High-rate composting

High-rate industrial composting – what is it?

High-rate industrial composting — the scientific management and enhancement of the natural biodegradation process — is a method of accelerating the decomposition of organic wastes.

Whether processing food waste, sludge, vegetation or any other organic-based material, the use of an accelerated composting system can convert biodegradable waste to a high-value product much faster than more traditional, low-tech composting technologies.

Why is acceleration significant?

Rapid breakdown of target compounds results in a smaller facility footprint able to recycle about 10 times the volume of a windrow operation in the same space.  But also significant is the fact that high-rate composting can produce 10 times the volume of a high-grade, salable product.   Can any responsible facility owner — public or private sector — afford to disregard revenue potential multiplied by a factor of 10?

For a commercial operation, the ability to generate 10 times the revenue can mean the difference between profitability and business failure.  For a public-sector project, designing and managing a composting facility to generate meaningful revenue can offset operating costs and represent a real savings to those who foot the bill — the taxpayers.

For urban settings, the facility design and high level of control inherent to high-rate systems also protects sensitive receptors from environmental breaches that lead to nuisance complaints.  Preemptive design, process selection, and siting can all but eliminate things like leachate, odors, and flies when combined with “good neighbor” management practices.

What makes high-rate industrial composting scientific?

Unlike more primitive composting methods, scientific processes establish and maintain an environment conducive to the proliferation of the specific microbial populations responsible for natural biodegradation.

Scientific composting does not put engineering first.  Instead, engineering plays second fiddle to biology, specifically, controlling the biochemical processes that take place during biodegradation.

Here’s how science plays a significant role:

Aerobic microbes — those responsible for biodegradation — prefer temperature, oxygen, and moisture levels within a certain range.   When these conditions are met in the presence of an ample food supply, microbes eat and reproduce at a rapid rate.

Microbes release enzymes as they feed on the sugars and proteins that make up the organic waste.  The enzymatic action breaks down compounds at the molecular level.  This feeding activity results in the release of water, heat and CO2.  The steam rising from a compost pile is a sign of this activity.

With windrow composting, every ingredient, batch or windrow is treated the same.  Wastes are blended and windrowed.  At minimum, the piles are turned as often as government regulations require.  Periodic temperature readings (usually, with a manual probe) may be used to monitor internal temperatures, with a windrow subjected to additional turns if it gets too hot or too cold for the aerobes.

But this hot/cold cycling, even for short periods, lengthens the time required to break down the material.  In addition, if history is any indication, most windrow processes are not sufficiently robust to breakdown more complex compounds like biodegradable resins/plastics.

Instead of just turning a pile and letting it sit until a regulation or periodic manual temperature reading says it’s time to turn it, scientific composting recognizes the individuality of every blended batch and creates the perfect environment, regardless of ingredients or ambient temperature.

The science in feedstock selection and blending

Process control starts with the choice of feedstocks for every processing batch.  The blending goal is to achieve optimum moisture levels and carbon-to-nitrogen (C:N) ratios, as well as ensure adequate porosity and homogeneity for each batch.

Moisture — provided there’s not too much of it — is used by microbes for transportation, making consistent moisture levels throughout the batch a critical blending and processing goal.  If there are dry pockets within the blend, microbes will not be able to move in to do their work.  If too wet, the wrong kind of microbes — anaerobes, the ones responsible for odor generation — will take over.

Pore space serves as the conduit for water migration and air flow.  Air flow is necessary to deliver oxygen to the microbes and prevent heat build-up during processing.

Homogeneity means the batch is well-blended with no marbling of wastes or other indicators of an improper mix.  Lumps and clumps in the blend can leave pockets of non-degraded waste within a “finished” processing batch.  A homogeneous mix equalizes opportunities for microbial contact with target compounds throughout the blend and speeds up the degradation process.

Sometimes, getting the blend just right may require the sourcing or purchase of specific amendment materials or even turning away waste products that either offer no benefit or prove detrimental to the process and/or quality of the finished compost.

In some cases, these undesirable wastes can be pretreated at the generation source to make them more amenable to composting.  Dewatering might be considered an example of a composting pretreatment for residuals and by-products that are mostly water.

The science in processing

Typically, high-rate industrial composting technologies will rely on some sort of process containment to achieve the level of control required for a quick and efficient composting.

The objective is to control as many process influences as possible — especially weather — which can wreak havoc on an outdoor composting facility.  While a well-constructed compost pile will continue to generate heat during cold months, it will do much better in containment.   The same may be said for a pile exposed to rainfall.  Excess moisture can kill an aerobic process, requiring extra turning or even re-blending to get the composting restarted.   Time issues aside, extra materials handling means higher equipment and labor costs per ton processed.

There are many containment options available, ranging from covering outdoor piles to banked, indoor composting to fully encapsulated processing bays.  Full containment, of course, offers the most control over the process.

Advantages (compared to open air processing) include:

  • Elimination of weather as a biodegradation influence.  While tarps and covers offer some protection from rain, they do little to negate the impacts of ambient temperature changes or exposure of the blended admixture during placement, turning or removal.  Temperature fluctuations will impact degradation rate.  Tarps and other covers also have a limited life and must, periodically, be replaced.
  • All-indoor operations allow for air extraction/biofiltration from both the processing bays and work zones.  This not only improves visibility and worker health, but also allows the operation to fit more comfortably into the host community.  Covers and tarps are little help in mitigating odors generated during off-loading and blending in open-air environments.
  • Automated monitoring and process control keep temperature levels constantly within the “zone” for aerobic microbes, regardless of blend constituents.  The more control exerted over the process, the faster the biodegradation rate.  When it comes to high-rate industrial composting, control equals speed.  This is the reason a high-rate composting system can have a throughput rate 10 times that of a windrow operation within the same footprint.