Rheological characteristics of faecal sludge from VIP latrines and implications on pit emptying

Abstract

This work aims at characterizing the rheological properties of faecal sludge from Ventilated Improved Pit (VIP) latrines and their implication on pit emptying. Faecal sludge was sampled from 3 pit latrines located in the eThekwini Municipality (Durban, South Africa). Samples were taken at different positions within the pit. For each of the samples, measurements in the rheometer in triplicates were performed in order to determine their rheological properties, and their moisture and ash content were measured also in triplicates. Experiments in the rheometer were performed for samples for which its moisture content was modified. In order to better understand the influence of water addition into the pit. During pit emptying, calculations were carried out from the experimental data, based in the criteria set in the Omni-Ingestor initiative, carried out by the Bill & Melinda Gates Foundation.Faecal sludge exhibited a shear thinning behaviour, i.e. a decrease in viscosity with increasing shear rate, and presented a yield stress comprised between 500 to 1000 Pa. This needs to be surpassed in order to overcome the elastic resistance of the sludge to flow. Similar viscosities were found for the samples from the different pits, irrespective of the position within the pit, except for the sample from the bottom of one of the pits for which it was not possible to induce a flow. This sample had a considerably lower moisture content (67% wet basis) compared to the other samples (around 80% wet basis), probably due to a higher biodegradation as it was the most aged sludge in the pit.According to the experimental results and calculations, the pumping requirements during pit emptying will decrease drastically by increasing the moisture content of the sludge. The addition of water into the pit would then facilitate the pit emptying operation by reducing the head and power required for pumping. However, this practice would require employing considerable amounts of water and handling higher volumes of sludge, which would lead to longer pit emptying times and increase the difficulty of the operation. For example, increasing the moisture content of the sludge from 75 to 90% will reduce the head and power of the pump by a factor 100, but will triplicate the amount of water in the sludge and, consequently, the time for pit emptying. Therefore, a compromise has to be made between increasing the pumping feasibility and adding water to the pit.

Managing Non-Sewered Human Waste Using Thermochemical Waste Treatment Technologies: A Review

“…Indeed, the physical and chemical characteristics of the solid waste (mainly faecal matter and/or tissues) generated and stabilised through these decentralised wastewater treatment systems are different from those stabilised in commercial/pilot-scale wastewater treatment plants. Most of the research in this area has been conducted in response to initiatives such as Bill and Melinda Gates' 'Reinvent the Toilet Challenge' (RTTC) [2,3,56]. The innovative sanitary systems built under this initiative are required to operate in a waterless mode, whilst pursuing sustainable operation from the energy intensity point of view by recovering energy from solid waste [52].…”

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

“…Pit emptying fees range from $35 - $95 per household in India and Kenya respectively, representing a substantive portion of an average monthly salary ranging from 12 to 125% ( Chowdhry and Kone, 2012 ; World Bank, 2019 ). Furthermore, 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 viscosity to facilitate a sufficient drop in pump head and power requirements ( Septien et al, 2018a ; Strande et al, 2014 ). This practice is, however, 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 ).…”

“…Characterisation of the rheological behaviour of faecal sludge is critical to inform on both the design and operation of dewatering or drying processes. Solids concentration is considered to be the defining factor which influences the rheology of faecal sludge (sewage sludge, pit latrine sludge, blackwater and faeces) by proportionally inducing shear stress to yield flow ( Cao et al, 2016 ; Septien et al, 2018a ; Thota Radhakrishnan et al, 2018a ; Thota Radhakrishnan et al, 2018b ; Woolley et al, 2014b ). However, faecal sludge rheological values differ according to type and literature on fresher unconsolidated faecal material is limited.…”

“…Faecal sludge also possesses adhesive properties attributed to solids concentration which correspond to regions requiring high stress to initiate sliding, described as the ‘sticky phase’ ( Peeters et al, 2011 ). Beyond 25% TS, faeces becomes a semi-solid material which is unable to flow, presenting a limit for conventional rheological characterisation ( Septien et al, 2018a ). The rheology of dewatered and dried faecal material is therefore unrepresented, which is equally required considering the importance of solids concentration.…”

The role of emptying services in provision of safely managed sanitation: A classification and quantification of the needs of LMICs

“…In contrast, ‘dry’ pits cannot be pumped using mechanical means. A rheological study of faecal sludge has shown that below 80% moisture content, pumping is not technically possible due to high frictional losses ( Septien et al, 2018 ).…”