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Review

Integrating the Design of Tall Building, Wastewater Drainage Systems into the Public Sewer Network: A Review of the Current State of the Art

Institute for Sustainable Building Design, Heriot-Watt University, Riccarton Campus, Edinburgh EH14 4AS, UK
*
Author to whom correspondence should be addressed.
Water 2021, 13(22), 3242; https://doi.org/10.3390/w13223242
Submission received: 3 August 2021 / Revised: 22 October 2021 / Accepted: 29 October 2021 / Published: 16 November 2021
(This article belongs to the Section Urban Water Management)

Abstract

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The design of above ground building drainage systems follows codes and standards that only give cursory recognition to the fact that this system connects, in the majority of cases, directly to a vast network of sewer pipes leading to a wastewater treatment plant. At the same time, for underground systems, airflow within as well as in and out of sewers is often neglected during the design of sewers, which depend on these building installed systems for pressure relief and venting. There is clearly an interaction between the two systems, yet this is not reflected in the design guidance, particularly inside buildings where air pressure fluctuations can lead to the destruction of water trap seals and the ingress of foul air containing sewer gases and potentially harmful pathogens. In this systematic review of historical research and design practice for both above and belowground drainage systems, we present the current state of the art and make recommendations for advancements that recognise the interaction between systems and present a view on how design could be advanced in a more holistic way.

1. Introduction

Building drainage and sewer systems are both integral parts of a larger network, which aim to convey and discharge their contents without causing unacceptable environmental nuisance and danger to public health [1]. The Victorian attitude to drainage was to get rid of unwanted wastewater as quickly as possible, hiding it away in underground pipes, using a one pipe system called the combined sewer system, without considering the impact this system has during a heavy rainstorm, which may lead to sewage-contaminated flooding and back up through toilets and baths into habitable space [2]. Additionally, any failure in sewer design and/or construction deficiencies and absence or inadequacy of maintenance may result in a reduction in hydraulic performance and blockages [3]. Although building drainage is designed in isolation from the sewer without considering the existence of the main horizontal pipe, which connects the whole building’s drainage system to the public network (Figure 1), any external pressure fluctuation and ventilation conditions may affect the system performance. There are several significant factors involved in integration between the above and below ground systems, such as air pressure, air flow movement [4,5], and ventilation [6].
Local and international codes provide essential information for designing drainage vent systems. In contrast, the current codes require further understanding of the integration of the building drainage system (BDS) of tall buildings into the public network and how system performance is influenced by the variability observed at this interface [7]. However, the number of tall buildings being constructed around the world is growing rapidly with increasing population, which introduces unique issues when detailing the design and specification of wastewater drainage and vent systems [8]. The concurrent need to adapt the design of sewers to these changes and developments is not easily addressed. Increasing the irregularity and complexity of structures requires thorough understanding of all the relevant building concepts, including drain and sewer systems. In particular, the need to focus on pressure transient phenomena within the fluid services associated with building operation has increased with the complexity of the built environment [9]. Different studies have shown that the effect of these air pressure (positive and negative) transients on the system can be devastating and lead to the ingress of foul air into the habitable space [10,11].
The design and operation of existing sewers may require to be enhanced to cope with the sewers’ capacity. Factors that contribute to overloading the system and may cause some combined sewer systems and storm tanks to spill for longer periods than usual include environmental factors (climate change), urbanisation, and customer demand and usage. Therefore, with increasingly tall buildings, especially in industrialised countries, when the systems are more than 150 years old, as in London and Manchester, they may be difficult to replace and renew [12]. Urban drainage networks usually operate in a free surface condition with a filling ratio of 0.7–0.8. However, severe rainfall events and unexpected conditions such as pipe blockage and backflow resulting from pump stop and inappropriate design can lead to sewers overflowing with road flooding and possible flow back to houses [13].
Therefore, extensive research is required to prevent or reduce pressure fluctuations from the sewer due to trapped air compression in rapidly filling combined sewer overflow, which may have a devastating health impact and damage the building drainage. Hence, it is necessary to provide protection between the building drainage and sewer systems to ensure the system’s integrity and prevent flow back from the sewer into the building drainage when the system is subjected to extreme events to safeguard against potential cross-transmission of disease. This study, therefore, aims to review most of the topics related to building drainage and sewer systems in order to fill the knowledge gap related to the integration of BDS into the public networks, as well as highlighting the need for a re-evaluation of design standards and building codes, especially for high rise buildings.

