Can reverse osmosis clean borehole wellpoint water

The pretreatment of raw water for reverse osmosis Part 2: Systems for water extraction


1 The pretreatment of raw water for reverse osmosis Part 2: Systems for water extraction K. Gebhard, K. Nikolaus, S. Ripperger * Today, reverse osmosis is the preferred method for obtaining drinking water from brackish and seawater. Such systems are in operation in many countries around the world and their number will continue to increase. The permeation of the water through the membrane can be significantly hindered by interfering substances in the raw water. These substances cause so-called fouling and scaling and result in a reduced system throughput, a shortened service life of the membrane, an increased cleaning effort and thus also downtimes of the systems. In the first part of this series it was described that disruptive water constituents in the inlet of a reverse osmosis system have a decisive influence on the operation and profitability of a system. Therefore, a processing of the feed (a pretreatment) is of crucial importance. In the following, second part, systems for water extraction are described and it is shown that in some systems a filtration of the water and thus a pretreatment are already connected. 1 Introduction Part 1 of the series of articles / 1 / described the disruptive substances contained in the raw water and their negative effect on the reverse osmosis process. The explanations make it clear that a poor design or implementation of the water pre-treatment can result in system failures and major damage. Therefore a location-specific, i. H. The pretreatment design tailored to the raw water is of central importance. The selection of the membrane and the planned measures for membrane cleaning are also linked to the pretreatment of the raw water. The pretreatment or pretreatment includes pre-cleaning and / or conditioning of the raw water in such a way that the system can be operated economically in accordance with the planning. The prerequisite for this is that, on the one hand, the membrane used and other parts of the system are not damaged unexpectedly and, on the other, that maintenance costs are reduced as much as possible. According to / 2 /, the pre-treatment has the task of generating water that is characterized as follows: - low content of particles and total dissolved solids; - low scaling potential; - low biological activity; - low concentration of heavy metals; - Absence of components that can form particles through oxidation. Inadequate pretreatment can result in fouling and / or scaling of the membrane, which can result in a high cleaning frequency, lower water yield, high operating pressure, poor product quality and a reduced service life of the membranes. In order to prevent these disadvantages, the membrane manufacturers specify requirements based on empirical values ​​that must be adhered to when operating the membranes. The warranty is also linked to the fulfillment of the requirements. Table 1 lists requirements that relate to pretreatment according to / 3 /. For the membranes under consideration, the pre-cleaning must ensure that the permissible limit values ​​are not exceeded. Since the raw water quality can differ greatly at different locations, it is essential to adapt the treatment processes to the specific conditions at a location. 2 Extraction systems for seawater Various systems (intake systems) have been developed for the extraction of seawater, which, depending on the process, can have a decisive influence on the raw water quality. Accordingly, the subsequent pretreatment is also influenced by the type of removal. The selection of a system is largely determined by the site-specific circumstances. For this purpose, the topography of the coast, the geology and Table 1: Requirements for the pre-cleaned sea water for membranes made of spiral-wound elements with membranes made of cellulose acetate and polyamide according to / 3 / * Dipl.-Wirtsch.-Ing. Katrin Gebhard Dipl.-Ing. Kai Nikolaus Prof. Dr.-Ing. Siegfried Ripperger Chair of Mechanical Process Engineering TU Kaiserslautern Tel .: F & S Filtration and Separation Volume 26 (2012) No. 1

2 Hydrogeology of the coastal area, the interactions between the seawater and the coastal groundwater layer, environmental requirements and data on urban, agricultural and industrial activities in the coastal area are recorded and evaluated. Furthermore, the data on the tides and currents as well as the confluence of rivers and the discharge of industrial wastewater must be available and evaluated. Since the conditions are also subject to seasonal and meteorological fluctuations, the water extraction systems must be adapted to the most unfavorable conditions. You and the subsequent treatment must ensure that the flow to the reverse osmosis system is as constant as possible. According to Alvarado / 4 /, an ideal intake is characterized by the following properties: - optimal seawater quality without organic suspended matter, dissolved organic material, biological activity as well as dissolved heavy metals and interfering substances that cause scaling; - permanent