Today, water treatment plants not only feel the pressure of providing enough water to meet the needs of growing communities and customers, they are also faced with meeting the new Long Term 2 Surface Water Treatment Rule (LT2 rule) regulations. The US EPA defines the purpose of the rule “… to reduce disease incidence associated with Cryptosporidium and other disease-causing microorganisms in drinking water. The rule will supplement existing regulations by targeting additional Cryptosporidium treatment requirements to higher risk systems.  The rule also contains provisions to reduce risks from uncovered finished water storage facilities and to ensure that systems maintain microbial protection as they take steps to reduce the formation of disinfection byproducts. The rule will apply to all systems that use surface water or ground water under the direct influence of surface water.” This LT2 rule was published in the Federal Register on January 5, 2006.

Environmental factors, such as drought and the scarcity of easily treatable water, impose new challenges when evaluating upgrades and expansion, which will require careful consideration and justification. Aging equipment and systems becomes a factor to decide if repairing the equipment or replacing the equipment with one of a new technology is another justification that requires evaluation. This article describes how nine water treatment plants sought creative ways to overcome the problems they faced and what they did to resolve them.

Many of these facilities incorporated basic enhancements to optimize filter plant operation, minimize waste and reduce costs. The most common enhancements involved: upgrading sequential (or staged) air water backwash with simultaneous air-water scour; upgrading filter boxes to replace gravel packed layers with header and lateral backwash distribution components with new direct retention underdrains; upgrading washtroughs with media retaining baffles; converting existing filters to Granular Activated Carbon (GAC) basins; and incorporating pretreatment methods to handle more difficult waters while reducing Disinfection Byproducts (DBPs) and Total Organic Carbon (TOC).  Worth mentioning is the fact that each of these plants used a consulting engineer and worked with the state department of health officials (or other regulatory bodies) to evaluate the approach before proceeding with full scale implementation.

Cherokee, NC, Eastern Band of Cherokee Indians (EBCI) sought to increase its water production from 4 to 6 MGD to meet the needs of the expanding community and to better treat the variability of its source waters. In this case, the Oconaluftee River saw turbidities ranging from 4 to 600+ NTU, which reduced overall net production due to frequent clarifier flushed and filter backwashes. The existing plant had two packaged water treatment units that used an upflow clarifier followed by mixed media filters, which were Microfloc Trident TR840 units installed in 1994. Filter runs were approximately 20+ hours and clarifier flushes were 8+ hours on good, low turbidity water. During high turbidity spikes, the plant had to just shut down the units, drawing their water from system storage tanks. The Reservation was faced with increased water demand from the new super Wal-Mart, golf club and resort, and associated needs with increased demand from tourists. BP Barber of Columbia, SC as well as Summit Engineering of Spartanburg, SC worked with EBCI to add a third treatment unit and three new Trident HSR pretreatment units to improve the plant’s ability to meet demand and EPA regulations. The existing two units were upgraded to incorporate the latest design improvements of direct retention MULTIBLOCK underdrains, air water backwash and the replacement of hydro-mechanical level controllers with ultrasonic level controllers. When completed, this plant will be compliant with regulations and will provide enough water for the recent resort development needs of the area.

The Microfloc Trident HSR pretreatment system has been shown to reduce TOC by over 30-35% as compared to the existing Trident treatment units, and also reduces influent solids by 80-90% prior to the existing treatment systems. The 60 degree tube settlers, running at a rate of 5 gpm/sq ft., settle a majority of solids with the addition of an anionic
polymer, and are removed with a sludge collector located below the settling tubes. A portion of the solids formed are re-circulated to the influent supply to help generate new floc particles, much like a solids contact clarifier. The bulk of the influent organics are removed in this pretreatment stage. Additionally, those solids removed in this step enable the subsequent treatment units to operate with longer clarifier runs between flushes and longer filter runs due to the reduced influent solids load. Longer clarifier and filter runs reduce downtime, reduce waste production and optimize plant productivity.

