The latest water treatment, along with careful planning and design of intake and outfall, can minimize environmental impacts
With water scarcity an ongoing problem around the world, attention has focused on desalination, a process that can turn abundant seawater into drinking water. Yet desalination has issues that have suppressed many projects.
Initially, the process’s high energy requirements were an obstacle to adoption, but, as larger plants have been considered to meet growing demand, concerns over marine ecosystem impacts at plant intake and outfall have come to the fore. However, innovations are mitigating the possible impacts.
Seawater Intake
As water is drawn from the sea to be desalinated, the main impacts on marine life are impingement and entrainment (I&E). Impingement is when aquatic animals like fish or crabs are sucked against the intake screen mesh and can’t escape. Entrainment is when smaller organisms make it through the screen. Much of seawater intake regulation in the United States concerns I&E.
Another term, entrapment, is used in the context of offshore intakes that convey water through a long pipeline or tunnel with a screen at the onshore end. Open sea intakes typically use coarse bar screens followed by fine screens that keep most marine life from entering. The fine screens may be stationary or moving.
Seven Seas’ Trinidad team has nearly completed one of our environmental stewardship projects, adding a screen to the submerged seawater reverse osmosis intake at one of our desalination plants.
At Seven Seas Water Group, we perform environmental impact studies to prudently design intake pipelines to protect and preserve marine life and the environment. At one of our desalination plants that provides drinking water in Port Fortin, Trinidad, a team is working with Keith Downer, a Seven Seas engineer, on a net to cover the submerged raw water intake. The additional barrier between the ocean and the existing low-velocity pumps should stop impingement and entrainment of fish, endangered turtles, and other sea life.
In contrast to open-sea intakes, intake wells and infiltration galleries prefilter out most sea life through the natural subterranean separation barrier of seabed and coastal aquifer sediment.
Brine Discharge
As salt is removed from seawater, a hypersaline solution called brine is rejected. Brine can contain harmful chemicals from the pretreatment stage, antifouling agents, heavy metals, organics, chlorine, and acids. It also can be discharged at temperatures too high for the surrounding ecosystem. Some common methods of disposal are:
- Surface water discharge.
- Deep-well injection.
- Sewage discharge.
- Evaporation ponds.
- Land application.
Discharge into the ocean, however, is the most common method for plants near the shore. When heavier brine is discharged into the ocean without sufficient diffusion, it sinks and flows across the seabed, harming benthic (sea bottom) ecosystems.
Adequate subsurface diffusers or risers placed strategically according to current and flow modeling, however, can go a long way toward keeping sea life out of harm’s way. Environmental impact studies during the location and design process also can show ways of letting marine life and desalination coexist successfully.
Monitoring programs can help plant operators adjust brine discharges to spare sensitive ecosystems. In Trinidad, for instance, the Seven Seas plant has earned high praise for its low-impact siting and monitoring program.
Innovations in Environmental Protection
At Seven Seas, we only use non-harmful, National Science Foundation-approved chemicals that are approved for drinking water throughout our processes, ensuring that discharges are non-toxic and safe for the environment. Our wastewater treatment solutions remove harmful bacteria and chemicals in wastewater streams, preventing pollution of natural resources.
Technology is still evolving to improve water recovery and reduce brine volume with a goal of nearly waste-free zero liquid discharge desalination, which crystalizes salt and chemicals. In fact, innovative methods of recovering valuable resources, including sodium hydroxide and hydrochloric acid, from brine is helping to incentivize marine ecosystem protection at outfall.
As desalination is becoming increasingly necessary, risk to marine ecosystems can be minimized with thoughtful design and planning, and a procession of innovations is improving mitigation strategies. Contact Seven Seas for more information. We have long experience with desalination and are looking forward to a future in which sea life and desalination can thrive side by side.
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