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King County local limits are set for heavy metals and cyanide, listed below. The metal limits are for total metals, not dissolved metals. If a company discharges less than 5,000 gallons of wastewater per day, the company must comply only with the instantaneous limits.

Parameter Daily Average Maximum
mg/l (ppm)
Instantaneous Maximum
mg/l (ppm)
Arsenic 1.0 4.0
Cadmium 0.5 0.6
Chromium 2.75 5.0
Copper 3.0 8.0
Lead 2.0 4.0
Mercury 0.1 0.2
Nickel 2.5 5.0
Silver 1.0 3.0
Zinc 5.0 10.0
Cyanide* 2.0 3.0
 *Cyanide amenable to chlorination, not total cyanide

About heavy metals

Heavy metals pass through the sewage treatment plant into the treated effluent released to Puget Sound or through the biosolids used to improve soils. These limits are established to protect these resources.  

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