Water and Wastewater Treatment: Unit Processes and Design Parameters
Water and wastewater treatment questions combine process selection with design calculations for clarifiers, coagulation, disinfection and biological reactors. Expect one or two numerical items on loading rates or solids retention time.
Core rules and concepts
- Sedimentation: ideal settling is governed by surface overflow rate, SOR = Q/A, which equals the settling velocity of the critical particle; depth does not change removal in ideal theory.
- Coagulation-flocculation: charge neutralisation and sweep floc; alum or ferric dose set by jar test; flocculation intensity G = sqrt(P/(uV)), typically 10-75 s^-1.
- Filtration and disinfection: CT = disinfectant concentration x contact time; Chick-Watson inactivation; turbidity must be low before chlorination.
- Activated sludge: HRT = V/Q; SRT = VX/(QwXr + QeXe); F/M = QS0/(VX). Nitrification needs about 4.57 mg O2 and consumes about 7.14 mg alkalinity (as CaCO3) per mg NH4-N.
- Sludge and advanced options: thickening, digestion, dewatering; UASB, MBBR, SBR, MBR and RO for reuse.
Worked example
A primary clarifier receives 12,000 m3/d with a surface area of 400 m2. SOR = 12,000/400 = 30 m3/m2 d = 1.25 m/h. Particles with settling velocity above 1.25 m/h are removed completely in ideal theory; particles with lower velocity are removed in proportion to vs/SOR. If the flow doubles to 24,000 m3/d, SOR becomes 2.5 m/h and removal of the same particle distribution falls sharply. The reasoning point is that overflow rate, not detention time, controls clarification.
Traps that cost marks
Confusing SRT with HRT; using BOD5 as if it were ultimate BOD (BOD5 is roughly 0.6-0.7 of BODu for domestic sewage); ignoring chlorine demand when computing residual CT; and forgetting alkalinity and oxygen demand in nitrification design.