December 2012, Vol. 24, No.12

Problem solvers

Surmounting the challenge of removing COD from industrial wastewater

Problem: Elevated COD in industrial laundry wastewater.
Solution: Install a coagulant system to precipitate soluble COD.

Removing soluble chemical oxygen demand (COD) and biochemical oxygen demand (BOD) remains a challenge in the arena of industrial wastewater treatment. A southeastern U.S. industrial laundry encountered problems with elevated COD concentrations after attempting to reduce its water consumption.

The laundry cleans shop towels from a printer and a mechanic, as well as mats, uniforms, and aprons from a slaughterhouse and food-processing facility. This led to flows of about 265,000 L/d (70,000 gal/d). These items contain contaminants that must be washed out using high-alkaline and COD-laden surfactants before being returned completely cleaned to the customer.

The resulting washwater has to be treated to a level of compliance that may exceed a publicly owned treatment works permit. Substances of concern in the wastewater include COD, total suspended solids (TSS); fats, oil, and grease (FOG); toxic organic compounds; metals; and acidity.

The laundry’s initial wastewater treatment system included an equalization tank sized to hold one-quarter to one-half of the daily flow, a demulsification coagulant, a mixing chamber, a flocculent, dissolved-air flotation, and sewer discharge with flowmetering. The laundry sought to reduce its water use and soon achieved its goal of 204,000 L/d (54,000 gal/d). However, this led to elevated COD concentrations between 1200 and 1570 mg/L — beyond the laundry’s 1000 mg/L compliance limit.

The laundry decided not to use the Fenton’s solution method to remove the soluble COD because of its high fault rate and the associated large capital investment. Other alternatives, such as installing a sequencing batch reactor or adsorption columns, also were rejected because of the associated capital and operations investments.

To maintain its current level of performance and reduce COD concentrations, the laundry replaced its current coagulant with the GenChem International (Carmel, Ind.) coagulant Pitch Black.

Pitch Black is used in conjunction with dissolved-air flotation units, clarifiers, and centrifuges. In the product, carbon particles are enrobed in a coagulant that is used for water–oil demulsification. Once in the wastewater, the primary coagulant washes away from the particles and treats the oil and grease. The now-free carbon particles then can absorb sugars, toxic organics, surfactants, and other types of organics. Next, a flocculent is added to cause all of the treatment and pollution-bearing particles to agglomerate for flotation or settling. The clarified effluent can be discharged for compliant disposal, according to Stuart Davis, GenChem International principal.

“The resulting sludge can be dewatered with little to no additional treatment,” Davis said. “The BOD/COD taken out by Pitch Black is removed from the treatment sequence forever and not returned as concentrate or any other sidestream.”

After installation of this system at the laundry, sampling was conducted and compliance achieved. Plant effluent COD concentrations immediately dropped to between 500 and 800 mg/L.

The system, developed to reduce surcharges and meet low BOD and COD compliance limits, treats not only COD and BOD but also TSS, FOG, and toxic organic compounds, Davis said. Pitch Black is designed to efficiently remove these pollutants, be versatile in its applications, directly replace current coagulants, and eliminate the need to change pump size, Davis said. It also is intended to reduce compliance liability and surcharge taxation, have a similar or smaller solids footprint, and be comparable in cost with traditional treatment chemistries, he explained.

Costs for installing the system are comparable to other treatment options. In the long run, Davis said, overall costs may be lower, as solids production typically drops between 20% and 50%.

 

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