By: Nathan Staples
Water treatment plants must not only provide enough water to meet the needs of growing communities, but they also must meet regulations based on the new Long Term 2 Surface Water Treatment Rule (LT2 rule) published in the Federal Register on January 5, 2006. The purpose of the rule is to reduce disease incidence associated with Cryptosporidium and other disease-causing microorganisms in drinking water, by requiring additional Cryptosporidium treatment in higher risk systems. The rule also contains provisions relating to uncovered finished water storage facilities.
These new regulations, as well as environmental factors such as scarcity of easily treatable water, are among the challenges water utilities must face when considering plant upgrades and expansion. It's important to evaluate aging equipment and systems to decide when to repair and when to replace it with new technology.
Listed below are examples of how several water treatment plants found creative ways to overcome these challenges.
Many of the facilities incorporated basic enhancements to optimize filter plant operation, minimize waste, and reduce costs. The most common enhancements involved: upgrading sequential air-water backwash with simultaneous air-water scour; upgrading filter boxes to replace gravel-packed layers with header pipes and lateral backwash distribution components with new direct-retention under-drains; upgrading washtroughs with mediaretaining baffles; converting existing filters to granular activated carbon basins; and incorporating pretreatment methods to handle more difficult waters while reducing disinfection byproducts and total organic carbon (TOC). Each of these plants used a consulting engineer and worked with the state health department or other regulatory bodies to evaluate the approach before proceeding with full-scale implementation.
BIG CREEK WATER TREATMENT PLANT
This Thomson, Ga., plant's existing Microfloc Trident 420A units (installed in 1989) struggled to meet capacity demands. The town needed to expand it from 2 to 4 mgd. Two new units were added and the two existing units were upgraded to include Multiblock direct retention underdrain with air-water backwash. A stainless- steel plate fastened to the top of the underdrains requires no gravel and minimizes plugging and biological fouling.
The plant upgrade and expansion was designed by O'Brien & Gere of Alpharetta, Ga., who used two Microfloc Trident HSR-1400 units in an unusual configuration. The water treatment plant's water comes from Lake Strom Thurmond, where organics can be high; however, due to space limitations, the pretreatment system could not be installed close to the existing Trident units. The two pretreatment units located between the lake and the plant take water from the reservoir and remove some of the organics and solids before discharging the treated water to a smaller pond nearby.
CITY OF VERSAILLES, IND.
Versailles had two old surface water treatment units, installed in 1989, which could not easily treat seasonal algae growth and high turbidity (more than 400 Nephelometric Turbidity Units, or NTU) swings after rain events. The water is of variable quality, with turbidity in a normal range of 25 to100 NTU and spikes up to 400 NTU. The plant had difficulty meeting disinfection byproduct regulations due to high TOC.
The city and R.E Curry Inc., of Danville, Ind., investigated proposals both to rehabilitate and to replace the existing equipment. They determined that a new Trident HS-700A two-tank system would fit easily on the existing concrete pads. The design provided two-stage clarification and media filtration, followed by ultraviolet (UV) disinfection. The original plan involved removing interior components of the existing tanks, repairing them, and adding new treatment components ? a process that would allow one unit to remain in service while the other was rehabilitated. However, the cost difference for new stainlesssteel tanks was little enough that Versailles chose to proceed with new, corrosion-resistant tanks for its plant. During one rain event, influent turbidity spiked to 425 while effluent water remained at <0.1 NTU.
ILE-A-LA CROSSE, SASKATCHEWAN,CANADA
This water plant struggled to meet the water needs of the community and had high TOC levels such as 9 to 12 miligrams per liter (mg/l). They used excessive chemicals to combat this, which carried over to the filter, elevating headloss and placing the treatment unit into backwash. Short filter runs and unnecessary backwash waste production resulted in inefficient operation. The plant treats water that is typically low in turbidity (<5 NTU) but quite high in TOC. Also, due to high TOC levels, the Trihalomethane concentrations were out of compliance.
Trident HS-700A single-tank system was installed to treat the water, which reaches a temperature of only 35° F in the summer. Colder waters are sometimes more difficult to treat as the chemical reactions are slower. The Trident HS offered more detention time for the reactions to occur. The treatment was successful, with effluent turbidity of 0.15 NTU or less and Trihalomethane concentrations of ~78 micrograms per liter (?g/l), well within the limits of proposed regulations. Filter run length tripled, and backwash waste was greatly diminished.
