Structuring your company around safe environmental practices is becoming a necessity
BOD and COD The sorts of bacteria found in the sewage systems use oxygen as a source of energy for breaking down waste materials. If all that oxygen gets used up, the bacteria can no longer function. Even worse, so-called anaerobic (oxygen hating) bacteria can start to take over. The oxygen-loving bacteria are the friendly bacteria, while the anaerobic ones are definitely unfriendly. So, in order to keep the friendly bacteria around, those emitting effluents must ensure that they don’t provide so much food that the bacteria use up all the oxygen before they can consume the waste. Clearly BOD is the best measure of how much is being put down, but it’s a tricky, rather slow test to perform. COD is a much simpler, quicker test and is a good approximation.
The effluent from a screen-printing shop’s stencilmaking and screen-reclaiming areas will principally consist of water used to develop and reclaim polymer-based stencils and blockouts. These materials will contain polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH) solids, plus some stencil remover, haze remover, and degreaser. The effluent will also contain screen-wash solvent and ink residues that are carried over from the post-print cleaning operation, whether it be manual or automated.
Every industrial site will have its own discharge consent limits based on a mix of local, federal, and international criteria, so it’s difficult to generalize what are acceptable disposal limits for a particular company. In general, however, a typical printer may have a BOD consent level of 150-500 mg/l, a COD consent of 500 mg/l, and a total suspended solids (TSS) limit of 100-1000 mg/l with pH limited to 5-11. Surcharges may apply above the 250 mg/l level for BOD and 500mg/l for COD. But again, these values will vary depending on your classification and location. See the sidebar “COD and BOD Testing” on page 51 for an explanation of testing procedures.
Considering that a typical PVOH/PVA-based emulsion or blockout has a COD of around 400,000mg/kg, and a screen wash could be around 1,400,000 mg/kg or higher, it’s clear that significant dilution is required to enable a company to stay in compliance when releasing the material into the sewage system. This is why it is critical to carefully look at our practices and work to limit the waste we generate.
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