BioCycle June 2005, Vol. 46, No. 6, p. 36
Two month trial at a New Hampshire agitated bay biosolids composting facility evaluates the effects of positive versus negative aeration on factors such as ammonia release, compost quality and utility costs.
Richard Nicoletti and Jim Taylor
AMMONIA vapors are major concerns at an enclosed composting facility that processes biosolids, since they can be highly corrosive to the structures and equipment. Combined with high humidity, ammonia also creates an adverse working environment. For these reasons, compost pile aeration and process air handling are key design components that impact the success of any enclosed composting facility operation and maintenance program. This report examines the results of a negative aeration study conducted at an in-vessel, agitated bin composting facility in Merrimack, New Hampshire.
Forced aeration into compost piles optimizes the biosolids composting process. Positive aeration blows air up through the compost; Negative aeration draws air down through the compost. Both methods have advantages and disadvantages. Ammonia release inside the facility is greater from composting technologies that utilize positive aeration because of the difficulty capturing released ammonia. Negative aeration has operational problems associated with collection, removal and disposal of fines and moisture that are drawn into the aeration network.
The biosolids composting facility in Merrimack uses USFilter’s IPS Composting System (IPS), which historically has utilized the positive aeration method. A typical IPS facility incorporates a series of concrete bays that are approximately 220 feet long to retain the compost for 21 days. Each bay is subdivided into multiple aeration zones; each zone contains a perforated piping network beneath the compost pile that connects to a dedicated aeration blower in an aisle. The blower serves the dual purpose of cooling and aerating the compost. On average these blowers are designed to operate about 10 minutes per hour – thus only about 15 percent of the blowers (75 total blowers at the Merrimack plant) are running at any one time.
Each blower has a suction side and a discharge side. Piping connected to the discharge side creates positive aeration, pushing air up through the composting mass. Conversely, piping connected to the suction side converts the process to negative aeration by pulling air down through the composting mass. The goal of the Merrimack study was to assess the impact on all facets of the composting process by converting one bay from positive aeration to negative aeration. The converted bay was monitored daily for a two-month period and then periodically for one year. One positively aerated bay was similarly monitored as a control.
As expected, this study indicates there are advantages and disadvantages to operating in the negative aeration mode. Some of these results are specific to IPS facilities with its aeration system design, while other results are more generic to all composting technologies. These test results indicate that Class A compost can be made with either aeration method with no significant differences in quality and process performance.
DESIGN FEATURES OF STUDY FACILITY
The aeration study was conducted at the Town of Merrimack’s 15-bay composting facility that has been operating since 1994. It is designed to cocompost 60 wet tons/day of wastewater treatment plant (WWTP) sludge with sawdust and recycled compost. Over a seven day period, the facility loads approximately 240 cubic yards of blended infeed mixture into the 15 bays in 16 cubic yard charges. Three agitators simultaneously shred, mix and move the compost material down the length of the bays.
Each bay contains five aeration zones, each with a dedicated blower (3 HP, 800 cfm @ 6-inches water). The bays are 6.5-feet wide by 6-feet deep (to top of aeration stone). They are isolated behind plastic strip curtains to retain the malodorous process air over the bays. Transfer fans located in the front of the facility (loading area) move fresh air into the bays behind the strip curtains to further prevent process air from moving into the loading area where personnel typically operate. Six 50 HP building exhaust fans provide an estimated 12 air changes per hour in the area over the bays. Process air is treated through a biofilter.
For each zone, the aeration system consists of a 6-inch header with three 2-inch laterals that span the length of the bay. The aeration zone lengths vary from 30-feet to 50-feet throughout the bay. This pipe is covered in 18 inches of stone, designed to distribute the air evenly across the bays as well as protect the pipes from coming in contact with the compost. A 2-inch layer of wood chips on top of the stone prevents compost fines from migrating into the aeration stone. This wood chip layer is replaced when significant plugging occurs – indicated by exponential increases in blower running time. Merrimack’s bay infeed blend is a 1:1:1 ratio of biosolids, sawdust, and recycled compost from the end of the bays. Parameters of the infeed mixture are: bulk density of 940 lbs/cy; 60 percent moisture content; 81 percent organic matter; C:N ratio of 24:1; pH of 6.6; Total N of 1700 lbs/day; 1.1 lbs/day of nitrate and 200 lbs/day of ammonium.