In all of these cases, water utilities sought to expand treatment plant production capabilities and meet regulatory requirements by taking advantage of new technologies. All of them found that simple changes to filter boxes, clarifiers, and pretreatment steps helped them to meet these goals. ? Nathan Staples (firstname.lastname@example.org) is a sales support specialist for Conventional Filter Products; Ken Bridges (email@example.com) is the rehabilitation and retrofit sales engineer for Conventional Filter Products; and Richard Ross (firstname.lastname@example.org) is the technical sales manager, eastern region. All work for Siemens Water Technologies
Here's how four water treatment departments sought creative ways to overcome problems when retrofitting their plants, and what they did to meet capacity and regulatory requirements:
This town needed both to expand its water treatment plant to meet capacity requirements, and to evaluate refurbishing its inefficient existing treatment plant. Because it had to move quickly, the town brought in temporary water treatment trailers to provide water while the existing plant was reconfigured.
Bloomfield's engineering firm realized that the water source to feed this 3-mgd potable water treatment plant would be the San Juan River, which has highly variable water quality. They selected a Trident HS-2100A two-tank system for turbidity removal, which includes a UV disinfection system to inactivate pathogenic microorganisms. While designed to treat water with turbidity spikes up to 400 Nephelometric Turbity Units (NTU), the Trident HS has handled turbidity spikes up to 1,000 NTU while maintaining an effluent turbidity of 0.16 NTU.
Rock Hill, S.C.
The rapidly growing community of 66,000 people needed to upgrade and expand its conventional treatment plant from 24 mgd to 36 mgd. The plant was built in stages beginning in 1946.
In a region that has experienced frequent drought, efficient water usage and production are extremely important. As part of the plant upgrade, the filter cells were upgraded to include Multiblock underdrains and Multiwash baffled washtroughs. The Multiwash troughs reduced filter-cleaning time. Backwash frequency was reduced from daily to every 72 hours, significantly reducing water usage and waste while continuing to meet the community's water demands.
Wiedeman and Singleton, Rock Hill, S.C., has been responsible for the plant design and several upgrades, providing a plant capacity up to 60 mgd, as demand dictates. The project included air/water backwash filter renovations, construction of chemical feed facilities, piping improvements, a new clearwell, and upgrades to the existing facility. Pressure filters were also incorporated into the backwash recovery system to allow reuse for local irrigation needs.
The Southbridge Water Treatment Plant had two existing packaged treatment units with a capacity of 4 mgd. The existing units used heavy media in the clarifier, followed by a mixed-media filter equipped with a gravel-style underdrain and header lateral collection system. The plant needed to increase production to 6 mgd.
Weston & Sampson of Peabody, Mass., recommended adding a third Microfloc Trident TR-840 unit, plus an upgrade to replace the heavy media in the existing clarifier to a buoyant media design (Microfloc's Adsorption Clarifier). This clarifier reduces solids by 80% to 90% without irregular solids carryover. The heavy media configuration can "burp" solids onto the filter box, increasing filter headloss and prematurely placing the unit in backwash mode. Frequent backwashing increased water demand from the finished water clearwell, and took the unit off line, thereby reducing effluent production. The upgraded adsorption clarifiers reduce this excessive need for filter backwashing due to inefficient solids reduction. Along with modifications to the existing clarifiers, upgrades are planned to retrofit the existing filter underdrain system.
Huntsville's water treatment plant faced performance issues at its source water: a reservoir with seasonal algae. The facility was originally constructed with four heavy-media clarifiers feeding four filter media beds, which provided only minor reductions in turbidity through the clarifier. The "fines" from the clarifier created maintenance problems with instrumentation, physical restraints, and other structures at the plant, as well as resulting in short filter runs.
To improve the plant's operation, the clarifier was converted to an up-flow adsorption clarifier with buoyant media. While clarifier flush cycles did not change substantially, dramatic reductions in clarifier turbidity were seen all across the retrofitted clarifiers. As the adsorption clarifier efficiently captures coagulated particles, filter runs were increased from a typical 40 hours to more than 100 hours. The end result for the facility was a 40% increase in capacity and a 50% reduction of filter wash water.