Solids Handling, Thickening, Dewatering, and Digestion
Core concepts
Solids handling begins with thickening: gravity thickeners (typical 4–6% for primary sludge), dissolved air flotation (for light biological sludge), and gravity belt or rotary drum thickeners. Thickening cuts volume and reduces downstream tank and digester sizing.
Dewatering: belt filter presses, centrifuges, plate-and-frame presses, screw presses, and drying beds. Key metrics are cake solids percent, solids capture (recovery), and polymer dose (lb polymer per dry ton solids). Conditioning with polymer or inorganic coagulant improves release of water. Know the difference between filtrate (liquid) and cake (solid).
Anaerobic digestion: four stages - hydrolysis, acidogenesis, acetogenesis, methanogenesis. Methanogens are sensitive: pH 6.8–7.4, stable temperature (mesophilic ~95°F/35°C, thermophilic ~131°F/55°C), and no sudden loading. Indicators of upset: rising volatile acids, falling pH, dropping gas methane content (normal ~60–65% methane), and rising CO2. Aerobic digestion is simpler but less energy recovery.
Traps
Questions tempt "raise temperature quickly" to fix a digester - actually, sudden temperature swings cause upset. Another trap: confusing volatile solids destruction with total solids. Watch for dewatering questions where too much polymer creates a greasy, poor cake rather than fixing capture.
Worked example / rule of thumb
Digester gas at 55% methane and rising volatile acids with pH 6.5 = souring. Rule of thumb: stop or reduce feed, hold temperature steady, and let methanogens recover; adding lime to chase pH without reducing load often masks the problem. For dewatering, expect roughly 10–20 lb polymer per dry ton for anaerobically digested sludge.