What is the difference between BOD and COD?

Biochemical Oxygen Demand (BOD) measures the oxygen consumed by microorganisms biodegrading organic matter over a 5-day incubation period, reflecting only the biodegradable fraction of organic pollution. Chemical Oxygen Demand (COD) measures the total oxygen equivalent of organic matter oxidized by a strong chemical oxidant (potassium dichromate), typically in a 2-3 hour test, and captures both biodegradable and non-biodegradable organics. COD values are always equal to or higher than BOD for the same sample, and the BOD/COD ratio is often used as an indicator of a wastewater's biodegradability.

What is the difference between a detention basin and a retention basin?

A detention basin temporarily stores stormwater runoff and releases it at a controlled rate to reduce peak downstream flows, and is normally dry between storm events. A retention basin (also called a wet pond) maintains a permanent pool of water even during dry weather, providing both flow attenuation and, through the standing water volume, additional water quality treatment via settling and biological uptake. Retention basins generally require more land and a reliable water source or high water table to maintain the permanent pool.

Why does the Ten States Standards matter if my state has its own regulations?

The Ten States Standards (formally the Recommended Standards for Water Works and Wastewater Facilities) is not itself law, but many state environmental agencies formally adopt it by reference or use it as the baseline for their own design standards, especially for aeration tank sizing, clarifier surface overflow rates, and filter loading rates. Even where a state has its own manual, reviewers often expect designs to meet or exceed Ten States Standards minimums unless the state standard is explicitly more restrictive or more permissive with documented justification.

What triggers a TMDL for a water body?

A TMDL is triggered when a water body is placed on a state's Clean Water Act Section 303(d) list of impaired waters — meaning monitoring data shows the water body does not meet its designated water quality standards for one or more pollutants (commonly nutrients, bacteria, sediment, or low dissolved oxygen). Once listed, the state (or EPA, if the state does not act) must develop a TMDL establishing the total pollutant load the water body can assimilate while meeting standards, then allocate that load among contributing point and non-point sources, which often results in tighter NPDES permit limits for dischargers in that watershed.

What is the practical difference between nitrification and denitrification in a treatment plant?

Nitrification is an aerobic process (requires dissolved oxygen) in which bacteria convert ammonia to nitrite and then to nitrate — it removes ammonia toxicity but does not remove total nitrogen, since nitrate remains in the effluent. Denitrification is an anoxic process (requires the absence of oxygen but the presence of nitrate) in which different bacteria convert that nitrate to inert nitrogen gas, which off-gasses harmlessly to the atmosphere. A plant seeking to meet a total nitrogen limit needs both processes in sequence — nitrification followed by denitrification — typically achieved through alternating aerobic and anoxic zones or basins in a biological nutrient removal (BNR) design.

How is the Rational Method different from the NRCS curve number method for estimating runoff?

The Rational Method (Q = CiA) is a simple, single-peak-flow estimate best suited to small drainage areas (generally under 200 acres) and is widely used for sizing storm drains and small culverts. The NRCS (SCS) curve number method estimates a full runoff hydrograph (not just peak flow) based on soil group, land cover, and antecedent moisture, and is better suited to larger or more complex watersheds and to detention basin routing, where the volume and timing of the full runoff hydrograph — not just its peak — determines required storage.

Engineering·13 min read·August 29, 2026

💧 Environmental & Water Resources Terminology Glossary: 55 Essential Terms Explained

Plain-language definitions of 55 essential environmental and water resources engineering terms — BOD, NPDES, activated sludge, hydraulic loading rate, Manning's equation, MCL, TMDL, and more, with EPA, ASCE, and Ten States Standards references.

📖Environmental & Water Resources Engineering Handbook (Full Access)

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Why These Terms Matter

Environmental and water resources engineering sits at the intersection of public health, regulatory compliance, and hydraulic science. Practitioners move constantly between EPA and state regulatory language (NPDES, MCL, TMDL), unit-process terminology from treatment plant design (activated sludge, coagulation, disinfection), and classical hydraulic and hydrologic vocabulary (Manning's equation, time of concentration, hydraulic loading rate). Misreading a term in a permit, a design standard, or a plan review comment can mean a rejected submittal, an undersized basin, or a violation of an NPDES discharge limit.

