As more and more facilities are turning to zero liquid discharge (ZLD) for wastewater treatment and effluent reduction—whether it’s to comply with local discharge regulations or recycle and reuse as much water as possible in your process—more information is becoming available about the possible issues your facility might face while using ZLD technology. Because of the scope of ZLD systems being used and the difference in treatment depending on your industry and individual process, it’s difficult to pin down what you might be seeing specifically, but in a general sense, these are the five most common problems with zero liquid discharge and possible solutions:

RO membrane fouling

Reverse osmosis (RO) is an integral part of the ZLD process, but when pretreatment isn’t removing harmful solids properly prior to entering the RO phase, it is extremely likely you will see some level of fouling in your RO membranes.

Fouling is an accumulation of deposits in your filtration membranes as a result of contaminants such as:

• Calcium
• Magnesium
• Silica
• Metal oxides
• Bacteria
• And other colloidal and suspended solids

If left untreated, these contaminants can cause deposition severe enough to plug piping and reduce the efficiency of your ZLD process by increasing energy usage and causing damage to your membranes. These issues can even occur in a matter of hours, and cleaning these membranes proves difficult once they’re fouled, so it’s best to attempt avoiding this fouling by ensuring proper pretreatment methods in the first place.

Depending on which contaminants are present, your pretreatment options that avoid RO fouling might include:

• Ultrafiltration (UF). When you choose the right UF technology for your ZLD system, it is effective in reducing RO membrane fouling by allowing more control when it comes to membrane pore size, turbulence and polarization control, and backwash capabilities. Keep in mind that it is often difficult to pin down exactly where smaller particles are coming from, and it’s common to see them make it through UF (which can filter out particles well below 1 micron), especially for plants that draw their process water from surface or recycled water sources. Despite pretreatment efforts, it’s common to have to clean RO membranes every couple of days. A thorough evaluation and treatability study of your wastewater stream is helpful in designating the correct technology for your system and can’t be stressed enough. It is also advisable to have a complete set of membrane replacements on hand in case they’re needed.
• Physical/chemical pretreatment. Chemical pretreatment used in collaboration with ZLD systems can be tricky. Again, it’s extremely important to implement a treatability study to ensure your water treatment engineers understand exactly what contaminants are entering your wastewater stream and where. Many facilities have a difficult time coagulating smaller particles prior to RO due to certain chemicals used in the process that can hinder the effects of certain coagulants when being processed for ZLD. Some effective chemical treatment (depending on your process and contaminants) can include softening, precipitation, coagulation, and adsorption, among others. Where you place your chemical feed is imperative to avoiding RO fouling as well.  

System scaling

Similar to fouling, scaling occurs in the RO membranes and heat-related treatment (such as evaporation) when small particles that get through pretreatment and filtration form deposits, but scaling forms much harder deposits than what you see with fouling. The result is reduced water permeability, greater energy usage, and damaged membranes and equipment.

In order to lower the possibility of scale formation, these scale-forming minerals need to be removed in pretreatment and other parts of the ZLD process they might be present. Many facilities have good results when using cold-lime softening followed by ion exchange technology in pretreatment.  

Alkalinity (pH) can also play a major role on scaling. As pH increases, many scale-forming compounds decrease in solubility, precipitating out in higher rates when the water is heated.

Another factor that can affect how much scale formation takes place is the amount of the scale-causing contaminants present in the water. If the volume of these scale-causing contaminates becomes greater than their natural saturation point due to evaporation, scale is likely to occur regardless off the alkalinity or temperature, so you can see there are several factors that will determine how much scale your system will accumulate.

Treatments to prevent these issues might include pH control, chemical scale inhibitors, and degassifiers.

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Problematic balance of TSS/TDS 

With ZLD, it is important to remove minerals to the right concentration, especially when using evaporation or other thermal-related methods of treatment. Adding heat to solutions with an incorrect ratio of total suspended solids (TSS) to total dissolved solids (TDS) can quickly and easily result in corrosion of piping and fouling and/or scaling. As water evaporates, the solids remaining concentrate. If they are not properly removed in blowdown or prevented in the first place by properly pretreating the wastewater, these solids and hardness begin to build up on heat transfers and other internal piping. This can clog the system and lead to downtime or failure. In general, it’s best to ensure water and makeup water chemistry is being treated properly to avoid this issues in the first place, as once the scale is formed, it’s difficult to remove.

These ratios are of TSS to TDS vary depending on your system, as will the treatment options, but in general, ensuring your level of TSS and TSD to the proper concentrations will ensure a smoother and more efficient process.

Corrosion

During certain parts of the ZLD process, such as crystallization and dewatering, it’s important to be aware of the cause and effects of corrosion. When there are higher salt concentrates in the remaining liquor or brine, it can cause a breakdown in the piping or other mechanical parts of the system. Sometimes spray driers can help, as they can offset the solubility of certain salts, but in general, it’s important to use corrosion-resistant construction materials such as nickel- and titanium-based alloys.

Scale buildup can also cause corrosion and the loss of heat transfer, so again, much of the contamination that can cause corrosion in the thermal parts of the ZLD process need to be addressed based on the individual system. The chemical balance, concentrations, level of heat, and pressure can all contribute to this complex calculation.

Chemical feed

It is generally advisable to minimize chemical use with ZLD technology, as much as possible. Chemical feed on a ZLD system can add another level of complexity to an already complicated process. For example, many chemical treatments for cooling water include certain dispersants to prevent fouling and scaling, but the benefits these chemicals offer the cooling tower water treatment process make it more difficult to effectively use coagulants when they need to be removed via clarification or filtration.

Since wastewater is so complex because of the variety and level of contamination, the use of chemicals in your ZLD process need to be monitored very closely. Even the point of the process at which you introduce these chemicals can have an adverse effect on your process, and there are many facilitates that experience ZLD treatment issues because of this. Certain chemicals can also affect filtration membranes and other construction materials, so make sure to raise this issue when consulting your water treatment specialist.

In conclusion

Turning your facility into a ZLD plant, whether by necessity or choice, can be an extremely complex process with several moving parts that can’t be standardized.

SAMCO has over 40 years’ experience custom-designing and manufacturing these types of systems, so please feel free to reach out to us with your questions. For more information or to get in touch, contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your ZLD wastewater treatment system needs.

You might also be interested in reading more about wastewater treatment and ZLD in the following blog articles:

• What Are the Best Companies for Zero Liquid Discharge Equipment and Technology?
• How Much Will a Zero Liquid Discharge System Cost Your Facility?
• What Is a Wastewater Treatability Study and How Does it Work?
• How Much Does a Wastewater Treatment System Cost? (Pricing, Factors, Etc.)
• The Importance of Wastewater Treatment for Your Facility: Is it Necessary?
• How to Choose the Best Wastewater Treatment System for Your Plant
• How Do You Know If An Industrial Facility Needs a Wastewater Treatment System?