A comparison of biofilm growth and water quality changes in sewers with anoxic and anaerobic (septic) conditions

Changes in water quality in sewers may have significant effects on the performance of treatment plants. Experiments have been carried out in two pilot scale sewers for studies of biofilm growth and changes of water quality with anoxic (dosage of nitrate) and anaerobic (septic) conditions, respectively. The thickness of the biofilm in the anoxic sewer was 1-2 mm as compared to 0.3-0.6 mm in the septic sewer at a water velocity of 0.5 m/s. An increase in the water velocity from 0.002 m/s to 0.5 m/s gave thicker and denser biofilms. Sulphide formation was prevented in the anoxic sewer, and influent sulphide was efficiently removed. The denitrification rate in the anoxic sewer was 3.5-4.3 g NO₃-N/m²d (15°C). This gave a considerable reduction in both organic matter and phosphorus and an increase in pH and alkalinity. The potential to remove organic matter in sewers may be exploited as a pre-treatment to mechanical and chemical treatment plants in order to meet more stringent effluent standards for organic matter discharge. Removal of readily biodegradable organic matter in the sewer may, however, be a disadvantage for treatment plants based on pre-denitrification. A reduced phosphate concentration may give a reduced metal coagulant dose at chemical treatment plants, whereas an increase in pH and alkalinity may give an increased dose. In the septic sewer, there was a small reduction in the organic matter, but no significant net change in total phosphorus, pH and alkalinity on average. The production rate of sulphide was in the range of 0.3-0.7 gS/m²d (15°C). On average there was no change in the concentration of suspended solids in either of the sewers. A sporadic large sloughing of biofilm was observed, particularly in situations with a very diluted wastewater. Sporadic high concentrations of suspended solids may cause separation problems at treatment plants.