The increase in plastic wastes having a long term of natural decomposition has a serious impact on the deteriorating environmental situation and is an issue of global concern. One way to solve this problem is making more use of biodegradable polymers. However, their widespread availability is hampered by the high cost of raw materials, one of which is lactic acid. A promising source of stimulants and nitrogen, which reduces the cost of lactic acid production, is the filtrate of the culture fluid after the purification of municipal wastewater with microalgae Chlorella vulgaris. It has been experimentally established that using the culture fluid filtrate after municipal wastewater treatment with microalgae as a source of nitrogen and stimulating substances it is possible to increase the concentrations of: cells of lactobacilli bacteria of strains Bacillus coagulans and Lactobacillus casei 1.5-1.9 times on average; lactic acid in the culture fluid by 15-30 % (wt.) compared to the control samples. On the basis of the experimental data obtained using the equations of Verhulst and Monod- Yerusalimsky, a mathematical model of the process of culturing lactic acid bacteria has been identified, describing the kinetics of growth of lactic acid bacteria, the loss of the carbon-containing substrate and the accumulation of lactic acid.