Abstract

Disaggregating residential water use into components for indoor and outdoor use is useful in view of water services planning and demand management campaigns, where outdoor use is often the target of water restrictions. Previous research has shown that individual end-use events can be identified based on analysis of the flow pattern at the water meter, but such studies are relatively complex and expensive. A basic method to disaggregate the indoor–outdoor water use would be useful. In addressing this problem, a technique was employed in this study to disaggregate indoor–outdoor water use based on knowledge of the wastewater flow, with assumptions that link indoor use to wastewater flow. A controlled study site in a gated community, with small bore sewers, was selected to allow certain assumptions to be validated. The results provide insight into the monthly indoor and outdoor water use of homes in the study area, and show how wastewater flow could be used to assess outdoor use. Outdoor use was found to represent up to 66% of the total household water use in January, accounting for ∼58% of the total annual water use in the study area 2016.

INTRODUCTION

Background

Household water use consists of various indoor and outdoor components. Usually the outdoor water uses are not disposed of via the sewer system (Butler 1991), while most indoor uses are connected directly to sewers. Some of the indoor water uses may potentially be reused, impacting the relationship between water use and wastewater flow. A better understanding of the relationship between indoor use, outdoor use and wastewater flow is important in view of water services planning and demand management campaigns.

Measurement of household water use is made possible by a consumer water meter, with water use billed in many countries based on the actual monthly water meter readings. In contrast, measurement of household wastewater flow is complicated and uncommon. Household wastewater flow is generally considered to be a function of water use, explaining why consumer sewer tariffs are often derived directly from water meter readings.

Indoor and outdoor water use

Indoor water use has been researched in detail (Buchberger & Wu 1995; Blokker et al. 2010), including analysis of individual end uses such as shower events (Makki et al. 2013), bathing, toilet flushing and so forth. It is beyond the scope of this text to provide a comprehensive review of earlier studies into indoor use and end-uses of water. Total indoor water use is reasonably predictable, given that the required model input parameters are available.

Outdoor use is much more unpredictable than indoor use (Hemati et al. 2016), although models are available to estimate outdoor use (DeOreo et al. 2011; Du Plessis & Jacobs 2014). Some empirical studies have investigated outdoor use in detail. Outdoor use is mainly characterised by garden irrigation and irrigation of urban agricultural crops (Makwiza et al. 2018), swimming pool use (Fisher-Jeffes et al. 2014) and outdoor washing (DeOreo et al. 2011). Outdoor use is a function of climatic factors, thus explaining the seasonal fluctuation in potable water use at households, but also making outdoor use vulnerable to long term impact by climate change (Makwiza et al. 2018). Outdoor use could potentially be supplemented with alternative water sources (Jacobs et al. 2017), including treated effluent and greywater systems (Starkl et al. 2013), thus reducing the demand for potable water. Fluctuations over time and for different regions could also be introduced by different levels of water restrictions.

Household wastewater

Butler (1991) investigated wastewater flow from household appliances and found a strong correlation between water use and wastewater flow, especially in determining the resulting peak wastewater flow. Wastewater consists of sewage and the following extraneous components: stormwater ingress, groundwater infiltration and household plumbing leaks (Stephenson & Barta 2005; Erskine et al. 2011). Wastewater volume over a specified time interval would not equal indoor end use volume, because not all indoor water use is disposed of via sewers. For example, some potable water is consumed or is used for watering of indoor pot plants. Butler (1991) confirmed that the non-wasted portion is relatively small. Also, wastewater consists of wasted potable water (indoor use) plus added constituents, meaning that the wastewater flow could exceed indoor use (Jabornig 2014) when the above listed extraneous components are included. For example, Drangert (1973) reported a urine excretion volume of 1,370 mL per person per day that would be added to the wastewater stream. However, the volumes of added constituents are insignificant when compared to typical indoor water use. The contribution of flushed potable water to the household wastewater stream is considered to be the most significant part, suggesting again that the indoor use would approximate wastewater flow.

