The goal of every chemical manufacturer is to ship a quality product to his customer. One common requirement for waterborne products such as coatings is that the product remains microbiologically stable until used, and in some instances, for some period after its use (paint films for example). Being aqueous, these products are susceptible to microbial contamination and degradation. Anti-microbials (biocides) are routinely added to assist with the delivery and maintenance of product qualities so that they meet customer expectations. However, it has long been apparent that even adding an appropriate anti-microbial to the finished product may not be sufficient to accomplish this goal. It is now understood that the product must first, and foremost, be manufactured under a tightly controlled process that includes adequate plant hygiene. Doing so minimizes the final cost of the product and helps ensure that a quality product is manufactured and delivered.

The following document details some of the areas of concern in a "generalized" aqueous product (such as coatings, adhesives and caulk) manufacturing scheme. The intention is to highlight those areas that are most likely to cause microbial quality problems so that a plant hygiene program can proactively address them. Regular attention to plant hygiene will pay dividends to the manufacturing process of waterborne specialty chemicals.

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Plant Hygiene Philosophy

The philosophy of a plant hygiene process is to locate plant processes, procedures, practices, equipment and raw materials that can contribute to microbial contamination. With this knowledge, problem areas can be addressed economically and before serious quality issues impact the final product. In general, many of the steps of the chemical manufacturing process are common across the industry. This allows a generalized discussion of how to implement a plant hygiene program. Attention to these common areas of concern in the manufacturing process will eliminate the vast majority of microbial-induced problems. It is understood that more detailed analyses (usually still based on these same areas of concern) may be required to solve more difficult and unusual problems. Individualized plant hygiene programs will need to be developed for each particular manufacturing process; however, this should not be difficult in most cases. The general areas requiring attention are:
  • source water and source water-handling systems;

  • recycled water and recycled water-handling systems;

  • recycled raw material or recycled product;

  • raw material storage and handling systems;

  • mixing, milling and reaction vessels and their associated piping systems;

  • product packaging systems; and

  • product transportation and delivery systems.

Some common and basic questions can be asked at each of these potential locations for biological contamination. These include the following questions.

  • Is this location a source of microbial contamination or a location allowing contamination "levels" to increase?

  • Is microbial control required at this location or is it best handled elsewhere?

  • If anti-microbial control measures are present, are they adequate?

To answer these questions, more detail must be known about the status of the current location. Then a judgment can be made towards a path forward.

Guiding Principles

A regular hygiene survey (plant survey) as part of a plant's overall hygiene plan will help answer these questions and provide guidance for any required remedial action. There are two guiding principles.
    1. Spot surveys of suspected "problem" areas are seldom sufficient to uncover all areas requiring attention. A more thorough survey of the entire system is usually more productive.

    2. Any survey is just a "snapshot in time" of the process being monitored. Only with repeated surveys over time can a true picture of the system be assembled.

With a normal background established, future remedial actions are much more likely to be successfully completed, both more quickly and at lower cost. With time the person responsible for plant hygiene will know more about the operation of the manufacturing plant than any other one person in the facility.

The most effective way to handle the biological component of the plant hygiene program is to establish a microbial assay component into the plant's QA laboratory. This will allow microbial data to be collected on a regular basis so that "out of bounds" conditions can be quickly spotted and corrected. The testing methods and procedures applicable to this work are beyond the scope of this discussion, but they must be customized for each type of product manufactured and, to some extent, for each location sampled. Supplemental work can be done by outside consulting laboratories and, often, by one or more of the biocide vendors supplying the production plant.

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Locations/Processes to be Surveyed

Extensive biological audits (bio-audits) within diverse manufacturing plants have led to the generalized list of plant locations and production processes (above) that are common sources of biological contamination. This list covers the gamut of industrial manufacturing processes including coatings, adhesives, caulks, sealants, paper, textiles, polymer emulsions, etc. For this generalized list, a location/process specific hygiene plan can be developed for each manufacturing plant.

Bio-Audit (Survey) Details

1. Source water and source water-handling systems

Source water is one of the most common areas for the introduction of microbial contamination into a manufacturing process. Most environmental water sources can carry a rich supply of microorganisms into the facility. The water pH, temperature, microbial load, nutrient load, hardness, oxidative-reductive balance, etc., all can play a roll in determining if the water requires treating and, to some extent, determines the type of anti-microbial treatment that can be used. The ubiquitous problems that can be caused by water quality dictates that every plant should have a detailed understanding of the incoming water and a treatment plan for that water.

Fortunately, anti-microbial treatment, if required, is often most effective and least costly at this location. Treatment at other locations in the plant (see following sections) is more problematic and much more involved. If a treating program is used for the source water, it must be monitored appropriately to ensure that it is effective. Until sufficient baseline data is collected to show otherwise, testing for biocide addition or of residual biocide concentration should be done daily. Thereafter, biological testing should be done at least weekly. It is routinely found that proper monitoring and biological control of source water is lacking.

2. Recycled water and recycled water-handling systems

It is often necessary to use recycled process water in the manufacturing process. It must be recognized that the use of recycled water dramatically increases the potential microbial loading to the manufacturing process. Organic loading from the process, microbial contamination from within the plant, dirt and debris, and retention time all conspire to increase microbial numbers in this water.

For monitoring purposes, collect information on the same parameters as discussed above. In addition, a detailed "picture" of what the water contains, where it has been, what environmental changes have happened, etc., needs to be developed for this water. This water should be considered a "highly likely" contamination source until proven otherwise during any problem remediation. The more known about this water, the easier it is to solve future problems.

