Sludge treatment and biosolids

Wastewater treatment concentrates the pollutants it removes into sludge. Although sludge is only about 1 % of the volume of the wastewater treated, its handling and disposal can account for a third to half of a plant's operating cost. It contains organic matter, nutrients, pathogens, metals and, as recent monitoring has shown, trace organic contaminants including PFAS. Treated to the right standard, it becomes biosolids, a valuable fertilizer and soil conditioner; handled poorly, it is a nuisance and a hazard. This lesson follows the city plant's solids from the clarifiers to their end use.

Sources and quantities

Primary sludge is the settled solids from primary clarifiers: gray, odorous, readily digestible and typically 2 to 6 % solids. Waste activated sludge (WAS) is the excess biomass from the activated sludge process: brown, flocculent, harder to thicken and digest, and typically 0.5 to 1.5 % solids as wasted. Chemical sludge from phosphorus precipitation adds metal phosphates and hydroxides. Scum, screenings and grit are handled separately and landfilled.

The basic tool is the solids mass balance. The dry mass of solids, in kg/d, is conserved through thickening and dewatering apart from the small amount lost to the returned liquid streams; the volume falls sharply as water is removed. Because a sludge's density is close to that of water, its volume is approximately

where \(M_s\) is the dry solids mass and \(P_s\) the solids fraction. Thickening WAS from 1 % to 6 % solids reduces its volume by a factor of six; dewatering digested sludge from 3 % to 22 % reduces it again by more than seven.

Thickening

Thickening is the first step, to reduce the volume sent to the digesters and so their size. Primary sludge thickens well by gravity, in circular thickeners similar to clarifiers designed on solids loading rates of about 100 to 150 kg/(m²·d), reaching 5 to 8 % solids. WAS thickens poorly by gravity and is usually thickened mechanically: by dissolved air flotation (lesson 4), gravity belt thickeners, rotary drum thickeners or centrifuges, with polymer conditioning, reaching 4 to 7 % solids. The liquid separated, called filtrate, centrate or overflow, returns to the head of the plant.

Stabilization

Stabilization reduces pathogens, odors and the potential for putrefaction. Anaerobic digestion is the most common method at plants of this size. In a heated, mixed, sealed tank, a sequence of microorganisms hydrolyzes complex organic matter, ferments it to volatile fatty acids, converts these to acetate and hydrogen, and finally produces methane and carbon dioxide. The last step, methanogenesis, is the slowest and most sensitive, so digesters are operated to protect the methanogens: steady feeding, stable temperature, and enough alkalinity to buffer the fatty acids.

Mesophilic digesters operate at about 35 °C (95 °F) with an SRT of 15 to 20 days; in a digester without recycle the SRT equals the HRT. They destroy 40 to 60 % of the volatile solids and produce biogas of about 60 to 65 % methane, at about 0.75 to 1.1 m³ of biogas per kg of volatile solids destroyed (12 to 18 ft³ per lb). Thermophilic digesters at about 55 °C are faster and destroy more pathogens but are less stable and produce more odorous sludge. Temperature-phased systems combine the two.

Aerobic digestion, used at smaller plants, aerates the sludge for 40 to 60 days so that the organisms consume their own cell mass. It is simple but uses energy instead of producing it. Lime stabilization raises the pH above 12 to kill pathogens without reducing the organic content. Composting, mixing dewatered cake with wood chips and letting it heat to 55 °C or more for days, produces a high-quality product. Thermal drying produces pellets with more than 90 % solids.

