7 keys to successful composting
Composting “done right” can be easy or difficult. Easy if facility operators understand and manage according to a handful of scientific principles. Difficult if they do not.
The specific method in use or design of the physical plant may determine the level of difficulty and costs required to get it right, but both are secondary to what really matters – process management.
From the basic outdoor windrow to accelerated in-vessel processing, successful facilities are managed to meet goals linked to specific process influences. Without attention to these details common to every composting operation, even the most advanced, tricked-out facility can get composting wrong.
Here are 7 keys to successful composting, unlocking the full potential of any type of operation:
The choice of feedstocks, whether income generators or purchased amendment, can make or break a composting operation. For most private-sector operators, feedstocks are tied to revenues and, as such, influence profitability based on positive or negative dollars at the gate.
But for all facilities – commercial, government and non-profit — feedstocks also impact the financial health based on ease of processing and how that material will add to or detract from the market value of the finished product. Each ingredient is unique. Each adds different variables to the blend. Some feedstock influences can fluctuate from one load to the next.
It is the sum of these parts that must be considered when evaluating feedstocks and formulating processing blends. Every aspect of successful processing relies on a good blend, and that blend is based on types and volumes of individual feedstocks.
Carbon and Nitrogen ratios, a.k.a. C:N or C/N, reflect the proportions of each in the feedstock blend. The ratio, comparing the number of parts Carbon to 1 part Nitrogen, is sometimes expressed as a single digit, i.e, “20” as shorthand for 20:1 or “100,” meaning 100:1.
C:N ratios are critical considerations when blending. Carbon (non-mineral) and Nitrogen (mineral) provide microbes with energy and food. They also influence processing rate and odor generation potential.
High carbon materials include things like oak (200:1) and wheat straw (80:1), compared to nitrogenous materials like cattle manure (20:1) and alfalfa (13:1).
The ideal is 30:1 for blended feedstocks, but anything in the 25-35:1 range is considered good. 20-40:1 still works, but as numbers creep higher, the process slows. As numbers drop, the need for more aggressive odor management increases. (A strong ammonia smell can indicate a low C:N ratio.)
Feedstock testing and/or generic C:N ratio charts like this one, combined with manual calculations or computer modeling, can provide material types and volumes for determining the most efficient blends.
The microbes responsible for biodegradation – aerobes — require moisture, but not too much, since they can’t breathe under water. Excess moisture blocks air passageways, resulting in oxygen starvation. Too much water leads to anaerobic conditions, which cause odor and create toxic compounds.
Water is also used by microbes to move around the pile and is required for bio/chemical reactions. If the pile is too dry, biodegradation pretty much grinds to a halt for both aerobic and anaerobic processes.
Like a lot of factors influencing the composting process, moisture has its Goldilocks Zone. Shoot for a moisture content of 40-60 percent (by weight).
There are a number of testing methods for moisture (including one using the common microwave), plus lots of moisture meters on the market. Meters start at around $10 for the backyard variety and move on up into the hundreds of dollars. Compared to testing, which tends to be more precise, meters offer rapid feedback and the ability to quickly sample multiple pile locations to improve accuracy without having to wait for test results.
But the simple “Squeeze Test” can also be a good measuring tool. Simply grab a handful of the mix and squeeze. If water runs out, it’s way too wet. Does it crumble? Too dry. But if the handful holds its shape and dribbles little to no water, it’s probably okay.
Regrettably, none of the methods used to determine moisture content are spot-on, even laboratory testing. But the good news is that moisture level is a parameter with flexibility.
Just remember – the higher the volume of the composting mass, the higher the likelihood for error using crude estimations and monitoring techniques. Multiple sampling locations and sampling depths will help improve accuracy.
Uniformity – A compost blend needs to be a homogeneous mix of all ingredients with no multi-colored swirls, clumps, or pockets of individual feedstocks. Visually, the blend should look like a single material with both texture and particle size consistent throughout the admixture.
Failure to achieve a uniform blend prevents microbes from degrading target compounds at an equal rate throughout the composting mass, which can lead to zones of unprocessed, smelly, and even toxic material in finished product.
Pore space — The free movement of air and water throughout a composting mass is essential to a successful process. Microbes need oxygen, and that oxygen is delivered as air moves through the spaces between individual particles, a.k.a., pores. This air flow also removes excess heat. Microbes use those same pores (plus moisture) for migration.
Compaction is the enemy of pore space, and like the lack of uniformity, can retard or kill the process. Keep loaders and people off piles.
Compost piles can slump as the material degrades. Prior to curing, turning or screening will restore pore space. When using a loader instead of a windrow turner, don’t just dump the bucket. Use a shaking or sifting action to create, restore and/or preserve pore space, whether placing compost in a bay, building a new pile or turning.
Air flow – Composting is supposed to be an aerobic process. If air doesn’t flow, the process isn’t composting. It’s anaerobic biodegradation. Anaerobes create odors and toxins, so if a pile smells, lack of air flow is one of the chief suspects.
Keep the air flowing with an initial blend with good pore space and moisture level, augmented by turning or aeration — at the recommended volumes/velocities specific to the system in use.
However, too much air flowing through the pile can dry it out, so monitor moisture and add water, as needed.
Temperature – Heat buildup (not oxygen supply) is often the limiting factor in composting. When air delivery is sufficient to moderate temperature, microbes are being supplied with more than enough oxygen to maintain biochemical functions.
Therefore, keeping temperatures in the right zone for the specific phase of decomposition is composting’s equivalent of the Prime Directive.
There are precise time/temperature targets for pathogen kill. While originally written for biosolids management, an EPA 503-approved process is now required by many states to kill pathogens inherent in everything from food waste to yard waste.
While reducing pathogens may be a regulatory mandate, those time/temperature requirements represent just one process influence demanding temperature management. In fact, assuming the operator maintains control of temperatures from the beginning of the process to the end, pathogens cease to be a processing issue once the regulatory requirement has been met.
Far more important is the intentional management of temperature during both the mesophilic and thermophilic phases of composting.
Mesophilic microbes prefer moderate temperatures up to about 40 degrees C or 104 degrees F. They are the initial decomposers, and their feeding activity generates a lot of heat.
Whenever temperatures within the composting mass rise unchecked, the process grinds to a halt (all microbes die), crashes (there’s no decomposition) and must restart (microbial populations must rebuild). In nature, this happens repeatedly, which is one reason why Mother Nature’s system is so slow.
But a well-managed composting process stops this hot-cold cycling from happening. If the blend is right, temperatures will start to climb within a few hours of process initiation. As temperatures pass out of the mesophilic zone, microbial populations change to thermophiles – the heat-loving bacteria.
Thermophiles continue to work until temperatures reach about 65 degrees C or 149 degrees F – the ceiling of their comfort zone. The objective of composting is to manage air flow to maintain the level of heat required to meet the time/temperature requirements for pathogen kill (over 55 degrees C or 131 degrees F) without killing the thermophiles.
This thermophilic zone – 131 degrees F to 149 degrees F – is where rapid decomposition takes place. The longer the composting system holds temperatures within this zone, the faster the process will be. Systems that do this well are referred to as accelerated or high-rate processes.
Eventually, the thermophiles run out of food. As they die off, temperatures drop back into the mesophilic zone, where mesophiles slowly finish off remaining food supplies. This natural decline in temperature signals the beginning of the curing phase of composting.