| Activated sludge | Aerated wastewater with a suspended biomass. |
| Aerobic Condition | Where oxygen is present. |
| Ammonia | Should be called ammoniacal nitrogen. |
| BOD | Biochemical oxygen demand: the amount of dissolved oxygen consumed by microbial activity over usually a 5-day test at 20° C (BOD). |
| Biofilm | A film of biomass or microbial cells attached to a surface usually submerged. |
| BAF | Biological Aerated Filter that contains a submerged medium of a high surface area which is aerated. The excess biomass is cleaned or back washed by some method. |
| BF | Biological filter or trickling filter. This is usually a circular tank filled with media and a distribution system dispersing wastewater over the media. |
| Cesspool | An underground watertight tank without outflow used for collecting domestic wastewater. It is a watertight vessel that stores the raw sewage until collected by a tanker. |
| COD | Chemical Oxygen Demand is the amount of oxygen consumed by the chemical oxidation of the matter present in the wastewater sample. |
| Crust | Accumulated material that collects on top of the primary liquor and which is removed when the system is de-sludged. |
| Desludging | Method of removing by suction tanker the accumulated crust, sludge and deleterious material. Sludge removal reduces BOD loading. |
| DWF | Dry Weather Flow is the average daily flow received by the wastewater plant in a day to have originated directly from the premise connected to the plant. |
| EA/SEPA/EHS | Environmental Regulators (Environment Agency/Scottish Environment Protection Agency/ Environment and Heritage Service Northern Ireland). |
| Eutrophication | Enrichment of a watercourse or body by nutrients mainly phosphates and nitrates that usually leads to deterioration in the water quality, especially bacterial, algal and/or plant growth. |
| Full flow to treatment | The maximum flow a wastewater plant can treat and is usually expressed as a maximum flow for a set number of hours not repeated more than twice a day. |
| Humus tank | A settlement tank that follows secondary treatment. |
| Mixed liquor | The mixture of microbial solids and wastewater present in activated sludge aeration vessels. |
| Organic loading | The amount of BOD present in the wastewater expressed as grams or kilograms per day. |
| Per capita volume and loads | The wastewater volume in litres per person per day with the organic and ammonia load expressed in grams. |
| Population equivalent (pe) | The notional value is equivalent to a domestic resident usually used for comparisons or sizing a treatment plant which receives some non-domestic wastewater. |
| Reedbed | A gravel bed with reeds planted to provide some form of additional treatment. |
| RBC | Rotating biological contactor is a treatment plant that has discs that rotate in, but which are not completely submerged in, the settled effluent. The biological treatment takes place on the discs. |
| Septic tank | A compartmentalised settling tank that provides a limited amount of anaerobic digestion. |
| Sequencing batch reactor | A treatment process that undergoes a filling, aeration (biomass and wastewater treatment), settlement and discharge phases. |
| Submerged aerated filter (SAF) | A fixed film treatment process in which the media is submerged in the effluent and a mechanical device introduces oxygen. |
