Once the U.S. EPAs Disinfectants and Disinfection Byproduct Rule (D/DBP) became a fact of life last December, water systems across the United States began taking a closer look at their operations. Given the complexity of the regulation, many of them have asked for advice about how to meet its enhanced coagulation section. The immediate response usually is, "Look at your database, and dont be hasty."
The danger is that systems will change coagulants when they are not required to under the rule, perhaps jeopardizing their ability to meet other treatment goals and increasing costs. This article stresses how to approach the Enhanced Coagulation section of the D/DBP Rule and find the best approach for a given treatment situation.
Compliance with the part of the rule that involves enhanced coagulation can occur through one of three steps:
1. Determine if water quality meets the Step 1 matrix for TOC removal. Begin by looking at the plants historical TOC removal data. If treatment meets the percent TOC removal in the matrix, monthly samples must be collected, analyzed, and documented. If TOC removal is close to this level, slight adjustment or optimizations in treatment may bring the plant into compliance.
2. If the Step 1 removal levels cannot be satisfied, the six Step 2 alternative criteria should be examined by reviewing the plants historical database for TOC and SUVA data. Plants that document compliance with any of the six do not need to practice enhanced coagulation.
3. If the plant does not meet Step 1 or Step 2 criteria, a point of diminishing returns (PODR) should be set.
While nearly all systems understand how to use the Step 1 matrix, there is some confusion about working with the Step 2 criteria and the PODR. The following examples should help in this regard.
Meeting Step 2
Historical data at one plant (Table 1) showed that its raw water TOC averaged 2.23 ppm and its alkalinity averaged 83 ppm (as CaCO3). Although the Step 1 matrix sets removal at 25 percent TOC at this TOC and alkalinity level, the data showed that the plant could not remove more than 23 percent TOC on average. It thus needed a different compliance strategy.
Meeting Step 2
The data showed that the plant meets Step 2 of the regulation, because treated water TOC averaged less than 2 ppm. The data also showed compliance with the regulation based on an average treated water SUVA(TOC) of below 2 L/mg-m. However, a SUVA calculation based on DOC is required to meet the Step 2 criteria.
Meeting Step 2
Recent data for this plant showed DOC and TOC to be similar, but historically the data has not been collected and must be in order to support compliance. The plant has begun to gather this data by making monthly measurements of TOC, DOC, and UV254 in raw and treated waters a part of its normal operations to provide proof of compliance under the rule.
Meeting Step 2
In relation to evaluating DOC, some plants find their tests show higher DOC values than TOC. This indicates a testing error, because DOC is measured after insoluble organic particulates are filtered out and therefore should be less than TOC. If this occurs, look for sources of organic contamination in the filtration procedure.
Establishing a PODR
Another plant also began by evaluating it historical data (Table 2) to define a compliance strategy. With an average raw water TOC of 4.6 ppm and an average alkalinity of 150 ppm (as CaCO3), the plant needed to remove 25 percent TOC under the Step 1 matrix. The plant averaged less than 25 percent removal. Since it was optimized for many factors including TOC treatment, the plant could not easily improve TOC removal, so it explored its ability to meet Step 2 criteria.
Establishing a PODR
The average treated-water SUVA was 2.2 L/mg-m and treated TOC was 3.5 PPM during an 18-month period. Therefore, the plant needed to improve treatment performance to meet Step 2 criteria. At this point, it decided to demonstrate a "point of diminishing returns" for additional coagulant use.
Establishing a PODR
In the first set of jar tests, the plant determined the coagulant dose needed to lower the treated water pH to equal to or less than the target pH. Incremental coagulant doses found that about 115 ppm of the alum/polymer coagulant was needed to meet the target pH.
Establishing a PODR
This is the maximum amount of coagulant needed to satisfy the rule. To find the PODR or the point at which a set amount of aluminum metal, 0.91 ppm, removes less than 0.3 ppm of TOC, the plant ran another jar test series.
