14.4 Sludge Processing, Digestion, and Dewatering

Sludge Is the Other Product of Treatment

Treatment does not destroy most pollutants; it transfers them into screenings, grit, primary sludge, biological sludge, chemical sludge, scum, or dewatered cake. The April 2024 WRE specification explicitly includes solids treatment, handling, and disposal, and the analysis-and-design topic separately names solids loading and sludge. Expect both conceptual process questions and mass-balance arithmetic, where regulated biosolids must meet the U.S. EPA 40 CFR Part 503 Class A or Class B pathogen and vector-attraction rules before land application.

Sludge Stream Differences

The first calculation is almost always dry solids. Use lb/day = MGD x mg/L x 8.34 for a liquid stream in mg/L; for percent-solids data, dry solids = wet mass x solids fraction. Keep dry solids separate from wet volume: a sludge at 1 percent solids is 99 percent water, and even a 20 percent cake is still mostly water.

Thickening, Stabilization, and Conditioning

Thickening reduces water before stabilization or dewatering. Gravity thickeners suit primary sludge; dissolved air flotation (DAF), gravity belt, rotary drum, and centrifuge thickening suit dilute WAS, typically raising it from under 1 percent to 4-6 percent. Thickening reduces volume but does not stabilize.

Stabilization reduces odor, pathogens, and volatile solids. Anaerobic digestion runs without oxygen, commonly mesophilic at about 35 C with a solids retention time near 15-20 days; it destroys 40-60 percent of volatile solids and produces biogas of roughly 60-65 percent methane at about 12-18 cubic feet of gas per lb VS destroyed. Aerobic digestion uses blowers to oxidize biomass, is simpler at small plants, but consumes aeration energy and yields no usable methane.

Conditioning improves dewatering through polymers, ferric chloride, lime, or heat. The PE concept: conditioning changes water release and cake quality; it does not remove influent BOD.

Dewatering and Cake Calculations

Dewatering devices include belt filter presses, centrifuges, screw presses, plate-and-frame presses, drying beds, and lagoons; typical cake is 15-30 percent solids. The key exam quantity is cake mass. If a plant produces 6,000 lb/day of dry solids at 20 percent cake solids, wet cake = 6,000/0.20 = 30,000 lb/day = 15 wet tons/day. At 15 percent solids the same dry solids become 40,000 lb/day = 20 wet tons/day, a one-third jump in hauling for a 5-point drop in cake solids. Percent solids is the dominant disposal-cost driver.

Solids Workflow

1. List every sludge source (primary, secondary, chemical, grit, screenings).
2. Convert each liquid stream to dry lb/day.
3. Add or subtract dry solids for capture efficiency, wasting, or process yield.
4. Apply volatile-solids reduction only to the volatile fraction when digestion data is given that way.
5. Convert dry solids back to wet sludge or cake using percent solids as a fraction.
6. Confirm whether the answer wants dry mass, wet mass, volume, percent solids, or disposal rate.

Exam Diagnosis

Sludge issues surface as plantwide symptoms. Too little WAS raises mixed-liquor age, deepens sludge blankets, and loses solids over the clarifier weir; too much wasting cuts biomass and raises effluent BOD. Poor digestion brings odor, high volatile solids, foaming, and poor dewatering. Poor dewatering overloads hauling and returns high-strength filtrate or centrate to the plant head, spiking the nitrogen and BOD load on the secondary process. Treat sludge as a core process, not a side note: it is where removed solids, phosphorus precipitates, and many contaminants ultimately concentrate.

Digester Loading and Sizing

Anaerobic digesters are sized by volatile-solids loading rate and solids retention time. A common standard-rate to high-rate VS loading is 0.1-0.4 lb VS per cubic foot per day, and mesophilic high-rate digesters target an SRT of roughly 15-20 days; below about 10 days, methanogens wash out and the digester sours. If a problem gives VS feed in lb/day and a design loading rate, divide to get required volume, then check that the resulting hydraulic detention time meets the minimum SRT. Two-stage systems separate active digestion from gas storage and supernatant decant.

Temperature control matters because the methanogenic rate roughly doubles per 10 C rise up to the mesophilic optimum near 35 C, with a separate thermophilic regime near 55 C.

Capture Efficiency and Recycle Loads

No dewatering device captures 100 percent of solids. Solids capture efficiency (often 90-98 percent for a well-run centrifuge or belt press) sets how much dry solids leaves in the cake versus how much returns in the filtrate, centrate, or pressate. That recycle stream is high in ammonia and BOD and is sent back to the plant head, where it can add 15-30 percent to the influent nitrogen load. The exam may ask for cake dry solids given a feed rate and a capture percentage: cake dry solids = feed dry solids times capture fraction, and the recycled solids equal the remainder.

Sidestream treatment of this ammonia-rich return (for example, deammonification) is an advanced option when nitrogen limits are tight, and it is the right answer when a question describes a nitrification bottleneck traced to dewatering recycle rather than to influent.