Strategies for Reducing Disinfection Byproducts
The Environmental Protection Agency (EPA) developed the Surface Water Treatment Rule (SWTR) to create a safeguard for public drinking water. While this rule requires plants to treat surface waters with disinfectants, the disinfectants they use can react with natural organic matter (NOM) in their source waters to form disinfection byproducts (DBPs) that may be carcinogens. As a result, the EPA has implemented DBP rules that regulate DBP compounds such as trihalomethanes (THMs) and haloacetic acids (HAAs).
Utilities must therefore simultaneously comply with two sets of EPA rules. The first set requires them to disinfect source waters. The second set requires them to reduce likely DBP precursors, such as total organic carbon (TOC), that could potentially lead to the formation of disinfection byproducts. This latter ruleset applies to surface water treatment and to groundwater treatment for groundwaters that are influenced by surface waters.
The Chemistry of Disinfection Byproducts
The extent to which, and types of, DBPs that form in any given system depend on several variables:
The amount of TOC: A measurement of NOM, TOC is arguably the most significant variable. The greater the concentration of TOC in the source water, the greater the probability that DBPs will form when the source water is subjected to disinfectants (if the TOC is not removed).
The type of disinfectant: Several types of disinfectants are available for water treatment, the most common of which is chlorine (either as chlorine gas or sodium hypochlorite). Chlorine is inexpensive, a powerful oxidant, and an efficient disinfectant. However, it can lead to the formation of DBPs. This disinfectant, along with others, requires effective pretreatment for TOC removal to minimize DBP formation.
More and more communities are using alternative disinfectants, including ultraviolet (UV) light, chloramines, chlorine dioxide, and others. UV light works well to control pathogens but does not provide a residual disinfectant within the distribution system (an SWTR requirement). Thus, utilities that use UV light need a secondary disinfection system.
Chloramine is an effective disinfectant that has the advantages of being inexpensive and of producing reduced DBP formation. Its disadvantages include additional contact time and complexity in feeding. Chlorine and ammonia are both required to make chloramine, and balancing the dosages complicates the process.
Chlorine dioxide is an effective oxidant and disinfectant, and using it results in fewer issues with DBP formation. However, it is generated on site, requires additional equipment, and can be very expensive.
Disinfectant contact time: Per the SWTR, contact time with the disinfectant before distribution is required for proper disinfection. However, the longer TOC is in contact with certain disinfectants, the more DBP-producing chemical reactions occur.
Water temperature: Water temperature affects chemical-reaction kinetics. Reactions occur faster in warmer waters.
Water pH: pH influences the effectiveness of the disinfectant. It also affects chemical reactions and often plays an important role in determining the type of disinfection byproduct that forms. Disinfectants in low pH waters tend to create more HAAs, while in high pH waters, they tend to form more THMs.
Disinfectants and Disinfection Byproducts Rules
The EPA’s Stage 1 and Stage 2 DBP rules apply to all community and nontransient noncommunity water systems (CWSs and NTNCWSs) that
use any type of disinfectant other than UV or deliver disinfected water, and transient noncommunity water systems (TNCWSs) that add chlorine dioxide. (Source: EPA Comprehensive Disinfectants and Disinfection Byproducts Rules.) These rules currently focus on a handful of DBPs – namely, THMs, HAAs, bromate, and chlorite. However, additional DBPs are under consideration, including chlorate, nitrosodimethylamine (NDMA), and brominated and iodinated compounds. The following table contains the maximum contaminant levels (MCLs) and MCL goals (MCLGs) of currently regulated disinfection byproducts.
Utilities have several options for complying with DBP rules. The option they select depends upon their source waters, treatment trains, preferred disinfectant, and so forth. For example, if testing has indicated that a plant’s source waters demonstrate seasonal increases in TOC that its existing processes are unable to efficiently reduce, it might choose to add granular activated carbon (GAC) contactors as a final step in its treatment train. This option would allow seasonal operation of the GAC system to address TOC increases while optimizing overall plant performance.
The Case for TOC Removal
Because TOC is the most significant variable in DBP formation, it isn’t surprising that the EPA’s DBP rules include TOC removal. EPA bases TOC removal levels on source-water alkalinity.
