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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 nonhazardous 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 production, resulting in 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 ZLD means 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 most facilities will find themselves in a situation where the ELGs for its industry are changing, and you will 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.  

ProChem strives to help their customers establish the highest level of credibility and a positive reputation within the regulatory community. Their goal is to significantly reduce the amount of fresh water that manufacturers require by providing sustainable solutions that will also benefit the customer’s bottom line.

When running an industrial wastewater treatment system, the floc that forms can sometimes float. If your treatment system contains floating floc, you know it can be a challenge to eliminate. The floc actually contains the contaminants that are in your wastewater, so if it isn't removed before the wastewater is discharged, you may be out of compliance with your discharge permit. There are many things that cause floating floc. Some of the more common causes are described below, along with troubleshooting tips that you can use to help you determine what may be causing floating floc in your system.

Water flow that is too high

When the water flow rate is at or near the maximum flow rate that your clarifier was designed to handle, floc does not have time to settle the clarifier like it should. This can cause the floc to float. If you have recently increased the flow rate, try decreasing it to see if the floc settles. If you have not increased the flow rate recently, you should investigate other causes.

Oxidants introduced into the waste stream

Oxidants like hydrogen peroxide (H2O2) can cause floc to float. You can conduct a jar test to determine whether or not an oxidant is causing the floating floc. When you do so, a look for the floc to be at the top of the beaker rather than the bottom. Sometimes it can take as long as 15 to 30 minutes for floc to float. If you confirm that oxidants are causing the floc to float, you can add a reducing agent such as bisulfate or a proprietary metal precipitant to your waste stream. You should consult your wastewater treatment services vendor about the best procedures for adding reducing agents to your waste treatment program.

Floc that is too light and too large

In some cases, floc can become too fluffy and lightweight during treatment. There are several options for correcting this. You can try using a heavier coagulant, changing to a more compatible polymer, or altering how the polymer is dosed. Changing the polymer dosing method can be as simple as altering the dost rate. You can also change the polymer concentrations that are being used. For example, if you use an emulsion or dry polymer at 0.3% by volume, try 0.15% by volume instead. You may also want to consider changing where the polymer is dosed within the water treatment process. Sometimes dosing in the last reaction tank in addition to the clarifier can prevent floating floc.

Polymer overdose

Overdosing polymer can also lead to floating floc that will not settle. If your floc is very small and light (often called pin-floc), that is an indication that your polymer is being overdosed. You can conduct a jar test to determine the optimal polymer dose for your wastewater.

Biological outgassing

As anaerobic bacteria grow, they will outgas or release oxygen. This causes bubbles to rise to the surface of the water and for floc (or sediment) to float in large chunks, even when the system isn't running. If you see bubbles and larger chunks of floc, biological outgassing is likely the culprit. To correct this problem, you will want to first flush out the clarifier with an air sparger, and then you can treat it with biocide. The biocide will inhibit the bacteria growth. If your wastewater treatment system has floating floc, it is always a good idea to consult your wastewater treatment service provider before attempting to correct it yourself. They can help you find a solution to the problem quickly so you can avoid discharge compliance violations.  

ProChem strives to help their customers establish the highest level of credibility and a positive reputation within the regulatory community. Their goal is to significantly reduce the amount of fresh water that manufacturers require by providing sustainable solutions that will also benefit the customer’s bottom line.

Whoever said that "oil and water don't mix" never treated treated industrial wastewater. Oils can become mixed in wastewater, and the way it's treated varies based on the oil and water mixture. Most local POTWs have Fats Oil and Grease (FOG) discharge limits in place, so manufacturers who have oil in their water have to address it before discharging. Oil has two basic forms in wastewater:

  • Non-emulsified: Oil that floats on the surface of the water and is more easily removed before wastewater treatment.
  • Emulsified: Oil that was subjected to chemical or mechanical action and dispersed into the water. It does not float on the surface and requires more sophisticated techniques for removal before wastewater treatment.

Treating non-emulsified oil

Non-emulsified oil can be removed from water by skimming or an oil water separator. Skimming can be done at the source by "skimming off" the oil over a tank weir. An oil water separator can be as simple as feeding the water into a storage tank with an exit pipe placed a short distance below the top of the tank. This allows the oil to be decanted from water. There are many kinds of oil water separators. Each has their own idiosyncrasies but all accomplish the same goal. Some of these are:

  • Tube skimmers
  • Belt skimmers
  • Disc skimmers

Treating emulsified oil

Emulsified oil can be removed by decantation and skimming as well, as long as it is separated from the water first. This usually requires chemical processing, depending on the types of oils. Here are a few methods for separating oil from water using chemical processing:

  • Lower the pH to the acidic side, and add metal-based coagulant.
  • Raise the pH to neutral/alkaline, and add metal-based coagulant.
  • Add a chemical de-emulsifier.
  • Use any of the preceding without metal-based coagulant.

Following the chemical processing, the water can be treated using an oil water separator method (as with non-emulsified oil). Emulsified oil can also be treated without chemically pre-treating and without use of oil water separator, by using physical/filtration methods such as:

  • Membrane filtration
  • Activated carbon
  • Clay based materials
  • Evaporation/distillation

In all of these instances, the separated oil should be evaluated for moisture content to determine if it can be reused. If it can be reused, there are many companies that will actually pay you for this resource and haul it off at their expense. Once the majority of the oil is removed, it can then be chemically treated to remove other metals and solids by using a process like this:

  1. Lower the pH to 3.5 (if not already there).
  2. Dose with organic coagulant.
  3. Raise the pH to 9.0 - 10.0.
  4. Add an ionic polymer to produce floc.

In some cases, emulsified oil might be present in low enough concentrations that pretreating it with a separator is unnecessary. Other times, pretreatment may not be rigorous enough to remove all emulsified oil. In both cases, it may be possible to utilize an organic coagulant to remove oil during chemical treatment for removal of metals and solids. Such a process looks like this:

  1. Lower the pH to 3.5 (if not already there).
  2. Dose with inorganic coagulant.
  3. Raise the pH to 9.0. - 10.0.
  4. Dose with organic coagulant.
  5. Add an ionic polymer to produce floc.

Water and oil do mix, and treating water containing oil can be accomplished using various methods, depending upon the form, type, and concentration of oil. Contact ProChem by clicking Contact Us to help you determine the best treatment method for your wastewater.  

ProChem strives to help their customers establish the highest level of credibility and a positive reputation within the regulatory community. Their goal is to significantly reduce the amount of fresh water that manufacturers require by providing sustainable solutions that will also benefit the customer’s bottom line.