PROBLEM STATEMENT

It is increasingly important in regions of water stress to distinguish between indoor and outdoor water use, because water restrictions typically target outdoor water use (Hemati et al. 2016). The total consumer water use is normally measured with only one water meter at the property boundary. Detailed end use analysis of the flow pattern, recorded at the single water meter, would allow identification of individual end-uses to be extracted. Flow trace analysis has been widely used in previous research, but could be relatively expensive. Some of the notable studies include DeOreo et al. (2011) and Beal et al. (2011). This study addressed the problem of disaggregating indoor and outdoor water use components with limited information.

OBJECTIVE

If wastewater flow volume (outflow) could be compared to the total water use (inflow) volume, the outdoor use could be estimated by incorporating specific assumptions as set out in this paper. The research objective was to estimate outdoor water use by investigating the difference between the total water supplied into a specific district metered area and the wastewater flow from the same area, over a specific time interval.

APPROACH

Monthly water meter readings are often recorded and are available for research purposes in South Africa (Jacobs & Fair 2012). The monthly water use is actually recorded and used for billing consumers. In this study the water use of residential properties in a gated community (GC) and the total bulk supply to the same area was obtained from water meters. Pumping records could be obtained for the wastewater pump station to which all the properties in the study area drain and were used to derive the wastewater flow from the same area.

The derived wastewater flow was used to estimate the indoor and outdoor water use components of all the homes in the study site combined – no attempt was made to investigate individual homes. Of course, the water meter only provides the total water use to a household, which would include the indoor use, outdoor use and also plumbing leaks on the property. Employing the wastewater flow as a means to disaggregate indoor and outdoor use proved useful and relatively inexpensive.

STUDY SITE

The residential area investigated was a GC, located in the Western Cape province, South Africa. The study site location is shown by the highlighted area in Figure 1. The GC consists of 371 individual properties, or plots, which primarily range in size from 400 to 1,200 m2. During the last year of the study 338 plots were developed and occupied. The study site also included the following non-residential consumers: the management offices, a clubhouse, tennis and squash courts, a gymnasium, swimming pool and a putting course.

Figure 1

Location of the study site.

Figure 1

Location of the study site.

All homes are serviced with potable water and a small bore wastewater collection system, also called solids-free sewers (Little 2004). Small bore sewers have relatively low extraneous flows, potentially resulting in wastewater flow from a study site with small bore sewers more accurately representing indoor use than for conventional gravity sewers. The wastewater system in the study area drains to a single wastewater pump station, for which telemetry data were available. A separate stormwater drainage system collects and drains rainwater and surface runoff in the study area. In this study small bore sewers reduced the number of unknowns because wet weather flow is limited. However, it would be possible to extend the work to regions with gravity sewers. Wet weather flow events would be identified, and infiltration rates would be estimated from available rainfall records; alternatively the data could be recorded during dry periods.

Potable water is supplied to the study area by the water service provider via one bulk supply connection, with water use recorded by a magnetic flow water meter coupled to a GSM-based data logger. The study site was confirmed to be discrete with no cross boundary connections. Communal landscaped areas in the study site were irrigated by a non-potable private borehole and dual supply system. Private home gardens were irrigated with potable water, supplied to each home by the service provider via the water distribution system. No on-site storage is available – all water supplied to the study site via the potable pipe network was used in the study site, and wasted water would flow away under gravity via the piped wastewater system. Each property in the study area had a water and sewer connection.

DATA ACQUISITION

Water use

The bulk water supply and wastewater pumping records of the study site were acquired. The bulk water meter readings were obtained from the municipal online platform. The data were abstracted for the period 1 January 2013 to 31 December 2015. Due to an erroneous telemetry system, data capture was interrupted from (1) 12 April 2013 to 2 September 2013, (2) 25 September 2013 to 8 October 2013, and (3) 24 March 2015 to 23 April 2015. The bulk water meter reading was recorded every 30 minutes. Values were recorded in L/s, averaged over the 30 min interval. The 30-minute readings were converted to daily and monthly averages for further analysis. For the three periods where data were unavailable, the average monthly flow rates were determined from the remaining data in an applicable month.

