Effects of combined carbon on biological nutrient removal

A combination of carbon sources was tested for biological nutrient removal in a tertiary pilot plant. The tertiary nutrient module followed a MLE activated sludge pilot plant treating primary settled wastewater. The activated sludge final effluent was the feed for the tertiary pilot plant that was supplemented with nitrogen and/or phosphate. The tertiary pilot plant consisted of an anoxic, oxic and final settling tank. The anoxic zone was divided into two anoxic zones to feed methanol in the first anoxic zone and acetic acid in the second anoxic zone. The settled biological solids were recycled back to the first anoxic zone.; The combined carbon continuous runs were divided into carbon ratios totaling 50 mgCOD/l of methanol/acetic acid at 10/40, 20/30, 30/20 and 40/10. The sole chemical runs were both at 50 mgCOD/l. When the influent was supplemented with nitrate and phosphate, the combined carbon addition run exhibited a maximum TIN (Total Inorganic Nitrogen) removal efficiency of 97%, corresponding to 0.63 mgN/l at a COD ratio of 40/10. The sole chemical run for methanol produced a TIN more than 4 mgN/l. The final effluent TIN was above 5 mgN/l in the sole acetic acid run. All runs at 50 mg/l of total COD resulted in poor removal of supplemented phosphorus with more than 3.5 mgP/l in the final effluent when influent phosphorus was above 6 mgP/l. The optimum combined carbon level for the highest removal of supplemented nitrate and phosphorus was at a total COD of 80 mg/l at a ratio of 30/50 with a TIN removal efficiency of 98.3% and complete P-removal.; Batch tests were also conducted with sludge taken from the optimum combined carbon addition run. The denitrification rate increased with an increase in acetic acid but was not affected by an increase in methanol. Under an anoxic and carbon limited condition, 1 g of nitrate was reduced by the denitrifying fraction of PAOs for 1.19 g of P-uptake. P-release and uptake rates were proportional to the acetic acid level. Kinetic constants, Ks, K, Y and Kd were 43.5 mg/l COD, 3.41/day, 0.92 mgVSS/mgCOD and 0.22/day, respectively.