Will greywater reuse really affect the sewer flow? Experience of a residential complex in Al Ain, UAE

Use of water efficient appliances, reuse of wastewater/greywater and harvesting and reuse of rainwater/stormwater are the commonly applied urban water demand management practices. Water demand reduction leads to a reduction of wastewater flow and subsequently increases the wastewater strength. A reduction of 30% to 55% of water demand can reduce about 15% to 16% of wastewater flow (DeZellar & Maier 1980). As the flow decreases, the concentration of biochemical oxygen demand (BOD) and total suspended solids (TSS) is reported to increase by 25% to 40%. Parkinson et al. (2005) found that adoption of 6 litres and 4.5 litres capacities of toilet flush instead of 9 litres increased the TSS, BOD, chemical oxygen demand and ammonia nitrogen (NH₄-N) concentrations by 10% and 24%, respectively. According to Marleni et al. (2012), there are three sewer problems associated with changes of wastewater quantity and quality. These are sewer blockage, formation of odour and corrosion of sewer pipes. Previous studies informed that the occurrence of sewer blockage is determined by the wastewater flow velocity and its quality (Arthur et al. 2008).

The United Arab Emirates (UAE) is one of the most water-scarce countries, but their water consumption rate is significantly high. Water consumption in traditional villa-type detached houses in the Abu Dhabi Emirate ranges between 270 and 1,760 litres per capita per day (lpcd) with an average of more than 600 lpcd (Environment Agency – Abu Dhabi 2015). The security of its urban water supply requires an augmentation of water supply sources and reduction of water demand. While treated sewage effluent (TSE) is used for landscape irrigation in the Abu Dhabi Emirate, the TSE is not reused for domestic non-potable purposes because of the high expenses of the dual reticulation system. However, the decentralized greywater reuse scheme is comparatively less expensive than the centralized treated wastewater scheme. In a recent study, Chowdhury et al. (2015) explored the potential of decentralized greywater reuse for water demand reduction in the study area. The average greywater generation rate in the study area was found to be 192 lpcd, which was about 69% of the average internal water consumption. The sources of greywater in the study area are showers (49%), ablution (18%), laundry (10%) and wash basins (23%) (Chowdhury et al. 2015). It was shown that the generated greywater is sufficient to fulfil the non-potable water demand in houses. Therefore, the research question is how much greywater can be extracted for reuse without affecting the existing water and sewer pipe networks. The objectives of the study were to estimate the total mains water consumption, and investigate the water end uses and the quantity of downstream sewer flow. This study only focused on the quantitative analysis of greywater.

There are many challenges to reusing greywater. Presence of salts in treated greywater is an issue and presents a barrier to their reuse for irrigation. Stevens et al. (2011) assessed the environmental risk posed to Australian and New Zealand ecosystems by the presence of powdered laundry detergents in greywater used for garden irrigation. They observed an increased alkalinity, sodicity and salinity in plants and soils irrigated by treated greywater. The presence of phosphorus and boron was also identified in some detergents that was considered to be a high risk for some plants. This would be especially the case in very low rainfall areas such as the UAE.

In addition to chemical hazards, there are some social challenges to greywater reuse. Some of these challenges are lack of public awareness, public perception, unavailability of regulations and standards, and lack of social and political willingness (Khatun & Amin 2011). Greywater has low quality in comparison with municipal water, so there are many social challenges to reusing it on-site (McNeill et al. 2009). Social acceptance is the primary issue for utilizing this unconventional resource. Without public acceptance, reuse of greywater cannot take a step forward (Nitivattananon & Sa-nguanduan 2013). Social attitude has a great influence as well. In particular, water and wastewater have a religious value in some cultures. A study conducted in Palestine showed that about 75% of the residents of the West Bank did not agree to accept treated wastewater for irrigation purposes because of their local social–cultural values (Halalsheh et al. 2008). On the other hand, in Jordan, more than 50% of farmers agreed to accept treated wastewater for irrigation purposes. A comprehensive greywater reuse education and awareness plan can raise this acceptability level (McNeill et al. 2009). The cost of greywater treatment varies according to its utilization and the type of treatment selection. The payback period of greywater treatment systems depends on the volume of treated greywater and the savings from municipal water charges (Li et al. 2010). Usually, the greywater treatment system used for irrigation purposes exhibits a shorter payback period.

