Implications of different toilet flushing solutions on the precipitation potential of urine

Abstract

Cape Town, South Africa has been experiencing its worse drought in decades. As a result, the City of Cape Town introduced water restrictions and encouraged residents to save water. One solution was to reduce toilet flushing and allow urine to stagnate in toilet bowls. Another solution focused on using greywater, specifically shower water, to flush toilets. This exploratory work focused on the impact of such solutions in terms of the precipitation potential of urine. Precipitation processes, such as urea hydrolysis, are governed by pH and since greywater and cleaning products can increase the pH of a solution, these could also result in precipitation occurring. We found that flushing with greywater results in pH values of 9.2 being reached within 2 days and significant precipitation occurring. The solid build-up of precipitates would add an estimated 893 tons of extra solids to the sewer network in Cape Town if everyone flushed their toilets with shower greywater only. This is unlikely to negatively affect the sewer network, but the excessive build-up of solids could affect the immediate toilet and piping infrastructure, causing blockages. In addition, adding bleach to stagnated urine should be avoided because of the harmful by-products that could form. As a result of these findings, we developed a guideline for toilet flushing with greywater that aims to reduce the precipitation potential of urine. Greywater can be used to flush toilets but bleach or another disinfectant should be added immediately after the flush to limit excessive bacterial growth and pH fluctuations.