Compost Exhaust to Provide Nutrients for Plants in Biofilter and Heat for Greenhouse

Methods as proposed:

Farm partner, Cherie Whitten, seeks to record and analyze data to minimize formation and leakage of CH4 and N2O into the environment and to understand how to manage exhaust through a biofilter to positively affect nutrient availability and temperature for the worms and plants.  

The biofilter consists of a 30" deep bed of material layered in the bottom of the greenhouse which includes the root zone and the plants growing therein.  The bottom layer of wood chips creates an aeration plenum, so the nutrients in the compost exhaust move evenly into the upper layers.  

We endeavor to match the compost aeration and resulting moisture, gaseous N and CO2 to the needs of plants and worms, and to maintain a healthy enclosed workspace.  Excessive accumulation of nutrients in growing matrices would be detrimental to crop health and yield, and the air quality within the greenhouse must be safe for occupants. 

High-yield greenhouse crops will be selected in our trial to optimize removal of nutrients from the biofilter.  A combination of supplemental lighting will be provided to ensure vigorous crop production and efficient nutrient uptake throughout the winter.

These trials will be conducted in plots of varying biofilter media. This media will include zeolite, aragonite, biochar, clay, rock phosphate, bone char and sawdust.  We endeavor to find the most advantageous mix of media and understand how to adjust it as needed.

Representative sampling and analyses of feedstocks will establish baseline nutrient mass balances to be composted. Aeration regimes and temperature data will be tracked in the composting batches. A control and monitoring package to measure temperature, air flow rate and oxygen content of the compost exhaust will be assembled by Agrilab Technologies, programmed, and tested for this project.  Exhaust gases, biofiltration growing media, plants, and population and nutrient accumulation of Eiseniia foetida will be sampled and analyzed to determine the efficacy and key performance factors within the integrated biofiltration system.

Nitrogen dynamics are highly variable in active compost, soils, exhaust ductwork, and biofiltration media.  As feedstock N decomposes, NH4 and N2O will be generated in varying concentrations based on feedstock composition and compost management. The air handling system, heat exchanger, and condensate removal ductwork will remove some of the NH4 and entrain it as NH3. When ammonia in the exhaust touches the soil, it converts to ammonium, a form of nitrate that bonds to the soil so plants can use it. When too much ammonia enters the soil, or the biofilter becomes saturated, ammonia odors can enter the greenhouse, which can become toxic for the plants. Therefore this approach to biofiltration in greenhouses needs to be done carefully.  The biofilter will capture most or all of the remaining NH4, and mineralize it into plant available N.  The high carbon content of the biofilter soil should buffer the heavy loading rates of NH3 by providing colloidal surface area and increasing the cation exchange capacity.  The worms will digest carbonaceous material and the other amendments and nutrients incorporated into the biofilter.  In this way, some of the N will be converted into worm biomass within the biofilter.  

Nitrification and denitrification processes within the biofilter media will be tracked via representative sampling and analyses in the varying treatments.  Greenhouse atmosphere will be monitored for CO2, temperature and N2O to optimize plant growth and control ventilation to ensure workplace health and safety.  Due to the worms sensitivity to high levels of nutrient build up sampling the movement and health of the worms will help us determine media saturation as well.

Bruce Fulford will review data regularly and will be contacted when there are outliers.  Biofilter media manipulation will produce the mineralization of plant available N.

We will record the following data:

1. Compost quantity and recipe
2. Heat provided by the compost
3. Ammonia in the compost exhaust  
4. Soil nitrogen at various points in the biofilter
5. Moisture at various points in the biofilter
6. Carbon dioxide at various points above the biofilter
7. Location and condition of the worms
8. Health of the plants
9. Yield of vegetables from various plots
10. BRIX levels of vegetables and plants

With this information we will:

• Analyze increases or decreases in production quantity and quality in correlation to nutrient and temperature factors.

• Analyze the cost savings in heating a greenhouse, as well as the reduction in carbon footprint.

• Estimate the amount of greenhouse gases that were sequestered during this project.

• Demonstrate improved productivity and reduction of costs.

• Produce reports that will enable other farmers to duplicate this process, or scale it up.

Progress Reports:

Report 1: Jan, 2023 - Since launching the project in August, 2022, ANCA purchased a biogas monitoring system from Agriblab in Vermont on behalf of partner farmer Cherie Whitten from Whitten Family Farm for $5,000. Cherie has just completed construction of the building that will be housing the composting process – a 30x40 timber frame structure (covered by a different grant). The structure is immediately adjacent to a greenhouse that will be the location of this experiment. Cherie has been collecting food waste from regional entities for three months and will be composting these materials with wood chip mix in early February. Gases from the aerated static pile (ASP) system will be bio filtered and funneled into the adjacent greenhouse this spring and summer. She will be starting plants in the greenhouse in February.  It will be these plants that will be monitored to determine the quality of the exhaust gas entering the greenhouse. Cherie’s next purchase will be for a lighting system to go in the greenhouse.

