Log Yard Stormwater Treatment System

When the Oregon Department of Environmental Quality lowered the National Pollutant Discharge Elimination System (NPDES) stormwater permit benchmarks, Hampton Affiliates (Hampton) and many industrial facilities in Oregon had to modify their stormwater systems to comply with the new permit.

After an initial assessment of Hampton’s lumber mill and log yard in Willamina, MFA presented several alternative designs for treating stormwater at the site and assisted Hampton with the selection of a preferred alternative that included modifications of the conveyance system to route stormwater from several outfalls into a new pump station that conveyed stormwater from a large part of the site into an on-site, passive stormwater treatment system. The treatment system included a settling pond with a pretreatment forebay where settleable solids are removed via sedimentation. Pretreated stormwater then flowed into an open sand filter, where the remaining pollutants were filtered prior to discharge.

This system operated for several years until the space where the pond and filter were located was slated for the construction of a new mill building.

MFA subsequently designed a new, larger settling pond, a chemical injection system and pressurized mixed media filtration to treat stormwater from most of the site, including the log yard. The new treatment system was relocated to the log yard and new pump stations were installed to convey runoff to the new system. Log yard runoff is now injected with treatment chemicals and routed to a pretreatment forebay and settling pond. Stormwater from the mill is routed directly to the settling pond. The combined pretreated stormwater is then filtered using pressurized mixed media filters that reduce the remaining contaminants. The improved stormwater treatment system met NPDES stormwater permit Tier 2 requirements.