Effect of vegetation in a dissolved metal passive compost stormwater treatment system.

A better understanding of the flow and sorption properties of vegetated compost roadsides is needed before such passive, low cost treatment beds can be recommended as a Best Management Practice (BMP) to treat metals in stormwater runoff during the non-vegetative growth and high rainfall period between Oct. and April in the Pacific NW. The effect of four different flow rates between 70 and 900 mL/min on retention time in 12° sloped compost and grassed compost beds (1.02 m x 0.254 m x 0.10 m deep) were examined using bromide as a conservative tracer. Zinc sorption (at 3.0 mg/L, pH=5.6, 11°C) buffering potential and turbidity levels were examined in both beds at 264 mL/min. The mean particle size of the compost was 0.0063 m. Roots that had amassed for 7 months prior to the experiments increased retention time only at the low flow rate of 70 mL/min. At this flow rate, most of the flow was concentrated along the bed bottom where the root mass was thickest. Excepting at the two lowest flow rates, the drainable water volume in the compost bed was higher than in the grassed compost bed. Bed composition in duplicated trials did not affect zinc breakout at 264 mL/min when retention time and percentage of the bed utilized were similar in both bed types. Breakout time in the beds was approximately 230 hr. Both bed types buffered influent pH to as high as 7, and this ability decreased over time (F=50-200, p<<<0.0005). Initial turbidity readings for both treatment beds were found to be significantly higher than the runtime turbidity. The grassed compost bed produced lower initial and runtime effluent turbidities (tstat=3.18-. 5.14, p=.0005 to <