Below is our A-Z list of all the key terms you might need to know when considering the installation or maintenance of your septic tank or sewage treatment plant.

Activated sludge: Aerated wastewater with a suspended biomass which breaks down much of the substances in sewage and wastewater.

Aerobic: Condition where oxygen is present.

Ammonia: A nitrogen containing compound, strictly should be called ammoniacal nitrogen.

BOD-Biochemical oxygen demand: the amount of dissolved oxygen consumed by microbial activity over  5-day test at 20° C (BOD5)

Biofilm: A film of biomass of microbial cells attached to a surface usually submerged.

BAF: Biological Aerated Filter that contains a submerged medium of high surface area on which biomass can accumulate and which is aerated. The excess biomass is cleaned off (or back-washed) and settles to from sludge.

BF-Biological Filter: or trickling filter which 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: The removal of the accumulated crust, sludge and other solid material by suction tanker. NB Sludge removal reduces BOD loading.

DWF: Dry weather Flow is the average daily volumetric flow received by the wastewater plant in a day which has originated directly from the premise connected to the plant.

EA/SEPA/NIEA: Environmental Regulations – Environment Agency (for England and Wales), Scottish Environment Protection Agency, Northern Ireland Environment Agency Department of Environment.

Eutrophication: This is the enrichment of a water course or body by nutrients mainly phosphates and nitrates that usually leads to deterioration in the water quality by excessive growth of plants, algae and bacteria.

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: As settlement tank that follows secondary treatment.

Mixed Liquor: The mixtures of microbial solids and wastewater present in activated sludge aeration vessel.

Organic Loading: The amounts of BOD present in the wastewater expressed as grams of kilograms per day.

Package sewage treatment plant: A self-contained sewage treatment plant that is manufactured in a factory and transported to site for installation. It can be more than one module.

Per captia volume and loads: The wastewater volume in litres per person per day with the organic (BOD) and ammonia (N-NH4) load expressed in grams.

Population equivalent (pe): The notional value of wastewater equivalent to a domestic resident usually used for sizing a treatment plant which receives some non- domestic wastewater is given the equivalent ‘’pw’’ value.

Reed bed: A gravel bed with reeds planted to provide some additional treatment.

RBC: Rotating biological contactor is a treatment process that has a group of discs that rotate in the settled effluent to be treated; they may not be completely submerged. The biological treatment takes place in the discs.

Septic tank: A compartmentalised settling tank that provides a limited amount of anaerobic digestion.

Sequencing batch reactor (SBR): A treatment vessel for a process that undergoes the sequence of filling, aeration (bio-mass growth and wastewater treatment), settlement and discharge, a sequence which is repeated successively.

Submerged aerated filter (SAF): A fixed film treatment process in which the solids media is submerged in the effluent and on which the biofilm grows, a mechanical device introduces oxygen.

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