Abstract <p>Qualitative and quantitative characteristics of the aqueous phase obtained as a result of hydrothermal liquefaction of biomass of excessive activated sludge from municipal and domestic sewage treatment plants in Kaliningrad have been investigated experimentally. The content of organic compounds averaged 11 592 ± 571 mg O/dm³, and the biochemical index was 0.57. Aromatic compounds, carboxylic acids, and heterocyclic nitrogen-containing compounds were found in the composition of the aqueous phase. From activated sludge of treatment facilities of PJSC Nizhnekamskneftekhim, Republic of Tatarstan, 11 isolates–destructors of components of the aqueous phase were isolated. Their cultural and morphological characteristics were evaluated, and the necessary dilution of the aqueous phase for their cultivation was determined. It was noted that, in the process of cultivation of isolates, S2, S7, and S11 are able to neutralize components of the aqueous phase effectively in comparison with other isolated cultures: the efficiency of degradation by COD was 69, 73, and 75%, respectively. Based on 16S rRNA gene sequencing, isolate S2 had the highest similarity to <i>Pseudomonas glycinae</i> MS586 (99.52%), S7 with <i>Paenarthhobacter nicotinovorans</i> DSM 420 (99.8%), and isolate S11 with <i>Comamonas testosteroni</i> KS 0043 (99.6%). The most active isolates can be used for the treatment of highly concentrated wastewater before preliminary discharge into the sewerage network and natural water bodies.</p>