During the 15-plus years that the plant operated, it conducted some of the most comprehensive odor control and related research done in the industry. MCRCF received the U.S. Environmental Protection Agency’s Beneficial Use of Biosolids Award four times, most recently in 1998 for Research Activities.
Over the years, MCRCF developed analytical techniques to measure reduced organic sulfur compounds, which are principally responsible for off-site odor complaints. It also developed odor monitoring and scrubbing technologies. The most recent research sought to discover the positive odor reductions that might be achieved by using a consistent and stable biosolids as feedstock for the composting process. It compared odor emissions from prelimed (before dewatering) biosolids to the postlimed biosolids the facility usually receives. The facility also recently conducted research on the odor reducing effect of adding wood ash to the biosolids/wood chip mixture and on methods for controlling and reducing odor and VOC emissions.
The following report on the most recent research was adapted from MCRCF’s award nomination application submitted to EPA in 1998.
EFFECTS OF BIOSOLIDS STABILIZATION AND DEWATERING PROCESSES
Biosolids from the Blue Plains WWTP consist of primary and secondary sludge at a 1:1 ratio, which is stored at four to six percent solids for various periods of time. As a result, the primary solids are allowed to putrefy in storage to various states of stability. Lime is added just before and immediately after dewatering in an effort to reduce odors during the dewatering process and transportation to the MCRCF. However, lime addition is not always successful in controlling odor emissions, and the MCRCF often received batches of highly odorous biosolids. In addition, the postliming process is not uniform in mixture or lime content, and the heat of lime hydration raises the biosolids to high temperatures. Thus, the MCRCF receives biosolids in various states of temperature, putrefaction, and lime content. As a result, odor emissions from the biosolids, and subsequently from the composting process, have been highly variable.
To study the impact that a more consistent and stable biosolids product might have on odor emissions, the MCRCF received prelimed biosolids from the Piscataway (Maryland) WWTP for two months. The biosolids consist of approximately 60 percent primary sludge and 40 percent waste activated sludge (WAS). The liquid primary sludge and WAS are stored in gravity thickeners for at least two hours at pH 12. Lime and ferric chloride are added to the stabilized mixture just before it is dewatered with vacuum filter presses. The prelime stabilization and vacuum filter press processes yield a very consistent and stabilized biosolids product. In addition, the heat of lime hydration occurs in the gravity thickeners. Thus, the biosolids were close to ambient temperature when they were discharged at the MCRCF.
The Piscataway solids were mixed with Blue Plains biosolids at a 1:1 volumetric ratio because there was an insufficient amount to fulfill the capacity of the MCRCF. Mixing the two sources reduced the differences in odor emissions. After 1.5 months of accepting Piscataway biosolids, the MCRCF staff and several environmental consulting engineers conducted comprehensive field sampling programs to assess the emissions of odor, ammonia, and reduced organic sulfur compounds from the facility due to the two different compositions of biosolids (biosolids from the Blue Plains WWTP only (BP) and the mixture of the two (PW/BP)). The sampling strategy included measuring the flux of odor, ammonia, and reduced sulfur compounds from the delivery trucks, mixer discharge (biosolids mixed with wood chips), 14-day static compost piles (nonventing and venting areas), breakdown compost piles, and the mixer building stack. All of these are area sources of emissions except the mixer stack.
Because the biosolids receiving and mixing sources at the MCRCF were operated only four to eight hours/day and the composting piles and scrubbers emit 24 hours/day, mass emission rates must be used to compare the relative value of various sources to odor emissions. Mass emission rates (pounds of compound released per hour) of sulfur compounds and ammonia were calculated to account for the operational and spatial variability associated with composting facilities. Results of these comparisons are summarized in Table 1.
As shown at the top of Table 1, the raw BP biosolids sources (e.g. receiving trucks and fresh mixed compost) emitted approximately two times the quantities of dimethyl disulfide (DMDS), total reduced sulfur (TRS), and ammonia than samples from sources of raw biosolids from the BP/PW mixture. This correlates well to the 1:1 volumetric ratio of the BP/PW mixture. The higher emissions are probably due to the higher temperatures and putrefaction associated with the BP biosolids. Those were received at 94ƒF, while Piscataway biosolids were received at 63ƒF. However, Table 1 also shows that the active composting piles emitted much more odorous compounds than the raw biosolids receiving and transfer sources. Once again, the piles composed only of BP biosolids released higher concentrations of DMDS and TRS. This was especially true for samples taken from vent areas in the active composting piles. The vents are areas on the top of the piles where convective forces are greater than the vacuum forces created by the aeration system at the bottom of the pile. As a result, these vent areas have much higher temperatures than the bulk compost pile, and steam is emitted at the top of the pile. The steam carries much larger quantities of odorous compounds than other lower temperature area sources.
