Impact of Lime Dose and Mixing Quality on Odor Generation by Lime-Stabilized Biosolids

Odor generated from lime-stabilized biosolids with various lime doses and mixing regimes was studied during two bench scale experiments over four weeks of storage to determine the correlation between the lime stabilization process and odor strength. For the first bench scale experiment, dewatered solids were mixed with various lime doses (4.5% and 11.7% by wet weight) under various mixing conditions (poorly mixed and thoroughly mixed). For the second bench scale experiment, odors generated by various lime doses that were thoroughly incorporated were analyzed.

The bench study indicates the various roles of lime dose and mixing quality on odor generation over time. During the initial week of storage, lime dose was more important than mixing quality. A low lime dose had a weaker odor strength (lower dilution-to-threshold) and less offensive odor (more neutral hedonic tone) than a high lime dose. However, mixing quality became more important over the course of time. A well-mixed biosolid product had weaker odor strength (lower dilution-tothreshold) than a poorly mixed product. The well-mixed biosolid product also had a relatively stable hedonic tone whereas the poorly mixed products became increasingly more offensive over time.

Lime stabilization is used to treat dewatered solids from municipal wastewater treatment plants so that biosolids can then be beneficially recycled as a soil amendment at local farms (Erdal et al., 2004; Murthy et al., 2001). Biosolids stabilized with lime may generate offensive odors, which mainly contain ammonia, amines and sulfides (Murthy et al., 2001). There are two reasons for the odors: 1) anaerobic conditions are present during the upstream solids process and 2) lime is poorly incorporated into the biosolids (Murthy et al., 2001). Odors that result from organic amines and sulfides can be difficult to control and treat.

Since the odor is capable of traveling miles downwind, neighbors of the wastewater treatment plants and farms often complain of the offensive odor (McGinley and McGinley, 2004). Offensive odors from land-applied biosolids generate public opposition to biosolid land application programs (Kim et al., 2001). Poor incorporation of the lime sometimes yields an offensive product. A thoroughly mixed product often has very little odor and may smell like soil or compost.

The objective of this study was to determine how mixing quality and lime dose affect the odor of biosolids over a 4-week time period. Mixing quality was represented by mixing time. A short mixing time was suspected to provide insufficient time for the lime to be uniformly mixed with the dewatered solids and represented poor mixing quality, while a long mixing time was suspected to provide a uniform mix of lime and dewatered solids and thus represented good mixing quality. Mixing times were determined in a pre-bench scale study where calcium uniformity was used to determine poor mixing and good mixing. Calcium uniformity was measured by taking 15 samples from different sections of a batch and measuring the calcium concentration in each sample. The standard deviation of the calcium concentration for the 15 samples represented the uniformity of the batch. A mixing time that produced a lime-stabilized biosolid sample with a Ca standard deviation that was below 1 was used to represent good mixing quality.

For samples that were well mixed, it was expected that a moderate lime dose would be better than a low or high lime dose. A low lime dose was anticipated to be unable to maintain the high pH even if adequately mixed and thus would contain higher levels of reduced sulfur compounds. The high lime dose was expected to generate more ammonia into the headspace and increase offensiveness as compared to other lime doses.

Two wastewater treatment plants in Pennsylvania were chosen to supply unlimed dewatered solids for this study. Plant 1 was selected due to its relatively non-offensive biosolid product as compared to typical wastewater treatment facilities in the area. In the study of Plant 1, two lime doses were examined: a low lime dose of 4.5% by wet weight and a high lime dose of 11.7% by wet weight. The 11.7% lime dose was equivalent to the dose being used at the plant. Two mixing times were also examined, a short mixing time of 15 seconds (representing poor mixing quality), and a long mixing time of 45 seconds (representing good mixing quality).

Plant 1 is located northeast of Harrisburg, PA and is noted for having infrequent odor complaints about their biosolids. If they occur, they are about the odors generated from stored biosolids at the plant. Very few complaints are received about the odors that occur at the site of land application.

The plant dewaters its solids using a belt filter press and then adds approximately 11.7% (by wet weight) powdered quick lime. The dewatered solids come off the belt press and the quick lime is mixed in as a screw auger moves the dewatered solids. Mixing takes place for approximately 45 seconds to a minute. The auger moves the solids for approximately 20 feet before the solids are deposited in a storage pile.

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