Thomson, GA Big Creek Water Treatment Plant was faced with a very similar problem to EBCI’s. The existing Microfloc Trident 420A units (installed in 1989) struggled to meet capacity demands, and the town needed to expand its plant 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. This underdrain system assures good distribution of air and water during the backwash process to aggressively remove entrained contaminants of the mixed media filter. A unique precision laser cut stainless steel plate fastened to the top of the underdrains requires no gravel and minimizes plugging and biological fouling, which is not uncommon to porous bead cap style underdrains.  The plant upgrade and expansion was designed by O’Brien & Gere of Alpharetta, GA who used two Microfloc Trident HSR-1400 units in a rather unique configuration. The Big Creek WTP water source comes from Lake Strom Thurmond. Organics from this surface water source can be high, and due to space limitations, the pretreatment system was installed.. These two pretreatment units take water from the reservoir and remove a significant amount of the organics and solids prior to discharging the treated water to a smaller pond located on the WTP property. The pretreated water then flows by gravity to the 4 MGD Trident units. The removed solids are sent to the backwash waste holding basin to help reduce the load on the installed Trident units, increase plant productivity and help meet the future EPA LT2 regulations.

The following water treatment plants had older, conventional style packaged water treatment units, which incorporated flocculation, tube settlers and mixed media filters. They were replaced with the newly designed Microfloc Trident HS units that include high rate tube settlers and adsorption clarification followed by mixed media filtration. This concept of multiple barriers of treatment, including UV disinfection, provides better protection from process upsets without sacrificing effluent water quality. MULTIWASH media retaining baffles are mounted on the washtroughs to allow aggressive filter media cleaning during backwash, while at the same time minimizing media loss commonly found with standard packaged treatment unit troughs. 

The City of Versailles, IN had two old conventional style surface water packaged treatment units installed in 1989. These units were not able to easily treat seasonal algae growth and high turbidity (400+ NTU) swings after rain vents. They had difficulty meeting DBP regulations due to high TOC.  Initially, the City and R.E Curry Inc, of Danville, IN, investigated rehabilitation proposals to minimize building disruption, yet also evaluated the cost of installing new units. It was determined in the end that a Trident HS-700A two-tank system would fit easily on the existing concrete pads and was designed for a flow rate of 575 gpm, to treat surface water from their reservoir. This design consisted of two-stage clarification and media filtration, followed by ultraviolet (UV) disinfection. The proposed plan would involve removing the interior components of the existing tanks, repairing them and adding new treatment components. This would allow one unit to remain in service while the other was rehabilitated.  The cost of new stainless steel tanks was also compared with that of rehabilitating the existing steel tanks. The difference in cost for the new tanks was close enough to the original estimate such that Versailles decided to proceed with the new tanks.

The project shifted from rehabilitation to replacement, providing the city new corrosion-resistant tanks for its plant. The water is of variable quality with turbidity in a normal range of 25-100 NTU with spikes up to 400 NTU. During one rain event, influent turbidity spiked to 425 while effluent water remained at < 0.1 NTU turbidity.

Ile-a-la Crosse, Saskatchewan, Canada. This Water Plant struggled to meet the water needs of the community, and had high TOC levels (9-12 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 (THM) concentrations were out of compliance. At 250 gpm flow, a Trident HS-700A single tank system was installed to treat the water, which, in the summer, reaches a temperature of 35°F.  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 THM concentrations of ~78 μg/l, which is well within the limits of future proposed regulations. Filter run length tripled, and backwash waste was greatly diminished. 

