation soon made them an innovative wastewater management technology. The innovation of this technology is that it has the ability to treat liquid waste efficiently with minimal land use. The required installation area is about 0.1 m²/p.e. due to the characteristics of the filter material with a high specific surface area. TF material is rated for an average lifespan of 20 years, in design conditions, and can then be replaced at a relatively low cost. A comparison of the performance of TF vs. AS indicated almost ten times better efficiency. Data from residential and commercial textile media packed bed filters show that these provide consistent, high quality wastewater treatment. Small compact systems are manufactured for units of capacity from 20 to 100 p.e. [96]. The technology benefits from low energy and operational costs and minimal reliance on complex mechanical equipment [31]. Today there are more than 20,000 TF units ranging in size from 20 to 2000 p.e. installed. In Crete, a facility of a 10,000 p.e. capacity is currently under construction. A TF system is shown in Figure 9.

Filtration technology can be applied to both water and wastewater purification, which is equally applicable to centralized and decentralized facilities and has been in existence in modern times since the 18th century [31]; however, archeology showed that ceramic clay filters were already used in Minoan times to produce potable water from seawater, so these could be considered precursors of the present-day ceramic membranes [97]. A highly innovative process that dramatically improved the efficiency of treatment systems and their future development was introduced in the form of modern synthetic membrane media around the middle of the 20th century. Membranes were originally intended for industrial separation purposes in laboratory and industry and have been successfully used in the water sector for the past 60 years, taking an increasingly important role in the provision of safe water supply and treatment and the reuse of wastewater. After the introduction of cellulosic RO membranes in the 1960s, these were applied to desalination in water-scarce areas and other niche water-related applications. The first applications in water treatment and reuse started in the 1980s and membranes have since seen a dramatic growth in applications across the entire water sector, particularly in the past 20 years due to the introduction of tighter regulations, water scarcity and technological advances [98]. In supply water treatment, RO is now the predominant method of desalinization and micropollutants removal, including most pathogens [10]. RO is currently capable of providing pure drinking water at a cost lower than 0.50 EUR/m³ [97]; while this cost is fully justified in situations where no freshwater sources are readily available, it is still higher than the local treatment of secondary effluents for potable water production.

Membrane bioreactors (MBRs) technology integrates the biological degradation of wastewater pollutants with a high rate of filtration, ensuring the effective removal of contaminants and nutrients from domestic and industrial wastewaters. By achieving high biomass retention and thus high solid concentrations in the reactor, MBRs allow for higher pollutant removal rates and have become a proven more compact alternative to traditional AS systems. Micro (MF), ultra (UF) and nanofiltration (NF) treatment provides high quality effluents by removing small particles up to large ions size (NF) by means of a physical barrier with pore sizes ranging from 1 to 0.001 µm. NF can remove solids, protozoa, bacteria and most viruses from solutions, achieving not only the removal of almost any pollutant, but also effective effluent disinfection. For this reason, MBRs are considered state-of-the-art technology for wastewater treatment and reuse, both at a centralized and decentralized scale [10,99,100].

Major cities in water-stressed areas currently rely on the combination of MBR wastewater treatment and RO potable water processing to provide “toilet to tap” water reuse. Singapore is perhaps the most notable example since it is committed to this strategy to reach water independence [10].

Although MBR technology is currently the most efficient biological treatment in terms of pollutant removal capability, it also has some drawbacks that could limit its application: energy requirements for the generation of the necessary Transmembrane Pressure (TMP) to draw water through the filtration medium are high, and maintenance requirements can be quite demanding. Membrane media are in fact subject to fouling by biological solids, organic matter and inorganic precipitates, which progressively reduce their permeability, making them require frequent periodic chemical cleaning. Additionally, higher aeration rates compared to an equivalent AS process are required, since in addition to biological requirements, excess air is needed to scour membranes’ surface. Furthermore, not all the fouling is reversible, and in due time (usually 8–12 years) the membrane medium must be replaced at a substantial cost [101]. MBR systems have been successfully used in decentralized applications [102] but with energy requirements about three-fold those of traditional AS systems [103].

