Compost & Mulch Print E-mail
Friday, 09 July 2010 00:00

Grinding Compost and Mulch

Quick Links: 2700C2710C4700B4710B5710C
Size reduction machines are manufactured to reduce large material pieces to smaller sizes and are usually necessary as the first step in converting raw feed stocks into usable compost or mulch. Compost and mulch products are increasingly valued and becoming widely recognized for their contributions to soil enrichment, moisture retention, erosion control, organic farming and other values in agriculture, road and building construction, landscaping, and environment enhancement. Depending on the desired end product, the processes following initial grinding or size reduction can vary widely, heavily influencing which process is most cost efficient.
To select the most cost efficient size reduction method, it’s helpful to understand the entire process, from the raw feed stock to the point of sale of the end product.


•Green Waste
•Logging Residue
•Woodlot Thinning
•Agricultural Products & By-products



1. Grinding, Chipping, Shredding
2. Separating, Screening, Sorting
3. Mechanical or Chemical Treatments
4. Handling: Loaders, Conveyors, Baggers, etc.
5. Stockpiling, Packaging, Bulk or Bagged
6. Transporting to Point of Sale


• Bulk
• Bagged
• Specialty
• Colored
• Bark Mulch
• Hardwood Mulch
• Erosion Control
• Stormwater Mgmt
Figure 1. Feed Stocks Converted to Mulch


•Green Waste
•Logging Residue
•Woodlot Thinning
•Agricultural Products & By-products
•Food Waste


1. Grinding, Chipping, Shredding
2. Separating, Screening, Sorting
3. Thermal, Biological, Mechanical, Chemical Treatments
4. Handling: Loaders, Turners, Conveyors, Baggers, etc.
5. Stockpiling, Packaging, Bulk or Bagged
6. Transporting to Point of Sale


• Bulk
• Bagged
• Boutique
• Crop Specific Blend
• Soil Amendments
• Filter Berms
Figure 2. Feed Stocks Converted to Compost
The charts in Figure 1 and Figure 2 briefly outline some common steps in converting various feed stocks to valuable compost or mulch, with the first step in all cases being a size reduction process.
All end products undergo some post-grinding material handling processes that may contribute value. Turning compost, for example, aerates the pile, contributes to product quality, and shortens the composting cycle. Stockpiling, bagging, and transporting mulch or compost are often necessary elements in competing for end use customers, but can also contribute to brand loyalties.

Size Specifications

Many size reduction or grinding opportunities have both “minimum fines” and maximum particle size specifications. The size reduction process is thus limited by economic considerations on both ends of the particle size spectrum.
In the final product too many particles that are too large or too small can reduce the value of the material. A machine that makes “right sized” particles out of randomly sized feed materials contributes the most value.


What Is Mulch and What Are Its Common Purposes?
Mulches are often made of woody material such as wood chips and ground-up wood particles but can include straw, pine needles, shredded paper, and other finely ground organic materials that have not yet decomposed*. To serve various purposes, mulch needs to be placed as a layer on the surface of the soil. It must be absorbent, yet light in weight, somewhat porous, allowing both water and air to penetrate. For many uses, the layer of mulch must be opaque to sunlight and insulative, reducing extremes of temperature in the soil below it while preventing sunlight from reaching the soil surface.
• Mulches retain moisture, preventing loss of water from the soil by evaporation.
• Mulches suppress growth of weeds, when the mulch material itself is weed-free and applied deeply enough to shade the ground and prevent weed germination or to smother existing weeds. However, some mulches are purposely mixed with seed to promote growth of selected plants while suppressing growth of unwanted weeds.
• Mulches insulate, keeping the soil cooler in the summer and warmer in the winter, maintaining a more even soil temperature.
• Mulches add to the aesthetic appeal or appearance of a landscape by providing a cover of uniform color and texture.
• Mulches prevent soil splashing, to reduce erosion and keep soil-borne diseases from splashing up onto plants.
• Mulches help prevent soil compaction and crusting of soil surface, improving absorption and movement of water into the soil.
• Organic mulches can improve the soil structure. As the mulch decays, the material can become topsoil. Decaying mulch also adds nutrients to the soil.
• Mulches help prevent damage to the trunks of trees and shrubs by lawn equipment.
• Mulched plants tend to produce additional roots in the surrounding mulch.

How Is Mulch Produced?

Woody or other plant feed stocks are fed into size reduction machines, where grinding contributes to the mulch’s value by making the particles small and relatively uniform in size. Mulches with highest value impose both minimum and maximum particle size specifications. Too many fines reduce the value.
Minimum particle size specifications are driven by the economics of volume-to-weight ratios in handling and marketing mulches. Controlling mulch density is a key to increased profitability. A mulch product with higher volume-to-weight ratio means having more bags to sell from the same weight feed stock.
Maximum particle size specifications are driven by down-stream material handling process requirements and by considerations of moisture control, packaging demands, and rate of organic deteriorization. Good mulch is light weight, contains a minimum of fines, is uniform in size, and has particle characteristics that help it do its job.
Most mulches are valued for their ability to absorb and hold water. Those mulches are made up of individual particles that attract and hold water. Particles that are “fuzzy” at the surface do this job well. Water tends to run off or evaporate from smooth-surfaced particles too quickly. Grinders that produce particles of the right size that are fuzzy or rough surfaced are best for this type mulch.
For a few mulches, however, shedding water quickly is helpful: wood chip mulch made especially for walking paths is an example. This type of mulch is best made with a cutting action, like a chipper, instead of a grinder, which tends to break the material into chunks that are less smooth at the surface.
Specialty mulches include those made for use as playground cover, which must be clean, springy and soft, and splinter-free.
Producing mulch with the desired characteristics is important to users and to producers who want higher yields, higher prices, lower transport and bagging costs, and reduced colorant costs.
Mulch, designed to form a layer at the surface of the soil, differs from compost, which is designed as amendment to soil, often to be mixed below the surface level with existing soil.


