Improvement of sludge dewaterability using filamentous fungi and energy recovery from thermal treatment of sludge

Abstract

Nowadays, the increasing production of municipal sludge requires effective and suitable disposal methods. The commonly used methods include thickening, dewatering, anaerobic digestion, composting, and thermal treatment. This PhD research aimed at assessment of energy recovery from waste sludge. It consisted of four major research contents: first, improvement of sludge dewaterability using fungal conditioner was studied; second, the performance of the fungal conditioned sludge in the filtration process was investigated; then, thermal treatment of sludge was modelled by chemical equilibrium modelling, and finally, a self-sufficient thermal treatment of sludge was proposed for energy recovery. The fungal conditioners were developed from Aspergillus niger (A.niger). The fungal conditioner helped the formation of sludge pellets and thus enhanced sludge dewaterability. The specific resistance to filtration (SRF) of sludge was decreased by 25.0% and 47.1% at 4% and 8% (v/v) dosage of fungal inoculum, respectively, after 30 mins of incubation, in comparison with the SRF of raw sludge. Conditioners containing activated mycelia were preferable to enhance sludge dewaterability. A filtration model was developed to study the dewatering process of sludge. The driving force, resistance and velocity in the filtration process were discussed. The model was then verified with experiments conducted at constant pressure filtration. The filtration constant B in the filtration process increased from 2.736 × 10-6 to 7.224 × 10-6 m2/s by inoculating 16% (v/v) of liquid fungal conditioner. A mathematical model was established to evaluate the performance of a sludge-fed gasifier based on the mass conservation, chemical equilibrium and energy balance. The model was built in Matlab and Cantera. By using this model, the equilibrium temperature, syngas production and composition, syngas heating value and cold gas efficiency (CGE) can be studied, in addition with the relationship between the syngas properties and the sludge properties and equivalent ratio (ER). Based on this model, a self-sufficient thermal treatment of sludge was assessed. The process included 1) a drying stage to reduce the water content of fungal conditioned sludge to 25%-55%, 2) a gasification/combustion stage, and 3) a combined heat and power (CHP) unit to recover energy. The results showed that under gasification conditions, the syngas would have a heat value of 3.2 MJ/Nm3 . The electricity recovered can account up to 0.079 kWh/m3 wastewater treated, providing 28.2-31.6% of the energy consumption in a conventional WWTP. Reducing the moisture content to ≤ 70% allows for the self-sufficient thermal treatment of sludge based on a WWTP of the similar scale of the Ringsend WWTP in Dublin, Ireland.

The research outcome will advance the understanding of the role of filamentous fungi in sludge dewatering, the sludge filtration process, and the performance of a sludge-fed gasifier. This PhD research will be beneficial to the development of sustainable sludge treatment using filamentous fungal conditioners and thermal treatment of sludge for energy recovery. The modelling results will provide guidance for the design of sludge gasification system.