Sludge Treatment & Biosolids Engineering

Eighty-five vocabulary flashcards summarizing essential terms, materials, processes, and regulations related to sludge treatment, stabilization, conditioning, dewatering, and biosolids management.

Sludge

Concentrated impurities from wastewater separated as a semi-solid mass.

Biosolids

Treated or stabilized wastewater sludge suitable for safe disposal or beneficial reuse.

Screenings

Large organic and inorganic materials removed on bar racks during preliminary treatment.

Grit

Heavy inorganic particles that settle rapidly in grit chambers.

Scum / Grease

Floatable oils and solids skimmed from settling tanks and other units.

Primary Sludge

Odorous gray slurry from primary clarifiers; readily digested anaerobically.

Activated Sludge

Brown flocculent biomass from aeration basins; “earthy” odor when healthy.

Trickling Filter Sludge (Humus)

Brown, flocculent, relatively inoffensive sludge from trickling-filter clarifiers.

Aerobically Digested Biosolids

Brown flocculent solids with musty odor that dewater easily after aerobic digestion.

Anaerobically Digested Biosolids

Dark brown-to-black gas-laden solids with burnt-rubber odor when fully digested.

Waste Activated Sludge (WAS)

Excess biomass withdrawn from secondary clarifiers, 70-80 % volatile solids.

Sludge Thickening

Process that increases solids concentration by removing a portion of water.

Gravity Thickening

Uses sedimentation tanks and scraper arms to settle primary sludge to about 10 % solids.

Flotation Thickening (DAF)

Air bubbles attach to particles causing them to float for removal.

Mechanical Thickening

Centrifuges, rotary drums, or filter belts concentrate sludge mechanically.

Centrifuge

High-speed rotating equipment that separates sludge solids by centrifugal force.

Gravity Belt Thickener

Moving porous belt drains water from polymer-conditioned sludge by gravity.

Rotary Drum Thickener

Rotating screened cylinder that drains water from flocculated sludge.

SVR (Sludge Volume Ratio)

Volume of sludge blanket in a thickener divided by daily volume of sludge withdrawn.

Air-to-Solids Ratio (DAF)

Mass ratio of dissolved air to solids; typical 0.02–0.06 : 1 for sludge thickening.

Hydraulic Loading (DAF)

(Feed + recycle flow)/flotation area; commonly 30–120 m³ d⁻¹ m⁻².

Solids Loading Rate (DAF)

Mass of solids per hour per flotation area; 2–5 kg h⁻¹ m⁻² for WAS without polymer.

Sludge Stabilization

Conversion of putrescible sludge to inert material by chemical or biological means.

Alkaline Stabilization

Raising sludge pH ≥ 12 (usually with lime) to inhibit pathogens and odors.

Lime Stabilization

Addition of Ca(OH)₂ or CaO for at least 2 h at pH ≥ 12.

Anaerobic Digestion

Stabilization in absence of oxygen producing methane-rich biogas.

Aerobic Digestion

Long-term aeration that oxidizes organics to CO₂, H₂O, and nitrate.

Composting

Aerobic thermophilic biodegradation of sludge mixed with bulking agents to humus.

Hydrolysis (Digestion)

Extracellular enzymes split complex organics into soluble monomers.

Acidogenesis

Fermentative step converting monomers to volatile acids, alcohols, CO₂, and H₂.

Acetogenesis

Conversion of volatile acids/alcohols to acetate, CO₂, and H₂ by acetogens.

Methanogenesis

Final anaerobic step where methanogens produce CH₄ from acetate, H₂/CO₂, or methyl compounds.

Acetoclastic Methanogens

Microbes (e.g., Methanosarcina) that split acetate into CH₄ and CO₂.

Hydrogenotrophic Methanogens

Methanogens reducing CO₂ with H₂ to form CH₄.

Methyltrophic Methanogens

Archaea that produce methane from simple methylated substrates like methanol.

PFRP (Processes to Further Reduce Pathogens)

Class A treatments that lower pathogens to below detectable levels.

PSRP (Processes to Significantly Reduce Pathogens)

Class B treatments that markedly, but not completely, reduce pathogens.

Class A Biosolids

Products meeting strict pathogen limits (FC < 1000 MPN/g TS or Salmonella < 3 MPN/4 g).

