Sediment retention devices (SRDs) include permeable mediums that protect storm drains, staged barriers with filtration, settling basin decanters for drainage from the top of the water level, and in-ground filtering boxes. They take up much less space than a settling basin and have a removal rate of 80%, but don’t remove heavy metals, hydrocarbons, and other contaminants.
You can increase SRD effectiveness, or entirely avoid their expense, by introducing coagulants and/or flocculants into the stormwater or wastewater flow stream. Depending on chemical mix and application method, they remove more than 99% of contaminants. Thus, choosing the right coagulant or flocculant — or mix thereof — is key to a treatment’s long-term efficacy.
What they are and how they work
Coagulants and flocculants assist in the solids/liquid separation of suspended particles in solution. Such particles are characteristically very small and their suspended stability (also called colloidal complex) is due to both their small size and to the electrical charge between particles. Coagulation and/or flocculation condition a solution to promote the removal of suspended particles.
Coagulants neutralize the repulsive electrical charges (typically negative) surrounding particles, allowing them to “stick together” and create clumps — flocs — that form a small- to mid-size particle (sometimes called a pin-floc). This usually occurs at a pH of 6 to 9. Typical coagulants include polymers, alum, ferric sulfate, ferrous sulfate, and ferric chloride.
Once the floc has formed, a second chemical — a flocculant — is required to make even larger particles. Flocculant addition occurs at a pH of 8-10. Flocculants act like a net, gathering up smaller coagulated particles into one large particle that quickly drops to the bottom of a container, forming a sludge.
One more ingredient: precipitants
Many stormwater and wastewater streams contain metals, which can be either soluble or insoluble. Soluble means the metals have gone into solution and are actually part of the water. Insoluble means there are small particles floating in the water that are too light to settle out without aid.
Stormwater, wastewater, and groundwater also frequently contain chelants and other complexing agents that surround metal ions, preventing the chemical reaction that converts the ions to insoluble particles. Using a precipitant — which chemically “breaks” the chelating/complexing rings surrounding metal ions — along with a coagulant/flocculant usually succeeds in separating metals from the water.
Finding the right combination
So as you see, a primary coagulant by itself may require pH adjustment using additional chemicals along with the additional treatment of precipitants for metals removal and various chemicals to break oily emulsions. Coagulants are also sensitive to water temperature; in addition, heavy flow rates can shear — or break — polymer strands, causing contaminants to re-suspend. When that happens, treatment must be repeated. Because traditional chemistries require a several-step process requiring multiple chemicals, mixing tanks, and additional equipment, it’s not uncommon for contaminant removal to take several hours.
Depending on what pollutants are present, treatment may require one, two, or all three chemicals applied at various times and ways. Obviously, a product that combines coagulants, flocculants, and precipitants would reduce flocculation time, remove soluble and insoluble metals, break oily emulsions (hydrocarbons), and eliminate having to pre-treat with acids or salts to achieve proper pH.
Used in industrial wastewater settings for more than 10 years, montmorillonite clay-based flocculants are one such product. Capable of removing oils, grease, surfactants, dyes, inks, coolants, solvents, metals, heavy metals, and suspended solids, they’re blended with other basic items that cause encapsulation of oils and greases; removal of other organics by partition and chemical bonding; and removal of heavy metals by precipitation, ion exchange, and chemical bonding. The result is a dense, non-leachable sludge that usually passes the toxic characteristic leachate procedure (TCLP) test.
Best of all, they’re one-half to one-third the cost of typical coagulants/flocculants.
—Melissa Schrand (firstname.lastname@example.org) is president of Innovative Turf Solutions (www.innovativeturfsolutions.com) of Cincinnati. The company’s flocculating agents and equipment are marketed under the brand name FLOC (www.floc.co).