THE INFLUENCE OF GREYWATER REUSE ON DOMESTIC WASTEWATER QUANTITY AND QUALITY
Abstract
Pressure on freshwater resources and increasing pollution to the environment as a result of inefficient sewer systems has led to the emergence of innovative water management strategies. One such strategy is wastewater recycling in the form of household greywater reuse (GWR). While previous literature offers insight into potential benefits of GWR, it has generally overlooked its effects on domestic wastewater. The objective of this paper is to quantify the extent to which GWR affects domestic wastewater quantity and quality when employed on a single-house scale.
For the analysis, three scenarios were modelled, each with a varying degree of GWR. Diurnal patterns of flow volumes, pollutant loads, and pollutant concentrations were examined to gain a better understanding of not just what the effects of GWR is on these parameters, but also to identify the times in the day when the most significant changes to wastewater quality and quantity occur. Wastewater streams were categorised according to the household appliances responsible for generating them, as well as the type of greywater produced. This analysis revealed that the greatest contributors to domestic wastewater by volume were bathtubs, showers and kitchen sinks.
As expected, the results showed that domestic wastewater flows are significantly reduced as a direct result of onsite GWR. The greatest reduction to wastewater flows was found to occur during the evening and morning peaks, coinciding with the period of peak wastewater generation. Contrary to initial expectations, it was observed that pollutant loads were reduced only slightly with an increase in GWR. Pollutant concentrations, however, were found to increase markedly with an increase in GWR. The greatest changes in concentration occurred during the morning peak hours.
Sewer systems are responsible for the drainage of two types of unwanted water: stormwater and wastewater.
Stormwater refers to water resulting from any form of precipitation that falls on an urbanised area. It may consist of pollutants from the air or catchment surface. Failure to properly drain this water could lead to damage of property, inconvenience and serious health risks to people living in the area.
Wastewater on the other hand begins as clean water that has been supplied to residential areas, commercial areas, and industries, and after such water has been put to the necessary use, it becomes waste water and is considered hazardous to the environment including public health if not properly drained.
Unwanted water has long been dealt with in different ways. For example, in small isolated and sometimes low-income areas, wastewater may be treated locally, and storm water directed into the natural drainage systems. Such systems are usually sufficient considering the minimal scale of pollution to watercourses involved. Some of them also have relatively small population size. However, urban areas tend to have much more complex sewer systems, consisting of an intricate network of pipes and drainage structures.
The methods of dealing with unwanted water have also been changing rapidly owing mainly to technological as well as engineering developments. Over time, therefore, the types of challenges faced by drainage engineers involved in the design of artificial drainage systems have led to the development of various types of sewer systems. Broadly, these include combined, separate, as well as hybrid sewer systems.
In earlier times, combined sewer systems were more commonly used in the design of towns. In these types of drainage systems, wastewater and stormwater are drained from a built-up area and carried together in the same pipe, as shown in Fig. 1.1. Pipes used in these systems are very large, to accommodate the two flows. Majority of the time, however, the pipes carry only wastewater (also known as dry weather flow – DWF). Since the ratio of stormwater to wastewater is such that the stormwater could be 50 to 100 times the DWF (Butler & Davies, 2000), and since it would be un-economical to provide the pipework and capacity at the Waste Water Treatment Plant (WWTP) required to treat all the flow, provisions are made for stormy periods when flow is significantly higher by incorporating combined sewer overflows (CSOs) at some convenient point in the system, where excess water (a dilute mixture of wastewater and stormwater) can be released to a nearby watercourse. The rest of the flow is carried on to a WWTP, where it is treated and released to the watercourse. This water disposal system has been found to be inefficient in terms of environmental impact and sustainability, particularly due to the spillage of contaminated flows into waterways during stormy weather through CSOs. These spills inevitably lead to pollution over time. These systems are commonly found in old towns, and account for 70% of all sewers by length in the UK (Butler & Davies, 2000).
Figure 1.1 – Schematic plan of combined sewer system (Butler & Davies, 2000)
Figure 1.2 – Schematic plan of separate sewer system (Butler & Davies, 2000)
1.3 Conventional Urban Drainage Systems
Conventional urban drainage systems are quite inefficient in water management. The water supplied to a typical UK household is treated to drinking standard, only to be used largely for non-potable uses such as flushing toilets. This leads to wasteful use of precious resources and contamination of previously unpolluted water. One way of reducing the negative effects that separate and combined sewers pose with regard to pollution of waterways is to substantially reduce household water consumption, as well as improve the quality of the wastewater entering the sewer. Wastewater recycling is emerging as an integral way of preserving freshwater supplies as well as potentially reducing the pollutants entering the environment. One method of doing this is the installation of a greywater system that reuses a proportion of the less polluted wastewater streams produced in a household for non-potable applications.
