Sea Water Reverse Osmosis As A Technology For Desalination Plants In Australia

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I. Introduction

Seawater reverse osmosis (SWRO) is a promising, sustainable technology for desalination plants in Australia, as the environmental impacts of the process can be carefully monitored and managed. Treated water for consumption from a SWRO desalination plant in Darwin, would be required to meet many industry standards including but not limited to;

  • The Australian Drinking Water Guidelines (2006)
  • WHO Guidelines for Drinking-water Quality (2017)
  • Northern Territory (NT) Water Act 1992
  • Northern Territory (NT) Waste Management and Pollution Control Act 1998

II. Environmental issues and impacts for the intakes

Seawater reverse osmosis industries usually operate with open surface intake systems depending on the location as it seems to be more feasible and profitable. Unfortunately, this type of systems reflects on some environmental issues known as impingement and entrainment. Impingement and entrainment refer to small living-marine organisms that are transported to feedwater. The report discusses in detail impingement and entrainment environmental effects and mitigation that should be considered.

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Nowadays, many desalination plants operate with water intake systems (Kress, 2019). There are two types of intakes, the first one being open intakes consisting of open surfaces and submerged surfaces; the second one being subsurface intakes including wells and filtration galleries that are located below the seafloor and generally doesn’t have significant effects on living marine organisms (Kress, 2019). The location of open water intakes can be nearshore that supplies surface water or offshore that are located away from the seawater surface and withdraw feedwater from 2-6m above the seafloor (Kress, 2019); while submerged and subsurface intakes are located offshore only. The selection of the intake system depends on the hydro-geomorphological conditions, costs, plant size, technology and environmental impacts. (Kress, 2019). Seawater reverse osmosis (SWRO) plants frequently use surface open water intake systems.

When selecting a surface intake system, the seawater quality must be evaluated due to the living organisms as it may be reflected along within the plant process operations, resulting in membrane failure and obstructions of seawater flow (Kress, 2019). Surface intake systems are generally supplied with active screen wire mesh panels to prevent small organisms or particles travelling to the feedwater; they are found in different shapes and sizes varying from 9.5mm to 13mm (Kress, 2019).

Although desalination plants have implemented techniques to avoid marine animals passing through the process, they haven’t been able to mitigate the problem satisfactorily as they still impact the environment and marine life. The primary environmental problem that desalination industries are encountering is the impingement and entrainment, which are usually associated with open intake systems only (Kress, 2019).

Impingement refers to adult’s sea animals such as fish, crabs and in general any more prominent size than 14mm to be retained by the intake screen mesh. These organisms are dragged by the seawater velocity flowing through intake pipes until fish screens trap them causing injury or death. Entrainment is the transport into the desalination plant of smaller organisms known as planktonic, including fish eggs, larva and juveniles that are dragged by intake pipes and are entirely passive by the screen mesh system (Petersen, Frank, Paytan, & Bar-Zeev, 2018) (Missimer & Maliva, 2018), and get removed from their habitat. The main environmental impact is due to the reduction of the mortality rate of fish and living marine organisms (Missimer & Maliva, 2018).

During the design of a desalination plant, mitigations for environmental impact need considering. One of the most effective strategies to mitigate the problem is to use subsurface systems; however, this may not be feasible for all environmental conditions, it is also expensive and may not deliver enough feedwater for a large size desalination plant (Kress, 2019). Other mitigations for open-intake systems that are not limited to; include a reduction of feedwater velocity to 0.15m/s, establishing initial barriers such as nets to diminish the number of organisms reaching the intake water and lastly introducing a bypass system to return the living impinged animals back to their habitat (Kress, 2019). Generally, entrainment organisms are difficult to mitigate, therefore, if a smaller screen mesh is introduced, other complications can reflect upon it, such as coagulation issues and reduction of feedwater supply.

Desalination industries use open intake systems, unfortunately, issues due impingement and entrainment of small marine organisms reflect along the process. The report outlined a primary solution which is using a different type of intake system known as subsurface intakes. However, it can incur high operational costs. In order to mitigate the problem some other control measures have been studied, suggesting a decrease of inlet velocity and bypassing living animals back to the environment

III. Environmental issues and impacts for the outfalls

Key environmental concerns regarding the discharge of the desalination plant include the increase in dissolved oxygen levels, temperature and salinity of the brine discharge and the overall effect it has on marine life, surrounding ecosystems and water quality. The three key features that directly affect the diffusion of brine discharge into seawater include tidal averages, wind direction and wave heights (Jenkins & Wasyl, 2005). When any of these elements have a large influence, the area of risk within the ocean is decreased due to swift dispersion from turbulent forces. However, with a larger area of diffusion, there will consequently be an increase in the area of ocean exposed to low risk (Danoun, 2007).

The extensive discharge of brine over time can cause an increase in the concentration of salinity in the seawater around the point of discharge (Roberts, Johnston, & Knott, 2010). It is widely understood that changes to natural salinity levels can impact marine ecosystems by directly affecting the areas in which certain species congregate. Reverse osmosis (RO) desalination plants can double the salinity levels in the brine discharge in comparison to the seawater used. This is a significant impact on the brine discharge, in contrast to the temperature levels, which is not nearly affected as strongly with this filtration method (Tularam & Ilahee, 2007).

The average temperature of brine discharge is on average 60% higher than the seawater that enters the desalination facility (Ahmed & Anwar, 2012). For the desalination plant proposed in Darwin, this would result in an average annual temperature of 44°C around the brine disposal port, resulting in thermal pollution. It is explored that there is a direct connection between the varying temperature of the seawater and the location of the brine discharge point (Wahab, 2007). A change in ambient temperature of an ecosystem can subsequently impact marine life in both a negative and positive way. It can increase reproduction rates and population growth, decrease the time during larvae development and reduce the time taken to reach maturity in other species (Danoun, 2007). As the temperature increases, there is an inversely proportional relationship between the surge of salinity and depleted dissolved oxygen levels in the seawater (Ahmed & Anwar, 2012).

