The Problem
The addition of lime forms an important part of water treatment. Lime is used to adjust the ph of drinking water and is added at various stages of the process. The control of ph is important as the effectiveness of coagulants and polymers varies greatly depending on the ph of the water.
Lime mixing and lime dosing present particular problems, lime is particularly aggressive and can be dangerous if released into the work area especially in a dust form before it is hydrated into solution.
Two principle methods of lime dosing can be employed:
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Fixed rate dosing of variable strength lime solution.
The amount of powdered lime mixed into solution is controlled thus regulating the strength of the lime solution, once mixed in this manner the lime is dosed into the water stream at a fixed rate.
Changes in the ph of the water are adjusted for by changing the amount of lime powder introduced into the lime solution, the lime is mixed on a continuous basis in accordance with ph of the incoming water.
High-level corrections are made using stronger lime solutions and low-level corrections are made using weaker lime solutions.
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Variable rate dosing of fixed strength lime solutions.
The lime is mixed at a set strength in a bulk batch and is then dosed into the water stream at a flow rate which is controlled in relation to the ph of the water.
Introducing higher volumes of lime solution makes high-level corrections; low-level corrections are made introducing lower volumes of lime solution.
For the first method fixed speed fixed flow pumps are employed for the second method variable speed variable flow pumps are utilised.
Once in solution lime presents further difficulties to the pumps and transfer systems.
It has a tendency to coat or line the pumps and pipe work system, if pipe work velocities are not maintained the lime tends to drop out of suspension, settle in the pipe work resulting in blockages.
In the pump the coating process builds up layer on layer affecting the performance of the pump, ultimately resulting in failure that requires dismantling and cleaning of the pump. Traditional metering pumps that employ ball valves suffer particularly from this problem as a result of the ball valves and seats becoming choked with lime.
Seals are another area that are prone to failure again because of the coating problem jamming up the spring mechanisms as well as the seal faces suffering from abrasion from the particles of lime. In addition when the pump stops the lime solution tends to dry out on the seal faces and sticking the faces together.
The Solution
The Verderflex pump has proven an ideal solution to these problems due to its peristaltic design. Having no requirement for valve or seal systems two major problem areas are eliminated. Because of the squeezing action of the shoes on the hose there are no interference fit high friction areas as is the case with progressing cavity pumps as a result abrasion is not of a particular concern. The removal of such wear factor means that he pumping capability of the pump remains stable over the life of the unit and no adjustment or trims to the operating speed of the pump have to be made to compensate for wear.
The Water Research Council made an evaluation of best practice for handling lime solutions and used a Verderflex pump in their trails to assess the suitability of peristaltic pumps. Their trial found the peristaltic principle to be the most suited method of pumping lime solution.
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