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

Abstract

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.

Inhibition of Sulfide Generation in a Reclaimed Wastewater Pipe by Nitrate Dosage and Denitrification Kinetics

“…There was not a big difference between the nitrate removal with each nitrate dose (2.5 and 3.5 mg/L NO 3 ‐N). In this pipeline stretch, the consumption ratio between COD S and nitrate was in the range of 1.2 to 2.4 g COD S /g NO 3 ‐N in both series of experiments, which is far below the consumption ratio of 4.8 g COD S /g NO 3 ‐N between dissolved organic matter and nitrate when readily biodegradable organic matter is available ( Æs⊘y et al, 1997 ). However, it should be taken into account that the study by Æs⊘y et al (1997) was made in a sewer, with wastewater and biofilm characteristics quite different from the sewer under study.…”

“…In this pipeline stretch, the consumption ratio between COD S and nitrate was in the range of 1.2 to 2.4 g COD S /g NO 3 ‐N in both series of experiments, which is far below the consumption ratio of 4.8 g COD S /g NO 3 ‐N between dissolved organic matter and nitrate when readily biodegradable organic matter is available ( Æs⊘y et al, 1997 ). However, it should be taken into account that the study by Æs⊘y et al (1997) was made in a sewer, with wastewater and biofilm characteristics quite different from the sewer under study. Because the removal of COD S corresponded to only a part of the theoretical consumption of COD based on the nitrate removal, it may be concluded that the other part was taken from particulate compounds (the average total COD consumption was above 4.8 g COD/g NO 3 ‐N with both doses).…”

“…Because the removal of COD S corresponded to only a part of the theoretical consumption of COD based on the nitrate removal, it may be concluded that the other part was taken from particulate compounds (the average total COD consumption was above 4.8 g COD/g NO 3 ‐N with both doses). According to Æs⊘y et al (1997) , a net removal of total COD higher than the net removal of COD S indicates a high hydrolysis activity, which was the case under study. The TSS depletion in the anoxic stretch (Figures 5 and 6) confirms this assumption.…”

“…( Boon et al, 1977 ; Cadena and Peters, 1988 ; Murray and Sims, 1979 ; Newcombe et al, 1979 ; Tomar and Abdullah, 1994 ). Another possible method is nitrate addition ( Æs⊘y et al, 1997 , 1998 ; Bentzen et al, 1995 ; Murray and Sims, 1979 ) or, in the case of reclaimed wastewater transportation, nitrification in the treatment plant. The presence of oxidized nitrogen compounds (NO X ‐N, nitrate, and/or nitrite) induces anoxic conditions and denitrification.…”

Studies on decreasing Chemical Oxygen Demand (COD) on artificial laundry wastewater using anaerobic-aerobic biofilter dipped with bio ball media

“…Biofilms in blackcolored anaerobic reactors with slightly rougher (grainy) visible texture are coating the surface of the bio ball medium. According to [17], characteristics of the biofilm that grows in anaerobic conditions seen black colored, more rugged and compact. Biofilm in aerobic reactor looks brownish color with the texture that looks more slippery and thicker coated the surface of bio ball media.…”