Halo acetic acids (HAAs): The Chlorine Byproduct Threat, and How to Get Rid of Them

Haloacetic acids (HAAs) are another group of disinfection by-products (DBPs) that form when chlorine or other halogen-based disinfectants react with natural organic matter (NOM) in water. Like trihalomethanes (THMs), HAAs are regulated due to their potential health risks, including carcinogenicity and reproductive effects.

1. Formation of Haloacetic Acids (HAAs)

The formation process of HAAs is quite similar to that of THMs:

A. Presence of Organic Matter (NOM)

Natural organic matter, which includes compounds such as humic substances, fulvic acids, and amino acids, is present in most source waters. These organic precursors are often derived from the decay of plant and animal matter in surface waters and can react with disinfectants to form HAAs.
B. Chlorination and Halogenation
When chlorine (Cl₂) or chloramine (NH₂Cl) is added to water, it reacts with the NOM. This reaction typically involves the substitution of hydrogen atoms in the organic compounds with halogen atoms, primarily chlorine or bromine (if bromide is present). This halogenation process results in the formation of HAAs.
C. Key Factors Influencing HAA Formation
Several factors influence the formation of HAAs:
Chlorine Dose: Higher chlorine doses can increase the formation of HAAs.
– Contact Time: Longer contact times between chlorine and NOM can increase HAA formation.
– pH: HAAs typically form in slightly acidic to neutral pH conditions. Higher pH levels favor the formation of other by-products like THMs rather than HAAs.
– Bromide Presence: If bromide is present in the water, brominated HAAs such as dibromomethane (DBAA) can form.
– Temperature: Higher temperatures accelerate the reaction rates, leading to increased HAA formation.

D. Types of Halo Acetic Acids

There are five major regulated HAAs:

• Monochloroacetic acid (MCAA)
• Dichloroacetic acid (DCAA)
• Trichloroacetic acid (TCAA)
• Mono bromo acetic acid (MBAA)
• Di-bromoacetic acid (DBAA)

Together, these are referred to as HAA5, and water quality regulations generally focus on controlling the sum of these five compounds in drinking water systems.

2. Removal of HAAs

To manage the formation of HAAs, water treatment plants often focus on a combination of precursor removal (to reduce organic matter), alternative disinfection strategies, and post-treatment to remove formed HAAs. Below are the methods for controlling or removing HAAs:

A. Reducing Organic Precursors (Source Control)

Enhanced Coagulation and Flocculation: By using coagulants like alum, ferric chloride, or ferric sulfate, water treatment plants can remove organic matter before the chlorination stage. Enhanced coagulation is a technique where coagulant dosing is optimized to target NOM and reduce the potential for HAA formation.

– Activated Carbon (GAC/PAC): Granular activated carbon (GAC) or powdered activated carbon (PAC) can adsorb organic precursors in water, which reduces the potential for HAA formation. Activated carbon is particularly effective in removing dissolved organic carbon (DOC), which is a key precursor for HAAs.

– Pre-oxidation with Ozone or UV: Ozone and UV treatment can be used before chlorination to break down organic matter and reduce HAA formation. These methods oxidize organic compounds into smaller, less reactive molecules that are less likely to form HAAs when exposed to chlorine.

B. Alternative Disinfection Methods

– Chloramines (NH₂Cl): Using chloramines instead of free chlorine reduces the reactivity with NOM and significantly lowers the formation of HAAs. Chloramines are less reactive and form fewer halogenated by-products compared to free chlorine.

– Chlorine Dioxide (ClO₂): Chlorine dioxide is another disinfectant that does not form significant levels of HAAs. It is often used in specialized applications where controlling by-products like HAAs is important.

– Ozone and UV Disinfection: Ozone or UV light can be used as primary disinfectants in water treatment. These methods don’t produce HAAs, as they do not involve halogen-based chemistry. Chlorine can still be used for secondary disinfection to maintain residual disinfection, but in much lower doses.

C. Post-Treatment Removal of HAAs

If HAAs have already formed, certain treatment processes can be used to remove them:

– Granular Activated Carbon (GAC): GAC can adsorb organic contaminants, including HAAs, after they have formed. It is a commonly used post-treatment method to reduce HAA concentrations in drinking water distribution systems.

– Air Stripping: Air stripping can be used for the removal of volatile HAAs, but it is less effective compared to its use for volatile THMs. Only certain HAAs, such as monochloroacetic acid (MCAA), may be removed by this method, and the process is generally not considered a primary method for HAA removal.

– Advanced Oxidation Processes (AOPs): Advanced oxidation processes, such as those that use ozone combined with hydrogen peroxide or UV light, can break down HAAs through oxidation. These processes generate hydroxyl radicals, which are highly reactive and can degrade HAAs into smaller, less harmful compounds.

D. System Management Practices

– Chlorine Dose Optimization: Reducing the chlorine dose to the minimum required for effective disinfection can help limit HAA formation. This can be achieved by real-time monitoring of chlorine levels and carefully adjusting doses based on the water’s organic content.
Reduce Chlorine Contact Time: Minimizing the contact time between chlorine and organic matter by shortening residence times in storage tanks or reservoirs can reduce HAA formation.

– pH Control: Adjusting the pH during chlorination can help reduce the formation of HAAs. Acidic pH levels tend to reduce the formation of HAAs, while neutral to slightly alkaline pH levels may promote their formation.

3. Summary of HAA Control Strategies

– Precursor Removal: The most effective way to reduce HAA formation is by removing natural organic matter before chlorination. Enhanced coagulation, GAC/PAC filtration, and ozone/UV pre-treatment can all reduce organic precursors.

– Alternative Disinfection: Disinfectants like chloramines, chlorine dioxide, ozone, and UV can reduce or eliminate HAA formation by avoiding the use of free chlorine.

– Post-Treatment: Methods like GAC filtration, air stripping, reverse osmosis, and advanced oxidation can be used to remove HAAs after they form.

– Operational Adjustments: Optimizing chlorine dosage, minimizing chlorine contact time, and controlling pH are practical ways to reduce HAA formation during water treatment.
By combining these strategies, water treatment facilities can effectively manage HAA levels, ensuring compliance with regulatory limits and reducing health risks associated with these by-products.