The consumed water volume was divided by the estimated population of the complex and by the numbers of days per year in order to estimate the per capita per day water consumption. Comparison was made between the calculated actual water consumption in each house and the guided consumption prescribed by local regulations and the Vision 2030. Tables 1 and 2 show this comparison, partially extracted from the Environment Agency Abu Dhabi (EAD) and the Regulations and Supervision Bureau (RSB). The Vision 2030 sets the priority threshold target for domestic water consumption (Table 1). The RSB, an independent body, supervises the water quality and supply standards in the Abu Dhabi Emirate, and provides the water demand and sizing criteria for water distribution companies in the Emirate (Table 2).

The primary purpose in performing the hydraulic modelling of the existing water network in the study area is to identify the effects of estimated greywater reuse, as reduction in total water consumption and change in water flow rate. The hydraulic modelling was carried out using the EPANET software (www.epa.gov). The water network pipes and their characteristics were collected from the AADC. Three scenarios were performed in order to estimate the effects of greywater reuse on the pipe and sewer network (Table 3). The first scenario represents the actual water consumption. The second and third scenarios consider a reduction of total water consumption by 5% and 10%, respectively. It was found that the indoor water consumption was only 15% of the total water consumption. At the 192 lpcd greywater generation rate, theoretically, a maximum of 8% of municipal water could be saved if all the generated greywater was harvested and reused (Chowdhury et al. 2015). The adopted scenarios in Table 3 identified the pressure and velocity drop and rise on the existing water DN.

The first scenario (Table 3) was based on the actual water consumption of 867,434 m³ (no reduction), which was estimated at the consumers’ connection points linearly converted into a flow rate of 27.51 litres/second (L/sec). The total wastewater produced by indoor water activities was estimated at 127,131 m³ annually, which was 15% of the total municipal water consumption (867,434 m³). In the literature, it was found that about 69% of wastewater is greywater (Chowdhury et al. 2015), and its generation rate was found to be about 192 lpcd. Therefore, a maximum possible water-saving target for the study area was about 8% of total water consumption (indoor and outdoor) and considering the 69% of wastewater is greywater, a maximum of 10% of total water could be saved. Hence, the other two scenarios (the second and third scenarios) assumed in this study were reductions of total water consumption by 5% and 10%, respectively.

The Hazen–Williams equation was used in the EPANET software for modelling the water network and to estimate flow friction loss due to the physical properties of the pipe. The equation simply shows the relation between mean velocity of water in a pipe and the geometric properties of the pipe and slope of the energy line: V = kCR^0.63S^0.54, where V is velocity (m/s), k is a conversion factor (k = 0.849 for SI units), C is the roughness coefficient, R is the hydraulic radius (m) and S is the slope of the energy line (head loss per length of pipe). The coefficient, C = 130, was considered for new DI pipes in the study area. The existing pressure of 1.5 bars at the source was entered in the fixed grade node or boundary node in addition to their elevation. The elevation of all junctions was entered according to the AADC built profile drawings and the demand for every junction was entered according to Table 4. After successfully running the model for hydraulic simulation, time series graphs for pressure and flow velocity were recorded. The tabulated results were also prepared for evaluation of the optimal selection.

The average plot and building size of houses in the complex is about 2,000 m² and 600 m², respectively. This indicates that about 70% of plot area is kept as open land area and more than 80% of houses occupy garden and amenity plantations in the open spaces. The estimated median family size was found to be ten persons per household. The average water consumption was found to be about 10,608 m³/year. It was found that only 20% of supply water is discharged into sewer pipes, and the remaining 80% is used for outdoor purposes (irrigation to plantation and car washing). Statistically significant (at 95% significance level) correlation was not identified between family size and annual water consumption per house. This is because water consumption is mostly influenced by outdoor water uses, which is independent of family size. From the model analysis, it was found that if greywater is reused at a 190 lpcd rate, about 30% of sewer flow is reduced, but total water consumption (indoor and outdoor) is reduced by less than 10%. Currently, only 8% of generating TSE is used for street landscape irrigation and the rest is discharged to the Arabian Gulf. Therefore, from a sewer flow perspective, the establishment of TSE connection pipes to every house/complex may serve better performance than the greywater reuse scheme.

