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TARRANT REGIONAL WATER DISTRICT'S FIELD-SCALE WETLAND PHASE 1 OPERATIONS ndash; LESSONS LEARNED

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The Tarrant Regional Water District (TRWD) has completed more than three years of operation of the 243-acre field-scale wetland constructed to facilitate further research regarding the treatment expectations and verification of performance capabilities documented during the eight year pilot wetland study, as well as refinement of design criteria for full build-out of the George W. Shannon Wetlands Water Recycling Facility (GWSWWRF) at Richland-Chambers Reservoir. The GWSWWRF concept was developed by the TRWD to meet future water supply requirements. The phased development of the wetland system was formulated by the TRWD to research financial aspects, operation and maintenance issues, as well as treatment performance and facilitate refinement of design criteria for the full-scale wetland system. The initial operation of the field-scale wetland, designated as Phase 1, includes operations from June 3, 2003 through January 9, 2007.

The TRWD's approach to augmenting the water supply with reclaimed water involves diverting Trinity River water into two of the TRWD's reservoirs located downstream of the Dallas-Fort Worth Metroplex. The GWSWWRF, which will provide polished water to Richland-Chambers Reservoir, is located on the Richland Wildlife Management Area and involves a working partnership between the TRWD and the Texas Parks and Wildlife Department (TPWD). A Memorandum of Understanding was formalized in 1996 between the TRWD and TPWD which setup the project's long-term goals and objectives of the field-scale wetland system. These include research studies and confirmation of pollutant removal/performance observed at the pilot-scale wetland system; evaluation of management issues at the field-scale level; establishment of vegetation and habitat for wildlife; water level management over larger wetland cells; water balance issues; sedimentation rates; projected frequency and management of accumulated sediments; and flow distribution through larger wetland cells.

Analyses of the data collected during the Phase 1 operational period included flow balance and mass balance calculations, kinetic modeling, and review of routine and special studies data. Mass balance calculations were performed to determine removal efficiency of total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) within the sedimentation basin and the field-scale wetland train. The field-scale system functioned as designed with the majority of the suspended solids removed within the sedimentation basin with some associated removal of nutrients. Overall, the field-scale system provided effective removal of suspended solids (96%) comparable to the performance documented at the pilot-scale system (95%). The field-scale system also provided effective removal of both TN (67%) and TP (44%) although the removals based on percent mass removed were not at the performance levels exhibited in the pilot-scale system (82% for TN and 66% for TP). Differences related to the design and operation of the field-scale system versus the pilot-scale system include substantially higher hydraulic loading rates at the field-scale system resulting in higher mass loading rates, reduced density of vegetation and higher water depths.

Field-scale system flows from the Trinity River were typically between 12 and 15 MGD, but ranged higher than 15 MGD on several occasions. Based on the measured flows, the overall average flow from the Trinity River pump station for the Phase 1 operational period was 13.51 MGD (based on daily flow data), and from the sedimentation basin was 12.69 MGD (based on weekly flow data). Hydraulic loading rate (HLR) calculated for the entire field-scale system over the Phase 1 operational period was 5.88 cm/day. Due to drawdowns of wetland cells for maintenance requirements and moist soil management, there were substantial periods where the system was operated with one or more wetland cells off-line. As a result of various cells being off-line, HLRs ranged from 4.97 cm/day to 11.52 cm/day for the system.

The k-C* model as proposed by Kadlec and Knight (1996) was used to evaluate the wetland performance data from both the pilot-scale study and the field-scale system. The effect of reductions in operating wetland area due to maintenance drawdowns and/or moist soil management for habitat enhancement on the functional performance rate constants for TSS, TN, and TP were also evaluated. Reduced removal efficiency for TP was determined when the effect of cell drawdowns on wetland operating area was included.

The Phase 1 operational period of the field-scale wetland has been a period of ongoing learning and changes including physical modifications and operational adjustments to address issues identified early during this stage. This period has been characterized as combined periods of start-up and restart-up as well as periods of relative extended operations when treatment equilibrium representative of long-term performance might be expected. As such, this period has provided substantial information regarding cell design, construction, planting needs, and operational limitations that will be utilized to further the design criteria for future phases of the GWSWWRF full-scale wetland system. Construction of the field-scale wetland system with full size Trinity River diversion pump station has enabled demonstration of hydraulic and mass loading rates beyond what was possible at the pilot-scale wetland system, but the treatment performances demonstrated by the eight year pilot-scale study were a good indication of performance capabilities of the large-scale system. Despite less than optimal flow distribution, treatment area utilization, and vegetative cover, performance of the field-scale wetland system in terms of mass removal rates has matched or exceeded the performance exhibited by the pilotscale wetland system. Based on the data analysis from the two wetland systems, it is likely that TN and especially TP removal effectiveness may be improved at the field-scale system with increased cover of emergent vegetation, reduction in average water depths, improvements in flow distribution, and correlation of diverted flows and mass loading rates with wetland treatment area in operation.
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Document Type: Research Article

Publication date: 2008-01-01

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