Decreased dissolved oxygen levels in seawater due to brine discharge at the outlet, can cause harm to marine life. A decrease in dissolved oxygen levels can occur from an increase in temperature at the brine discharge point, or through oxygen-consuming chemicals utilised within the RO plant or from an increase in biomass from entrainment (Lattemann & Hopner, 2008). There is a limitation on previous literatures in respect to this matter. Despite the aforementioned short-term effects of brine discharge in the ocean, there is a considerable lack of information regarding the long-term effects of the waste product from SWRO plants on marine ecosystems. It is recommended that further research be undertaken to understand these effects in order to further design sustainable desalination plants for the future.

SWRO plants use chemicals at various stages that might eventually be disposed of with the brine. These chemicals and the entrained biotic debris are released by cooling waters from the power plant at the shoreline and further offshore through outfalls, either by itself or mixed with other discharges (Kress N., Galil B., 2018). At the pre-treatment stage, coagulants such as iron or aluminium salts and polymers are added. Toxic biocides like chlorine, neutralisers (sodium sulphite) and anti-scalants such as polyphosphates, poly-phosphonates, polyacrylic acid, poly-maleic acid are also used to prevent fouling of the membranes. Detergents, acidic and alkaline solutions are further applied as cleaning solutions for RO membranes. The lime water requires additional pH and hardness adjustors to achieve consumable products (Kress N., Galil B., 2018).

The discharge of particles like coagulant ferric chloride increases water turbidity and water discoloration, and decreases light penetration, which results in a reduction of primary productivity (RPS Environment and Planning Pty Ltd, 2009). Benthic communities are also affected by coagulants and anti-scalants. Past studies confirmed that coagulants and polyphosphate-based anti-scalants reduce the diversity and compositions of bacterial and eukaryotic communities (Thomas M. M., Robert G. M., 2018). Antiscalant chemicals such as sulphuric acid, polyacrylic acid and polymeric acid assist in maintaining the pH level that inhibits carbonate scale formation. However, the toxicity of anti-scalants to aquatic life is generally low (RPS Environment and Planning Pty Ltd, 2009). Due to natural buffering of seawater, changes in pH level might have a minor impact to the standard marine life. However, organism groups like coral reefs and algae symbiotes are easily threaten by the minor increase in ocean acidification (Australian Government, 2020).

Chlorine is known to be a strong oxidant and an effective biocide that can be toxic to the environment even in diluted concentrations. The chemical compound is usually added in both desalination and power plants to prevent fouling. On the contrary, RO plants apply chlorine oxidation to prevent damage to the membranes from chlorine residual. When reacting with seawater, toxic complexes with bromide and nitrogen-containing organic seawater constituents are formed with the chlorine discharge (Kress N., Galil B., 2018). Experiments revealed that bromoform or dibromochloromethane is accumulated in the liver of marine organisms and long term intake can cause liver and kidney cancer (Agency for Toxic Substances and Disease Registry, 2005).

Cleaning solutions for RO membranes such as Sodium metabisulphite results in acidification and hypoxia even when marine species are in short-term contact to low concentrations. Toxicity bioassays indicated that mortality can occur at concentrations equal to or higher than 50 ppm. Deaths of soft bottom fish species are to be seen in broader areas (Thomas M. M.r, Robert G. M., 2018).

In general, SWRO plants contribute to the escalation of chemicals discharged in the environment with inadequately diluted brine and further accumulate the number of pollutants in enclosed areas (Thomas M. M., Robert G. M., 2018). On the contrary, pre-treatment chemicals are applied for the improvement of RO plant performance.

Last but not least, the negative influence of metals discharge from corrosion of the plant are to be discussed. Some of the main metal elements of the manifold corrosion materials are copper, nickel, iron, chromium and molybdenum. Studies have found that copper is present naturally in the environment and the discharge of copper is doubted to have an adverse effect. For most organisms, copper is an important micronutrient and should only become toxic if overexposed (RPS Environment and Planning Pty Ltd, 2009).

Heavy metals can accumulate in sediments and tissues of the marine communities, redistribute metals and cause changes in the aquatic life if excessed amounts were to release into the ocean. However, SWRO plant is not likely to dispose heavy metals as they are potentially constructed of corrosion resistance stainless steel. Treatments in RO process usually remove low level of metals such as iron, nickel and molybdenum (Lenntech, 2020). Overall, the concentration of these metal disposals are relatively small to negatively impact the ecosystem. On the other hand, there are nothing that can be done to entirely prevent the corrosion of heavy metals add to the brines (Lattemann S., & Hopner T., 2003).

IV. Recommendation

Advanced desalination technologies further support in reducing the amount of chemicals added. In particularly the use of micro filtration (0.1–10 µm) and ultra-filtration (0.1–0.01 µm) that can replace the chemical coagulation in the pre-treatment process and bio-flocculation to replace chemical coagulants. Residues and significant components can be disposed on land to be isolated by suitable methods (Kress N., Galil B., 2018). Seawater intake volume could also be minimized by improving the processing plant efficiency. Accordingly, optimization techniques and strategies are essential for the reduction of impingement and entrainment as well as to decrease hyper-salinity stress by better diluted brine discharge or salt extraction for reusing. Moreover, environmental friendly offshore desalination plants at areas that are less sensitive than nearshore habitats are recommended in near future.

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