TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: In (I), the necessary biomass for the purification is produced in a non-sterilized growth medium comprising tap water and 0.5-4 g/L soap. A mixed population comprising all 3 strains is used. The method may further comprise purifying the water using microorganisms selected from Pseudomonas azelaica DT-6 (DSM13516), Azospirillum sp. DT-10 (DSM 13517), Ancylobacter aquaticus DT-12 (DSM13518) and/or Xanthobacter sp. DT-13 (DSM13519).Preferred Bacterial Population: The population (IX) may further comprise (V), (VI), (VII) and (VIII).Preferred Bioreactor: (X) preferably comprise all 3 bacteria (II), (III) and (IV), and may further comprise (V), (VI), (VII) and (VIII). The bioreactor preferably contains all 7 bacteria. The bioreactor as 1 or more separating walls arranged to force water to circulate in the reactor. The bacteria are immobilized on a plastic carrier medium with a specific density of 0.8 g/cm3. EXAMPLE - Household black water was purified in a bioreactor system comprising 5 interconnected tanks with a total volume of 6.5 m3. The system had 2 aerobic tanks without a carrier (into which bacterial strains Bacillus Sp. DT-1 (DSM12560), Pseudomonas azelaica DT-2 (DSM12561) and/or Rhizobium sp. DT-5 (DSM12562) were added), 1 aerobic tank with a carrier (to which bacterial strains Bacillus Sp. DT-1 (DSM12560), Pseudomonas azelaica DT-2 (DSM12561) and/or Rhizobium sp. DT-5 (DSM12562) were attached) and 2 sedimentation tanks. The temperature was 8-15 degreesC and the flow rate was 0.5-5 m3 waste water/day. Aeration was conducted by recycling the water through the carrier. The energy consumption was 0.05-0.5 kWh. The treatment reduced: (a) the total nitrogen content of the water from 100-300 mg N/L to 1-5 mg N/L; (b) the total phosphorous content of the water from 10-25 mg P/L to 0.2-2 mg P/L; (c) the Thermo-tolerant coli-form content of the water from 108-109 colony forming units (cfu)/ml to less than 20 cfu/ml; and (d) the pH of the water from pH 7-8 to pH 6.5-7.