Environmental Best Practices, Part 3
Structuring your company around safe environmental practices is becoming a necessity
The drain is the ubiquitous black hole where waste goes, and, once it’s gone, most of us tend to think nothing more about it. Out of sight is out of mind. Well, not anymore. The drain-disposal route is a significant waste outlet we must consider in our environmental best practices journey. We encourage printers to be aware of their responsibilities to their drain systems and to the ever increasing restrictions on what can go down them.
Among the various catch phrases prevalent in our industry today, one is “drain safe,” which is a misleading term at best. Water can be thought of as drain safe, but too much of it down a drain can cause problems. Assurances made about the safety of products that will inevitably find their way to the drain are meant to offer printers wrestling with disposal issues a sense of security. But ignorance of true waste-disposal requirements is no safety net when the local water authorities decide to inspect your company.
Every region has focuses on different aspects of what goes down a drain, but these aspects are all variants of four key factors:
• volume of effluent
• percent solids and biological oxygen demand (BOD) and chemical oxygen demand (COD) of the effluent
• effect on bacteria in the effluent system
• effect on the outflow of the sewage system
We’ll discuss each in turn after a short description of how an ideal sewage system functions.
A stable sewer
The basic idea is that any waste sent down the drain should get converted into harmless byproducts by the bacteria in a sewage system. This degradation takes time, so a balance must be found. A very large sewage system will cope with nearly anything you can throw at it—but such systems represent a large investment, and regulators will still control what you can send down the drain. A small sewage system might cope well with average loads of effluent, but they can be overwhelmed by big surges of waste. So the ideal waste stream is a steady, modest load that gives the bacteria ample time to do their job.
But complications do exist. Bacteria can’t get rid of everything. They can’t cope with heavy metals, and they struggle with hydrocarbon solvents that are not water miscible. Therefore, high quantities of these substances should not be going into the sewage system. That’s why printers have strict no-go limits on certain types of chemicals.
If you throw a lot of water down your drain, you can make your waste look rather good because it’s diluted. But the sheer volume of water you use can create a double problem. First, supplying everybody with nice, clean water is becoming increasingly difficult in many parts of the world, and bills for water are rising sharply to reflect this fact. Secondly, all that water can overwhelm the sewage-treatment facilities, which prefer a slow and steady process to keep the bacteria happy. Therefore, high dilution is not the solution.
Percent solids and BOD/COD
Assuming you are not attempting to mask issues with excess water, you have three criteria by which you can measure your load on the sewage system.
Percent solids The higher the percentage of solids in your effluent, the more solid waste you’re putting into the system. Some of this will eventually be eaten by bacteria in the treatment systems, and some will simply get filtered out. In both cases, the more solid waste there is, the harder the sewage system has to work and the more you’ll be charged to become or stay in compliance. Incorporating a filter system into your own plant to process waste is en effective way to reduce the waste load and your disposal costs. This can be a simple mat/pad filter or a more extensive flocculating system to remove so-lids from the liquid waste.
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.
Chemicals going down the drain must be readily biodegradable, which means that the bacteria are happy to consume them. Readily biodegradable is defined by a set of tests (e.g., OECD 301B, 301D, 301E, 301F, modified Sturm test), which show that a large proportion (>60%) of the solvent is converted into carbon dioxide from the sewage sludge in a short time, typically about 10 days.
In theory one way to reduce BOD in the effluent is to use solvents that aren’t readily biodegradable, but this simply means that the solvents will end up being discharged from the sewage outfall back into rivers and streams. Also, don’t be confused by products labelled as inherently biodegradable, which are significantly less biodegradable than materials classified as readily biodegradable.
The effect of waste on bacteria
It should be clear by now that killing off friendly bacteria is not a good idea. What is not so obvious is that the bacterial conversion of waste starts right at your facility. The sewage pipe leaving your operation should be full of friendly bacteria. They are happy to be there because you are providing them with a great source of food. So the journey from your facility to the main sewage-treatment facility provides an opportunity for a large amount of your waste to be consumed, which in turn means that the sewage treatment plant doesn’t need to be as large.
If you’re not careful, however, you can kill off the bacteria along your section of the sewer line. What kills them? If you are throwing buckets of hot water down the pipe, you’re not just wasting precious heat, you’re also killing off bacteria. If you are throwing down lots of bleach or acid or alkali (such as caustic haze removers), then you are also wasting precious chemicals and killing bacteria. Ideally, you should monitor both the temperature and pH (measure of acidity or alkalinity) of your effluent on a continual basis to avoid overloading the bacteria. Most facilities don’t throw down a constant stream of alkali, so spot checks of pH may be enough to tell you whether you’re doing fine.
Effect on the outflow
It’s worth reminding yourself and your staff that the outflow leaving a sewage- treatment facility isn’t just something you can forget about. That outflow might be going into a local river that, in turn, is the source of your drinking water. Thinking of it like that helps concentrate the mind on the real problems. There are basically three effects to be concerned with that can create a potentially dangerous out-flow situation:
Overload There’s nothing the sewage system can do about too much effluent coming in, and when the system is overwhelmed, raw sewage can get into the local river or other outflow site. For this reason, authorities do their best to take account of not only on your average load, but also the extreme loads you can produce.
Chemicals bacteria can’t consume Some chemicals show a high degree of environmental persistence (i.e., they are very stable and don’t decompose easily or are insoluble in water) and they may accumulate and harm the bacteria. Organo-halogen and organophosphorous compounds are typical examples of such chemicals, with the latter frequently found in industrial detergents. Pesticides or biocides, such as Aldrin, DDT, and Lindane, create similar problems.
