Techno-economic and environmental assessment of wastewater management systems: Life cycle approach

Highlights

• Selected wastewater management strategies were compared throughout their life-cycle.

• Conventional sewers had the largest environmental footprint of the tested systems.

• Cost-integrated life-cycle assessment proved SBS to be the most feasible system.

• 67% smaller treatment plant was adequate to serve the SBS compared to other systems.

• The septic/tanker strategy was the least financially viable of examined strategies.

Abstract

The septic-tank and tanker (STT) system is a traditional wastewater management practice commonly used in many developing countries. The system is based on septic tanks that are pumped out by sewage tankers on a regular basis. Although the STT system is gradually being replaced by conventional gravity sewers (CGS), the high capital cost of this shift remains a major obstacle. This research aims to investigate the economic feasibility and environmental footprint of an alternative wastewater management system compared to existing sanitation systems. The study examines the small-bore sewers (SBS) system which utilizes the existing septic tanks to separate solids from gravity-conveyed effluent. A comparative assessment of the three systems (STT, CGS, and SBS) along with their treatment facilities was conducted on a residential area in the United Arab Emirates. Local design criteria of the SBS system were proposed based on current CGS guidelines and international SBS standards. A cost-integrated life cycle assessment (LCA) was carried out in order to evaluate the environmental and economic aspects of the three management strategies. The total present value of the SBS strategy was found to be significantly higher than those of STT and CGS, respectively. Moreover, a 67% smaller treatment plant was sufficient to serve the SBS effluent. The LCA results revealed that the CGS strategy imparted the highest damage to the environment in all impact categories considered including global warming potential, whereas, the STT system produced a higher impact than SBS in six out of the eleven tested impact categories. Overall, while the STT strategy was the least financially feasible, the CGS had the largest environmental footprint. The eco-efficiency assessment revealed that the SBS strategy was the most favored among the examined strategies.

Introduction

Population in the United Arab Emirates (UAE) has been rapidly increasing in the last decade, reaching 9.4 million capita in 2017 at an average annual growth rate of 1.39% (World Bank, 2018). The high standard of living, among other factors, has resulted in one of the top per-capita water consumption rates worldwide, 550 L/capita/day (Szabo, 2011). This, in turn, has resulted in extreme wastewater generation rates. Wastewater collection systems comprising septic tanks and sewage tankers have traditionally been implemented in several parts of the country, and still widely used to date. Septic tanks in such systems function as storage units from which the accumulated wastewater is pumped out and transported to treatment facilities on a daily basis. However, local communities suffer from the negative impacts of this system, such as: major traffic delays, as well as multiple environmental and health risks caused by emissions and leakages. Therefore, to serve the increasing sanitation needs, the septic-tank/tanker (STT) system has been replaced by conventional gravity sewers (CGS). While the CGS system is successfully applied throughout the world, its high capital cost hinders its complete roll out throughout the UAE. Moreover, the system typically requires deep excavation which is impractical for densely populated areas. Hence, it is essential to explore unconventional cost-effective wastewater collection systems such as pressurized sewers, vacuum networks, small-bore sewers, and simplified sewers. In order to cope with the increasing consumption pattern, it is crucial to explore innovative, cost-effective, and eco-friendly wastewater management approaches (Ghulam et al., 2017; Tahir and Sagir, 2019). The present research is focused on the small-bore sewer (SBS) system which has been proven to be efficient in several countries and can be implemented in areas where the SST system is currently in use.

The SBS system is a solid-free pipe network that transports the liquid portion of the sewage under gravity from septic tanks to the wastewater treatment plant (WWTP). The system is also termed solid-free sewers, settled sewers, small diameter gravity sewers (SDGS) in the United States, and septic tank effluent drainage systems (STED) in Australia. The SBS consists of key components including: house connections, septic tanks, sewer conduits, and cleanouts. The wastewater solids effectively settle in the septic tank during a retention period of one to two days, thus eliminating sludge and scum from the liquid stream and preventing potential clogging of downstream sewers. As raw sewage flows through septic tanks, it undergoes primary treatment in the form of physical settling and anaerobic decomposition processes, resulting in 80% reduction of the sludge layer (Butler and Payne, 1995; Mills et al., 2014). A well-maintained septic tank can efficiently remove 30 to 50% of biochemical oxygen demand (BOD) and 60 to 80% of total suspended solids (TSS) (USEPA, 2002). Since the receiving sewers run free of solids, self-cleansing velocity and partially-full flow are no longer required. Hence, smaller conduits can be used at mild gradient, leading to lower material and excavation costs. Moreover, fewer inspection points, in the form of simple cleanouts rather than the bulky manholes, are required for maintenance (Otis and Duncan Mara, 1985; Tilley et al., 2014).

