Disinfection Technologies: How Do You Choose?

Effectiveness

The first and most important thing to consider when comparing disinfection technologies is how effective they are at disabling pathogens, such as cists, viruses, and bacteria. In a water treatment system there are two measures of disinfection: immediate and residual.

Immediate disinfection refers to the initial ability of the chemical or UV rays to kill microorganisms. Under normal circumstances, chlorine, PAA, ozone, and UV may appear equal in effectiveness. However, turbidity can shield pathogens from disinfectants. For UV systems, turbidity is an even bigger issue because it can prevent UV rays from penetrating the water entirely, rendering it largely ineffective. This may be especially true with industrial wastewater or in systems where public and industrial wastewater are mixed.

Residual disinfection occurs downstream as the disinfectant travels through the distribution system, and only chlorine offers residual disinfection. Ozone and PAA react too quickly, dissipating within minutes of being administered. UV cannot provide any disinfection beyond the lamp system. This may be acceptable in many applications, particularly for wastewater that is discharged to sensitive bodies of water. This is why it’s important that decision makers have a clear understanding of the demands that will be put on the system, and the results they expect to achieve.

Safety Factors

No disinfectant is without safety concerns. Ozone is administered as a gas and is a powerful irritant that can damage lungs. While ozone has a distinct smell, the odor is not always strong enough to detect.

Chlorine is also a powerful irritant to lungs, skin, and eyes. As such, operators must wear appropriate personal protective equipment (PPE). However, chlorine also has a powerful odor that is immediately noticeable, giving operators ample warning of a leak or spill.

UV light can be harmful to eyes and skin, although the lamps are typically enclosed during operation and rarely pose a threat.

PAA can cause damage to the skin, eyes, and lungs. Those handling the chemical should do so in a well-ventilated room and wear appropriate protective gear. It is also corrosive.

Dosing Levels

Neither ozone nor UV can be overdosed. UV disinfection does not add chemicals. Meanwhile, ozone dissipates quickly, breaking down into oxygen within minutes. PAA breaks down into acetic acid, water, and oxygen. This makes it safe to treat effluent that is discharged into source water, or other bodies with high levels of organic content. It oxidizes almost instantly, so there is rarely any residual chemical after discharge.

The same is not true for chlorine. While some downstream chlorine is beneficial for its residual disinfection, operators must use precise dosing via advanced chemical metering (Figure 1).

Cost And Availability

UV systems have a high upfront cost but are often easy and affordable to maintain –– they do not require any chemicals, and the only operating cost is electricity and the occasional lamp replacement.

Ozone, which is provided in and administered using compressed gas cannisters, has a moderate upfront cost but is very expensive to run as the gas is difficult to make. PAA is also expensive and the number of manufacturers and distributors is far fewer. However, as adoption of PAA as a disinfectant is increasing, these factors are likely to change. It is also regulation agnostic and has a far longer shelf life than chlorine.

Chlorine is the most affordable. Not only is it inexpensive and easy to obtain, but chlorine can be easily dosed using efficient and costeffective metering pumps. Even when combined with ancillary equipment such as pH and chlorine analyzers, flow meters, and chlorine level sensors, chemical metering / dosing pumps are often more cost effective than other disinfection technologies. Advanced features such as maintenance alerts and failure warnings can help ensure that any given pump continues to run effectively for a long time.