Reference
Wastewater Treatment Glossary
Concise, engineering-accurate definitions of the terms used across secondary treatment — from BOD and SRT to MBBR, MBR and SBR.
- BOD₅
- Biochemical Oxygen Demand over 5 days — the oxygen microorganisms need to break down biodegradable organic matter. The Urban Wastewater Treatment Directive sets a secondary-treatment standard of 25 mg/L (70–90% reduction).
- COD
- Chemical Oxygen Demand — the oxygen equivalent of chemically oxidisable matter, including non-biodegradable fractions. The directive standard is 125 mg/L (75% reduction).
- TSS
- Total Suspended Solids — particulate matter in the water. The directive sets a typical secondary standard around 35 mg/L; effluent TSS carries associated BOD and phosphorus.
- MLSS
- Mixed Liquor Suspended Solids — the concentration of biomass (activated sludge) held in the aeration basin.
- SRT
- Solids Retention Time (sludge age) — the average time biomass stays in the system. It governs nitrification capacity and sludge production and is a central design lever.
- HRT
- Hydraulic Retention Time — the average time the water itself spends in a tank or process.
- F/M ratio
- Food-to-Microorganism ratio — the organic load applied relative to the biomass available to treat it; a key operating indicator for activated sludge.
- SVI
- Sludge Volume Index — a measure of how well activated sludge settles. A high SVI signals bulking and poor settling.
- DO
- Dissolved Oxygen — oxygen available to the aerobic biology. A setpoint around 2 mg/L in the aerobic zone is typical for BOD removal and nitrification.
- Nitrification
- The aerobic, two-step biological oxidation of ammonia to nitrate by autotrophic bacteria. It is temperature- and SRT-sensitive.
- Denitrification
- The anoxic biological reduction of nitrate to nitrogen gas, used for total-nitrogen removal; it also recovers alkalinity.
- EBPR
- Enhanced Biological Phosphorus Removal — uses an anaerobic selector to favour phosphorus-accumulating organisms that store phosphorus in the biomass, removed with the waste sludge.
- Activated Sludge
- The conventional suspended-growth secondary process: an aeration basin plus a secondary clarifier with sludge return.
- MBBR
- Moving Bed Biofilm Reactor — a fixed-film process in which biomass grows on suspended plastic carriers, adding treatment capacity without a larger clarifier.
- MBR
- Membrane Bioreactor — combines biological treatment with membrane filtration, replacing the secondary clarifier and producing very high-quality, low-solids effluent in a compact footprint.
- SBR
- Sequencing Batch Reactor — a fill-and-draw activated-sludge process where all steps happen in one tank across timed phases.
- IFAS
- Integrated Fixed-Film Activated Sludge — a hybrid adding biofilm carriers into an activated-sludge basin to boost capacity, especially for nitrification.
- RAS
- Return Activated Sludge — settled sludge recycled from the clarifier back to the aeration basin to maintain the MLSS.
- WAS
- Waste Activated Sludge — the surplus sludge removed from the system for treatment and disposal; wasting rate sets the SRT.
- Secondary Clarifier
- The settling tank downstream of the aeration basin that separates treated water from the activated sludge.
- Bulking
- Poor sludge settling, often caused by filamentous organisms, leading to high SVI and solids carryover to the effluent.
- PE
- Population Equivalent — a unit expressing pollution load relative to one person; used to size plants and set directive thresholds.
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