2. Methods

A systematic review was carried out on articles relating to historical and state of the art background studies on both above and below ground drainage system design and operation. The primary sources of literature were Scopus, Google scholar, Web of Science and PubMed. The search included a broad range of terms and keywords related to air pressure, building drainage, sewer system, dropshafts, climate change, urbanisation, and tall buildings. Due to limited research on the integration between above and below ground drainage systems in terms of the air pressure, the authors searched for below ground and above ground drainage systems separately and used a general search for finding articles. The first search revealed 650 articles; 400 were rejected that did not refer directly to the interaction above ground and below ground drainage design. Of the remaining 250 articles, 91 papers were chosen for relevance and rigour.

3. Understanding Air Pressure Transients within Building Drainage

The water film inside a vertical stack represents the air/water interaction responsible for driving air movements that are the potential source of the positive and negative pressure transients inside the building drainage system. The pressure fluctuations can lead to the destruction of water trap seals and the ingress of foul air containing sewer gases and potentially harmful pathogens. In addition, the drainage system is interconnected through all parts of a building and hence there is high potential for contaminated air to travel through a building. Therefore, the contents of this paper focused on air pressure inside building drainage, disease transmission, air pressure in the sewer system, and interaction between above and below drainage system.

3.1. The Mechanisms of Air Pressure Transients in the Building Drainage System

Air pressure transients are air pressure fluctuations caused by the discharge of appliances connected to the system and the consequent induced air flows, which propagate through the pipe network at the appropriate acoustic velocity and are subject to both reflection at the system boundaries and attenuation during propagation [14]. Pressure transient theory addresses what is now referred to as water hammer or pressure surge, a condition relevant to the design and operation of all fluid carrying systems, depending on their usage, such as public utility water distribution networks, sewage networks, and oil and gas transportation [15]. The air pressure transients propagate throughout the drainage system as a result of accelerating and decelerating flow conditions, which generate negative and positive pressure transients or imposed air pressure fluctuations from outside the systems such as wind shear over the roof stack termination or sewer air pressure fluctuations arising from remote sewer surcharge or pump selection [11,16]. Moreover, air pressure variations in a drainage stack can derive from the wind blowing across the opening, operation of a mechanical fan, or the opening and closing of a washroom window or door [17]. To understand the mechanisms leading to the generation of air pressure transients, it is first necessary to review the conventional view of water and air flow under steady state conditions in buildings’ drainage, waste, and ventilation systems (Figure 2). Buildings’ drainage networks are designed to carry discharges from a horizontal branch connection then downwards through a vertical stack, which runs the full height of the building, collecting discharges from each floor, and finally to a sewer network through a horizontal main drain. This operation involves an unsteady flow of water, where the appliance discharges to the system are time dependent and random. In response to air pressure distribution within the stack, the air pressure changes from atmospheric pressure at the upper termination and pressure falls due to pipe friction loss in a ‘dry’ stack. A local pressure loss is experienced due to the effect of the airflow being drawn through the water curtain formed at the discharging branch, which is followed by a pressure recovery zone within the lower ‘wet’ stack where the traction effect [5] of the water annulus on the entrained airflow gradually raises the pressure profile [9,15,18,19]. Increase in annular downflow leads to an increase in the entrained airflow, which therefore generates transmission of negative pressure transients, while reductions in the entrained airflow generate positive air pressure transients that propagate throughout the system [9]. At the base of the stack, the entrained airflow is forced by the pressure differential generated by shear force in the wet stack and the differential between this local terminal pressure and the drain pressure, normally taken to be close to atmosphere [15]. Air pressure rises close to the base of the stack due to the periodic flow ‘closure’ of the vertical to horizontal transition [14].
It can be seen that many studies have identified the mechanisms of air pressure fluctuation profiles within vertical stacks and horizontal branches in building drainage. However, there is still a lack of knowledge and understanding of the air pressure profile within the horizontal drainage pipeline because the building drainage is designed in isolation from the sewer, and it has been assumed that the sewer is at atmospheric pressure without considering the existence of the main horizontal pipe. Additionally, the influences of sewer air pressure fluctuations arising from remote sewer surcharge or any external conditions on the whole system’s performance have not yet been addressed for high rise buildings.

