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The plant effluent coliform count fails to meet required standards for disinfection. The chlorine residual is normal. Inspection of the disinfectant supplies and equipment reveal no problems. What is the most probable cause of the elevated effluent coliform count?

A. Capacity of chlorinator is too low
B. Too little solids in the effluent
C. Chlorine demand is too high
D. Contact time is too short

Answer: D. To effectively kill the coliform bacteria, an adequate dosage of the disinfecting chemical is needed with a long enough contact time. In the case shown above, the chlorine residual is normal and the chlorine supply and equipment are working well. Had the chlorine demand been too high (C), the chlorine residual would not be normal; it would be very low, and that would affect the ability of the chlorine to disinfect. Some state regulatory agencies require specific minimum hydraulic retention times in chlorine contact tanks. For example, Florida Administrative Code requires no less than 15 minutes of contact time at peak hourly flow to ensure effective disinfection of coliform bacteria.


Which condition causes faster formation of total trihalomethane?

A. Low pH and warm water temperature
B. High pH and warm water temperature
C. Low water temperature and high pH
D. High hardness and low water temperature

Answer: B. The formation of total trihalomethane is dependent on pH and temperature, among other factors. Trihalomethanes form in water treatment and water distribution systems when organic material containing humic and fulvic acids reacts with free chlorine residual. Free chlorine residual can be in the form of hypochlorous acid (HOCl), the hypochlorite ion (OCl-), or both. Higher pH levels, over 8.0 for example, tend to favor the formation of the hypochlorite ion over hypochlorous acid. The hypochlorite ion is very reactive with organic acids and can form various combinations of trihalomethane. The four trihalomethanes of concern are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Warm water temperature will certainly allow faster and more thorough chemical reactions, thereby creating total trihalomethanes faster than in colder water conditions.

About the author

Ron Trygar, a certified environmental trainer, is the senior training specialist for water and wastewater programs at the University of Florida’s TREEO Center. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. He holds a Florida Class A wastewater treatment operator license and a Florida Class B drinking water operator license.


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