Improving Water Quality and Biosolids

Since 1969, the Industrial Waste Program has required many industries to pretreat wastewater before discharging it into the sewer. The result has been a continuous improvement in water quality and biosolids.

Biosolids are nutrient-rich organic solids produced by wastewater treatment processes. King County biosolids are used to enrich soil for agriculture, forestry, and landscaping. Our goal is to produce the most contaminant-free biosolids practically achievable.

The photo below shows how biosolids spread on forest land increased the growth per year of a tree, as revealed in the wider annual growth rings.

One type of waste removed by wastewater pretreatment is heavy metals. The graph below shows the decline in heavy metals (cadmium, chromium, copper, nickel, lead, and zinc) in wastewater received at King County's West Point Treatment Plant in Seattle and South Treatment Plant at Renton.