Solids Handling and Biosolids Management
Thickening and stabilization
Thickening reduces volume before stabilization using gravity thickeners, dissolved air flotation, gravity belt thickeners, or rotary drums, typically raising solids to 3–6%. Anaerobic digestion proceeds through hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Mesophilic digestion runs near 35 °C (95 °F); thermophilic near 55 °C. Well-run digesters achieve 40–55% volatile solids reduction and produce biogas that is roughly 60–65% methane and 30–35% carbon dioxide. Digesters need stable temperature, pH near 6.8–7.4, adequate mixing, and consistent feed; upset shows as low pH, rising volatile acids, ammonia toxicity, or foaming.
Aerobic digestion is simpler but achieves less volatile solids destruction and needs continuous aeration.
Dewatering
Sludge is conditioned with polymer or with ferric chloride and lime, then dewatered on belt presses, centrifuges, plate-and-frame presses, or drying beds. Cake typically reaches 15–30% solids. Trap: confusing percent solids with percent moisture - 25% solids equals 75% moisture. Centrate and filtrate return to the headworks carrying high ammonia and solids, so recycle loads matter.
Regulations and safety
40 CFR Part 503 governs land application, surface disposal, and incineration. Class A biosolids require fecal coliform under 1,000 MPN/g TS and Salmonella under 3 MPN/4 g; Class B is less stringent but restricts site access. Vector attraction reduction is a separate requirement. Methane is explosive between about 5 and 15% in air, and digester gas is corrosive and toxic in confined areas.
Rule of thumb
If cake is 22% solids, each wet ton contains 440 lb of solids; doubling cake solids halves the hauling trips for the same solids load.