This glossary collects 55 of the terms most frequently encountered in water treatment, wastewater treatment, stormwater management, hydrology, and environmental permitting practice, organized alphabetically with the standard or regulatory body that defines or governs each term where applicable.

A

Activated Sludge — EPA / Metcalf & Eddy: A biological wastewater treatment process in which a mixed population of aerobic microorganisms (the "mixed liquor") is suspended in an aeration tank and continuously mixed with air or oxygen to break down organic matter. Treated mixed liquor flows to a secondary clarifier where the biomass settles and a portion is returned to the aeration tank (return activated sludge, RAS) to maintain the microbial population; the remainder is wasted (waste activated sludge, WAS) to control solids retention time.
Aeration — General: The process of adding air or oxygen to water or wastewater, either to support aerobic biological treatment (activated sludge, trickling filters) or to strip dissolved gases and volatile compounds from drinking water (such as radon or hydrogen sulfide) and add dissolved oxygen.
Alum (Aluminum Sulfate) — Water Treatment: A common coagulant used in water treatment, Al₂(SO₄)₃, that hydrolyzes in water to form positively charged aluminum hydroxide flocs that neutralize the negative surface charge on colloidal particles (clay, organic matter), allowing them to aggregate and settle during coagulation-flocculation.
Anoxic — Wastewater Treatment: A condition in which dissolved oxygen is absent or present only in very low concentrations but nitrate (NO₃⁻) is available as an electron acceptor. Anoxic zones in biological nutrient removal (BNR) processes are where denitrifying bacteria convert nitrate to nitrogen gas, achieving biological nitrogen removal.
Aquifer — Hydrogeology: An underground layer of permeable rock, sand, or gravel that can store and transmit significant quantities of groundwater to wells and springs. A confined aquifer is bounded above and below by impermeable layers and is under pressure; an unconfined (water table) aquifer has a free upper surface exposed to atmospheric pressure through the overlying soil.

B

Best Management Practice (BMP) — EPA / NPDES: A structural or non-structural technique used to reduce the quantity or improve the quality of stormwater runoff before it reaches a receiving water. Structural BMPs include detention basins, bioretention cells, and permeable pavement; non-structural BMPs include street sweeping, erosion control ordinances, and public education programs.
Biochemical Oxygen Demand (BOD) — EPA / Standard Methods: A measure of the amount of dissolved oxygen consumed by microorganisms as they biologically decompose organic matter in a water sample, typically measured over a 5-day incubation period at 20°C (BOD₅). BOD is the primary parameter used to characterize the organic strength of wastewater and to size biological treatment processes; NPDES permits commonly set effluent BOD limits (often 30 mg/L monthly average for secondary treatment).
Bioretention — Stormwater / Low Impact Development: An engineered stormwater practice consisting of a shallow depression filled with an engineered soil media and planted with vegetation, designed to capture, filter, and infiltrate runoff while removing pollutants through filtration, adsorption, and biological uptake. Rain gardens are a common residential-scale form of bioretention.
Backwash — Water Treatment: The process of reversing flow through a granular media filter (sand, anthracite, GAC) to lift, expand, and scour the filter bed, dislodging accumulated solids and restoring filter capacity. Backwash is typically triggered by a set head loss across the filter or a scheduled time interval.