With each daily pass of the agitator, the compost pile is moved approximately 12 feet down the length of the bay. After about 21 days, the compost is discharged out of the bay by the agitator, then moved to the curing area of the facility for approximately 30 more days prior to being marketed
Prior to the US Filter/Town of Merrimack study in 2003, facility operators experimented with negative aeration in one of the bays over a three-year period in an effort to reduce building and equipment corrosion. Standard temperature monitoring was conducted and there were no problems meeting the required temperatures. Because there were no drains in the bay, staff periodically checked leachate levels. Although leachate built up to about 2-inches throughout the bay, visual inspection of the finished compost indicated it was not wet. The results of the 3-year trial demonstrated to staff that running the bays on negative aeration would be feasible and not impact compost production or quality.
TWO MONTH TEST
To further analyze the pros and cons of negative versus positive aeration, Merrimack agreed to assist USFilter in conducting a formal study. The initial 2-month test commenced in September 2003 and concluded in November 2003. Bay 15 was selected to be operated in the negative aeration mode. Bay 1 was selected to be the positive aeration mode control. These bays are on opposite sides of the building and are located adjacent to aisles. Holes were drilled through the bay walls so that leachate could drain into the aisles. New, aluminum blowers were supplied for Bay 15 zones A – C to minimize corrosion from the wet air stream. (Existing blowers are painted steel, which are more prone to corrosion.) Air pulled through the material from these negative blowers was piped to the overhead exhaust fan suction duct. It was anticipated that there would not be much ammonia remaining in the compost after Zone C so the existing blowers in Bay 15 (D & E) were simply rotated and vented into the building. (This proved to be an incorrect assumption.)
Prior to beginning the testing, the wood chip barrier over the aeration system was replaced in Bays 1 and 15 to ensure that the conditions of the two bays were identical. Throughout the test, the two bays were charged daily with the identical infeed compost mixture (1:1:1 of biosolids, sawdust and recycle compost).
One characteristic of this infeed mixture is its propensity to heat up very quickly, regardless of the aeration method. Average compost pile temperatures during the trial were measured with a 6-foot Reotemp thermometer for each of the 5 aeration zones for Bays 1 and 15 (taken in the geometric center of each zone). For the center of Zone A, which represents about two days retention time, the average temperature was 65°C in both bays. This jump start to the composting process is generally seen when large amounts of recycle are used, as is the case in Merrimack. What was noticeably different between the bays was that negatively aerated Bay 15 tended to hold the heat in the pile more than the positively aerated Bay 1 (61°C vs. 47°C in Zone C, 57°C vs. 47°C in Zone D and 51°C vs. 44°C in Zone E). It is believed this is due to the relatively less effective ability to aerate the piles via the blowers operating in the negative mode, and thus cool them.
Five key areas were analyzed during the trial (and continued to be monitored through September 2004). These include: 1) Quantify ammonia, hydrogen sulfide and acetic acid within the facility; 2) Confirm ability of the compost to achieve and maintain US EPA Part 503 compliance temperatures; 3) Measure impact on oxygen levels of negatively aerated compost; 4) Assess impact on marketability of the finished compost; and 5) Assess impact on operational and maintenance costs of the facility.
AMMONIA, HYDROGEN SULFIDE AND ACETIC ACID EMISSIONS
Several aeration schemes were used to measure the effect on ammonia emissions over the bays: 1) Bay 1 aeration blowers running in the positive mode – both “worst case” (all blowers on) and “normal” (blowers on 15 to 20 percent of the time); 2) Aeration blowers off (Bay 14 was selected); and 3) Bay 15 aeration blowers running in the negative mode. The ammonia concentrations were measured directly over the surface of the compost with a Kitigawa Model 8014 gas sampler. The Kitigawa device is limited to gas streams with temperatures less than 50°C. For measurements made in the discharge of the negative blowers, where the gas temperatures were as high as 70°C, a Drager 6400000 Accuro gas pump was used with a temperature correcting probe.
The measurements are shown in Table 1. With all aeration blowers running in the positive mode (worst case situation), Bay 1 had significant releases of ammonia into the building. The quantity released decreased down the length of the bay in correlation with the number of days material was retained in the active composting process.
