Introduction
Seawater reverse osmosis (SWRO) is a promising and sustainable technology for desalination plants in Australia, offering a viable solution to water scarcity. The environmental impacts of SWRO can be closely monitored and managed, ensuring the preservation of natural resources. In particular, the implementation of a SWRO desalination plant in Darwin would necessitate compliance with various industry standards and regulations, 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
Adhering to these standards is essential to ensure safe and high-quality drinking water production while minimizing any adverse effects on the environment. Therefore, understanding and meeting these regulatory requirements are paramount in successfully implementing SWRO desalination plants in Australia.
Environmental Issues and Impacts of Intake Systems
Seawater reverse osmosis (SWRO) industries often utilize open surface intake systems, chosen based on feasibility and profitability. However, these systems present environmental challenges such as impingement and entrainment, which involve the transport of tiny marine organisms into the feedwater. This report delves into the ecological effects of impingement and entrainment, along with mitigation strategies that should be considered.
In contemporary desalination practices, various water intake systems are employed. These include open intakes comprising both open and submerged surfaces and subsurface intakes, such as wells and filtration galleries located beneath the seafloor. Subsurface intakes typically have minimal impacts on marine organisms. The selection of intake systems depends on factors like hydro-geomorphological conditions, costs, plant size, technology, and environmental considerations. SWRO plants commonly utilize surface open-water intake systems.
When opting for a surface intake system, it is imperative to assess seawater quality, as the presence of marine organisms can adversely affect plant operations, leading to membrane failure and seawater flow obstructions. To mitigate these risks, surface intake systems have active screen wire mesh panels, preventing tiny organisms or particles from entering the feedwater. These screens come in various shapes and sizes, typically 9.5mm to 13mm.
While desalination plants have tried to prevent marine animals from passing through their processes, they face challenges in effectively mitigating the impact on the environment and aquatic life. The primary environmental issues encountered by desalination industries are impingement and entrainment, which are predominantly associated with open intake systems (Kress, 2019).
Impingement involves larger marine animals, such as fish and crabs, trapped by intake screen mesh with a size limit typically exceeding 14mm. The seawater velocity carries these organisms through intake pipes until they are captured by fish screens, often resulting in injury or death. Entrainment, on the other hand, involves the passive transport of smaller organisms, including fish eggs, larvae, and juveniles, through intake pipes and into the desalination plant, where they are removed from their natural habitat (Petersen et al., 2018; Missimer & Maliva, 2018). The primary environmental impact of impingement and entrainment is the reduction in the survival rate of fish and other marine organisms (Missimer & Maliva, 2018).
During the design phase of a desalination plant, measures to mitigate environmental impact must be considered. One effective strategy is subsurface intake systems, although this may only be feasible in some environmental conditions and can incur high costs, particularly for large-scale desalination plants (Kress, 2019). Other mitigation measures for open-intake systems include reducing feedwater velocity to 0.15m/s, installing initial barriers such as nets to reduce the number of organisms reaching the intake water, and implementing a bypass system to return impinged animals to their habitat (Kress, 2019). However, mitigating entrainment organisms proves more challenging, as reducing the screen mesh size can lead to complications such as coagulation issues and decreased feedwater supply.
While subsurface intakes offer a primary solution to mitigate impingement and entrainment, they may only sometimes be practical due to cost constraints. As such, other control measures, such as reducing inlet velocity and returning impinged animals to their environment via bypass systems, should also be considered to minimize the environmental impact of desalination operations.
Environmental issues and impacts on the outfalls
Environmental concerns regarding the discharge of a desalination plant primarily revolve around the impact on dissolved oxygen levels, temperature, and salinity of the brine discharge, as well as their effects on marine life, ecosystems, and water quality. The diffusion of brine discharge into seawater is influenced by tidal averages, wind direction, and wave heights (Jenkins & Wasyl, 2005). Increased influence of these factors leads to reduced risk areas in the ocean due to rapid dispersion from turbulent forces. However, a larger diffusion area results in more ocean areas being exposed to lower risk (Danoun, 2007).
The continuous brine discharge can elevate salinity levels in the surrounding seawater over time (Roberts et al., 2010). Elevated salinity levels can disrupt marine ecosystems by altering the habitats of various species. Reverse osmosis desalination plants can significantly increase salinity levels in brine discharge compared to seawater intake, impacting marine environments (Tularam & Ilahee, 2007). Although temperature levels are not as profoundly affected by this filtration method, the average temperature of brine discharge tends to be approximately 60% higher than seawater intake (Ahmed & Anwar, 2012).
