PRD Tech, Inc.

Pilot-Scale Biofilter Test for Treating Zimpro Odors


Courtesy of PRD Tech, Inc.

The thermal sludge conditioning process of U.S. Filter/Zimpro Corporation, Rothschild, Wisconsin (Zimpro process) is primarily used to improve dewatering characteristics of the waste sludge. The process entails a partial breakdown through oxidation and hydrolysis of complex, high molecular weight organic compounds, such as, proteins, fibers, carbohydrates, and fats. The partial breakdown of chemical bonding results in the formation of volatile compounds, mainly aldehydes, ketones, volatile organic acids, alcohols, esters, and some alkanes (primarily methane). These compounds, present in the off-gases from the sludge tank have a characteristic odor, the primary source of Zimpro process odors.

Traditionally odor control has been achieved by oxidation using chemical scrubbers and conventional b1ofiltration methods using compost media. The chemical scrubbing methods to treat (oxidize) the odorous organic compounds commonly use oxidizing agents such as chlorinated water, sodium hypochlorite, hydrogen peroxide, and potassium permanganate solutions. The hydrogen peroxide is a weak oxidizing agent requiring considerable reaction time (15 to 20 minutes) and effective treatment is achieved only if added upstream of the headworks. The permanganate solution is a strong oxidizing agent, which is difficult to handle as a dry powder and would require a high dosage to be effective. The hypochlorite solutions produce chlorine gas as a byproduct, which has its own distinctive pungent odor. The addition of chlorinated water for chemical scrubbing operation is generally effective when treating odors containing hydrophilic compounds (high mass transfer rates to water). When the odorous compounds are hydrophobic, such as the branched chain compounds which usually exhibit low aqueous solubility, mass transfer into the liquid phase limits the odor removal efficiency. The chemical handling hazards, potential release of halocarbons into the environment as a result of the chemical reaction of chlorinated water with the odorous gases, and pumping costs are other operating considerations with a chemical oxidation treatment system.

Other treatment methods, such as, thermal incineration, or catalytic oxidation have even greater disadvantages when compared to chemical oxidation which includes:

  • Higher investment and operating costs (natural gas consumption);
  • Generation of nitrogen oxides from the nitrogen present in odorous compounds and air;
  • Increased generation of carbon dioxide, a greenhouse gas; and
  • Production of fine particles.

A pilot-scale biofilter test was conducted to determine the efficacy of biotreatment for eliminatincy odors from the Zimpro process. Figure I shows a schematic of the pilotscale thebiofilter system, that was operated for over three months, with air being withdrawn from headspace of two Zimpro sludge tanks. The biofilter system used a proprietary synthetic support media that provided high surface area for growth of active biofilms and exhibited low gas-phase pressure drop.


The biofilter system consisted of two cylindrical beds (2 feet diameter and 7 feet height) through which the odorous gas flowed sequentially. A blower was used to draw the gases from the sludge tank through a condensate tank, to separate any condensed water. The first bed was filled with 5 feet height of glass-filled 2 inches polypropylene Jaeger Tripack material and water was sprayed at the top of the bed to cool the incoming gases from an Inlet temperature of 180OF to ambient temperature. The cooled gases then flowed down the second bed, which operated as the biofilter. Mineral nutrients at neutral pH were sprayed at the top of the proprietary biofilter support media and the nutrients from the bottom of the biofilter bed were pumped into a nutrient tank. The pH in the nutrient tank was maintained at 7.2 by automatic injection of sodium hydroxide by the pH controller. Periodically, spent nutrients were withdrawn from the nutrient tank and fresh nutrients were added, to maintain the ammonium and phosphorus concentrations in the nutrient liquid.

Gas samples were withdrawn from the inlet of the cooling tower and the biofilter outlet and analyzed to determine the Detection Threshold (DT), Recognition Threshold (RT), and the relative value of odor unpleasantness (range of 0 for no odor and 10 for maximum odor) by a seven-member human odor panel. The odor panel data was calibrated with respect to biofilter treatment efficiency by obtaining the average odor panel response (range 0-10) for various dilutions of raw Zimpro odors with carbon filtered air. The 100% biofilter removal efficiency represented odor panel response for carbon filtered air. The odor panel calibration result is shown in Figure 2.


A Membership Function, also shown in Figure 2 was developed, based on the odor panel calibration data, to obtain biofilter removal efficiencies from average odor panel responses. These responses were also correlated with the DT and RT values as shown in Figure 3.


The biofilter removal efficiency is shown in Figure 4. The biofilter efficiency varied mainly due to variations in odor panel responses.


The performance of the biofilter was better characterized by its ability to reduce the DT and RT values as shown in Figure 5.


The biofilter performance was controlled by the ammonium concentration in the liquid nutrients, supplied to the biofilter.

Preliminary cost analysis has shown that biofilter treatment of Zimpro odors is very cost effective, when compared with other treatment methods.

Pilot Scale Test of the Biotreatment of Odors from Zimpro Sludge Conditioning Process, Final Report submitted by PRD Tech to Sanitation District No. 1 April 1998.

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