How to pump ferric chloride safely, reliably and cost-effectively

Ferric chloride is gaining in popularity for the primary treatment of waste water. Compared with alternatives such as alum (aluminium sulphate) or polymers, ferric chloride is often more effective both as a coagulant and for the removal of odour-inducing sulphides. However, the problem with traditional pumping and dosing technologies, in particular diaphragm metering pumps, is that the pump size has to be closely matched to the current need – leaving little room for manoeuvre if the dose rate has to be increased significantly – and the diaphragms can fail without warning and contaminate the pump and surrounding area. Considering the highly aggressive nature of ferric chloride, this can be a major problem.

An alternative that has proved to be very successful is peristaltic dosing pumps. The pumps are capable of accepting tubing with many different bore sizes, and by varying the tube diameter and motor speed, users can achieve a flow range turndown of 875,000:1. In other words, installing a suitably sized pump from the outset will enable almost any conceivable dose rate to be catered for, whatever the future requirements of the waste water treatment plant.

One of the advantages of peristaltic pumps is that the only wetted part is the inside bore of the tube, so problems of erosion and corrosion of the pump by the pumped fluid are eliminated. Indeed, virtually no maintenance is required during the long lifetime of a peristaltic pump, other than occasionally replacing the tubing at very low cost and with minimal downtime. Moreover, the pump’s brushless DC motor will normally be specified to run continuously at approximately 50 per cent of its rated maximum, so the motor needs little attention and consumes far less energy than a comparably-sized diaphragm metering pump driven mechanically, hydraulically or by solenoids.

A further benefit of peristaltic pumps is that they give a smooth output; in comparison, diaphragm metering pumps often require pulsation dampeners to be installed, plus inline strainers, artificial backpressure valves and, quite probably, a concrete base.

While the initial purchase price of a peristaltic pump is similar to that of a mechanical diaphragm metering pump, the overall cost of installation tends to be significantly lower due to the need for less ancillary equipment, and the cost-of-ownership is minimal thanks to the low maintenance and operating costs.