Solids Handling: Thickening, Digestion, and Dewatering

Why Solids Handling Matters

Every water and wastewater treatment plant generates solids as a byproduct of cleaning water. These solids—called sludge or biosolids—must be stabilized, reduced in volume, and ultimately disposed of safely. Solids handling typically accounts for a large share of a plant's operating cost, so operators must understand each step: thickening, digestion, and dewatering.

Gravity Thickening

Before sludge can be digested or dewatered efficiently, its water content must be reduced. A gravity thickener is a large circular tank, similar in appearance to a secondary clarifier, where dilute sludge is fed at the center and allowed to settle. A slowly rotating rake mechanism gently stirs the settling solids, breaking up gas pockets and moving thickened sludge toward a central hopper at the bottom. Overflow, called filtrate or supernatant, returns to the plant headworks for retreatment.

Feed sludge may arrive at 0.5–1% solids (mostly water). After gravity thickening, underflow solids concentration can increase to 4–6% or more, depending on sludge type. This reduction in volume makes downstream processes far more efficient. Operators monitor feed flow rate, solids loading (pounds of solids per day per square foot of thickener surface area), and the clarity of the overflow.

Polymer is sometimes added to the feed to improve settling. The polymer addition point is typically at the feed pipe or a mixing chamber just upstream of the thickener.

Anaerobic Digestion

Anaerobic digestion stabilizes thickened sludge by using microorganisms that work without oxygen. The process destroys pathogens, reduces odor potential, and breaks down volatile (organic) solids into carbon dioxide and methane gas. That methane—called digester gas or biogas—can be captured and used to heat the digester or generate electricity.

A conventional anaerobic digester operates in a closed, heated tank, typically at mesophilic temperatures (around 95–100 °F / 35–38 °C). Mixing keeps solids in contact with active microorganisms. The key operating parameters operators watch include:

• Volatile solids loading: pounds of volatile solids fed per unit of digester volume per day
• Hydraulic retention time (HRT): how long sludge remains in the digester
• pH: anaerobic digestion is sensitive; a stable pH near neutral is essential
• Volatile solids reduction (VSR): the percentage of organic solids destroyed—a primary measure of digester performance
• Alkalinity and volatile acids: imbalance between these signals digester stress

A well-operated mesophilic anaerobic digester typically achieves 38–50% volatile solids reduction or higher, depending on design and feed characteristics. State operator manuals and WEF guidance provide facility-specific benchmarks.

Calculating Volatile Solids Reduction

The standard formula for VSR compares volatile solids (VS) entering and leaving the digester:

VSR (%) = [(VS in − VS out) / VS in] × 100

Where VS in and VS out are expressed in the same units (pounds per day or kg per day). You must account for both the concentration of volatile solids in the sludge and the flow rate of that sludge stream. See the worked examples below for a step-by-step walkthrough.

Mechanical Dewatering

Digested (or sometimes raw) sludge still contains 90–97% water after digestion. Mechanical dewatering removes additional water to produce a semi-solid cake that can be transported, composted, or land-applied. Common dewatering equipment includes:

• Belt filter press (BFP): sludge is conditioned with polymer, then sandwiched between two porous belts that squeeze water out through a series of rollers. Cake solids of 15–25% are typical.
• Centrifuge: spinning force separates solids from liquid. Often achieves higher cake solids than a belt press.
• Screw press and plate-and-frame filter press: used at smaller plants or for specific sludge types.

The liquid removed during dewatering—called filtrate, centrate, or press water—is high in solids and nutrients and is returned to the plant headworks.

Biosolids and the EPA Part 503 Rule

Stabilized, dewatered sludge that meets regulatory standards is called biosolids and may be beneficially used, most commonly by land application as a soil amendment. The EPA's 40 CFR Part 503 rule governs biosolids quality and land application. Key concepts:

• Class A biosolids must meet more stringent pathogen reduction requirements; they can be applied with fewer restrictions, including to home lawns and gardens.
• Class B biosolids meet a lower level of pathogen reduction and have site restrictions (such as limiting public access and certain crop harvesting intervals after application).
• Pathogen reduction can be achieved through processes such as anaerobic digestion, aerobic digestion, composting, heat treatment, and others specified in the rule.
• Vector attraction reduction requirements minimize the ability of flies, rodents, and other vectors to be attracted to biosolids.

Operators do not need to memorize every numeric limit, but must understand which class applies to their plant's process and what general obligations exist for recordkeeping and site management.

Safety in Solids Handling

• Confined space: digesters, thickener hoppers, and wet wells are permit-required confined spaces. Never enter without a completed confined-space entry permit, atmospheric testing, a trained attendant, and rescue equipment on site.
• Digester gas / methane: highly flammable. No open flames or ignition sources near digesters. Gas detectors must be maintained and calibrated.
• Hydrogen sulfide (H₂S): can be present when sludge is disturbed. Use a calibrated multi-gas detector. H₂S is immediately dangerous to life at relatively low concentrations.
• PPE: nitrile or chemical-resistant gloves, safety glasses or face shield when handling polymer or sludge, slip-resistant boots.
• Lockout/tagout (LOTO): before performing any maintenance on thickeners, belt presses, or centrifuges, follow your facility's LOTO procedure to de-energize and isolate all energy sources. Rotating belt-press rollers and centrifuges are serious pinch and entanglement hazards.