Waste Management

In this article, a number of references are made to laws and procedures that have been formulated in the United States with respect to waste management. An engineer handling waste-management problems in another country would well be advised to know the specific laws and regulations of that country. Nevertheless, the treatment given here is believed to be useful as a general guide.

Multimedia Approach to Environmental Regulations in the United States Among the most complex problems to be faced by industry during the 1990s is the proper control and use of the natural environment. In the 1970s the engineering profession became acutely aware of its responsibility to society, particularly for the protection of public health and welfare. The decade saw the formation and rapid growth of the U.S. Environmental Protection Agency (EPA) and the passage of federal and state laws governing virtually every aspect of the environment. The end of the decade, however, brought a realization that only the more simplistic problems had been addressed. A limited number of large sources had removed substantial percentages of a few readily definable air pollutants from their emissions. The incremental costs to improve the removal percentages would be significant and would involve increasing numbers of smaller sources, and the health hazards of a host of additional toxic pollutants remained to be quantified and control techniques developed.

Moreover, in the 1970s, air, water, and waste were treated as separate problem areas to be governed by their own statutes and regulations. Toward the latter part of the decade, however, it became obvious that environmental problems were closely interwoven and should be treated in concert. The traditional type of regulation—command and control—had severely restricted compliance options.

The 1980s began with EPA efforts redirected to take advantage of the case-specific knowledge, technical expertise, and imagination of those being regulated. Providing plant engineers with an incentive to find more efficient ways of abating pollution would greatly stimulate innovation in control technology. This is a principal objective, for example, of EPA's "controlled trading" air pollution program, established in the Offsets Policy Interpretative Ruling issued by the EPA in 1976, with statutory foundation given by the Clean Air Act Amendments of 1977. The Clean Air Act Amendments of 1990 expanded the program even more to the control of sulfur oxides under Title IV. In effect, a commodities market on "clean air" was developed.

The rapidly expanding body of federal regulation presents an awesome challenge to traditional practices of corporate decision-making, management, and long-range planning. Those responsible for new plants must take stock of the emerging requirements and construct a fresh approach.

The full impact of the Clean Air Act Amendments of 1990, the Clean Water Act, the Safe Drinking Water Act, the Resource Conservation and Recovery Act, the Comprehensive Environmental Responsibility, Compensation and Liability (Superfund) Act, and the Toxic Substances Control Act is still not generally appreciated. The combination of all these requirements, sometimes imposing conflicting demands or establishing differing time schedules, makes the task of obtaining all regulatory approvals extremely complex.

One of the dominant impacts of environmental regulations is that the lead time required for the planning and construction of new plants is substantially increased. When new plants generate major environmental complexities, the implications can be profound. Of course, the exact extent of additions to lead time will vary widely from one case to another, depending on which permit requirements apply and on what difficulties are encountered. For major expansions in any field of heavy industry, however, the delay resulting from federal requirements could conceivably add 2 to 3 years to total lead time. Moreover, there is always the possibility that regulatory approval will be denied. So, contingency plans for fulfilling production needs must be developed.

Any company planning a major expansion must concentrate on environmental factors from the outset. Since many environmental approvals require a public hearing, the views of local elected officials and the com-munity at large are extremely important. To an unprecedented degree, the political acceptability of a project can now be crucial.

Plant Strategies At the plant level, a number of things can be done to minimize the impact of environmental quality requirements.

These include:

1. Maintaining an accurate source-emission inventory
2. Continually evaluating process operations to identify potential modifications that might reduce or eliminate environmental impacts
3. Ensuring that good housekeeping and strong preventive maintenance programs exist and are followed
4. Investigating available and emerging pollution-control technologies
5. Keeping well informed of the regulations and the directions in which they are moving
6. Working closely with the appropriate regulatory agencies and maintaining open communications to discuss the effects that new regulations may have
7. Keeping the public informed through a good public-relations program.
   

It is unrealistic to expect that at any point in the fore seeable future Congress will reverse direction, reduce the effect of regulatory controls, or reestablish the preexisting legal situation in which private companies are free to construct major industrial facilities with little or no restraint by federal regulation.

