By combining desalination with wastewater recycling, energy consumption can be reduced by 67 percent



"While drinking water reuse and desalination have traditionally been considered separate parts of the water supply mix, it makes sense to consider how we can mix wastewater treatment and desalination to meet water and energy goals while ensuring environmental standards are met," said Childress, a professor in the Department of Civil and Environmental Engineering at Sunne-Astani.

To understand this opportunity, one must consider the current context. First, the salinity of wastewater is increasing, in part due to water conservation. This higher salinity water is more expensive to treat and may require desalination. Advances in wastewater treatment facilities mean that desalination processes, such as reverse osmosis, which filters contaminated water through semi-permeable materials to make it clean, can help treat high-salinity streams relatively efficiently.

Use existing water pressure

Salinity in wastewater is increasing due to water conservation and other reasons. For example, in coastal areas, seawater can invade the infrastructure of wastewater recycling facilities and also increase salinity. The immediate effect of increased salinity is that you may need to run existing desalination processes at higher pressures, or you may need to introduce new desalination processes to treat water.

Traditionally, higher salinity streams have been a lower priority water resource because of how energy-intensive it is to desalinate such streams and purify the water to meet environmental and regulatory standards. However, if existing desalination processes can be modified or new ones added, high-salinity streams utilizing desalination capacity become more viable streams to meet water supply needs.

There are technologies that can be added to the facility. These technologies include: energy recovery devices (ERDs), which harness the energy from the brine output during desalination and apply it to the newly treated water flow, and closed-circuit reverse osmosis (CCRO), which maintains pressure in the system rather than releasing it into the resulting brine. This helps to reduce the additional salt burden without adding an additional energy burden.

Energy management strategies for water recovery

The discharge of saltwater is regulated by certain standards, which means that the salinity of the discharge stream must be below certain levels, most likely similar to the salinity of sea water, i.e. 35 grams per litre. Initially, Wei, a doctoral student, focused on the mixing of streams from different water sources from the perspective of meeting regulatory standards for salinity concentrations in streams. Recently, however, she has recalibrated her research to consider different angles.

"Think about it the other way around, if we can meet the requirements by using wastewater in a potable water reuse way, rather than just mixing wastewater streams and discharging it into the ocean, can we reuse it and take water resources so that we have this additional water supply?" Childress said.

In advanced water purification facilities, the use of reverse osmosis membranes - which apply pressure to move water through a semi-permeable material while filtering out contaminants - to purify water has become the industry standard, providing an opportunity to treat high-salinity wastewater streams.

High energy costs in the water sector have led many water and wastewater treatment facilities to include energy management strategies. For example, energy recovery units are often used in conjunction with high-salinity reverse osmosis processes to reduce the energy consumption of the desalination process, the researchers said.

Energy recovery devices reduce energy consumption by transferring the remaining pressure from the (already treated) concentrated brine stream to the intake stream. The researchers say the energy recovery device can reduce the energy consumption of seawater reverse osmosis desalination facilities by up to 67 percent, depending on operating conditions.

Advanced water purification facilities are considering high-recovery reverse osmosis processes, such as closed-circuit reverse osmosis, to improve water recovery while maintaining low energy consumption. Membrane processes with saltier influents require higher pressure (or energy). In a conventional reverse osmosis process, the pressure is fixed at a high level that can overcome the final pressure of the concentrate. In closed-circuit reverse osmosis, the pressure is gradually increased so that it is just above the required pressure. Using time-varying feed pressures, closed-circuit reverse osmosis may provide greater energy savings than energy recovery devices. Another benefit of closed-circuit reverse osmosis is that it can discharge less water.

Childress says we're trying to achieve flexibility in water treatment - assessing differences in water quality and using different approaches to treat that particular stream for maximum efficiency and minimum waste.

The future of water

As drought caused by climate change continues to threaten traditional water sources, considerations of how to deal with water resources flexibly and sustainably are becoming increasingly important.

Childress says we're not creating a new technology or a new process, but looking for synergies that might exist in coordinating adjacent facilities - something that's not currently being done.

Looking ahead, looking at water differently could be the key to using it as efficiently as possible. Cities like Los Angeles have begun to adopt an initiative called "One Water," which aims to treat all of the city's water resources as one entity and strive to manage them in a more environmentally friendly, economical, and socially beneficial way.

Instead of treating water as rainwater and wastewater and seawater, how about saying it's all the water that needs to be treated? Then we can look at our systems and assess what we need to achieve that goal. The ultimate goal for coastal cities like Los Angeles is to close the water cycle - not send water into the ocean, but identify every valuable resource in the discharge stream and find ways to reuse it. Right now, it's too expensive to do that, but hopefully that's where we're headed.