IntroductionFor more information, select one of the following topics:
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Overview of Treatment Process
Courtesy of US Filter
The Trident Water Treatment System design consists of the following features:
Coagulant choices include alum, ferric salts, sodium aluminate, and cationic polymers. Coagulants are added to the raw water by an Aquaritrol® solid state controller, taking a signal from an effluent turbidity meter. This automated device continually monitors raw water quality and makes any necessary changes to the chemical dosage to maintain the plant's desired water quality parameters. Coagulation control is also available by an optional streaming current meter.
The adsorption clarifier process combines coagulant mixing, contact flocculation, and solids removal. Coagulated water passes through a packed bed of buoyant adsorption media. The clarifier bed consists of polyethylene beads that are retained against a screen. Flocculation results from solids contact as the water slowly flows upward through the bed. The flocs are removed by adsorption to the clarifier media.
Once the solids have built up in the clarifier, the bed is flushed by passing air and raw water up through the clarifier. During the flushing process, the media expands and is agitated, allowing the sorbed solids to be removed from the clarifier bed in the raw water piped to the waste.
UNDER CONSTRUCTIONFiltration in the Trident Water Treatment System is the same as that used at most conventional water treatment plants. The mixed media filter incorporates three or more granular materials (usually anthracite coal, silica sand, and garnet) of varying sizes and specific gravities to allow for deep bed filtration. Deep bed filtration, a process in which particles are removed throughout the entire depth of the bed, occurs because media pore spaces at the bottom of the filter are smaller than those at the top.
The left graduated cylinder in
the photo shows the media as installed before being placed in operation. Anthracite coal is on the top, silica sand is in the middle, and garnet is on the bottom.
By using three different media, the gradation of grain sizes allows the largest grains to remain near the top while the
smallest grains stay close to the bottom. Incorporating a mixed media filter into the Trident system allows for longer filter runs
and production of a superior quality finished water. The distribution of the filter media is made possible because the different specific gravities of the media allow for intermixing of the grains. Intermixing creates
a filter that has a progressively finer grain size gradation throughout the entire filter depth. The graduated cylinder on the right side of the photo
shows what an intermixed multimedia filter looks like during filtration.
Triton Direct Retention Underdrain
The Triton direct retention underdrain is a hemispherical perforated plastic drain
which allows the filter media to be place directly on top of it, eliminating the need for support gravel. This underdrain system allows for uniform collection of
the filtered water over the entire bed area while providing an excellent backwash for cleaning the filter. This underdrain allows for simultaneous air/water backwash, reducing the amount of backwash to be treated by up to 50%.
Treatment ApplicationsMost of the turbidity removal occurs in the adsorption clarifier. Turbidity levels of finished water generally range between 0.1 and 0.3 NTU. For a high-quality source water, the Trident effluent may have turbidity levels as low as 0.05 NTU.
According to Matt Dickson of US Filter CPC, the system is capable of treating source waters with turbidity levels up to 100 NTU with spikes twice that. However, the system works most efficiently for source waters at moderate turbidity levels. At levels of 100 NTU and greater, effluent quality remains high but run times of the adsorption clarifier and filter decrease significantly. Short run times result in frequent filter backwashing, reducing the amount amounts of finished water. For consistently high turbidity source waters, the Trident system should only be used if the raw water undergoes pre-sedimentation.
Removal of Giardia lamblia and Cryptosporidium
The presence of Giardia lamblia and Cryptosporidium is a major concern in the field of drinking water treatment. The clarification and filtration processes of the Trident water treatment system remove these dangerous protozoans. The system is capable of 2.5-log removal of Giardia lamblia and Cryptosporidium. In some cases, 4-log removal can be achieved.
Reduction of iron and manganese
An added benefit of the Trident water treatment system is the capability for iron and manganese reduction. This step is important to improve the taste of the finished water and prevent staining of household fixtures. Iron and manganese reduction requires pre-oxidation. The actual removal of these minerals occurs in the clarification and filtration steps of the treatment process. The Trident system performs better if the source water has lower concentrations of iron and manganese. Trident units can treat raw water with iron levels up to 10 mg/L.
Color removal is necessary in the production of aesthetically pleasing drinking water. The first step in color removal is the addition of coagulant. After a floc is formed, it is removed in the clarification step. According to the manufacturer, the Trident system is capable of treating source waters up to 100 color units. However, the total turbidity and color units may not exceed 150.
Operations at Roanoke County's Trident Water Treatment Plant
(1) Pre-oxidation with chlorine dioxide
(2) Addition of ferrous sulfate coagulant and in-line mixing
(3) Clarification and filtration in a Trident water treatment unit
(4) Contact with granular activated carbon bed for taste and odor control
(5) Disinfection with chlorine and addition of fluoride.
Jeff Booth, Water Production Supervisor for Roanoke County, is very happy with the quality of the finished water. According to Mr. Booth, the Trident plant offers several advantages over a conventional treatment plant. For the Roanoke County facility, Mr. Booth does not see any disadvantages in the system. Overall, he is pleased with the performance of the Trident units.
Click here to hear Mr. Booth's comments about Roanoke
County's Trident Water Treatment Plant.
According to Bob Benninger, Assistant Utility Director for Roanoke County, the decision to construct a Trident water treatment plant resulted in significant cost savings. The Trident plant reduces capital costs because it eliminates the tank costs of sedimentation basins. The Roanoke County facility also sees savings in an approximately 25 to 40% reduction in coagulant usage. The direct retention underdrain can result in 20 to 30% savings in operation and maintenance costs as compared to other underdrain systems.
The advantages of the Trident plant, as expressed by Mr. Booth and Mr. Benninger, and summarized below:
Acknowledgements
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