As the worldwide use of Reverse Osmosis (RO) to treat and recycle waste water streams expands rapidly, it follows that there is a need to reduce the cost of RO systems and hence the cost of the water reuse schemes as a whole. In some parts of the world, land for new plant construction is at a premium, and so as RO plants become larger in capacity it is important to minimise footprint requirements. Additionally, existing plants where there is little, if any, room to expand, may need to be retrofitted to help meet the additional demand.
The 8-inch diameter x 40-inch long RO element was introduced to the market as long ago as 1975 and is clearly not ideal for many of the large systems currently under consideration. As a result the RO membrane manufacturers faced a challenge to come up with a large diameter RO element to meet the needs of an expanding market place. In fact there is nothing really new in the concept of large diameter RO elements. A 12.75 inch diameter product was produced in 1978 for the famous Yuma De-Salting plant in the States and 15 inch x 50 inch long elements have been used commercially since 1990. However the world’s largest commercially available RO element is 18 inch x 60 inch long.
Development of large elements
As the growing need to reduce both the installed and operating cost of RO systems was recognised by manufacturers it was clear that one potential way was to increase the diameter and length of the individual RO elements. Large diameter elements offer savings in footprint and building costs which in turn lead to savings in the installed cost/m³ produced of the system. Obviously the availability of large diameter elements offer opportunities for alternative plant arrangements and layouts, as the optimum design approach for 8 inch elements is not necessarily the optimum approach for large diameter elements. Designers need to be aware and consider different design approaches for different size elements.
The first spiral RO element was introduced to the world in 1964, and the first commercially available product was a 4 inch in diameter by 40 inches long element which progressed to 8 inch diameter by 40 inches length in 1975. While there is nothing new in the concept of large diameter RO elements, the challenge for the manufacturers was to overcome the element construction and manufacturing issues, make the product up-scalable from 8 inches and create a simple procedure for loading and unloading.
The issues surrounding the physical construction of such elements have been widely discussed, but the choice of element diameter is worth examining in more detail. While it could be argued that a natural progression would be 4 inch to 8 inch to 16 inch, the pressure vessels for this type of element did not exist and needed to be newly developed, thus the nominal diameter of the element was clearly arbitrary and it made sense to optimise it based on manufacturability and vessel design limitations. As a result of this optimisation process the commercial product now available from Koch Membrane Systems has a diameter of 18 inch. This represents a 30% gain in area over the “natural” 16 inch for only a two inch increase in vessel diameter.
Element handling
The membrane area of the 18 inch diameter product is 3,050 square feet or 283 square metres but more importantly, the weight of the element is around 150 kg – much more than can be handled by a single workman. At first some expressed the opinion that weight would be a problem, but in fact it has been resolved quite easily with just a few simple tools and a hand-operated winch and pulley system. A five elements long vessel can be loaded – under optimum conditions – in 20 minutes. It can be safely stated, that it will never take longer to load a 18 inch system than to load a same capacity – 8 inch system, and in most of cases it will be much faster.
Using a scissor jack, an element is positioned at the entry point to the vessel where it is connected to a pulley cone via links. It is then pulled into the vessel using the hand operated winch. Skis on the ATD are there to minimise friction. The second element is then lifted into position and connected to the first element. This is similarly pulled into the vessel. The process is then repeated until all elements (up to 5 per vessel) are coupled together. Another major advantage of the design of the new large diameter elements is that loading and unloading is carried out from the same end again helping to reduce the footprint of the overall plant.
The smaller footprint and reduction in the number of interconnecting components of large diameter systems leads to significant savings in construction and installation costs, but another recognised saving is in the shipping costs because large diameter element systems require fewer shipping containers. Finally, maintenance costs are reduced simply because there are fewer vessels, pipe connections and o-ring seals which could be potential leak points. The actual savings are always project specific and should be calculated accordingly. However, in general terms only, overall savings of 15 to 30% have been observed.
To date there are twelve currently in operation as far afield as Australia, the Middle East, Spain and the Ukraine for potable water and industrial water and wastewater reuse applications accounting for over 900 MegaMagnum elements in total.
The Western Corridor recycled water project (Australia)
Australia's largest recycled water project utilises this element. This water reclamation system is part of the Au$2.5 billion Western Corridor Recycled Water Project, one of the largest recycled water projects in the Southern Hemisphere, which will eventually provide 280 mld of recycled water to reduce the load on the region's water supply. The scheme consists of three Advanced Water Treatment Plants (AWTPs). Two are located in the Brisbane area and the third outside the city of Ipswich at Bundamba. This plant comprises two stages – 1A and 1B – with a total capacity of 66 mld which will be delivered as purified, recycled water to local power stations at Swanbank and Tarong. At the heart of each AWTP is an RO system using 18 inch diameter elements.
Bundamba 1A comprises four trains, each with 65 x 18-inch elements, meaning a total of 260 elements. The pre-packaged systems ensured speedy installation and start-up; installation began in May 2007 and the plant became operational in August 2007, three weeks ahead of schedule.
“Koch Membrane Systems’ equipment started arriving at the Stage 1A site on May 14th, with the last pieces of equipment arriving in the first week of June,” said Sandy Berman, senior project manager at KMS. “Thiess Black & Veatch was contracted by the Queensland government to build, commission and start-up the plant very quickly, with firm dates required for water production. KMS was able to provide the resources necessary to complete the majority of the start-up in four weeks rather than the seven that had been scheduled.”
Bundamba 1B started up successfully in April 2008 and as expected the 18-inch diameter elements significantly reduce the footprint and installation time of RO systems, thereby reducing the overall cost. Each element contains 3,050 square feet (283 square metres) of membrane area, compared with the 400 square feet (37 square metres) in the standard 8 inch product.
Due to the increasing water shortage in Australia, a need for advanced systems of wastewater treatment and water recycling was identified. Clearly this challenged the wastewater treatment industry to develop technology and processes for municipal and industrial wastewater treatment to meet the drinking water standards for reuse. The AWTP treats existing biologically treated municipal effluent using Microfiltration, reverse osmosis and advanced oxidation. The recycled water will be used initially for supply to two power stations nearby, freeing up an equivalent amount of water for potable use. Phase 1A if the plant was commissioned in August 2007 and produces 30,000 m3/d. This phase comprises 4 equal trains of 12 vessels in a 7:4:2 array. Phase 1B has since been commissioned, this comprises another 5 identical trains to produce a further 36,000 m3/d. Figure 5 shows a 1A skid being loaded into position.
Outlook
In conclusion there is no doubt that large diameter RO elements have proven their capabilities – the first references are successfully in operation and they are here to stay. The scepticism which existed five years or so ago regarding the manufacture of the elements, loading and unloading issues, availability of pressure vessels and so on has been overcome. The challenges have been met for brackish systems up to 30 bar operating pressures and the next challenge for the industry is to commercialise the large diameter sea water product.
Progress is certainly being made here too, as evidenced by the imminent start-up of the first full scale 18” inch seawater MegaMagnum RO pressure vessel demonstration at a Red Sea desalination plant. The project will set a benchmark for the long term performance of large RO elements – as will the others planned for Europe and the US in the first two quarters of 2009.