Concentrate Management for Reverse Osmosis
Desalination is becoming an increasingly important water technology as naturally occurring freshwater supplies dwindle. With shifting rainfall patterns and surface water sources at capacity, other sources of drinking water are required. Low salinity groundwater, the traditional alternative to surface water, is becoming increasingly saline through overextraction and the ongoing effects of climate change. Saline groundwater above the palatability limit (600-1000 ppm) requires treatment before it can be used as a source of drinking water. Additionally, coastal communities that have long relied on rainwater have seen prolonged drought conditions and storm surges that overwhelm rainwater harvesting systems. The only alternative water source for many of these communities is seawater.
Reverse Osmosis is the cheapest and most energy efficient method for producing potable water from brackish (>1,000 ppm) and seawater sources (>32,000 ppm). Reverse Osmosis works by overcoming the natural osmotic pressure of salty water by applying external hydraulic pressure. This pressure forces clean water through a semi permeable membrane leaving the salt behind. Reverse Osmosis technology is mature, widely adopted, readily scalable and can be powered using renewable energy.
One of the drawbacks of Reverse Osmosis is that while it produces a stream of low salinity water for consumption, the remaining water (that does not transit through the membranes) exits the system with increased salinity (as water is removed but not salt). So a Reverse Osmosis system will produce two streams of water – the potable water for drinking and the reject stream with salinity higher than the feedwater. Reverse Osmosis design must include not only the production of the potable water stream but also a plan for the disposal of the reject stream (also known as concentrate or brine).
Coastal Concentrate Management
Concentrate management, much like Reverse Osmosis design itself, is location dependent. Seawater systems by necessity dispose of the reject back in the ocean. Low recovery rate (percentage of the feedwater converted into potable water) seawater systems can dispose of the concentrate directly back to the ocean as the concentrate salinity is not much higher than the salinity of the ocean. These systems operate with recovery rates as low as 15-20%. Low recovery rate systems also have the advantage of not requiring chemical dosage on the front end to protect the membranes from salt precipitation (scaling).
Higher recovery seawater systems, such as municipal scale seawater plants, can operate at recovery rates up to 55%. These systems require feedwater chemical dosing to prevent membrane scaling due to the high recovery rate. As the concentrate stream is higher salinity with high recovery rate systems (compared with low recovery rate systems), this can pose a risk to marine life such as seagrass. High recovery rate seawater reverse osmosis systems use brine diffusers to release the concentrate underwater at different locations to minimize the disruption to the local ecosystem and spread the reject across a large area. Other locations will mix the brine with seawater prior to discharge to reduce the salinity.
High volume throughput seawater reverse osmosis plants also have issues with low oxygen levels and elevated temperatures in the brine stream. Low oxygen levels can suffocate marine life and elevated temperatures causes stress for marine organisms. Disposal mitigation designs must be carefully considered for a waste stream containing reduced oxygen and elevated chemical concentration, salinity and temperature to minimize the effect on the marine environment.
Inland Brine Disposal
Inland Reverse Osmosis systems have limited options available for concentrate management as disposal via the oceans is not an option. The main options are:
- Surface water disposal
- Sewer disposal
- Deep well injection/aquifer reinjection
- Evaporation basins
- Brine crystallization
- Silviculture
Surface water disposal is similiar to ocean discharge. However instead of to the ocean, the concentrate is discharged into rivers, creeks and streams. Although inland brackish water Reverse Osmosis systems have lower feedwater salinity than seawater systems, discharge into surface water sources will depend on local regulations and the local environment. Sewer disposal represents a good option (if available). The salinity of the concentrate is reduced by mixing with treated effluent prior to discharge.
Remote Inland Reject Disposal
For remote inland applications, the two most common methods are deep well or aquifer reinjection and evaporations basins. In concentrate injection, the concentrate is pumped back underground into a separate aquifer or into the same aquifer a specific distance from where the feedwater was drawn from. Extensive geotechnical data is required for this solution as well as a modelling of groundwater recharge rates. Evaporation basins use solar irradiation to evaporate the concentrate, leaving a salty residue which can be disposed of in landfill. One issue with evaporation basins are the engineering and regulations required to design them. There is also a risk of uncontrolled releases when the capacity of the basin is overwhelmed by extreme weather events.
Brine crystallization is a similar method to evaporation basins. Instead of using solar irradiation to evaporate the water, evaporation is achieved using thermal desalination. As with evaporation basins, brine crystallizers will leave a salty residue requiring disposal. The main drawback of brine crystallizers is that they are extremely energy intensive. However brine crystallisers are often used for inland locations where there are no other options (i.e. remote gas fields).
Circular Solutions
Silviculture, or the irrigation of salt tolerant plants, is an excellent method for concentrate disposal. With this method, a crop of salt tolerant plants like Saliconia (saltbush) is irrigated using the concentrate. Salt tolerant plants can be an excellent source of minerals for livestock. However salt tolerant plants have an upper salt limit (around 10,000 ppm). If the concentrate salinity is above this level, an alternative concentrate management method will be required.
Each Reverse Osmosis system will have a range of concentrate management options depending on location, feedwater salinity and concentrate volume. Moerk Water are experts at designing reverse osmosis systems, including concentrate management, for all locations and industries. Contact Moerk Water today to discuss your unique desalination requirement.
