Faecal Sludge Drying: Can We Fake It?

  • Naomi Deering

Student thesis: Doctoral ThesisPhD


The new emphasis on safely managed sanitation brought about by the Sustainable Development Goals era has placed faecal sludge management (FSM) firmly at the centre of the development agenda. FSM refers to everything in the sanitation after containment i.e. the removal, transport and reuse or disposal of faecal sludge (FS). A failure at any point in the FSM stream can have significant impacts on public and environmental health due to exposure to untreated human waste. The causes of failure at each stage in the FSM stream are often associated with coast, location (of the sanitation facility or the treatment/disposal site) and a lack of understanding regarding the risks involved in emptying untreated faecal sludge (FS). Many countries attempt to solve the FSM problem with large-scale, centralised wastewater and sewerage systems. However, these solutions are often unsuitable and unsustainable in the context of developing countries due to construction, operation and maintenance costs. Instead, a stepwise, context-specific approach to FSM is required.

The focus of this thesis is the treatment of FS on unplanted sludge drying beds (USDBs) with a filter. USDBs are simple, low-cost FS treatment options for rural and peri-urban areas. As such it is desirable to improve their design and efficiency. Two primary mechanisms are responsible for the dewatering of FS on USDBs: filtration and evaporation. Several factors affect the efficiency of FS treatment on USDBs including climatic conditions, the operating parameters of the bed itself (sludge loading rate and thickness and quality of the filter media) and the characteristics of the influent sludge. The type of sludge plays a key role in whether it can be treated on an unplanted drying bed. It has been found that stabilised sludges dewater well while fresh sludge is often unsuitable for treatment on USDBs due to its resistance to settling. As such, it requires mixing with more stabilised FS prior to being deposited onto the drying bed.

It has been observed in practice that up to 80% of influent FS volume deposited on a drying bed will emerge as effluent. It is common for this to occur within the first 24 hr – 48 hr. This effluent requires treatment prior to discharge into the environment. It is desirable to be able to predict the likely volume of effluent runoff based on the characteristics of the influent FS in order to prevent the over or under designing of the effluent treatment technology. Additionally, it is often the case that sludge dryness is estimated by operators based on observations; however, this can result in premature or late removal of the sludge solids. Both scenarios can cause problems. Premature removal of sludge solids may affect the efficiency of further treatment while late removal of the biosolids may result in the USDB not being available for new influent sludge which may then be disposed of untreated into the environment.

It would be useful for practitioners to combine practical observations with a model based on external conditions and influent sludge properties capable of predicting the necessary sludge drying time required to achieve a particular moisture content. Such a model should then prevent the under or over designing of treatment facilities as well as ensuring efficient secondary treatment following dewatering. To achieve this goal an in-depth understanding of the FS properties governing its dewatering behaviour is necessary. However, FS is highly variable and concentrated, as such it is difficult to perform repeatable tests to assess dewatering behaviour and the properties affecting it. Therefore, the purpose of this thesis was the development of a synthetic FS simulant which would replicate the key properties affecting dewatering of stabilised FS deposited on an unplanted drying bed. This simulant could then be used to perform safe and repeatable drying tests under varying conditions to develop a useful model for the prediction of drying time required to achieve a given moisture content.

To date, no simulant exists which replicates the key properties affecting the dewatering and drying of real, stabilised FS. A comprehensive literature review was conducted encompassing the well-developed wastewater sludge dewatering theory. A list of key properties found to affect the dewatering of stabilised FS in terms of filtration and evaporation was collated. These properties were used in the development of eleven new FS simulants based on two simulants developed by PRG (2014) and Penn et al (2018). The simulants were characterised, and drying tests were conducted to assess the representativeness of the simulants in terms of the key properties and drying behaviours.

Results showed that the simulants performed marginally better than previous simulants in terms of replicating the properties of stabilised FS. Shrinkage and crust formation were observed during the drying tests, but no cracking as would be expected for real FS. A successful simulant was not found but several key observations were made. A research gap was identified in terms of a lack of research regarding the properties of sludge which affect moisture loss by evaporation. This is significant as solar drying and thermal drying of FS are promising new treatment options. Therefore, research into the properties of influent sludge which will determine its drying rate will aid the efficiency of these technologies. Furthermore, to the best of the author’s knowledge, this is the first thesis to provide a comprehensive list of the properties which are likely to affect the drying of FS by filtration and evaporation. This will assist in future research regarding simulant development to replicate the key properties affecting the dewatering of sludge.
Date of Award24 Mar 2021
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorKevin Briggs (Supervisor), Lee Bryant (Supervisor) & Celia Way (Supervisor)

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