Question 11 of 20: Selecting Aerobic, Anaerobic, Chemical or Physical Treatment

Question 11: Selecting Aerobic, Anaerobic, Chemical or Physical Treatment (6 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.

The choice among these four basic process families is driven primarily by the waste's strength, biodegradability, and the specific pollutant class present. Aerobic biological treatment (activated sludge, aerated lagoons, trickling filters) is favoured for moderate-to-low-strength, readily biodegradable organic wastes (BOD typically under a few thousand mg/L), where oxygen can be economically supplied and where good-quality effluent (low residual BOD) is required, but it is energy-intensive to aerate and produces a relatively large biosolids volume. Anaerobic biological treatment (UASB, anaerobic digesters, lagoons) is preferred for high-strength organic wastes (BOD/COD in the thousands to tens-of-thousands mg/L, e.g. food-processing or brewery waste) where aeration cost would be prohibitive; it produces less biosolids, generates biogas that can offset treatment energy cost, but yields a lower-quality effluent that typically needs aerobic polishing afterward. Chemical treatment (precipitation, oxidation, neutralization, coagulation) is selected specifically for non-biodegradable or biologically-toxic constituents — heavy metals (precipitated as hydroxides/sulphides), cyanide (chemically oxidized), or a waste stream whose pH must be adjusted before it can even be biologically treated. Physical treatment (screening, sedimentation, flotation, filtration, adsorption) is used for suspended/floatable solids removal, oil-water separation, and as pretreatment ahead of biological or chemical steps, or as a polishing step after them. In practice, most industrial wastewaters need a combination: physical pretreatment to protect downstream units, chemical treatment for any toxic/non-biodegradable fraction, and biological (aerobic and/or anaerobic, depending on strength) treatment for the bulk biodegradable organic load. Sequencing matters as much as selection: physical pretreatment ahead of chemical treatment ahead of biological treatment is the usual order, because each earlier step protects the process capital investment of the step that follows it from solids, toxic shock, or a pH excursion it was not designed to handle.