C

Chlorine Demand — Disinfection: The difference between the amount of chlorine added to water and the amount of free or combined chlorine remaining after a specified contact time, representing chlorine consumed by reactions with organic matter, ammonia, iron, manganese, and other reducing substances in the water.
Coagulation — Water Treatment: The chemical process of destabilizing colloidal particles in water by adding a coagulant (alum, ferric chloride, or polymer) that neutralizes the particles' negative surface charge, allowing them to collide and stick together during subsequent flocculation. Coagulation is a rapid-mix unit process, typically lasting seconds under high-energy mixing.
Combined Sewer Overflow (CSO) — EPA / NPDES: A discharge of untreated or partially treated wastewater and stormwater that occurs when the capacity of a combined sewer system (one carrying both sanitary sewage and stormwater in the same pipe) is exceeded during wet weather, causing the mixture to bypass treatment and discharge directly to a receiving water. CSOs are regulated under the EPA's CSO Control Policy and typically require a long-term control plan (LTCP).
Curve Number (CN) — NRCS / Hydrology: A dimensionless parameter (0–100) used in the NRCS (formerly SCS) runoff method to estimate direct runoff from rainfall, based on hydrologic soil group, land cover, and antecedent moisture condition. Higher curve numbers (e.g., 98 for pavement) indicate greater runoff potential; lower numbers (e.g., 30 for woods in good condition on well-drained soil) indicate greater infiltration.

D

Denitrification — Wastewater Treatment: The biological process by which facultative bacteria convert nitrate (NO₃⁻) to nitrogen gas (N₂) under anoxic conditions, using nitrate as an electron acceptor in place of oxygen. Denitrification is the second step of biological nitrogen removal, following nitrification, and requires a carbon source and the absence of dissolved oxygen.
Detention Basin — Stormwater: A stormwater management facility designed to temporarily store runoff and release it at a controlled rate to reduce peak discharge to downstream systems, typically dry between storm events (as opposed to a retention basin, which maintains a permanent pool of water).
Disinfection Byproducts (DBPs) — EPA / Stage 2 DBP Rule: Chemical compounds formed when disinfectants (chlorine, chloramine, ozone) react with naturally occurring organic matter in source water. The two regulated classes are trihalomethanes (TTHMs, MCL 80 µg/L) and haloacetic acids (HAA5, MCL 60 µg/L), both regulated under the EPA Stage 2 Disinfectants and Disinfection Byproducts Rule.
Dissolved Oxygen (DO) — Water Quality: The concentration of oxygen gas dissolved in water, expressed in mg/L, essential for supporting aquatic life and aerobic biological treatment processes. Low DO in receiving waters (often below 5 mg/L for warm-water fisheries) indicates oxygen depletion from excessive organic loading, commonly assessed through BOD/DO modeling such as the Streeter-Phelps sag curve.

E

Effluent — General: Treated or partially treated wastewater discharged from a treatment facility, unit process, or outfall, as distinguished from influent (the untreated water entering a process).
Eutrophication — Water Quality: The enrichment of a water body with nutrients, primarily nitrogen and phosphorus, that stimulates excessive algae and aquatic plant growth. Subsequent decomposition of this biomass consumes dissolved oxygen, potentially causing fish kills and degrading water quality. Cultural eutrophication accelerated by human nutrient inputs (agricultural runoff, wastewater discharge) is a leading driver of TMDL development for lakes and estuaries.
Evapotranspiration — Hydrology: The combined loss of water from a land surface through direct evaporation and transpiration from vegetation, a major component of the water balance equation used in hydrologic modeling and stormwater volume calculations.

F

Filtration — Water Treatment: A physical treatment process that removes suspended solids, floc, and pathogens by passing water through a granular media bed (sand, anthracite, dual-media) or membrane. Rapid sand filtration is the conventional approach following coagulation-flocculation-sedimentation; the Surface Water Treatment Rule requires filtration (or an approved alternative) for all surface water and groundwater under the direct influence of surface water.
Flocculation — Water Treatment: The physical process following coagulation in which destabilized particles are gently mixed at low velocity gradients to promote collisions and the formation of larger, settleable floc particles, typically occurring over 20–30 minutes in a dedicated flocculation basin with slow paddle or turbine mixers.
Freeboard — Hydraulic Design: The vertical distance between the design water surface elevation (in a channel, pond, tank, or levee) and the top of the containing structure, provided as a safety margin against wave action, settlement, debris, and design uncertainty. Detention basin freeboard is commonly specified as 1 foot above the 100-year design water surface.

