Treat the Source
New emphasis on maintenance, sampling, and monitoring in the Municipal Phase II National Pollutant Discharge Elimination System (NPDES) permit has sparked attention to identifying and controlling the source of pollutants contributing to the system. The upshot is a greater need for advanced stormwater treatment techniques that meet water quality standards.
Most water quality issues are not related to single contributors, oil spills, or catastrophic events, but are related primarily to nonpoint source and construction site pollutant contributions. What's more, studies have shown the majority of hydrocarbon, metal, and pesticide contributions in stormwater to be associated with the fine sediment suspended in the stormwater.
Based on these and other findings, municipalities have imposed numeric limits and benchmark values for pollutants. These locally prescribed values are often based on receiving waters or a watershed-focused pollutant loading perspective. The implementation of numeric standards in categories such as values above background or percent removal heightens the challenge of selecting the correct best management practice (BMP) or treatment train to apply to achieve the prescribed standards.
In many cases, conventional BMPs are incapable of reducing turbidity and other pollutants to acceptable levels. Traditional installation of straw wattles, silt fences, and similar measures are insufficient for removing suspended particles to levels suitable for the new NPDES permit requirements.
Plan of Attack
Advanced stormwater treatment typically requires chemical treatment of the stormwater. Chemical treatments work at the molecular level and cause particles that are otherwise too small to be caught by conventional filtration methods to readily settle. Extremely fine particles, typically silts, and clays can stay in suspension for decades without settling because of their relative size and ionic charge.
The chemical, or flocculant, acts on an ionic level to draw soil particles together and create a larger, heavier particle, or floc. The floc can then be removed through gravity settling or filtration. Gravity settling is typically used in batch treatment processes where discharge rate and footprint are not limiting factors. Filtration is commonly employed to reduce processing time or provide continuous, high-rate discharge.
The Benefits of Chitosan
Using chitosan as a treatment agent is gaining in popularity and regulatory prescription. Chitosan is made from chitin, a naturally occurring biopolymer derived from recycled crustacean shells. Chitosan's appeal is based on its ability to biodegrade in the environment and the fact that it is made from a waste product of the shellfish industry. Some jurisdictions have classified it as a biological treatment rather than a chemical treatment.
Chitosan is a versatile and effective treatment option for applications ranging from batch treatment to high-rate flow-through systems. Asingle chitosanenhanced sand filtration (CESF) system can reduce extremely high turbidities (up to 1000 nephelometric turbidity units, or NTU) to below benchmark standards (less than10 NTU) at high flow rates (as nuch as 600 gallons/minute). Simply put, this technology can quickly remove suspended particles from stormwater to manage rain events that cause significant water quality issues and high pollutant loads to municipal systems and receiving waters.
Chitosan treatment can extend the construction season through the rainy season while maintaining site compliance with the NPDES permit requirements. Chitosan treatments allow projects to stay on schedule and minimize the potential for water quality-related stop work orders, fines, and lost profits.
Chitosan is also impacting the industrial community. A unique aspect of chitosan is its ability to chelate dissolved metals in water. Chelation is a process by which multiple binding sites along the polymer chain bind with the metal to remove it from solution. In studies, chitosan reduced metal concentrations in water by more than 75% to nondetectable limits.
Some applications where chitosan has shown positive results are on remediation sites, log sort yards, mining industry, and boatyards.
Because of chitosan's unique properties, several regulatory agencies and municipalities in the United States and Canada have specified chitosan as the only flocculant that can be used in advanced treatment processes when discharging to surface waters.
Nathan Hardebeck is environmental communications/marketing manager for Clear Water Compliance Services, Lynnwood, Wash.