Researchers Seek a New Method for Determining SPN Quality to Help Control Odor Emissions

Research Coordinator Robert Wallace checks DAF skimmings quality.

Recurring nuisance odor complaints can slowly erode community support for a facility located in close proximity to a residential area. For poultry processors, this frequently is a challenge that is ever changing. Nuisance odors frequently spawn from refrigerator truck drippings or tanker truck underflows in open holding areas. For rendering plants, a very important allied industry, nuisance odors also emanate from poor quality material being received from poultry processing facilities.

Secondary protein nutrients (SPN), also called dissolved air flotation (DAF) float or skimmings, is a wastewater byproduct that is often rendered. Georgia Tech researchers recently worked with the rendering industry to evaluate new methods for monitoring odors from SPN during poultry processing wastewater treatment. The effort was part of a broader assessment of wastewater pretreatment systems.

“People familiar with wastewater treatment operations know DAF skimmings can be a source of fugitive odors. Most facilities have enclosed the operations and keep the area well ventilated. However, tanker trucks are parked outside in the sun while being loaded. We know of no one who has documented any relationship between nuisance odor indicators and SPN quality degradation over time. Aging SPN affects the bottom line for the processor and the renderer, as well as generating nuisance odors that make neighbors very unhappy,” says John Pierson, Georgia Tech senior research engineer and project director.

With an estimated 9 billion birds processed annually, approximately 2.8 billion pounds of SPN (wet) must be disposed of or treated nationally by the poultry industry. While a valuable raw material, SPN fat content can easily become rancid as it is collected and concentrated by gravity dewatering throughout the processing day.

To prevent fat rancidity and maintain SPN quality for rendering, commercially available antioxidants are dripped into SPN during tanker loading. Antioxidants are well tested and effective, although they do not readily dissolve in water. Instead, antioxidants combine with fats to prevent oxygen from degrading SPN. Poor antioxidant mixing with fats due to high SPN water content can lead to SPN quality problems. Antioxidants remaining in tanker underflow water can drain into the plant’s wastewater system. Any unprotected SPN can rapidly spoil, leading to nuisance odor issues for renderers.

The chemical reactions related to degrading fatty materials first produce peroxides, known as primary oxidation products. Peroxides trigger further chemical reactions that produce aldehydes and ketones or secondary oxidation products. For example, hexanal is an aldehyde that is a known nuisance odor byproduct produced from degrading poultry fat. Free fatty acids are also produced as proteins and other lipid-containing feedstuff degrade, and these are measured as an acid value or FFA.

Measuring peroxide values for product quality can be problematic because as peroxide levels increase and then decrease, they produce secondary compounds. Some methods for getting around this problem include observing weight gain, conjugated dienes, active oxygen method (Swift test), 2-thiobarbituric acid (TBA) value, and carbonyls (hexanal). However, no standard method has been developed for distinguishing all oxidative changes in lipid-containing materials.

An alternate approach investigated by Pierson and his team was to evaluate fat degradation and thus SPN quality using TOTOX. TOTOX (total oxidation) is the measure of the combined limit of primary and secondary oxidation products as reflected in the peroxide and anisidine values of the sample (a TOTOX value is based on [2 x PV + AN]). During evaluations the team observed pigment interferences resulting from SPN sample preparation, which made visual-based methods for measuring peroxide values impractical.

The research team then conducted accelerated aging of SPN (i.e., heating at 77, 95, and 104 °F for 24 hours) with and without antioxidants. Sample monitoring included peroxide value (PV), p-Anisidine (p-A), thiobarbituric acid (TBA), and free fatty acid (FFA) analyses. Hexanal formation was monitored as an indicator of nuisance odors and product quality. To assess the potential impact on wastewater pretreatment systems, anaerobic digestion of antioxidants was conducted to evaluate impacts on biologic pretreatment systems.

“ We found that FFA values increased with time and temperature, as expected. However, the other parameters we examined varied with time, temperature, and with regard to other indicators. When we compare the results from the other indicator tests with FFA concentrations, by time and temperature, we do see definite trends that we believe can assist with monitoring SPN quality and associated nuisance odor production,” notes Pierson.

Pierson says that the team did note that antioxidant addition (250 ppm as ethoxyquin) inhibited FFA and secondary product formation, especially at the highest temperature of 104 °F (40 °C).

Hexanal concentrations also increased with temperature and were higher without ethoxyquin addition. The greatest rate of hexanal production was at 35 °C, with a decrease at 40 °C. Data also indicated that hexanal formation follows time and temperature relationships similar to that of anisidine, one of the secondary oxidation product tests.

“ We think monitoring hexanal production has real potential. Using a chemical biosensor that is being developed by our colleague David Gottfried [Georgia Tech senior research scientist and lead developer of the Georgia Tech interferometric biosensor], I believe real-time monitoring of SPN quality can be developed around this concept. The other good news is that the presence of ethoxyquin derived from the supernate of a 40 °C accelerated testing sample did not have adverse effects on anaerobic waste treatment systems,” adds Pierson.