Report 2: March, 2024 - 

Pictures

Project update:

The Whitten Family Farm (WFF) compost barn has (5) covered shed bays for reverse aerated compost located on the north side of the barn. Each bay is 12’ wide and 16’ deep with a 12” channel in the center of the bay with 4” Perforated HDPE pipe which contains air that is drawn down through the piles and through a manifold, heat exchanger and blower.  The warm and moist air contains nutrients from the compost.  Some of the moisture is shed from the air as it cools, at specific points, including when leaving the channels under the bay and entering the insulated building as well as at the heat exchanger.  Condensate traps collect the moisture that is released at these points.  

The air still retains heat after the heat exchanger and is then exhausted into a biofilter that is inside the south side of the insulated building.  The south side of the building has polycarbonate on an angled wall and on the roof, creating passive solar heat gain in this greenhouse area.  

On February 10, 2023 we received a control and monitoring package assembled by Agrilab Technologies, programmed, and tested for this project.  It measures temperature, aeration regimes, air flow rate and oxygen content of the compost exhaust.  

We installed the exhaust system, biofilter and Agrilab Tech system during March.  This was completed on March 27th and we began composting on March 28th.  We used all sources available for compost inputs and were only running at 18% capacity.  We tracked the feedstocks we composted for further analysis.  At this point the biofilter consisted of 4” perforated HDPE pipe running in two continuous loops on either side of the center, covered in 24” of wood chips layered in the bottom of the greenhouse creating an aeration plenum.  This was designed so the heat, moisture and nutrients in the compost exhaust should move evenly throughout the biofilter and then dissipate evenly into the upper layers.  The heat from the compost that was exhausted into this biofilter was sufficient to quickly warm the building in early April, which was still very cold from the unheated winter.  By the end of April we were experiencing temperatures and moisture in the building which were too high and required us to ventilate, and divert the exhaust from the biofilter into an exterior bay of compost and woodchips.   This still captured the nutrients, but in another compost and woodchip pile as opposed to the biofilter we were looking to test from.  Because the compost pile will work as usual and be cycled from the bay, we took turns exhausting into several piles, and the tracking of nutrient build up is not possible.  Because our compost inputs in large part come from several local colleges, in May, capacity decreased to 10% and between lack of food waste inputs and needing to ventilate, we discontinued the experiment until September.  

September food waste collection produced food waste at 20% of capacity.  These trials are being conducted in plots of varying biofilter media, as the biofilter was now amended with an additional 3” of woodchips, sawdust, Aragonite, Zeolite, Soft Rock Phosphate, BioChar and Bone Char in five differing plots per the attached sketch.  

By early October the compost pile size and recipe was sufficient enough to begin emitting enough heat to keep the building warm with October temperatures.  We had varying results with pile temp throughout the winter as shown in the following chart:

We were able to heat well until December 3rd.  At this point we were losing some ground.  Beginning December 6th we had colder temperatures and were bringing trailers filled with 10-15 frozen totes of food waste into the building by opening an overhead door.   We now realized that without sufficient compostables volume the temperatures in the building would dip very close to freezing (see attached air temp report).  On December 10th we built compost batch 12, with a higher carbon percentage, lacking sufficient greens. This pile failed to heat properly, and we began installation of a basic 40 gallon 4500 watt electric water heater.  We ran this heater from Dec 26th through January 24th, using an estimated 1489 kWh of electricity to boost temps in this 2800 sq ft area during this time.  

We were very diligent in trying to increase food waste and compostable products to increase the volume in the compost bays.  Because the capacity was less than anticipated, the nutrients and moisture in the biofilter were not close to saturation levels in the biofilter.  

We believe that we have demonstrated that composting and greenhouse production can be mutually beneficial endeavors, diverting food waste from landfills and decreasing heating costs.  The minimal boost that the greenhouse needed at only 20% of capacity, shows that heating costs can be substantially reduced.  The resulting compost provides increased nutrient input to farms, which increases production and therefore will increase profitability and quality of life for farmers.  

