Rheological characterisation of synthetic and fresh faeces to inform on solids management strategies for non-sewered sanitation systems

Edwina Mercer,Shane P Usher,Ewan J Mcadam,Brian Stoner,Yadira Bajón-Fernández

doi:10.1016/j.jenvman.2021.113730

Edwina Mercer, Shane P Usher + Show 3 more

Open Access

https://doi.org/10.1016/j.jenvman.2021.113730

Copy DOI

Abstract

In order to obviate the economic issues associated with pit latrine emptying and transport such as high water additions and rheologically difficult sludge properties, the implications of prompt solid/liquid separation were investigated. This was achieved through rheological characterisation of fresh human faeces and synthetic faeces, and comparison with aged faecal sludges. Shear yield stress, thixotropy and post-shear structural recovery were characterised for a total solids (TS) concentration range of 5–35% total solids (TS) and stickiness yield stress was determined for concentrations up to 100% TS. Fresh faeces rheology proved to be favourable when compared to aged matrices, evidenced by a lower shear yield stress and higher gel point solids concentration, suggesting that aging could alter the physico-chemical properties of faecal sludge. Fresh and synthetic faeces exhibited similar shear thinning, thixotropic behaviour with the majority of structural breakdown occurring at a low shear rate of 10 s−1, and the extent increasing with higher solids concentrations. At 32% TS, fresh faeces shear yield stress was permanently reduced by 80%, suggesting that low shear pumping could reduce the energy demand required for faeces transport. The sticky phase, which represents the region to avoid faecal transport and mechanical drying processes, was identified to range from 30 to 50% TS, with 25% TS as ideal to commence dewatering processes. This also coincides with the average solids concentration of faeces, which is achievable by source separation. This study has identified that handling of fresh faeces as opposed to aged faecal sludges would result in economic and environmental benefits, with energy, water and labour savings.

Highlights

The United Nations (UN) estimates that 61% of the global population lack safely managed sanitation and 29% lack clean water services (United Nations, 2018)
In the dense urban setting, where limited space prevents the use of large capacity sludge tankers, intermediate small-scale vehicles restricted by volume and distance alternatively take sludge to a fixed transfer station to be collected by larger tankers
Shear yield stress was investigated to provide values of the minimum energy requirements for faeces to flow from stasis (Fig. 1)

Summary

Introduction

The United Nations (UN) estimates that 61% of the global population lack safely managed sanitation and 29% lack clean water services (United Nations, 2018). Due to the long storage periods, pit latrine sludge is stratified, with the fresher liquid layers towards the top and degraded consolidated sludge towards the bottom, reported to have densities of up to 1750 kg m−3 (Radford and Sugden, 2014) where a considerable amount of added water is required to reduce sludge vis cosity to facilitate a sufficient drop in pump head and power re quirements (Septien et al, 2018a; Strande et al, 2014) This practice is, restricted by pit volume and water availability and incurs longer emptying times and greater transport costs which constitute 25% of faecal sludge management costs (Steiner et al, 2002). In Dhaka, Bangladesh, only 0.3% of faecal sludge reaches treat ment with residents preferring to manually empty hazardous faecal sludge directly into the surrounding drains in order to avoid emptying and transport costs (Ross et al, 2016)

Objectives

Methods

Results