G

Grit Chamber — Wastewater Treatment: A preliminary treatment unit that removes heavy inorganic solids (sand, gravel, eggshells, coffee grounds) from raw wastewater by slowing flow velocity enough to settle grit while keeping lighter organic solids in suspension for downstream treatment, protecting pumps and downstream equipment from abrasive wear.
Groundwater Under the Direct Influence of Surface Water (GWUDI) — EPA / SWTR: A regulatory classification for groundwater sources (typically shallow wells or springs near surface water bodies) shown through hydrogeologic assessment to be significantly influenced by surface water, subjecting the source to the same filtration and disinfection requirements as surface water under the Surface Water Treatment Rule.

H

Hydraulic Loading Rate (HLR) — Water/Wastewater Treatment Design: The flow rate of water applied per unit surface area of a treatment unit (clarifier, filter, or land application area), typically expressed in gallons per day per square foot (gpd/ft²). HLR is a key design parameter for sizing sedimentation basins and filters — conventional rapid sand filters are typically designed for 2–4 gpm/ft².
Hydraulic Retention Time (HRT) — Wastewater Treatment: The average time water or wastewater remains within a treatment tank or basin, calculated as tank volume divided by flow rate. HRT (distinct from solids retention time, SRT, in activated sludge systems) governs the contact time available for chemical reactions, biological treatment, and disinfection (CT values).
Hydrologic Soil Group (HSG) — NRCS: A classification (A, B, C, or D) assigned by the Natural Resources Conservation Service based on a soil's infiltration and runoff potential when saturated. Group A soils (deep, well-drained sand) have low runoff potential; Group D soils (clay, shallow to bedrock, high water table) have high runoff potential. HSG is a required input to the NRCS curve number method.

I

Impervious Surface — Stormwater: Any surface (pavement, rooftops, compacted soil) that prevents or significantly reduces infiltration of rainfall into the ground, increasing the volume and rate of stormwater runoff. Percent impervious cover is a primary input to hydrologic and water quality volume calculations and a common regulatory threshold for triggering stormwater management requirements.
Infiltration — Hydrology / Wastewater: In hydrology, the downward movement of water from the surface into soil. In sanitary sewer engineering, infiltration refers to groundwater entering a sewer system through cracked pipes, joints, or manholes (as distinguished from inflow, which is stormwater entering through direct connections such as roof drains or sump pumps); combined, infiltration and inflow (I/I) can overload treatment plants during wet weather.
Influent — General: Untreated water or wastewater entering a treatment plant or unit process, as distinguished from effluent leaving the process.

L

Land Application — Wastewater/Biosolids: The controlled application of treated wastewater effluent or biosolids to land as a beneficial reuse and disposal method, regulated under EPA 40 CFR Part 503 for biosolids, with restrictions based on pathogen reduction class (Class A or Class B) and pollutant concentration limits.
Lift Station — Wastewater Collection: A pump station within a gravity sewer collection system used to raise wastewater to a higher elevation when gravity flow is impractical due to topography, typically consisting of a wet well, submersible or dry-pit pumps, and a force main discharging to a higher point in the collection system.

M

Manning's Equation — Open Channel Hydraulics: An empirical formula, V = (1.49/n) R^(2/3) S^(1/2) (U.S. customary units), used to calculate flow velocity in open channels and partially full pipes based on the hydraulic radius (R), channel slope (S), and Manning's roughness coefficient (n), which depends on channel material and condition. It is the standard method for sizing storm drains, culverts, and open channels.
Maximum Contaminant Level (MCL) — EPA / Safe Drinking Water Act: The legally enforceable maximum concentration of a contaminant permitted in water delivered to any user of a public water system, set as close to the health-based maximum contaminant level goal (MCLG) as is feasible considering treatment technology and cost. Examples include an MCL of 10 µg/L for arsenic and 15 pCi/L for gross alpha particle activity.
Mixed Liquor Suspended Solids (MLSS) — Activated Sludge: The concentration of suspended solids (primarily microbial biomass) in the aeration tank of an activated sludge process, typically maintained between 2,000 and 4,000 mg/L for conventional activated sludge, used together with the food-to-microorganism (F/M) ratio to control process loading and performance.