Microbiological Activity on Waste Treatment

Microbiological monitoring is seldom needed in hot water loops that operate continuously above 140°F (60°C) or in chilled systems where the glycol concentration is above 20%. However, when these conditions are not met, bacterial monitoring should be part of the testing program, especially with readily biodegradable treatments such as nitrite. Testing can either be via plate counts/dip slides (giving results in colony forming units (CFUs) per mL) or ATP (adenosine triphosphate) assay, which gives a measure of all types of micro-organisms and provides results within minutes. The ATP test result is in ng of ATP/mL, although some companies use the less accurate and not consistent units of RLUs (relative light units: light output relative to a particular instrument and a particular batch of reagents).

Any monitoring program is only as good as the frequency with which it is carried out. Corrosion coupons and corrosion product determination (in the system fluid) are most effective at illustrating a trend rather than providing absolute values. A regular program of sampling will allow one to verify the level of protection that is being achieved and to determine if it is changing over time.

Product/Treatment Level

Monitoring the product concentration is normally a routine part of the treatment program. It can be done either by plant personnel or the chemical supplier. The objective of the chemical testing is to confirm that the corrosion inhibitor is present in an adequate amount and that the pH is buffered to the right level. In the case of glycol loops, verifying that the glycol concentration is more than 20% is critical. At levels less than this (for chilled and out of service hot loops), rapid biological degradation of either ethylene or propylene glycol to an assortment of organic acids and intermediate products, will take place.

Performance Standards

Considering what can happen if a system is not treated to a reasonable standard, closed systems must be treated to give the lowest possible corrosion rate and to control microbiological activity.

Corrosion

Using effective inhibitor packages, it is possible to obtain corrosion rates of less than 0.2 mpy on mild steel and less than 0.1 mpy on copper and copper alloys. With these corrosion rates and schedule 40 piping, a system life of more than 50 years is a realistic expectation. While the overall, or general corrosion rate is important, a successful program must also control pitting or localized corrosion.

In systems where low flows or stagnant conditions can exist for extended periods, protection from localized corrosion is equally important. The amount of iron and copper present in the system fluid (whether water or water/glycol mixtures) should be quite low. In well maintained systems, it is common to find iron and copper concentrations at or below, 0.2 mg/L and 0.1 mg/L, respectively.

Microbiological

Microbiological activity will vary from zero in an operating hot water circuit to being present in chilled water loops treated with nitrite. In chilled water systems, it is generally agreed that <103>

The Future

At a time when the need for new, innovative approaches to treat closed loops appears to be growing, little new technology is coming to the marketplace that can match the performance of 20-year-old technologies.

As restrictions on metal-based and nitrite programs continue to grow, the demand for effective “green” alternatives will expand. Newer chemistries, such as neutralized dibasic acids, phosphonates, triazines, etc., either do not provide the level of protection of traditional programs or are not applicable to a broad range of applications.

At a minimum, treatment programs for the next millennium will need to provide:

• Low environmental impact, with good long-term biodegradability;
• Biological stability (not readily biodegradable) under use conditions;
• Mild steel and copper corrosion rates of less than 0.5 mpy and 0.1 mpy, respectively; and
• Easy testing by plant personnel.

The company that can develop and market a product with this performance profile will be well positioned to capture a low portion of this market.

Corrosion Inhibitor

Control of copper corrosion is critical in any closed loop. While copper and its alloys are quite corrosion resistant, the impact of even low corrosion rates can be dramatic. When copper corrodes, soluble copper ions plate out onto mild steel components. When this happens, the more inert copper metal becomes a “permanent” cathode on the metal surface. At this point, the corrosion process, which had been spread over the entire steel surface, now becomes localized and continues at an accelerated rate. As this proceeds, instead of having a low general corrosion rate, high local corrosion rates will be seen. Azoles are used to prevent the initial corrosion of copper alloys, as well as to inhibit copper deposits on mild steel surfaces.

MBT (mercaptobenzothiazole), a low cost, effective inhibitor, has been used for many years with good results. More and more commonly, TT (tolyltriazole) has become the inhibitor of choice due to cost considerations and its superior resistance to the corrosive effect of chloride ions. In contrast to precipitating agents, the nitrogen atoms in the azoles bond to the copper metal via copper oxide molecules on the surface. The protective layer that is formed enhances the natural corrosion resistance of copper and copper alloys.

Monitoring
Monitoring of closed loops entails verifying that the treatment program is meeting the goals for corrosion, deposits, microbiological activity, and product/treatment level. The corrosion monitoring should also have some provision to show that localized or pitting type attack is not taking place.

