Treatment of shower greywater for toilet flushing
The objectives of a cited pilot study were to characterize graywater (GW) treatment technology and to verify the suitability of the reclaimed water for toilet flushing [S4]. The reuse of wastewater is one effective approach to solving the problem of water resource scarcity, although deterioration in the quality of reused water, such as increased odor and bacterial growth, restricts its reuse [S4]. A broader review of building-scale greywater use describes greywater as making up 50–89% of total domestic wastewater volume, with flow variability depending on residents, building type, and user habits [S3].
Treatment technologies investigated
The pilot study compares two main biological greywater treatment trains and a hybrid option for reuse in toilet flushing [S1]. The trains examined are a membrane bioreactor (MBR), a biological aerated filter (BAF), and BAF followed by ultrafiltration (BAF + ultrafiltration), together with purified mixed wastewater used as a comparison [S1]. Greywater treated by both MBR and BAF met the standard for reusing water for toilet flushing, as reported in the study [S1].
Verification of suitability for toilet flushing
Suitability was verified by comparing treated effluent against a reuse standard and by simulating actual toilet flushing conditions [S1]. The effluent from MBR, BAF, and BAF + ultrafiltration treatment, and purified mixed wastewater, was used to simulate toilet flushing at 28 °C, with the addition of 5 mg/L NaClO to the reused water [S1]. The authors state that the study provides a scientific basis for the popularization and application of reclaimed water for toilet flushing [S1].
Building-scale and source variability considerations
Separate academic work has examined improving a greywater reuse unit for the purpose of toilet flushing in a student residence at the University of Johannesburg, South Africa, showing that building-scale deployment has been studied in a residential-type setting [S2]. The review of current trends and challenges in greywater use in buildings discusses quantitative and qualitative characteristics of greywater, including its generation sources, share in total domestic wastewater volume (50–89%), and flow variability depending on residents, building type, and user habits [S3]. The same review places particular emphasis on toilet-flushing applications within the building context [S3].
Decision factors supported by the evidence
- Treatment train performance: MBR and BAF have both been shown to meet the reuse standard evaluated in the pilot, while BAF + ultrafiltration provides an additional membrane barrier [S1].
- Disinfection dose and test temperature: Simulated reuse was carried out with 5 mg/L NaClO added to the reused water at 28 °C, indicating that a chemical disinfection step is part of the demonstrated reuse scheme [S1].
- Building-scale context: A study on a greywater reuse unit for toilet flushing in a student residence at the University of Johannesburg, South Africa, is documented in the literature [S2].
- Source variability: Greywater generation sources, its 50–89% share of total domestic wastewater volume, and flow variability depending on residents, building type, and user habits are described as key inputs to system design [S3].
Limitations of the evidence
Evidence-based takeaway: The cited work shows that greywater can be treated with MBR, BAF, or BAF + ultrafiltration, disinfected with 5 mg/L NaClO, and used to simulate toilet flushing at 28 °C, with MBR and BAF effluents meeting the reuse standard evaluated in the pilot [S1]. The broader review positions greywater as 50–89% of total domestic wastewater volume, with flow variability depending on residents, building type, and user habits, and emphasises toilet-flushing applications in buildings [S3]. Building-scale deployment has been studied in a student residence at the University of Johannesburg, South Africa [S2].