Biosolids Lagoon Sampling Methodology and Data Interpretation

The City of Portland, Oregon, maintains a 37-acre solids treatment and storage lagoon known as Triangle Lake. Since construction in 1970, the lagoon has served to provide temporary storage of biosolids and treat sidestreams from dewatering and digester cleaning. The lagoon provides tremendous operational flexibility, treatment of sidestreams, increased stabilization of solids, and improved solids dewaterability. Recently, the original unlined construction of the lagoon was questioned as a potential source of groundwater contamination. While groundwater monitoring has been inconclusive in terms of demonstrating adverse effects, the decision was made to upgrade the lagoon to current design standards including a full synthetic liner. A predesign of the reconstructed facility included the HDPE liner system and compartmentalization. Before construction can proceed, existing solids in the lagoon need to be removed. Because solids date back to the 1970s when source control was less stringent, the decision was made to construct an on-site monofill as part of the reconstruction project. Previous sampling had shown that solids exceed the Part 503 ceiling concentration for lead and high concentrations of dioxin were suspected. As such, land application was not feasible and landfill disposal was determined to be prohibitively expensive. To define required capacity for the monofill and facilitate solid waste permitting, an updated survey was needed to quantify solids quantity and quality. The new survey results confirmed these expectations and revealed that the total quantity of solids was greater than anticipated. Survey methodology and the strategy for handling solids not considered suitable for land application are described.