Waste Water Treatment and Industrial Outfalls
Outfalls to the marine environment fall into three categories. Outfalls from waste water treatment (WWT) plants which process waste water from the mains sewerage system; industrial sites which take waste from manufacturing plants; and some private domestic sources. These categories of outfall also discharge to the riverine freshwater environment which eventually reach the sea.
All sewerage systems that also collect rainwater (combined sewers) need overflow outlets (combined sewer overflows) that deal with the extra water collected during some rainstorms. Without these safety valves there would be a flooding risk at both the sewerage works and in other properties. Sewage in these overflow discharges is diluted with significant amounts of rainwater.
There are four types of treatment undertaken at waste water treatment plants:
Preliminary treatment is the first stage of the process and involves the removal of coarse solids and large materials.
Primary treatment is a physical and/or chemical process involving settlement of suspended solids.
Secondary treatment is a process generally involving biological treatment either with secondary settlement or another process. It is designed to reduce the suspended solids and oxygen demand. It also reduces the bacterial content of sewage.
Tertiary treatment is the final stage of the process and can include filtration, removal of nutrients such as phosphorous and nitrogen, and disinfection, for example by UV light.
The Urban Waste Water Treatment Directive 1991 sets out treatment levels for waste water based on the population equivalent of the plant catchment and the sensitivity of the water receiving the discharges.
WWT plants and outfalls are found all around the coastline near villages, towns and cities. Those with the larger capacity and higher treatment levels being concentrated in the major firths, near large cities or near sensitive receiving waters.
Scottish Water provides sewerage services to the majority of households in Scotland, although some households have private discharges. Data on private discharges such as septic tanks are not included in the spatial data.
Industrial effluent discharges are licensed if the volume of effluent is above certain thresholds and are subject to control placed on the amount and nature of the discharge.
