Resolving the relative contributions of cistern and pour flushing to toilet water usage

4 Poster Track: Applied Research » 4 Innovative technology aspects

Speaker: Sonia Grego

Summary – for publication in conference brochure:


It has been widely proposed that non-potable water reuse for toilet flushing could alleviate pressure on water resources in urban areas. We conducted a detailed sensor-based field investigation of water use for cistern flushing and pour flushing in bathrooms plumbed with a water line for personal cleansing.  The data indicate low utilization of cistern flushing and a preference for pour flushing.  These user practices may significantly lessen the impact of reclaimed water reuse for cistern toilet flushing as a water conservation strategy in countries such as India where personal toilet cleansing by water is the common practice.

Introduction, methods, results and discussion:


Water treatment and reuse is emerging as a key strategy to address the increased pressure on water resources globally and, in addition to graywater, treated blackwater (the effluent of toilets) is a potential source of non-potable water that can be recycled for toilet flushing and irrigation to address water scarcity.

In India and as well as much of Southeast Asia and other regions, bathrooms are plumbed with a water line for personal cleansing (i.e., toilet paper is not commonly used). For reuse applications, reclaimed water can only be used for the cistern flush, while the personal cleansing water line must be sourced from fresh water that meets requirements for personal hygiene.  The typical amount of water used for cleansing is not well understood, with available data from the literature suggesting a wide range of 0.5-3L per toilet use [1]. A frequent assumption is that the amount of water used from the cleansing tap is negligible compared to the 6-10 liters used for cistern flush.

Preliminary observations from field investigations of two separate onsite blackwater treatment technologies indicated pour flushing was extensively practiced even in toilets equipped with cistern flushing.

The objective of this study was to quantify the relative contribution of cistern flush water and pour relation to the blackwater volume and to elucidate whether bathrooms with squat plates received more pour flushing water than western pedestals.

Toilet flush water volumes from cistern and from the personal wash tap were measured by recordings from multiple digital water flow meters in connection with the operation of two different treatment technologies designed to treat blackwater for reuse. Two units of The Closed Loop Advanced Sanitation System (CLASS) were installed in two separate apartment buildings [2] and the Duke blackwater treatment system connected to a single toilet stall of a shared toilet facility [3]. Water measurements were logged with time resolution of 10 minutes (CLASS) and 5 seconds (Duke) for a period of months in 2017 and in 2018.   

All of the 3 sites in this study had approximately 20 users each, all bathrooms had a cistern flush, and one apartment building had a pedestal toilet, while the two other sites has Indian squat plates.  

The fraction of flush water from the cistern was around 40% for both the apartment building sites and it was much lower, 14%, for the shared toilet site. This result indicates that the contribution of pour flush water in toilet use is significant and represents 60 to 86% of the source of blackwater. This finding is consistent with the fact that the additional water is used not only for personal cleansing, but also for pour flushing. The water/user/day ranges from 30 to 50 L/user/day and is comparable across sites and agrees with literature values of 35 L/capita/day for India [4]. Because one of the apartment buildings feature pedestals and the other squat plates (site 2), these data suggest that the type of appliance does not affect the pour flushing volume [5]. 

Conclusions and implications:


It has been widely proposed that non-potable water reuse for toilet flushing could alleviate pressure on water resources in urban areas. However, our findings were that cistern flushing contributed no more than 40% of the water flushed, and was as low as 14%, with a significant contribution from pour flushing. Additionally, the type of toilet (squat plate vs western pedestal) did not appear to affect this practice. While this study was limited in terms of geographic coverage, data were collected over several months and use trends were consistent.

These results should inform the design of onsite blackwater treatment systems for markets where pour flushing is practiced. The low utilization of cistern flushing may limit the effectiveness of treatment technologies designed to use reclaimed water for toilet flushing and intended household water savings would be limited. This study suggests that in washing cultures particularly, alternative water reuse applications such as garden irrigation should be considered. This study did not consider any interventions to limit the pour flushing practice, such as providing a spray bidet instead of a tap and bucket. Such interventions may be necessary to improve the impact of household water reuse technologies, particularly in water-scarce regions.


Relevant references:

[1] Tilley
et al.
Compendium of Sanitation Systems and Technologies – 2nd Edition
in Swiss Federal Institute of Aquatic Science and Technology (EAWAG). 2014: downloaded from www.wsscc.org
[2] Varigala
et al.
Field testing of an onsite sanitation system on apartment building blackwater using biological treatment and electrochemical disinfection. Environmental Science: Water Research & Technology
2020. 6(5): p. 1400-1411.
[3] Welling
et al.
Field testing of a household-scale onsite blackwater treatment system in Coimbatore
India. Science of The Total Environment
2020: p. 136706.
[4] Manna
Treatment of Gray Water for Reusing in Non-potable Purpose to Conserve Water in India. International Journal of Applied Environmental Sciences
2018. 13(8): p. 703-716.
[5] Welling
et al.
Resolving the relative contributions of cistern and pour flushing to toilet water usage: Measurements from urban test sites in India. Science of The Total Environment
2020: p. 138957.

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