Introduction
Industries such as refineries, power plants, and chemical plants use evaporative cooling via cooling towers for temperature control. Excess heat is transferred to a coolant to protect equipment and maintain optimum process temperatures. A commonly used type of cooling tower is an induced-draft tower. The hot water is sprayed through nozzles at the top of the tower and flows through fill media to maximize the contact area with cool air in either a counter-current or crossflow arrangement. The descending water cools and is collected at the bottom of the tower before being reused in the cooling process. This recycled water stream, known as cooling tower blowdown, becomes part of the facility’s industrial wastewater and must be properly treated before discharge or reuse. Membrane filtration technologies such as VSEP can be used to treat cooling tower blowdown, removing dissolved solids and contaminants to enable recycling or environmentally compliant disposal. Without a cooling tower and recycling of the coolant, a constant cool water source would be required and the volume required would not be feasible in most situations.
A common source of coolant is ground water. Over time, evaporation will increase concentrations of mineral content in the source water and promote bacterial growth. Buildup of dissolved solids, such as calcium, barium, and chloride can cause mineral scaling and reduce efficacy of cooling and lead to frequent maintenance issues. Chemical additives help but cannot provide a complete solution. To maintain efficiency and increase cycles of reuse, a portion of the water is purged when the mineral concentration helps but a set conductivity. The purged liquid is referred to as ‘blowdown’. Fresh water called “makeup water” is added to the process loop to replace the blowdown and any additional water loss due to evaporation and drift.
Wastewater Management
The cost of hauling wastewater for offsite disposal can be expensive and, in some cases, can be environmentally irresponsible. Because the blowdown contains high concentration of TDS, it must be treated to meet discharge permit limits.
Rather than discharging the blowdown, a more favorable option is to treat it for reuse. While recycling cooling tower blowdown will improve sustainability metrics anywhere, plants located in areas facing water scarcity challenges can derive even more immediate benefits from reuse.
Treatment Options
The challenge in treating cooling tower blowdown is the solids. Treatment options for solids disposal such as crystallizers require a large amount of thermal energy, a large footprint, and expensive corrosion-resistant materials. Evaporation ponds must be lined and may not be feasible in locations with limited space and low net evaporation rates; furthermore, no water is reclaimed in the process.
Wastewater can be collected and treated at offsite facilities when no onsite treatment is available. While hauling requires no upfront capital, it can be much more expensive over time. Hauling costs can also fluctuate wildly depending on the cost of fuel. Surface water discharge is an option but must Discharging to surface water can lead to large fines when the quality is not within permit limits.
Reverse Osmosis membranes are used to separate dissolved ions and produce a high quality permeate. Typical membranes require pre-treatment such as TSS removal, softening, antioxidants, and pH adjustment. The clean permeate generated can be reused, but sometimes multiple stages of filtration are often required to meet quality standards. The recovery of the RO stage can be 45-70% for conventional crossflow membranes. Because traditional RO membrane systems are not designed to tolerate extremely high solids, the recovery of clean water is limited by membrane fouling.
There are many other combinations of treatment methods to achieve the goal of a Zero Liquid Discharge (ZLD) process. Each approach will have unique requirements and treatment strategies.
VSEP Solution
In contrast to the conventional technologies, New Logic Research has developed a unique membrane system called VSEP (Vibratory Sheared Enhanced Process). VSEP uses membranes to achieve the desired separation, but the membrane stack is oscillated above a torsion spring that moves the stack back and forth approximately½inchat a rate of 54 times per second. This motion is analogous to the agitator of a washing machine but occurs at a much higher speed.
The oscillation produces a shear at the membrane surface of about 150,000 inverse seconds (equivalent to over 200 G’s of force), which is approximately ten times the shear rate of the best conventional crossflow systems. More importantly, the shear in a VSEP System is focused at the membrane surface where it is cost-effective and most useful in preventing fouling, while the bulk fluid between the membrane disks moves very little. Membranes in the range of microfiltration to reverse osmosis can be installed on the VSEP and have higher flux and recovery compared to conventional cross flow membranes.
Cooling tower blowdown can be treated in a single stage of VSEP RO and achieve recoveries of 75-90%. VSEP is able to concentrate beyond solubility limits of compounds and operates at economical flux rates due to the benefits of high shear. Similar pre-treatment chemicals used for conventional RO systems such as antioxidants and pH adjustment may be required depending on the feed quality, but softening or particulate removal is not required. The permeate from the system can be used for makeup water, boiler feed, or it could be discharged. The concentrate can be sent to disposal wells, hauled offsite, or further treatment in an on-site ZLD system.
It is common to have multiple sources of wastewater in a single plant. Owing to its versatility, a single VSEP system is often called upon to separate disparate wastewater streams such spiral RO reject, boiler blowdown, and storm drain runoff, thus increasing its operational value tremendously.
The VSEP system is modular and can be used for a wide variety of process flow rates. Modularity also allows for convenient expansion. Every system is custom-built and engineered by New Logic Research to meet each client’s needs. Systems are available fully automated with clean-in-place systems and chemical dosing skids. Full-scale VSEP cooling tower blowdown recovery systems are operating around the world at coal, biomass, gas-fired power plants and gas production plants among others.
For example, a gas production plant treats cooling tower blowdown with VSEP at 5,000 BPD from 2 different towers. The blowdown is collected and processed continuously in alternating tanks. The blowdown is processed 24hours/day with a single module taken offline for cleaning when required. Rudimentary pretreatment required for this process includes carbon filters for membrane oxidation protection along with anti-scalant and pH adjustment for maximum flux and minimum cleaning frequency. The system produces permeate that is blended with other waste streams to improve quality before discharge.
Summary
As regulations become more stringent, the need for environmental responsibility is becoming a top priority. VSEP is able to recover water for reuse and reduce water consumption and reduce blowdown volume for treatment and/or disposal. New Logic Research strives to provide the highest level of support at every level of testing and through each phase of engineering to provide an optimum system design. Contact a New Logic Research representative to learn more and to develop an economic analysis for your project.
For more information about how V✧SEP can help you, please contact us at info@vsep.com.
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