3.2. Studies on Pressure Transients along the Building Drainage and Vent Systems

Many studies have been undertaken to address various issues related to the prediction of the pressure response of building drainage and vent systems based on computer simulation models, experiment rig, and site test. The authors of [14,20] discussed the mechanisms of transient propagations and characterisation of boundary conditions: air admittance valve (AAV), water traps, water curtains, and terminations, based upon the use of method of characteristics technique to solve the defining St. Venant equations for continuity and motion and validation of the model against full scale networks subjected to pressure transient generators (PTG). Then, the research was extended to develop this model using a computer model, AIRNET, through the link between pressure transient and unsteady descending water film, which helped to simulate air pressure within the network through connecting the branch pipe to the PTG for introducing transients and testing the curtain boundary condition, the bend at the base being used to generate back pressure and test the water film pressure effect [4]. The same mathematical model was used for unsteady flows in a partially filled drainage system [21] to deal with appliance discharges. The numerical simulation model, AIRNET, can be used to simulate the range of possible flow conditions within the single stack system through defining ‘traction force’ to enable the simulation of both single and combined downflows in different sections of the wet stack [5]. Moreover, a pressure pulse technique was used at the base of the stack to identify the pressure transient of a depleted appliance trap seal, using the same method followed in the laboratory and site test to validate this technique [22]. In addition, collaborative work was undertaken between HWU and the National Science Council of Taiwan to make comparisons using an AIRNET simulation model to run the system and operating parameters of the experimental tower at NTUST, aiming to validate the accuracy of developed numerical models [23].
In Taiwan, an experimental tower of 40 m in height (13-storey building) was used to investigate air pressure fluctuations within single point discharge and steady flow conditions depending on a prediction model and empirical parameters. For (i) a single stack vertical drainage system [24], it was revealed that this prediction model can reproduce the mean air pressure distribution value in a vertical drainage stack, and for (ii) a 2-pipe vertical drainage stack and vent system [25], using different ventilation conditions (vent-loop types), the result confirmed the necessity of good vent design to balance and control air pressure fluctuation. Another study on the same tower investigated the air pressure fluctuation frequency, as well as the maximum and average air pressures with their respective standard deviations; the result showed that water seal failure would be likely to occur at some heights below the discharge locations [26]. Liao et al. (2011) examined air pressure variations and potential of installing a real-time drainage monitoring system for an in-use high rise building with unsteady discharging flow rates at a 78 m drainage stack (17-storey with three basements for car parking). The results showed the maximum air pressure at the drainage stack described by the probability density function of the measured data using the above proposed mathematical expressions for steady discharging flow rates with a modified constant.
In Hong Kong, a three-storey drainage test rig in the industrial centre at Poly U [27] and the 18-storey Li Ka Shing building were used for assessment of air pressure fluctuation in the system. The experimental results showed the occurrence of hydraulic jump as a result of differences between horizontal and vertical speed. The outcome from the site test showed the air pressure distribution did not satisfy a kind of normal distribution, pressure in the upper stack being more stable than that at lower level. The researchers suggested using a pressure attenuator near the stack bottom and double cross- vent pipes at the tee joint of related floors to ensure the performance of the drainage system [28]. In addition, investigations were undertaken by [29,30], cited in [24], based on experimental work and theoretical calculation in Japan to simulate air pressure distribution for a 30 m high single drainage rig.

3.3. Methods of Control and Suppression of Air Pressure Transients

It is worthy of mention that most of the solutions for public health protection are available inside the building, and the main reason is to minimise, or as much as possible control the pressure fluctuations occurring as a result of appliances’ random discharges that contribute to the time dependent water-flow conditions within the system to generate negative pressure as a result of increased entrained air. In addition, this occurs as a result of system surcharge at the base, at offsets, or at discharging branches that can cut off the air path down the stack, which generates positive pressure, and finally, the external factors that were mentioned before, such as a wind, sewer surcharge, and pump selection [10,11].
Based on Joukowsky’s analysis, surge protection depends on a reduction in the rate of change in the flow conditions. This will be achieved through reducing the speed of the valve closure using controlled valve closure or providing an alternative route for the flow via methods such as outward relief valve. In the case of building drainage and vent systems, traditional approaches have been used to protect the water trap seal, which represents a primary defence against the ingress of contaminated sewer gas into habitable space. Passive solutions include providing a traditional vent that will hold the pressure transient excursion to a minimum, but the growth in building heights has led to an increased need to innovate new techniques that are cost effective and applied as close as possible to the source of the transient, using methods such as the air admittance valve (AAV), waterless trap, and positive air pressure transient attenuator (PAPA). The control device must be positioned between the source of the transient and the site to be protected from pressure [10].
Negative pressure transients generated can be alleviated via providing local vents and using inwards relief air admittance valves [16]. However, these mechanisms do not address the problems with positive transients. The authors of [11] developed various volume containment attenuators to deal with positive pressure transients, but these devices were not a satisfactory solution to reduce the pressure transients to zero. Different research showed the effectiveness of the PAPA device, which was introduced as a flexible bag that allows air to leave the stack under positive pressure conditions, this air re-entering the stack once a negative pressure regime is re-established. Together, the use of AAVs and the PAPA device allows consideration of a fully sealed building drainage and vent system [31]. Extensive scientific research has addressed the effectiveness and necessity of positive air pressure attenuator in designing a building drainage and vent system [10,11,16,31] to control the problems close to the source of transient rather than not waiting for the relief reflection to returning from the vent open to the atmosphere. However, while these mechanisms have been used for control and suppression and are currently available to designers, they are not universally accepted by national code bodies.
Previous studies provided valuable information on ventilation systems. Controlling air pressure fluctuation is one of the important factors to ensure the performance of the building drainage system, and it is clear that using the active solutions of both AAV and PAPA represents a viable design option to alleviate the effects of air pressure transients created by normal system operation; nevertheless, the complex drainage and vent designs needed for high rise buildings require further simulation and validation of the models with experimental work and site test [31].

