About 90 % of global lactate production is derived from bacterial fermentation of sugars via pure homofermentative cultures in batch mode. Acid whey, which is a lactose-rich wastewater from the yogurt industry, can be used as an alternative substrate for commercial lactate production. Operating reactor microbiomes reduces the lactate production costs by circumventing sterilization, while continuous operation with biomass retention achieves higher productivity at shorter production times. To find the best reactor configuration with biomass retention for lactate production from acid whey, we operated three different reactor configurations: (1) an upflow anaerobic sludge blanket (UASB) reactor
(2) an anaerobic filter reactor (AFR)
and (3) an anaerobic continuously stirred tank reactor (CSTR) with a hollow-fiber membrane module. We operated at different hydraulic retention times (HRTs) to find the optimum production parameters at a temperature of 50 °C and a pH of 5.0. We did not use an inoculum but enriched the endogenous D-lactate-producing Lactobacillus spp. that later dominated the reactor microbiomes (>
90 % relative abundance). Undissociated lactic acid concentrations of more than 60 mmol C L