Since December 2015 no bulk meter flow data were available, because the magnetic water meter and data recording equipment were stolen. Problems relating to theft and vandalism are not uncommon in developing countries (Purzycki 2014). Subsequently the bulk water meter readings were unavailable for the period corresponding to wastewater pump station records. The problem was not insurmountable. Water meter readings of all the individual households in the study area were subsequently obtained for the period 1 January 2016 to 31 December 2016. The household water meter readings were collated to represent the 2016 bulk water use.

Pumping records and pump station dimensions

The wastewater flow from the study site was derived by considering pumping duration and event volume, linked to the wastewater inflow rate. Pump station event records from telemetry were available from 1 August 2015 to 31 July 2016, identifying all pump starts and stops. A total of 14,908 pump events were recorded in this period. The pump station houses two identical pumps, which under normal conditions operate in an alternating fashion. For the purpose of this study, it did not matter which pump was in operation. A physical survey of the pump station sump was conducted to determine the sump volume in that section of the sump between the level switches used for switching the pump on or off.

During operation, a pump would operate continuously and empty the volume between the level switches, plus the volume flowing into the sump during the particular event. The pump sump volume was physically determined to be 1.57 m3. Determining the additional inflow volume required a few assumptions, because the wastewater inflow rate was not measured during the field experiment. The initial average wastewater inflow rate was determined by considering the pump event duration of 3 minutes and the known pump sump volume. An iterative procedure was employed to obtain a final estimate of the inflow rate, but an assumption had to be made regarding the wastewater inflow duration – inflow to the pump sump was assumed to be directly linked to water use. Water use (and thus wastewater inflow) during the period 2300–0500 h was relatively insignificant compared to the use over the remaining period of the day. The inflow volume that occurred during a pump event was thus determined to be 0.32 m3 per event, by considering inflow over an 18 h day. The total pump event volume was thus found to be 1.89 kL/event.

ANALYSES RESULTS

Water use and seasonal pattern

The 30-minute bulk water meter readings for the period 1 January 2013 to 31 December 2015 were collated annually for 2013, 2014 and 2015, thus representing the total supply to the GC. Water meter readings of all the individual households were subsequently obtained for the period 1 January 2016 to 31 December 2016. The household water meter readings were collated to represent the 2016 bulk water use. The non-domestic component supplied to the management office and club house complex (with ∼700 m2 total floor area), represented about 8% of the total annual GC water use and was excluded from further analysis. Figure 2 illustrates the total water use by all residential consumers in relation to the average temperature and average rainfall. A clear seasonal variation is observed, with relatively hot, dry summers and cold, wet winters.

Figure 2

Monthly bulk water supply, average temperature and average rainfall at the study site.

Figure 2

Monthly bulk water supply, average temperature and average rainfall at the study site.

Analysis of wastewater pumping records

The number of pump events were lifted from the data set and converted to monthly average volume – in order to match the flow to the monthly water use. For the purpose of this research, wastewater flow volume was assumed to directly represent indoor water use. The average monthly waste water flow is illustrated in Figure 3.

Figure 3

Monthly average wastewater flow (2015/2016) from the study site.

Figure 3

Monthly average wastewater flow (2015/2016) from the study site.

Water leakage and losses

A study by Knox (2016) assessed the water losses in the distribution systems of two GCs, one of which included the study site for this paper. Accurate water balances for the two GCs were developed and analysed by Knox (2016) by comparing household water meters with a totalling bulk water meter. It was found that the majority of the water losses in both the distribution systems were attributed to the physical leakage, with low levels of apparent loss.

Non-revenue water in the study area was determined to range between 6% and 12% during 2012 and 2013, with an average of 7% found during a detailed water balance conducted with the 2013 data. The non-revenue water was assumed to approximate real losses, because no unbilled authorised consumption was reported in the study area. Also, apparent losses are influenced by the presence of unlawful pipe connections, water meter accuracy and age, and the effectiveness of data transferral. As the study site is a relatively small and well managed, high-income area inside a GC, the apparent losses were negligible, with the 7% thus ascribed exclusively to real losses. These real losses in the distribution system were subtracted from the bulk inflow readings during the analyses.

Water use components

All data sets were superimposed so that the bulk water supply to the study site could be compared to the indoor and outdoor water use components, assuming that the wastewater flow approximates indoor use. The system input volume (average bulk water supply) and wastewater flow are presented in Figure 4, with the losses and non-domestic components also added.