N

National Pollutant Discharge Elimination System (NPDES) — EPA / Clean Water Act: The permitting program established under Section 402 of the Clean Water Act that regulates point-source discharges of pollutants to waters of the United States, requiring dischargers (treatment plants, industrial facilities, and — under Phase I/II stormwater rules — municipal separate storm sewer systems and construction sites) to obtain a permit specifying effluent limits, monitoring, and reporting requirements.
Nitrification — Wastewater Treatment: The two-step biological oxidation of ammonia (NH₃/NH₄⁺) first to nitrite (NO₂⁻) by ammonia-oxidizing bacteria (e.g., Nitrosomonas), then to nitrate (NO₃⁻) by nitrite-oxidizing bacteria (e.g., Nitrobacter), an aerobic process requiring adequate dissolved oxygen, alkalinity, and solids retention time in the activated sludge process.
Non-Point Source (NPS) Pollution — EPA / Clean Water Act: Water pollution originating from diffuse sources rather than a single identifiable discharge point, such as agricultural runoff, urban stormwater, and atmospheric deposition. NPS pollution is not directly regulated under NPDES permits (which cover point sources) but is addressed through TMDLs, watershed management plans, and best management practices.

O

Outfall — General: The point at which a pipe, channel, or conduit discharges effluent, treated wastewater, or stormwater into a receiving water body, typically the location where NPDES permit compliance monitoring is conducted.
Ozonation — Disinfection: A disinfection and oxidation process using ozone gas (O₃), a powerful oxidant generated on-site by passing dry air or oxygen through a high-voltage electrical field, effective against chlorine-resistant pathogens such as Cryptosporidium and Giardia and used to control taste, odor, and color without producing chlorinated disinfection byproducts (though it can form bromate as a byproduct in bromide-containing source water).

P

Peak Flow / Peak Discharge — Hydrology: The maximum instantaneous rate of flow occurring during a storm event or design condition, typically expressed in cubic feet per second (cfs), used to size culverts, storm drains, and detention facilities for a given design storm return period (e.g., the 10-year or 100-year peak flow).
Point Source — EPA / Clean Water Act: Any discernible, confined, and discrete conveyance from which pollutants are or may be discharged, including pipes, ditches, channels, tunnels, and conduits. Point sources require an NPDES permit, in contrast to non-point sources, which are diffuse and unpermitted.
Potable Water — Drinking Water: Water that is safe for human consumption, meeting all applicable federal and state drinking water standards, as distinguished from non-potable water (such as reclaimed water) which may be safe for irrigation or industrial use but not for drinking.

R

Rational Method — Hydrology: A simplified empirical formula, Q = CiA, used to estimate peak stormwater runoff rate for small drainage areas (typically under 200 acres), where Q is peak flow (cfs), C is a runoff coefficient reflecting land cover and soil type, i is rainfall intensity (in/hr) at the time of concentration, and A is drainage area (acres).
Reclaimed Water — Water Reuse: Treated wastewater effluent that has received an additional level of treatment (often filtration and high-level disinfection) to make it suitable for beneficial non-potable reuse, such as landscape irrigation, agricultural irrigation, industrial cooling, or toilet flushing, governed by state-specific water reuse regulations (e.g., California Title 22).
Return Activated Sludge (RAS) — Activated Sludge: The portion of settled activated sludge biomass from the secondary clarifier that is pumped back to the head of the aeration tank to maintain an adequate microbial population (MLSS concentration) for ongoing biological treatment.

