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Ferric chloride is gaining in popularity for the primary treatment of waste water, mirroring what has been occurring in the USA for many years.
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.
Flow range turndown of 875,000:1
An alternative that has proved to be very successful in the USA at plants such as Las Vegas, Nevada, and Blue Plains and Washington DC and which offers exactly the same benefits in the UK and Europe, is peristaltic dosing pumps.
The pumps can accept 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.
No maintenance, no downtime
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% 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.
Low cost of ownership
A further benefit of peristaltic pumps is that they give a smooth output; in comparison, diaphragm metering pumps often require pulsation dampers to be installed, plus inline strainers, artificial back-pressure 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.
Wide range of control options
Watson-Marlow Bredel has a wide range of peristaltic pumps and tubing suitable for dosing ferric chloride. Depending on the application requirements, customers can either specify a stand-alone pump with integral controller and readout, or pumps that can be connected to a SCADA (supervisory control and data acquisition) system.
Even when operated manually, a pumping accuracy of 0.5 per cent is easily achieved.
When integrated within an automated water treatment plant, input signals are usually 4-20mA, and there are 4-20mA outputs for signalling to a PLC and onwards to the SCADA system.
Other options include 24V signalling, TTL signals, remote start/stop signals and automatic/manual control switching.
Also, a second 4-20mA input can be available to provide scaling of the main speed input. Full status monitoring can be achieved through four user-configured 24V relay outputs.
Sophisticated chemical metering pump
Watson-Marlow Bredel’s recently introduced 520 series, for example, incorporates a four-line LCD display that indicates data such as flow rate, motor speed and direction of rotation.
A polyester powder-coated pump housing resists attack from most chemicals, including ferric chloride.
This pump is believed to be the most sophisticated chemical metering pump available. It comes with a three-year performance warranty.
Watson-Marlow Bredel’s Marprene tubing is usually specified due to its chemical resistance and mechanical resilience.
Excellent flocculant
Ferric chloride (also known as iron chloride hexahydrate, ferric trichloride hexahydrate, ferric trichloride and iron chloride) is extremely aggressive, readily attacking most metals.
However, it is an excellent flocculent. When added to water, it hydrolyses into electropositive ferric hydroxide, attracting negatively charged colloidal substances and thereby forming flocs that clump together and precipitate out.
Through this mechanism, ferric chloride reduces BOD (biological oxygen demand) as well as removing heavy metals, insect eggs, pathogenic content, suspended solids and colloidal content. It also precipitates phosphates, which is a major advantage in the fight to avoid eutrophication of rivers and lakes.
Many users find that ferric chloride provides better performance than alum, with a floc that forms more quickly, is denser, settles more quickly and is not as easily broken up. Ferric chloride is widely viewed as the most cost-effective flocculating agent.
For more information on Watson-Marlow Bredel peristaltic pumps or to discuss ferric chloride dosing applications, please contact Heather Beale on 01326 370370 or visit www.watson-marlow.com.
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