Effect of ageing on the physicochemical properties of fresh faeces.

Abstract

Conventional onsite sanitation facilities, like pit latrines and urine diversion toilets, and innovative reinvented toilets with in-situ excreta treatment involve onsite storage of faecal matter for a certain period, where biological and physicochemical degradation, drying and dewatering processes may occur. The study was conducted from an improved sanitation perspective and investigated how faeces transformed from when they are generated and deposited in a repository. Fresh human faeces were left to age for sixteen weeks under ambient conditions (temperature ~20°C and ~60% relative humidity) This was experimentally determined by how the moisture content, drying rate, dewaterability, rheological, physicochemical, and thermal properties were affected as the faeces aged. Fresh faeces samples, collected from voluntary donations from healthy individuals at the University of KwaZulu-Natal, were aged in a ventilated environment under ambient conditions. The previously mentioned characteristics were evaluated weekly for sixteen weeks. Faeces dried with time since moisture content decreased from 79.1 to 26.9% (wet basis). Faeces could not be dewatered to enhance dehydration to reduce sample volume. The drying curves generated indicated a uniform rate of drying uninfluenced by the sample age and initial moisture content. Moisture-dependent characteristics such as rheological and thermal properties were mainly affected during storage and water activity. Despite an unexpected decrease in the first week, possibly due to biodegradation, the waste exhibited increased viscosity and yield stress due to the sample drying. The particle size did not change during the storage and did not influence the rheological properties. Also, the faeces' thermal conductivity and heat capacity significantly reduced with storage time as the sample was dried and showed a fair dependence on the sample water activity, which decreased during the process, suggesting an increase of the moisture boundedness along with the faeces dehydration. The minor variation in volatile solids, COD, carbon, nitrogen, ammonium and nitrates indicated that faecal matter remained rich in organic and nutrient content without undergoing considerable biodegradation. The dried samples maintained a calorific value of around 22 MJ/kg regardless of age.