With thanks to Water UK, Yorkshire Water and Lenntech BV.

Or click here to view all terms ordered from A-Z.


Sand filtration

Sand filtration is a frequently used and very robust method to remove suspended solids from water. The filtration medium consists of a multiple layer of sand with a variety in size and specific gravity. Sand filters can be supplied in different sizes and materials both hand operated and fully automatically.

Saturated zone

The area below the water table where all open spaces are filled with water.


The precipitate that forms on surfaces in contact with water as the result of a physical or chemical change.


Use of screens to remove coarse floating and suspended solids from sewage.


The gross solids, including rags and plastics, in sewage are intercepted by screens and removed manually or by raking mechanisms.


Strategic Direction Statement, produced by all water companies in the UK for Ofwat, stakeholders and customers, setting out a long term framework for delivering a service to consumers and the environment over the next 25 years.

Secondary treatment

The removal or reduction of contaminants and BOD of effluent from primary wastewater treatment.

Secondary wastewater treatment

Treatment involving the biological process of reducing suspended, colloidal, and dissolved organic matter in effluent from primary treatment systems . Activated sludge and trickling filters are two of the most common means of secondary treatment and is accomplished by bringing together waste, bacteria, and oxygen. This treatment removes floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids.

Security of supply

The extent to which water supplies to customers are reliable and not subject to possible restrictions because of a lack of rainfall over a period of time.

Security of Supply Index (SoSI)

Assesses each appointed water company’s ability to supply customers in dry years without imposing demand restrictions such as hosepipe bans. Companies with higher index score bands have better security of supply.


The process by which settleable solids are removed from sewage by passing it through a tank at a rate that allows the solids to gravitate to the floor to form sludge.


Soil, sand and minerals washed from land into water, usually after rain.


Security and Emergency Measures Directive.

Semi-confined aquifer

An aquifer partially confined by soil layers of low permeability through which recharge and discharge can still occur.

Sensitive areas

A water body or area where more than secondary treatment is required for wastewaters discharged under EC directive 91/271/EEC.

Septic tank

A type of sedimentation tank in which the sludge is retained sufficiently long for the organic content to undergo anaerobic digestion. When sludge is eventually removed to a sewage treatment works, some is left in the tank to act as a 'seed' to initiate further digestion. Used for receiving the sewage from houses and other premises which are too isolated for connection to a foul sewer.

Service reservoir

A reservoir where treated water is stored.


The capability of a system of assets to deliver a reference (ie, expected) level of service to consumers and to the environment now and into the future.

Settleable solids

Those suspended solids in wastewater that will settle over a certain period of time and are removed in that way.


Waste fluid in a sewer system.

Sewage (or wastewater)

Water-borne wastes from domestic uses of water, derived from households or similar uses in trade and industry. Storm sewage is that flowing to a treatment works in wet weather or discharged from storm overflows, when the sewage is diluted to a greater or lesser extent with rainwater.

Sewage sludge (also known as 'biosolids' or 'sludge')

A by-product arising from the treatment of sewage or from septic tanks or similar installations. (See Article II of EC directive 86/278/EEC for full definition.)

Sewage treatment plant

A facility designed to receive the wastewater from domestic sources and to remove materials that damage water quality and threaten public health and safety when discharged into receiving streams or bodies of water. Most facilities employ a combination of mechanical removal steps and bacterial decomposition to achieve the desired results.


A pipe conveying wastewater or sewage discharged into it from two or more house drains.


A system of pipes and mechanical appliances for the collection and transportation of domestic and industrial wastewaters.


A semi-solid residue, containing microoroganisms and their products, from any water treatment process.

Sludge cake

Sludge that has been dewatered to the extent that it can be handled as a solid, usually containing more than 15 per cent dry solids, depending on the type of sludge and method of dewatering.

Soft water

Water which forms an immediate lather with soap and has a total hardness which is typically less than 60 milligrams per litre (as CaC03).


The removal of calcium and magnesium from water to reduce hardness.

Source station

A pumping station and/or treatment works which supplies treated drinking water from a specific water supply source.


Sewage Pumping Station.


Service Reservoir.


Short Sea Outfall.


Sites of Special Scientific Interest.

Statutory water company

A company authorised immediately before the passing of the Water Act 1973 by any local statutory provision to supply water, or a company in whom the assets of any company so authorised have subsequently become vested.

Statutory Water Quality Objective (SWQO)

Water quality objective set down in legislation.


Sludge Treatment Facility.

Storage reservoir

A reservoir for storing untreated water.

Storm sewage overflow

A device to prevent overloading of the sewerage system during storm events.

Storm sewage tank

A tank into which, in wet weather, is diverted all the sewage and rainwater reaching a treatment works in excess of that which is to receive biological treatment. Its purpose is to store as much of the storm sewage as possible, for return to the works inlet after the flow has returned to normal, and to remove settleable solids from the remainder which overflows from the tank to a receiving water.


Bacterial indicator of faecal contamination of water.


Sewage Treatment Works.


Sustainable Urban Drainage Systems.

Supply pipe

That portion of pipe which conveys water from the main to the customer's house and which is on the customer's property.

Supply/demand balance

The balance between the amount of an appointed water company’s available water resources and the demand for water by customers. Any imbalance between supply and demand can be met through resource enhancement or demand management strategies.

Surface water

The run-off from paved and unpaved roads, buildings and land.

Suspended solids

Solid organic or inorganic particles that are held in suspension in a solution.

Sustainable Urban Drainage Systems (SUDS)

SUDS offer an alternative approach to drainage in developed areas, that keeps water on site longer, prevents pollution and allows storage and use of the water. SUDS include one or more of the following: inlet structures, outlet structures, swales, constructed wetlands, ponds, filter trenches, attenuation tanks and detention basins (together with any associated pipes and equipment).


Shellfish Water Directive.


Surface Water Management Plan.

Back to top