Description

Thomas/Wright, Inc. prepared the design for closure of the existing MSW ash trenches Nos. 1 and 2, cell development of a new ash trench, and leachate management system. The project involved evaluating different landfill liner and cover options based on site-specific ground water, soil, other site conditions and operational needs. Leachate management options included conventional treatment systems and reuse of leachate in the incineration process. The firm worked with the County's air quality consultant to evaluate potential effects of leachate injection on emission control needs. Thomas/Wright, Inc. helped Coos County select appropriate options, negotiate with DEQ, and monitor construction. The firm prepared site development, design plans, a leachate management plan, landfill gas monitoring plan, and an emergency leachate plan for regulatory approval.

Thomas/Wright, Inc. prepared engineering design plans for construction of a new 5-acre ash disposal trench and leachate management system. The landfill is adjacent to Seven Mile Creek, and receives about 60 to 80 inches of precipitation annually. Ash quench water from the municipal solid waste incineration system was disposed using spray irrigation. Coos County wanted to replace spray irrigation with a recirculation system that incorporates leachate collected from the new disposal trench. The leachate recirculation system includes primary and secondary leachate collection layers with sumps in the trench, a pump station and force main to convey leachate to an above-ground 100,000 gallon storage tank, and a recovery sump and pump to direct ash quench water to the storage tank.

In addition to the trench liner and leachate collection system, Thomas/Wright, Inc. also designed the surface water drainage controls including ditches, catch basins, and outfalls. The firm prepared the grading plans, contract documents, and engineer's cost estimate for construction. During construction, Thomas/Wright, Inc. administered the contract and worked with County staff to monitor construction. T/WI prepared the construction monitoring report and provided general assistance with regulatory negotiations for the site's operating permit.