Establishing a PODR
For the alum/polymer blend used as a coagulant at this plant, the incremental dose of 23 ppm (liquid) is equal to the addition of 0.91 ppm of aluminum metal (Table C in sidebar/page 74). The PODR occurred at 68 ppm, where the required 23 ppm increment dosage increase removes less than 0.3 ppm of TOC. At this point, 3.7 ppm of TOC remained in the treated water. Since the raw water contained 4.6 ppm of TOC, this point correlates to an alternative TOC removal of 19.6 percent. This alternate TOC removal percentage must be obtained in full-scale plant operations and must be negotiated and set with the governing state.
Enhanced Coagulation Considerations
Use of enhanced coagulation requires an informed approach, since it can pose other significant operational concerns. For instance, coagulation can be enhanced by operating at a lower pH, but decreasing treated water pH can increase corrosion of equipment and concrete treatment basins, as well as increase costs associated with post- treatment pH adjustment. The corrosion control program for the distribution system may also be drastically impacted by changes in coagulation practices.
Enhanced Coagulation Considerations
Enhanced coagulation also can mean higher coagulant doses than those needed to lower turbidity and particulates. This can generate more chemical solids, higher settled and finished water turbidities, and increased plant solids disposal issues/costs.
Enhanced Coagulation Considerations
The EPA has gone to great lengths to make this section of the D/DBP Rule a way to improve or optimize water quality with regard to organic removal while balancing other water quality and plant operating issues. The rule was not written to force plants to change coagulants or coagulation practices. Overall water quality remains the primary goal.
Enhanced Coagulation Considerations
TOC removal can be optimized by choosing the right coagulant, dose and treatment pH, among other factors. Non-traditional coagulants, like aluminum chlorohydrate (ACH), polyaluminum hydroxychloride (PACl), PACl/polymer formulations or alum/polymer formulations, also have a role to play because they carry more positive charge than traditional coagulants. These coagulant families can provide the required aluminum metal in an effective form that helps utilities meet the rule as well as other important treatment goals, such as cold water performance, decreased solids production, and decreased alkali demand.
Enhanced Coagulation Considerations
Acidified alum coagulants, which typically have 1 percent to 15 percent free sulfuric acid, can aid in TOC removal by depressing pH more significantly than straight alum and utilizing a lower treated-water pH for more effective TOC removal.
Enhanced Coagulation Considerations
Even though early results showed iron-based coagulants to be promising for TOC removal, the initial wave of plants considering iron salts has slowed significantly due to ancillary factors. For instance, the amount of ferric metal that must be added incrementally to meet PODR tests and requirements is more than twice that of any aluminum coagulant (e.g., alum, ACH and PACl) on a metal basis. Use of ferric metal thus gives a greater increase in chemical solids, making these coagulants undesirable for many plants at a time when disposal costs are rising.
Enhanced Coagulation Considerations
The incremental metal dosage difference produces 42 percent more ferric hydroxide than aluminum hydroxide. This is without considering any additional sludge produced in raising the pH back to acceptable levels with lime. Also, feed equipment changes are required if the feed system was designed for alum. Only plants built in the shadow of the rule have feed systems constructed of material compatible with and sized for ferric and aluminum coagulants. Some exceptions do exist, but they are rare. Ferric salts also increase time needed to maintain operating equipment, clean stained basins and remove solids, all of which increases costs. And some plants find they cannot meet their year round turbidity and other water quality goals with ferric salts.
Enhanced Coagulation Considerations
TOC removal is site specific because of unique source waters, plant factors and such events as runoff, ice melt and algae blooms. The D/DBP Rule challenges water systems to increase TOC removal and still meet all existing treatment objectives as raw water properties shift.
Enhanced Coagulation Considerations
It is tempting to try to meet the rule by altering coagulants or coagulant practices, but systems should do as much as they can to comply without making changes that can raise costs and undermine goals for sludge reduction, pH control, corrosion control, and finished water turbidity and metal residuals. If a change is needed to boost TOC removal, plants should consider specialized coagulants like PACl and acid alum that can help them balance D/DBP requirements and other treatment goals.
