Organic transformation, lactic acid metabolism, and membrane performance in high-rate methanogenic treatment of dairy processing wastewater using thermophilic high-solid anaerobic membrane bioreactor

Dairy processing wastewater (DPW) is an easily biodegradable substrate rich in lactic acid (LA) and native lactic acid bacteria (LAB), which can threaten the efficiency and stability of anaerobic treatment. This study introduced a high-solid anaerobic membrane bioreactor (AnMBR) to explore methanogenic efficiency, metabolic processes, and membrane performance in DPW treatment. A high microbial concentration rapidly realized the highest organic loading rate (OLR) of 26.3 g chemical oxygen demand (COD)/L/d with a methane yield of 317.1 ± 5.2 mL/g COD. Mass balance showed 90.6 %–98.8 % COD of DPW transferred into methane at different OLRs. A syntropic bacterium of Tepidanaerobacter syntrophicus achieved over 4.5 g/L/d LA loading rate, with relative abundance increasing from 0.36 % to 4.06 %. LAB were gradually eliminated because of unfavorable environmental conditions and high OLR. High OLR regulated methanogenesis transformation from hydrogenotrophic to mixed hydrogenotrophic and acetoclastic types. Lactic/acetic acid production was proposed as a central pathway in the metabolic process. Enhanced mixed liquor total solid concentration (20–50 g/L) limited membrane flux because of the exponentially increased sludge viscosity. Also, the colloidal particle percentage in the sludge increased from 3.5 % to 10.5 %, which promoted membrane fouling for pore blocking and cake/gel layer formation. Organic foulants dominated the membrane resistance and resulted from the high rejection efficiency of organic matter. This study showed a novel and effective approach to achieving high-rate treatment of DPW by high-solid AnMBR and provided significant guidance for the dairy industry in future wastewater treatment development.