Removing metals from wastewater is a major concern of almost any metal finishing operation in order to meet local and state regulations.
Robin DealTim Pennington, Editor-in-Chief of FinishingandCoating.com, spoke with wastewater treatment expert Robin Deal, product leader for Aquapure brand for Hubbard-Hall, on best practices for running metal finishing operation’s system.
What would a perfect ideal wastewater treatment system look like from front to end?
RD: So, for a metal finishing or a plating shop, it’s probably a little bit more complicated than a city or a POTW in that we have metals that are very toxic to our environment and can kill the environment. In fact, in 2021, a facility in Indiana released cyanide-bearing wastewater to their local stream, and there was a rather large fish kill as a result of it. The pump dosing of the cyanide destruction system failed them and resulted in the accidental release. So, depending on what your metal finishing operations look like, you may have to have a cyanide destruction system,d you may have to have a hexavalent chrome reduction system, and those two processes are done independently of a main wastewater treatment system. So, cyanide destruction is accomplished through alkaline chlorination. That is a two-stage process where you increase your pH to 10.5 or higher with hydroxide, then you introduce a hypochlorite to a positive 550 or higher ORP or millivolt reading, and you allow the process to occur that will destroy the cyanide. Hexavalent chrome reduction works at the other end of the pH scale. You have to make sure your pH is at 2.5 or lower, and you introduce a reducing agent such as bylfite until you see a positive 250 ORP reading. So those two systems work independently of the main wastewater system. When their processes are complete, they generally co-mingle back into the main wastewater stream. Does that make sense so far?
What is used to remove the metals?
RD: So, when you remove metals, the most common process is a process called hydroxide precipitation, where you increase your pH with the sodium hydroxide, and then you introduce a flocculant, or a polymer, layman term. In wastewater, we call it snot because it’s extremely slimy. And then you allow the solids to settle out, and your clear supernate goes out as you’re effluent. So that’s the easiest process to do. Unfortunately, it’s not always that easy with plating water or some cleaning applications because there’s a thing called chelation and chelating agents that are introduced on the manufacturing side to assist those processes. So, there’s a very extensive list. There are numerous chelating agents out there in the world, but some of the strongest ones that we see in wastewater are citric acid, EDTA, and pyrophosphate, even though on a chemical engineering side or chemical side, pyrophosphate is considered a sequestering agent, not a chelating agent. There’s a difference between those two. But when it comes to wastewater, if you have pyrophosphate and it touches nickel, it will not release nickel. So that’s why we call it a chelating agent. So, when you have chelates present in your waste stream, and you have metals present in your waste stream, the chelates lock onto the metals, and they will not let them precipitate out. So, then you introduce a coagulant, and coagulants can be iron, aluminum, calcium, or now we’re seeing rare earth blends as well. Typical coagulants are just a metal salt. So, you introduce those. Depending on the metal affinity to the base of the coagulant, you may find that if you have high chrome in your wastewater, you want an iron-based coagulant, whereas if you have nickel in your wastewater, you want an aluminum-based coagulant. So, you introduce your coagulant, and you adjust your pH up to the desired hydroxide curve chart reading; different metals have different optimal PHs, then you introduce your flocculant. But again, some chelates that’s not all you need. So if you find that introducing a coagulant along with the pH adjustment doesn’t work, you will have to use what’s called a metal precipitant, and again, there’s a whole list of those. The most common are DTC, sulfide, and thiocarbonates, or a blend of those. And how that work is when you introduce your coagulant, and you adjust your pH up, you introduce the metal precipitant while you’re watching your ORP or your millivolts, and when the ORP or millivolts drops 50 points, you know you’ve introduced enough of your metal precipitant.
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Some shops still get in trouble. So, where are they missing the point? Where are they having issues along that way that you’re talking about? Where do you see it mostly that the issues bring up?
RD: It’s actually a combination. So, wastewater in a lean manufacturing environment is probably last on the list of where money is spent. Wastewater treatment is never going to make the facility money. It may break even with things like sludge recycling, water recycling, and things like that, but it’s never going to make plant money. And manufacturing plants are not in the wastewater treatment business, so it’s thought of last.
