CONSTRUCTION SITE STORMWATER TURBIDITYREMOVAL USING CHITOSAN ACETATE AND SANDFILTRATION AT THE LAKESIDE CONSTRUCTIONPROJECT IN REDMOND, WASHINGTON:A CASE STUDY

摘要

Construction site managers are finding it increasingly difficult to meet local and stateturbidity standards when discharging stormwater runoff to streams, lakes and rivers. Evenwith diligent attention to the design, implementation, and maintenance of erosion andsediment control BMPs, site stormwater runoff is commonly an order of magnitude moreturbid than regulatory limits will allow. Another contributing factor is the increasingenforcement of environmental standards on construction sites. These pressures are creatinga need for innovative stormwater treatment methods that will allow for continued constructionthrough the rainy season without negatively impacting the environment.One such innovative technology is Chitosan-Enhanced Sand Filtration (CESF).Derived from waste crab and shrimp shells, chitosan can coagulate colloidal sedimentsuspensions in water that can be subsequently sand filtered producing a clear filtrate thatmay be legally discharged directly to surface water. Unlike most water treatment agents,chitosan becomes bound to the sediment particles and is completely captured in the sandfilter so that only clear, pure water is discharged. Chitosan contains no polyacrylamide,polyamine, aluminum, or toxic byproducts and is biodegradable.This case study, carried out by Natural Site Solutions, LLC., in Redmond,Washington, monitored the performance of one 500 gpm CESF system operating betweenMarch 2003 and March 2004 on an 80 acre mixed residential construction project located inRedmond, Washington, called Lakeside Estates. A total of 17,300,000 gallons of turbidstormwater was treated.Over the monitoring period the treatment system reduced turbidity, on average, by98.4 percent with a standard deviation of 1.29 percent and a coefficient of variation of 0.013.The average starting turbidity was 248 NTU with a high of 917 NTU and a low of 42.3 NTU.The average effluent turbidity was 2.98 NTU with a high of 5.18 NTU and a low of 0.58 NTU(standard deviation of 1.04 NTU and coefficient of variation of 0.348). All of the 1050 effluentsamples tested met the state discharge standard of ≤10 NTU and all whole-effluent aquatictoxicity tests (rainbow trout and Daphnia magna) passed, demonstrating that the treatedeffluent was non-toxic and safe to discharge.