To evaluate the impact of the blowers on ammonia release, the same measurements were duplicated over Bay 14 with all blowers off. In that mode, the ammonia levels dropped by a factor of 4 to 6, again decreasing down the length of the bay. Ammonia levels in the exhaust fan discharges were reduced to 15 to 20 ppm. Ammonia levels measured in the aisles with the blowers off and agitators running were on the order of 10 ppm.
In the negative mode, Bay 15 ammonia concentrations measured over Zones A and B are significantly lower than Bay 1, and up to 50 percent lower than Bay 14 with no aeration in those zones. At the same time, there were significant concentrations of ammonia in the blower discharge of Bay 15, even in zones 15D and 15E.
This data clearly indicates that operating in the negative mode should result in lower levels of ammonia within the entire process building. One way to measure conditions inside the building is to test the exhaust fan discharges by drawing a gas sample directly from the exhaust fan port using the Kitigawa. Under normal operating conditions, ammonia levels in the building exhaust fan discharges ranged from 50 ppm for the fans over the front of the bays down to 20 ppm for the fans over the back of the bays. In tests run with all the aeration blowers off, and all exhaust fans running, the ammonia concentrations in the aisles, behind the strip curtains, did not exceed 10 ppm and the maximum ammonia concentrations measured in the exhaust fan outlets did not exceed 20 ppm.
This scenario is a conservative estimate of the concentrations that would be present in the building because the ammonia release would be less with the blowers operating in negative mode. This finding should be generic for any IPS type composting operation. Assuming a design basis of 12 air changes per hour over the bays and all bays operating in the negative mode, it is estimated that the building air changes can be reduced by as much as half and still maintain ammonia concentrations in the aisles below 25 ppm.
Based on what is known about corrosion caused by ammonia release, building and equipment life would be extended by operating in the negative mode. While quantifying extended building life was beyond the scope of this study, data shows that ammonia levels within the building can be reduced by approximately 50 percent by going from positive to negative aeration with all exhaust fans running on high (i.e., 12 air changes per hour).
The Kitigawa also was used to measure hydrogen sulfide (H2S) and acetic acid. Neither was detected in the Bay 15 blower discharge piping, within the facility or over the negative aeration bay during the test. Measurable amounts of either gas could indicate problems with the compost process, such as the existence of anaerobic conditions.
MEETING PART 503 COMPLIANCE TEMPERATURES
During the nine-week test period, there was no indication of failure for the negatively aerated bay to maintain temperatures sufficient to meet Part 503 requirements (55°C for 15 days). In fact, due to elevated temperatures, blower run times (controlled by temperature feedback) in Zones A and B in Bay 15 were significantly higher than the same zones in positively aerated bays (18 hours vs. 9 hours in Zone A; 9 hours vs. 3 hours in Zone B). This may be unique to Merrimack due to its infeed mixture. While the temperatures get equally hot under normal operating conditions, the positive aeration method is so much more efficient at cooling the pile that the blowers don’t need to run as long. Therefore based on the findings of this study, IPS facilities choosing to use negative aeration would need to increase the blower capacity for Zones A and B to 1200 cfm @ 12-inches water column (for narrow bays) to address the cooling needs.
IMPACT ON OXYGEN LEVELS
Oxygen levels were measured with the Kitigawa in the compost throughout the bays and were identical between the positively and negatively aerated bays. Also as mentioned above, the lack of H2S indicates no signs of anaerobic conditions. Proper aeration of the compost via negative aeration does not appear to be an issue in terms of sufficient oxygen. (Daily agitation of the pile is designed to provide sufficient oxygen to maintain aerobic conditions.)
Another operational issue seen in the negative bay was the relatively rapid degradation of the wood chips above the aeration stone into a dense, wet hard pack layer. At the end of the nine week test, the chips in Zones 15A – 15C were wet and dense whereas the chips throughout Bay 1 showed no signs of degradation.
Over time, the wood chip layer in positively aerated bays deteriorates and a dry hard pack forms, preventing proper aeration of the pile. Typically, IPS facilities operating in the positive mode replace the wood chips once every two to three years. In the negative mode, the wood chip layer may need to be replaced one to two times per year. In addition, there was evidence of some migration of fines into the top layer of stone. There was no evidence of fines migrating into the aeration piping, as condensate in the blower drains was colored but clear. These issues of chip degradation and fines migration are further reason to increase the aeration blower capacity in zones A and B when operating in the negative mode. Essentially, operating blowers in the negative mode is inherently a less effective way of cooling the pile. A wetter, denser wood chip layer and fines migration exasperates the problem. Therefore more powerful aeration blowers are needed.
