Events

HPL Engineering presents the design of a pilot-scale treatment system for the Tillamook Closed Landfill (TCL). The system’s applicability to a full-scale treatment process is also considered. The TCL ceased operations and was converted to a transfer station in 1992. It is monitored and maintained by the Tillamook County Department of Solid Prevention and Recycling (TCSW). Low-strength leachate flows from the landfill year-round, with daily discharges varying from 6 to 100 GPM. Iron and ammonia are present in the leachate at elevated concentrations. The leachate is currently pumped from a collection well to a nearby field where it is spray irrigated using a sprinkler system. The land application is not a sustainable long-term solution, and TCSW is seeking proposals for treatment options that facilitate direct discharge of the leachate to a vegetated swale located at the bottom of the property.

A comprehensive alternatives analysis was performed to evaluate treatment options that target the removal of iron and ammonia. Alternatives investigated for treating iron included cascade aeration, mechanical aeration, coagulant addition, pH adjustment, and ion exchange. Alternatives investigated for treating ammonia included  air stripping, filtration, waste stabilization ponds, anammox treatment, and constructed wetlands. Alternatives were evaluated in decision matrices using weighted design criteria. Design criteria were rated on a scale of importance, based on process effectiveness, stakeholder desires, and other project constraints. Criteria evaluated for the alternatives were iron removal effectiveness, ammonia removal effectiveness,  passiveness, maintenance cost, sustainability, capital costs, scalability, land use, and public perception. Criteria were given weights of 3, 2, or 1. Criteria of highest importance were weighted at 3, lower importance criteria were given a weight of 2, and the  lowest importance a 1. The treatment processes with the highest scores were selected. This lead to a treatment system which includes cascade aeration, pH adjustment, sedimentation, and a horizontal subsurface wetland. 

A cascade aerator was selected to oxidize the dissolved iron due to its ability to effectively raise the dissolved oxygen content of water while operating as a passive system. The aerator is followed by a sedimentation basin where iron continues to oxidize, precipitates, and is removed by settling. The final selected unit operation was a horizontal subsurface flow wetland to remove the ammonia prior to discharge of the leachate. An initial analysis indicated that the oxidation of iron is more effective at higher pH values, so pH adjustment using lime was added to the design with the aim of reducing the required settling basin size. A pilot-scale system was designed to treat the leachate at a flow of 8 GPM. 

Analysis suggests that changes need to be made to both the pilot and full-scale design to make it suitable for implementation at the TCL. Although the cascade aeration system should be able to effectively treat the wide range of flows that the full-scale design would see, both the sedimentation basin and the constructed wetland appear to require too much space to make them feasible. Chemical pH adjustment did not result in a sufficiently downsized sedimentation basin, and strong consideration is being given to suggesting coagulant addition to the leachate to improve iron removal.