Treatment of digestate from swine sludge continuous stirred tank reactor to reduce total carbon and total solids content

Anaerobic digestion is a biological process that can partially convert organic matter into gases with potential to energy generation. However, in case of continuous stirred tank reactor (CSTR) for the treatment of sludge from swine manure, a digestate with a high concentration of solids, undigested carbon, nitrogen and phosphorus is usually obtained, which can limit the direct application to the soil or other direct uses or disposal. Therefore, an additional treatment of digestate is still needed to meet environmental requirements for the viability of an anaerobic digestion plant to enable the disposal of the liquid effluent. In this study, solid-liquid separation (SLS) strategies were studied for treatment of digestate from swine sludge CSTR biodigester. The following processes were tested: settling, centrifugation and chemical flocculation.The performance of each process was evaluated by monitoring the concentrations of the following parameters in the raw digestate and after treatment: total carbon (TC), phosphorus, total kjeldahl nitrogen, ammonia nitrogen, total solids (TS), volatile solids and fixed solids. The factorial design was used for the optimization of centrifugation and chemical flocculation tests. Total carbon, solids and phosphorus were significantly reduced from digestate using the different SLS processes. However, higher removal efficiencies were obtained by centrifugation, being more expressive to phosphorus, TC and TS (reduction of 95, 90 and 83%, respectively). Furthermore, the sludge production by centrifugation was 3.8 and 7.3 times lower than by settling and chemical flocculation, respectively. Thus, centrifugation could be considered the most appropriate SLS process to enable digestate treatment and the liquid efluent is suitable to be treated by deammonification process for nitrogen removal.

Automated summary

Anaerobic digestion can convert organic matter into gases, but a high concentration of solids, undigested carbon, nitrogen, and phosphorus in the digestate from swine manure CSTR requires additional treatment. Solid-liquid separation strategies were tested, with centrifugation showing higher removal efficiencies for carbon, phosphorus, and solids, making it the most appropriate process for digestate treatment.