In the case of a proposed desalination plant in Darwin, the average annual temperature around the brine disposal port could reach 44°C, leading to thermal pollution (Wahab, 2007). The location of the brine discharge point directly affects the seawater temperature. Ambient temperature changes can positively and negatively affect marine life, reproduction rates, population growth, larvae development time, and maturity rates (Danoun, 2007). As temperature rises, there is an inverse relationship between salinity surge and depleted dissolved oxygen levels in seawater (Ahmed & Anwar, 2012).
Reduced dissolved oxygen levels in seawater from brine discharge at the outlet can threaten marine life. This decrease can stem from elevated temperatures at the discharge point, oxygen-consuming chemicals used in the RO plant, or increased biomass from entrainment (Lattemann & Hopner, 2008). Despite acknowledging the short-term impacts of brine discharge, there remains a significant gap in understanding the long-term effects of waste products from SWRO plants on marine ecosystems. Further research is warranted to inform the future design of sustainable desalination plants.
SWRO plants utilize various chemicals throughout their processes, which may ultimately be discharged with the brine. These chemicals and entrained biotic debris are released into the cooling waters from the power plant, both at the shoreline and further offshore through outfalls (Kress et al., 2018). Coagulants like iron or aluminum salts, polymers, toxic biocides such as chlorine, and anti-scalants are employed to prevent membrane fouling during pre-treatment. Additionally, detergents and acidic and alkaline solutions are used for RO membrane cleaning, while lime water necessitates pH and hardness adjustors to ensure product quality (Kress et al., 2018).
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Dealing particles like coagulant ferric chloride can increase water turbidity and discoloration, reducing light penetration and primary productivity (RPS Environment and Planning Pty Ltd, 2009). Benthic communities may also be affected by coagulants and anti-scalants, reducing the diversity and composition of bacterial and eukaryotic communities (Thomas et al., 2018). While anti-scalants aid in pH maintenance to prevent carbonate scale formation, their toxicity to aquatic life is generally low (RPS Environment and Planning Pty Ltd, 2009). Nonetheless, minor pH fluctuations resulting from seawater buffering may impact organisms like coral reefs and algae symbiotes, which are sensitive to ocean acidification (Australian Government, 2020).
Chlorine, recognized as a potent oxidant and biocide, can pose environmental hazards even at diluted concentrations. It is commonly employed in desalination and power plants to prevent fouling. Conversely, chlorine oxidation safeguards membranes from residual chlorine damage in RO plants. When chlorine reacts with seawater, it forms toxic complexes with bromide and nitrogen-containing organic constituents, resulting in chlorine discharge (Kress et al., 2018). Studies have shown that marine organisms accumulate bromoform or dibromochloromethane in their liver, which can lead to liver and kidney cancer with prolonged exposure (Agency for Toxic Substances and Disease Registry, 2005).
Cleaning solutions like Sodium metabisulphite, used for RO membrane cleaning, can induce acidification and hypoxia, causing mortality in marine species even at low concentrations. Toxicity bioassays have demonstrated mortality at concentrations equal to or exceeding 50 ppm, particularly impacting soft-bottom fish species (Thomas et al., 2018).
SWRO plants contribute to environmental chemical escalation through inadequately diluted brine discharge and compounding pollutant accumulation in confined areas (Thomas et al., 2018). However, pre-treatment chemicals are applied to enhance RO plant performance.
Furthermore, the negative impact of metal discharge from plant corrosion warrants discussion. Copper, nickel, iron, chromium, and molybdenum are the leading metal elements susceptible to corrosion. While copper naturally occurs in the environment and its discharge's adverse effects are disputed, excess exposure may render it toxic (RPS Environment and Planning Pty Ltd, 2009).
Heavy metals can accumulate in marine sediment and tissues, causing alterations in aquatic life if released into the ocean in excessive amounts. However, SWRO plants are unlikely to discharge heavy metals as they are typically constructed of corrosion-resistant stainless steel. Additionally, RO processes typically remove low levels of metals like iron, nickel, and molybdenum (Lenntech, 2020). Although the concentration of metal disposals is relatively tiny and impacts the ecosystem significantly, preventing heavy metal corrosion in brines remains a challenge (Lattemann et al., 2003).
Recommendation
Advanced desalination technologies offer a promising avenue for reducing the reliance on added chemicals. Microfiltration (0.1–10 µm) and ultrafiltration (0.1–0.01 µm) can potentially replace chemical coagulation in the pre-treatment process, while bio-flocculation may substitute chemical coagulants. Residues and significant components can be disposed of on land and isolated using appropriate methods (Kress et al., 2018). Additionally, minimizing seawater intake volume by enhancing processing plant efficiency is imperative. Optimization techniques and strategies are crucial for reducing impingement and entrainment and mitigating hypersalinity stress through improved diluted brine discharge or salt extraction for reuse. Moreover, the future establishment of environmentally friendly offshore desalination plants in less sensitive areas is recommended.
