Most people believe that the water, in the drain tank of their refrigerant dehumidifiers, is clean. The liquid water that is collected, as a result of condensation, is expected it to be similar in purity to distilled water.
Unfortunately, there is also the potential that airborne particles and bioaerosols may become mixed with the water. As the fan moves air across the AC coils of a refrigerant dehumidifier, aerosolized particulates travel with it. Some of these particles impact onto the wet surfaces of the coils and become trapped in the water film which then drains into the holding tank. Over time, particles can accumulate in the tank. A residue may adhere to the wall of the tank and not be pumped out. Other particles can accumulate on the coils, reducing their ability to remove moisture efficiently.
Airborne particles may include bacteria, mold spores and other organic material that may be a food source for the bacteria and molds to grow. On a sewage loss it is possible that some of these bioaerosols can be toxigenic, pathogenic or allergenic. Left unattended, there is also the remote possibility of creating a potential health risk. As these organisms grow, they may produce odors in the form of microbial volatile organic compounds (mVOCs). These mVOCs are byproducts of digestion, in the form of gases, which can accumulate and be dispersed in high concentration when the unit starts up, creating that burst of odor you mentioned.
Growth on AC coils is slowed by the cold temperature, but when the unit is turned off the coils can warm up. As long as moisture and a food source are present, microbial growth can occur.
Water pumped from the drain pan may have some potential contamination, from exposure to the air that may be contaminated or from a poorly maintained drain pan. Realistically this situation is unlike to result in a major exposure risk, especially if the water was continuously being pumped out of the tank and back into the sanitary sewer system. In addition, if the air is contaminated sufficiently to create a problem in the dehumidifier drain water, the air itself would normally be considered a much greater hazard.
Many refrigerant dehumidifiers today have filters that trap some of these aerosolized particles before they get into the unit. The more efficient the filter, the fewer and smaller the particles that will make their way into the machine. This should result in fewer problems with microbial growth and resultant odors and potential health effects; however, few if any commercially available refrigerant dehumidifiers have HEPA or other high-level filtration systems, which would be necessary to prevent this problem completely.
To keep these potential problems from getting out of hand, you need to implement two common-sense rules for the prevention of microbial growth:
- Keep it dry, and nothing will grow.
- If you cannot keep it dry, and the surface is non-nutritive, then keep the surface clean to prevent or at least minimize microbial amplification.
While refrigerant dehumidifiers are operating, you cannot keep the exposed surfaces dry, as condensation is necessary for the machine to perform as they are designed. However, since the exposed surfaces are non-nutritive, all you need to do is keep them clean to minimize microbial amplification.
I recommend you clean dehumidifiers thoroughly after each job and before they go out to the next job. Exact procedures vary by model, but a thorough cleaning/decontamination process would generally involve disassembly; thorough cleaning of the coils; drain pan and exposed surfaces; cleaning or replacement of the filter; and reassembly. Consult the manufacturer instruction for cleaning/decontamination procedures suitable for that particular model. Inappropriate cleaning procedures may damage the unit.
When the dehumidifier has been used on a bacterially contaminated project, you might want to consider using, as part of the cleaning process, a biocidal product registered with the EPA for use on equipment. Follow the label instructions and wear appropriate personal protection equipment if required.
Refrigerant dehumidifiers vary significantly in how difficult they are to disassemble, clean and reassemble. You might want to take this into account when purchasing equipment. In a severely contaminated environment, you might also want to preferentially use equipment that is easier to decontaminate.
— Jim Holland