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 Thames Water Cat 1-2 Pollution incidents 2011/12

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David Harvey

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PostSubject: Thames Water Cat 1-2 Pollution incidents 2011/12    Wed Jul 11, 2012 12:58 am

We asked Thames Water for further details on the category 1-2 pollution incidents as shown in their annual report. They are slightly vague about details as investigations/ prosecutions are pending but have offered to have an 'off the record' meeting which we will set up.

What we have got is below, the 'Other Causes' are the contentious ones and will need to be proved if TW were at fault.


Pollution Incidents 2011

Thames Water regrets all pollution incidents and are working towards eliminating such incidents.

The Environment Agency categorised 20 pollution incidents in 2011 as significant. All resulted in discharges of wastewater to watercourses.

• Four were consented storm discharges to river.
• Three were due to third party blockages.
• Seven were due to fat, oil and grease blockages.
• Six were due to other causes in the network or on process sites.

Consented storm discharges

Storm discharges occur when the sewer network has reached capacity. The sewer network was designed to operate in this way so the Environment Agency categorise these incidents as consented discharges. The Thames Tunnel will reduce storm water discharges to the tidal River Thames from central London Combined Sewer Overflows (CSOs) by increasing capacity in the sewer network.

During storm discharges wastewater is diluted which minimises the impact on river water quality. However large volumes of wastewater discharge can cause deterioration in river water quality. In these circumstances Thames Water have the capability, in conjunction with the Environment Agency, to use aeration vessels and hydrogen peroxide dosing to raise dissolved oxygen levels in rivers.

Tidal River Thames, London

o In June heavy rainfall resulted in multiple consented storm discharges to the tidal River Thames from Mogden Sewage Treatment Works and the central London CSOs. Many thousands of fish were killed in the River Thames due to a delay in the use of the aeration vessel and hydrogen peroxide dosing.

o In August heavy rainfall resulted in a consented storm discharge from the central London CSOs that significantly affected river water quality in the tidal River Thames.

o In September heavy rainfall resulted in consented storm discharges from Mogden STW and the central London CSOs that significantly affected river water quality in the tidal River Thames.

River Lee, London

o In May heavy rainfall resulted in a consented storm discharge from Deephams Sewage Treatment Works in London significantly affected river water quality in the River Lee.

Third party blockages

Hatfield Hyde Brook, Welwyn Garden City

In March wastewater entered a fen in The Commons Local Nature Reserve and our Ecology and Heritage Team worked closely with the park warden to ensure damage to habitat and amenity value was minimised, this included the introduction of new breeding sites for newts and a new footpath. Whilst some wastewater did enter the Hatfield Hyde Brook the impact was localised.

River Beane, Stevenage

In May wastewater entered the River Beane in Stevenage, this reduced water quality in the river but there were no signs of distress in the fish population.

River Rom, Romford

In June wastewater entered the River Rom in a Site of Importance for Nature Conservation. Heavy rainfall following the incident rapidly raised dissolved oxygen levels and improved water quality.

Fat, Oil and Grease blockages

Thames Water continues to raise awareness of the problems caused by Fat, Oil and Grease with customers and businesses.

Bishopswood Stream, Tadley

In March wastewater entered a pond and the Bishopswood Stream via a private sewer. In order to protect the Bishopswood Stream Thames Water aerated the pond. The main downstream impact was to the amenity value of the Bishopswood Golf Course.

Mimmshall Brook, South Mimms

In April wastewater flowed over arable land into the Mimmshall Brook. Some fish were seen in distress in the brook but dissolved oxygen levels recovered sufficiently to prevent further impact to the fish population.

Chalvey Brook, Slough

In June wastewater entered a balancing pond and the Chalvey Brook. The death of approximately 80 fish had occurred by the time Thames Water arrived on site, in order to prevent further loss of fish life Thames Water aerated the balancing pond.

Kenton, Harrow

In July wastewater entered the Wealdstone Brook. Thames Water remains committed to improving the overall water quality of the Wealdstone Brook through multiple projects that aim to reduce the number of illegal misconnections to the surface sewers in the catchment.

Alders Brook, London

In July wastewater entered the Alders Brook, the wastewater was diluted as it entered the downstream River Roding.

River Brent, Ealing

In July wastewater entered the River Brent. The main downstream impact was to the amenity value of a park. Thames Water remains committed to improving the overall water quality of the River Brent through multiple projects that aim to reduce the number of illegal misconnections to the surface sewers in the catchment.

Turkey Brook, Enfield

In September wastewater entered the Turkey Brook. This incident resulted in the death of approximately 100 fish, Thames Water cleaned the river to prevent further loss of life.

Other Causes

River Lee, London

In May a consent compliant discharge of wastewater occurred from Deephams Sewage Treatment Works resulting in elevated ammonia levels in the River Lee. Thames Water are working closely with the Environment Agency to improve final effluent quality at Deephams Sewage Treatment Works.

Stanway Green Balancing Pond, Bourton-on-the-Water

In June wastewater percolated through a field from a rising main to a ditch and a balancing pond. The balancing pond was aerated and isolated to protect the downstream watercourse. This incident occurred in an Area of Outstanding Natural Beauty and Thames Water’s Ecology and Heritage Team worked closely with Natural England to ensure there was minimal impact to habitat impact during the repair of the sewer.

Un-named ditch, River Thames tributary, Farringdon, Oxfordshire

In August sewage from a rising main entered a ditch and subsequently a watercourse resulting in the death of hundreds of fish. Unfortunately a third party had damned a section of the watercourse which blocked the migratory path of the fish from the sewage to cleaner downstream water. Due to the severity of the impact Thames Water undertook an extensive environmental assessment; the results showed the incident did not impact the River Thames.

River Crane and Duke of Northumberland River, Middlesex

In October a penstock in a trunk sewer failed in the closed position. This caused a discharge of a large volume of wastewater to the River Crane and regrettably resulted in the death of thousands of fish. Extensive environmental assessments were undertaken to identify areas for remediation. Thames Water continue to work closely with the Environment Agency, local councils, London Green Corridor, Thames Angling Conservancy, FORCE (Friends of the River Crane environment) and London Wildlife Trust with the aim of sustainability restoring the River Crane.

Engine River, Walton

In November a burst rising main caused wastewater to flow over arable land and enter the Engine River. This resulted in a fish kill, the river was aerated to prevent further loss of fish life. Thames Water worked closely with the farmer to restore the arable land.

Danson Park Ponds, Bexley

In November wastewater entered the Danson Park ponds and caused the death of approximately three thousand fish. Thames Water recognise the significance of this incident and is undertaking a full environmental assessment of the ponds.


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