Cooler cleaning
Question: I would like to know how to clean
the drinking water machine in my office. Many people drink distilled water, which is dispensed from this drinking water machine. But I wonder how frequently should we clean the water machine inside? How do we clean it? Which kinds of disinfection solvents should we use to clean it? I would appreciate answers to these questions.

Murphy Chung
Hong Kong, China

Answer: Typically, water dispensers such as this should be cleaned at least once a month.  Assuming no one with a waterborne disease contaminates the unit, it will likely grow only non-pathogenic bacteria; however, high concentrations of certain kinds of these bacteria will produce objectionable taste and/or odor over time if the unit is not properly sanitized and maintained. Typically, soapy water and a clean cloth is all that’s required. If the buildup is extensive, though, you can make up a dilute solution of chlorine (sodium hypochlorite) and water to rinse out the cooler. Concentration of about 10 ml per liter of water will disinfect the unit effectively. Let us know if we can be of further assistance.

Filters: Double or nothing
Question: I am doing my Ph.D. studies on filtration. I require some information:

1.    What is the effect on pressure drop and flow rate if two cartridges are joined in series to form a single cartridge?
a.    Will pressure drop double?
b.    Will flow rate double?
2.    What is the effect on pressure drop and flow rate if two filter cartridges are placed in parallel?
a.    Will pressure drop double?
b.    Will flow rate double?

Also, if you could, tell me about some relevent books and literature. Your assistance is highly appreciated.

Stephen Flemming

Answer: When two filters are combined in series and operated at the original flow rate, the pressure drop is doubled if they are the same type of filter. If flow rate is doubled across the combined filters, then pressure drop increases by a factor of four. When two filters are combined in parallel and operated at the original flow rate, the pressure drop is reduced in half if they are the same filter. If the flow rate is doubled across the combined filters operating in parallel, then the pressure drop returns to the same pressure as a single filter operating at the original flow rate. In general, filters should be operated in parallel when possible. Filters are operated in series only when one filter serves as a “guard” for the other filter in a critical application.

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