Zero Liquid Discharge (ZLD) Wastewater Treatment
What is Zero Liquid Discharge (ZLD)?
Zero Liquid Discharge (ZLD) describes a wastewater treatment that completely eliminates liquid discharge from a system. The goal of any well-designed ZLD system is to minimize the volume of wastewater that requires further treatment, process wastewater in an economically feasible manner, while also producing a clean stream suitable for reuse elsewhere in the facility.
Interest in ZLD water treatment technology has grown in the industrial manufacturing sector over the past decade. Companies may begin to explore ZLD because of ever tightening wastewater disposal regulations, company mandated green initiatives, public perception of industrial impact on the environment, or concern over the quality and quantity of the water supply.
The equipment needed to achieve ZLD varies depending on the characteristics of the wastewater as well as the wastewater volume. Typical waste streams in an industrial setting include wastewater treatment reject typically from reverse osmosis (RO) or ion exchange, cooling tower blow down, spent coolants, DI regenerant, metal finishing wastewaters, tank or equipment washing wastewaters, and other miscellaneous industrial wastewaters such as compressor condensate and floor scrubber wash waters. The first step to achieving ZLD is to look for ways to limit the amount of wastewater that needs to be treated. Reducing the amount of wastewater almost always provides quick payback. For example, pre treating the water going to a cooling tower to reduce hardness and silica that can increase the amount of time the water can be used before it becomes spent.
Once wastewater generation is minimized and the volume of wastewater that needs to be treated is known, you can then explore what equipment is needed.
A traditional approach to ZLD is to use some sort of filtration technology, funnel the reject waters to an evaporator, and send the evaporator concentrate to a crystallizer or spray dryer.
The downside to this is that the equipment to de-water the concentrate slurry tends to be very large and can be extremely expensive, which limits the cost effectiveness to only those with very large waste streams.
ZLD Technology Using Evaporation
In a typical layout for the ENCON's ZLD solution, wastewater is fed to the ENCON MVC Evaporator followed by dewatering of the MVC concentrate by an ENCON Thermal Evaporator and a centrifuge.
In the first stage, an MVC Evaporator is fed the wastewater and it yields distilled water and concentrate. The concentrate goes to a concentrate storage tank, which separates into slurry and supernatant layers. The supernatant layer gets pumped to an ENCON Thermal Evaporator, which further concentrates the liquid while exhausting water vapor to atmosphere. The concentrated slurry from the thermal evaporator is pumped back into the concentrate storage tank. The slurry layer from the tank is pumped into a centrifuge.
The ENCON approach is unique because it uses cost effective, time-tested equipment that makes it economically feasible for even moderately sized waste streams.
At the core of the ENCON zero liquid discharge solution is evaporation technology. Evaporation always figures prominently in most ZLD solutions because it has always been more “hands off” than other wastewater treatment methodologies resulting in a dramatically lower labor cost. Evaporation technology can handle a much wider range of waste streams compared to membranes and traditional physical / chemical treatment methodologies. Finally, evaporation does a much better job of concentrating waste streams compared to other methods, thereby yielding a lower cost for disposal.
Two types of evaporation technology are used in the ENCON ZLD solution. First, at a typical operating cost $0.01 - $0.02 per gallon of distillate, the ENCON Mechanical Vapor Compression (MVC) Evaporator is also very energy efficient and yields a high quality distillate. Models are available that can process 40 to 4,000 gallons of distillate per hour. More information on the ENCON MVC evaporator can be found here in our Mechanical Vapor Compression section. Second, the ENCON Thermal Evaporator can be used with a variety of heat sources, such as natural gas, propane, #2 fuel oil, waste oil, steam, and electricity. Capacities range from 8 to 400 gallons per hour. More information on ENCON Thermal Evaporators can be found here in our thermal evaporator system section.
Achieving Zero Liquid Discharge Over Time
The ENCON approach to Zero Liquid Discharge is that it can be implemented in stages over time. For example, a company may choose to install an ENCON MVC Evaporator to minimize their wastewater hauling expenses. A year or so later, they find that feeding an ENCON Thermal Evaporator from the MVC’s concentrate holding tank to further reduce the volume of liquid waste makes financial and environmental sense. When the company finally decides to become a Zero Liquid Discharge facility, it is a simple matter of adding a filter press and accessories to the current wastewater process.