Dissolved-air flotation system helps small utility achieve Stage 2 DBPR compliance
The city of Buffalo, Wyo., is among the many smaller U.S. water systems for which reliance on surface water means treatment changes are needed to comply with the Stage 2 Disinfectant and Disinfection Byproduct Rule (Stage 2 DBPR). The U.S. Environmental Protection Agency (EPA) mandate expands the scope of an earlier Stage 1 rule aimed at minimizing consumer exposure to disinfectants and disinfection byproducts (DBPs) in a community’s drinking water.
This is particularly true of the total trihalomethanes (TTHMs) and five haloacetic acids (HAA5s) created by the reaction of chlorine-like disinfectants with minute amounts of residual organics in a water treatment plant’s (WTP) effluent in the distribution system. A reaction in the distribution system creates the need to eliminate targeted DBPs, which are suspected carcinogens.
Starting in September 2008, the EPA rule required utilities that rely on surface water to perform initial distribution system evaluations (IDSEs) of the finished water reaching the public. Deadlines to complete these evaluations were based on the size of population served by the water utility. With a population of 3,900, the city of Buffalo ranks in the category of small systems serving less than 10,000 customers; it had to complete its IDSEs by March 2010.
The Stage 2 DBPR requires utilities to monitor their distribution systems to ensure a maximum contaminant level not exceeding 80 ppb for TTHMs and a combined residual not exceeding 60 ppb for the five HAA5s. In the case of the Buffalo utility, the distribution monitoring occurs at the most distant point of the distribution grid, according to Plant Superintendent Ron Young. The maximum thresholds correlate in large part to the achievable level of total organic carbon (TOC) removal from plants that rely on surface water for their source and the surface water’s alkalinity. The rules dictate monitoring for TOC in the raw water influent and in the finished water effluent prior to distribution.
Deciding on DAF
The Buffalo WTP receives raw water by gravity through an 18-in. line that taps Clear Creek, which flows down from the higher-elevation Tie Hack Reservoir. The Wyoming Department of Environmental Quality (DEQ) notified the utility on repeated occasions, starting about six years ago, that EPA’s targeted DBPs in the utility’s distribution system exceeded allowable limits. This inspired an upgrade program to remove TOCs ahead of distribution through improved clarification and filtration links in the process chain. The upgrade also increased plant capacity from 3 million gal per day (mgd) to 7 mgd.
Several methods can remove combined insoluble and soluble materials comprising TOCs, including ozone, potassium permanganate, peroxide and the dissolved-air flotation (DAF) method. Due to exceptional efficiency and more acceptable initial capital and operating cost, the city’s consultants recommended DAF pretreatment to instill pretreatment clarification immediately ahead of microfiltration (MF) membranes and add capacity to the process.
The Leopold group within Xylem was contacted to test its Clari-DAF system as a solution to the Buffalo plant’s TOC issue. Historically, the WTP’s TOC would spike during spring runoff or algae blooms that can affect the utility’s source water. The plant’s earlier influent was subjected only to flash-mixing before passing through an upflow clarifier. This less efficient process frequently and irreversibly clogged the upflow clarifier and demanded replacement of the existing dual-media filtration, according to Young, who oversees the water utility.
The utility eventually would engage Leopold to perform two pilot studies using a tractor-trailer equipped with a mobile Clari-DAF system. The resulting pilot studies tested not only the clarification efficiency of the technology but also how it performed in tandem with various filtration alternatives, according to Rob Wiley, Leopold process engineer on the pilot evaluations.
“The Clari-DAF system has gained widespread recognition as an innovative, high-rate DAF pretreatment technology,” Wiley said. “The effluent produced from the system has a typical turbidity value of 0.3 NTU at the Buffalo WTP. During piloting, it was shown that the system—in combination with dual-media filtration—could reliably produce consistent filter effluent turbidities of less than 0.1 NTU. Also shown was that equally low effluent turbidities could be reliably achieved with the Clari-DAF system followed by MF membranes.”
“Due to the low alkalinity of Buffalo’s raw water, low concentration of suspended solids and elevated levels of dissolved organics, the water proved in the past to be extremely difficult to coagulate and exceptionally challenging for the previous upflow clarifier system,” Wiley continued. “This was not the case with Clari-DAF system, which is efficient at removing even micro-sized floc. Based on our pilot studies and the previous plant’s performance, I would not expect removals nearly as good from any other type of pretreatment on this source water.”
