Maximum uptime realised through in-line turbidity at Franklaw WTW

Dr Patsy Rigby, Hach Lange UK discusses a water treatment application at a Preston in North West England

A key United Utilities water treatment facility has significantly reduced the risk of failure to comply with the regulatory requirements of the Badenoch Bouchier report and reduced the number of maintenance hours spent by installation of a continuous in-line turbidity probe. As an additional benefit advanced warning of de-sludging issues can be immediately identified enabling the plant operations to rectify the situation in minimal time.

United Utilities Group PLC forms the largest listed water business in Britain, controlling through its subsidiary United Utilities Water (UUW), the licence to provide water and wastewater services to 7 million customers in the North West region. Between the period 2010-2015, UUW is investing £300m improving treatment at 45 of their facilities promising the development of innovative operational solutions to optimise water treatment efficiency beyond the scope of existing processes.

Franklaw WTW based just outside Preston is a key treatment plant in the North West catchment area drawing raw water from various sources including the rivers Wyre and Lune, Fylde borehole, Barnacre reservoir and Thirlmere aqueduct. As a key facility, the plant supplies treated water to a population of 650,000 within Blackpool, Preston and the surrounding districts, providing up to 220 million litres of water per day (equating to a staggering 93,000 glasses of water per second). The years 2001-2004 saw £35 million investment by United Utilities to upgrade and develop the washwater treatment processing at the plant providing lamella based clarifiers for efficient washwater treatment.

North Lancashire Water Technical Officer James Bowsher (see photograph) explains the current treatment process at Franklaw WTW: “Treatment begins with water drawn from 4 inlet sources which are mixed and course-screened to remove large debris. The coloured raw water is dosed and mixed with Aluminium Sulphate and Polyelectrolyte. The coagulated waters then enter the upward flow super-pulsator units which hold the floc within baffle plates while the cleaner water flows upwards and on to the next processing stage. From here, clarified water enters one of 12 rapid gravity filters to trap any remaining particulates. Periodically these filters are automatically backwashed with air and then water to prevent blockage, and the backwash water passed to dirty backwash water tanks for blending and balancing. These waters then pass through a series of lamella clarifiers for further treatment.

Franklaw WTW Site Manager Michael Tillery and North Lancashire Water Technical Officer James Bowsher

“The lamella plate settlers maximise settling (owing to significantly enhanced surface area of multiple stacked angled plates compared to the surface area of a flat horizontal surface) while demanding only 10% of the space taken up by traditional settling units. Deposited sludge from the plates is drawn off and dewatered via sludge press units into 22% solids cake further downstream and ultimately sent out for landfill. The treated supernatant water from the lamella units is returned to the inlet tower for re-treatment. A maximum 10% proportion of the total plant inlet volume may be recycled by return to the inlet of the treatment works. However, regulation demands that recycled backwash water must contain a turbidity of ≤10NTU in accordance with the Badenoch and Bouchier reports.”

Continuous recycling of a small percentage of the total works throughput has dual benefits: 1. there is no need to discharge excess volumes to the water course – eliminating the imposition of discharge consent limits by the Environment Agency and 2. internal recycling reduces the water draw from local resources with river and water stocks sustainably preserved.

Backwash water heavily loaded with sticky organic residues (which deposit over the lamella plates during settlement) must be carefully monitored to ensure that turbidity complies with the 10NTU limit imposed by the Badenoch and Bouchier reports. As James Bowsher continues “In the past we used off-line turbidity analysers with the sample pump fed from the supernatant holding tank post the 6 lamella clarifiers. This holding tank combines a strategic sampling point with an inbuilt overflow unit to carry excess volumes back to the lamella for recycling.

With previous systems, water from the tank was pumped into a separately housed kiosk to the inlet of the turbidity analyser and this gave a delayed turbidity reading. Off-line instruments give good accuracy by enabling a controlled sample environment at optimum temperatures, with bubble traps to reduce interference from air pockets which can scatter the light path and cause false readings. However the nature of the process water in the lamella application was found to cause organic deposits to build up in the sample lines causing serious blockage in the joints and tubing of the flow path. As a result, turbidity readings were found to frequently flat-line.

“To strip the system down, hose all joints and tubing, then re-calibrate the system before a final flush was demanding several hours of maintenance every couple of days. The demand on operator time and the risk of sending high turbidity water back to the inlet with non-continuous measurement far outweighed the reliability of results. Without regular assessment by the operational team the flat-lining could theoretically be un-noticed for several days.”

United Utilities challenged Hach Lange to provide a reliable plug and play, low maintenance in-situ system to avoid the issues arising with off-line systems. A self-cleaning Solitax sc turbidity probe was recommended, mounted on a sliding pole for simple probe removal and maintenance. Based on a dual infrared light scattering technique reliable, real-time monitoring in very low to highly turbid (up to 15% solids) and coloured water samples is afforded. With an automatic cleaning unit, drift free operation is assured despite continuous operation in the wastewater stream and with maintenance limited to an annual change of the wiper blade maintenance is almost entirely eliminated. The system is factory calibrated and requires just a simple factor against a single laboratory result before use eliminating the need for the commonplace dilution based calibration of contemporary systems.

The Solitax sc probe was pole-mounted approximately 1m below the water surface in the combined holding tank of the backwash water (alongside the overflow pipe). The system operates such that if the turbidity approaches the user-set threshold a high range alarm is triggered to stop the supernatant pump returning back to the inlet and diverting the water back to the lamella for re-treatment. Should the water level drop below the height of the probe, an alarm is raised by localised level sensors and the subsequent rise in turbidity from exposure to excess light provides a secondary alarm of high turbidity providing a dual safety approach.

Initial trial data was verified alongside a portable turbidity analyser and proved that when the lamella were overloaded, causing excessive valve head pressure (which increases the sludge retention and reduces the transfer of water) the set alarms promoted immediate intervention, saving many hours of maintenance and re-treatment costs.

The installation in the combined lamellae outlet

Site Manager Michael Tillery describes the immediate benefits of the system: “The in-line Solitax probe gives instant, accurate turbidity results and much quicker than we could achieve with pump fed off-line systems. The particular advantage in the sampling set up is that it is virtually maintenance free, with automatic self-cleaning of the optical surface every 12-24 hours eliminating the maintenance we previously faced to hose down and remove organic deposits. A precautious wiping frequency keeps the turbidity reading cycling below the target threshold which is much better than letting the value creep towards the consent limit.

“With strategic location of the Solitax on the combined outlet of the lamella clarifiers we now get an instant alarm should levels rise ensuring we are always within compliance and with no pump stoppage for maintenance. With operators freed up to undertake other critical jobs around the site it has greatly improved the plant efficiency. On this basis, the system has not only been purchased by this site but is being rolled out across other sites.”

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