By Tim O’Neill & Geoff Hill
High nitrogen feedstocks include biosolids, digestate, and food waste. These feedstocks are often in abundant supply but are challenging to process with rudimentary processes like static piles or windrows due to their tendency to off-gas ammonia or remain in their anoxic state dominated by smelly anaerobic microbiology. This whitepaper outlines the steps necessary to get these challenging feedstocks into a BMP mix that can be maintained in aerobic conditions to minimize odor release and maximize decomposition rate.
There are a few simple Best Management Practices (BMPs) that can be regularly performed onsite at any composting facility and used as Key Performance Indicators(KPIs). With regular collection and an understanding of the target and range within each BMP, site operators can use the data to make immediate changes, predict emissions, and understand end product quality. The most effective times to collect data and adhere to BMPs (and use these as KPIs) is on the mix going into primary composting and during the primary composting process. This short paper provides our recommendations for BMP data collection at these two stages and a short explanation of why these parameters are critical.
A BMP mix is critical to the entire compost process. There are three key measurable parameters in a BMP mix: moisture, density, and C/N ratio. Since ground woody amendments are generally
with high nitrogen residual feedstocks (such as biosolids, manures, and digestate) a fourth qualitative parameter is “mix uniformity.” These parameters set the stage for conditions on the biofilm where the decomposition takes place.
If all are within BMP ranges, the subsequent composting process will have the greatest chance of proceeding under optimal process conditions. If one or more parameters falls outside the BMP range, the process will become inhibited due to anoxic/anaerobic conditions (poor gas exchange) or excess ammonia (low C/N) resulting in the generation of strong ammonia odor and dramatically slower bio-stabilization.
The table below outlines the three mix parameters we think should be measured and logged regularly to provide assurance that a BMP mix is being made each and every batch of compost.
| Key Process Variable | BMP Target | Method | Frequency |
|---|---|---|---|
| Bulk Density | 750-950 lb/cy | Bucket and scale | Composite sample each batch |
| Moisture Content | Moist not drippy | Squeeze test | 3-4 grab samples each batch |
| Moisture Content | 52-63% | Lab test to verify squeeze test | Quarterly |
| C/N Ratio | >25 | Lab test | Quarterly |
While the production of a BMP compliant mix requires the operator’s attention, the sustaining BMP oxygen and temperature levels during active composting is largely dependent on the design of the aeration and control system. Consistently maintaining BMP conditions requires feedback-controlled aeration that automatically adjusts to meet the highly variable cooling demands of the biological process and achieve user defined process goals (such as PFRP). Further, the mechanical design of the aeration system must provide adequate peak flow rates (for cooling) and flow uniformly to keep the majority of composting volume within the BMP temperature and oxygen ranges. The peak design flow should be matched to the degradability of the feedstock mix; airflow rates between 5 – 10 cfm/cy of mix can be periodically required early in the composting cycle. Windrow composting airflow rates are generally less than 0.1 cfm/cy. Fabric covered systems usually restrict airflow rates to less than 1 cfm/cy.
Ammonia inhibition is commonplace in biosolids composing despite being quite easy to control and prevent. Both pH and temperature act as forces driving ammonium (NH4+) into free ammonia gas (NH3).
Uncontrolled, the process can be severely impacted by free ammonia gas, dissolved into the liquid of the biofilm of each compost particle, and the process can be inhibited by its own toxicity (ammonia gas is toxic). However, with good temperature control and woody bulking agent to offset the alkaline pH of biosolids, the majority of the ammonium can be kept in ionized form (NH4+) where it is available to bacteria for cellular growth and oxidation of carbon compounds.
The figure below displays the relationships between ammonium and ammonia as affected by pH and temperature. As temperature increases, the fraction of unionized ammonia gas (NH3) increases and negatively impacts the composting process. A process at 50°C and pH 8.3 can have <50% of its nitrogen in ammonia gas phase, whereas that same process at 80°C would have >85% of its nitrogen in ammonia gas. With ammonia inhibition, we would expect high odor generation and much slower rates of compost stabilization.
High temperatures have been shown to inhibit the general (non-feedstock specific) composting in two ways. First, is the fact that bio-oxidative process has been shown to roughly increase by a factor of 2x up to around 65°C, and then drop off sharply at higher temperatures due to temperature inhibition.
Second, is due to the inverse relationship between temperature and oxygen solubility in the liquid film layer. The explanation is as follows: each composting particle is covered in a thin water layer, or biofilm, as shown in Figure 1. The combination of oxygen concentration in the air space and the temperature in the biofilm determine the amount of oxygen that the biofilm can carry (this relationship is shown in Figure 3 published by the UK Environment Agency). The aerobic bacteria that do the composting reside in the biofilm. High dissolved oxygen levels (>3.5ppm) have been shown to provide rapid stabilization (conversion of bio-available solids to CO2) and low odor production.
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