Non-sterile lactic acid (LA) fermentation of highly viscous food waste was demonstrated in batch and continuous flow fermentations. With Streptococcus sp., an indigenous consortium, and/or applied glucoamylase, food waste was fermented without addition of external carbon or nitrogen sources. Experimental results were used for economic and energy evaluations under consideration of different catchment area sizes from 50,000 to 1,000,000 inhabitants. During batch mode, addition of glucoamylase resulted in a titer (after 24 h), yield, and productivity of 50 g L ⁻¹, 63%, and 2.93 g L ⁻¹h ⁻¹, respectively. While titer and yield were enhanced, productivity was lower during continuous operation and 69 g L ⁻¹, 86%, and 1.27 g L ⁻¹h ⁻¹ were obtained at a dilution rate of 0.44 d ⁻¹ when glucoamylase was added. Both batch and continuous flow fermentations were found economically profitable with food waste from 200,000 or more inhabitants.