4. Transmission of Disease through the Drainage System

Extensive studies have investigated mechanisms of cross transmission, improvements in system design, and innovations in system monitoring, including confirmation of the wastewater plumbing system as a reservoir for pathogens [32,33]. Communicable diseases spread fast via plumbing and drainage systems in buildings. In 2003, an outbreak of the SARS virus occurred in high-rise residential blocks in the AmoyGardens housing estate, Hong Kong, China, with a total of 321 residents infected. Depleted trap seals and oversized bathroom extractor fans were identified as a significant cause of transmission of the virus [34]. The mechanism and route of spread of the SARS virus have been outlined by the Hong Kong SARS expert committee [35] and simulated by [36] through using AIRNET based on St. Venant equations of continuity and momentum, solvable via a finite difference scheme utilising the method of characteristics technique. The simulation results showed a minimal level of suction pressure is sufficient to induce significant air movement into the habitable space.
In addition, coronavirus disease (COVID-19), which was first reported in Wuhan, China, in December 2019, as with the SARS-CoV-2 virus, might move through the wastewater plumbing network. Despite there being no evidence to show that the COVID-19 virus is transmitted via sewerage systems with or without wastewater, research confirmed that plumbing or drainage is interconnected through all parts of a building, and hence there is high potential for contaminated air to travel through a building if the plumbing and drainage are defective [37], as shown in Figure 3 below:
This research also provided clear evidence regarding air entering the space from the main sewer via the drainage pipe network; however, to what extent the state of the sewer (during rainfall or dry events) will affect the building system is still not clear. The authors of [32] suggested a need for a secure protection between the building drainage/sewer network and habitable space within a building to minimise any potential cross-contamination and consequent risk to human health. In addition, Ref. [38] provided some recommendations to ensure that transmission through wastewater plumbing system is minimised.

5. Understanding Transient Flow in the Sewerage System

5.1. Two Phase Flow in the Horizontal Pipe

Two phase flow phenomena are often encountered in various industrial applications such as petroleum, power plant, nuclear reactor technology, food production, chemical process, aerospace and automotive industries [39,40], and sewage pipelines [41]. Two-phase flow can be solid–liquid flow, liquid–liquid flow, gas–solid flow, and gas–liquid flow. These mixtures may produce different geometric configurations, which are usually referred to as regimes or flow patterns. The flow regimes occur due to instability of flow and are influenced by physical properties of the fluid such as gravity and density viscosity, surface tension, and the flow system geometry [40]. One of the key issues in the design of urban drainage systems is the integrity of the system structure under hydraulic overloading. Uncontrolled air–water interactions can potentially result in severe infrastructure damages and may pose a hazard to the public [42].
A number of researchers have investigated the transition from gravity flow to pressurised flow during drainage sewer surcharging, and they indicated that hydraulic instability and strong pressure oscillations occur during the transition and air may be trapped and released either at upstream or downstream manholes (Yevjevich (1975), Yen (1978), Valentin (1981), Zech (1985), [43,44], and Baines (1991), cited in [45]).
Undoubtedly, entrapped air (air pocket) flow counts as one of the most complex issues in the transient flow scenario and may have detrimental effects such as blowbacks, reducing capacity, and inducing surge [46]. Transient flow with air entrained in a horizontal pipe occurs as a consequence of rapid change in system conditions: filling [42,47] or/and emptying operations of pipes [48] and differences between the densities of the water and air phases [49] may cause unexpectedly high pressure and/or geyser events [50].
Different experimental studies have attempted to identify the transient flow conditions through manipulating the outflow and inflow conditions. The authors of [45] stated that the establishment of pressure transients is due to a sudden gate closure, pump failure, or sudden rise in level at the downstream manhole and the subsequent release of trapped air bubbles at the upstream manhole; moreover, the pressure transients were observed to become more severe as the flow increased. Experimental investigations were conducted by [43,44] to examine the mechanism of transition between free-surface flow and pressurised flow conditions in a circular pipe, and the researchers found that a sudden change in boundary conditions will result in positive and negative interfaces. In addition, they observed that air pocket effects become important when the velocity of the air being forced ahead of the interface is large relative to the velocity of the water surface. Other experimental and numerical work was performed by Ionsson (1985) to address the impact of air pockets on pressure transients caused by pump 323 station operation, and the result showed that the sucking of air into the conduit through the pump led to production of high-pressure oscillation. Moreover, Valentin (1981) investigated the sewerage system’s surcharging issues and identified pressure pulse generation after escaping from the trapped air pocket [46].

