A question from a customer: We are a zinc-chloride plater and flow 15,000 gallons per day through a membrane system. We struggle with keeping the membranes running properly. We average between 36 and 48 hours of run time before we must stop and clean the membranes. What can we do to improve the run time?
Wastewater Specialist: “That’s a great question, and in my personal experience as a MF membrane wastewater operator for over 15 years, I have found that the best way to improve MF process run time is to adapt and execute a full offensive approach with a focus on the upstream production processes.”
This offensive approach would include the evaluation of all negative impact factors that are being introduced into your waste stream. The goal is to remove these components from the waste stream to the best of our ability before they are introduced to the filters. These negative impact factors would include:
- Organic components and or other surfactants are usually found in heavily chelated cleaners;
- Brightener/wetting agents;
- Oil-based production processes.
If this offensive approach is not an option for you — and for any number of different reasons that may be out of the operator’s control — we can also achieve improved run time by focusing on three main MF cleaning processes. The three main cleaning processes include a freshwater push rinse, cleaning solution soak, and cleaning solution circulation.
Freshwater Push Rinse
A freshwater push rinse is a process of pushing freshwater through the filters — and each train — from front to back. Most of these systems have a freshwater feed already designed into the system for this reason. If you do not have this option, it can also be manually pumped through the system from a separate freshwater tank. This will allow most of the heavily concentrated solution sitting in the filters to be pushed from front to back and through each train. This process will only take a few minutes from start to finish, and its benefits could potentially help work to fight fouling and or promote more consistent and efficient cleaning results
Cleaning Solution Soak
The first step in the actual cleaning process is the cleaning solution soak, which is designed to attack and help break up the concentrated solution that has built up on the membrane surface. A successful soak for many hours will help ensure a proper circulation process that will follow. As a zinc plater, I would start with a 5% to 8% hydrochloric acid solution that would be pumped through the filters until each train was completely full. I would recommend a soak time of 8 to 12 hours based on the daily flow of 15,000 GPD. If possible, it is also a good idea to drain the spent hydrochloric acid solution and replace it with fresh HCL if time allows after a freshwater push rinse is again executed.
Cleaning Solution Circulation
The next step is the cleaning solution circulation process that allows you to circulate fresh cleaning solution that is feeding from the clean holding tank through the membrane filters inside to out under a small amount of desired pressure by adjusting the drain values slightly. Do not exceed the MF vendor’s specifications. If this circulation process is executed correctly and the proper equipment is available, the circulation can run automatically and safely for extended amounts of time and free up the operator to do other things. If time allows — and you are able to do so — it is always beneficial to drain the spent cleaning solution and conduct a freshwater push rinse before replacing it with a fresh cleaning solution.
Once both the initial acid circulation and soak processes have been completed, I would then recommend specifically targeting other components that are common in the waste stream, such as other metals or organic components. It is also typical for plating facilities to follow the HCL solution with other acids such as sulfuric to ensure that other trace metals do not build up in the filters.
I would again recommend multiple freshwater push rinses to flush out the system if it is possible. I have learned that the extra push rinses also help in the removal of foreign material that may not have been visible to the naked eye during processing. Once you have determined the acidic portion of the cleaning process is completed, I would then safely and properly repeat both the soak and circulation processes with cleaning receipts such as peroxide or bleach to target the organics surfactants that have built up inside the filters from the plating processes additives. The organic cleaning is just as important as the metal cleanings; if the organic and metal concentrations have not been properly and completely cleaned out, they will assist in further fouling of the filters and a reduction in discharge flow and run time.
I have found that in the world of wastewater, a one-size-fits-all approach is never successful. Every MF system is different in its design and capabilities; therefore, the above recommendations will need to be tailored to fit your specific situation. The implementation of these techniques and good practices will help promote improved MF process run times while reducing the number of cleaning cycles per week.
As seen in Finishing & Coating