Hodder WTW - Actiflo for Cryptosporidium removal


Courtesy of

United Utilities
83-week Programme
112,000 m3/day Max Flow

The Client
United Utilities are the 2nd largest water company in England / Wales. They are responsible for an area in the north west of the UK, which includes major cities such as Manchester and Liverpool.

The Client's Needs
United Utilities' Hodder Water Treatment Works, perched high above Slaidburn in the beautiful Forest of Bowland, supplies 120Ml/d of drinking water (enough for half a million people) to the surrounding area of Lancashire as far as Blackpool. The original works were built in the 1920s using a battery of pressure sand filters with in-line coagulation. In the original process, colour was removed from the water by coagulation, with the addition of aluminium sulphate ('alum') to produce a precipitate or 'floc' which clarifies the water in much the same way as finings are used to clarify beer and wine. The floc was removed by sand filtration but, when large amounts of floc were produced, the filters were susceptible to blocking.

A second stage of filtration was added under AMP 2 for manganese removal but subsequent changes in water quality standards meant that more modifications were necessary under AMP 3 to meet the new consent standards.

The existing potable water treatment plant provides water to the Hodder Aqueduct across a broad flow range of 35 Ml/d to 105 Ml/d. There is an impounding raw water reservoir, which is used by the general public for fishing, making this a high profile project. The reservoir, just like any natural lake, is susceptible to contamination by Cryptosporidium. Coagulation is an effective barrier to Cryptosporidium but, in order to ensure compliance with the latest water quality standards, it was necessary to increase the alum dose at Hodder. The increase in floc production meant that the filters were in danger of becoming overloaded and United Utilities needed to add a clarification stage to the works prior to filtration.

The works site was already congested and planning permission in an area of outstanding natural beauty, like the Forest of Bowland, was not easy to obtain, so the clarification process had to be compact and efficient.

VWS Solution
United Utilities awarded the construction contract to VWS-Birse JV. Veolia Water Solutions & Technologies provided the process design, construction, installation, testing and commissioning of three (3) ActifloTM High Rate Clarifiers and three (3) MultifloTM for backwash water recovery. The project included the sand storage, make up, transfer and recovery system, polymer make up and dosing system, MCC and software for the process.

All three ActifloTM streams are nominally in operation at the same time but two can handle the full works flow, allowing the third to be taken out of service for maintenance.

ActifloTM is a very compact process that utilises microsand as a seed for floc formation. The microsand provides surface area that enhances flocculation and also acts as a ballast or weight.

The resulting microsand ballasted floc display unique settling characteristics, which allow for clarifier designs with high overflow rates and short retention times. These designs result in system footprints that are between 5 times smaller than classic lamella clarifier or dissolved air flottation and up to 20 times smaller than conventional clarification systems of similar capacity, with an ability to start up from a 'standby' mode of operation to a steady state operating mode within minutes.

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