Further analysis of nutrients was expected to be done as follows:

3. Capture carbon dioxide, ammonia, methane and nitrous oxide in a biofilter instead of allowing them to escape into the atmosphere.
4. Record and analyze nutrient data to minimize formation and leakage of CH4 and N2O into the environment.
5. Understand and document the effects of exhaust nutrients on worms and plants.
6. Experiment with different biofilter mediums, plants and lighting arrangements to determine best practice for producing profitable and nutritious crops.

At this point, the nutrient level is low enough that we are not seeing any escape or build up.  This shows that we are capturing these nutrients and they are not escaping into the atmosphere.  However, we can’t quantify their value at this lower-than-expected level. In order to analyze this, we need a longer time-frame of an additional winter and we need to increase our volume of compost.

We sought to not just to minimize formation and leakage of CH4 and N2O into the environment but also to understand how to manage exhaust through a biofilter to positively affect nutrient availability and temperature for the worms and plants.  This we haven’t been successful doing during this period of time.

This was what we hoped to track with a higher volume of composting:

Excessive accumulation of nutrients in growing matrices would be detrimental to crop health and yield, and the air quality within the greenhouse must be safe for occupants. Due to projected likely high levels of nutrients, High-yield greenhouse crops were to be used in our trial to optimize removal of nutrients from the biofilter. Exhaust gasses, biofiltration growing media, plants, and population and nutrient accumulation of Eiseniia foetida are to be sampled and analyzed to determine the efficacy and key performance factors within the integrated biofiltration system, because nitrogen dynamics are highly variable in active compost, soils, exhaust ductwork, and biofiltration media. 

When too much ammonia enters the soil, or the biofilter becomes saturated, ammonia odors can enter the greenhouse, which can become toxic for the plants. This has not occurred, but is unlikely at the low levels we have.  The biofilter should capture most or all of the remaining NH4, and mineralize it into plant available N. The high carbon content of the biofilter soil should buffer the heavy loading rates of NH3 (BUT- we don’t have heavy loading rates) by providing colloidal surface area and increasing the cation exchange capacity. The worms will digest carbonaceous material and the other amendments and nutrients incorporated into the biofilter. In this way, some of the N will be converted into worm biomass within the biofilter. Nitrification and denitrification processes within the biofilter media were planned to be tracked via representative sampling and analyses in the varying treatments, but with the low levels this testing will not show much deviation from the compost and amendments in the biofilter.  Greenhouse atmosphere will be monitored for CO2  and N2O (but levels are too low to do this) to optimize plant growth and control ventilation to ensure workplace health and safety. Due to the worms’ sensitivity to high levels of nutrient build up (which there isn’t) sampling the movement and health of the worms will help us determine media saturation as well. Bruce Fulford will review data regularly and will be contacted when there are outliers. Biofilter media manipulation will produce the mineralization of plant available N.

We will record the following data:

1. Compost quantity and recipe -DONE
2. Heat provided by the compost-DONE
3. Ammonia in the compost exhaust -not yet
4. Soil nitrogen at various points in the biofilter -we can test, but levels are not high- results won’t mean much yet
5. Moisture at various points in the biofilter - needing to water about 25%
6. Carbon dioxide at various points above the biofilter -not yet
7. Location and condition of the worms -noted, no specific trend
8. Health of the plants -noted, no important observations with lower nutrient level
9. Yield of vegetables from various plots -not yet
10. BRIX levels of vegetables and plants -not yet

With this information we will:

• • Analyze increases or decreases in production quantity and quality in correlation to nutrient and temperature factors. -not possible with low levels
  • Analyze the cost savings in heating a greenhouse, as well as the reduction in carbon footprint -yes, we can complete this for the past winter.
  • Estimate the amount of greenhouse gases that were sequestered during this project-we can estimate what was sequestered.  
  • Demonstrate improved productivity and reduction of costs. -we can prove reduced heating costs
  • Produce reports that will enable other farmers to duplicate this process, or scale it up.

Report 3 - March, 2025 Update: 

The Whitten Family Farm (WFF) compost barn has (5) covered shed bays for reverse aerated compost located on the north side of the barn. Each bay is 12’ wide and 16’ deep with a 12” channel in the center of the bay with 4” Perforated HDPE pipe which contains air that is drawn down through the piles and through a manifold, heat exchanger and blower. 

The warm and moist air contains nutrients from the compost.  Some of the moisture is shed from the air as it cools, at specific points, including when leaving the channels under the bay and entering the insulated building as well as at the heat exchanger.  Condensate traps collect the moisture that is released at these points. The air still retains heat after the heat exchanger and is then exhausted into a biofilter that is inside the south side of the insulated building.  The south side of the building has polycarbonate on an angled wall and on the roof, creating passive solar heat gain in this greenhouse area. 

