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The odors produced when lime stabilizing biosolids are some of the most commonly cited problems that the public has with accepting land application. The nature of these odorants has been thoroughly studied, particularly the sulfur and ammonia compounds. What is still poorly understood is the relationship these malodorous compounds have to the amount of lime addition and the degree of lime incorporation into the bulk biosolids. Twenty-one plants producing Class B biosolids in the state of Pennsylvania were examined to evaluate their dewatering and lime incorporation processes. From samples collected and analyzed at each of these plants, nine treatment facilities were chosen to undergo more thorough analysis, including headspace testing and human odor panel analysis. These nine plants were chosen on the basis of comparable products/processes and high vs. low offensive odors.

The underlying objective of this study was to determine if there was some type of master variable (such as the degree of lime incorporation) that could be related to most or all of the other observed changes in the biosolids, such as odor quality, quantity, and the physical characteristics of the end product. Methods for evaluating the biosolids were chosen for their simplicity, so that treatment plant operators could use these tests to quickly examine their biosolids' product quality and make any necessary process changes. Another objective was identifying links between specific variables in the liming process and classes of odorants, since samples from one particular waste water treatment plant had the highest standard deviation in calcium content (at a reasonable dosage) and also possessed the most offensive odor.

The odor panel used both dynamic dilutions to threshold and odor characterization/hedonic tone tests to evaluate the biosolids samples. Work from the first phase of this project showed that the odor panelists had a tendency to perceive the biosolids odor as either positive or negative. The positively perceived smells were those of the ammonia and earthy categories, while the negatively perceived odors were those in the sulfide, rancid, and offensive categories. Most of the individual categories had very little correlation to any non-odor variable, which lead to the simplification at the bench scale to hedonic tone measurement. The offensive category had the strongest correlation to the total solids content of the material, which is reasonable, given that higher total solids will generally be more difficult to mix, leading to unlimed pockets capable of generating higher levels of odorants.

Headspace testing was conducted by the PA Department of Environmental Protection Laboratory using gas chromatography. All the other testing was carried out at the Penn State Capital College labs. Samples were tested for pH, calcium content and uniformity, total solids, volatile solids, and ammonia. The results were analyzed using SPSS analytical software, allowing us to make comparisons within one plant's biosolids product and between different products. During the bench scale, samples were evaluated to determine the minimum lime dosage required to meet the regulatory limits for pathogens and vector attraction. The pH and calcium testing revealed that in some samples, even though this minimum dosage was sometimes greatly exceeded, the degree of variance was high enough to allow areas of unlimed biosolids to persist. These areas, whether by chemical or biological means, may be continually increasing the level of odorous compounds associated with the material. Evaluations are being conducted to identify what classes of odor compounds can be controlled depending on the lime dose and degree of mixing. Potentially valuable results were obtained from the use of a flat-tip probe for pH measurement, rather than the current slurry method. The current method is thought to mask areas of poor incorporation, while the standard deviation in the flat-tip measurements may be a more worthwhile control. The flat tip probe correlated well to the calcium uniformity at poor mixing, but further research is needed to validate a connection between the flat tip pH and calcium content in improved mixing scenarios.

Document Type: Research Article


Publication date: 2006-01-01

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