QUALITY OF FINISHED COMPOST
The Soils Control Lab in Watsonville, California analyzed one composite sample per week of the compost discharged from both Bay 1 (+) and Bay 15 (-). They performed the standard United States Composting Council Seal of Testing Assurance analyses on the compost samples. Most of the data are identical (plus or minus 15 percent) within the accuracy of this test (C:N – 17 in both; pH – 7.3 in the positive and 7.4 in the negative; Total N – 2.1% in positive and 2.2% in negative; Ammonia – 1850 mg/kg in positive and 1871 mg/kg in negative). However there are some differences with the following:
Moisture Content: The moisture content for Bay 15 averaged 39 percent and was consistently lower than Bay 1 which averaged 42 percent. This is attributed to the relatively higher temperatures seen in zones 15 C – E, compared to the same zones in Bay 1. The higher temperatures resulted in more moisture being driven off. Again this may be unique to Merrimack but it appears reasonable to conclude that operating in the negative mode will not result in wetter compost.
Nitrate: The nitrate levels are significantly higher in the positively aerated compost. This is due to one test result that came back very high and is believed to be an anomaly.
It also should be noted that in either the positive or negative aeration mode, the Respiration Rate – 5.8 for positively aerated bay and 6.9 percent for negative – and Biological Available Carbon tests – 3.7 for positive bay and 4.1 for negative bay – indicate that the discharged compost is well within the “stable” range.
Impact On Operational and Maintenance Costs
The final part of the study evaluated the impact to the operational and maintenance costs of the facility if operated in a positive versus negative aeration mode. Factors assessed include: Ammonia (measured by nitrogen balances); Moisture; Blower Discharge Piping Condensate; Electrical costs; and Wood chip replacement.
Ammonia Issues – Nitrogen Balances: A mass nitrogen balance was calculated for both modes of operation. Measuring points for the negative mode included infeed mix, exhaust fan discharge, condensate and bay drains and the finished compost. Measuring points for the positive mode were the same, except for the condensate and bay drains. Total nitrogen (lbs/day) was measured in the bay infeed mix as well as the bay discharge compost; it was assumed that the predominant source of nitrogen in the exhaust fan discharge and aeration blower gas streams was ammonia. Table 2 summarizes the findings.
The positive mode fan discharge gas stream nitrogen data was measured at the exhaust fans with 12 of the 75 (16 percent) aeration blowers running as is historically the amount that run at any given time at Merrimack (and most IPS facilities). The agitators were also running at this time to ensure maximum ammonia release.
The negative mode fan discharge gas stream nitrogen data was estimated two ways with the agitators running as well: 1) Based on the operating hours of the blowers required in the negative mode, ammonia measurements were taken at the exhaust fans with the following number of aeration blowers running to mimic an all negative facility plus whatever was freely releasing off of the compost: 75% of Zone A; 33% of Zone B; 13% of Zone C; and 8% of Zones D & E. This resulted in an estimated 412 lbs N/day released; 2) With all aeration blowers off, ammonia measurements were made at the exhaust fans to mimic the amount of ammonia freely releasing. We then measured the amount of ammonia in the discharge of the negatively operated blowers and calculated the total amount of ammonia that would be discharged with the same quantity of blowers running as in item 1 above. This resulted in 404 lbs N/day being released.
Moisture Balance: A moisture mass balance was conducted for the negative and positive modes of aeration (Table 3). Moisture levels were measured in the bay infeed compost as well as the bay discharge compost. The exhaust fan discharge moisture levels were estimated based on assumed indoor relative humidity of 65 percent in the negative mode and 75 percent in the positive mode. Operating under the negative mode, it is estimated that the Merrimack facility will generate approximately 1,500 gallons/day of leachate, which contains suspended solids and ammonia. Since the compost facility floor drains are returned to the wastewater treatment plant, the flow will not be an issue. However, at facilities not located at the WWTP, there could be a cost to collect and treat this liquid discharge.
Blower Discharge Piping Condensation: Assuming a 5° F gas stream temperature drop in the piping from the aeration blower discharges to the exhaust fan intakes in the negatively aerated bay, it is estimated that about 2,000 gallons per day of condensate will be generated. This amount depends upon the ambient conditions, pipe lengths and air velocity. It is estimated that approximately 600 gallons per day of the condensate could be reused within the Merrimack facility, via a compost irrigation system. This would raise the final moisture content from about 40 to 45 percent. However the balance will need to be returned to the WWTP. At new facilities, blower condensation could require a cost to collect and treat.
