Zero liquid discharge (ZLD) is a water treatment process that results in absolutely no water byproduct. Instead, solids are produced that can be processed either as hazardous or non-hazardous waste. This can be accomplished using techniques like solidification and evaporation, where the liquids are crystallized or evaporated. Such treatment techniques are gaining popularity with industrial facilities who face more stringent effluent limitation guidelines (ELGs) on the wastewater that they discharge from their facility. This is a growing trend, especially for power generation plants.
ELGs at power generation plants
When coal is burned to make energy, a gas is produced that contains sulfur dioxide. The gas is released into the atmosphere via the flue. Before the gas is released, however, it is cleaned using a scrubber system that includes a water solution. This cleaning process is called Flue Gas Desulfurization or FGD, and the wastewater from the FGD scrubber produces what is called FGD wastewater. The EPA has ELGs specifically for FGD wastewater. One of the main reasons for these rules is the concentration of Selenium found in FGD wastewater. While small amounts of Selenium are actually needed by wildlife for better health, higher concentrations can be detrimental to wildlife and even fatal. The levels of Selenium typically found in water surrounding and downstream of power plants are very high, which is dangerous to wildlife such as fish and birds who eat from that water source. In birds, for example, too much Selenium can lead to weaker eggshell, resulting in a lower birth rate. All power generation plants have to meet the ELG for Selenium that is mandated by the EPA. Moving to a ZLD water treatment process allows these facilities to meet ELGs by avoiding discharge altogether. This is the only 100% guarantee that their discharge permits will be met now and in the future.
Reasons to consider zero liquid discharge
ELGs do change and that requires the manufacturer to act or to face the consequences of not meeting the guidelines. More often, ELGs become stricter over time, and the manufacturing facility ends up needing to purchase additional water treatment equipment and chemicals or to replace their wastewater treatment system altogether. Adding or replacing water treatment equipment often means adding personnel to operate it and capital money to purchase it. The additional water treatment costs also contribute to higher operating costs overall.
A zero liquid discharge system eliminates the need to react to changing ELGs because there is no longer any wastewater that has to meet the limits. These systems can be used as a stand-alone treatment following wastewater treatment, or they can be used in conjunction with an industrial water reuse system. Combining water reuse and ZLDmeans that not only will you never have any wastewater to discharge, but you still have the added benefit of cost savings on water purchases. What’s more is that with the right pretreatment process, the solid waste from the ZLD system can be disposed in a landfill as non-hazardous waste.
The reality is that your facility will find itself in a situation where the ELGs for your industry are changing, and you need to find a way to meet them. You can take your chances and invest in additional equipment to support a treat and discharge process. It is likely, however, that you will have to modify your treatment process again in a number of years to keep up with EPA mandates. On the other hand, you can invest capital funds into a ZLD and water reuse process now so that you never have to discharge again.