So there are a lot of moving points, and it can get confusing. And I think just a lot of people don’t understand what some of these regulations and guidelines are about.
RD: So, from an understanding level, you have two types of discharge permits that you could be issued. You can be issued an NPDES permit, which means you are directly discharged back to the environment, or you can be issued a pre-treatment permit. So, the NPDES permits, we don’t see a lot of those in our industry, but they’re written by the EPA, and they have really stringent limits because they’re the last line of protection for the environment. And there’s another thing called a sewage usage ordinance. They can be the same, or they can be separate, and those are issued by local authorities such as your city or your county; and some cases, the city will issue the pretreat permit, and the county will issue the sewage usage ordinance, and they won’t match up. I always tell my customers to err on the side of caution and go with the least amount that they can discharge. So, what happens is when you are looking at your permit, even on your permit, you have a monthly average and a daily average, and they’re different. So always err and try to target your wastewater to be less than whichever one is those least amounts because as long as you’re meeting that, you will be good. Then, when you look at your overall system, what I’ve seen is a lot of industries will introduce new processes and new production lines. Their production will ramp up tremendously, but they don’t. When wastewater systems are overwhelmed, we have what’s called pass-through. If you’re putting more water in the system than the system was designed to handle, it’s not going to be able to remove all the contaminants, and they’re going to go out the door. Introducing new processes in manufacturing, you always need to take into account what your system is rated to handle and how you’re going to help it handle these new influences. Then you also have to remember, if you go from flowing 15 gallons an hour to flowing 30 gallons an hour, your system is set up to handle that, so you’re handling it fine, but your chemistry is set up to handle treating a certain amount of water, and if you don’t upgrade your dosing to match the new flow, you’re actually underdosing what the water flow is. A problem example was we had worked with a company a number of years ago that was originally flowing 15,000 gallons a day over the course of 16 hours, and they were flowing that amount because that’s all that their wastewater system could handle using the chemistry that they were using at the time. So, we worked with them; we got them compliant with their discharge permit because they were not compliant. They were releasing chrome and copper to the local city authority. So we got them compliant with that, and during the course of doing that, they were able to work with me to make some upgrades to their system so that it could flow more water in an hour. And in doing that, we doubled their flow from 15,000 gallons a day to 30,000 gallons a day, and by allowing them to do that, they were able to double their manufacturing processes.
We’ve talked about heavy metals and medical particles; what are those, and what are the requirements to get rid of those from the wastewater system? Things like chrome, nickel, zinc, cadmium, and lead?
RD: So how you remove those, you have two options. The most common is hydroxide precipitation. There’s another option chemically that’s called sulfide precipitation, so you introduce a sulfide such as sodium sulfide to your waste stream at a much lower pH, and it works similar to hydroxide precipitation in that you introduce the sulfide, and you keep the pH at a lower level. Why we don’t see that as much in our wastewater treatment industry is because sulfides smell really, really bad, and they will also damage the environment. Specifically, if you go below 6.5 pH and sulfides are in the water, you’re at risk of producing hydrogen sulfide gas, which kills people, and it’s also extremely corrosive. That’s the two chemical ways you can remove metals. You can also remove metals through things like ion exchange. Metal removal can also occur with membrane filtration, such as a nano filter or a microfilter. You can remove the water from the metals through evaporation, which dehydrates the metals, and now you’re just left with a dried metal sludge that you have to dispose of, and you don’t discharge water.
How do the shops know if these are actually working effectively for them? Is it just by seeing the levels that they’re sending out, or how do they actually know that they’re using it and using it correctly?
