Open-ocean Intake Research Articles

Organic carbon movement through two SWRO facilities from source water to pretreatment to product with relevance to membrane biofouling

The presence of algae, bacteria, various fractions of natural organic matter (NOM), and transparent exopolymer particles (TEP) in the raw water, after each pretreatment process and in the permeate and concentrate streams, were measured at two SWRO plants to assess biofouling potential. It was found that the most significant process controlling the concentration of algae, bacteria, and the biopolymer and humic substances was the intake type with the subsurface intake discharge showing significant reductions. The mixed media filtration process was marginally useful in removing some TOC and NOM, but had little effect on TEP removal. Some bacterial regrowth may be occurring in the cartridge filters, but the evidence is inconsistent. Significant quantities of the biopolymer and humic substance concentrations were found to be retained in the membranes, but the concentrations were significantly greater in the facility using an open-ocean intake. Bacteria and TEP were found in the permeate stream, which may document bacterial regrowth and TEP production downstream of the membrane process. Measurements of the organic carbon passage through SWRO facilities can be successfully used to evaluate pretreatment process effectiveness and to make SWRO plant operational improvements.

Subsurface intake systems: Green choice for improving feed water quality at SWRO desalination plants, Jeddah, Saudi Arabia

An investigation of three seawater reverse osmosis facilities located along the shoreline of the Red Sea of Saudi Arabia that use well intake systems showed that the pumping-induced flow of raw seawater through a coastal aquifer significantly improves feed water quality. A comparison between the surface seawater and the discharge from the wells shows that turbidity, algae, bacteria, total organic carbon, most fractions of natural organic matter (NOM), and particulate and colloidal transparent exopolymer particles (TEP) have significant reductions in concentration. Nearly all of the algae, up to 99% of the bacteria, between 84 and 100% of the biopolymer fraction of NOM, and a high percentage of the TEP were removed during transport. The data suggest that the flowpath length and hydraulic retention time in the aquifer play the most important roles in removal of the organic matter. Since the collective concentrations of bacteria, biopolymers, and TEP in the intake seawater play important roles in the biofouling of SWRO membranes, the observed reductions suggest that the desalination facilities that use well intakes systems will have a potentially lower fouling rate compared to open-ocean intake systems. Furthermore, well intake system intakes also reduce the need for chemical usage during complex pretreatment systems required for operation of SWRO facilities using open-ocean intakes and reduce environmental impacts.

Feasibility of using a subsurface intake for SWRO facility, south of Jeddah, Saudi Arabia

The Kingdom of Saudi Arabia is the largest producer of desalinated water with about 13% of the global desalination capacity. Most of these desalination plants use the open-ocean intakes to deliver raw seawater to the desalination facility. Recently, some of the private desalination plants have shifted to subsurface intake systems, either wells or galleries, in order to obtain better water quality with a minimal environmental impact (e.g. minimal entrainment and impingement). The use of these intake types has improved the raw seawater quality extracted from the Red Sea and Arabian Gulf, providing better protection for the membrane component by eliminating/reducing algae, bacteria and organic matter concentrations from the seawater source. One of these desalination plants is located south of Jeddah city which is the second largest city in Saudi Arabia. The plant shifted from an open-ocean intake to beach wells to improve the water quality at the site. Currently, the plant employs 10 vertical wells to extract enough water to produce 10,000 m3/d of product water via the reverse osmosis process. Studies show that quality of seawater significantly improved after shifting to the well system. The use of a larger capacity well system or a seabed gallery intake was investigated at this site for a proposed additional 20,000 m3/d future expansion of the facility. More than 60 sediment samples were collected from the seabed along five different transects in an area of 25,000 m2, starting from shoreline and moving seaward. Grain size analyses, hydraulic conductivity and mud percentage were analyzed in order to determine the characteristic of marine sediments at the studied site. The marine bottom at the selected site contains carbonate sediments which have a high potential of reducing the natural organic matter concentration in the raw seawater. In this study, the laboratory measurements showed that this site has low mud content and moderately high hydraulic conductivity, which make it feasible for seabed gallery construction as an intake. It is concluded that use of a seabed gallery intake for the future expansion will overcome the problem of limited water capacity produced by individual wells, but additional alignments of wells is also technically feasible.