Corrosion
Monitoring relies on using a model for what is happening to a system. It is meant to prevent surprises such as in Figure 3. Instead of having to cut out pipes to see if corrosion is taking place, a well-designed monitoring program will provide the same information about the efficacy of the water treatment program with considerably less effort. While a physical inspection of boilers and chillers (or other related components) is the most effective way to determine overall system performance, other approaches can provide the same information on the cleanliness of the equipment.

Corrosion coupons are the most common type of monitoring, since they provide information on overall corrosion rates as well as the type of corrosion that is taking place. A coupon will give the following data:

• General/overall corrosion rate,
• Pitting corrosion rate,
• Indicators of biological attack, and
• Evidence of galvanic attack.

In a well-maintained system, the corrosion rate should be virtually nil, and there should be no sign of localized attack. In chilled water loops, it is also important to assess whether microbiologically induced corrosion (MIC) is occurring. Coupons are one of the easiest ways to look at this aspect. Most closed loops contain a variety of alloys, and yellow metals are quite common. If they are not adequately protected, the dissolved copper ions (from the corrosion process) can deposit onto steel components and cause galvanic attack. If this problem is suspected, the copper content on the surface of a steel coupon will confirm if it is an issue or not. To be effective, corrosion coupons need to meet a number of criteria to mimic a closed system:

1. The coupons should be of similar metallurgy to the components in the system. Mild steel (typically 1010) and copper (although brass and other more specialized alloys may be appropriate) coupons should be used.
2. Exposure periods should be varied, with some as short as 30 days and some being allowed to remain in for up to a year.
3. The flow rate through the coupon rack should be close to what various sections of the loop experience. This could range from the normal flow velocities of 1.2 to 1.5 m/s (4 to 5 fps) to as low as 0.03 m/s (0.1 fps).
4. Since temperature has a large impact on corrosion rates, the coupons should see the same temperature (or as close as possible to it) as the hottest section of the system. A good location is on the supply header, shortly after the boiler. With chilled water, the return header is an ideal location.

If coupons are left in for short periods (less than 30 days), the corrosion rate will be artificially inflated. The real value of the corrosion rate is partially the level itself, but the major value is the trend over several months or years. Consistently low corrosion rates, with no localized areas of metal loss, is the goal.

If localized attack occurs, it is normally a good idea (assuming that obvious causes such as low chemical residuals are not the cause) to have the coupon checked for copper plating and MIC. Most water treatment companies provide this sort of service as part of their service program. In some cases, consultants also can arrange to have this type of testing done, though they typically use outside laboratories, which might be less experienced in what to check for. Installing coupons to copy conditions in zones where the water velocity is low is important. It is rare to find a closed loop circuit that does not have low flow sections or is periodically stagnant. Although the high inhibitor levels used in closed systems should reduce the effect of stagnant or low flow conditions, it is necessary to ensure that the program being used meets this critical performance criteria.

Figure 4 shows the effect that reducing water velocity has on corrosion rates for a conventional borate-nitrite program. Although the corrosion rates should only be taken as relative indicator of what might happen in a system, the important point is that going from a water velocity of 1.5 m/s (5 fps) to 0.3 m/s (1 fps) allowed corrosion rates to more than triple. Higher inhibitor levels did minimize this effect, yet the general trend is still apparent. Not taking into account the effect that water velocity has on the inhibitor program has caught more than one installation by surprise.

Low-cost electrochemical measuring devices (to determine instantaneous corrosion rates) are becoming more common and accessible. Electrochemical monitoring provides rapid results (probes can generate accurate rates after only a few days) and makes it possible to trend corrosion rates not only using monthly averages but also on a day by day basis. Being able to get reliable data on corrosion rates over such short periods allows one to determine if system operation (e.g., periodic shutdowns, etc.) is affecting the corrosion protection being provided. Although not as high tech as electrochemical monitoring methods, a simple and effective technique to follow corrosion trends is to measure iron and copper concentrations in the closed loop fluid. As system metal corrodes, it goes into the fluid. Although it is present in a precipitated form, the metal concentration in the water does serve as a good trend indicator of what is happening. While it does not tell what is causing corrosion, it does provide a quick indication of when things begin to go wrong or confirmation that the program is continuing to meet agreed upon standards.

Deposits
Deposits are rarely a concern in a closed system. The best way of monitoring deposits is to record makeup rates. If the system is operating within normal limits (makeup 10% of system volume per year) the risk of scaling is minimal. It should be noted that some treatment programs, specifically phosphate inhibitors used in some glycols, are quite sensitive to hardness in the water used. If good quality water is not used, the phosphates can react with the hardness present and form deposits.