Managing milk-plant odors

H.P. Hood pasteurizes, processes, and packages milk and nondairy products at its plant in Oneida, New York. The operation discharges wastes from processing, cleaning, and sanitizing operations. Wastewater is treated at the Plant's pretreatment facilities and then discharged to the city's sanitary sewer. Pretreatment facilities comprise two covered and aerated 149,000-gal flow equalization tanks (FETs), pH adjustment, flow control, spill recovery system, and a submersible effluent pump station. Air is released through a vent located atop each FET and from the spill tank and pump station adjacent to the FETs.

The FETs at Hood

Shortly after the FETs were put in service, neighbors began to complain about odors. The plant's operators responded by adding chlorine and increasing aeration, but their actions resulted only in minimal odor reduction. It became clear that an odor control system was necessary. Barton & Loguidice Engineers was asked to find an odor control system. Working closely with Jim Sylvester of H.P. Hood, B&L developed a plan.

Source of odors

Organic compounds, primarily, were responsible for the odors, and they were mostly generated by the accelerated chemical decomposition of lactose in the caustic/acid high-temperature sanitizing processes. Since the FETs were aerated to maintain dissolved oxygen, hydrogen sulfide odors were essentially absent. Complaints about odor coincided with elevated tank pH and late summer afternoons when neighbors were likely to be outdoors.

Control technologies and complications

B&L investigated technologies that were proven to remove organic-based odors. Although the pretreatment facilities were originally designed to accommodate odor control systems (generally considered to be accomplished by scrubbing or adsorption), available space was limited. Any solution would have to fit in an existing small control room along a small strip of land adjacent to the existing building. Two added complications were the known build-up of a gelatin-like material that would plug the vent screens and the formation of milkstone (milk-product minerals) on surfaces exposed to the wastewater or wastewater aerosols. Four odor control technologies were evaluated for treating the air released from the pretreatment system:

• Biofilters
• Adsorption and activated media
• Packed bed scrubbers with oxidizers
• Vapor-phase neutralization.

Biofiltration

With biofiltration, media adsorption and biological oxidation remove air-borne odorous compounds. The air passes through porous organic media, and microbes in the media metabolize the odor-producing compounds adsorbed on the moist filter particles. The media typically is a combination of compost, wood chips, and nutrients. To maintain the biological process, temperature, pH, and moisture content must be carefully controlled.

Prepackaged biofiltration odor-control technology systems consist of shallow media trays that are stacked to promote uniform air distribution through the media. Prepackaged biofilter systems generally include several parallel-connected biofilters, a blower, humidifier/scrubber (particulate and mist eliminator), control panel, and piping.

A site-built conventional earthen biofilter system was also considered. Such an approach requires two large subsurface biofilter beds containing drainage piping, air distribution piping, media, and a wood chip or mulch cover. As with a packaged system, moisture, pH, and temperature must be accurately maintained. Because earthen biofilters are exposed, they would require additional maintenance such as weed removal and watering. Conventional biofilters use one deep layer of media (4 ft) which is susceptible to compaction and short-circuiting over time.

Packaged biofilters rather than conventional biofilters were focused on for the Hood pretreatment system because they are compact, require less installation effort, and are less prone to media short-circuiting. Moisture level and temperature are generally automatically adjusted by the system. Biofilter media would likely have to be replaced every 3-5 yr, and the scrubber/humidifier and ductwork would have to be cleaned periodically to remove the gelatinous organic substance and milkstone deposits.

Adsorption and activated media

Activated media adsorption technologies such as activated carbon air treatment vessels were evaluated for potential applicability to odor control. These technologies are generally effective in removing air-borne organic gasses and odors, but we were concerned that the gelatinous substance in the FET and pumping station would build-up in the activated media and cause plugging. Additionally, the malodorous constituents represent only a fraction of the total organic compounds that would be adsorbed by the activated media. Because of the large loading of organic substances, media such as activated carbon would have to be regenerated or replaced frequently. We, therefore, judged media adsorption technologies to be incompatible with this application.

Scrubber

A packed bed scrubber uses a physical-chemical process by which odorous gas is transferred to the scrubbing fluid and oxidized. Such a system consists of a blower, scrubbing vessel packed with randomly dispersed plastic media, oxidizing solution, solution pump, and spray manifold. During operation, odorous gas is pulled through the scrubber and contacts the solution-saturated scrubber media. The oxidizing agents, such as sodium hypochlorite or hydrogen peroxide solution, oxidize odorous organic compounds. Typical decomposition by-products are carbon dioxide and water.

Prepackaged scrubber systems are available and generally include a low profile packed bed scrubbing vessel with packing, spray manifold, recirculating pump, blower, control panel, and chemical feed system. Systems are typically automated and require little labor. Scrubber solution level and concentration is automatically maintained by the system. Oxidizing agent inventory must be maintained, however, and the system must be routinely inspected for proper performance and component function. The scrubber and related ductwork must be cleaned to remove gelatinous substances and milkstone deposits.

Neutralization

With vapor-phase odor neutralization, gaseous odor-neutralizing compounds react or interfere with odor molecules to change their structure and render them inodorous. Since this technology works in a vapor phase, the need to transfer gaseous odors into a liquid phase, such as with the scrubber or biofilter, is eliminated. Vapor-phase reactions are nearly instantaneous; therefore, the residence time is short.

Internal vapor-phase neutralization uses the existing covered tank headspace as a "reaction vessel," thus eliminating need for construction of an external contact vessel. For applications with insufficient headspace, an external contact vessel can be constructed. External contact vessels are similar to traditional scrubber systems but are typically smaller because of the faster reaction rates.

Odor neutralizing compounds vary depending on the application; many operators prefer products from essential plant oils not only for their effectiveness but also because they are generally nonhazardous and benign to the environment.

High-pressure chemical feed system

Decision-making

Although packed bed scrubber systems and biofiltration systems would remove odorous compounds from the air stream, their high cost and the probable build-up of organic material and milkstone in air handling equipment and treatment vessels made them less desirable for this application. We similarly dismissed media adsorption; adsorption would have had the added liability of costly media regeneration or replacement.

The technology that satisfied our criteria was the internal vapor-phase misting system. It provided effective odor treatment, limited construction, low operating expense, and low maintenance. This is the system that H.P. Hood installed. A single high-pressure pump (500 psi) and misting nozzles and piping at the flow equalization system made up the installed system. An inverted 3-ft diameter by 5-ft high vent stack was installed beneath the existing vent of each FET.

Odor control system

Two misting nozzles were installed in each tank's headspace under the existing dome-style covers, and a third nozzle was installed in the stack. The stack minimizes short-circuiting of untreated air and provides redundant treatment by channeling exhausted air through the mist created by the center-mounted conical-spray pattern nozzle. We recommended an essential oil based odor neutralizing solution because it is neither hazardous nor corrosive.

To date, the system has proven to be effective and reliable. Odors released by the facility have been eliminated.
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At Barton & Loguidice, PC in Syracuse, Nicholas J. Pinto, PE is a senior vice president, Richard A. Straut, PE is a senior managing engineer, and Eric A. Pond, PE is a project engineer. Jim Sylvester was H.P. Hood's Project Engineer and oversaw installation.

H.P. Hood web site