Technical feasibility of a seabed gallery seawater intake at Ras Abu Ali Island, Arabian Gulf, Saudi Arabia

Open-ocean intake systems require extensive and advanced pretreatment unit operation to produce feed water with low membrane fouling potential in seawater reverse osmosis (SWRO) facilities. Alternatively, subsurface intake systems tend to produce high quality raw seawater even before pretreatment. Subsurface intakes extract seawater indirectly through the geological structure of shoreline or nearshore sediments. Water percolation through geological units provides physical and biological treatment, so that the raw seawater is microbiologically stable with relatively low particulate and organics content. Overall, utilization of subsurface intakes will reduce the intensity of pretreatment, which reduces operating cost, lowers chemical and energy consumption, and reduces environmental impacts. An important aspect in the feasibility of a subsurface intake is the compatibility of the local geological environment. In this study, a field investigation was conducted at Ras Abu Ali Island in the Arabian Gulf. This location currently contains an of existing oil company facilities and a proposed governmental marine fish hatchery facility. Recreational, commercial, and domestic potable water uses require the need to use the SWRO process to meet demands. Characterization of the shoreline and marine offshore bottom were performed as well as observation of tidal fluctuations and wave heights. A specific grid area was chosen where 35 sediment samples were collected from the seabed floor for laboratory analysis of grain size distribution, sediment porosity, and hydraulic conductivity. Onsite observation showed that the marine bottom has a low slope creating a wide intertidal area. The lowest tidal zone is more than 150 m from the shoreline defining a far seaward boundary for the intake construction point. A relatively thin layer of mixed-type sediment (carbonate and siliciclastic) covers the marine hardground bottom. The unlithified bottom sediment contains a low mud percentage (less than 1%) with porosity ranging between 0.29 and 0.41 and hydraulic conductivities up to 22.5 m3/d. It was determined that seabed gallery development is suitable at this location. Preliminary design for a seabed gallery filter was developed using a series of cells. Each gallery cell represents a unit that can be simply duplicated to meet the overall intake capacity requirement. Each gallery cell is designed to have minimum 8 m/d infiltration rate through five layers of engineered sand and gravel. The total thickness of the filter bed is 4.5 m (2 m top layer). The dimensions of the proposed cells are 100 × 30 m and each cell will conservatively provide 24,000 m3/d of filtered water. The design is flexible to meet the required capacity. For example, a SWRO desalination plant which produces 54,000 m3/d product water from 38,000 mg/L salinity seawater at a 45% conversion rate will require a minimum of 6 cells using the preliminary design.

Impact of well intake systems on bacterial, algae, and organic carbon reduction in SWRO desalination systems, SAWACO, Jeddah, Saudi Arabia

The intake system can play a significant role in improving the feed water quality and ultimately influence the performance of downstream components of the seawater reverse osmosis desalination processes. In most cases, open-ocean intakes produce poor feed water quality in terms of the abundance of naturally occurring organic matter, which increases the risk of membrane fouling. An alternative intake is the subsurface system, which is based on the riverbank filtration concept that provides natural filtration and biological treatment of the feed water prior to the entry of the water into the desalination plant. The use of subsurface intakes normally improves the raw water quality by reducing suspended solids, algae, bacterial, and dissolved organic carbon concentrations. Therefore, the risk of biofouling caused by these substances can be reduced by implementing the appropriate type of intake system. The use of well intake systems was investigated along the Red Sea shoreline of Saudi Arabia in the Jeddah region. Data were collected from a seawater reverse osmosis (SWRO) plant with a capacity of 10,000 m3/d. The well system produces feed water from an artificial-fill peninsula that was constructed atop of the seabed. Ten wells have been constructed on the peninsula for extracting raw seawater. Water samples were collected from nearby surface seawater as a reference and from selected individual wells. The percentage of algae and bacterial removal by induced filtration process was evaluated by comparison of the seawater concentrations with the well discharges. Transparent exopolymer particles and organic carbon fractions reduction was also measured. The quality of raw water extracted from the well systems was highly improved compared with the raw seawater source. It was observed that algae were virtually 100% removed and the bacterial concentration was significantly removed by the aquifer matrix. The detailed analysis of organic carbon fraction using liquid chromatography-organic carbon detection instrument showed a high-percentage removal of the organic fractions consummate with the molecular weight. The results of this study can be used to improve the intake system design of existing SWRO facilities that might require expansion in future or new facilities that will be located along the Red Sea coastline.

Use of beach galleries as an intake for future seawater desalination facilities in Florida and globally similar areas