5.2. Air Pressure Transient and Ventilation in Sewer Systems

In general, sewer systems are designed for carrying sewage flow without consideration of air movement. Consequently, the pressure in a sewer network at a certain location can build up, and air can be released uncontrollably [51]. The conveyance of wastewater through the sewer systems results in anaerobic conditions (oxygen deficient), which lead to generating of hydrogen sulphide (H2S) and cause serious odour problems [52]. The sewer system is similar to the building drainage system regarding the necessity of providing sufficient ventilation. Improving ventilation in sewer networks can be defined as an important approach to maintain aerobic conditions in wastewater and control odour problems and sewer pipe corrosion [53]. The ventilation must be properly designed and regularly maintained [54]; therefore, when responsibility for operation, inspection, and maintenance of vent and sewer systems is not clear, the cost of ventilation will be very high [55]. Many technologies and strategies are available to address odours and air pressure in the sewer networks, including liquid phase treatment, vapour phase treatment, and hydraulic improvements.
To determine the ventilation locations and air extraction rates, the fundamental physics of air movement in sewer networks must be understood [56]. The authors of [51] mentioned that Pescod and Price (1978) were perhaps the first to present scientific experimental studies on air movement in sewer systems. In 1982, they measured the air velocity profile for a single pipe as a result of wastewater drag and pressure gradient. They also considered in their research the factors affecting air flow in the pipe, such as the air pressure force, wastewater drag, the friction force by the pipe wall, and natural factors such as the wind across the stacks, temperature difference, fluctuation of water level, and barometric pressure [57]. Several studies have been conducted on modelling air movement in a sewer pipe. Edwini-Bounsu and Steffler (2003) used CFD models to calculate air flow in circular sanitary sewer headspace resulting from barometric pressure and wind speed, and in subsequent studies [51,58], studied air flow due to the events of pressure gradient and wastewater drag. In addition, Qian et al. (2018) developed a steady state model to simulate the air pressure and movement in a prototype sewer pipe relying on the air pressure force, wastewater drag, and the friction force by the pipe wall.

5.3. The Main Causes and Consequences of Flow Transients in Urban Drainage Sewers

Flow transients within urban drainage sewers are attributed to certain conditions, including severe rainfall events in which the system may be completely or partially submerged, sudden change in the boundary conditions, sudden change in flow velocity 363, and failure of pump operation, improper alignment of the pipelines, and air entrapment in pipelines. These factors may damage the systems through blowing off of manhole covers and basement flooding [43,45]. In addition, [59] point out that the pressure 366 transient may cause backflow in the tunnel overflow at the shafts, flooding and damage to the sewer system in the form of blowoff of dropshaft and manhole covers.
Such an incident occurred in 1980, in the city of Hamilton, Ontario, Canada, where large pressure transient was documented for a 429 m length sewer box (10 × 10 ft) connected to a drop pipe from upstream and interceptor weir in downstream with bed slope of 0.02. The pressure was sufficient to cause basement flooding and popping manhole covers. To prevent future basement flooding, a parallel sewer was built for the house connection [43].
Furthermore, in July 1995 in the city of Edmonton, Canada, during an extreme storm event, with a return period in excess of 1:300 years, surcharged storm and combined sewers resulted in surface and basement flooding through the entire manhole structure being blown off from the pipe, along with other structures including a 300 mm force main, a 1200 mm trunk sewer, a 600 mm water main, and a 400 mm gas line (Ibid).
Another severe pressure transient occurred in a Canadian sewage force main, where even through the system was designed to prevent over pressurisation, water hammer following pump shutdown caused pipe displacement at a pump station as a result of insufficient installation of surge protection [60].

