Experimental study on rheological characteristics of high solid content sludge and it is mesophilic anaerobic digestion

Compared to conventional low solid content anaerobic digestion, high solid content anaerobic digestion can offer attractive advantages such as higher biogas generation and smaller reactor volume demand. However, high solid content sludge has the negative impacts on the anaerobic digestion for mixing and mass transfer efficiency, which should be investigated in order to improve mixing and mass transfer effects. In the first part of this study, the rheological characters of high solid content sludge was studied using the DNJ-1 rotating viscometer. The second part of the experiment was conducted by using an anaerobic reactor (10 l) to evaluate the potential biogas production and mesophilic anaerobic process stability. This was operated at mesophilic condition (37±1)°C with high solid content (11% total solid). The results showed that the sludge belongs to the pseudoplastic non-Newtonian fluid which has the characteristics of the shear thinning. Under the optimal mixing and mesophilic conditions, the experiment was operated for 42 days. The total biogas production of this experiment was about 45.5 l, i.e., biogas production per gram volatile substance was about 0.30 l/g volatile solid (VS). Besides, VS reduction ratio was 49.5%, and the methane content of biogas was about 68%. During the operation, the pH value in the reactor changed between 6.4 and 7.6; the total alkalinity was kept between 6000 and 12 000 mg/l. The results showed that high solid mesophilic anaerobic digestion process for waste activated sludge could be operated steadily, which could reduce the volume demand of anaerobic reactor dramatically. This concept has a great potential to be used as a practicable alternative for activated sludge anaerobic treatment.