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

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.

Effect of ageing on the physicochemical properties of human faeces in the context of onsite sanitation

“…This exponential relationship is similarly observed by Septien et al. (2018) for partially digested pit latrine sludge (ash content ∼30%) and Mercer et al. (2021) for fresh faeces.…”

“…The implications from the reduction of moisture content and water activity could provide significant benefits for subsequent treatment. Firstly, the sticky phase which is one of the leading causes of sludge dryer failure from fouling and characterised between a MC of 50 to 70%wt for faeces ( Mercer et al., 2021 ), is bypassed after 12 weeks of storage (MC < 41%wt). Secondly, studies on pathogenic faecal bacteria such as E. coli, salmonella and vibrio cholerae reported inactivation between a water activity of 0.62 - 0.91 ( Mujumdar and Devahastin, 2000 ).…”

“… Septien et al. (2018) and Mercer et al. (2021) noted that it was not possible to induce flow in faecal sludge and fresh faeces below a moisture content of 60%wt.…”

“…4 b) is shallower for aging faeces than for fresh faeces and faecal sludge. Mercer et al. (2021) compared fresh faeces yield stress to non-sewered faecal sludge, and sewered wastewater sludges at identical solids concentrations.…”

“…The extent of drying after 16 weeks provided sufficient pre-treatment for the combustion of faeces as autothermal combustion conditions were met (MC <30%wt, Onabanjo et al., 2016a ), which would further treat the material to a friable, safe to handle product, while recovering calorific energy which also presents an option for direct household reuse. This reduction of moisture also bypassed the sticky phase of faeces (50-70%wt, Mercer et al., 2021 ), which mitigates fouling in downstream solids treatment processes.…”