5.4. Hydraulic Structure Dropshafts

Within a building drainage system, depletion of the water trap seal is mainly related to pressure fluctuations in a vertical drainage stack, but with sewer systems, most of the studies focus on air entrainment processes inside the dropshaft structures, which are commonly used in municipal drainage systems. Reviewing the dropshaft structure will help to understand some of the parameters intended to increase the air entrainment, such as flow rate, pipe size, and height of the structure or inlet.
Dropshafts can entrain a large amount of air and pressurise the air space of downstream sewers, leading to odour complaints when odour escapes from the sewer system [56]. Due to the difficulty in obtaining field measurements, the flow in dropshafts has been studied quite extensively in laboratories focusing on hydraulics of water flow [61] and air entrainment [62]. However, experimental results on air entrainment in the laboratory cannot reliably be converted to prototype values because of the lack of similitude criteria [63].
Several experimental works have been performed to find the relative air demand (the ratio of entrained air flow rate to the water flow rate), which was reported to be about 1.4 in a 3 m high dropshaft model [64], up to 40 in an 8 m high dropshaft [61], and about 160 in a prototype dropshaft of 25 m high. The authors of [61] measured air induction rates of dropshafts with two drop heights (7.7 and 6.3 m) in different water flow conditions and found that a larger drop height resulted in a higher air demand. In addition, A research [56] found that using a retrofitted dropshaft (air circulation pipe) could result in reducing the downstream pressure by about 40–50% compared to that before the retrofitting. Furthermore, a laboratory study in Canada showed that the pressurisation inside the dropshaft could be reduced by up to 26% at large water flow rates, while a field monitoring result showed that the air pressure in the upstream/downstream manholes and the airshaft varied in response to the variation of the sewage flow rate. Additionally, an air flow model was developed and showed that increasing the size of the air circulation 412 pipes has more effect on reducing the downstream air pressure compared to increasing the number of the pipes [65].

6. Connections with the Public Network

6.1. Link between Air Pressure in the Sewer and Building Drainage

Limited research has been found on the linkage between the sewer system and its influence on the building drainage and vent system and vice versa. A study by [6] demonstrated the relationship between air entrance (inhaled air) from the top of the vertical stack termination and entrained air in the inspection wells close to the building in order to enhance the ventilation of the sewer. The results were based on measured and observed models, and it was concluded that approximately 94% of the air would flow into the headspace of the sewer during each branch discharge, which would be sufficient to enhance the sewer ventilation by reducing the concentration of H2S and CH4 in the sewer to zero. This research was based on specific buildings of 7 and 10 floors, which were directly linked to the municipal pipe network by a connection of just 2 m, and made no attempt to address the pressure variations along the discharge pipe.
Additionally, an experimental study demonstrated different scenarios between ventilation conditions at the top of the vertical stack (sealed, fully open, and half open) and outlet conditions (fully open, half open, and fully submerged) to simulate sewer surcharge, and the result indicated that the inlet height had little influence on pressure fluctuation in the main horizontal drainage pipe [66]. While this study provides a good starting point to address the pressure variations along the horizontal pipe, the experiment was based on single stack and specific inlet height and might not be typical of or applicable to other types of building such as high-rise buildings with complex structures.
On the other hand, [16] used the computer simulation model AIRNET to simulate the building drainage system through applying external pressure at a top termination and sewer generated surge, creating a model that was capable of predicting air pressure within a network that would cause trap seal oscillation and possible trap depletion with a high- level appliance discharge. Combination of this simulation with a real building test for HBDS and laboratory test to mimic real HBDS would improve understanding of the pressure variations along both the vertical pipe, on which work has already been done, and the horizontal main pipe, which still needs further investigation.