A new type of membrane-like reactor, the Biomass Concentrator Reactor (BCR), was proposed to overcome the main drawbacks of MBR systems by adopting a filtration medium that can be assimilated to a membrane, although the much higher porosity (5–20 μm) does not fall into the traditional porosity range defining membranes (below 1 μm) [104]. The BCR process, working by gravity flow only, positions itself energetically much closer to a traditional system than to an MBR while still maintaining a good degree of biomass filtration capacity. Fine bubble aeration provides enough scouring energy to keep the filter surfaces clean for long operational periods. BCR systems were tested for treating domestic [104] industrial [105] wastewater and contaminated groundwater [106] with excellent results. The BCR concept was subsequently improved by introducing a weak electric field in the reactor, which improved the filtration efficiency and diminished fouling by exploiting the electric polarization of sludge flocs [107,108].

Overall, MBRs (and BCRs) could represent a suitable solution for compact decentralized wastewater treatment installations in periurban and urban areas combining biological pollutants removal with high rates of physical filtration. The cost and operational and energy requirements, however, still limit their application in more remote decentralized locations. In these cases, an old, newly improved technology may be applied with success.

5.3.1. Anaerobic Treatment Technology

Anaerobic fermentation is a naturally occurring process that causes the reductive degradation of organic matter to CO₂ and methane. Anaerobic degradation occurs in nature and in septic tanks in the absence of free dissolved oxygen. It has been used for decades for the stabilization of excess sewage sludge; however, due to low kinetic rates, it was not considered suitable for the treatment of large volumes of wastewater, for which faster aerobic degradation processes were preferred. The greatest breakthrough in anaerobic process technology was the development of the UASB reactor, in which a high biomass concentration is achieved by the principle of autoflocculation though the generation of granular biomass [109]. This allows for the processing of relatively diluted substrates, with substantial energy savings due to the lack of aeration, while recovering CH₄-rich biogas even in nonoptimal process conditions [110]. Recent developments have highlighted the operational flexibility of these systems [111]. UASB reactors are simple to design and operate and do not require complex and expensive mechanical equipment. Nowadays they are considered a consolidated process technology that overcomes the limitations of conventional anaerobic processes and, as such, have found many applications in urban sewage treatment in developed and developing countries alike and have been indicated as possible alternative sustainable technology for decentralized systems [109].

5.3.2. Recent Advances in Water and Wastewater Treatment Technology

Energy consumption and greenhouse gases (GHG) emissions are becoming central issues in the current quest for future development sustainability, and this is also true for the water/wastewater sector. Consequently, research in wastewater processing technology is constantly aiming at reducing these inputs and maximizing resource recovery from wastewater streams.

Sequencing batch reactor (SBR) technology has perhaps been the most promising of many proposed AS process modifications and, due to its operational simplicity, has been applied in many small systems, particularly in rapidly growing regions of developing countries. In SBRs, wastewater is treated in a batch reactor by a sequence of repeated steps: filling, reacting, settling, decanting and idle. In SBR processes, both anaerobic and aerobic phases can occur sequentially in the same unit, achieving carbon removal as well and nitrification and denitrification [112]. The breakthrough in SBR technology was made possible by the Aerobic Granular Sludge (AGS) process [113], based on the formation of fast settling, compact sludge granules in which aerobic and anaerobic reactions can occur simultaneously with highly efficient organics and nutrients removal. The settling characteristics of AGS could partly replace the energy-intensive MBR process, although micropollutants and pathogens may still be present in the effluent, requiring its postprocessing for specific uses. This process is commercially known under the proprietary name NEREDA [114]. Granular sludge is also at the basis of the ANAMMOX^® process, a highly energy efficient denitrification process [115]. Both NEREDA and ANAMMOX substantially reduce the structural and mechanical design complexity and the environmental footprint of WWTPs, both as land occupancy and energy requirements, improving plant efficiency.

