(101b) Arizona American Water Coagulation Filtration Process Evaluation – the Significance of Scheduling and Process Variable Selection during Pilot Testing

Pilot testing plays a key role in confirming process performance, process selection and equipment sizing. However, validation and implementation of test results may be significantly impacted by the selection of pilot plant operating parameters: namely the timing of the tests and control of process variables. An illustrative example of the impact of the two requirements on plant design and construction is a series of pilot tests carried out during design of two Coagulation/Filtration (CF) arsenic removal facilities located in Arizona. The plants are being constructed in response to the EPA mandated arsenic regulation, which will take effect in January of 2006. As a result, both projects run on a fast-track schedule leaving little schedule float for additional process, design or construction work.

Since the project has been fast-tracked, the timing of the pilot testing was critical. The project was delivered by a design-build approach, which placed stringent demands on the time available to the designers for equipment evaluation and selection. In order to meet the design-build schedule, many major pieces of process equipment were procured early to ensure that they were available at critical points during construction. Much of the process design criteria had already been defined, so the additional information gained from the pilot tests needed to be incorporated into the project with minimal re-design work. Proper timing was crucial for both process modification and verification.

For the process verification to be valid, good control of process variables must be maintained. A good example is the effect of hydraulic loading rate on filter performance, which came into question during review of the filter design concept. Based on the filter design concept, much higher operating hydraulic loading rates were required when a filter was offline for backwashing. For the CF process, another key parameter of arsenic removal is pre-filtration coagulant residence time. Since coagulation and arsenic adsorption occur within a pipeline reactor, it would be difficult to select residence time and mixing values without some knowledge of the space available for the plant and the length of pipe between the coagulant injection point and the filters. Using results from a pilot study that did not adequately mimic the design of the full-scale facility could significantly limit the full-scale facility's ability to meet design treatment objectives.

Many of the performance objectives of a facility are related to parameters that will be affected by the design. Often, the pilot testing plan has been developed before such items have been addressed. For pilot tests to provide valid results, the test plan must address key parameters and be developed at the right time. The AAW arsenic removal pilot study was effective for validating the process design. This pilot study will help solve a major public safety concern by ensuring that safe drinking water is provided to the public. The study also helped develop and validate an emerging process for arsenic removal from drinking water. The results of this study provided valuable insight into the performance of the process and resulted in design enhancements expected to optimize performance and lower the overall operating costs for the full scale facility.