# Solids balance: thickening, anaerobic digestion, biogas and dewatered cake
UNITS = "US"
SI = UNITS == "SI"
prim = 4320.0                                  # kg TSS/d primary sludge (60 % of 7,200 kg/d)
was = 3300.0                                   # kg TSS/d waste activated sludge (biomass, inerts, alum P solids)
streams = [("primary", prim, 0.04, 0.80), ("waste activated", was, 0.06, 0.75)]   # thickened solids fraction, VS/TS
tot = vs = vol = 0.0
for name, m, frac, vsf in streams:
    v = m / (frac * 1000)                      # m3/d at density about 1000 kg/m3
    tot += m; vs += m * vsf; vol += v
    print(f"{name:16s} {m:6,.0f} kg/d at {100*frac:.0f} % solids -> {v:5.1f} m3/d" if SI else
          f"{name:16s} {m*2.2046:6,.0f} lb/d at {100*frac:.0f} % solids -> {v*264.17:6,.0f} gal/d")
print(f"digester feed {tot:,.0f} kg TS/d, {vs:,.0f} kg VS/d, {vol:.0f} m3/d" if SI else
      f"digester feed {tot*2.2046:,.0f} lb TS/d, {vs*2.2046:,.0f} lb VS/d, {vol*264.17:,.0f} gal/d")
# mesophilic digestion: 20 d SRT (no recycle, SRT = HRT), 50 % volatile solids destruction
SRT, VSR = 20, 0.50
Vd = vol * SRT
gas = 0.9 * VSR * vs                           # m3 biogas per kg VS destroyed, typical 0.75 to 1.1
CH4 = 0.64 * gas
energy = CH4 * 35.8 / 3.6                      # kWh/d at 35.8 MJ/m3 methane
print(f"digester volume {Vd:,.0f} m3 (two tanks of {Vd/2:,.0f} m3)" if SI else f"digester volume {Vd*264.17/1e6:.2f} MG (two tanks)")
print(f"VS destroyed {VSR*vs:,.0f} kg/d; biogas {gas:,.0f} m3/d; methane {CH4:,.0f} m3/d; energy {energy:,.0f} kWh/d" if SI else
      f"VS destroyed {VSR*vs*2.2046:,.0f} lb/d; biogas {gas*35.315:,.0f} ft3/d; methane {CH4*35.315:,.0f} ft3/d; energy {energy:,.0f} kWh/d")
digested = tot - VSR * vs
cake_frac = 0.22
cake = digested / cake_frac / 1000             # wet t/d (metric)
print(f"digested solids {digested:,.0f} kg/d; cake at {100*cake_frac:.0f} % solids {cake:.1f} wet t/d, {cake*7/25:.1f} truckloads of 25 t a week" if SI else
      f"digested solids {digested*2.2046:,.0f} lb/d; cake at {100*cake_frac:.0f} % solids {cake*1.1023:.1f} wet tons/d, {cake*1.1023*7/27.5:.1f} truckloads of 27.5 tons a week")
# time and temperature for a Class A batch process (US Part 503, solids 7 % or more): D = 131,700,000 / 10^(0.14 t)
for t in (50, 55, 60, 70):
    D = 131700000 / 10 ** (0.14 * t)
    print(f"{t} C: {D:8.2f} days = {D*24:7.1f} h")

primary           9,524 lb/d at 4 % solids -> 28,530 gal/d
waste activated   7,275 lb/d at 6 % solids -> 14,529 gal/d
digester feed 16,799 lb TS/d, 13,075 lb VS/d, 43,060 gal/d
digester volume 0.86 MG (two tanks)
VS destroyed 6,538 lb/d; biogas 94,254 ft3/d; methane 60,323 ft3/d; energy 16,986 kWh/d
digested solids 10,261 lb/d; cake at 22 % solids 23.3 wet tons/d, 5.9 truckloads of 27.5 tons a week
50 C:    13.17 days =   316.1 h
55 C:     2.63 days =    63.1 h
60 C:     0.52 days =    12.6 h
70 C:     0.02 days =     0.5 h

The city plant's thickened primary sludge and WAS together carry about 16,800 lb/d of dry solids and 13,100 lb/d of volatile solids in about 43,000 gal/d. At 20 days SRT the digesters need about 0.86 million gallons, matching the city's two existing digesters once they are cleaned of years of grit. Destroying half the volatile solids produces about 94,000 ft³/d of biogas carrying about 17,000 kWh/d of energy, enough to heat the digesters and, with a cogeneration engine, generate a third or more of the plant's electricity. After digestion about 10,300 lb/d of solids remain; dewatered to 22 % solids they make about 23 tons of cake a day, roughly six truckloads a week.