Current technologies allow for the removal of conventional pollutants from water and wastewater to very high levels; unfortunately, this is not the case for many emerging compounds. While it is a priority to remove them from effluent and especially from drinking water, no effective removal technologies that are capable of simultaneously removing all the contaminants of emerging concern have been discovered to date [116]. These compounds are of anthropic origin and hence of recent development, and they have been detected in waste and supply water even more recently; the most common processes for their removal are advanced oxidation processes (AOPs) [117] and advanced reduction/oxidation processes (AORPs) [63]. These are based on the generation of highly reactive radicals (·OH and others) by means of chemical (e.g., ozone, hydrogen peroxide, dithionite, sulfite, etc.) or direct energy (e.g., UV, ultrasounds, high energy irradiation) addition to the treated solution. In addition to providing potable water disinfection at a relatively low intensity, they are highly effective for the degradation of most emerging pollutants; however, they may result in the formation of degradation/transformation byproducts that are not well known or even identifiable by typical screening water and wastewater approaches [118]. These processes are energy intensive, which may limit their applicability to highly specific situations. Since the technologies are quite different, performance ought to be parameterized in terms of energy efficiency. The EEO (electric energy per order) index has been proposed to establish the energy needed by a given technology to reduce a contaminant by an order of magnitude (i.e., 90%). Among these systems, high-energy water radiolysis (electron beam and γ-rays irradiation) is a high-energy process that, compared to chemical methods, uses a clean technology that minimizes the formation of hazardous byproducts [119]. The effects of UV irradiation on pathogenic organisms was well known in ancient times, as proven by the ancient use of solar disinfection, which was used since 2000 B.C. when sanitation rules stated that water ought to be purified by exposure to sunlight and charcoal filtration prior to drinking. These rules also stated that impure water ought to be purified by boiling and dipping a piece of copper in it seven times before filtering [120]. This confirms the full awareness of the disinfection/germicidal benefits of the sun (UV rays), of the removal of contaminants/germs by filtration, and of the effect of heavy metals (copper) for the elimination of infectious organisms in ancient times.

5.3.3. Considerations on Centralized vs. Decentralized Approaches in Present Times

Centralized water/wastewater systems operate at a large scale, using extensive collection and distribution networks requiring large infrastructural investments. For example, Athens’s wastewater treatment plant in Psyttalia Island (about 2 km west of the Port of Piraeus) is the largest one in the country, designed for 5,600,000 p.e. and a hydraulic capacity of 265 million m³/yr. The plant cost 80 million EUR in the first stage in 1994 and 205 million EUR in in the second in 2004 [121]. In addition, the cost of O&M in an average year is about 27 million EUR.

A centralized approach could make use of some economies of scale in water/wastewater processing depending on the adopted technology, but the impact of the initial infrastructure (pipe network) could constitute a major fraction of total investment, even increasing with the degree of centralization (the more centralized the system, the larger the flows involved and the pipe size required), and it could be unaffordable especially in developing countries. On the other hand, decentralized “clustered” systems will limit the length of the pipes required and the flow collected at any cluster facility. In addition, a decentralized approach can help sanitation in more distant communities and industrial sites by bringing the precisely needed service where it is needed with significantly reduced initial costs. Most process technologies initially intended for centralized applications could be downscaled with no loss in efficiency. The appropriate choice of technology both for drainage and treatment will then affect the actual total cost of the investment and O&M, including energy. The technological process described above, according to its suitability for application to centralized or decentralized wastewater treatment approaches, is summarized in

Table 2. Suitability of the most common current treatment technologies to decentralized and centralized applications.

