Achieving Reduced-Cost Wet Weather Flow Treatment Through Plant Operational Changes

Despite documented I/I reductions in the collection system1, the dynamic hydraulic model for the Trinity River Authority of Texas’ (TRA) Central Regional Wastewater System (CRWS) predicted an increase in the Q2-HR PEAK/QADF ratio at the WWTP from 2.5 to 3.30.2 The 2.5 ratio historical value was known to be artificially low, the result of unintended in-line storage created by hydraulic bottlenecks in the collection system. The 3.30 ratio is the year 2020 predicted value derived from the hydraulic model. This assumes the completion of the scheduled collection system improvements intended to remove the bottlenecks.

TRA intends to implement the collection system improvements and to treat all wastewater flows at the CRWS Wastewater Treatment Plant. The improvements recommended to handle the projected 3.30 ratio peak flows included 1) wet weather treatment facilities and 2) additional final clarifiers. Capital cost of the improvements to treat the peak flow was estimated at $21.1 million.3 The wet weather treatment facilities – High Rate Clarification - have been deferred pending final regulations/policy by the U.S. Environmental Protection Agency (USEPA) with regard to blended flows and the preparation of additional technical and economic data by the Authority. The final additional final clarifiers were budgeted for construction in 2010.

The Authority began investigating operational modifications at the plant to allow CRWS to handle increased peak flow until either or both of the planned improvements could be brought on line. The two operational modifications determined most feasible were the addition of step-feed capability for all of the secondary treatment basins and conversion of the existing in-line equalization basins to off-line peak storage basins. This paper describes these operational changes, the quantification of the increased peak flow capability of the CRWS plant and the associated capital cost reduction of $6.2 million.

The Trinity River Authority of Texas is a political subdivision authorized by the State of Texas to provide a variety of services in the Trinity River basin including planning, construction and operation of wastewater treatment facilities. The CRWS Wastewater Treatment Plant is the flagship facility of the Trinity River Authority of Texas, serving 23 customer cities in the Dallas/Fort Worth Metroplex depicted in Figure 1.

With an estimated population equivalent of 1.41 million persons2 the plant is permitted to discharge 162 MGD (Average Daily Flow) and 405 MGD (2-hour peak flow) with a 7/15/(2/4)/6 – CBOD/TSS/NH3/DO effluent quality set, although the plant historically has discharged a much better quality effluent. CRWS was constructed in multiple phases with the most recent being the completion of the Phase I/II Expansion in 1976 and the Phase III expansion in 1993. The phased expansions inclusive of the master planned improvements through site build-out (Phase V) are presented in Figure 2.

The current design flows and projected flows through Phase V are presented below in Table 1. The hydraulic model indicated an increase in the Q2-HR PEAK/QADF ratio to a maximum of 3.30 and a maximum Q2-H PEAK of 623 MGD. The initial increases in the peak flow ratio result from the scheduled improvements in the collection system that are to be phased in between now and 2020 to eliminate the hydraulic restrictions. Despite the anticipated benefits of the Inflow/Infiltration improvements that the Authority and its customer cities have committed to implement each year between now and then, the impact of the removal of the hydraulic restrictions overcomes those reductions. The maximum peak ratio of 3.30 occurs in the year 2020, the design year for the Phase IV Improvements. All of the hydraulic restrictions will be removed by that year and the continuing I/I improvements are projected to produce a subsequent decrease in the peak flow ratio. The projected peak flow for Phase IV ranges from 594 to 623 MGD. The projected peak flow for Phase V ranges from 628 to 657 MGD.

The varying flows used by the Authority for planning are a function of whether the higher, more conservative State of Texas population projections approach or the less conservative, lower recent historical trend projections are used. For the facility design the higher population flows are used but for implementation planning (timeline for phasing of improvements) the lower recent historical flows are applied. For the plant facilities, inclusive of the improvements for
handling peak flows and this paper, the 623 and 657 MGD values will be used for Phase IV and V, respectively.

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