6.2. Urbanisation, Climate Change and High-Rise Buildings

Urbanisation has the essential role of increasing the urban impermeable surface area, leading to increasing stormwater runoff. The urban drainage networks’ inability to deal with this increasing amount of surface runoff causes flooding events and carriage of urban pollutants to the receiving waterbodies [67]. The main sources for urban flooding are poor drainage systems, lack of maintenance, and poorly controlled growth of urban areas, especially in developing countries [68]. Put another way, urbanisation with climate change may reduce the hydraulic performance of sewer pipes [69] (Figure 4 and Figure 5).
A large body of literature has approached the impacts of climate change and rapid urban development on a global scale [70,71,72,73], but the specific literature area of the interconnection between combined pressures (urbanisation and climate change) and sewerage networks is less well covered.
Increase in the population of the city of Los Angeles led to a subsequent increase in the volume of wastewater between 1950 and 1960. At that time, the principles of ventilation were not understood, and significant air entrainment was observed in sealing maintenance holes that resulted in increased pressure in the sewer and caused sewer odours to escape back to the houses, released through the roof vents of homes or/ and through other maintenance holes nearby. To alleviate this odour problem, the government created a master plan to reduce the high air pressures occurring in the sewer due to hydraulically overloaded pipes. This plan consisted first of the construction of relief sewers, such as the East Central Interceptor Sewer and North East Interceptor Sewer Phase1, repair of trap maintenance holes, and construction of local sewers [74]. In addition, increasing urbanisation in recent decades has meant an upturn in the construction of high-rise and tall buildings/multi-storey buildings worldwide, particularly in emerging economies. In the city of London, UK, in recent years, building heights have reached 150 m and more, the Shard being an example with a height of approximately 310 m. Although SuDS usage is being actively promoted for new developments, limited attention has been paid to existing urbanised areas with old sewerage systems [75]. Tall buildings are increasingly designed as mixed-use structures such as commercial buildings, residential buildings, or built for specific purposes such as hotels, hospitals, educational facilities, etc. The increasing number of these special buildings with the increasing irregularity and complexity of structures requires thorough understanding of all the relevant building concepts, including drainage systems. Just as it is necessary to consider earthquakes, wind, and fire in structural design, building drainage is a substantial issue that needs to be addressed, especially in these types of buildings, to prevent odour ingress and cross-transmission of disease from the sewer network into habitable space.

6.3. The Need to Review Design Codes

The main aim of updating the design of the wastewater drainage system is to cope with the exceeding of capacity and to ensure the effectiveness of removing wastewater from building without threatening people’s health and safety in or around the building. Many of the codes of practice and regulations for sanitary pipework such as the UK code (1256-2:2000), Australian and New Zealand code (AS/NZ 3500.2:2003), and two from the United States (UPC 1-2003-1 and IPC), which are used internationally to specify systems, are essentially based upon a theoretical responsive pressure regime that may be described as steady state. However, the need to conserve water, the design of high-rise buildings, the complexity of users, and rapid changes in operating conditions, which give rise to inherently unsteady pressure and airflow propagation throughout the network [76], have led to an ongoing research requirement based on experiment and real site work. The design for buildings up to 14 storeys is well understood and based on essential guidance from international and local codes; however, there is still a lack of knowledge about the performance of these guides when they are applied to high rise buildings [7].
High density population, flourishing of the economy, development of high-rise buildings with complex drainage and vent systems, and climate change have all reinforced the necessity to redefine the operation of building drainage and vent systems to ensure that infection spread and odour ingress are avoided [77]. The authors of [78] stated that the complexity and size of modern buildings have increased the difficulty of construction and risk of performance failure or design faults. The design of the building drainage system is often interpreted without properly taking into account that increased building heights render the sizes of the drainage stacks and ventilation stacks inadequate in terms of pressure relief [27]. The substantial problems of various existing buildings should be identified to prevent inconvenience and health risks [78]. At the same time, the constantly changing urban environment and landscape may have a significant impact on increasing the volume of wastewater discharges on a building level that enter the sewer, thereby affecting the capacity of the sewer network, particularly in existing sewer design flow [79]. Therefore, poor performance of sewerage design may lead to backflow of excess wastewater into inside buildings via the main horizontal pipe.
In Scotland, in cases where appliances are below flood level, around 500 buildings are flooded each year due to sewer surcharge, the consequence of which is the backing up of the flow through the pipe into the building due to blockage in the system. Based on BS EN-12056-4:2002, the wastewater drainage system should be connected to wastewater lifting plants or via an anti-flooding device [80]. The applicability of this connection may vary from one country to another due to system complexity, maintenance, and economic issues and to what extent this system is able to cope with higher sewage flows when the sewer pumping station fails. Therefore, further research is required to understand the effect of flood on the building drainage system.
When the system is connected to a public sewer, the design standard will be changed; in the UK, the design code is based on BS EN 752:2008. This means that the wastewater building drainage system is designed in separation from the sewerage system outside the building without considering the interaction between networks and their impacts on each other.
Most of the drainage systems constructed after the 1940s have been designed, based on both probability of a flood happening and its impacts, to cope with rainfall events that occur with a one-in-thirty-year probability. However, there is no doubt that the capacities in older parts of the sewer systems (combined sewer) will sometimes be exceeded [81]. In addition, the fact that the main sewer uses building drainage as a ventilation system has not been taken into account in building drainage design.