Technology	Centralized Systems		Decentralized Systems
	Pros	Cons	Pros	Cons
Earth/Soil		High areal footprint. Hygiene and groundwater contamination issues.	Low infrastructure impact. Suitable for remote, low populated areas.	High areal footprint. Hygiene and groundwater contamination issues.
Natural Systems		High areal footprint. Disinfection required.	Low infrastructure and energy impact. Suitable where land cost is not an issue. Easy to operate. Robust and resilient.	Low resources recovery (possible with algal harvesting). High water loss in hot climates. Public acceptance.
Biological systems (aerobic)	Very efficient mainstream technology. Low areal footprint. Opportunity of recovery of nutrients and energy from sludge.	Require post-treatment and disinfection. High investment and O&M costs (energy, sludge management). MBBR systems may have more complex operation. Filtration systems may have fouling issues.	Same as centralized, can be operated remotely with automated control.	Same as centralized. Energy issues may be more critical in remote areas.
Biological membrane systems (aerobic)	Very efficient technology. Very low areal footprint. Robust against flow transients. Opportunity of recovery of nutrients and energy from sludge. Disinfection may not be needed.	Higher investment and O&M costs thanks to nonmembrane systems (energy and maintenance). Complex operation and fouling issues. Periodic media substitution needed. Lower sludge management impact.	Same as centralized, can be operated remotely with automated control. Easy to implement in “packaged” facilities.	Same as centralized. Energy and maintenance issues may be more critical in remote areas.
Biological anaerobic	Efficient technology. Low areal footprint with UASB and related technology. Energy savings and recovery. Low capital and O&M costs. Opportunity of recovery of nutrients.	Lower pollutant removal efficiency than aerobic systems, especially with dilute streams. May require post-treatment. Possible nuisance from odors.	Same as centralized, can be operated remotely with automated control. Easy to implement in “packaged” facilities. Local energy recovery in form of biogas (methane)	Same as centralized.
Advanced Oxidation processes	Highly efficient against emerging pollutants.	High investment and energy costs. Requires expert operators.	Same as centralized.	Same as centralized. Operating and control issues may be critical.

.

6. Sustainability of Water and Wastewater Management

Water and wastewater management in the next future will be affected by dramatic changes due to a number of factors, both anthropic and not. Increasing population, urbanization and industrialization and the expectation of higher living standards in developing countries will drive the demand for water resources to higher levels. Since natural resource availability does not always match the predicted future demand, increasing attention to their recovery, reuse and extraction (e.g., from marginal sources and oceans) will be necessary. Furthermore, the Nexus between water, energy and food has become obvious; therefore, the increased integration of approaches and technologies will be essential in order to sustainably address all pressures. These could be tackled only by increasing the resilience of current water management paradigms to external pressures, which are still uncertain to a large degree. In primes, climate variability is expected to affect the availability of water resources in time and space; already, the occurrence of extreme events with longer dry periods and more intense short precipitations spells has manifested, highlighting the need for stormwater storage management through adequate approaches and infrastructure. The lessons of ancient practices (cistern storage and percolation) must be adapted to present urbanization schemes. At the same time, the mitigation of extreme events is needed according to totally new paradigms, as the approaches of LID and “Sponge Cities” have shown preliminary positive results [122,123].

Decentralization strategies have shown potential to optimally address urban water systems resilience requirements. In fact, distributed systems are not only prone to better answer local resources reuse needs [11], but due their complementary nature, they allow for the higher overall antifragility of water management systems, since the failure of one component will not bring down the entire network [124]. Wastewater reuse close to the source of emission can be achieved with adaptive technology according to the objectives of reuse, and the integration of various processes can enhance the efficiency and reuse possibility [10]. In remote touristic locations in many developing and developed countries, seasonal large population increases further increase the water supply and disposal systems stress. Flexible, decentralized solutions would limit this impact [125].