RD: We always recommend bench testing, whether it’s independently done by the operators or you bring an expert in, such as myself or one of my team members. Bench testing gives us a snapshot. So we’ll take a thousand mil sample of your wastewater before anything is done to it, and we will treat it in a beaker and do everything that is happening in your wastewater system in that one beaker, and then we can test the supernate off of that beaker to see if we’re removing the metals properly or if there are other contaminants such as maybe you have oil and grease in your wastewater. Are we removing that? Maybe there’s a dye in your wastewater, and your water’s pink from an anodizing process, and the city doesn’t want your pink water. So, bench testing is how you see the overall health of your wastewater system. It’s how you determine if you’re dosing your chemistry correctly, then you scale that up. If you look at a bench test, and a bench test shows that you need a mil per gallon of your coagulant, and you need your pH to go to nine, and you need two mils per gallon of your flocculant, then you go to your wastewater system itself, and you check your feed rates on your pumps. Is your pump pumping at a mil per gallon of a flow rate of 20 gallons per minute? So, then you should be pumping 20 mils per minute. So, you check that and make sure that’s what’s happening; then you look at your pH. Are your pH probes clean and properly calibrated and reading accurately? Is your pH control set to maintain that pH of nine? If that’s correct, then you move down to your flocculant. Is your flocculant made up properly at 0.2%? Is it dosing right? Are you maybe overdosing it, in which case you might see flocc floating in your clarifier? Are you underdosing it, in which case you don’t see floc at all? Everything you do in a bench test, you then correlate to your actual wastewater system, walk your wastewater system and make sure that what’s happening is what’s supposed to happen.
What are two or three really good traits you’ve seen that shops do or best practices two or three to really keep their wastewater system in good shape so that maybe they could think about adopting it?
RD: First is making sure you have someone dedicated to wastewater. In most cases, for shops, it’s not. In most cases, it’s a maintenance guy or the maintenance department, or maybe it’s the plating line manager, and their focus is on keeping the manufacturing running. And the reason why is a lot of times, wastewater operators look like they’re not doing anything. I’m going, to be honest. When you have a dedicated wastewater operator, their whole job is ensuring that the water that leaves your facility, your effluent, meets every set guideline. The training that they receive is not just for wastewater treatment. They also receive first responder and spill response treatment training. So they know how to handle if your plating line and your hexavalent chrome spills and hits wastewater. They know what to do and how to correct that problem, so you don’t have a pass-through event. They receive maintenance training, so they know how to rebuild pumps, and they receive plumbing training so they know how to pipe properly and how pipes should flow and how to repair leaky pipes. They receive IT training, so they know how to handle the PLC or SCATA unit that your system may be operating under. And if 90% of the time they look like they’re not really doing anything, you can trust and believe they are. They’re going through a checklist in their head. They’re listening to the pumps to make sure that they’re pumping properly because you can tell the difference when a pump doesn’t work. They’re smelling the air to make sure that the air smells the way it normally does because if there’s a spill of ammonia, you can definitely tell it in your wastewater. And a best practice is to have an operator dedicated to wastewater but give him a checklist for the wastewater system to check every hour, check to make sure the floc is forming, check to make sure the pH is right, check to make sure the chemicals are feeding right, that there are chemicals to last. And then you also give him another list of things to do, such as cleaning drums and storing them properly for disposal, helping move products around, and things like that.
What would be one more that you would recommend? Is it investing in equipment? Investing in technology?
RD: I’m going to actually give you two. One is definitely working with a chemical supply partner who fully understands wastewater and who’s not just trying to sell you a commodity chemical at 10 cents a pound that’s going to produce 50 cubic yards of sludge a month that you have to dispose of. You want to work with a partner who’s going to help you reduce your sludge, reduce your chemical usage, and help you improve your wastewater processes. And you have to trust them enough that you believe them when they say, “You need a new tank,” “You need a larger clarifier,” or “You need to expand your filter press.” So, working with a partner like that is always good. And then I’m going to say the last thing is understanding why wastewater works with your manufacturing processes. And what I mean by that is we had one customer in eight years that I’ve been with Hubbard-Hall, I’ve only seen one customer do this, but I really wish all of our customers would. This customer knew that for every gallon of wastewater that left his plant, he produced 10 finished parts. And off of those 10 finished parts, after he paid all the bills for manufacturing them and what he paid to treat that wastewater to leave his plant, he knew how much profit he made.