overview of the physical and chemical characteristics of the sea water, such as temperature and salinity; - constant flow of water; - Location of the system as close as possible to the reverse osmosis system; - Possibility of adapting the sea water inflow to the respective water demand; - Environmental friendliness and thus avoidance of negative impacts on the sea and coast; - Obtaining licenses and permits that allow access to the beach or the coast in the first place. Against this background, various extraction systems are presented below and their advantages and disadvantages are shown. The procedures can be divided into direct and indirect procedures. In direct processes (surface water intakes), the water is taken directly from the sea, whereas in indirect processes (non surface water intakes) the water first crosses natural soils or sand beds before it is pumped out / 5 /. This subdivision is further broken down by Cartier and Corsin / 6 / as shown in Figure 1. The structure of the chapter follows this structure. 2.1 Direct extraction Targeted seawater extraction for desalination In large desalination plants, the seawater is usually extracted directly from the sea. The water passes Fig. 1: Structure of the various extraction systems (according to / 6 /) with a filter tube or a sieve, the opening widths of which prevent larger living beings and dirt particles from entering. Often these are larger backwash filters. In order to minimize damage to fish, special constructions were developed in which the inflow velocity was minimized and / or deposits were largely avoided by continuously moving the filter / 7 /. A continuous inflow of the feed is guaranteed with these direct systems and the withdrawal of very large amounts of water is not a problem. The quality of the water drawn in, on the other hand, can be subject to large fluctuations. In this regard, the depth of the intake plays a crucial role. A distinction is made between shallow (0-15 m) and deep (20-35 m) extraction. Since less light penetrates to the sea floor, less photosynthesis takes place during deep extraction, so that in addition to the reduced amount of algae, the number of suspended matter is also lower. However, the water temperature decreases with depth, which reduces the water permeation through the membrane. The more constant water quality with deep withdrawal is associated with this disadvantage. Another disadvantage is that filter tubes and lines provide a good growth environment for marine organisms such as mussels. The use of plastic extraction systems can inhibit the build-up. In the event of vegetation, shock chlorination is often required along with periodic maintenance work to reduce biofouling / 8 /. It must be taken into account that maintenance work under the sea surface is complicated and costly and requires the use of specialists. The installation of the systems is also associated with large investments, in which the costs for the pumps and the associated inlet chambers as well as the pipelines must also be taken into account. It is advantageous that these are usually relatively long-lived systems with a lifespan of 30 to 50 years. Often, tubular casing pumps of an axial or semi-axial design are used. They are installed in inlet chamber structures, which must be designed in such a way that the flow is fed in as smoothly as possible. Unfavorable flow conditions in the inlet area to the pump, e.g. B. as a result of vortex formation or an asymmetrical speed distribution, can result in considerable mechanical loads on the pump. Direct deep extraction is only possible if there is a steep coast and the corresponding depth can be reached in less than 50 m from the shore. As this condition is seldom met, it is often taken from shallow water, although there is a higher content of suspended solids due to the currents and waves. In warm seasons, algae and the content of microorganisms grow more and more - F & S Filtrieren und Separieren Volume 26 (2012) No. 1 7

3 men and nutrients increases. The temperature is also subject to strong fluctuations in this case / 6 /. As a direct effect of the rain, the salt content can fluctuate in shallow extraction / 8 /. Due to these unfavorable conditions, a costly and complex pretreatment is usually required for shallow extraction. Extraction system divided with a power plant Often, large reverse osmosis systems are built in combination with or close to a power plant to generate electricity. A power plant with once-through cooling has a great need for cooling water, which also requires a water extraction system. Using this intake to feed the reverse osmosis system drastically reduces the investment costs in the case of a large system. This also significantly reduces the impact on the environment. In addition, the water previously used for cooling has warmed up, which has a positive effect on the permeation rate. By mixing the concentrate from the reverse osmosis system with the excess cooling water from the power plant, the salt content of the water flow introduced into the sea is reduced. In contrast to reverse osmosis, the pretreatment of the cooling water is less expensive. Often it is only treated chemically, which must be taken into account in further processing as a feed for a reverse osmosis system. While z. If, for example, continuous chlorine dosing is carried out for use in a power plant, the chlorine content in the feed of a reverse osmosis system must often not exceed low limit values ​​(see Table 1). In principle, once-through cooling in power plants is no longer considered to be the best available technology due to the negative impact on the environment. According to / 9 /, it is questionable that joint use of the intake would justify the continuation of this outdated procedure, especially since long-term contracts are often concluded. 