Bloomfield, NM. This town needed to expand its water treatment plant to meet capacity requirements of its growing community, and at the same time evaluate the refurbishment of the existing inefficient treatment plant. The town had to move quickly, within 13 months, and therefore brought in temporary water treatment trailers to provide water while the existing plant was reconfigured for new packaged water treatment units. The engineering firm realized that the source water would be from the San Juan River, which has highly variable water quality to feed this 3 MGD potable water treatment plant. The treatment system selected consisted of a Trident HS-2100A two-tank system for turbidity removal, and includes a UV disinfection system to inactivate pathogenic microorganisms. While designed to treat water with turbidity spikes up to 400 NTU, the Trident HS has handled turbidity spikes up to 1,000 NTU while maintaining an effluent turbidity of 0.16 NTU.  Figure 1 shows the influent versus effluent turbidity at Bloomfield.

Rock Hill, SC, a rapidly growing community of 66,000 people, found the need 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, water usage and production is extremely important. As part of the plant upgrade, the filter cells were upgraded to include MULTIBLOCK underdrains and MULTIWASH baffled washtroughs.  The MULTIWASH troughs allowed them to thoroughly clean the filter in a shorter period than with the previous conventional plant design. Backwash frequency occurred every 72 hours rather than daily, which significantly reduced water usage and waste, as well as continuing to meet the water demands of the community. Wiedeman and Singleton, Rock Hill, SC, has been responsible for the plant design and several upgrades, providing the plant configuration with a final build out of 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, including a 54-inch diameter raw water main to ensure adequate water supply. Pressure filters were also incorporated into the backwash recovery system to supply reuse for local irrigation needs.

Southbridge WTP in Southbridge, MA had two existing packaged treatment units and plant capacity of 4 MGD. The existing packaged treatment units used a heavy media in the clarifier followed by a mixed media filter equipped with a gravel style underdrain with header lateral collection system. This plant needed to increase the production to 6 MGD.
Weston & Sampson of Peabody, MA, recommended adding a third Microfloc Trident TR-840 Unit. In addition, the existing units required an upgrade to replace the heavy media in the clarifier to a buoyant media design (Microfloc’s Adsorption Clarifier). This clarifier reduces solids by 80-90% without the irregular solids carryover commonly found in heavy media clarifiers. The heavy media configuration has been known to “burp” solids onto the filter box, increasing filter headloss and prematurely placing the unit in backwash mode. Backwashing frequently increased water demand from the finished water clearwell, and took the unit off line, thereby reducing effluent production. The upgraded adsorption clarifiers effectively reduce this excessive need for filter backwashing due to inefficient solids reduction.  Along with modifications to the existing clarifiers, upgrades will be made to retrofit the existing filter underdrain system to a design similar to the one used in the new Trident TR840 unit.

Huntsville, PA. Water Treatment Plant was faced with performance issues at its source water, which is 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 substantially change, 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 over 100 hours. The end result for the facility was a 40% increase in capacity and a 50% reduction of filter wash water.

Somersworth, NH struggled with DBP issues and increased capacity needs for its water treatment plant. Wright Pierce of Topsham, ME, conducted a number of modifications to this plant, replacing the existing clarifier-filter configuration with a new ballasted treatment step, and made creative use of the existing filter plant. The new clarifier pretreatment was followed by filtration. The three existing clarifier/filter basins were converted solely to filters, and a new fourth filter basin was added. This increased the capacity from 3 MGD to 4 MGD. They also used MULTIBLOCK direct retention style underdrains and a MULTIWASH media retention trough, which ensured aggressive media cleaning of chemicals and contaminants, and also made use of the trough’s design to prevent media loss.

Conclusion

In all of these cases, each plant sought to expand their water treatment plant production capabilities by taking advantage of some relatively new technologies and by evaluating what was needed to meet current and future regulatory requirements.  Each engineering firm and water treatment plant strove to reduce waste, meet water demands of their service area and optimize filter runs.  Several incorporated new pretreatment technologies to reduce solids loading on existing treatment units and evaluated those technologies to reduce TOC/DBP issues.

In some cases, pilot studies were conducted and town or city worked closely with state and provincial health authorities to confirm the process selection.  Each benefited from the use of simple changes to filter boxes, clarifiers and pretreatment options.

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