S

Sanitary Sewer Overflow (SSO) — EPA / NPDES: An unpermitted discharge of raw or partially treated sewage from a separate sanitary sewer system (one that does not also carry stormwater), typically caused by pipe blockages, infiltration/inflow overloading, pump station failure, or pipe collapse. SSOs are prohibited discharges under the Clean Water Act and are subject to reporting requirements and, often, a consent decree requiring system rehabilitation.
Sedimentation (Clarification) — Water/Wastewater Treatment: A physical treatment process in which suspended solids and flocculated particles settle out of water under the influence of gravity in a quiescent basin (clarifier), typically the unit process following flocculation in water treatment or following biological treatment (secondary clarification) in wastewater treatment.
Solids Retention Time (SRT) — Activated Sludge (also called Sludge Age): The average time solids (microorganisms) remain in an activated sludge system, calculated as the mass of solids in the system divided by the mass of solids removed (wasted) per day. SRT is the primary control parameter for activated sludge process performance — longer SRTs favor nitrification and sludge stability, while shorter SRTs favor faster-growing organisms and reduce sludge production.
Stormwater Pollution Prevention Plan (SWPPP) — EPA / NPDES: A site-specific document required under NPDES construction general permits and industrial stormwater permits that identifies potential pollutant sources, describes best management practices to control erosion and sedimentation, and outlines inspection and maintenance procedures to prevent stormwater contamination during construction or industrial operations.
Suspended Solids (Total Suspended Solids, TSS) — Water Quality: The dry weight of particulate matter suspended in a water or wastewater sample, retained on a filter of specified pore size, measured in mg/L. TSS is, along with BOD, one of the two primary conventional pollutants limited in NPDES permits for secondary treatment (commonly 30 mg/L monthly average).

T

Ten States Standards — Great Lakes / Upper Mississippi River Board: A widely referenced set of recommended design standards for water and wastewater treatment facilities, formally titled "Recommended Standards for Water Works" and "Recommended Standards for Wastewater Facilities," developed by a board representing ten (now more) U.S. states and Ontario, frequently adopted or referenced by state regulatory agencies as minimum design criteria.
Time of Concentration (Tc) — Hydrology: The time required for runoff to travel from the hydraulically most distant point in a drainage basin to the point of interest (outlet), a required input to the Rational Method and NRCS TR-55 peak flow calculations. Tc is typically computed as the sum of sheet flow, shallow concentrated flow, and channel/pipe flow travel times.
Total Maximum Daily Load (TMDL) — EPA / Clean Water Act Section 303(d): The maximum amount of a pollutant that a water body can receive and still meet water quality standards, allocated among point sources (as waste load allocations), non-point sources (as load allocations), and a margin of safety. TMDLs are developed for water bodies listed as impaired under CWA Section 303(d) and often drive more stringent NPDES permit limits.
Trickling Filter — Wastewater Treatment: A fixed-film biological treatment process in which wastewater is distributed over a bed of rock or synthetic media colonized by a biofilm of microorganisms; as wastewater trickles through, organic matter is absorbed and degraded by the attached biomass, with excess biofilm ("sloughing") removed downstream in a secondary clarifier.
Turbidity — Water Quality: A measure of the cloudiness or haziness of water caused by suspended particulate matter, measured in Nephelometric Turbidity Units (NTU) using light scattering. The Surface Water Treatment Rule requires filtered water turbidity to be less than 0.3 NTU in at least 95% of samples each month, as turbidity can shield pathogens from disinfection.

V

Volatile Organic Compounds (VOCs) — EPA / Drinking Water: Synthetic organic chemicals with high vapor pressure that readily evaporate from water, including solvents such as trichloroethylene (TCE) and benzene, regulated in drinking water under EPA MCLs and commonly removed by air stripping or granular activated carbon (GAC) adsorption.

W

Waste Activated Sludge (WAS) — Activated Sludge: The portion of activated sludge biomass removed (wasted) from the treatment process daily to control solids retention time and MLSS concentration, typically directed to sludge thickening and digestion processes for further treatment and disposal or beneficial reuse as biosolids.
Water Table — Hydrogeology: The upper surface of the saturated zone in an unconfined aquifer, below which soil and rock pores are fully saturated with groundwater, and above which lies the unsaturated (vadose) zone. Water table elevation fluctuates seasonally with recharge and pumping.
Watershed (Drainage Basin) — Hydrology: The total land area that drains to a common point, such as a stream, lake, or outfall, delineated by topographic divides. Watershed area and land use are fundamental inputs to hydrologic modeling for stormwater design and TMDL development.
Wellhead Protection Area (WHPA) — EPA / Safe Drinking Water Act: The surface and subsurface area surrounding a water supply well or wellfield through which contaminants are reasonably likely to migrate toward and reach the well, delineated as part of a wellhead protection program to guide land-use restrictions and source water protection.

Topics covered

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