There are many steps involved in properly treating wastewater so that it meets regulatory requirements and can be safely reused or discharged. Disinfection is a critical step in the process. Disinfection kills or disables pathogens that may harm humans, the ecosystem, or both.
While Sodium Hypochlorite is the most well-known and widely used disinfectant, other technologies are being used successfully, and may be worth considering. These include Peracetic Acid (PAA), Ozone and UV disinfection.
It is critical for engineers, project managers and operators to understand the pros and cons of each disinfection technology before incorporating them into a water treatment system.
Effectiveness
One of the most important considerations 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.
With UV systems turbidity is an important factor because it can prevent UV rays from penetrating the water entirely, which renders 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 delivers residual disinfection. Ozone reacts too quickly, dissipating within minutes of being administered. UV cannot provide any disinfection beyond the lamp system, PAA does not provide residual disinfection.
This may be acceptable in many applications and is another example of 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 some 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, but the lamps are typically enclosed during operation and rarely pose a threat.
Peracetic Acid is corrosive and can cause injury to the skin, eyes, and lungs. Those handling the chemical should do so in a well-ventilated room and wear appropriate protective gear.
Dosing Levels
Neither ozone nor UV can be overdosed. UV disinfection does not add chemicals, and ozone dissipates quickly, breaking down into oxygen within minutes. The additional oxygen adds a pleasant taste to water with no adverse effects.
The same is not true for chlorine. Excess chlorine can dramatically change the taste and odor of water, making it unpleasant or harmful to drink. While some downstream chlorine is beneficial for its residual disinfection, operators must use precise dosing via advanced chemical metering pumps and systems.
Peracetic Acid, 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. PAA oxidizes almost instantly, so there is rarely any residual chemical after discharge. PAA does not form any known DBPs.
Unintended Effects
In addition to the impact on taste and odor, chlorine has two other secondary effects that can impact usage. The first is corrosion. Unlike UV and ozone, chlorine can, over time, damage pipes and equipment. While this impact is often minimal, and depends largely on the amount of chlorine in the water, it should still be considered.
The second is disinfection byproducts (DBPs). These substances can form when natural organic matter reacts with chlorine and can be harmful if consumed or inhaled. DBPs can be mitigated with good upfront filtration and treatment.
Cost And Availability
UV systems have a high upfront cost but are often easy and affordable to maintain –– they do not require chemical use, and the only operating cost is electricity and 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 because the gas is difficult to make.
Chlorine is the most affordable. Not only is it cheap and easy to obtain, but chlorine can be dosed using efficient and cost-effective metering pumps. Even when combined with ancillary equipment, chemical metering / dosing pumps are more economical than other disinfection technologies.
Peracetic Acid is more expensive than chlorine due, in part, to the fact there are fewer manufacturers and distributors. 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.
Working Together
Disinfection does not have to be all or nothing. For example, it is possible to add small amounts of chlorine after initial UV or ozone treatment in order to take advantage of residual disinfection.
The final decision will depend on a variety of factors, including total cost, operator expertise, downstream operations, and infrastructure conditions, among others.
It’s advisable to work with knowledgeable vendors to design a system that meets the requirements of the application and provides dependable service.
Written by:
Blue-White® Industries
714-893-8529
