A biological secondary treatment process that uses living microorganisms suspended in wastewater to remove dissolved organic matter (BOD). Organisms are aerated in a basin, settled in a secondary clarifier, and a portion of settled sludge is returned (RAS) to maintain the microbial population.

The tank where wastewater is mixed with activated sludge and continuously supplied with air or oxygen. The air keeps organisms suspended, supplies oxygen for respiration, and promotes contact between the microbial community and organic matter.

A condition or process requiring the presence of free dissolved oxygen. Aerobic bacteria use oxygen to break down organic matter. Most biological secondary treatment processes are aerobic. Contrast with anaerobic (no oxygen).

The capacity of water to resist changes in pH (buffering capacity), primarily from bicarbonate, carbonate, and hydroxide ions. Expressed in mg/L as CaCO₃. Critical in anaerobic digestion — declining alkalinity with rising volatile acids is an early warning sign of digester upset.

A condition or process occurring in the complete absence of dissolved oxygen. Anaerobic bacteria use alternative electron acceptors or ferment organic matter. Used in sludge digestion to stabilize solids and produce biogas (methane + CO₂).

A condition where dissolved oxygen is absent but nitrate (NO₃⁻) is present. Denitrifying bacteria use nitrate as an oxygen substitute in anoxic conditions, converting it to nitrogen gas. Distinct from anaerobic (no oxygen AND no nitrate).

A measure of the dissolved oxygen consumed by microorganisms decomposing organic matter over 5 days (BOD₅). Primary indicator of organic pollution strength. High influent BOD = heavy treatment load. High effluent BOD = treatment failure or permit violation.

A layer of microorganisms attached to a surface, embedded in a polymer matrix. The biological mechanism in fixed-film processes like trickling filters and RBCs. Also called slime layer or zoogleal film. Uneven or missing biofilm on a trickling filter often indicates distributor arm problems.

A condition where activated sludge fails to settle and compact normally, caused by overgrowth of filamentous bacteria. Indicated by high SVI (above 200 mL/g). The sludge appears fluffy and diffuse. Common causes: low DO, low F:M ratio, nutrient deficiency, septic influent. Not to be confused with rising sludge.

The formation and violent collapse of vapor bubbles inside a centrifugal pump caused by insufficient suction pressure. Produces a crackling or rattling noise, reduces flow, and damages the impeller over time. Caused by excessive suction lift, undersized suction piping, or a partially closed suction valve.

A measure of the total oxygen demand of both biodegradable and non-biodegradable organics, determined chemically. Results available in hours vs. 5 days for BOD. COD is always ≥ BOD for the same sample. Often used as a rapid process control surrogate for BOD.

A settling tank that separates solids from liquid by gravity. Primary clarifiers remove settleable solids before biological treatment. Secondary clarifiers separate treated biological sludge (MLSS) from the treated effluent after the aeration basin.

An enclosed space large enough to enter but not designed for continuous occupancy, with limited means of entry or exit. A permit-required confined space contains a known hazard. Wet wells, digesters, and manholes are common examples at wastewater plants. Governed by OSHA 29 CFR 1910.146.

The product of disinfectant Concentration × Time, used to quantify disinfection dose. A required CT value is specified by regulators to achieve a certain log-reduction of target pathogens. Higher CT = more effective disinfection. UV disinfection uses UV dose (mJ/cm²) rather than CT.

The removal of residual chlorine from effluent before discharge to protect aquatic life. Typically accomplished with sodium bisulfite (NaHSO₃) or sulfur dioxide (SO₂). Required by most NPDES permits for plants that chlorinate.

The biological conversion of nitrate (NO₃⁻) to nitrogen gas (N₂) under anoxic conditions. Removes total nitrogen from wastewater. Also the cause of rising sludge — when sludge stays in the clarifier too long, denitrification produces N₂ gas that floats the sludge to the surface.

The concentration of oxygen dissolved in water, expressed in mg/L. Aerobic biological treatment requires 1.0–3.0 mg/L in the aeration basin. Low DO promotes filamentous growth and bulking. High DO wastes energy and can cause pin floc via shear.

Treated wastewater leaving a treatment process or facility. Effluent quality is regulated by the plant's NPDES permit. Contrast with influent — wastewater entering the plant.

A holding tank used to dampen flow and load variations before treatment. Collects excess flow during peak hours and releases it during low-flow periods, protecting downstream processes from hydraulic and organic shock loads.

Food-to-Microorganism ratio — mass of organic food (BOD/COD) applied per day relative to the mass of active biomass (MLVSS). Controls the metabolic state of the sludge. High F:M = overfed organisms, poor floc. Low F:M = starved organisms, risk of filamentous growth.

Microorganisms that grow in long thread-like strands rather than compact clumps. Small numbers provide floc structure; overgrowth causes bulking sludge. Common genera include Nocardia, Microthrix, and Thiothrix. Detected by microscopic examination of mixed liquor.

Clumps of microorganisms and organic particles formed in the aeration basin. Healthy floc is dense and settles quickly. Poor floc — pin floc, dispersed growth, or filamentous floc — results in high effluent TSS and treatment problems.

The active disinfecting form of chlorine in water — primarily hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). HOCl is far more effective (40–80×). Lower pH favors HOCl; higher pH shifts equilibrium toward the weaker OCl⁻ form.

Heavy inorganic particles in wastewater — sand, gravel, cinders, eggshells, coffee grounds. Removed in a grit chamber during preliminary treatment using controlled flow velocity. If not removed, grit abrades pumps, fills digesters, and accumulates in basins.

The average time the liquid fraction spends in a treatment unit. HRT = Volume ÷ Flow Rate. Distinct from SRT (which tracks solids). In activated sludge, SRT is intentionally longer than HRT — the biology stays in the system longer than the water, thanks to RAS recycling.

