Ramping up water recycling in metal finishing
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Jar testing provides valuable information about processing wastewater. Chemistry, dosages, sludge volumes, and operating costs can all be determined with some certainty with proper testing done up front.
Purpose-built for recycling, this 35-gpm RO system is overbuilt in terms of membrane count, pump sizing, plumbing, control and pretreatment because of high salt loading and feed stream variation. Redundant pumps (foreground) provide recycled water back to the plant.
Water recycling system at a plant in Reynosa, Mexico currently provides 35 gpm of pure water after receiving approximately 27 gpm in process wastewater. The difference is drag-out and evaporation. City water is used for make-up and is run through a potable water filtration system before entering the recycle loop.
The last few years have seen some huge changes in the world of manufacturing. Things that never used to be significant have become critical parts of running a profitable business.
Water is no exception. Compliance costs, and the cost of non-compliance, have gone through the roof as regulators have combined fund raising with the concept that a hefty penalty is a good incentive for better behavior. The same is true of rates for water and sewer services: Municipal services have become semi-for-profit enterprises supporting the rapid expansion of growing cities. We can’t blame them—it’s our city too, and we know that infrastructure has to be built before the growth can be completed—but then again, stretching resources is everyone’s responsibility, and if we can cut our costs in the process, so much the better.
So it goes that recycling of water in the metal finishing shop has finally gotten to the mainstream. Technology and the numbers have converged on known solutions. Many companies exist with the capability to provide the solutions. In the last year the ROIC for water recycling on a two-shift finishing operation has come under 12 months in many cases.
Like many relatively new technologies, water recycling has been the subject of a great deal of hype and expectations. So what’s reasonable? Well, it’s a fact that water can be absolutely recovered as pure H2O, with almost no contamination. But the cost of that level of recovery is so high as to make it completely unrealistic in almost all cases. The effective, efficient level of recovery in a given application will depend on many variables such as: Cost of incoming water; sewer cost; electricity cost; gas cost if the water is heated, or if heat is recovered; skill within the staff operating the systems; capital cost of the equipment; operating cost; recovery percentage; disposal cost for sludge and wastewater.
The goal of this article is to give the reader both a general idea of what can be done and specific information on how to evaluate ideas and proposals for water recycling. The educated consumer, with proper expectations and a good idea of what he wants, is much more likely to be successful.
It doesn’t necessarily go without saying that all the variables listed earlier (and more) must be considered in deciding what level of water recovery is efficient and what specifications and goals should be set for a project. The unreasonable promises and expectations of managers on the front end of a project can destroy the perception of even a successful project. So it’s important to specify early on what the actual expectations for the recycling project are.
Knowing the exact quality requirements for water at each step in your process can make a huge financial difference in a recycling application. For example, if half the washer doesn’t require perfect water, then part of the stream can be removed from the recycling equipment early, reducing the capital and operating costs of the equipment in the process.
Take a holistic approach to your water infrastructure when considering recycling. The pure water systems may need to handle recovered wastewater, and the chemistry of that water will be very different from other sources. Be sure to have a competent person model the chemistry of the process so you know what things are effectively removed, what contaminants will build up, and what the equilibrium points are for them. The better you understand this part, the better your system design will be, and the project will run better for it.
Deciding what makes sense for your application begins with clearly defining the absolutes. In other words, get as many constraints clearly defined up front as you can. Doing that will often make most of the variables define themselves. Some of the absolutes that can normally be defined are shown in Table 1.
|Table 1. Water Recycling Process Variables|
|Water used in process||5–12 gpm × 24 hr × 5 days
|Flow depends on part configuration and dragout.|
|pH required for process||6–8, consistent||Not critical, just needs to be consistent.|
|Total dissolved solids (conductivity)||<50 ppm for rinses,
<1000 ppm for chem-bath makeup
|Wastewater flow||3–9 gpm × 24 hr × 5 days
|2–4 gpm loss in drag-out and evaporation is normal for a
|Wastewater analysis||Need complete analysis of the starting point for recycling.||
“Complete” is a relative term.
|Available space, power, facilities||Be sure space, power, drainage, and other facility constraints are known up front.||Specifying these things can save a lot of time later in the process.|
|Cost / benefit analysis||Current costs, benefits expected, corporate ROIC rules, availability of funds, timetable.||These items will price the process for you. Anything outside the required ROI rule is a non-starter.|
As you can see, once your rules are defined the RFQ gets pretty easy to write. A good performance specification, including operating cost, capital cost, throughput, product quality and the level of automation required, is a very good starting place. From that information a creative company can get you their best offering to match your application.
Another critical consideration is choosing the right type of water recycling technology for your application. Some of the options that will come up in discussions about recycling include reverse osmosis (RO), precipitation, deionization, filtration, centrifuges, oil separation, ultrafiltration (UF) and nano-filtration.