On February 10, 2023 we received a control and monitoring package assembled by Agrilab Technologies, programmed, and tested for this project.  It measures temperature, aeration regimes, air flow rate and oxygen content of the compost exhaust. 

We installed the exhaust system, biofilter and Agrilab Tech system during March.  This was completed on March 27th and we began composting on March 28^th.  We used all sources available for compost inputs and were only running at 18% capacity.  We tracked the feedstocks we composted for further analysis.  At this point the biofilter consisted of 4” perforated HDPE pipe running in two continuous loops on either side of the center, covered in 24” of wood chips layered in the bottom of the greenhouse creating an aeration plenum.  This was designed so the heat, moisture and nutrients in the compost exhaust should move evenly throughout the biofilter and then dissipate evenly into the upper layers. 

The heat from the compost that was exhausted into this biofilter was sufficient to quickly warm the building in early April, which was still very cold from the unheated winter.  By the end of April we were experiencing temperatures and moisture in the building which were too high and required us to ventilate, and divert the exhaust from the biofilter into an exterior bay of compost and woodchips.   This still captured the nutrients, but in another compost and woodchip pile as opposed to the biofilter we were looking to test from.  Because the compost pile will work as usual and be cycled from the bay, we took turns exhausting into several piles, and the tracking of nutrient build up is not possible.  Because our compost inputs in large part come from several local colleges, in May, capacity decreased to 10% and between lack of food waste inputs and needing to ventilate, we discontinued the experiment until September. 

September food waste collection produced food waste at 20% of capacity.  These trials are being conducted in plots of varying biofilter media, as the biofilter was now amended with an additional 3” of woodchips, sawdust, Aragonite, Zeolite, Soft Rock Phosphate, BioChar and Bone Char in five differing plots per the attached sketch. 

By early October the compost pile size and recipe was sufficient enough to begin emitting enough heat to keep the building warm with October temperatures.  We had varying results with pile temp throughout the winter as shown in the following chart - see attachment below:

We were able to heat well until December 3^rd.  At this point we were losing some ground.  Beginning December 6^th we had colder temperatures and were bringing trailers filled with 10-15 frozen totes of food waste into the building by opening an overhead door.   We now realized that without sufficient compostables volume the temperatures in the building would dip very close to freezing (see attached air temp report).  On December 10^th we built compost batch 12, with a higher carbon percentage, lacking sufficient greens. This pile failed to heat properly, and we began installation of a basic 40 gallon 4500 watt electric water heater.  We ran this heater from Dec 26^th through January 24^th, using an estimated 1489 kWh of electricity to boost temps in this 2800 sq ft area during this time. 

We were very diligent in trying to increase food waste and compostable products to increase the volume in the compost bays.  Because the capacity was less than anticipated, the nutrients and moisture in the biofilter were not close to saturation levels in the biofilter. 

We extended the duration of the grant to collect more data, hoping to achieve composting volumes closer to capacity.  The following batch data from the 24-25 fall and winter season depict better heating results from 34% capacity.  The batch data and probe temps are below.  Simple issues with varying pile ingredients or outside temps, and even with aeration pipes being clogged with grease, can easily upset the heating regiment.  When we didn’t have enough ingredients to build another pile, we moved Batch 2 and added it with Batch 1 in Bay 2, in order to keep getting the little bit of heat we could out of the pile in January.

The electric hot water heater was used to boost temps for only 10 days this winter, between Jan 23^rd & Feb 7^th.  Even without this boost, the building would likely not have gotten below freezing, but the frozen food waste bins were very slow thawing. The minimal boost that the greenhouse needed at only 20% of capacity, shows that heating costs can be substantially reduced.  In year 2 at 34% of capacity it is clear that we could heat a greenhouse above freezing. We also realize that it would be difficult to operate at expected capacity even if sufficient food waste was available, due to cycling time and weather, mechanical and technical issues we did not foresee.  This winter’s 9713 cf volume is probably actually about 83% of what is really feasible for this design to handle. 

Analysis of nutrients:

Excessive accumulation of nutrients in growing matrices would be detrimental to crop health and yield, and the air quality within the greenhouse must be safe for occupants. Due to projected likely high levels of nutrients, High-yield greenhouse crops were used in our trial to optimize removal of nutrients from the biofilter. Exhaust gasses, biofiltration growing media, plants, and population and nutrient accumulation of Eiseniia foetida were to be sampled and analyzed to determine the efficacy and key performance factors within the integrated biofiltration system, because nitrogen dynamics are highly variable in active compost, soils, exhaust ductwork, and biofiltration media.  