Class B Biosolids

Biosolids with reduced pathogens suitable for controlled land application.

CFR Title 40 Part 503

U.S. EPA rules governing use and disposal of sewage sludge biosolids.

Sludge Conditioning

Treatment (chemical or physical) to enhance sludge dewatering performance.

Heat Treatment (Thermal Conditioning)

Pressurized heating of sludge to 177–240 °C for 15–40 min to improve dewaterability.

Elutriation

Washing digested sludge with water to remove soluble components before conditioning.

Chemical Conditioning

Use of lime, ferric chloride, alum, or polymers to coagulate sludge solids.

Buchner Funnel Test

Laboratory measure of sludge specific resistance and dewatering behavior.

Capillary Suction Time (CST) Test

Quick test measuring water release from sludge onto filter paper.

Standard Jar Test (Sludge)

Bench test to select conditioner type and dose via mixing, flocculation, and settling.

Sludge Dewatering

Physical separation that reduces sludge moisture by vacuum, pressure, or drying.

Cake (Dewatering)

Solid output from a dewatering device, typically 15–50 % dry solids.

Belt Filter Press

Continuous dual-belt system squeezing conditioned sludge through rollers.

Filtrate

Liquid drained or pressed out of sludge during dewatering.

Pressure Filter Press

Batch plates and frames that dewater sludge under up to ~225 psi.

Fixed-Volume Recessed Plate Press

Conventional plate press with rigid chambers of constant volume.

Variable-Volume (Diaphragm) Plate Press

Press using inflatable membranes to squeeze sludge for higher cake solids.

Vacuum Filter

Rotary drum under vacuum that forms, dewaters, and washes a sludge cake.

Solid Bowl Decanter

Horizontally oriented centrifuge where scroll conveys solids toward discharge ports.

Centrate

Clarified liquid effluent from a centrifuge.

Polyelectrolyte

High-molecular-weight polymer used to flocculate sludge for centrifugation or pressing.

Sand Drying Bed

Layered sand/gravel bed that dewaters sludge by drainage and evaporation.

Paved Drying Bed

Concrete or asphalt slab (drainage or decant type) facilitating mechanical sludge removal.

Artificial Media Drying Bed

Drying bed that replaces sand with wedge wire or polyurethane panels for drainage.

Vacuum-Assisted Drying Bed

Bed with porous plates and applied vacuum to accelerate dewatering.

Drying Lagoon

Large earthen basin where sludge dries by long-term evaporation and seepage.

Amendment (Composting)

Organic material (e.g., sawdust, straw) added to sludge to adjust moisture and porosity.

Bulking Agent

Material (often wood chips) providing structural support and air voids in compost pile.

ATAD (Autothermal Thermophilic Aerobic Digestion)

Insulated aerobic process that self-heats to 55–60 °C for rapid solids and pathogen reduction.

High-Purity Oxygen Aerobic Digestion

Aerobic digestion using pure O₂ to enhance reaction rates, particularly in cold climates.

Cryophilic Aerobic Digestion

Low-temperature (< 20 °C) aerobic stabilization requiring longer sludge ages.

Low-Rate Anaerobic Digester

Unmixed, unheated tank with 30–60 day SRT relying on stratification.

High-Rate Anaerobic Digester

Mixed, heated digester with 10–20 day SRT and often staged operation.

Egg-Shaped Digester

Tall steep-sloped anaerobic digester minimizing grit deposition and improving mixing.

Primary Digester

Heated, mixed vessel where main anaerobic reactions and gas production occur.

Secondary Digester

Unheated tank for sludge settling, gas storage, and supernatant return.

Gas Production (Anaerobic)

810–1120 L biogas per kg volatile solids destroyed; ~65 % methane typical.

Surface Disposal (Monofill)

Dedicated land placement of biosolids in piles or sludge-only landfills.

Beneficial Land Application

Use of biosolids on agricultural or non-agricultural land to supply nutrients and organic matter.

Incineration (Sludge)

Thermal destruction of dewatered sludge to inert ash for disposal.

Cake Solids (Press)

Percentage dry solids in press cake; belt presses typically 18-30 %, plate presses 35-50 %.