Wastewater Treatment Processes and Microbiology
The treatment train
Wastewater treatment is a sequence of unit processes chosen to remove suspended solids, biodegradable organics, nutrients, and pathogens. Preliminary and primary steps are physical; secondary treatment is biological; tertiary treatment and disinfection polish the effluent. A candidate should be able to place any unit process in that train and state what it removes.
Measuring strength
BOD5 measures oxygen consumed by microorganisms over five days at 20 °C and represents biodegradable organic matter. COD measures the oxygen equivalent of nearly all oxidizable matter, so COD is always greater than BOD5 in raw wastewater. For typical domestic wastewater, BOD5 is roughly 0.4–0.6 of COD, and ultimate BOD is about 1.4–1.5 times BOD5. TSS, VSS, and ammonia are the other routine numbers. Trap: questions that treat COD and BOD as interchangeable, or assume BOD5 captures all organics.
The organisms
Heterotrophic bacteria oxidize organics and build floc. Autotrophic nitrifiers (Nitrosomonas, Nitrobacter) oxidize ammonia to nitrite and nitrate, grow slowly, and are sensitive to low DO, low temperature, and pH. Ciliated protozoa graze dispersed bacteria and indicate a mature sludge; rotifers appear in older, well-nitrified sludge; filamentous bacteria are structural at low abundance but cause bulking when dominant. Sulfate reducers produce H2S; methanogens finish anaerobic digestion. Coliforms are indicator organisms, not pathogens - a frequent exam trap.
Rule of thumb
To get loading, multiply concentration × flow × 8.34 to obtain lb/day. Use BOD5 in BOD-based ratios such as F/M; never substitute COD into a BOD-based F/M calculation.