The definition of what constitutes greywater has often been termed a “grey issue” with the relevant regulations differing based on geography. In the UK, the Environment Agency defines greywater as “Wastewater (WW) from showers (SH), bath tubs (BT) and wash basins (WB) only”. It particularly excludes the more contaminated wastewater streams from washing machines (WM), kitchen sinks (KS) and toilet flushing (WC). (Environment Agency, 2011). In California, the Uniform Plumbing Code gives a similar definition (Reschke, 2006). In Israel, water from kitchen sinks and washing machines may sometimes be considered greywater (Friedler, 2011).
The general consensus is that greywater is wastewater that does not contain any form of faecal matter, and in some places, also no wastewater from kitchen applications and washing machines (particularly when washing soiled diapers) can be classified as greywater. As this project will be investigating the influence of GWR in UK urban areas, the first definition will be used henceforth.
Treatment of greywater is necessary prior to reuse. Greywater is most commonly used for toilet flushing and garden irrigation.
2.4 Scenarios of Greywater Reuse
GWR is frequently used for toilet flushing and/or garden irrigation. The following three scenarios will provide the framework for comparison as proposed by (Friedler E. , 2011), based on the extent of GWR.
- No GWR
There is no reuse practiced at all, and all appliance outputs go directly to the sewer in a single stream without treatment, as in Fig. 2.3.
- GWR for toilet flushing only
Light GW is treated upon exiting the house and is reused for toilet flushing. As supply exceeds demand, excess Light GW is allowed to overflow directly into the sewer without treatment. This is illustrated in Fig. 2.4.
- GWR for toilet flushing and garden irrigation
This scenario is identical to scenario 2 with the exception that excess Light GW is used for garden irrigation after treatment instead of overflowing into the sewer. During winter when garden irrigation is not required, the excess overflow is allowed into the sewer. This is illustrated in Fig. 2.5.
Due to the fact that sewers perform under unsteady flow conditions, the flow quantities to be used in the analysis are the sub-daily diurnal flows as opposed to the average diurnal flows. Additionally, it is the sub-daily instantaneous flows which are responsible for sediment movement in sewers.
A dataset containing the number of discharges per appliance per hour per inhabitant (Appendix 1 Table 1) was obtained from a study carried out by (Matos, Sampaio, Duarte, & Bentes, 2009) in Portugal. The data was collected by distributing questionnaires to 11 different dwellings. Inhabitants recorded the number of times each appliance was used per hour throughout the day for a period of three weeks.
Volume of wastewater generated per use for each appliance was obtained from (Butler D. , 1991) – Appendix 1 Table 2. The average number of inhabitants in a typical UK household was obtained from (Environmental Change Institute, 2005). The typical UK household was reported to have 2.3 inhabitants.
With the data from the previous two sections, it was possible to calculate the momentary volume for each appliance.
This made it possible to know in detail the distribution and frequency of usage of each appliance throughout the day. Moreover, the relative contribution of each appliance to the total volume discharged per day could be found as follows:
The total volume discharged to the sewer for all appliances could then be found as follows:
3.1.4.1 Scenario 1
The momentary wastewater discharged into the sewer for scenario 1 was found as follows:
3.1.4.2 Scenario 2
In scenario 2, the momentary volume of wastewater discharged to the sewer consisted of three streams:
- Dark greywater
- Blackwater
- Overflow of light greywater
First the momentary available light raw GW is the sum of the momentary volumes produced from the shower, bathtub and washbasin.
The optimum capacity of the system per hour was determined by iterations, ensuring momentary storage was never less than zero, and that the amount of greywater left over in the tank at the end of the day did not exceed 3 litres. The reason for this is that greywater should never be stored for longer than 24 hours, because it may begin to putrefy due to the presence of organic solids and nutrients. The optimum capacity was found to be 5.2 litres/hour. This means that each hour the system could take a maximum of 5.2 litres to be treated.
The momentary light greywater overflow was then found as the difference between the momentary light greywater available, and the momentary light greywater inflow into the system.
The momentary wastewater discharged into the sewer for scenario 2 could then be found by summing up the results of these three streams.
3.1.4.3 Scenario 3
In scenario 3, momentary wastewater discharged was calculated as above in scenario 2, with the exception of light greywater overflow. It was assumed that the overflow is used for garden irrigation, and none was diverted to the sewer. The total light greywater overflow per day was found to be 88 L/hh/d, which would amount to about 16m3 during an irrigation season (May – October).
