Chemical Solution Feeders

A wide variety of chemical solution feeders may be used to introduce solutions into the household water supply for treatment purposes. Chlorine or permanganate for the oxidation of iron and manganese; alkalis for the neutralization of acid water; and polyphosphates for the control of soluble iron can all be used with these feeders. The following material is aimed at chemical feed pumps, but many of the same principles will apply to eductor and suction devices, differing only in the installation and method of operation.

Chemical feed pumps are usually installed in connection with private well systems. Coordination of the feed pump with the good pump provides a most satisfactory method of proportioning a solution into the waterline. When the motor of one of these pumps is wired to operate with the good pump, the solution is fed at a quite consistent ratio to the water drawn.

Solutions are normally introduced into the water line between the pump and the pressure tank. As most of these pumps use a reciprocating piston-like action, solutions are not fed continuously, but in intermittent shots as the pump cycles.

The pressure tank can serve as an excellent mixing unit to even out the intermittent feed provided by the pulsating action of the pump. For good results, all the water must pass through the tank. Also, the inlet and the outlet must be placed so as to avoid a direct flow of water through the unit if possible. To do this, connect the outlet and inlet at opposite sides of the tank and at different levels. It is advisable to install a drain valve in the tank, separate from the inlet and outlet, to permit periodic flushing of the tank for removal of sludge.

It is highly important for all of the water to pass through the pressure tank as shown above. The installation shown below is not acceptable because the pressure tank is on a blind leg. Here water enters and leaves the pressure tank through the same pipe. Mixing is poor. And, if taps are opened with the pump in operation, it is probable that some untreated water will flow into the household water lines.

Recent tests show that properly installed pressure tanks produce mixing which approaches theoretically perfect curves. However, these tests also show that it is not wise to depend upon a pressure tank to provide much "contact time" if this is necessary for a chemical reaction to take place.

However, where contact time is to be provided, two simple auxiliary devices may be used. One is a simple coil of plastic pipe. The other is an ordinary tank loaded with coarse gravel. To calculate the contact time provided, divide 38% of the actual water capacity of the gravel-filled tank, or 75% of the volume of the pipe, in gallons, by the maximum flow rate of the system in gallons per minute. The results will give the minimum contact time provided by these devices in minutes. The choice in each instance will depend upon the local costs for these two devices.

In many cases where two chemicals are fed into the water, they can be mixed and fed from a single container. When chemicals are not compatible and might produce an undesirable reaction in the solution tank, a feed pump with two separate pumping heads and two solution containers are necessary. An alternate method is the use of two separate pumps.

Pumps with two heads are also used to increase capacity beyond that possible with a single head unit. Two pumps or two heads can be used to pump double the volume of solution from a single tank.

Because of the versatility of chemical feed pumps, they have applications in a wide variety of water conditioning problems. Their disadvantages are initial cost; the necessity for periodic preparation of solutions; and, the initial trial and error period commonly needed to establish the proper feed rate, as feeds cannot always be calculated precisely.