Most decentralized systems, operating at limited ranges from wastewater sources, may exploit at best the advantage of gravity flow rather than pumping, and could easily be scaled up to the needed size in communities with rapid growth, thus wisely using energy, financial resources and land. Advanced decentralized systems can achieve treatment levels comparable to centralized ones, can be designed to meet specific treatment goals, handle unusual and peculiar site conditions and address local environmental protection requirements [10]. Therefore, they are the best option to comply with new paradigms for sustainable urban development.

Source separation may be a suitable strategy for decentralized applications where freshwater resources availability is limited [44]: greywater may be treated locally for non-potable reuse (e.g., WC flushing, irrigation) [126] while concentrated blackwater can be treated anaerobically, for example with UASB systems, with energy recovery in the form of biogas [109]. New collection system paradigms, based on in-house source segregation, could allow cities to locally close the water/energy loop and could result in significant energy savings for conveyance, treatment and supply [127].

Potable reuse, already applied in several water-stressed cities, will be a critical element in the development of sustainable strategies for water supply, especially in high density urban areas, and will act as an alternative to water withdrawal from remote sources. In Athens (Greece), for example, water is drawn from inland regions (up to 220 km away) and is distributed and discharged into the sea coastal waters in an unsustainable fashion [45]. The quest for resilience could be achieved by reinterpreting and reapplying old practices and philosophical approaches in a modern key [128].

7. Epilogue (Conclusions)

In this paper, wastewater and stormwater management technologies in ancient and modern times are presented and discussed. In ancient Greece, storm- and wastewater management in urban areas, including disposal practices, were characterized by simplicity, the robustness of operation and the absence of complex mechanical equipment. In summary, they were resilient. Modern sanitation systems, established since the second half of the 19th century in European and American cities, on the other hand, are characterized by the fast removal of waste and stormwater, limited reuse and added technical complexity due to higher objectives and expectations. The purely technical approach, however, is already showing several limitations and vulnerabilities: modern sewerage systems are poorly resilient to events that, once considered rare, are nevertheless becoming more frequent due to climate variability.

Throughout the ages, innovation has played a key role, as achieving progress required individuals to meet emerging challenges; however, modern era water management paradigms have relied more and more on hard technology rather than adaptation. In other words, the resilience of ancient systems has been foregone in favor of punctual efficiency. In the future, waste- and stormwater management systems will be faced by more severe challenges induced by higher pressures and ongoing climate evolution, some of which could be addressed by improving technology and some by enhancing adaptation and resilience.

In addition, the development of megacities has rendered the traditional paradigm of highly centralized wastewater management untenable, as it requires massive resource investments and strongly limits reuse applications. This situation is no longer bearable: there is an urgent need for sustainable and cost-effective water supply and sanitation facilities, particularly in large cities of the developing world. Thus, the applicability of the selected ancient water management concepts to the contemporary developing world should be considered. Learning from technologies and practices implemented in the past, such as design philosophy, adaptation to the environment, decentralized management of water and wastewater projects, architectural and operational aspects and sustainability of design can be guiding principles. Sustainable wastewater management is defined as the collection, treatment and reuse of water in a way that will not adversely impact the health of humans or other species, will preserve environmental quality and the integrity of ecological systems, recover the energy and nutrients present in waste and utilize resources efficiently. The rapid growth and development of urban areas has increased both the importance of sustainable wastewater approaches and the complexity of their implementation. The predicted increase in urbanization will have impacts on the future wastewater management philosophy and especially on the development of collection and treatment infrastructures, which will evolve in self-supporting, small, resilient, resource recovery oriented systems. In a highly urbanized world, the development of cost-effective water supply and wastewater sustainable technologies to support all water harvesting, storage and reuse, in order to increase availability and minimize flood and disaster risks, will be of utmost significance.

This review was written with the belief that historical research showing the collective experience and “philosophy of sanitation” can provide inspiration to face future challenges.

Όμοια γάρ ως επί το πολύ τα μέλλοντα τοις γεγονόσιν, i.e., The events to come extensively resemble to those of the past

UNESCO’s ‘Aristotle Anniversary Year’ 2016.