Overdosing can become an expensive habit

Precise chemical dosing is an essential element of water treatment. Bran + Luebbe believes automated make-up plants are a way of making the grade.

Throughout the water treatment industry, chemical dosing plays an essential role in ensuring water is fit for use and wastewater can be discharged safely. Although not all the chemicals involved are expensive or hazardous, for them to be effective it is essential that there is precise dosing control. One approach to ensure effective use of water treatment chemicals is to employ automated make-up plant, whereby raw materials are mixed in specific concentrations on demand and dosed as required. Bran + Luebbe has now integrated its liquid dosing, powder handling and mixing technologies to develop a standard make-up and dosing package, the Tomal system, which can be used in many stages of water and waste treatment.

Southern Water has just installed a Tomal system for bentonite dosing at its Testwood water treatment works in Hampshire. Previously Southern Water had used poly-aluminium chloride (PAC) for assisting the flocculation stage when treating raw water, which is taken from the River Test.

Although effective in terms of helping to achieve good settlement in the flat-bed and circular clarifiers, together with assisting in the removal of pesticides, PAC can be quite expensive. By switching to a bentonite slurry and employing an automated make-up and dosing plant, the process has been made more cost-effective.

The bentonite plant consists of an 11m high powder storage silo, two screw conveyors, two make-up tanks, two dosing tanks, dosing pumps and pipework. The purpose of the plant is to produce and dose 5-bentonite slurry.

A feature of the design is the mounting of both the silo and make-up tank on load cells. Both dosing tanks have a greater capacity than the make-up tanks, when the level of slurry in the dosing tanks drops to around 1.5m3 the entire content of the make-up tank is pumped across to the dosing tanks and another batch make-up is activated. Operating at maximum capacity, each multi-screw twin outlet feeder is rated at 200kg/hr, whilst the transfer pumps will handle 10m3/hr and the dosing pumps between 160-320 litres/hr.

Scotland Water has installed a Tomal polyelectrolyte dosing system at its Shieldhall STW, which serves west Glasgow. It is the largest built in the UK by Bran + Luebbe, and can produce 12,000 litres of 0.5 solution per hour.

Whilst the purpose of each application influences the design of the Tomal set-up, the operating principle remains constant. The system takes the powder either from a bag or silo via gravity feed into the multi-screw feeder mechanism, from where it enters a dissolving cone or dissolving tank with a metered water supply. This arrangement creates a solution with a highly accurate and constant strength where it can be transferred to maturing or holding tanks and dosed as required. Tomal multi-screw feeders can meter powders to +/-1 at constant bulk density.

Aggressive liquids
Moving away from the make-up area of water treatment, it is necessary to employ dosing pumps, which are capable of handling potentially aggressive liquids. Dosing sodium hypochlorite in water treatment can be problematic, for not only is it potentially hazardous if not handled carefully, it is also highly corrosive.

Thames Water uses sodium hypochlorite at its Wilmington and Darenth WTW in the primary dosing stage. Experience with dosing pumps used for this application showed they were adversely affected by the corrosive liquid, creating the possibility of overdosing and frequent breakdown. Overdosing with sodium hypochlorite can often lead to an imbalance in the treatment process. Coupled with the problem of overdosing, pump breakdowns resulted in excessive downtime and considerable expenditure on maintenance. The pumps have now been replaced with Bran + Luebbe's ProCam dosing pumps, and Thames has now reported a major improvement in process efficiency.

The ProCam is described by Bran + Luebbe as an economical and reliable pump, capable of handling highly aggressive liquids. The pump head needs no maintenance and with double diaphragms made of poly-tetrafluroethene (PTFE), the ProCam offers accurate, leak-free operation.

Measured dose
ProCam was used by Thames to help deal with a similar situation arising at a water supply company in the USA. At the Westchester County STW in Blind Brook, New York, operatives had been left with no option but to overdose sodium hypochlorite in order to comply with national pollutant discharge elimination system (NPDES) standards. This was due to malfunctioning of the metering pumps, which had an unreliable stroke length.

By switching to ProCam dosing pumps, the overdosing was stopped and consumption of sodium hypochlorite halved. Although the ProCam seemed quite expensive, the return on investment in this case was less than two months. After 18 months in service, no downtime had been necessary.


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