There are two different but related issues: the problem of untreated storm discharges and nutrient phosphorus pollution by treated (final) effluent.
Standard waste-water treatment plants such as the one in Ilkley are designed to remove organic matter from the waste-water arriving at the plant before discharging the treated effluent to a water course.
Ashlands STW waste-water includes surface water from rainfall. Such “Combined Sewer systems” are a problem as the flow of waste-water into the plant, especially after heavy rainfall events, can quickly exceed its capacity. As a result, combined sewer overflows (CSOs) are placed on the network to relieve pressure in the system and prevent internal and external flooding.
When treatment capacity is exceeded, waste-water is pumped into storm-water tanks where the untreated water is held and released back for treatment once excessive inflow has subsided. Ashlands has four tanks designed for this purpose with a total capacity of 826 cu m.
If the storm tanks fill to capacity before the flow has subsided the waste-water overflows untreated directly into the river.
In very high flow conditions when the capacity for the incoming waste-water is exceeded, in Ashlands the inflow capacity is 246 litres/sec, waste water is released, untreated but under permit, directly into the Wharfe.
Following prolonged rainfall raw sewage can be discharged simultaneously both directly into the river (when the inflow rate is exceeded) and indirectly (once the storm tanks are full).
Some mitigation has been introduced at Ashlands since the summer of 2018. Screens and pumps, not required by the permit, have been upgraded. The new pump lifts additional flows into the storm tanks and the new screen will replace the current screen on the overflow to retain more debris.