If needed, anti-microbial treatment of this water requires higher concentrations than does most supply water, but timely treatment is more economical than trying to preserve the final product and recover from poor water handling. Often engineering controls such as reducing dead legs, increasing circulation rates, adding filtration, etc., can be helpful in reducing the impact of this water on the manufacturing process. Monitoring biocide addition and/or residual biocide concentration should be done daily until adequate data shows that a weekly schedule is sufficient. Biological testing should be done at least weekly once enough background data is collected to show this extended time schedule can be justified.

3. Recycled raw material or recycled product

Some manufacturing processes involve the recycle of quantities of spilled product and or raw materials, washouts of product/raw material handling systems, washouts of packaging systems, railcar washing, container cleanouts, etc. These materials can become quite contaminated in many situations. It is necessary to understand, in general terms, the makeup and condition of this material. Contamination levels should be measured, pH and temperature should be recorded. In general, these process streams tend to be more heavily laden with solids, which can in turn have an impact on the redox potential that develops over time. Understanding the changing redox potential of the material is often necessary as it has an impact on determining what biocides will be effective. At this point in the system, oxidizing biocides become less valuable and preservatives are usually required. Again depending on background data, it may be possible to test only weekly to ensure quality control. Loss of biological control should trigger immediate daily (or even more often) testing (along with remediation steps) until control is again demonstrated.

4. Raw material storage and handling systems

After the source water and recycled materials, the next most important factor to monitor in maintaining plant hygiene is the raw material storage and handling systems in the plant. It is important to have a keen understanding of the chemical and physical properties of the raw material being used. What is the pH? How was it made? How was it treated before it arrived and since its arrival? Was a biocide added in production of the material? Was a biocide added since? Is that biocide compatible with the processes and chemicals to be employed in the plant?

With a good understanding of the material and its handling history, the next issue is to ascertain the current microbial condition of the material. Is the material stable or is it deteriorating? Does it require treatment? Standard analysis methods, including target results, should be established for each raw material. It is important to recognize that a one-time measurement is not sufficient. A routine monitoring plan needs to be in place to measure the biological stability of all raw materials over storage time.

This information must be known for each raw material in the plant. Some raw materials will be simple to handle and pose little risk; others will be significant issues for plant hygiene. Once the appropriate background information is gathered, weekly routine biological monitoring may be sufficient. A treatment plan should already be established in advance for each material in the event monitoring shows an "out of bounds" condition.

5. Mixing, milling and reaction vessels and their associated piping systems

As the raw materials, recycled materials and water come together in the processing required to produce a product, a quality plant hygiene program begins to pay off. If the raw materials are in good biological condition, as well as any water used, the problem of biological control in the final product is much more manageable. The plant hygiene concern for this part of the process can center on using only treated ingredients and protecting those ingredients from contamination during processing. In a typical batch process, every batch should be tested for microbial contamination until sufficient background data has been gathered to allow a more relaxed testing schedule.

Following processing, the concern is transferred to ensuring complete removal of each batch from the system, including elimination of ‘heels' of product in tanks and low points in piping. Intermittently used equipment requires an established cleaning and disinfection program. It should be cleaned immediately and stored dry, or if not practical, have a biocide added and circulated around the equipment.

As with other areas in the plant, look for and eliminate dead-end zones with low/no circulation. Another obvious step is to keep vessels, drums, bags, etc., covered and protected from foreign materials, including dust, dirt and water. Another is to keep the plant as a whole clean and dry to avoid fostering the growth of microorganisms that can become airborne. Scheduled shutdowns should be used for more thorough cleaning and disinfection of the whole plant (floors, walls, ceilings, railings etc.). A routine plant survey of the equipment should be used to assess the thoroughness of the cleaning process. This is often best done immediately upon the plant returning to service where the first product batches are good indications of the cleanliness of the plant. Experience will dictate the time recommended between shutdowns and cleaning; often as seldom as semi-annually is possible with a good plant hygiene program.

6. Product packaging systems

Once the product is manufactured, it must be packaged for delivery to the customer. This is the traditional point where most think of adding a biocide. However, without adequate plant hygiene, it may not be possible to economically eliminate (or even control) microbial contamination present in the product, let alone prevent any subsequent contamination during packaging, shipment and storage. With proper plant hygiene, the addition of biocide at this point should be for "insurance" reasons.

A product sampling schedule should be set up to monitor the condition of product ready for delivery. Experience will dictate how often to sample, starting with each batch and ending with perhaps weekly samplings when sufficient data is collected to justify this extended schedule. It is routine to retain small aliquots of each batch for QA/QC purposes, which should include a microbial assurance component.

7. Product transportation and delivery systems

At this point, plant hygiene has done all it can do to allow the manufacture of a microbial clean product. The biocide added during packaging is supposed to see the product through to its use by the customer. In some cases, there are off-loading issues in which partial batches of product are used over an extended period of time. In other cases, the shipping containers are returned for refilling. The proper cleaning and care of these containers obviously impacts the delivery of the next batch of product shipped in these containers.

As with the packaging systems, it is necessary to ascertain that biological contamination is not entering the product at these locations. Random samples should be taken, perhaps weekly, until sufficient data is collected to confidently reduce this schedule. Once the product reaches the customer, then a program must be in place to handle customer complaints about contaminated product. With a good plant hygiene program in place, it should be possible to rapidly locate and eliminate problem areas in the plant.