Desalination of seawater using the reverse osmosis process can be made less costly by the use of subsurface intake systems. Use of conventional open-ocean intakes requires the addition of a number of pretreatment processes to protect the primary RO process. Despite using the best designs possible for the pretreatment, seawater RO membranes tend to biofoul because of the naturally-occurring organic material and small bacteria present in seawater. These materials are not completely removed by the pretreatment system and they pass through the cartridge filters into the membranes, thereby causing frequent and expensive cleaning of the membranes. Quality of the raw water can be greatly improved by the use of subsurface intakes which can substantially reduce the overall treatment cost. There are a number of possible subsurface designs that can be used including conventional vertical wells, horizontal wells, collector wells, beach galleries, and seabed filters. The key selection criteria for the type of subsurface intake most suited and most cost-effective for a site are based on the required volume of raw water and the local geology. The active shorelines of Florida are very well-suited for the development of beach gallery intake systems. These systems are installed beneath the active beach between the high and low tide zones of the beach. Since they are constructed with a depth to the screens between 3 and 5 m, they cannot be observed at surface and persons using the beach would be unaware of their existence. These galleries are simple to construct and they tend not to clog because the active wave action within the intertidal zone provides mechanical energy that continuously cleans the filter face. They also have other advantages, including: the water quality is seawater unaffected by substances present in freshwater aquifers occurring landward of the shoreline, the salinity of the water is generally constant, and there are no impacts on water users located inland from the shoreline. A comprehensive study of the grain size characteristics of Florida beaches has allowed an assessment to be made of the hydraulic conductivities of the Florida beach sands. Hydraulic conductivity values generally range from 1.8 to 24 m/day, which is more than sufficient to allow the design and construction of high-capacity galleries at a reasonable cost. This type of intake is particularly relevant to the northeast Florida shoreline adjacent to an area being considered for development of a large-capacity seawater desalination system.

Subsurface intakes for seawater reverse osmosis facilities: Capacity limitation, water quality improvement, and economics

The use of subsurface intake systems for seawater reverse osmosis (SWRO) desalination plants significantly improves raw water quality, reduces chemical usage and environmental impacts, decreases the carbon footprint, and reduces cost of treated water to consumers. These intakes include wells (vertical, angle, and radial type) and galleries, which can be located either on the beach or in the seabed. Subsurface intakes act both as intakes and as part of the pretreatment system by providing filtration and active biological treatment of the raw seawater. Recent investigations of the improvement in water quality made by subsurface intakes show lowering of the silt density index by 75 to 90%, removal of nearly all algae, removal of over 90% of bacteria, reduction in the concentrations of TOC and DOC, and virtual elimination of biopolymers and polysaccharides that cause organic biofouling of membranes. Economic analyses show that overall SWRO operating costs can be reduced by 5 to 30% by using subsurface intake systems. Although capital costs can be slightly to significantly higher compared to open-ocean intake system costs, a preliminary life-cycle cost analysis shows significant cost saving over operating periods of 10 to 30years.

Technical feasibility of using gallery intakes for seawater RO facilities, northern Red Sea coast of Saudi Arabia: the King Abdullah Economic City site

The Kingdom of Saudi Arabia is dependent on desalination of seawater to provide new water supplies for the future. Desalination is expensive and it is very important to reduce the cost and lower the energy consumption. Most seawater reverse osmosis facilities use open-ocean intakes, which require extensive pretreatment processes to remove particulate and biological materials that cause operating problems such as membrane fouling or shutdown during algal blooms. Subsurface systems, using the concept of riverbank filtration, can be used as intakes. These systems include wells of various designs and galleries that provide natural filtration and biological treatment to improve the quality of feed water before it enters the desalination plant. This reduces operating cost, lowers chemical and energy consumption, and reduces environmental impacts. Technical feasibility of gallery-type intakes, beach and seabed types, for use as intakes to seawater reverse osmosis (RO) facilities was evaluated along the northern Red Sea shoreline of Saudi Arabia. The geological characteristics of the offshore ocean bottom were found to be favorable for the development of seabed gallery systems, but the shoreline geology was not adequate for the development of beach gallery intakes. One of the potentially favorable sites for a seabed gallery system was located in the nearshore area at King Abdullah Economic City (KAEC). Detailed investigation of the site hydrology (tides and wave action), sediment grain size characteristics, and sediment hydraulic conductivity, and access for construction were assessed. It was determined that seabed gallery development is favorable at the site. Based on the seawater that has a salinity of about 41,000 mg/L and a conversion rate of 40%, a conservatively designed gallery cell with dimensions of 100 by 50 m would produce about 25,000 m3/day of filtered seawater and seven cells (6 primary and 1 standby) could support a 60,000 m3/day (permeate) seawater RO plant.

Update Evaluation of Under-Ocean Floor Seawater Intake and Discharge

AbstractThe Long Beach (Calif.) Water Department (LBWD) is aggressively investigating seawater desalination as a potential source of potable water. In addition to testing the operational feasibility of various desalting membrane technologies, including the patented two-pass nanofiltration process and traditional seawater reverse osmosis, LBWD is researching an alternative intake and discharge system to address potentially negative environmental impacts associated with open-ocean intakes and disharges. Regulations require a more environmentally friendly process for seawater intakes to mitigate against impingement and entrainment, as well as point-source high-salinity discharges. LBWD research on an under-ocean floor system is aimed at addressing these concerns.