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A gronomy J our n al • Volume 101, I s sue 2 • 2 0 0 9 323

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a b s t r a c tPhosphorus (P) recovery and re-use will become increasingly important for water quality protection and sustainable nutrient cycling as environmental regulations become stricter and global P reserves decline. The objective of this study was to examine and characterize several magnesium phosphates recovered from actual wastewater under field conditions. Three types of particles were examined including crystalline magnesium ammonium phosphate hexahydrate (struvite) recovered from dairy wastewater, crystalline magnesium ammonium phosphate hydrate (dittmarite) recovered from a food processing facility, and a heterogeneous product also recovered from dairy wastewater. The particles were analyzed using ''wet" chemical techniques, powder X-ray diffraction (XRD), and scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy (SEM-EDS). The struvite crystals had regular and consistent shape, size, and structure, and SEM-EDS analysis clearly showed the struvite crystals as a surface precipitate on calcium phosphate seed material. In contrast, the dittmarite crystals showed no evidence of seed material, and were not regular in size or shape. The XRD analysis identified no crystalline magnesium phosphates in the heterogeneous product and indicated the presence of sand particles. However, magnesium phosphate precipitates on calcium phosphate seed material were observed in this product under SEM-EDS examination. These substantial variations in the macroscopic and microscopic characteristics of magnesium phosphates recovered under field conditions could affect their potential for beneficial re-use and underscore the need to develop recovery processes that result in a uniform, consistent product.

This study investigates the differences between field-based odor assessment methods that may be used to discriminate odors from livestock and food processing facilities. Field olfactometers have been praised for their low cost ($500 -$1200) and portability but criticized for their lack of control of inhalation rates by different panelists, the discomfort of glass inhalation tubes and the odor fatigue caused by poor nasal sealing or removing the scentometer between samples as compared to laboratory dynamic, triangular forcedchoice olfactometers ($30,000). This study evaluated the variability of responses using these three field olfactometers compared to laboratory olfactometry, as well as field and lab odor intensity. Panel responses using laboratory dynamic, triangular force-choice olfactometry and Nasal Ranger field olfactometers were found to exhibit the least amount of variability across odor sources. Significant differences with poor correlation were found between field and laboratory odor intensity methods. Findings of this study should caution regulators, policy makers and investigators about establishing regulatory thresholds or reporting scientific data based on single odor analysis methods or techniques due to the variability in the performance of odor determination methods across the odorous sites investigated.

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