The SBS system is considered a potentially viable alternative to improve the current wastewater management practices. This is evident in several studies that documented its successful application in various developed and developing countries such as Canada, Australia, USA, Nigeria, Zambia and Egypt (Bakir, 2001; Hailu, 1988; Harindi and Kamil, 2011; Hass, 2007; Otis and Duncan Mara, 1985; Palmer et al., 2010). Hass (2008) examined case studies in Canada where SBS was installed and proved its cost-effectiveness compared to septic systems. It was found that only 50% of the WWTP capacity was utilized when the SBS was implemented. Nawrot (2010) focused on the financial aspect of the SBS system installed in Poland compared to the CGS and pressurized systems. Based on capital and operating costs, it was found that the SBS system was three times less costly than the other systems. A similar research was conducted in Botswana to scrutinize the feasibility of replacing the CGS system with either SBS or vacuum sewers (Little, 2004). The study concluded that both alternatives were more cost-effective than the conventional system. Moreover, Abdul Alim (1997) discussed the successful implementation of SBS in a 15,000-inhabitant village in Egypt. The results showed that the SBS system was 34% more cost-effective than the conventional system. On the other hand, Norman et al. (2011) discussed the performance of the SBS system installed in Dakar, Senegal. The outcomes of the project were unsatisfactory due to poor management of the construction activities.

Although there are few studies that covered the technical and economic aspects of SBS, the research conducted on its environmental impacts has been limited. Life cycle assessment (LCA) is frequently utilized in evaluating environmental impacts of conventional wastewater systems and comparing various alternative solutions in terms of estimated environmental loads (Nogueira et al., 2009; Tomei et al., 2016). Existing LCA studies of sanitation systems primarily focused on gravity sewer networks and treatment plants. For example, Risch et al. (2015) assessed the environmental performance of the construction and operation of sewer systems and WWTPs, including pipe materials, civil works, and road rehabilitation. Results indicated that construction of the sewer infrastructure was more damaging to the environment than the construction and operation of the WWTP. Few LCA studies have been conducted on the CGS system in comparison to other wastewater collection systems. For example, black water separation systems were found to cause less environmental impacts than the conventional system on the short- and long-terms (Lundin et al., 2000; Remy, 2010). Another study reported the LCA results obtained using comprehensive life cycle inventories for the construction and renovation of sewers (Morera et al., 2016). The results suggested that material type and lifespan, site-specific characteristic such as soil conditions, and civil works such as pipe laying and backfilling demonstrated the largest influence on the environment. Few studies analyzed different pipe materials used in the construction of sewer networks. For example, Akhtar et al. (2014) compared concrete, polyvinyl chloride (PVC), vitrified clay, and ductile iron (DI). The study concluded that PVC pipes demonstrated the maximum sustainability in terms of both environmental and economic aspects. Another study developed a model to assess the life cycle cost and environmental impacts of DI and PVC pipe materials. It was found that DI is more cost effective and environmentally sustainable (Thomas et al., 2016). A similar study analyzed the environmental impacts of various pipe materials (concrete, high-density polyethylene (HDPE), and PVC) embedded in different trench designs (Petit-boix et al., 2016). The concrete pipes in granular trenches were found to cause the lowest impact on global warming potential and cumulative energy demand. Other studies have analyzed the life-cycle environmental impacts of wastewater treatment plants (Corominas et al., 2013; Réka et al., 2019; Rodriguez-Garcia et al., 2011). Raghuvanshi et al. (2017) reported that the electricity required for the treatment processes, including collection, sludge activation, treatment, purification, and re-distribution had the highest environmental impact. Another study suggested that the effluent quality and electricity consumption during the operation phase had the most negative environmental impact (Li et al., 2013). Lorenzo-Toja et al. (2015) noted that although large WWTPs demonstrated higher performance levels, poor environmental profile was observed compared to small or medium WWTPs Petit-Boix et al. (2018) examined the combined financial and environmental impacts of wastewater using eco-efficiency framework. Based on the eco-efficiency indicators, an optimum location of the WWTP has been selected. Other studies have also explored economic and environmental impacts of small, decentralized plants, as well as waste to energy technologies and sludge stabilization activities in WWTPs (Mills et al., 2014; Nogueira et al., 2009; Tomei et al., 2016).

It is clear that the existing wastewater management practices, particularly STT and CGS systems, in the UAE do not always provide the optimum solution due to financial, environmental and/or sanitary limitations. Based on many successful international experiences, the SBS system seems to be a potential alternative that needs further investigation under local operating conditions compared to current systems. To date, the life-cycle cost and environmental impacts of the SBS system have not been addressed in previous studies. The present study presents the first cost-integrated LCA for the SBS system as part of an integrated wastewater management strategy. This research comparatively analyzes the techno-economic and environmental life-cycle aspects of three integrated wastewater management strategies involving the STT, CGS, and SBS systems. The study assesses the applicability of SBS in the UAE compared to current systems, each as part of a comprehensive wastewater collection, treatment, and disposal strategies. The assessment is conducted on a residential development in Sharjah, the third most populous emirate in the UAE after Dubai and Abu Dhabi. A customized version of the SBS design criteria is established based on local CGS standards and international SBS guidelines. A modelling software for gravity sewers is used for the hydraulic analysis of the CGS and SBS networks. The life-cycle cost analysis (LCCA) and LCA are carried out according to the ISO guidelines 14040 and 14044 in order to provide a thorough financial and environmental evaluation of the examined strategies. Furthermore, the environmental and financial aspects of the three sanitation strategies were combined under the eco-efficiency assessment framework based on ISO guidelines 14045. This study is intended to enrich the literature on unconventional wastewater management systems, particularly in developing countries, towards improved sanitation and sustainable development. The findings support stakeholders in making environmentally conscious decisions about cost-effective wastewater management systems for unserved communities.