3. Many well-operated activated sludge systems have a small layer of light brown foam covering up to 30% of the aeration tank. However, excessive foaming is often an indication of operational problems e.g. high or low F/M ratio, poor aeration, and low MLSS concentration. A thick layer of white foam was observed in the aeration tank at Newstead wastewater treatment plant. As an environmental engineer, you are asked to investigate the cause of the problem. Table Q3.1 shows the healthy operational ranges of parameters of an activated sludge system. Parameter Unit Hydraulic detention time hours F/M ratio Sludge MLSS Age BOD removal efficiency (Kg BOD/d)/ (Kg MLSS) days Mg/l % Healthy 4 14 0.2-0.5 3 10 2000- 90-95 range 4000 Table Q3.1: Loading and operational parameters for activated sludge systems The following data from Newstead wastewater treatment plant was gathered to assist you in your investigation: Population equivalent: 175,000 Flow per capita: 225 1/day Influent BOD into secondary treatment: 275 mg/l Enforced effluent standard of BOD: 15 mg/l in treated water Enforced effluent standard of SS: 15 mg/l in treated water Suspended solids concentration in aeration tank: 3500 mg/l Suspended solids concentration in secondary clarifier sludge: 15000 mg/l Yield coefficient: 0.55 Decay rate: 0.075 /d Maximum growth rate: 0.1/day Mean cell residence time: 3.5 days (a) Determine: (i) the volume of the aeration tank, (ii) the hydraulic detention time (iii) F/M ratio (iv) mass of secondary sludge (20 marks) (v) Flowrate of wasted sludge, and (vi) BOD removal efficiency. Compare the values obtained to the operational ranges in Table Q3.1. What is the likely cause of white foaming in the aeration tank? OTU Dublin 2022 4 (b) Without carrying out any calculations, describe measures you would take to rectify the problem. (5 marks) Total marks for Question 3 25 marks