When AQUAMOVE set up its mobile water treatment plant facility in the UK, it was aimed at providing temporary solutions for plant breakdowns, planned maintenance and commissioning. It was ultimately for temporary requirements, and that is still the main application, but over recent years a new trend has been developing: long term plant rentals.
Initially the mobile fleet was equipped with just reverse osmosis and ion exchange demineralisation to cover mechanical breakdown of on-site systems, and these are still the two most widely used packages. Deionisation trailers typically provide two streams of cation, anion and mixed bed ion exchange which are capable of producing 100 m3/h of 0.1 µS/cm high purity water.
When the ion exchange resins are exhausted, the complete trailer is returned to the central regeneration facility, which means zero discharge on site and no problems of handling or disposal of regenerant chemicals. Since the plant footprint is bigger than ion exchange, containerised reverse osmosis units are generally capable of producing up to 25 m3/h of permeate of conductivity typically less than 2 0µS/cm in a single pass. If higher quality treated water is needed, two units can be configured in series as'twin pass' giving permeate of less than 5 µS/cm.
No standby capacity
Financial constraints often mean that there is no standby capacity in the on-site water treatment plant, so when a vessel internal fails or there's a fire in the plant room, production is at risk. When an emergency like this does occur the simplest solution is to hire a trailer mounted mobile water treatment system while repairs are carried out. But the introduction of common carriage and extended water supply networks has brought a different kind of problem. Mains water, unlike gas, oil and electricity, is not always the same: seasonal algal growth on a reservoir can increase the organic content causing rapid fouling of ion exchange resins and reverse osmosis membranes.
Another possibility is that the water supplier may have to change the source of the water supply from river to borehole or vice versa, with a sudden and dramatic change in total dissolved solids. The water still meets the requirements for drinking water but the effect on an industrial water treatment plant can be devastating: Ion exchange plants designed for thin, upland waters may have insufficient capacity to treat thicker borehole waters whilst those designed for hard, borehole waters may suffer from fouling by higher concentrations of natural organic matter in supplies derived from moorland reservoirs.
To meet this demand, the mobile fleet was expanded to include a range of pre-treatment processes including coagulation and flocculation, clarification, filtration, activated carbon adsorption and softening all in modular configurations to provide the right process route to treat any raw water quality to achieve any treated water quality and quantity required.
While many of the applications for mobile units are emergencies not all of them are about the nasty things in life. An unexpectedly full order book often creates an urgent need to expand production quickly, and a mobile plant can provide the additional treated water you need while you're waiting for a permanent plant to be installed. Sometimes companies need to carry out production trials, and may want to look at the effects of water quality on their production processes.
Temporary deionisation plants provide a cost effective means of finding out if it is worthwhile making substantial capital investment in permanent plant. And, of course, many mobile customers hire temporary plants to provide treated water during planned maintenance shutdowns of the water treatment plant and similar'non-emergency' situations. Over the last year the service has provided, among many others, a filtration and reverse osmosis plant to treat an unexpectedly high volume of landfill leachate resulting from high rainfall; a reverse osmosis system to upgrade a poorly performing river water treatment plant; and a reverse osmosis and deionisation plant to feed a power and steam utilities plant at a major chemical complex during a planned outage.
But, perhaps surprisingly, an increasing number of customers are making a case for long-term hire. This may be because the capital investment in a permanent plant would not pay back within the life of the project - a nuclear power station decommissioning project and a limited life power station refurbishment were real examples where this approach proved to be cost effective.
The temporary plant approach also provides other advantages. The quality of the treated water is guaranteed and the treatment process is tailored to meet current water quality needs, whether that is for dealkalised water for soft drinks manufacturing, Purified Water for pharmaceuticals, demineralised water for high pressure boilers or even ultrapure water for semiconductor manufacturing. If the treated water quality needs to be upgraded, it is an easy matter to change the trailer for one using a more advanced treatment process.
Waste plant example
Energy from one waste plant incinerates 225,000 tonnes per year of non-recyclable domestic waste to produce 400°C superheated steam in a 46 bar water tube boiler. This steam drives a turbine which generates electricity for export to the National Grid. Make-up water for the boiler is produced in an on-site ion exchange demineralisation plant consisting of cation exchanger, anion exchanger and mixed bed polishing but this was failing to meet the 20µg/l silica water quality requirement.
The initial solution was to provide three mobile temporary mixed bed polishers to polish the water from the demineralisation plant mixed beds and reduce the silica to less than 10µg/l. Continuous treated water quality monitoring for conductivity and silica was also provided. The ionic load on to the mobile units is very low and exhaustion is infrequent. They are not regenerated on site but returned to the central regeneration station in exchange for a regenerated unit. The "temporary" solution proved so reliable and cost effective that it is still in use more than twelve months later.
Long term rental of mobile water treatment solutions means that the plant is available immediately, has no associated capital cost and does not require any housing: the equipment is all installed in a standard 40 ft trailer which is insulated and supplied with heating and lighting so that, inside the container, the environment is just like most plant rooms. The trailers are also kitted out with appropriate safety equipment and complete with all necessary instrumentation and controls. They can be positioned to make the best use of available space and connected to existing pipework using flexible hoses.
In terms of process technology, the range is almost unlimited. Mobile units are completely modular, so any configuration of processes can be combined to make up a complete system to suit any application, which means that borehole, river and reservoir waters can be handled as easily as mains waters.
A typical temporary or mobile plant might consist of pre-treatment by multi-media filtration, organic scavenging or granular activated carbon adsorption in pre-treatment trailers; reverse osmosis in a second trailer and even mixed bed ion exchange polishing in a third. Temporary tanks and pumps can also be provided in trailers together with all interconnecting flexible hoses, water meters and fittings, and, of course, mobile generators to make the system completely stand-alone.
The latest addition to the range is REMOX, a membrane degassing process which removes carbon dioxide and oxygen from water. Dissolved carbon dioxide reduces the efficiency of ion exchange and continuous electrodeionisation processes. Membrane degassing works by passing the water across a gas-permeable membrane, on the other side of which is a flow of atmospheric air. Carbon dioxide transfers across the membrane from the gas rich water to the carbon dioxide lean inert gas. The process reduces dissolved carbon dioxide to less than 0.3 mg/l. Replacing the air with an inert gas - either nitrogen or water vapour generated under vacuum - allows dissolved oxygen to be removed.
Oxygen causes corrosion in boiler systems so it has to be removed from boiler feed water by adding oxygen scavenging chemicals like sulphite or carbohydrazide. Remox removes dissolved oxygen to less than 0.01 mg/l, dramatically reducing the consumption of oxygen scavengers. Dissolved oxygen in semiconductor rinse water can result in wafer surface defects and, in food and beverage products; it can impair shelf life and taste. Remox is the first membrane degasser to be available on the rental market and can treat up to 100 m3/h of water. The degassing system is fully automatic with on-line monitoring for complete operational security and is complete with all necessary compressed air, nitrogen and vacuum equipment.
Operational problems with mobile plants are rare, there are hardly any chemical additions and no chemical handling and disposal issues, and regular maintenance service visits are included in the hire contract. This means that total operational costs can be forecast with a high level of certainty, and with no requirement for capital investment. If the raw water quality changes, the mobile plant can easily be reconfigured. No wonder that more and more temporary solutions are becoming permanent.