Buffalo WTP’s new DAF system is engineered to use flotation for removing insoluble and precipitated soluble organics—TOCs—only tens of microns in size from source water. It was shown to operate best within a pH range of 5.6 to 5.9 to enhance precipitation of dissolved organics during coagulation. The small floc formed during coagulation and flocculation is large enough for the system’s base-level diffusers to carry it to the surface, where it can be skimmed off by mechanical means. The system’s ability to float even very small floc inherently reduces chemicals and mixing otherwise needed to capture organic particles by other methods. By removing the organics during the clarification stage with more minute traces during the subsequent filtration, the plant reduces the formation of DBPs during chlorine disinfection in the distribution system.
Another quality that made the system attractive was its high loading rate, which typically requires a smaller footprint for the necessary clarification. This often enables a retrofit into an existing clarifier basin with room to spare for other plant processes. Also, with the typical sludge solids averaging between 2% and 5%, there generally is a savings in solids handling and disposal cost.
Pilot Study Results
The first of the two pilot tests of the Clari-DAF system at the Buffalo WTP took place in May 2004. It lasted for approximately one month, but the algae blooms that fouled the previous plant’s clarifiers were less vigorous than in past years. Nonetheless, the system demonstrated excellent removal of turbidity when algae content was spiked artificially by adding a sample collected from an offline holding basin. Even with the simulated increase, turbidity removal occurred without any appreciable degradation in the pretreatment efficiency.
Unit filter run volumes during the four-week pilot test exceeded 18,000 gal per square foot, which is considered exceptional performance, according to Wiley. The pilot study revealed consistently low clarified turbidities of less than 1 NTU; an average color reduction of 76% in the pretreatment effluent and true color reduction of 85%; dual-media filter effluent turbidity less than 0.1 NTU, consistently less than the plant’s typical values; and the high filter flow volume.
The following year, DBPs detected in the distribution system led the state DEQ to notify the Buffalo utility that TTHMs and HAA5s in distribution exceeded the newly enacted Stage 2 limits for utilities of its size and customer base. This led the city to engage Leopold for a second two-week pilot study to evaluate TOC removal using the Clari-DAF system in tandem with several filtration alternatives, including dual media, granular activated carbon (GAC) media and MF membranes.
The final test included the system with both MF membrane and GAC contactor simulations. Ferric chloride dosing to a pH of slightly less than 6.0 was found to produce the highest levels of TOC removal of the coagulants screened in preliminary bench-scale jar testing. Coagulation with ferrous or aluminum salts at a pH of less than 6.0 for precipitation of dissolved organics is referred to in the water treatment industry as enhanced coagulation.
The results for the combination of the enhanced coagulation with ferric chloride in the Clari-DAF system and MF membranes resulted in a greater than 60% TOC removal, with formation potentials of TTHMs reduced by more than 70% and HAA5 formation by 82%. The reductions in DBP formation potential with the DAF system followed by membrane filtration rivaled the results attained with GAC.
These results eliminated the need for a GAC contactor as an added measure for TOC removal. A GAC contactor or GAC media filtration would have required periodic regeneration or GAC replacement if adopted for the eventual operation, in addition to increased initial capital and operating costs. The utility has since replaced the original use of ferric chloride with ferric sulfate and hydrochloric acid to manipulate the pH and enhance coagulation to comply with EPA permitting. Beyond the lower cost, efficiency and durability, the MF membrane with the Clari-DAF pretreatment also presents an absolute barrier to Cryptosporidium and Giardia, according to Wiley.
Compliance & Citizen Confidence
The integration of the DAF system with MF membrane pretreatment delivers added benefits to the utility. The Buffalo WTP was able to utilize a gravity feed from the creek’s source water, eliminating the need for pumping equipment and associated operating costs. Eliminating the frequently used GAC for TOC removal delivered major initial and life-cycle operating savings. Only a modest addition to the WTP’s physical facilities was necessary to enclose the upgrade, yet it nearly doubled the capacity of the plant’s two treatment trains. Furthermore, the excellent clarification provided by the system should extend the membrane life and reduce cleaning costs when compared to membrane filtration lacking pretreatment. Only low doses of pretreatment chemistry are needed to form the small floc required for clarification with the Clari-DAF system, which translates into lower operational costs.
Most importantly, the utility now can meet the TOC and DBP numbers the state wants during spring runoff—when the plant’s raw water is charged with organics—and Buffalo’s citizens can rest assured that they are receiving the safest, highest-quality water from their taps via a state-of-the-art treatment system.