By Nick Nicholas
Tertiary reverse osmosis (RO) wastewater recycling has become a suitable solution to augment water supplies to combat water scarcity across the world. RO desalination in tertiary wastewater processes has proven to provide a cost efficient way to reuse water for both municipalities and industrial organizations.
Experience has shown that tertiary RO wastewater recycling with proper pretreatment in the primary and secondary stages has significantly reduced total dissolved solids, trace heavy metals, bacteria, viruses and other dissolved contaminants.
Current best practices have demonstrated that pretreatment processes such as electrocoagulation, bio-organic flocculants, new low fouling membrane chemistries, appropriately selected antiscalant chemicals and disinfection are critical to prevent membrane scaling and fouling. These processes allows for optimization of membrane recovery rates and membrane flux rates to provide a cost efficient process generating a quality effluent water.
Numerous mid size and large scale plants across the world from Singapore to California, USA and Kuwait provide the evidence that tertiary RO wastewater recycling is an applicable solution. These advanced treatment systems provide both industrial organizations and municipalities looking for a sustainable solution to augment their water supply in these clean water challenged areas.
How does tertiary RO wastewater recycling work?
Well if you’re familiar with the process of osmosis, you know that given a system divided by a water permeable membrane, water will naturally flow from the side with a low concentration of solids, to the side with a higher concentration. This creates an equilibrium within the system.
However, we want to do the opposite of this to get rid of the wastewater contaminants. Therefore, we use another osmosis mechanism: pressure. Divided by a membrane, water will naturally flow from an area of high pressure to one of low pressure, no matter the concentration on either side. So, if pumping our secondary treated wastewater on one side of a membrane and pressurizing it, the water will flow through the membrane to the opposite side, leaving behind all the trace contaminants that we do not want.
Voilá! Treated quality water applicable for both municipal and industrial process applications.
Below, we have listed several pros & cons of tertiary RO wastewater recycling for industrial organizations and municipalities:
High TDS removal
Tertiary RO wastewater recycling is one of the most effective methods of TDS reduction due to the selectivity of these low fouling membranes for contaminant removal.
The compact, modular systems are ideal for applications in communities and industrial organizations. These systems can typically be integrated into preexisting treatment system processes or new installations with relative ease. Additional modular systems can allow for treatment capacity expansion.
Reduction in color, trace tannin, COD, heavy metals
Similar to TDS, other trace dissolved contaminants can be significantly reduced using a tertiary RO wastewater recycling system.
Requires efficient pretreatment
Reverse osmosis is an effective tertiary treatment under the right conditions. The membrane can become clogged with solids or scaled with precipitates if proper pretreatment is not provided in previous wastewater treatment steps.Typically treatment systems that use RO wastewater recycling as a tertiary step have enough prior treatment stages that any foulants are handled before they reach the RO treatment system.
If the membrane is fouled/scaled, treatment flow rate and water quality will be affected.
High power requirement
The constant flow of constant pressure of the system can draw a decent amount of power. However, there is a way to reduce this somewhat. It is possible, through pressure exchanger technology, to recycle the pressure that builds up in the system and using it to pressurize the influent water. This decreases the net requirement for power.
Along with pretreatment, the membrane needs to be carefully maintained in order to maximize efficiency. However, maintenance can be reduced if pretreatments are effective and a potential membrane cleaning cycle added. This also extends the longevity of the membrane, meaning they would need to be replaced less frequently.