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Learning from turbidity failures

Turbidity is a measure of the cloudiness of water caused by suspended particles. At treatment works, turbidity is an important operational and regulatory control because it provides evidence of whether raw water has been adequately treated before disinfection and supply. The treatment works standard for turbidity is 1 NTU. In 2025, there were 12 failures of this standard from 171,515 tests undertaken at treatment works in England.

There were also five zonal turbidity failures, where the standard is 4 NTU. The case studies below illustrate the importance of effective process control, maintenance planning, timely investigation and appropriate operational response.

Anglian Water – Pitsford treatment works

Pitsford works is a surface water treatment works located in Northamptonshire. In February 2025, a regulatory final water compliance sample recorded turbidity of 1.48 NTU, exceeding the treatment works standard of 1 NTU. Further analysis of the same sample identified iron at 335 µg/L, exceeding the iron standard of 200 µg/L.

The company’s investigation concluded that the most likely cause of the turbidity and iron exceedances was a transient hydraulic disturbance within the Boughton pumping main. This was associated with the return to service of Boughton high lift pump No. 1 following planned maintenance. The final water compliance sampling point at Pitsford works is located on the Boughton pumping main, on the delivery side of the high lift pumps. The pump was flushed and returned to service at 14:13, shortly before the regulatory sample was collected at 14:24.

The company had not undertaken an adequate risk assessment for returning the pump to service. A short length of cast-iron pipework containing stagnant water upstream of the pump was not flushed before the pump was returned to supply. This allowed iron-rich stagnant water to be mobilised, resulting in the turbidity and iron exceedances following the maintenance activity. The Inspectorate recommended that the company update the site risk assessment and method statement for the Boughton pump set at Pitsford works to include appropriate flushing arrangements following pump maintenance, ensuring that stagnant water is removed before the asset is returned to service.

The company’s investigation did not include interstage turbidity sampling at Pitsford works, on the basis that the treatment process was operating normally at the time. Although several routine indicators supported this conclusion, the absence of interstage turbidity monitoring limited the evidence available to demonstrate conclusively that treatment performance had not been affected. Interstage sampling would have provided stronger assurance and supported the company’s investigatory obligations under regulation 18. The Inspectorate required the company to ensure that learning from this event was effectively disseminated to relevant operational, maintenance and remedial teams, to reduce the likelihood of similar failures occurring elsewhere across the supply system. The event demonstrates the importance of fully assessing water quality risks associated with planned maintenance, ensuring that return-to-service procedures are supported by clear flushing requirements and appropriate verification.

Anglian Water – West Pinchbeck treatment works

West Pinchbeck treatment works, located near Grantham, treats groundwater from five artesian borehole sources. On 11 January 2025, a final water sample collected at the works recorded a turbidity result of 1.10 NTU, exceeding the treatment works standard of 1 NTU. The sample was not analysed until two days after collection, meaning the company was not aware of the result until the late afternoon of 13 January.

Delays in sample analysis have been identified previously as a recurring issue for the company. The Inspectorate has already recommended that the company review analytical turnaround times to ensure that timely, appropriate and meaningful investigations can be undertaken. Delayed analysis can compromise the ability to establish the cause and extent of a failure promptly and may also limit the effectiveness of any immediate protective action.

This is not consistent with the expectations of regulation 18, which requires companies to take immediate steps to investigate and establish the cause of any potential risk to water quality, and to take appropriate action to protect public health.

This was the second turbidity exceedance at West Pinchbeck works within a 20-month period. The first, in August 2023, was attributed to elevated iron concentrations. During that event, the works remained in supply following a site shutdown due to artesian pressure from the boreholes.

The Inspectorate had considered enforcement action requiring the company to implement a process to divert excess flow to waste during site shutdown. However, within three days of the 2023 breach, the company modified the shutdown control philosophy by programming an additional run-to-waste function into the Programmable Logic Controller (PLC), intended to prevent forward flow beyond the filtration stage during shutdown conditions.

Those controls were designed to ensure that, in the event of a shutdown, artesian flows from the boreholes would be diverted to waste before reaching the contact tank, thereby protecting treated water quality. The company also identified a further 35 groundwater sites with artesian flow and undertook checks to confirm that similar forward flow would not occur at those sites during shutdown scenarios.

Following the January 2025 exceedance, the company’s investigation found that the turbidity failure at West Pinchbeck works was associated with elevated iron and manganese concentrations above the relevant standards. A faulty ultrasonic level sensor incorrectly indicated a satisfactory level in one of the balance tanks while the actual water level was low during drain-down. The low level disturbed sediment at the base of the balance tanks. Additional artesian flow through the works then caused the run-to-waste valve to close once the high-flow shutoff threshold had been exceeded, allowing forward flow to occur once the balance tank level recovered. This chain of events demonstrated that the PLC control changes introduced following the 2023 exceedance were not sufficiently robust.

In response to the 2025 event, the company implemented further PLC changes to prevent forward flow through the works when either the pre-contact turbidity monitor or final water turbidity monitor entered a state of alarm. Under these arrangements, borehole water is diverted to either the pre-filter or post-filter run-to-waste route during a turbidity shutdown. These changes are intended to prevent forward flow through the treatment process during relevant shutdown conditions; however, they also mean that a representative sample cannot be collected from the final water tap while the works is in this shutdown state.

The Inspectorate required the company to take several actions in response to the 2025 event. These included reviewing PLC controls at all other works affected by artesian flow to determine whether additional safeguards were required. The Inspectorate also made three key recommendations: first, to improve turbidity sample analysis timeframes to support compliance with regulation 18; second, to undertake CCTV surveys of all five supplying boreholes, which had last been surveyed between 10 and 30 years previously despite the company’s risk-assessed survey frequency of 10 years; and third, to carry out full internal inspections of all four contact and balance tanks on site, which had last been inspected between seven and nine years previously, exceeding the company’s risk-assessed maximum inspection interval of seven years for works tanks. These actions are necessary to support ongoing compliance with regulations 4 and 27.

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