Sewer flow comes from indoor water consumption and it was found that about 15% to 20% of the total water supply discharges to the sewer network. It is obvious that extraction of greywater reduces municipal water consumption, which will not only help to decrease water consumption but also alleviate the sewer load on treatment plants. On the other hand, a decline in wastewater discharge may affect the sewer flow and can increase the risk of sewer lines clogging. Wastewater treatment plants may experience dense sewer loads that require frequent removal of sludge (Friedler & Hadari 2006). The main purpose in assessing the impacts of greywater reuse on the sewer lines and treatment plant was to review different aspects of flow reduction and the building up of dense sludge in the sewer network. The Abu Dhabi Sewerage Services Company (ADSSC) wastewater treatment plant was visited to obtain the required information. Informal consultation was conducted with the plant operation engineers in order to evaluate their impacts on the sewer lines.

After the extraction of greywater, the wastewater treatment plants may receive a similar load with more concentrated substances due to less flow, and the boiler may consume the same amount of electricity as the load is same. But, the hydraulic retention time (HRT) in the aeration tank might be more than the usual HRT. Furthermore, some equipment may become oversized due to a reduction in the flow volume. The sewer line's HRT can be increased and sewer-solid content may settle on the pipe due to long HRT and the smaller portion of sewer water. Moreover, prolonged HRT provides a favourable condition for anaerobic bacterial growth. An aerobic process that occurs over a long time produces odour in the pipeline. The effect can be evaluated by estimating the average daily generation of black water and kitchen wastewater during a 24-hour period. Chowdhury et al. (2015) estimated that the average daily water consumption for toilet flush and dishwashing is about 39 litres/person and 44 litres/person in the study area. This indicates that a person generates about 83 litres of wastewater daily and discharges into the sewer network.

The ADSSC guidelines for the design of sewer lines recommend a design flow of 180 litres per capita for residential houses. Hence, it can be concluded that if 100% of available greywater is captured and reused, the average daily sewer flow rate will be about 50% (83 ≈ 90 litres/capita/day) of the design flow rate. As a result, extraction of 100% greywater, which is 192 lpcd, can effect the sewer line flow. However, extraction and reuse of 50% of available greywater fulfils the design flow rate in the sewer network. In addition, there is a possibility of odour and the building up of dense sludge in the sewer pipes, which can be minimized by proper operation and maintenance of the sewer network.

Urban water management is one of the major challenges in the UAE. The per capita water consumption in the UAE is excessively high, which is because of outdoor water uses. In the study area comprising 100 traditional villas, municipal water is used in outdoor plantations in almost every residential villa. It was identified that about 20% of the total supplied water is returned back to the downstream sewer system, which indicates that about 80% of the supplied water is used for outdoor activities. Because of this significantly high outdoor water use, water consumption was found to be independent of the number of family members. Residential water supply in the UAE is solely dependent on expensive desalinated water. Without reduction of water demand and augmentation by alternative water sources, there is no doubt that the urban water supply in the UAE will be unsustainable in the future. Since rainfall and runoff are not an abundant source of supply, the only feasible unconventional water source is recycled wastewater. This study explored the potential of greywater reuse, and in particular, how much greywater can be extracted without affecting the upstream water distribution and downstream sewer networks.

The modelling results indicate that the extraction of greywater has positive impacts on the water DN, such as that the improved pressure and reduced high velocity in the pipes may be favourable for identifying leakage and reducing pipe erosion, respectively. Furthermore, this may improve the minimum residual pressure in the pipes and be advantageous for maintaining the RSB recommended pressure of 12.5 m in the network. However, 100% extraction of greywater affects the downstream sewer network by reducing the design flow rate. It was shown that about 50% of greywater (about 95 lpcd) can be extracted without affecting the design flow in the sewer network. This study considered the quantitative aspect of greywater reuse and concluded that 50% of greywater can be recycled and reused. A comprehensive assessment considering quality, treatment, cost, health risk assessment and social acceptability is particularly essential for its application.

The research is funded by the UAEU-SEED research grant (31N183) entitled ‘sewer network problems associated with urban water source management practices’ awarded to the first author.