The present project (K5/2897) was about the development, testing and evaluation of two prototype solar thermal drying technologies for the treatment of faecal sludge, namely a greenhouse-type solar dryer and a screw conveyer, and it included a pre-feasibility study.
The prototype testing showed that, temperatures between 35 and 45°C could be obtained with peaks up to 50°C, as well as relative humidities lower than 40%, leading to favourable conditions for drying. The prototypes were capable to handle material from viscoelastic consistency (paste-like aspect) to granular solids, but they were not adapted to a too watery feedstock. Under the most favourable weather and operating conditions, the feedstock could be dried in the prototypes at drying rates around 1-2 kg/h/m2, efficiencies up to 30-70% and a specific energy consumption of 100-400 kWh/ton, which is lower than the typical values found in conventional thermal driers (800 and 1000 kWh/ton). The drying rate drastically slowed down at the last phase of drying and the specific energy consumption increased to values superior than 800 kWh/ton, which was presumed to be due to remaining moisture being tightly bounded to the solid matrix. The screw conveyer solar drier shown a higher performance than the greenhouse, but it experienced serious stickiness issues that can compromise the long-term operation of the prototype. This stickiness issues were attempted to be mitigated by mixing the sludge with additives. So far, lime addition to the sludge was observed to diminish the stickiness problem but not yet in a significant way.
Based on a techno-economic analysis, it was estimated that the cost to treat one tonne of sludge would be approximately ZAR600 and ZAR150 for the greenhouse and screw conveyor solar thermal drying prototypes, respectively. Some potential technical challenges and areas of improvement were identified after the testing of the prototypes. The operation of the prototypes can be further optimised, which will lead to a further decrease of specific energy consumption, as well as the design specification for the reduction of the construction costs.
This study confirmed that solar thermal drying is an interesting cost-effect alternative for faecal sludge treatment. In the developed solar driers, the drying of faecal sludge is expected to occur at the optimal time and at low energy consumption when the weather conditions are favourable, and with the correct operating conditions. The process could be stopped at low moisture content before reaching a high level of moisture boundness where drying progresses very slowly and the process becomes inefficient.

Bibliographic information

Santiago SEPTIEN STRINGEL, Pareshin NAIDOO, Akhil RAMLUCKEN, Freddie INAMBAO, Craig MCGREGOR, Jon POCOCK, Anusha SINGH (2023). Development of an In-Situ Faecal Sludge Solar Dryer at Pilot-Scale WRC Final Report of the Project K5/2897 Water Research Commission

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Recommended by SuSanA (other than SuSanA publications)Sanitation systems and technology options (WG4)Faecal sludge treatment processesFaeces or faecal sludgeCamps (emergency or longer term)Peri-urbanRuralSchoolsUrban (entire city)Urban informal settlements (slums)PractitionersResearch publicationsEnglishEnergy: fuel (liquid or solid)Fertiliser

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