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Environmental Best Practices, Part 3

(October 2008) posted on Mon Oct 06, 2008

Structuring your company around safe environmental practices is becoming a necessity

By Neil Bolding, Steven Abbott

So what can you do to reduce solid waste, BOD, and COD? The most obvious trick is to flush more water down the drain. Unfortunately, this approach leads to excessive water consumption and tends to overwhelm sewage systems. The screenmaking and cleaning/reclaiming processes typically consume significant amounts of water anyway, so it is important to look at these activities and tightly control processes. One good first step is to invest in a high-pressure washing system (manual or automatic) that uses water more efficiently.

The next obvious solution would seem to use low BOD/COD solvents. The bad news is that such solvents don’t really exist. Simply put, all the solvents that are likely to be used in screen shops have BOD/COD levels that don’t vary by more than a few percent from one product to another. The good news is that anything with organic carbon in it is likely to be food for bacteria, provided we don’t overwhelm them with it.

Another trick is to use solvents with a high evaporation rate and hope that they go into the atmosphere rather than into your drains. This is really just swapping one problem (BOD/COD) for another: emissions of volatile organic compounds, which have many other drawbacks.

Water authorities regulate the minimum flashpoint of the organic solvents that can go down the drain—typically this flashpoint can’t be less than 140°F. They may also restrict specific solvents, such as benzene, xylene, and acetone. Other cleaning solvents quite widely used in our industry, such as d-limonene, may strictly be limited. For example, in Clearwater, FL d-limonene is restricted to no more than 26 mg/l.


Equipment solutions

The final, most effective means of controlling the contents of your effluent is to employ recirculation systems so that you use your solvent over and over again. Reduction in solvent use is number one on the EPA’s hierarchy for pollution prevention and is a key part of the green pathway.

The ideal recirculation system pulls all the solvent back into a recirculation tank, and only the water used for rinsing the screen is drained off as waste. An effective solvent-recirculation system will have an up-front cost, but it quickly negated by the considerable savings in solvent consumption and substantial reductions in BOD/COD discharge.


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