Question 7 of 19: Minimum Nutrient Requirements for Biological Treatment

Question 7: Minimum Nutrient Requirements for Biological Treatment (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Biological treatment needs nitrogen and phosphorus, in addition to the carbon supplied by the organic waste itself, to build new microbial cell mass; a commonly cited minimum ratio for aerobic systems is BOD₅ : N : P ≈ 100 : 5 : 1 (by mass). Anaerobic systems need proportionally less nutrient — roughly 100 : 2.5 : 0.5 — because anaerobic biomass yield (cell mass produced per unit substrate consumed) is much lower than aerobic yield. Trace amounts of micronutrients (iron, magnesium, calcium, potassium, and trace metals such as cobalt and nickel for some anaerobic populations) are also required, though these are usually present in sufficient quantity in industrial wastewaters and municipal water supplies without deliberate supplementation. Where the influent BOD:N:P ratio falls short — common for wastewaters low in nitrogen such as many food-processing or pulp-and-paper effluents — nutrients must be dosed externally (e.g. as urea or ammonium salts for N, phosphoric acid or a phosphate salt for P) or the biological process will be nutrient-limited and effluent quality will suffer regardless of organic loading control.