Enhanced Coagulation: A New Challenge

The recently promulgated Stage 1 of the Disinfectants/Disinfection Byproducts Rule (DBPR) includes sections dealing with the use of enhanced coagulation and enhanced softening to help reduce the concentration of disinfection byproducts (DBPs) in treated drinking water.

The recently promulgated Stage 1 of the Disinfectants/Disinfection Byproducts Rule (DBPR) includes sections dealing with the use of enhanced coagulation and enhanced softening to help reduce the concentration of disinfection byproducts (DBPs) in treated drinking water.

DBPs are formed by oxidation of organic or inorganic matter in a water source during disinfection. One method of reducing the amount of DBPs is to reduce the amount of organic matter available to react with the disinfectant. This can be accomplished through precipitation of humic compounds by coagulation or softening, adsorption of organics onto granular activated carbon, or physical separation of the organic material using membranes.

A common indicator of organic material in water is total organic carbon (TOC). The new rule requires all public water systems to reduce the amount of TOC in the raw water prior to disinfection, unless the water meets certain alternative compliance criteria. Another measurement of organic material in water is SUVA, the specific ultraviolet absorbance at 254 nanometers. SUVA of a water sample is calculated by dividing the ultraviolet absorption obtained at 254 nanometers by the concentration of dissolved organic carbon. Because accurate SUVA measurements can be obtained only before the addition of a chemical disinfectant, it normally would not be determined for treated waters, as most utilities add a disinfectant ahead of filtration.

Applicability

Although it initially applies only to water systems that serve more than 10,000 people, the Stage 1 DBPR will apply to all public water systems that treat their drinking water with a chemical disinfectant. Because ground waters normally do not contain substantial amounts of organics, the enhanced coagulation and enhanced softening requirements of the rule will affect only systems that use conventional filtration to treat a surface supply or a water source that is directly influenced by a surface water.

Applicability

Compliance with the new rule will normally be attained by removing a set percentage of TOC. There are alternative compliance criteria. They include:

Applicability

  • documenting that the source water has a TOC concentration less than 2.0 mg/L or a SUVA of 2.0 L/(mg)(m) or less;

Applicability

  • documenting that the treated water has a TOC concentration less than 2.0 mg/L or a SUVA of 2.0 L/(mg)(m) or less;

Applicability

  • maintaining total trihalomethane (TTHM) and haloacetic acid (HAA5) concentrations of 0.040 mg/L and 0.030 mg/L or less, respectively, when the source water has a TOC concentration less than 4.0 mg/L and an alkalinity higher than 60 mg/L; or

Applicability

  • having TTHM and HAA5 levels which do not exceed 0.040 mg/L and 0.030 mg/L, respectively, within the distribution system while using only free chlorine for disinfection.

TOC Removal

To meet the rules Step 1 removal criteria, systems must remove a set percentage of TOC based on the amount of TOC in their water, and the source water alkalinity. As an example, a municipality with untreated water which has an initial TOC of 4.63 mg/L and an alkalinity of 100 mg/L can comply with the Step 1 criteria by removing at least 35 percent of the TOC. Depending on conditions, removal can be accomplished with the addition of about 45 mg/L of alum. Day-to-day water quality changes will necessitate adjustments in the alum dose to consistently achieve a minimum TOC removal of 35 percent.

TOC Removal

Water systems whose current treatment process does not remove a sufficient percentage of TOC will be required to conduct either jar tests or pilot testing to determine whether increasing the coagulant dosage will result in additional TOC removal.

TOC Removal

The intent of the rule is not to compel treatment facilities to add large amounts of coagulant without a corresponding reduction in TOC. Consequently, there are limits to the amount of additional coagulant required by the rule. If coagulant addition depresses the pH below a certain target value or if less than 0.3 mg/L of TOC is removed for an additional 10 mg/L increment of alum, (The DBPR inadvertently expressed alum as aluminum. This technical correction is forthcoming from EPA.), then the water system can ask the state to certify an alternate TOC removal percentage that must be met.

TOC Removal

An example of compliance based on Step 2 criteria is shown on Figure 2. In this instance, a 35 percent reduction in TOC would normally be required; but the water system can comply by removing approximately 28 percent or more of the TOC because increasing the alum dose beyond 30 mg/L does not result in substantial additional TOC removal.

TOC Removal

The requirements of the rule can be met in still another way. If coagulant addition alone depresses the pH to a certain level (the Step 2 target pH), then the percent TOC removal reached at that pH can also be designated by the state as an alternate TOC removal percentage. Supposing that the pH in the example just discussed dropped to the target pH before the addition of 30 mg/L of alum. If so, whatever TOC removal was obtained with that particular alum addition will satisfy the requirements of the rule.

TOC Removal

As mentioned previously, either jar tests or pilot testing will be used to determine the amount of TOC required to be removed. Because Step 2 testing, rather than the alum dose or the target pH, determines the percentage of TOC that must be removed, it is essential that the water system conduct seasonal testing in order to determine TOC removals at varying water quality conditions.

TOC Removal

EPA understands that some water systems will not use alum as a coagulant, and minimum TOC removals can be established using either alum or ferric salts. The Guidance Manual which will accompany the Rule also gives a procedure which can be applied when other coagulants are used.

TOC Removal

Systems that soften must also comply with the requirements for TOC removal. Compliance can be accomplished in one of three ways if one of the alternative compliance criteria has not already been satisfied. First, if the treatment process can achieve the TOC removal percentages given in the last column of Table 1, the system is in compliance. If these TOC removals cannot be met, then either a minimum of 10 mg/L of magnesium hardness expressed as CaCO3 must be removed or the treated water alkalinity must be lowered to at least 60 mg/L. Systems that practice softening using a pH higher than 10.5 should not experience any difficulty in meeting the requirements of this rule, as the majority of these systems will remove at least 10 mg/L of magnesium as CaCO3.

Summary

Implementing the enhanced coagulation and enhanced softening requirements of the Stage 1 D/DBPR will be a challenge to many water purveyors. It is important to understand how the new rules will affect your water treatment process and to begin planning to implement any necessary modifications or operational changes.

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