Table 1 also shows that sulfur emissions from the mixer stack and scrubber (point sources) are much greater than all the area sources. Again, BP biosolids were more odorous than the BP/PW mixture. This is supported by data collected with an on-line TRS monitor which is used to pace chemicals added to the three stage misting process gas scrubber (used at MCRCF to treat odorous air). TRS concentrations declined in the process gas as the proportion of Piscataway biosolids increased. Piscataway biosolids were first received at the MCRCF on April 4, 1998. From that point, the proportion of Piscataway biosolids increased to approximately 50 percent by the middle of May. Total tons of biosolids also increased from approximately 900 tons/week in March, 1998 to approximately 1,200 tons/week in May, 1998. Despite the increase in tons of biosolids processed, the average and peak TRS concentrations continued to decline (Figure 1). Peak TRS concentrations create the most operating problems for the existing scrubber because the chemicals must rapidly change as the TRS concentration changes. The peak TRS concentrations were reduced substantially with the inclusion of Piscataway biosolids and the scrubber became easier to operate at peak efficiency.
The objective of testing Piscataway biosolids at the MCRCF was to determine if sulfur and odor emissions could be reduced by receiving a consistent and more stable biosolids. The results convincingly show that DMDS and TRS emissions from all sources were significantly less with the BP/PW mixture. However, the olfactory odor data is not so convincing. Odor data for all sources, except the vented areas of the compost piles, show reductions associated with the inclusion of Piscataway biosolids. The odor data for the vented areas of the compost piles, however, show higher odor associated with the BP/PW biosolids mixture. The odor data were reviewed to determine why odor measurements did not follow the sulfur findings for the vented areas of the compost piles. Ammonia emissions from the compost pile vents increased with the inclusion of Piscataway biosolids. While the reason for increased ammonia for this source was not determined during the study, it is believed the ammonia emissions are not responsible for off-site odor. The area sources are not significant as shown in Table 1 and the scrubber achieves very high removal efficiencies for ammonia (>99 percent removal). This data shows that reduced sulfur data is much more useful for monitoring odor emissions from composting sources than olfactory odor data.
EFFECTS OF ADDING LIME AND WOOD ASH
The MCRCF undertook studies to determine the effects of changing lime doses in the biosolids on odor and methanol emissions.
In addition, the use of wood ash was studied to determine if it could reduce emissions of both methanol and organic sulfurs.To obtain this data, the MCRCF designed and built bench-scale composters to test different biosolids, lime, and wood ash recipes. Process gas samples were collected and analyzed for reduced sulfur content and VOCs such as methanol, acetone, and methyl ethyl ketone (MEK).
Table 2 summarizes the results of two bench-scale experiments using a compost mixture of undigested biosolids and a 5:1 volumetric ratio of wood chips to wood ash. In summary, these experiments showed that lime is more effective than wood ash as a single amendment for controlling sulfur generation during the composting process. However, the addition of wood ash to limed biosolids appeared to further control the generation of sulfur compounds.
To verify the actual effectiveness of the lime and wood ash amendment combination, three full-scale tests were conducted. In each test, wood ash was added to limed biosolids (pH 12) at a reasonable volumetric ratio of 10:1, wood chips to wood ash, with the wood chips to biosolids at a volumetric ratio of 3:1. Overall, lime plus wood ash decreased the generation of sulfur compounds by 35 percent for the three tests.
To further investigate the chemical mechanisms responsible for reduced emissions resulting from increased pH and wood ash amendment, the following samples were taken: 1) Mixer discharge stack: Two samples were taken directly from the stack which pulls air from the sludge/wood chip mixers. One sample was untreated and the other was filtered through a cartridge containing wood ash; 2) Freshly mixed compost pile consisting of Blue Plains biosolids: Two samples were taken from a flux chamber situated over a fresh pile of compost mix. A sample was first drawn from the flux chamber and then a thin layer of wood ash was applied to the surface of the pile before taking the second sample; 3) Active composting pile vent (14-days into the composting cycle): Two samples were taken from a flux chamber situated over a vent area of an active composting pile consisting of a Piscataway/Blue Plains biosolids mixture. The first sample was collected directly from the flux chamber and the second was pulled through a cartridge of wood ash.
Table 3 summarizes the results for these samples, which were analyzed with a gas chromatograph located on site. For each of the three sources, the concentrations of both DMDS and TRS were reduced in the samples utilizing wood ash as filter or cover. Thus, it appears that wood ash quickly adsorbs these sulfur compounds. In contrast, the other highly volatile compounds analyzed for this study did not produce conclusive results regarding the capability of wood ash to reduce emissions. Therefore, adsorption appears to be the fundamental mechanism responsible for reduced sulfur emissions provided by wood ash.