According to the EPA’s rules, plants must achieve the TOC removal percentages outlined in the table above through treatment techniques such as enhanced coagulation or enhanced softening unless their systems meet one of the organization’s alternative criteria.
By officially recognizing enhanced coagulation and enhanced softening systems as methods for reducing TOC concentrations, the EPA is tacitly acknowledging that it is better to remove organic matter before disinfecting the water (thereby reducing the number of DBPs that form as a result of disinfection).
Enhanced Coagulation and Enhanced Lime Softening
Most plants know if they are not meeting TOC-removal requirements because the EPA requires them to test their raw and treated water’s TOC and alkalinity at intervals that are based on the size of the population they serve and the types of source water they use.
Success Depends on Chemistry
The first step toward meeting these requirements is to determine what mix of coagulants, polymers, oxidants, and occasionally pH adjusters are the most effective. Some equipment providers, including WesTech, offer on-site pilot testing and water treatment experts who can perform jar tests that indicate the effectiveness of several mixtures to determine the preferred chemical components. For example, they might test multiple coagulants (generally various aluminum and ferric salts). They also test various polymers (anionic, cationic, and occasionally nonionic) to determine which provides the most effective flocculation.
The chemistry for enhanced coagulation can include additives that adjust the water’s pH to the range that optimizes coagulant effectiveness. For example, aluminum sulfate (alum) prefers pH in the range of 5.5 to 6.5. Alum can reduce alkalinity, which might require additional chemicals to prevent pH from dropping below the acceptable range.
Experts can test more powerful oxidants as well, such as sodium permanganate (NaMnO4) and potassium permanganate (KMnO4), which can assist with TOC removal and with taste and odor issues.
The additives that plants use in enhanced coagulation will change with the temperature of the water as well as its pH. Both qualities change with environmental conditions. In other words, while an effective means of reducing TOC, enhanced coagulation is not a set-it-and-forget-it method. Plant operators must adjust their chemical dosages as conditions change. Plants that lack expertise in this area can contract with operations services professionals to ensure that the plant remains compliant.
Enhanced lime softening chemicals depend on ionic reactions that encourage TOC to adsorb to calcium or magnesium in lime softener precipitates. According to EPA, at a pH of 10.6 or higher, enhanced lime softening also precipitates other contaminants – such as magnesium, silica, uranium, and radium – allowing plants to address multiple issues within a single unit process.
Recirculating settled solids increases the likelihood that DBP precursors and suspended solids will have the opportunity to react with other particles, polymers, and coagulants to form settleable or filterable (or both) flocs. While enhanced coagulation and enhanced lime softening methods require increased chemical addition, continuously recirculating settled solids reduces the necessary chemical input.
Equipment for Enhanced Coagulation and Enhanced Lime Softening
Plants can implement enhanced coagulation and enhanced lime softening in traditional solids contact clarifiers or high-rate clarifiers.
Traditional Solids Contact Clarifiers
Solids contact clarifiers provide flocculation and sedimentation in a single tank. In the case of WesTech’s Solids CONTACT CLARIFIER™, a unique, low-shear, high-volume pumping impeller efficiently recirculates settled solids while swept-back, curved blades minimize both required horsepower and floc-particle shear. The WesTech CONTRAFLO® provides a similar process using a ducted marine propeller for sludge recirculation.
Plants can use this type of equipment for either enhanced coagulation or enhanced lime softening, depending on their source waters and additional treatment goals. Groundwater treatment, for example, often includes hardness reduction as a goal, so enhanced softening would serve a dual purpose – hardness reduction and TOC removal. Plants can also adjust the source water’s contact time with recirculated solids and chemical additions to increase the rate at which solids and TOC precipitate.
In addition to designing and building new Solids CONTACT CLARIFIERs, WesTech can retrofit plants’ existing clarifier basins with Solids CONTACT CLARIFIER internals.
High Rate Clarifiers
High-rate clarifiers have smaller footprints than traditional clarifiers. They are a good solution for plants that need to add TOC reduction capabilities to an existing treatment train, particularly where space is limited.
WesTech has a variety of high-rate options that range from its CONTRAFAST® high-rate thickener clarifiers to its package water treatment plants.