A colorless, toxic gas produced by anaerobic decomposition of sulfur-containing organics. Smells like rotten eggs at low concentrations but deadens the sense of smell at higher concentrations — making it a serious confined space hazard. IDLH = 100 ppm. Common in wet wells and digesters.

The primary active disinfecting agent in chlorinated water. Far more effective than hypochlorite ion (OCl⁻). Favored at lower pH values (below 7.5). Rising pH shifts equilibrium toward OCl⁻, reducing disinfection efficiency even at the same chlorine dose.

The amount of material (BOD, solids, or flow) applied to a treatment unit per unit of time or unit area/volume. Exceeding design loading rates leads to treatment failure. Common forms: organic loading rate (lbs BOD/day), surface overflow rate (gpd/ft²), and hydraulic loading rate.

The mixture of wastewater and activated sludge present in the aeration basin during treatment. Contains both the biological community and the organic matter being consumed. The solids concentration is measured as MLSS.

Total suspended solids concentration in the aeration basin, expressed in mg/L. Typical range: 1,500–3,000 mg/L. Controlled primarily by WAS rate. Low MLSS = insufficient biology; high MLSS = clarifier overload risk.

The volatile (burnable) fraction of MLSS — representing the active living biomass. Typically 70–80% of MLSS in a healthy system. Used in F:M ratio calculations. A declining MLVSS/MLSS ratio over time suggests inert solids accumulating in the system.

The biological conversion of ammonia (NH₃) to nitrite then to nitrate (NO₃⁻), carried out by slow-growing autotrophic bacteria (Nitrosomonas and Nitrobacter). Requires adequate DO and a long enough SRT. Plants with ammonia permit limits must maintain nitrification.

National Pollutant Discharge Elimination System permit — the federal permit authorizing discharge of treated wastewater to US waters. Specifies effluent limits (BOD, TSS, ammonia, etc.), monitoring requirements, and reporting. Exceeding permit limits can result in regulatory enforcement.

A measure of hydrogen ion concentration on a 0–14 scale. Neutral = 7.0. Below 7 = acidic; above 7 = alkaline. Affects chlorine speciation, digester stability, biological treatment efficiency, and chemical dosing. Most biological processes operate best at pH 6.5–8.5.

Very small, lightweight floc particles that don't settle well. Commonly caused by over-aeration (physical shearing), very high F:M ratio, or very young sludge age. Distinct from bulking — the particles are tiny rather than filamentous.

The first major treatment stage (after preliminary treatment). Removes settleable solids and floating scum by gravity settling in primary clarifiers. Typically removes 50–70% of TSS and 25–40% of BOD. Does not remove dissolved organics.

Settled sludge pumped from the secondary clarifier back to the aeration basin to maintain the microbial population. RAS rate is adjusted to control the clarifier sludge blanket level. Too low a RAS rate raises the blanket; too high depletes MLSS in the aeration basin.

Settled sludge that floats back to the surface in large, dark clumps — caused by denitrification producing N₂ gas under anoxic conditions in the clarifier. SVI is typically normal (the sludge settled fine; it re-floated afterward). Corrective action: increase RAS rate to reduce sludge detention time. Not the same as bulking sludge.

Biological treatment following primary clarification. Removes dissolved and suspended organic matter using microorganisms. Common processes: activated sludge, trickling filters, RBCs. Standard secondary treatment performance: ≤30 mg/L BOD and ≤30 mg/L TSS.

A measure of sludge settleability from the 30-minute settleometer test. Formula: SVI (mL/g) = [Settled volume (mL/L) ÷ MLSS (mg/L)] × 1,000. Normal range: 80–150 mL/g. Above 200 indicates a settling problem (usually bulking). Below 50 may indicate over-oxidized or old sludge.

The average time a microorganism spends in the activated sludge system before being wasted out. Also called sludge age or MCRT. Controlled by WAS rate. Short SRT = young sludge. Long SRT required for nitrification (slow-growing nitrifiers need time to accumulate). One of the most critical activated sludge control parameters.

Flow applied to a clarifier per unit of surface area (gpd/ft²). Exceeding design SOR reduces settling time and causes solids carryover. Taking one clarifier out of service concentrates flow on remaining units, increasing the SOR on them.

The concentration of solids in water retained on a filter (typically 0.45 µm), expressed in mg/L. A primary NPDES permit parameter alongside BOD. High effluent TSS indicates poor clarifier performance, bulking, or hydraulic overloading. TSS and BOD often rise together — solids carry BOD with them.

A fixed-film secondary treatment process where wastewater is distributed over media (rock or plastic) and trickles down through a biofilm. Microorganisms on the media surface consume the organic matter as wastewater passes. Effluent is collected at the bottom and sent to a secondary clarifier.

Short-chain organic acids (acetic, propionic, butyric) produced by acid-forming bacteria during anaerobic digestion. In a stable digester, methanogens consume these acids as fast as they're made. Rising volatile acids combined with falling alkalinity is a key early warning of digester upset (souring).

The fraction of suspended solids that combusts at 550°C — representing the organic and biological portion. Used to estimate active biomass. In activated sludge, MLVSS = volatile suspended solids measured in the mixed liquor.

Excess sludge removed from the system to control SRT and MLSS. The primary long-term process control lever in activated sludge. Increasing WAS decreases SRT and MLSS. Decreasing WAS increases SRT and MLSS. Operators adjust WAS for long-term process control; RAS for short-term clarifier control.

A notched or flat overflow structure at the effluent end of a clarifier over which treated water flows before leaving the tank. Weir loading rate (flow per unit length of weir) is a design and operational parameter. Sludge or foam appearing at the weir indicates a process problem upstream.