A country-wide survey into the composition and transformation of short-chain organic acids in anaerobic sludge digesters at wastewater treatment plants in Denmark.

Short chain organic acids (SCOAs) containing 1-6 carbons are important intermediates in anaerobic digestion (AD) of sewage sludge, and transformation of SCOAs is often the rate-limiting step. Comprehensive information about SCOAs in full-scale anaerobic digesters is thus crucial to better monitor and regulate the AD process. However, large-scale investigations and comparisons are still scare for industrial digesters, especially when considering the diverse operating conditions and AD techniques applied in the plants. To fill this gap, a country-wide survey was conducted into 34 full-scale anaerobic sludge digesters located at 19 Danish wastewater treatment plants (WWTPs), using ion chromatography to detect nine SCOAs at µM level. The generation and transformation of typical SCOAs were recorded under varying conditions and the kinetics were calculated. 549 SCOA composition profiles were generated and compared with 904 total acid concentrations provided by the WWTPs using a photometric method. Results showed that total SCOA concentrations maintained at levels lower than 5 mM, which were generally overestimated by the photometric method up to 32 times. Acetate, formate and lactate were the most abundant SCOAs detected in nearly all digesters. Formate demonstrated relatively high concentrations (0.1-1.0 mM), suggesting its importance in carbon turnover, which previously was usually ignored. Regarding the transformation dynamics, butyrate was shown to be degraded in a first-order kinetic, accompanied by isomerization to isobutyrate; while propionate, lactate and acetate followed zero-order kinetics within the studied concentrations (≤ 2.2 mM). Findings from this extensive survey provided new insights into SCOA occurrence and transformation patterns in full-scale anaerobic sludge digesters.