Note that the quantity analysis carried out in this paper refers to the summer months, when all overflow is used for irrigation. During winter, flow quantities for scenario 3 would likely be identical to scenario 2, since excess light greywater is fed into the sewer.
Fig. 4.1 shows the pattern of discharges per inhabitant for each domestic device along the day. There are two distinct peaks (morning and evening). The morning peak occurs at around 9:30, while the evening peak occurs at around 8:30. The results highlight the significance of the kitchen sink, as it is a major contributor to the total flow in terms of number of discharges.
Figure 4.1: Diurnal pattern of discharges per inhabitant per domestic device
The distribution of flow volumes released along the day per domestic device is shown in fig. 4.2. Once again a high morning peak and a low evening peak are present. These results also show the significance of baths and showers in terms of wastewater volume generated. For example, during the morning peak (8:00-9:00), the bathtub and shower generate 28Litres/household.
Figure 4.2: Diurnal pattern of WW volume generated per household per domestic device
The individual contribution of each appliance to the total flow volume is then found by summing up the momentary flow volumes for each appliance over the day. This is represented in fig. 4.3. As inferred from the last two charts, the highest contributors to the total flow volume are the bath and shower, followed by the kitchen sink.
Figure 4.3: Individual contribution of each appliance to daily total WW volume
Fig. 4.4 represents the diurnal variation in WW volume for the three scenarios described. Reduction in WW flows varies throughout the day, with the greatest reductions occurring during morning (8:00-11.00) and evening (20:00-23:00) peaks. During the morning peak, the maximum percentage reduction in flow volume for scenarios 2 and 3 compared to scenario 1 is 11% and 55% respectively. During the evening peak, these values are 13% and 42% respectively. Additionally, sludge is released during the morning peak period. The volume of sludge released is 0.6 L/hh/d for scenario 2 and 1.2 L/hh/d for scenario 3.
When no GWR is practiced, the total daily wastewater discharge per household is 441 L/hh/d. When GWR for toilet flushing only is practiced, this value is reduced by 19% to 356 L/hh/d. When overflow is further used for irrigation as in scenario 3, the value reduces by 39% (compared to scenario 1) to 268 L/hh/d.
Figure 4.4: Diurnal variation in WW volume for scenarios 1-3
Practicing onsite GWR for toilet flushing only, as in scenario 2, results in the reduction of household water consumption by 19% (441 L/hh/d to 356 L/hh/d). Additionally, reusing the excess light greywater for garden irrigation, as in scenario 3, reduces consumption by 39% (441 L/hh/d to 268 L/hh/d). The household appliances responsible for the highest contribution to daily wastewater flows are baths and showers (130 L/hh/d), followed by the kitchen sink (104 L/hh/d). Washing machines and toilets follow closely, contributing 82 L/hh/d each to the daily wastewater flows. The lowest contributor is the wash basin, which contributes only 42 L/hh/d.
The main reduction in wastewater flow occurs during the morning and evening peak. These are the times of peak wastewater generation. At these times the majority of the flow consists of blackwater and light greywater. In the morning, the reduction in wastewater flow relative to the base case amounts to 11% and 55% in scenarios 2 and 3 respectively. In the evening, the reduction relative to the base case amounts to 13% and 42% in scenarios 2 & 3 respectively.
Daily pollutant loads do not vary significantly from the base case when GWR is practiced. In scenario 2 there is a maximum reduction of 8%. For scenario 3, the maximum reduction in pollutant load is 17%. This suggests that removal of the light greywater stream has little effect on reducing the overall pollutant load of the wastewater stream; since light greywater contains a small proportion of pollutants relative to the other streams in the wastewater. It may also be as a result of sludge entering the system after treatment as it contains the pollutants removed from raw light greywater in the RBC.
Daily pollutant concentrations were increased significantly when GWR was practiced. The maximum increase in pollutant concentration relative to scenario 1 was 24% and 64% for scenarios 2 and 3 respectively. Diurnal variations in pollution concentration showed that the main changes occurred in the morning hours when the main streams in the wastewater were blackwater and light greywater. These changes coincided with the periods of peak wastewater generation. The removal of light greywater from the system means that dilution of blackwater is reduced; hence overall pollutant concentrations are increased. In scenario 3, the effect is magnified because instead of allowing the overflow of light greywater to go into the sewer, it is used for irrigation.
The findings of the research suggest that while GWR could have a positive impact on domestic wastewater quantity, by reducing overall household water demand, the relationship between GWR and domestic wastewater quality is more complex. Further research would be required to arrive at substantiated conclusions regarding the influence of GWR on the urban water cycle.
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