Physical composition-based energy models for co-management of solid wastes and faecal sludge from pit latrines

Abstract

Pit latrines are the main form of sanitation in unplanned areas in many rapidly growing developing cities. Pit latrines are containment structures for faecal matter with solid waste being part of the contents especially in informal settlements. Solid waste from pit latrines contains faecal matter and are hazardous to the environment. Their incompatibility with faecal sludge and wastewater treatment plants makes their treatment and disposal difficult since they are not allowed at sanitary landfills. This creates a problem of limited options available to manage solid wastes mixed with faecal sludge from pit latrines. This study was aimed at developing energy models based on physical composition for co-management of solid waste and faecal sludge from pit latrines. A total of 7 pit latrines (3lined and 4 unlined) from the parishes of Kamwokya II parish and Makerere III parish in Kampala capital city, were fully emptied and all the contents used for the study. Characterization of pit latrine contents was carried out based on physical composition, proximate, ultimate analysis and calorific value. Energy predictive models based on physical composition of pit latrine contents were developed and they were validated for their accuracy using experimental data and comparing them with existing models based on municipal solid waste. The study reveals that the solid waste fraction in pit latrines varies between 16.5% - 22.7% and the combustibles being 31% - 45% of the total solid waste fraction. Solid waste from unlined pit latrines have a higher moisture content, total volatile solids, and less ash content as compared to the lined ones. Carbon was the most dominant element with the highest concentration ranging from 28% - 70.3% followed by Oxygen, Hydrogen, Nitrogen and Sulphur. The wastes from pit latrines have a great potential for energy recovery with the minimum calorific value being 8MJ/Kg. Addition of faecal sludge to solid waste increases the moisture content, lowers the total volatile solids and increases the ash content for both lined and unlined pit latrines. This in turn reduces the energy content of solid waste from pit latrines. The study also developed predictive models for energy based on the physical composition for solid waste and solid waste mixed with faecal sludge all from pit latrines. A comparison of the calculated values of the developed models and experimental values of the energy content resulted in a good agreement as compared to the existing models based on physical composition of municipal solid waste. Model development is necessary for the accurate estimation of energy in order to minimize cost in the design and operation of solid waste from pit latrines.