Melting Away Odor Issues in Ice Cream Production Process

Ice cream brings back fond memories of childhood and hot summer days running after the clanging bell of an ice cream truck. Even today, as we reach into the grocer’s freezer, rarely do we think about the steps it takes to produce ice cream.

Manufacturing the flavorful treats involves many common methods used in dairy processing including blending, pasteurizing, homogenizing, flavoring and freezing. As with any production process, a waste stream is produced as a byproduct. In particular, dairy producers amass large amounts of high strength biological oxygen demand (BOD) waste.

At an ice cream production facility in southeast Missouri, this BOD waste discharged to the local publicly operated wastewater treatment plant contributed to odor complaints from hydrogen sulfide (H2S) formation and resulted in offensive odors around the collection and treatment process.

Contributing Factors

Any place or process where wastewater is collected, conveyed or treated has the potential to generate and release nuisance odors. These are the result of anaerobic or “septic” conditions. Such conditions occur when oxygen is deficient. Microbes known as sulfate reducing bacteria (SRB) thrive under septic conditions and convert naturally occurring sulfate ion to sulfide ion in a biochemical process. H2S, which has a strong offensive “rotten egg” odor, is a byproduct of this process.

While we love the taste of ice cream for its sugar content, this nutrient present in byproduct wastewater contributes significantly to BOD and is enjoyed by SRB to produce sulfide ions and H2S gas - causing the rotten egg odors at the city’s wastewater collection and treatment facilities.

To better understand factors causing odors, the city’s wastewater characteristics and treatment process were reviewed. The wastewater treatment plant process consists of a series of aerated lagoons. The average flow into the plant ranges from 250,000 to 300,000 gpd. The majority of flow is produced by the local dairy producer.

Nutrient levels as measured by BOD are greatly variable in the lagoon influent. The average BOD as measured at the raw water inlet in the lagoon was 4,930 ppm for a representative sample measured in early 2006. BOD has been reported in excess of 10,000 ppm.

The pH of the Lagoon North primary influent has fluctuated from 4.98 measured in April 2005 to 8.57 measured in December 2005. The total suspended solids (TSS) in the lagoon raw water fluctuates greatly. TSS averaged 5,486 ppm for 2005, ranging from 160 to 32,735 ppm. A process flow diagram is included in Figure 1.

The combination of nutrients present in the wastewater and elevated BOD encouraged the growth of SRB producing H2S gas. The nuisance level of odor from the lagoon was high. Gastec detector tubes were used to confirm presence of the gas. It was detected at a concentration of 40 ppm in the North lagoon and 10 ppm in the South primary lagoon when purged from a one liter sample of lagoon wastewater.

To combat the odor emissions, the facility looked at liquid-phase treatment options.

Odor Prevention Strategy

Several treatment techniques are available and marketed to prevent odor emissions due to H2S in a wastewater storage and treatment system. Vapor-phase treatment wasn’t considered because the process covers too much land surface area to be ventilated cost effectively.

Liquid-phase treatment options are summarized as follows:

1) Add oxidizing chemicals such as hydrogen peroxide to react with sulfide ions.

2) Add iron salts for direct reaction with sulfide ion to form ferrous sulfide precipitate.

3) Add nitrate salts to biochemically prevent sulfide from forming.

A formulated salt of calcium nitrate marketed as Bioxide® was selected as the best technique to prevent H2S for the city’s lagoon. The product was chosen because it is nonhazardous and cost effective for the application - and wouldn’t impact the treatment process. Readily available materials such as polyethylene tubing, PVC piping and single-walled polyethylene tanks can be used to handle and store the salts.

The process works biochemically to prevent odors. It involves addition of an alternative oxygen source in the form of nitrate ions. The salts are biochemically converted to nitrogen gas that’s harmlessly emitted and odorless. The prevention mechanism was attractive to the city because it would stop the formation of H2S gas.

An initial slug dose was proposed to quickly prevent the gas from occurring in the lagoon. The dose was added to the splitter box of the treatment basin to eliminate H2S immediately. Related odors ceased within days after the slug dosage was added. Testing confirmed elimination of the H2S.

Conclusion

Following the initial slug of Bioxide, doses are dispersed on a daily basis. A constant dose provides a small residual of nitrate to the front end of the treatment process for prevention of sulfide mainly in the primary basins. Several factors can cause the dosage to be raised or lowered. Higher temperatures in the summer encourage higher microbial activity and more dosing is required to account for this change. Higher strength wastewater has a similar effect. The removal of hydrogen sulfide has eliminated most of the problem and complaints about offensive odors from the lagoon.

About the Author: Tim Matheis is an applications engineer for the odor control business of Siemens Water Technologies. Matheis is located in its Sarasota, FL, office. Contact: 800-345-3982 or [email protected]