We sought to not just minimize formation and leakage of CH4 and N2O into the environment but also to understand how to manage exhaust through a biofilter to positively affect nutrient availability and temperature for the worms and plants.  Due to lower-than-expected availability of compostable materials, the amount of exhaust and therefore nutrient level in said exhaust is lower than anticipated.  However, we did not change the bio-filter between seasons.  It operated at 20% in year 1 and 34% in year 2.  I would surmise that the biofilter is capable of capturing most all the nutrients at 54% capacity.  Based on the temperature data at 20% & 35% capacity, we know that we could easily heat the building at this level, to much higher temps than we recorded.  However, it is difficult to quantify the saturation of nutrients at this lower-than-expected level.  Although these numbers appear that they are within reasonable range, there is still concern regarding the amount of CO2 that could be omitted daily at higher levels.

When too much ammonia enters the soil, or the biofilter becomes saturated, ammonia odors can enter the greenhouse, which can become toxic for the plants. This has not occurred, but is unlikely at the low levels we have.  The biofilter should capture most or all of the remaining NH4, and mineralize it into plant available N. The high carbon content of the biofilter soil should buffer the heavy loading rates of NH3 (BUT- we don’t have heavy loading rates) by providing colloidal surface area and increasing the cation exchange capacity. The worms will digest carbonaceous material and the other amendments and nutrients incorporated into the biofilter. In this way, some of the N will be converted into worm biomass within the biofilter. Nitrification and denitrification processes within the biofilter media were planned to be tracked via representative sampling and analyses in the varying treatments, but with the low levels this testing will not show much deviation from the compost and amendments in the biofilter. 

Through monitoring for ammonia we found that there are no detectable levels of ammonia inside the building ever.  If the air is pulled from working piles and pushed back through an empty bay ammonia is detected in that outside bay at 160ppm, similar to how it would steam from a recently turned compost pile.  This shows us that we are capturing ammonia, and likely the methane or nitrous oxide (ammonia cycle components) in the biofilter instead of allowing them to escape into the atmosphere.

Worms had been introduced into the bio-filter, so they could help us determine if areas of the biofilter were too hot, either with heat or nutrients.  Worm distribution within the biofilter is substantially uniform.  This is probably because we failed to reach saturation level, and therefore didn’t have concern with any toxicity from nutrient buildup. 

We grew three crops in the greenhouse biofilter during this time.  The initial crop of lettuce performed very well until it got too hot in April and we began diverting some of the exhaust to outside bays. The second crop of tomatoes and cucumbers performed well across all biofilter mediums.  This crop grew during the 1^st spring, so at 20% there wasn’t enough nutrient excess to try to determine how the biofilter amendments functioned.  The cucumbers and tomatoes were both planted across all biofilter zones.  Both crops did very well, but the cucumbers performed exceptionally well.  They were by far the best crop of cucumbers we ever grew, and they wow’ d many visitors.

Lighting was added for this past winter’s growing season.  Although the lighting is expensive to run, it is necessary in our area during the winter due to short daylight hours.  We ran lights on a timer to mitigate day length issues, creating a minimum of 12 hours of daylight.  Our lettuce was planted on February 1^st after the lights were installed.  Brix reading from the lettuce are relatively consistent across the different biofilter mediums.  We will continue testing through maturity as nutrient differences will become more apparent if there are any.

Greenhouse atmosphere was monitored for CO2  to optimize plant growth and control ventilation to ensure workplace health and safety.  The highest CO2 level reading we had was 4,930ppm, just above the biofilter area.  This is getting close to the 5000ppm maximum for 8-hour exposure, where there would begin to be safety concerns for exposure.  CO2 levels over 1000ppm enable increased photosynthesis efficiency, so plants can produce more glucose and grow faster.  We have achieved this.  Although our levels are in a safe range, this is an important factor to continue to observe.

About 75% of plant and soil moisture needs were provided by the moisture contained in the exhaust, blowing into the bio-filter and root zone.  This decreased water frequency.  The exhaust moisture is how the nutrients are carried from the compost.

We still plan to:

• Estimate the amount of greenhouse gases that were sequestered during this project
• Estimate reduction of heating costs

 We believe that we have demonstrated that composting and greenhouse production can be mutually beneficial endeavors, diverting food waste from landfills and decreasing heating costs.  The resulting compost provides increased nutrient input to farms, which increases production and therefore will increase profitability and quality of life for farmers.  Nutrient toxicity within the biofilter is unlikely with the building’s functional capacity and the with the current bio-filter design, however I suggest that further CO2 analysis would be profitable.