Site description and sampling procedure

The samples were obtained from three wastewater treatment plants (WWTPs) in the Moscow region (Russia). Each WWTP treats 50,000–60,000 m³ of wastewater daily. The treated effluent is discharged into the tributaries of the Moskva-river basin. We selected three WWTPs with incoming wastewater of different chemical compositions. The first WWTP (WWTP-C, city) received only domestic wastewater. The second WWTP (WWTP-S, slaughterhouse) received effluent from slaughterhouses in addition to domestic wastewater, and the third WWTP (WWTP-P, petroleum) received domestic wastewater and refinery effluent with high petroleum-product content. These WWTPs used the same technical approach: mechanical treatment, a primary sedimentation tank, an aeration tank with complete biological treatment (without nutrient removal), and a secondary clarifier.

Sampling for BC structure studies was performed three times from October 2013 to December 2013 (1 month interval) in the chambers of incoming sewage and in the aerotanks with the AS. The samples were taken from the same locations in each WWTP. The similarity of the structures was estimated for samples that were collected in 1 day but with a time delay (15 min). Next, we compared the structure of the AS and the IW samples from the three WWTPs mentioned above. In accordance with the wastewater treatment scheme, aerotanks of all WWTPs are aerated all over their length. Samples for BC studies were taken in BD Falcon tubes from the central part of the aerotanks, and placed in liquid nitrogen within 20 min. Samples for chemical studies were taken from the same locations in the aerotanks and immediately transferred to the WWTP's laboratory.