Thickening
WAS is thin (often 0.5 to 1.0% solids). Gravity belt thickeners, dissolved air flotation, rotary drums or centrifuges raise it to about 4 to 6%, cutting digester volume needs. White River thickens WAS on gravity belt thickeners; digester feed averages about 5% total solids.
Digestion
Anaerobic digesters at about 95 to 100 F stabilize sludge and produce biogas (about 60 to 65% methane). Key monitoring: temperature, pH, volatile acids to alkalinity ratio (VA/ALK), gas production and composition, and volatile solids reduction.
White River runs mesophilic digesters at 98 F with feed at about 79% volatile solids and digested sludge at about 63%. Volatile solids reduction by the Van Kleeck formula:
VSR = (VSin minus VSout) / (VSin minus VSin x VSout)
= (0.791 minus 0.63) / (0.791 minus 0.791 x 0.63) = 0.161 / 0.293 = 55%
In September, a hauled FOG slug and a mixing pump failure pushed Digester 2's VA/ALK from about 0.04 to 0.33; cutting feed and restoring mixing and heat brought it back below 0.10 within a week.
Dewatering
Centrifuges, belt filter presses and screw presses use polymer to dewater digested sludge to cake (often 18 to 30% solids). White River's centrifuge runs about 99 gpm at 2.9% feed solids, producing about 24% cake with about 96% capture.
Centrate from digested sludge is high in ammonia (White River: 700 to 900 mg/L ammonia-N) and returns to the headworks, adding oxygen demand and ammonia load to the activated sludge process. Schedule dewatering for low load periods when you can.
Biosolids
Stabilized, dewatered biosolids are land applied, composted, landfilled or further processed, depending on quality and local rules.
Why operators of the liquid process should care
- Thickening and dewatering capacity limits how much you can waste.
- Return streams add ammonia, phosphorus and solids.
- Poor capture returns solids to the plant (White River's September 9 polymer pump failure sent centrate above 6,000 mg/L TSS back to the headworks).
- Foam formers in WAS can cause digester foaming.