Wastewater Treatment: Primary, Secondary, Biological Nutrient Removal, Solids Handling
From grit to cake
Preliminary treatment removes grit and screenings. Primary clarifiers remove settleable solids with detention of roughly 1.5 to 2.5 hours and typical TSS removal of 50 to 65 percent, protecting downstream biology and reducing aeration demand.
Activated sludge is managed through solids retention time (SRT or MCRT), food-to-microorganism ratio (F/M), mixed liquor suspended solids (MLSS), and sludge volume index (SVI). Conventional systems run F/M near 0.2 to 0.5 per day; SVI from about 80 to 150 mL/g indicates good settling, while above 150 suggests filamentous bulking. Low dissolved oxygen, low F/M with long SRT, nutrient deficiency, and septicity promote filaments. Return activated sludge controls MLSS; wasting controls SRT. Trickling filters, RBCs, and MBBRs are fixed-film alternatives with their own organic and hydraulic loading limits.
Nitrification needs long SRT, dissolved oxygen above 2 mg/L, and about 7.14 mg/L of alkalinity as CaCO3 per mg/L of ammonia nitrogen oxidized; cold temperatures slow it sharply. Denitrification in an anoxic zone consumes nitrate and readily biodegradable carbon, recovering about 2.86 mg of oxygen equivalent per mg of nitrate nitrogen and returning alkalinity. Enhanced biological phosphorus removal requires an anaerobic selector receiving VFAs with no nitrate or oxygen, followed by PAO uptake in the aerobic zone; chemical polishing with alum or ferric is the backup.
Solids handling includes thickening (gravity, dissolved air flotation, centrifuge), anaerobic digestion at mesophilic 35 to 38 C with 15 to 30 day SRT, dewatering, and cake hauling. Struvite and foaming are classic digesters problems.
Worked example
A 30-minute settle test reads 250 mL/L and MLSS is 2500 mg/L. SVI = (settled volume, mL/L x 1000) / MLSS = (250 x 1000) / 2500 = 100 mL/g - good settling.