Speaker: Santiago Septien Stringel
The Pollution Research Group (PRG), at the University of KwaZulu-Natal (Durban, South Africa), has conducted collaborative research on the area of fecal sludge drying, in order to provide useful data to sanitation practitioners, to characterize the process, and to better understand the fundamentals aspects. It is expected that this information will allow to improve the current faecal sludge technologies and to develop further innovations. This paper presents a summary of the main findings from these investigations.
Drying is an essential operation unit in the treatment of faecal sludge for the removal of moisture found in the sludge. This presents several benefits, such as (i) deactivation of pathogens; (ii) reduction of mass and volume of the waste, decreasing the transport and storage costs; (iii) increase of calorific value, increasing its value as a biofuel. Typically, moisture is removed from faecal sludge in drying beds, but this method leads to poor performance in terms of moisture removal and pasteurization (Cofie et al., 2006) and to a high surface footprint.
Thermal drying is a more efficient option for faecal sludge treatment. Different types of thermal drying technologies have been identified for faecal sludge treatment such as several contact and hot-air driers in faecal sludge treatment plants as the Omniproccesor (Villarreal, 2015), the LaDePa infrared dryer (from “Latrine Dehydration Pasteurization”) in the eThekwini municipality (Harrison and Wilson, 2012) or a microwave drying under development (Mawioo et al., 2017). Faecal sludge drying is a challenging process to put into application with several technological gaps to overcome. Sanitation practitioners have affronted considerable difficulties for the implementation and operation of thermal drying systems. One of the major issues resides in the lack of data and understanding about the process, which has limited the deployment of efficient technologies adapted to the faecal sludge characteristics.
The Pollution Research Group (PRG), at the University of KwaZulu-Natal (Durban, South Africa), has extensively investigated faecal sludge drying for its understanding and application of the generated knowledge in the field. The aim is to support sanitation practitioners and technology developers by providing useful data and insight into the faecal sludge drying process. Most of the data has been generated through experimental investigations in the laboratory using faecal sludge that has been collected from different types of onsite sanitation facilities (improved pit latrines, urine diversion dry toilets, small-scale decentralized wastewater treatment plants) and fresh faeces that were obtained from the voluntary and anonymous donations of healthy adults. This data includes the determination of moisture boundedness in the sludge, the measurement of drying and disinfection kinetics, the evolution of the morphological and mechanical properties (rheology, stickiness, consistency), the comparison of the drying the behaviour between different types of sludge, and the characterization of the physicochemical properties of the dried product for its reuse. The outcomes from these investigations offer knowledge, guidance and tools for the improvement of current technologies and the development of innovations.
Temperature is an important parameter to control during faecal sludge drying, as it has a strong influence on the performance of moisture removal and disinfection. Higher temperatures yield to faster moisture removal and pathogen die-off, but the solid structure can be degraded if the temperature exceeds a threshold (approximately around 200ºC).
Faecal sludge exhibits high viscosities that can be reduced by shearing (shear-thinning behaviour) with a stickiness peak at around 50% moisture content, which has to be considered for the design of drying operation units. Drying at 30% moisture content seems reasonable as most of the pathogens cannot develop under these conditions of dryness and the removal of the remaining moisture could be too high energy intense due to its high boundedness.
Drying does not affect the nutrient content nor the calorific value of faecal sludge for reuse as agricultural product or biofuel, except if the material has undergone thermal degradation, which affects the quality of the final product. Nonetheless, drying modifies the thermal properties of the sludge, such as the heat capacity and, thermal conductivity resulting in the enhancement of the heat transfer, as well as the nitrogen chemical form leading to a nutrient slow-release fertilizer.