2.2 Indirect extraction In indirect extraction, the seawater is pre-filtered due to a passage through the natural soil or through a bed of sand. The extraction can take place on- or offshore. Underground extraction from coastal areas For the underground extraction of seawater from a coastal area, vertical or horizontal filter wells or bank well galleries are installed in the coastal area. - Vertical wells Vertical filter wells in close proximity to the sea and with depths of 30 to 50 m are the most frequently used indirect extraction systems. They are driven into the water-bearing layer and are lined in the upper part with an impermeable pipe, which is provided with slots in the lower part through which the water from the aquifer can penetrate. There is usually a layer of gravel between the borehole wall and the filter tube. It guarantees sand-free continuous operation / 10 /. The depth of the well is chosen according to the geology and the permeability of the ground. The water quality can vary with the layer of soil / 9, 11 /. When the sea water penetrates the sand of the beach or other geological formations, it is already pre-filtered and, according to Cartier et al. / 6 / only a slight turbidity (0.5 NFU), a small colloid content (SDI 2), a few microorganisms and nutrients. Due to the dissolved organic substances that may still be present, fouling can occur when a reverse osmosis system is fed directly / 12 /. Regardless of this, the low oxygen content and the physical and chemical properties are usually advantageous. These are constant, as neither changes in the weather nor the swell have an effect on the extraction. The water quality means that the pretreatment only comprises a few procedural steps / 13 /. The suitability of the site must first be checked for the construction of a vertical well. Carrying out hydrogeological investigations provides information about the permeability of the layers in the ground, their thickness and the depth of the aquiferous layers. The effects of the special sediment on the water composition must also be checked, as this can influence not only the filtration but also the manganese and iron content / 14 /. The volume flow is also influenced by the permeability of the soil under the prevailing pressure conditions. At the end of 2007, only four systems were in operation that pumped more than m 3 / d of water. The largest took in Pembroke, Malta, at this time m 3 / d / 15 /. To ensure the necessary flow of water, many wells are drilled next to each other / 4 /. In between, a certain distance must be maintained so that the withdrawals from the individual wells do not interfere with each other. Vertical filter wells are arranged along the coast, as an intake installed further inland harbors the risk of fresh water being pumped. Vouchtov / 14 / illustrates that a large area is required, which can have a visual and aesthetic impact on the coast. The proximity to the sea makes it necessary for pumps and auxiliary equipment to be placed higher up to protect against flooding. - Horizontal filter wells Horizontal filter wells are an alternative to the systems described above. They have several horizontal filter tubes in the ground, into which the water can penetrate and which lead into a watertight vertical well that also serves as a pump sump can. The star-shaped filter pipes can be arranged around this shaft with a diameter of up to 300 m. Since the water is collected in several pipes, withdrawal volume flows of up to 500 m 3 / h can be achieved / 6 /. The yield corresponds to 6 to 8 vertical filter wells. These are usually more expensive than building a horizontal filter well. - Shore well gallery (Beach Galleries) A ​​well gallery built parallel to the shoreline is possible if the layer of extraction is not deep and only thin. It is necessary if a rock layer prevents the use of the types of wells described so far. In this process, filter pipes are laid horizontally in a construction pit before the pit is filled with a filtering material (e.g. layered gravel and sand with different grain sizes as well as sand from the beach). The penetrating water is often collected at one end of the pipes and passed on by a pump / 16 /. The arrangement of several galleries next to each other takes place while maintaining a certain distance. Missimer / 17 / lists the following advantages of the process: - Large amounts of water can be extracted, - only low maintenance costs are incurred, - the water collected from several units is passed on by a pump and - the galleries are cleaned out - use of the wave motion itself. According to Maliva et al. / 18 / the normal swell in systems that have been built at a corresponding height can ensure that the system is self-cleaning. Due to the swell, the area between sediment and water above 8 F & S Filtration and Separation Volume 26 (2012) No. 1