Figure 4

Derived water use components of the study site.

Figure 4

Derived water use components of the study site.

The results were recalculated exclusively for the indoor and outdoor components, thus excluding water loss and non-domestic use not returned to the sewer (Table 1).

Table 1

Recalculated result for indoor and outdoor water use exclusively

ComponentIndoor use (kL/month)Outdoor use (kL/month)Indoor + Outdoor (kL/month)Indoor share (%)Outdoor share (%)
Aug-15 2,325 951 3,276 71 29 
Sep-15 2,446 1,159 3,605 68 32 
Oct-15 2,587 2,285 4,872 53 47 
Nov-15 2,490 3,184 5,674 44 56 
Dec-15 2,451 6,768 9,219 27 73 
Jan-16 2,378 5,993 8,371 28 72 
Feb-16 2,223 4,691 6,914 32 68 
Mar-16 2,349 3,784 6,132 38 62 
Apr-16 2,127 3,471 5,598 38 62 
May-16 2,011 2,615 4,626 43 57 
Jun-16 2,225 1,738 3,962 56 44 
Jul-16 2,284 1,343 3,627 63 37 
Average 2,325 3,165 5,490 42 58 
ComponentIndoor use (kL/month)Outdoor use (kL/month)Indoor + Outdoor (kL/month)Indoor share (%)Outdoor share (%)
Aug-15 2,325 951 3,276 71 29 
Sep-15 2,446 1,159 3,605 68 32 
Oct-15 2,587 2,285 4,872 53 47 
Nov-15 2,490 3,184 5,674 44 56 
Dec-15 2,451 6,768 9,219 27 73 
Jan-16 2,378 5,993 8,371 28 72 
Feb-16 2,223 4,691 6,914 32 68 
Mar-16 2,349 3,784 6,132 38 62 
Apr-16 2,127 3,471 5,598 38 62 
May-16 2,011 2,615 4,626 43 57 
Jun-16 2,225 1,738 3,962 56 44 
Jul-16 2,284 1,343 3,627 63 37 
Average 2,325 3,165 5,490 42 58 

DISCUSSION

The results show that outdoor water use contributed notably to the total household water use in the study area. Indoor water use remained fairly constant throughout the year, while outdoor water use varied significantly and followed a seasonal pattern. It is worth noting that no water restrictions were in place in the study area during this investigation.

In the study site it is expected that outdoor water use predominantly consists of garden irrigation, because from inspection of the aerial photography no private swimming pools were present (a communal pool facility is available to all residents). The notable variation of outdoor water use by season is ascribed to garden irrigation that is a function of seasonal parameters such as rainfall, evaporation and transpiration. Outdoor water use made up 73% of the domestic water use during the peak summer month (December) and decreased substantially in winter months, contributing only 22% to the total domestic use in August.

CONCLUSION

The wastewater return flow, aggregated with estimated water losses and non-domestic water use, was compared to total water use in order to derive proxy indoor water use estimates for the study site. Results show that the month to month indoor wastewater return flows remained fairly constant while the outdoor use component contributed notably to the total water use of the GC – about 58% of the annual average water use was applied outdoors. The seasonality of the total water use can be attributed to the outdoor water use component, with garden irrigation probably the most notable end-use. The indoor use comprised 42% of the annual average, with 71% of the total water used occurring in August, the winter month with the lowest water use. Integrated analysis of the water supply and wastewater flow allowed for segregation of indoor and outdoor use, without the need to measure at household level. The approach would be valid for any relatively homogeneous fully serviced residential community with a single source and sink node.

ACKNOWLEDGEMENTS

The Overstrand Municipality, in particular the continued technical assistance provided by Mr H. Blignaut and Mr P Robinson over the period 2012 to 2017, is acknowledged. Also, the research would not have been possible without cooperation of the particular Gated Community, the former Estate Manager and the administrative staff. The authors would like to acknowledge funding provided by the South African Water Research Commission as part of an earlier project whereby the water use in the study area was reported via an online platform (Project K5/1995/3; 2013); as well as later financial support by the South African National Research Foundation to post graduate students involved in this project.

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