Nitrogen content Lots of nitrites and nitrates in the outflow can cause problems, and the solvents you put down the drain can affect the concentration of these substances. A complex nitrification test (ISO 9509) can be used to divide solvents into two types—those that affect the balance and those that don’t. Presently, few countries are alert to the nitrification issue. But screen-cleaning products that have no negative effect on the nitrification process are available and will become more common as more countries adopt stricter standards concerning this issue.
Other problems lurking in the water
Within the screen industry, the use of nonylphenol surfactants in cleaning products was once widespread. Unfortunately, these materials mimic natural hormones, causing sterility in fish and other aquatic life that ingests them. So many cleaning-chemical manufacturers have removed these materials from their products. Note, however, that some less reputable suppliers of cleaning chemicals still include nonylphenol surfactants in their products, and the burden falls on the printer to avoid such formulations.
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.
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.
One issue to be aware of with recirculation systems is that they require low-volatility solvents, otherwise you’ll experience rapid solvent loss within the unit through evaporation during the recirculation and extraction processes. Also, if you don’t get good separation of ink solids from the solvent, the solvent will quickly get too dirty to be useful, and the cost of your system won’t get offset by reduced solvent use and lower BOD/COD. Separation of ink solids and solvent depends on good system design and good control of the chemicals. Even a modest change in the solvent blend can tip the balance away from good separation.
Finally, comprehensive staff training and working practices are essential to ensure you get maximum use out of every drum of solvent and that BOD/COD discharge is minimized. A simple procedure, such as removing excess solvent from the screen with a window cleaning squeegee before rinsing, can offer significant benefits with minimal cost.
Stencil materials are typically acidic and will generate effluent with a pH of approximately 5, which is the low end of most regulatory limits and generally doesn’t create pH-related problems. The area of most interest to printers is at the high end of the pH scale. This is because many haze removers contain high percentages of sodium or potassium hydroxide. These are strong alkalis and will take the pH past 11 very easily.
So what can you do to control high pH? The key is to understand why we use haze removers as much as we do. Some inks are inherently tough and difficult to clean, but much ink haze is created when we use volatile solvents for screen cleaning. These solvents fuse the ink to the mesh. So looking for less aggressive solvents can be part of the cure. Additionally using an effective screen wash at the screen-reclaiming phase may also lighten haze.
Other substitutes for using highly caustic haze removers including using an effective pressure washer that delivers a high-pressure stream of water. Additionally, you may be able to turn to a less caustic haze remover, although using such a product may require you to change reclaiming procedures and will likely add to the time it takes to reclaim the screens.
Let sustainability sink in
The waste you release into your sewer system can have an impact on local water quality as well as your operating costs. By looking into alternate chemistries for screenmaking and screen reclaiming, adopting sound procedures, and employing quality equipment, you’ll bring sustainability to the screen room and achieve higher quality with less regulatory scrutiny.
COD and BOD Testing
Chemical oxygen demand (COD):
The organic (carbon-based) material going down the drain is all going to be rendered harmless by using oxygen. This is either by direct chemical oxidation to carbon dioxide, water and byproducts, or by bacteria eating the chemicals. The bacteria are aerobic, meaning they need oxygen to do their work. The amount of oxygen needed relates to the amount of material in the effluent they must consume. COD measures how much carbon is in the water, which is a measure of the maximum amount of solvent that could, in principle, be eaten by the bacteria. Testing for COD takes approximately two hours.
Principle of COD testing: Use a chemical that reacts strongly with organic chemicals in water and changes color on reaction.
Practice: React your water sample at 148ºC for two hours with a known amount of the strong oxidizing agent potassium dichromate in sulphuric acid with silver sulphate as the catalyst. Measure the color of the resulting solution at 585 nm—the higher the value, the more potassium dichromate has been consumed by the COD. This is all done using a standardized reagent kit and a spectrometer.
Biological Oxygen Demand (BOD):
This indirectly measures how much oxygen gets consumed by a representative sample of bacteria.
Principle of BOD testing: Allow the bacteria to eat the organic chemicals in your waste water, consuming oxygen and producing carbon dioxide. Measure the amount of carbon dioxide produced.
Practice: Put a known cocktail of bacteria into your waste water sample at 20ºC, and seal the vessel along with a pellet of sodium hydroxide. Over five days the carbon dioxide produced by the bugs is absorbed by the pellet, creating a vacuum. Measure the vacuum with a manometer to calculate BOD.
Examples taken from King County (Seattle), WA and Clearwater, FL publicly owned treatment works.
Neil Bolding is MacDermid Autotype’s business-support manager. He is a 25-year veteran of the printing industry with experience in quality control and technical customer support. He has written articles for a variety of trade publications, spoken at numerous industry events, and regularly contributes to SGIA training programs. He currently sits on the SGIA’s Environmental Committee and the Membrane Switch Council. In 1994 he was the industry co-chair (product testing subcommittee) for the US EPA’s Design for the Environment Program. Bolding also is a member of the Academy of Screen Printing Technology.
Professor Steve Abbott is technical and research director at MacDermid Autotype, Wantage, England. After receiving a PhD in chemistry at Oxford for work he carried out at Harvard and serving in a post-doctoral position in Strasbourg, he went to work at ICI on new product development. In his role as research & technical director of MacDermid Autotype, Ab-bott has been responsible for ensuring a constant stream of new products and also for providing the science behind the coating and printing techniques used. He frequently collaborates with researchers at the University of Leeds, where he serves as a visiting professor, and is a frequent speaker at international conferences related to coating and printing. Abbott is a recipient of the Swormstedt award for technical writing from the Specialty Graphic Imaging Association.