Biotechnological wastewater treatment in small-scale wastewater treatment plants

The article analyses the results of ten months of operations of two small-scale wastewater treatment plants. Both wastewater treatment plants were one-block facilities, operating on the basis of activated sludge in accordance with the same technological scheme and having an identical hydraulic load. The difference between the plants was that one was 1.3 times smaller than the other, was better insulated and had a longer sludge retention time, with excess sludge having to be removed less often. The smaller plant purified waste more effectively in terms of the parameters of chemical oxygen demand, biochemical oxygen demand over five days, suspended solids, ammonium nitrogen, and total phosphorus. The efficiency level in regard to removing ammonium nitrogen in the small plant given the winter temperature conditions was high (98–99%). In the treated effluent, the amount of remaining ammonium nitrogen was just 0.1–0.6 mg/L. Helpful factors were the long sludge retention time and the concentration of activated sludge, which reached up to 6.5 g/L. Excess sludge was removed from the small plant only twice in ten months. The amount of phosphorus remaining in the treated effluent was on average 1.06 mg/L. Comparing the two plants, a 20% greater effectiveness in elimination of phosphorus (82%) was achieved in the smaller plant, which used four times less electrical energy. The analysis results are useful for the further improvement of small biological wastewater treatment plants.