Solids Handling and Biosolids Management
Thickening
Gravity thickeners, dissolved air flotation, gravity belt thickeners, and rotary drums reduce volume before digestion or dewatering. Gravity thickening takes primary sludge to roughly 4–8% and WAS from about 1–3% to 3–5% with DAF or belt thickening. Traps: solids loading rate (lb/day/ft²) matters more than hydraulic rate, polymer can be overdosed, and hydraulic overload washes solids over the weir.
Digestion
Anaerobic mesophilic digestion runs near 35–38 °C with an SRT of about 15–30 days, 40–55% volatile solids destruction, and gas yield of roughly 12–18 ft³ per lb VS destroyed, at about 60–65% methane and 30–35% carbon dioxide. Watch pH (about 6.8–7.4) and the VFA-to-alkalinity ratio; rising VFA, falling gas production, and dropping pH signal upset. Aerobic digestion reduces VS more slowly and needs steady aeration.
Dewatering
Belt presses, centrifuges, screw presses, and drying beds produce cake of roughly 18–30% solids depending on feed and equipment. Solids capture (recovery) matters as much as cake dryness, because solids in the filtrate recycle to the head of the plant.
Disposal and use
Biosolids are regulated under 40 CFR 503 as Class A or Class B, with vector attraction reduction and pollutant limits. Options include land application, landfill, and incineration under air permits.
Worked example
10,000 gallons of 4% sludge contains 10,000 × 8.34 × 0.04 = 3,336 lb of dry solids. Dewatered to 25%, that is 3,336 ÷ 0.25 = 13,344 lb of cake, about 1,600 gallons - the same solids in far less water.