4 of the gallery kept clean. It prevents clogging by constantly moving and removing fine-grained material and marine organisms. With appropriate planning, such a system also overcomes weak points in well galleries that were built above the flood water level (scaling, erosion). However, the construction of such systems is expensive and complicated, since sheet piling is required and the system has to be drained from time to time. In addition, the process can only be used to a limited extent and there is a risk of danger from storms / 17 /. Also a progression of the beach, i. H. A seaward growth of the sediment in the area of ​​the beach can, over time, adversely affect the function of the galleries / 18 / Underground extraction from areas below the sea Extraction systems can also be arranged directly below the seabed near the coast.These can be withdrawals made using horizontal drilling methods or well galleries in the sea floor. - Horizontal drilling method In the horizontal drilling method, a pilot hole is first carried out with a jet lance or a rock drill head with a borehole motor. Exact control of the drill head is possible through the interaction of the angular position of the drill lance, the drill hole flushing and the feed in connection with the technology for location. In the second step, the canal is widened with cutting and broaching tools. Finally, a pipe is pulled through the borehole / 19, 15 /. This process is also used for the extraction of seawater and for the installation of the necessary filter pipes. With the neodren technique, as a special variant of this directional drilling, a special filter pipe, which is usually 250 to 300 meters long and is located under the sea, is connected to a closed pipe. The latter sucks water through the borehole on land. With the pipe laying technique, several pipes are installed in a fan shape from a location on the coast, usually four to seven meters below the sea bed. With a corresponding number, up to several hundred thousand cubic meters of water can be withdrawn daily in a limited coastal area / 20 /. A short backwash can increase the capacity again if the capacity has decreased. The advantage is that waves cannot damage F & S Filtration and Separation. The removal reduces the ecological and visual impairment to a minimum. The actual extraction can be arranged underground and a few hundred meters behind the bank / 21, 22 /. Alvarado / 4 / confirms some positive properties of the technology (high withdrawal volume, good and constant water quality, low environmental impact during the construction phase, simple water treatment). However, it is also pointed out that the process is a new technology for which not many references are yet available. It is also shown that organic material can be present in the filtrate. Peters / 20 / points out that official requirements as well as geological and logistical boundary conditions must be taken into account. - Seabed well gallery (Seabed Filtration) The construction of a well gallery has already been shown in section. The system described is also used on the seabed / 7, 21 /. Here, too, the natural sand or the deposits of the soil are partly replaced by other materials. When deciding whether high filtration only the best comes through PACO Spinpack sieves In filter technology it is like everywhere in business: only the best make it through. For 5 decades, PACO has been among the finest in filter technology with elements and components made of metal wire mesh, request information: Tel .: Fax: Metal fiber fleece, cans, perforated sheets, and much more. a. has to offer. Regardless of what your needs are: PACO always guarantees you the highest filtration value individually. PACO Paul GmbH & Co. P.O. Box Steinau a.d. Street Germany

5 Table 2: Advantages and disadvantages of different extraction systems (according to / 5 /) If the system is to be installed onshore or offshore, the following factors must be taken into account: - The water yield is higher when it is arranged below the sea. However, the yield can decrease over time as the permeability decreases due to sedimentation. - Galleries that only border the sea generally produce water with less turbidity and fewer bacteria / 23 /. - It is fundamentally easier to build a plant onshore. Maintenance and repairs can also be carried out more easily there / 24 /. - According to a publication by the US Department of Sustainability and Environment / 25 /, at least at the time of August 2008 there was only one filtration system located in the sea, located in Fukuoka, Japan. For this reason, the authors state that the operational safety of such a system has not yet been proven at the time. Advantages and Disadvantages of the Systems The most important advantages and disadvantages of the systems discussed were determined by David, et al. / 5 / summarized according to Table 2. 