“We use well water. It’s very clean; and it tastes good.” “Our local water district says the water is high quality.” “We know we have clean water, because little fishes swim around in our rinse tank.” “Our sales rep told us to dilute the cleaning agent with tap water.” Clients and colleagues share problematic comments about the water they use for critical product cleaning. Water is a precious commodity; water is a valuable chemical for critical product cleaning. Water is taken for granted. Excellent drinking water may not be right for the cleaning process. With the correct controls and process management, aqueous cleaning processes can run effectively. To clean effectively, you must manage water before, during and after the cleaning process. Keys to achieving consistent process water include understanding the attributes of your product, understanding the properties of incoming water, understanding how incoming water varies over time, and understanding how clean the water needs to be.
Just-right water
Do you need 18 megohm water for all cleaning processes? No! in fact, in many instances water can be too clean. Water doesn’t want to be pure; it looks to attract ions. In fact, in most instances, deionized water does not remain deionized during cleaning or other manufacturing processes. Water will pick up ions from process equipment and, more importantly, from product, particularly ferrous product. Using water that is too clean can result in flash-rusting of ferrous product. Ultra-pure water is an aggressive chemical that can etch fine optics.
At the same time, tap water or well water may not be clean enough, or, even more important – it might not be consistent. The right water quality begins before the wash, rinse, or dry steps of the cleaning process. We strongly suggest not using tap water. High TDS (Total Dissolved Solids) can interfere with effective cleaning. Water in Southern California tends to have high TDS. We’ve run into exceedingly high TDS. For example, areas in Texas have water with exceedingly high TDS; we’ve been to site visits where visible “sand-castle” residue is deposited next to even tiny leaks in faucets.
Even where the water quality appears to be acceptable; and might give consistent results, pre-treating the water can avoid unexpected, intermittent problems, starting at the wash step. Sometimes, tap water is recommended for diluting the cleaning agent. We disagree with this recommendation. Too often, we see that a process may work for a time, but, then, we get a call saying that the cleaning chemistry is no longer working or the parts are spotting or discoloring. This is because tap water varies. Variables include seasonal vegetation, agricultural practices, or other manufacturing practices up-stream from the water you are using. On the East Coast, one client ran into unexpectedly high level TDS associated with agricultural harvests. We received an urgent call from a client in a rural area of the mid-West. Suddenly, and without warning, plated products were purple. While this could have been an interesting design statement, customers did not want purple product. We never pinpointed the source of the purple color. However, switching to D.I. water for both the wash and rinse steps resolved the problem – and we’ve been told that the problem has not recurred for decades.
Wash
At the wash step, water must be clean enough that it does not interfere with the cleaning chemistry, including surfactants, builders, and chelators. You want the ingredients in the cleaning chemistry to be devoted to removing the soil from the part and keeping it away from the surfaces being cleaned.
Rinse, Dry
The product is only as clean as the last rinse step; and the last rinse step depends on consistent cleanliness of the water. Contaminants in the rinse water can deposit on the part. Consequences include “water spots” that appear after drying and defects that appear only after surface finishing. Multiple rinses are usually needed in order to achieve a clean surface. Reverse cascade, where the cleanest water is used for the final rinse and the rinse water flows “upstream” to replenish the earlier rinse baths, is an effective way to minimize total water use.
Disposal (or reuse)
Managing water from process baths brings a separate set of challenges that may include process equipment. In many, if not most, instances, you can’t just dump the rinse water, let alone the bath of spent cleaning agent, down the drain. What’s required? It depends – and it varies a lot, depending on water requirements for your exact location. Sometimes, the outgoing water has to be cleaner than the incoming water.
Even if the additive package for the cleaning agent could be directly released to the Publicly Owned Treatment Works (POTW), the contaminated bath is likely to contain process fluids, metal fines, and dissolved metals. Costs of using a reliable outfit to dispose of the of the spent wash bath can be significant. For this reason, the longer you can use the wash solution, the more economical the cleaning process. Cleaning agents that hold oils in suspension have become overloaded; the bath has to be changed. Oil-splitting chemistries prolong the bath life by allowing separation of oils from the cleaning agent. Coping with the oil can itself require process controls. Simply lifting the parts out of the tank can recontaminate the parts; the recovered oils are analogous to fat at the top of a bowl of chicken soup – your spoon gets greasy. There are several methods to skim the separated oil from the top of a bath. A weir consists of slowly flowing the top layer over an edge into a separate container. A sparger uses directed liquid or gas to blow the surface layer over an edge. Some have found small “ferris wheel” devices to be effective. Depending on the process, it can take time for the oils to separate out, so a holding tank may be needed.
You can’t assume that the rinse water is clean enough for release to the POTW; in many areas, the rinse water must be tested. Carryover is a costly culprit. Depending on the shape and configuration of the parts, significant amounts of the wash bath can end up in the rinse tank, resulting in rinse tanks that require significant treatment, or even more costly, have to be managed as hazardous waste. There are tactics to keep the contents of the wash tank out of the rinse tank. For example, parts can be lifted and held over the wash tank. Adding a short shower or mist spray at this point can be helpful and can help avoid flash drying and corrosion.
Avoiding or reducing the need for water disposal can significantly reduce the cost. Especially with the rinse water, it might be more economical to re-condition the water for re-use rather than for disposal. This way you can save both the cost of the water itself as well as costs to prove to the POTW that the water meets requirements or the costs to have it hauled off by a disposal company.
Avoid cleaning “gotchas”
Water is a valuable precision cleaning chemical. Don’t take it for granted. Invest in up-front process design to manage water before, during and after cleaning. The time and money spent results in consistent, cost-effective manufacturing.
