Reverse osmosis or RO is one of the most effective water filtration processes. However, one major downside of RO systems is that they generate a significant amount of wastewater.
For every 1 unit of purified water produced, 2 units of wastewater is generated. Dealing with this wastewater can be problematic both environmentally and economically.
However, there are some practical solutions we can implement to reduce the wastewater generated in our RO systems. In this blog post, we will cover 6 effective solutions to minimize wastewater from reverse osmosis systems.
Also, read our blog post Comprehensive Guide about Reverse Osmosis
How to Reduce Wastewater in RO System - 6 Effective Solutions
Here are six effective solutions to reduce wastewater in reverse osmosis systems.
Solution 1 - Improve Membrane Efficiency
The main culprit behind wastewater generation is the efficiency of the reverse osmosis membrane. Older and lower-quality membranes allow more water to pass through without undergoing reversal osmosis, thereby increasing wastewater volumes.
Using a high-quality thinner film composite (TFC) membrane can significantly boost water production rates and lower wastewater.
Some RO membrane manufacturers are continuously working on membrane innovation to offer even higher rejection rates while maintaining good flow rates.
Membranes with nominal rejections of 99% for total dissolved solids can reduce wastewater by 10-15% compared to older 95% rejection membranes. It's a good idea to periodically replace aging membranes every 3-5 years to maintain optimal production efficiency.
Another option is using a membrane with a slightly higher salt passage than the nominal rejection rating. For example, a membrane rated at 95% rejection may actually achieve 93-94% rejection in practice.
This small compromise in product water purity allows 5-7% more water to be harvested before exceeding the salt concentration threshold. The RO system runs slightly longer before diverting water to drain, lowering wastewater.
Solution 2 - Increase Inlet Pressure
Keep in mind that, the higher the inlet pressure, the more complete the osmotic reversal and the higher the purified water recovery rate. Most residential RO systems work between 50-80 psi but operating at the higher end of this pressure range can save wastewater.
Every 5 psi increase in operating pressure could potentially save 5-7% more water from becoming waste. However, higher pressures require a more robust pump and components rated for those pressures.
It may also lower membrane life expectancy over time. The ideal approach is to run the system at the maximum pressure possible without disturbing component integrity and product water quality.
Solution 3 - Control Permeate Flow
The product water flow rate is determined by a flow restrictor that regulates outlet pressure. A lower permeate flow allows more time for osmosis to take place, increasing water recovery.
Many RO systems are set to deliver 2.5-3 GPM permeate flow which is satisfactory for most uses. However, during non-peak hours when high flows aren't needed, the permeate flow can be reduced temporarily.
Even decreasing flow by 0.5 GPM for certain intervals can cumulatively lower wastewater volumes over time. Automatic controls can be installed to activate this reduction during non-usage periods.
Of course, this has to be balanced against the need to meet minimum daily water demands from the system. Targeting a recovery boost of just 2-3% through fewer hours of operation at reduced flows still makes a difference.
Solution 4 - Reject Recovery Systems
Rejecting over 50% of feed water as waste is wasteful, especially in areas facing water scarcity. Reject recovery systems provide options to partly reclaim this wastewater before discharge. A simple method is running the reject stream through a lower-pressure re-filtration unit that extracts some "wastewater" from the rejects.
Some systems like the Clack CW3000 add a second reverse osmosis stage to process reject water from the first pass membrane. Rejecting rejects a second time can harvest 15-25% more water, significantly lowering true wastewater volumes.
More advanced systems add an energy recovery device or pressure exchanger that transfers pressure from the reject stream to boost inlet pressure in the first stage. It is said, the reject recovery can help save water by 10 to 15%.
Solution 5 - Use RO Water for Non-Potable Purposes
Not all uses in the home require ultra-purified drinking water. RO rejects contain mainly salt and can still be suitable for many non-potable applications after an appropriate level of polishing filtration. Uses like watering outdoor plants/lawns, washing cars, rinsing floors, etc. do not strictly need potable quality water.
By setting up RO reject water plumbing routes to outdoor taps, approximately 30% of the reject stream could potentially find alternative non-potable uses. This minimizes true wastewater that needs disposal or additional post-treatment.
Proper signage & identification are a must to avoid any confusion over waterline uses. Non-potable reuse requires careful implementation but offers great wastewater reduction potential.
Solution 6 - Improve Pretreatment
Poorly pretreated water causes scaling, fouling, and membrane degradation over time, reducing permeate production rates. It also requires more frequent cleaning and replacements, increasing wastewater from cleaning activities.
Investing in an efficient 5-stage sediment and carbon filter system ensures feed water meets RO system requirements. Automatic filters that backwash during non-usage hours further remove particles that cause issues. Pretreating with phosphate-based anti-scalants inhibits scaling on membranes.
Maintaining proper prefilters through timely changes prevents unnecessary pressure losses from clogging. All of this results in longer-lasting membranes that sustain optimal recoveries and flow rates and minimize wastewater generation.
Proper cleaning and sanitization are also critical. Periodic chemical cleans remove accumulated biofilms, organic matter and deposits that compromise RO performance if left unchecked.
Cleaning as per manufacturer recommendations keeps the system running at maximum efficiency and uptime over its lifetime. Less frequent and more thorough cleaning translates to fewer chemicals wasted during maintenance.
Conclusion
While wastewater is an inherent part of the reverse osmosis process, conscious efforts can significantly reduce its volumes through optimal system operation and component selection.
The 6 solutions discussed above - improving membranes, increasing pressures, controlling permeate flows, adding reject streams, using RO water non-portably, and enhancing pretreatment - when implemented together, have the potential to lower true wastewater by 30-50%.