The CONTRAFAST is available in two designs: a concentric steel tank configuration that can treat up to 2 million gallons per day per unit – or 7,600 cubic meters per day per unit – and an eccentric configuration for larger flowrates. Both configurations include an external sludge recirculation pump for solids contacting. Chemical feeds for enhanced coagulation and enhanced lime softening are available as options. Filters that provide further TOC and contaminant reduction, such as the CenTROL® filter or concrete filters, are also available as options. WesTech offers CONTRAFAST pilot options for plants that want to test its effectiveness with their particular source waters.
Existing filtration plants that have low net production and struggle to meet effluent requirements due to turbidity and TOC spikes can upgrade by adding an Adsorption Clarifier® ahead of the existing filters. The Adsorption Clarifier is an upflow clarifier that uses contact flocculation with buoyant media. It has the advantage of a high hydraulic loading rate (10 gallons per minute per square foot or 24 meters per hour) and acts as an efficient clarifier in a small footprint.
Plants that require equipment with an even smaller footprint can opt for a package water treatment plant that is designed to work with their raw water characteristics. The Trident® offers two stages of treatment with an Adsorption Clarifier in the first stage and a mixed media filter in the second stage, all within a single tank. Trident units are packaged, steel tank systems designed to replace conventional processes in an area that is about one-fifth the size of conventional systems. The Trident typically introduces chemical feed in line with the aid of a static mixer. The dosed water then flows into the unit’s adsorption-clarification (AC) section, which contains small plastic beads that aid in floc creation and capture. The floc particles, in turn, help capture organics from the water.
The clarified water flows to a section that contains a mixed-media filter capable of removing remaining floc particles and parasites such as Giardia lamblia and Cryptosporidium. For smaller plants, WesTech offers the same functionality in its Tri-Mite® equipment. Tri-Mite is a factory-assembled model that offers plug-and-play operation.
Package water treatment plants that offer two stages of clarification, such as the Trident® HS, increase retention time and are therefore suitable for plants with colder source water and waters with higher turbidity and TOC. (Retention time is still short relative to that of solids contact clarifiers.) The first stage of the Trident HS consists of settling tubes that function like a lamella clarification system. Solids collect at the bottom of the tubes as sludge. The unit includes WesTech’s Sludge Sucker™ to remove settled solids and has the ability to send some of the sludge back to the front of the tube settler clarifier for improved solids contacting. At this point, plants can add more chemistry to optimize the process that follows. The unit pumps water from the tube settlers to the AC clarifier. As with the Trident and Tri-Mite, the water then flows to a mixed-media filter that removes remaining particulate matter, including parasitic cysts. Both the Trident and Trident HS are available for pilot testing, which WesTech recommends.
WesTech’s RapiSand™ and RapiSand Plus™ provide solids contacting in the form of ballasted flocculation, a process that mixes microsand with polymers and coagulated solids to form a dense floc that settles rapidly. These packaged water treatment plants can reduce total detention time to as low as 20 minutes. The RapiSand first mixes raw water with coagulants. The water then flows to consecutive flocculation tanks, where it mixes the dosed water with polymer and recycled microsand. The flocculated water flows into the clarification tank, where the floc settles and clarified water passes up through tube settlers (as it would in a lamella clarifier) at much higher loading rates (more than 20 gallons per minute per square foot or 48 meters per hour) due to the addition of microsand. The ballasting process is ideal for low turbidity, high TOC source waters that normally have settleability issues.
Depending on the chemistry that plants add, this system not only supports enhanced coagulation for TOC removal, but also handles turbidity spikes. It is available for purchase, as a pilot unit, and as a temporary mobile/rental unit.
The RapiSand Plus adds mixed media filtration to RapiSand processes, further reducing floc particles and waterborne parasitic cysts. This model is available for pilot testing.
Municipalities must not only disinfect their waters but must also ensure that their finished waters meet EPA standards for DBP removal. The most efficient way of accomplishing this delicate balancing act is to reduce DBP precursors, such as TOC, in source waters before disinfecting them. The chemistry and processes that plants use to accomplish the required level of TOC removal depend on the characteristics of their source waters as well as their additional treatment goals.
Determining the chemical dosing required to achieve sufficient TOC removal requires testing to indicate the best combination of coagulants, polymers, and sometimes pH adjusters. Plants must also determine which type of equipment will best support TOC reduction for their source waters. WesTech engineers have the breadth of equipment-related knowledge to help plants make these determinations.