These components all have a place in the market. The trick is to decide which ones make sense for your application, what order to put them in, and how to size them all for effective treatment and efficient operation.
For example, putting an inexpensive component such as a 5-μm filter in the wrong spot in your process can cost you hundreds of dollars per year in extra filter cartridge costs. Other components have considerations that are not so obvious, and you really need to have an expert working on the design to be sure that the process you finally install and start up will do the job without wasting operating money after it’s running.
For metal finishing, hydroxide precipitation will almost always be part of the process. This is the simplest way to remove the vast majority of solids, including metals, from your waste stream. Hydroxide precipitation is an old, tested method which, done properly, will result in treated water not too different from your supply water in terms of suspended solids and salts. Hydroxide precipitation will not remove oils, although by the nature of the flow and treatment, some types of oils will be reduced.
Membrane processes are another major component of recycling systems. Ultrafiltration, nano-filtration, and reverse osmosis are all used. UF and RO are the most common, with UF being used to remove 100% of the oils and organic contaminants and RO employed to remove salts. In E-coat applications, it is common to use a second stage of RO to polish the product to within the very tight limits required for that process. Once the membrane systems have done their work, the water is ready for use in any part of the process.
While it is very easy to build equipment that will self-destruct in this application, it is also easy enough to build a robust process where every piece of equipment is doing its job and is protected by the equipment ahead. Putting wastewater directly into an RO system, for example, would be a good way to convince yourself that RO will not work. However, that same RO system, with a precipitation system and a UF system in front of it protecting it from suspended solids and oils, will last for many years. The filtration, chemistry, and membrane filtration steps all have to work together in the correct order, and be sized correctly, for the system to be successful.
In considering recycling solutions, be careful with terminology. A lot of people love to use the terms “zero-discharge” or “near zero-discharge.” These terms describe what’s possible, but not what’s smart in most cases. In metal finishing applications it is almost always better to send a small stream to drain, if you can, to remove the salts from your process. If the expectation is for no liquid discharge at all, then the chemistry and process control become critical and expensive, and an evaporation system may be the most efficient solution.
Given the cost of energy these days, we should do everything we can to avoid evaporation unless it’s absolutely necessary. Instead, do the things that need to be done, in the most effective way at each step, and stop when the water is good enough. That is the way to build a successful system, both for your production environment and for your business.
When you’re preparing to select a technology solution for water recycling, take the time to go and see an installation or two. Get a feel for the quality of work a potential supplier company does and for the level of competence they have in solving issues outside their “normal” niche. In my experience, it’s the creativity and experience of a company that makes all the difference in solving problems.
If you aren’t 100% comfortable with evaluating the proposals and solutions presented, or with defining the RFQ, you should get a third party to do it. In recycling it is very easy to build systems that self-destruct or that simply will not accomplish everything that is needed to make the water reusable.
Be sure that the people who will be using the equipment understand what it is supposed to do and not do, and be sure unreasonable expectations don’tcreep in.
Always evaluate recycling in business terms. It’s a green technology, yes, but it’s also capital equipment, and it is imperative that it make good business sense. Don’t forget facility modifications, operating costs, training and learning costs. All of these things must be evaluated to be sure that the solution you choose is one that will be successful.
One other point here: Dissect the sales and marketing of any proposals you receive. Don’t allow promises that don’t make sense, and be sure to question anything that sounds like an inflated promise. More projects get burned due to over-promising and unrealistic ideas than for any other reason. Force realism through the whole process.
Remember, certain things might make sense for one application that just don’t for another, even though the processes are alike. If the flow rates are significantly different the whole amortization goes out the window. Be sure to evaluate everything based on your own numbers.
There is a bright and certain future for recycling, not just of water but other materials as well. Imagine being able to recover part or all of your process chemistry in a form that allows you to re-use it, or sell recovered compounds as commodity chemicals or products from your waste stream.
This future is coming. Water recovery is already being done, and will saturate the market over the next five to ten years. For example, the pickling stage in some washers right now is based, or could be, on sulfuric or hydrochloric acid. These acids, when spent, are very useful in treating industrial wastewater when used in combination with good coagulant polymers and flocculants. It would not be at all out of line at all to use these spent baths in your waste treatment instead of buying new materials. It simply takes a person on-site who understands the chemistry and how to use them correctly. The result is money in the bank, and you’ve already bought the chemicals, so why not use them?
If you define your project correctly and do your homework, potential technology suppliers will have a much easier time putting together a creative solution to your water recuyling neews based on tested and proven ideas. Remember to evaluate all proposals with a critical eye toward implied performance versus specifications. Force potential vendors to specify exactly what their system will do, what the limitations are, and what the operating costs will be.
Once a proposal is selected, get the project defined in a professional way, with timeline, milestones, performance metrics, and scope definitions. These things may seem like overkill on smaller projects, but the more detail and realism that goes into the project up front, the less delay, uncertainty, and money that goes into it on the back end.
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