In the UK, unlike many other European countries, dosing disinfectant into water supplies has traditionally been carried out economically using chlorine gas. There has been a change of heart recently among many water companies, brought about largely by health and safety considerations, with many sites now actively pursuing a move to liquid disinfection. However, the benefits to be gained by a switch to liquid dosing of disinfectant are not without a certain amount of compromise. Some utilities who were originally advocating a move completely to liquid are now reassessing the situation on a site by site basis.

Traditionally, gas dosing has involved controlling the introduction into the untreated water supply of quantities of between 0.8 and 2.5 milligrams per litre of chlorine.

The chlorine is supplied as a gas, which is delivered to and stored on site in a pressurised drum or cylinder. A number of commercially available chlorinators are currently marketed which dose precise amounts of chlorine into the mains supply.

The control of dosing is effective and routine, and safety measures are incorporated to ensure that, in the unlikely event that a leak occurs, it is dealt with in the appropriate manner. The fundamental difference between gas and liquid dosing has, until now, been in the method of delivery. Gas dosers, of the type developed by Portacel, utilise a vacuum created by the flow of mains water in the injector chamber to draw disinfectant into the water supply using the venturi effect - similar to the mixing of petrol vapour in a carburettor. Unlike traditional pumped dosing (used extensively for liquids), which forces disinfectant into the supply under pressure, the vacuum technique does not allow disinfectant to leak from the supply pipe if there is a rupture in the dosing system.

Portacel currently supplies many of the water companies in England and Wales with a range of dosing systems for chlorine, sulphur dioxide and ammonia gas.

Recent changes in customer preference for liquid dosing have prompted Portacel to develop an innovative, pumpless system which can handle liquids with the same fail safe principle as the company's existing gas dosers.

Changing from gas to liquid dosing presents a number of challenges to water companies and disinfection equipment suppliers alike. The method of dosing is only one of the aspects to be considered, as Jon McClean, general manager at Portacel explains:

"A number of equipment changes are required in order to deal with liquid dosing rather than gas dosing. There are mechanical considerations related to accurate dosing in small volumes as well as chemical changes that need to be carefully controlled."

The use of sodium hypochlorite has become popular as the alternative to chlorine gas. The liquid chemical can either be generated on site, at 0.8% concentration, using the ECOS (electro-chlorination on site) method or is available commercially at 10% or 15% concentrations. Initially, ECOS found favour in many areas of the country. However, transporting large quantities of salt (which is the raw material used to. generate the sodium hypochlorire), particularly to remote sites which have only a small requirement for disinfection, is uneconomic. Another issue raised by ECOS is the disposal of waste. . Chlorine gas arrives in a pressurised cylinder or drum, from which it is dispensed. The empty container is simply removed from site during the next delivery. Waste produced during the ECOS process has to be disposed of, which can be problematic.

Because of the shortcomings of ECOS, direct delivery of commercially available hypochlorite is now the approach favoured by many water companies.

Portacel has carried out extensive research into the most appropriate method of liquid dosing. The pumping unit is the critical part of a conventional, liquid dosing system. The range of turn down rates has to be wide enough to cope with a large spread of dosing volumes - particularly for the small quantities of sodium bisulphite used as the de-chlorinating agent.

A big limitation of pumped sodium hypochlorite is that oxygen generation causes gassing off, this leads to air locks in the pump. Portacel's work in this area has concentrated on providing a continuous, vacuum controlled, pumpless dosing system. Good turn down performance and the ability to handle gassing off are two advantages of the new Trent Liquid Doser, which was announced at Aquatech recently. The main benefit though is the absence of a dosing pump.

The liquid disinfectant is pulled rough into the water supplying the vacuum created in the injector. This results in a high degree of inherent safety as with a pumped system, a break in the supply would not necessarily affect the pump, which would continue to dispense over the floor of the water treatment plant. Another major benefit would appear to be the cost of maintenance. Whilst water companies can capitalise their initial plant investment, on-going maintenance and spare parts come out of operational budgets. Pumps are running all the time liquid is being dosed, which inevitably leads to component wear and the need for routine maintenance. Using pumpless technology, once the flow has been adjusted by a V notch valve, controlled by the Liquid Trents dedicated control unit, there are no moving parts since it is liquid filling the vacuum in the injector which facilitates dosing.

There is also an aspect of process chemistry which must be considered.

Liquid dosing in hard water areas tends to lead to a build. up of calcium carbonate. Not only does this block pipes, it plays havoc with the injectors in the disinfection plant. .Water softening needs to be employed in some water treatment works, so salt delivery rears its head yet again. The need for salt softeners can be overcome by introducing carbon dioxide immediately prior to the disinfection injector. This raises the pH sufficiently to prevent calcium carbonate forming in the injector. Once the water and disinfectant have mixed thoroughly - which takes fractions of a second in the injector - water hardening is no longer an issue.

"At one time, water companies seemed committed to liquid dosing exclusively,' explained Portacel's David Grant. 'The situation is now slightly different. Where gas dosing is viable, and the health and safety issues have been addressed, it can be less expensive and much easier to maintain. Where liquid is used, continuous, pumpless dosing provides major advantages over pumped systems and this appears to be the way the market is moving. The change to liquid dosing will bring with it developments in disinfection technology. Above all, reliability and accurate control of dosing rates are the critical factors for water companies to consider".