6.4. Solids Transport

Another important interaction between above and below drainage systems is solids transport. Solids are essentially human waste and associated cleaning waste such as toilet paper and wipes. Extensive research has been carried out in the field of solids transport inside drainage systems over the past 44 years. The main purpose of these studies is to address issues regarding sufficient transportation of waste solids away from habitable space through the above and underground drainage systems to the collection points. The solids are considered to be discrete within above ground drainage systems, while this is not the case within the sewer system due to the initial breakdown of the organic matter that has occurred, leading to consideration of suspended solids [82]. The initial research in this area was undertaken by Swaffield (1975), who examined solid transport in above ground drainage pipes. This was followed by research by [83] that addressed the problems in the hospital drainage system.
The current international emphasis on water conservation strategy has led to a reduction in the quantity of water available to transport waste to the sewer and has had a major impact on the hydraulics associated with the transportation of discrete solids in near horizontal drains in buildings [84]. One study showed the effects of water conservation programmes (reducing wastewater flow) on the wastewater treatment plant and concluded that such programmes do not cause any major problems at existing plants and may, in fact, improve performance [85].
Research has indicated that drainage system conditions may make a larger contribution to the risk of urban flooding than the occurrence of heavy storm events [86]. The authors of [87] confirmed that solids have an effect on the pressure regime in a vertical stack and minimal impact on water trap seals of the short duration transients. Within a building’s drainage system, there are factors such as bends, joints, junctions, and design issues that should be taken into account when dealing with solids transport. The same factors, along with pipe gradient [88] and size of the pipe [89], have an impact on sewer blockages. The authors of [90] mentioned that sedimentation blockages occurring along the bottom section of the pipe in most building drainage collection systems result in reduction in cross-sectional area and change in profile. The authors of [91] identified three changes in household appliances and habits: increased fat, oil and grease (FOGs), food waste disposers (FWDs), and low flush toilets (LFTs), all of which have the potential to cause an increase in solid deposition at the extremities of the networks.

7. Conclusions

Based on the literature review, it can be distinguished that building drainage and sewer system are integral parts of a larger network, which aims to transport wastewater from residential, commercial, and industrial areas to the wastewater treatment plant. It is clear that there is a continuing need for research to enhance understanding of the air pressure regime within both the building drainage and the sewer system, and that full integration of these systems is far from satisfactory.
At the same time, previous research has shown the necessity of providing ventilation for both above and below ground systems in order to reduce the transient pressure phenomena. However, limited research has been found on the interaction between the two systems and their impacts on each other.
The key points from this paper can be summarised as follows:
  • There is limited understanding of the behaviour of pressure fluctuations in the main horizontal pipe, which connects the whole building drainage in high-rise buildings with the sewer system, generating from external factors and inappropriate ventilation conditions.
  • Even though there is clear system integration between the above and below ground drainage systems, the influence that the underground system has on the building drainage not been addressed yet.
  • High-density population, flourishing of the economy, development of high-rise buildings with complex drainage and vent systems, and climate change have all reinforced the necessity to redefine the operation of building drainage and vent systems to ensure that infection spread and odour ingress are avoided.
  • Providing ventilation for both above and below ground drainage systems is crucial to reduce the transient pressure phenomena.

Author Contributions

Conceptualization, M.G. and K.S.; methodology, M.G. and K.S.;resources, M.G.; data curation, K.S.; writing—original draft preparation, K.S.; writing—review and editing, K.S. and M.G.; supervision, M.G.; project administration, M.G.; funding acquisition, M.G. All authors have read and agreed to the published version of the manuscript.

Funding

Heriot-Watt University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Connection of waste pipes to main drainage.
Figure 1. Connection of waste pipes to main drainage.
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Figure 2. Single stack drainage profile with two active branches.
Figure 2. Single stack drainage profile with two active branches.
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Figure 3. Interconnection of entire building through the building drainage system [37].
Figure 3. Interconnection of entire building through the building drainage system [37].
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Figure 4. Wastewater collection system.
Figure 4. Wastewater collection system.
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Figure 5. Storm, tall buildings, and wastewater collection system pipeline.
Figure 5. Storm, tall buildings, and wastewater collection system pipeline.
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Sharif, K.; Gormley, M. Integrating the Design of Tall Building, Wastewater Drainage Systems into the Public Sewer Network: A Review of the Current State of the Art. Water 2021, 13, 3242. https://doi.org/10.3390/w13223242

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Sharif K, Gormley M. Integrating the Design of Tall Building, Wastewater Drainage Systems into the Public Sewer Network: A Review of the Current State of the Art. Water. 2021; 13(22):3242. https://doi.org/10.3390/w13223242

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Sharif, Khanda, and Michael Gormley. 2021. "Integrating the Design of Tall Building, Wastewater Drainage Systems into the Public Sewer Network: A Review of the Current State of the Art" Water 13, no. 22: 3242. https://doi.org/10.3390/w13223242

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