Digester gas is a fuel but also a hazard: methane is explosive between about 5 and 15 % in air, and biogas contains hydrogen sulfide. Digester buildings and gas handling areas are classified hazardous locations with explosion-proof equipment, flame arresters, pressure relief and gas detection; digesters are confined spaces that must never be entered without isolation, ventilation, testing and a rescue plan. Willowbend's design includes a gas holder, a waste gas burner for excess gas, and hydrogen sulfide and siloxane removal ahead of the cogeneration engine.

Energy recovery and codigestion

Many digesters have spare capacity, and adding high-strength organic wastes such as food processing residues, fats, oils and grease from restaurant interceptors, or source-separated food waste, can double or triple biogas production. This practice, codigestion, earns tipping fees and turns a disposal problem into energy, but it must be managed carefully: the added wastes change the digester's loading, alkalinity and ammonia, can bring plastics and grit, and increase the nitrogen returned in the centrate. Biogas can be burned in boilers to heat the digesters, used in combined heat and power engines that generate electricity and recover heat, or cleaned to renewable natural gas and injected into the gas grid, where it qualifies for renewable fuel credits in both countries. The city's food processor already sends fat skimmed by its flotation unit to a landfill; Willowbend's energy study recommends receiving it at the plant instead.

Dewatering

Dewatering takes digested sludge from 2 to 4 % solids to a cake of 15 to 30 %, which can be handled with a loader and hauled by truck. Centrifuges and belt filter presses are most common; screw presses use less energy and are gaining ground; plate-and-frame filter presses reach higher solids at greater cost. All need polymer conditioning, typically 5 to 15 kg of active polymer per metric ton of dry solids. Drying beds and reed beds serve small plants with land available.

The returned liquid from dewatering digested sludge, the centrate or filtrate, is rich in ammonia, often 800 to 1,500 mg/L as N, because digestion releases the nitrogen in the destroyed cells. It can add 15 to 25 % to the nitrogen load of the main process. At the city plant, Nadia's mass balance shows it adds about 250 kg/d of ammonia nitrogen, so the design includes a side-stream treatment unit, using anammox bacteria, that removes most of it before it returns.

Biosolids quality and use

In the United States, biosolids applied to land are regulated by EPA under 40 CFR Part 503. The rule sets pollutant limits for metals, management practices and two pathogen classes. Class A biosolids have pathogens reduced to below detectable levels and can be used with few restrictions, including in bagged products; Class B biosolids have pathogens greatly reduced and can be applied to farmland with restrictions on crop harvesting, grazing and public access for set periods. Vector attraction reduction, for example 38 % volatile solids destruction, must also be met. States add their own rules, and several now require PFAS testing; EPA's 2025 draft risk assessment for PFOA and PFOS in sewage sludge has increased attention to the issue.

Pathogen reduction is achieved by time at temperature. For batch processes on sludge of 7 % solids or more, the US rule expresses the Class A requirement as a minimum time \(D\) in days at temperature \(t\) in °C:

so that 50 °C requires about 13 days and 60 °C about half a day. Mesophilic digestion at 15 days or more and 35 °C meets the Class B pathogen standard; Class A needs thermophilic digestion, composting, heat drying, high-pH and high-temperature treatment, or another equivalent process. Canadian provincial categories use similar time-temperature criteria.

Land application returns nitrogen, phosphorus and organic matter to farmland, replacing commercial fertilizer, and is the most common end use in both countries. Application rates are set by the crop's nitrogen need, or increasingly by its phosphorus need, which limits the rate more and needs more land. Other end uses include mine and gravel pit reclamation, forestry, composting for landscaping, incineration with energy recovery, and landfill as a last resort. PFAS concerns have made some farmers and municipalities cautious; the city's program will include PFAS testing of the biosolids and source control of industrial dischargers that may contribute them.