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With the establishment of Total Maximum Daily Loads (TMDLs) for Oregon rivers and increasingly stringent regulatory limits on surface water discharges, municipalities are faced with mounting challenges on discharging effluent. Effluent containing relatively high temperature levels or nutrients can not be discharged to rivers during times of low flow, principally in the summer. To address this, municipalities are examining alternatives for treatment and discharge.


In accordance with the compliance schedule dictated by the City of Woodburn's new National Pollutant Discharge Elimination System (NPDES) permit, an evaluation was conducted by the City of Woodburn, Oregon to compare various alternatives for reduction of temperature in the effluent discharged from the municipal wastewater treatment plant (POTW) to the Pudding River during the dry season months.

The Pudding River supports a cold water fishery and has a summer temperature standard of 64.5 degrees Fahrenheit (18 degrees C). The POTW discharges treated effluent at about 72 degrees Fahrenheit (22 degrees C) in the summer and causes the river temperature to increase. Temperature discharges are now being regulated in POTW NPDES permits to minimize impacts of receiving waters. The previous NPDES permit included year-round limits for carbonaceous 5-day biochemical oxygen demand (CBOD5), total suspended solids (TSS), E. coli bacteria, pH, and CBOD5 and TSS removal efficiency. For the months of June through October, additional requirements for CBOD5, TSS, dissolved oxygen (DO) and ammonia-nitrogen (ammonia-N) were included. The new NPDES permit added new limits for summer temperature discharge.


The purpose of the project was to identify the most appropriate and cost effective alternatives for thermal reduction given the conditions present at the City of Woodburn. The alternatives considered by the City for temperature reduction consist of combinations of irrigation of existing and new poplar trees and other trees to consume the warm effluent; the use of wetlands for evaporative cooling before river or groundwater discharge; cooling towers; night-time evaporative cooling in ponds; and riparian vegetation for river shading as temperature trading throughout the watershed. All of the alternatives were developed to treat the total dry weather design treatment plant flow of 5.037 MGD, since the City prefers having the capacity to take all of the flow out of the river. In addition, the City is interested in implementing natural treatment systems to meet its new and upcoming regulatory waste load allocations and to enhance the environment and restore its riparian property. The alternatives could all be built in phases to meet current and future flows.


The evaluation determined that, among the individual methods of temperature reduction, the use of constructed treatment wetlands was the most desirable as an efficient and cost-effective solution to significantly reducing effluent thermal loads. In addition, there are supplemental benefits associated with treatment wetlands in their ability to reduce other constituents in the effluent.

Thermal modeling of treatment wetlands for Woodburn was calibrated with data from other treatment plants in the Willamette Valley of Oregon using constructed wetlands. The results confirm earlier modeling done for the City of Woodburn that approximately 6 to 8 acres of wetlands operating 6 inches to 1 foot deep can adequately cool 1 MGD of effluent to average ambient air temperature and will not exceed the maximum thermal load during the most restrictive periods of the summer.

Field monitoring data from the wetlands at The Oregon Garden in Silverton Oregon and the treatment wetlands for the City of Salem Oregon wastewater treatment plant further serve to confirm the cooling potential of wetlands. At Salem, the monitoring data confirm that water flowing at a depth of 1 foot through 4.1 acres of treatment wetlands will cool effluent temperatures by up to 5°C in August and up to 10°C in December with 4 to 8 days and nights of detention time.

An alternative matrix of the various alternatives and order-of-magnitude costs associated with implementing a combination of temperature reducing components was developed as a tool to select the optimal combination of solutions, in order to maintain maximum flexibility to deal with Excess Thermal Loads and Maximum Daily Loads both now and in the future.


The least cost alternatives of those considered were the irrigation of existing poplar tree crops and construction of wetlands in existing lagoons. For expansion, it takes less capital investment to add new wetlands for additional capacity than to add the other features, including new poplar trees. Higher rate irrigation on poplar trees for groundwater recharge is lower cost than expansion of agronomic rate irrigation, but cannot currently be implemented without further regulatory discussions. Adding riparian trading to existing poplar tree irrigation and new wetlands is higher cost than adding more poplar trees and contains the complication of purchasing or long term contracts for additional riparian property. Using cooling towers is the most costly of the alternatives evaluate
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Document Type: Research Article

Publication date: 2007-01-01

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