Facility Electrical Costs: Although the aeration blower runtime increases, exhaust fan run hours can be decreased when the facility operates in negative aeration mode due to the decreased number of air changes per hour required to maintain proper ammonia levels. The facility electrical consumption estimate projects the yearly electrical cost savings by operating in negative mode. By running fewer exhaust fans, or running all the fans at lower speeds such that the air changes over the bays reduces from 12 to six, it is estimated that about $50,000 per year can be saved.
Wood Chip Layer Replacement: As discussed above, visual inspections done in November 2003 as well as in January 2004 indicated the wood chip layer in the negatively aerated bay is compacting faster than in the positive bay due to accumulation of moisture and fines. This results in a wet, dense layer over the aeration stone. Replacement cycle time for the wood chip layer in negative bays will probably be more frequent than the typical two to three years for positive bays.
Impact On Exhausts Fans: It is assumed that the existing exhaust fans are capable of mechanically withstanding the saturated air stream conditions that occur with negative aeration. Therefore, increased maintenance would not occur.
PROS AND CONS – THE BIG PICTURE
One of the most significant advantages of operating in the negative aeration mode is a decrease in internal ammonia and fog levels within the facility, which would result in an improved working environment for the operations staff and would increase building and equipment life. From the perspective of reduced utility cost savings, however, ammonia levels would rise as cuts are made in air changes. For example, reduced ventilation rates, perhaps as high as 50 percent while still meeting the ammonia threshold exposure limit imits of 25 ppm in the aisles behind the strip curtains, would save money via reduced exhaust fan horsepower. At the other end of the spectrum, maximizing ammonia level reduction minimizes the power savings. While this wouldn’t result in electrical savings from the exhaust fans, it should result in a significant reduction in the corrosion rates and extend the life of the building and/or equipment.
On the disadvantage side, in negatively aerated bays, moisture and fines are drawn down toward the aeration system at a faster rate. As a result, it is anticipated that the wood chip barrier over the aeration system would need to be replaced more frequently in negatively aerated bays than the wood chips in positively aerated bays. When the wood chip barrier is wet and much harder, there is associated pressure drop, causing the blowers to operate longer and for air to be drawn down the sides of the bay walls versus through the compost pile. The worst case scenario would be if fines entered into the aeration system, potentially plugging it. To date, this has not occurred at Merrimack.
The aeration blowers supplied by USFilter for Zones A and B could be marginal for negative aeration. These non-aluminum blowers are also not best suited for high moisture applications (e.g. with the amount of condensate generated in the negative mode). Therefore Zones A and B blowers should be upgraded to handle higher moisture content and higher pressure drops when installed for negative aeration. It is assumed the exhaust fans that were purchased for the facility were designed to handle 100 percent humidity air streams and moisture.
The only apparent capital cost savings of operating in the negative mode would be less exhaust fan horsepower, i.e. smaller fans, and lower overall power consumption. However, there could be increased costs associated with the aeration blower discharge piping, condensate collection system, waste liquid disposal and the increase in the frequency of changing the wood chip layer. Additional increases to operational and maintenance costs could occur if the aeration stone and/or piping become plugged by fines migrating into it due to negative aeration operation.
After weighing the pros and cons, the Town of Merrimack decided to switch all of its bays to negative aeration. Reduced ventilation costs, reduced odors, protecting the integrity of the compost building, and no negative impact on compost quality were the driving factors in deciding to change aeration modes. The town allocated funds in its capital improvement program to make the conversion, which is scheduled to take place in 2006.
Richard Nicoletti, P.E. is a Product Manager with the USFilter IPS Composting Division in Sturbridge, Massachusetts. Jim Taylor is Chief Operator of the Wastewater Treatment/Compost Facility in the City of Merrimack, New Hampshire. USFilter would like to thank the Town and staff of the Merrimack WWTP/Compost Facility for their support throughout this test. Special thanks to Larry Spencer, Assistant Director of Public Works, Jim Taylor, Chief Operator and Lee Vogel, Facility Maintenance Supervisor. This paper was presented at the U.S. Composting Council Annual Meeting in January 2005 in San Antonio.