VOC EMISSIONS AND CONTROLS
MCRCF is located in a serious ozone nonattainment area. As such, the Title V VOC emissions threshold for major sources in this area is 25 tons/year. In addition, the Title V threshold for any single hazardous air pollutants (HAP) in this area is 10 tons/year and 25 tons/year for total HAPs. VOC and HAP emission data for biosolids composting facilities are quite limited and highly variable. Therefore, the MCRCF could not estimate total VOC or HAP emissions based on literature sources and thus did not know if it was a major source of VOCs or HAPs under the Clean Air Act Amendments of 1990.
To determine its status under the Clean Air Act, the MCRCF decided to measure VOC and HAP emissions from its three major point sources (process gas scrubber, mixer discharge, and composting building exhaust fans). The VOC and HAP removal efficiency of the existing three stage misting scrubber also was tested. Air samples were taken from the process gas stream before and after the scrubber and from the composting building and mixer discharge stacks.
Measurements found that the scrubber emits the vast majority of VOCs compared to the biosolids/wood chip mixing facilities. In addition, it is evident that most of the VOC emissions in the scrubber inlet are condensable VOCs, which is not surprising because the temperature of the process gas is approximately 125ƒF and contains high boiling point by-products of biosolids composting. However, the scrubber is removing approximately 93 percent of total VOCs. Removal appears to be most effective for the condensable fraction of the VOCs. Very few low molecular weight, noncondensable VOCs are removed by the misting scrubber. This suggests that the condensable VOCs are condensing at the surface of the mist drops inside the scrubber. (However, the temperature of the process gas does not appreciably change as the gas proceeds through the scrubber.) This condensation is believed to be responsible for the high VOC removal efficiencies.
HAP analyses indicated that the composting buildings and biosolids/wood chip mixing facilities emit very few HAPs or VOCs. Only methanol and acetone were detected in samples taken from these facilities. However, the process gas entering and exiting the misting scrubber contained significant quantities of methanol, acetone, MEK, and high boiling point terpenes. These compounds are biological by-products of biosolids and wood chip degradation. Acetone and MEK are produced by ketosis, which occurs when insufficient oxygen is present to complete the Krebs cycle (also known as the citric acid cycle, a series of chemical reactions that occur within a cell and break down food molecules to produce energy). Methanol is produced by oxidation of wood chips at high temperatures. Methanol emissions at the MCRCF are higher than measured at other composting facilities. Bench-scale tests have shown the increased methanol emissions likely are due to the addition of lime to the BP biosolids before composting. The three stage misting scrubber has a low HAP removal efficiency due to the highly volatile nature of the HAP compounds found in composting emissions.
Recent research conducted at the MCRCF has once again advanced our understanding of odor and VOC emissions from biosolids and composting facilities. In particular, this research has produced the following conclusions:
•The use of a more consistent and stable biosolids as feedstock for the composting process will produce less organic sulfur emissions throughout the facility. Less sulfur and odor is generated in the receiving and transfer areas, less odor and sulfur is generated by the composting process, and the scrubber is therefore easier to operate. Overall, odor and sulfur emissions were reduced by more than 50 percent.
•Lime addition to biosolids reduces odor and sulfur emissions from the composting process. Since the composting process is the most significant source at the MCRCF, lime addition to the biosolids provides a large dividend in terms of overall odor and sulfur reduction. However, the method of adding lime affects odor emissions in the receiving and transfer sources. Prelime addition (lime addition before dewatering) produces a more consistent and lower temperature product. Therefore, prelimed biosolids will emit less odor and sulfur during receiving and transfer operations.
•Lime addition to biosolids increases methanol emissions from the composting process. High pH and temperature conditions in the active composting piles are conducive to pulping of the wood chips. Cellulose, lignin, and other wood-based sugars are extracted from the wood chips under these conditions. These compounds then serve as the precursors to methanol formation.
•The addition of wood ash to the biosolids/wood chip mixture reduces sulfur and odor emissions from the composting process. The wood ash probably adsorbs a portion of the sulfur compounds created in the composting process.
•Convection within aerated static composting piles can create “vented” areas that allow large quantities of odor and sulfur emissions to be emitted. To reduce these vent areas, the aeration system should be designed to overcome the convection forces.
•The existing three stage misting process gas scrubber is very efficient in removing high molecular weight, condensable organic compounds and inefficient in removing low molecular weight, noncondensable organic compounds. Since composting process gas consists of mostly high molecular weight, condensable organic compounds, the existing scrubber achieves high removal efficiency for total VOC removal. This phenomenon is not likely to occur in packed tower scrubbers.
This article was adapted from the Montgomery County Regional Composting Facility’s beneficial use award nomination application submitted to the U.S. EPA in 1998.