3 Outlook After the description of the disruptive water constituents in the inlet of a reverse osmosis system / 1 / and the systems for water extraction, which are often already coupled with a filtration in the ground or an artificial sand bed, the next issue deals with the actual chemical and physical processes Water pre-treatment described. Literature / 1 / Gebhard, K., Nikolaus, K., Ripperger, S. 2011: The pretreatment of raw water for reverse osmosis. Part 1: The need for raw water pretreatment. Filtration and Separation, 25, No. 6, S / 2 / Water Desalting Committee of the American Water Works Association Water Desalting Planning Guide for Water Utilities. John Wiley & Sons, Inc., Hoboken, New Jersey. / 3 / Watson, I., Morin, O.J. and Henthorne, L Desalting Handbook for Planners. Desalination and Water Purification Research and Development Program Report No. 72. U.S. Department of the Interior, Bureau of Reclamation, Technical Service Center, Water Treatment Engineering and Research Group. / 4 / Alvarado, O Intake Systems in Sea Water Reverse Osmosis (SWRO) Desalination Plants. Presented at the International Congress on Water Management in the Mining Industry: WIM 2008 in Santiago de Chile. / 5 / David, B., Pinot, J.-P. and Morrillon, M Beach Wells for Large-Scale Reverse Osmosis Plants: The Sur Case Study. Presented at the IDA World Congress - Atlantis, The Palm - Dubai, November IDAWC / DB / 6 / Cartier, G. and Corsin, P Description of different water intakes for SWRO plants. Presented at the IDA World Congress, Maspalomas, Gran Canaria, Spain. IDAWC / MP / 7 / Reynolds, T. and Maley, M Desalination Intake Approaches: Open Ocean Intake vs Subsurface. [Online, accessed on:] WA% 20Intake% 20Presentation% web2.pdf / 8 / Hill, C.P., et al. o.y. Florida Utilities Considering Seawater to Meet Growing Demands, Diminished Supplies. [Online, accessed at:] display / _printarticle / articles / membranes / volume-2 / issue- 30 / Features / Florida_Utilities_Considering_Seawater_to_M eet_growing_demands Diminished_Supplies.html / 9 / UNEP / WHO, United Nations Environment Program, Regional Office for West Asia, Manama, and World Health Organization, Regional Office for the Eastern Mediterranean, Cairo Desalination Resource and Guidance Manual for Environmental Impact Assessments .: ov, Manama, Cairo. / 10 / Kauch, E.P Vertical filter wells - calculation. Lecture notes for the subject of groundwater use at TU Graz. [Online, retrieved at:] ad / lehre / vorlesung_grundwassernutzen / gwn3.pdf. / 11 / Pankratz, T An Overview of Seawater Intake Facilities for Seawater Desalination. [Online, retrieved on:] / 12 / Saeed, M.O., et al Biofouling Potential in Open Sea and Adjacent Beach Well Intake Systems. Presented at the International Conf. on Water Resources & Arid Environment. / 13 / Hassan, A.M., et al Investigating Intake System Effectiveness with Emphasis on Self-Jetting Well-Point (SJWP) Beachwell System. Presented at the Second Acquired Experience Symposium on Desalination Plants O&M in Al-Jubail. / 14 / Voutchkov, N Use of Beach Well Intakes for Large Desalination Plants. Newsletter European Desalination Society, Issue 22. / 15 / Unesco-IHE and Ben Gurion University A comprehensive critical evaluation on conventional, membrane-based and novel pre-treatment procedures for SW and BW desalination. Project number: / 16 / Dennis, E.W Research and Development for Horizontal / Angle Well Technology. U.S. Department of the Interior, Bureau of Reclamation, Technical Service Center, Water and Environmental Services Division, Water Treatment Engineering Research Team, Denver, Colorado. Desalination and Water Purification Research and Development Program Report No / 17 / Missimer, T.M. Alternative Subsurface Intake Designs for Seawater Desalination Facilities. [Online, accessed on:]% 20Subsurface% 20Intakes.pdf. / 18 / Maliva, R.G. and Missimer, T.M Self-cleaning beach gallery design for seawater desalination plants. Desalination and Water Treatment, Volume 13, Issues 1-3, S / 19 / DrillTec. o.y. Company homepage of the company DrillTec GUT GmbH. [Online, retrieved on:] / 20 / Peters, Dr.-Ing. T Extraction and partial preparation of seawater for desalination plants with HDD-based Noedren technology. [Online, retrieved on:] / 21 / Peters, T. and Pintó, D Seawater intake and pre-treatment / brine discharge environmental issues. Desalination, Volume 221, Issues 1-3. S / 22 / Peters, T., Pintó, D. and Pintó, E Improved seawater intake and pre-treatment system based on Neodren technology. Desalination, Volume 203, Issues 1-3. S / 23 / Department of Environmental Quality. Public Water Supply Construction Standards. [Online, accessed on:] / 24 / Screen Servives. o.y. Screen Services company homepage. [Online, accessed:] / 25 / Department of Sustainability and Environment Victorian Desalination Project Works Approval Application Supporting Documentation. [Online, access at:] data / assets / pdf_file / 001 5/15414 / EPA-Works-Approval-Application-Part-B.pdf. 10 F & S Filtration and Separation Volume 26 (2012) No. 1