Tertiary and Advanced Treatment
Filtration
Rapid granular filters polish secondary effluent and are rated by hydraulic loading (gpm/ft²), media size and depth, and run time to terminal headloss. Backwash uses air scour plus water at a rate that expands the bed. Traps: rising headloss means surface blinding; turbidity breakthrough points to media problems such as mudballs or loss of media; backwash water recycled to the head of the plant adds solids load.
Membrane processes
Pore size order: MF > UF > NF > RO. MF and UF remove particles and microorganisms; NF softens and removes some organics; RO removes dissolved ions. Track flux (gfd or LMH), transmembrane pressure, and recovery. Rising TMP that responds to cleaning indicates fouling; a sudden loss of quality suggests an integrity breach. Concentrate must be managed under the permit.
Nutrient polishing
Tertiary phosphorus removal uses alum, ferric chloride, or ferric sulfate with the dose set by jar testing; polymer improves solids capture. Denitrification filters or post-anoxic tanks with a carbon source (methanol, glycerin, acetate) polish nitrate. Other polishing steps include oxidation for sulfide and iron, and adsorptive media for metals or color.
Traps
Watch units - gfd versus gpm/ft² - and remember that a membrane's main job is not the same at every pore size. Recovery and concentrate handling are part of the design, not an afterthought.
Worked example
A membrane unit receives 1.0 MGD of feed and produces 0.85 MGD of permeate. Recovery = 0.85 ÷ 1.0 × 100 = 85%. The remaining 0.15 MGD is concentrate or reject and must be returned or discharged under the permit.