Comparison of Methods

Not all water disinfection methods are created equal. Some methods cause environmental and health problems even as they attempt to improve water quality.

Let’s look at a few of these alternatives in turn.

Disinfection Method: Chlorine

Chlorine has been the purifying chemical of choice since its first application in the early 20th century. Its disinfection properties have meant the virtual eradication of serious municipal drinking water-borne pathogens such as cholera and typhus. Now, in the 21st century, scientists have begun to catalog the risks of continued use of chlorine. With the availability of new, safer, and more effective disinfection alternatives, the pressure is increasing to reduce or even eliminate the use of chlorine in public water supplies.

The liabilities associated with chlorine disinfection are several:

• Chlorine is a highly toxic and hazardous substance in undiluted form.
• There are dangers associated with shipping, storage, and handling chlorine.
• Businesses that handle chlorine are faced with growing liability expenses.
• De-chlorination treatment requires use of other toxic chemicals, such as sulfur dioxide.
• Chlorine doesn’t meet EPA guidelines for killing the common pathogens cryptosporidium and giardia.
• Chlorine provides an unpleasant taste and odor.
• Chlorine application requires extensive civil works and land area.
• Chlorine application can produce carcinogenic disinfection by-products (DBPs).

Disinfection Method: Ozone

Ozone is a strongly oxidizing gas. Since it is a very powerful disinfectant, water managers have experimented with the application of ozone generators over the past 40 years. However, the technology has yet to achieve sizeable market acceptance due to concerns over both safety and cost. Some of those concerns:

• Ozone is toxic to life forms and highly explosive.
• Ozone generators (and associated gas safety containment equipment) are expensive to construct, install and maintain; they are susceptible to leaks between the time of generation and processing.
• Excess ozone presents a disposal/safety issue.
• Ozonation oxidizes certain types of naturally occurring organic matter in water, creating more hazardous compounds (bromates, a form of DBP), which have been linked to cancer and birth defects in humans.

  Disinfection Method: Membrane Filtration

  Filtration is a physical separation technique; disinfection by filtration requires that the pathogens be permanently removed from the process output stream. Unfortunately, this technique does not in itself kill the microbes and render them harmless; they are still viable and must be handled and disposed of in some other way. Forms of filtration include reverse osmosis and microfiltration.

Disadvantages of membrane filtration:

• Generally requires high water feed pressures, which increases complexity and cost.
• Wastes a portion of the water, and that portion is always contaminated, thus producing disposal issues.
• Upstream (high-pressure) side of fragile membranes collect material and can grow microbes, eventually clogging the filter.
• Fouled membranes are ineffective, and subject to rupture and subsequent loss of disinfection integrity.
• Membranes are expensive and require periodic replacement.
• Total operating and maintenance costs are very high.

The use of porous membranes and filters to trap bacteria and other pathogens in water has been viewed as a potential non-chemical alternative to other disinfection technologies. However, while membranes can be quite effective in other applications in water treatment, their use in disinfection-specific applications is compromised by the significant risk of membrane breakage or damage, increasing the probability of microbial contamination. Unfortunately, there is no practical method to quickly identify compromised membranes and restore integrity.