Process design
Seawater reverse osmosis (SWRO) process engineers will design each desalination system on a bespoke basis because each project is unique with a particular variation in feed water quality, temperature, site specific conditions and the local/national regulatory constraints that will apply. Engineers have a number of unit processes at their disposal, which can be designed into various configurations in order to determine the best design based on all these site-specific considerations. This allows the benefits of each unit process to be maximised and the overall system fully optimised. In our previous article on the future for desalination (see May/June 2012 issue) we recognised that many SWRO engineers and operators are facing ever increasing challenges in respect of pre-treatment requirements. This is not only to ensure the efficient operation of their plants but the long-term sustainability of their significant investment in RO membrane technology. Getting the pre-treatment right is critical in ensuring consistently stable and acceptable membrane feed conditions. Seawater resources typically have a higher tendency to cause membrane fouling than surface water or groundwater supplies and hence more extensive pre-treatment is required. If this pre-treatment is ineffective then SWRO membranes (see Figure 2) can become fouled, easily requiring more frequent and onerous cleaning regimes to be applied in an attempt to recover membrane flux rates and salt rejection levels. Effective pre-treatment will maximise RO membrane life and reduce cleaning requirements, thus reducing whole life costs. But various factors are leading to a higher tendency for SWRO fouling. Increased fouling is resulting from membrane design advances such as increased flux rates and improved salt rejection levels, which lead to lower cross flow velocity in the membrane elements, higher velocity across the SWRO membrane surface and longer residence times in the RO system. Hence, to determine the correct pre-treatment for any specific site is the critical aspect of determining an efficient overall system design. Engineers will take into account past operational experiences on similar supplies, and also bench scale tests and sampling regimes can be used in an attempt to fully characterise seawater quality and variation. However, the only sure way to maximise the effectiveness of any process design is to undertake sufficient pilot testing, using scaled down unit processes to mimic the unit process components of the proposed design. By operating a pilot system on site the engineer is able to use their expertise to change parameters and modify or re-order unit processes to determine the best overall system design. Pilot plants need to be of an adequate size to be representative and produce valid results but budgets do influence the size of the trail units and hence the validity. Often the competitive use of pilot trials is specified by clients at the pre-tender stage. The challenges present in the seawater supply resulting in fouling are varied in terms of their type, intensity, concentration and frequency. Apart from the normal contaminants associated with water treatment, such as but not limited to suspended solids, turbidity, natural organic matter (NOM) and oil and grease, all of which have to be efficiently and effectively removed or at least reduced, there is cellular and extracellular material to consider. These comprise organic components such as transparent exopolymer particles (TEP) and polysaccharides associated with algae. As reported in the May/June issue of Filtration+Separation, algae bloom events are becoming more frequent at the intake to SWRO plants. These are generally referred to as Red Tides or Harmful Algae Blooms (HABs) and such events are a major challenge facing the desalination industry. These algae blooms can result in the release of high levels of TEP, polysaccharides and other toxins, providing a significant challenge for the SWRO facility. HABs can not only result in severe disruption to the SWRO system but they can also cause illness and mortality amongst marine life and general chaos in the area affected. Unit processes available to the process design engineer to tackle the various challenges presented in seawater supplies can be grouped into three main categories: chemical pre-treatment, conventional pre-filtration and membrane pre-filtration. Part 2 will consider each in turn. Click here for Part 2.