What is compost? A quick search of internet resources yields many definitions of compost. Here are a few helpful ones.
Compost is, “A solid mature product resulting from composting, which is a managed process of bio-oxidation of a solid heterogeneous organic substrate including a thermophilic phase.” according to, the website of the Composting Council of Canada.
Composting is Nature's way of recycling...”
Decayed organic matter that can be used as a fertilizer or soil additive.” (EPA)
Composting is a process involving multiple steps. The first step is size reduction. The best compost is relatively sterile of seeds and has been converted through a series of thermal, biological, and/or mechanical processes to humus or near-humus condition. Compost is most valuable following biological decomposition processes that generate heat and sometimes odors. Successive growths and replacements of populations of micro-organisms eventually stabilize compost to a mature state. The process can take different amounts of time, depending on several controllable factors. Commonly accepted criteria for stable, mature compost include that the original feed stocks can no longer be recognized, the material has a pleasing, soil-like odor, and forms a loose, highly nutrient-rich base in which to sprout seeds and grow healthy plants.
Of course, compost can be laid at the surface and perform some of the functions of mulch, and mulch can be turned under and allowed to decompose, thus becoming compost-like, but despite these real possibilities, their highest values are realized when used as intended.

What Are the Benefits of Using Compost?

 Most composters stress the soil amendment value for plant heath, moisture retention, and for reduced fertilizer requirements.

How Is Compost Produced?

The composting process begins with size reduction of feed stock and later combines mixing, aeration, turning, temperature and moisture control to enhance growth of microbes that biologically break down the organic materials, to reach a mature or “humified” state that is beneficial to plant growth. The initial size reduction step, plus the subsequent biological micro-environment of the pile, are important to the completion time and quality of the final product.
Knowledge of the function of the end product is important in helping decide the best steps in making compost.
In grinding for compost, minimum size specifications are similar to those for mulches, but composting processes can accept a less stringent maximum particle size. Compost is likely to be screened before final packaging or delivery so larger particles can be separated in that process. Also, some over-sized pieces (often called “overs”) can help provide particle structure that aids in aeration, a critical function that speeds decomposition and reduces odors.
Both mulch and compost are improved by particles that are well-fragmented, meeting their size specifications, with few fines and having lots of surface area at each particle to hold moisture. Note the “fuzzy” surfaces and small, uniform particles in Figure 3.
Fragments wood particles
Figure 3. Well-fragmented Particles from a Peterson 4710B Horizontal Grinder

Well-fragmented and Externally Fibrillated Particles

Well-fragmented feed stock particles with a minimum of fines and lots of surface area promote two important aspects of the process: moisture and aeration. The selected size reduction process largely determines how well fragmented the particles can be and thus influence the ability of the pile or mass to aerate and to absorb or retain moisture. The best mix of particles includes some slender pieces that form small bridges and voids, permitting air to pass more freely throughout the pile.
External fibrillation occurs when some of the surface wood fibers (wood filament tubes) in the wood fragments are separated parallel to the grain**. During size reduction the wood is fragmented or caused to structurally fail. Some failures occur against the grain, while others occur at other angles, including parallel to the grain (aligned with the wood fibers). Failures perpendicular to or across the grain are necessary to result in pieces that are short enough to meet the sizing specifications. Failures parallel to the grain are desirable at the surfaces of the particles. This happens to some degree with all size reduction processes, but is enhanced significantly with certain processes. Although more research is needed, to develop technologies of the science of grinding for compost, external fibrillation, sometimes called “fuzzy surfaces,” are important features of well-fragmented particles. These failure modes parallel to the length of the wood fibers result in the properties that compost and mulch producers prefer because they support aeration and moisture control.

Moisture and Aeration

Moisture content and aeration can both be controlled mechanically, by adding or withholding water, introducing added air, and by turning the pile at intervals. Some operators install aeration piping to instill the lower reaches of the mass with air. Besides the evaporative and humidity control capabilities of aeration, a necessary threshold of oxygen must be available to the microbes acting on the feed stock in order to control odors.
Compost producers that regularly turn the piles have calculated that the rate of turning directly correlates to the speed of composting action. The more often the pile is turned, the sooner it becomes a mature, higher value product.

Heat-Loving Microbes

Under optimal conditions composting takes place in three phases involving different communities of microbes or bacteria. The first and third phases are moderate-temperature phases in which bacteria thrive that favor temperatures up to 104 °F (40 °C). During the first and second phases the temperature rises. When the pile temperature reaches 104 °F (40 °C), the mesophilic (moderate-temperature loving) microbes*** become less competitive and are succeeded by thermophilic (high-temperature-loving) microbes. Above 131 °F (55 °C) many microorganisms that are human or plant pathogens are destroyed. Above 140 °F (60 °C) the diversity of bacteria decreases, so it is in the interest of the composting business to maintain temperatures in optimal ranges for all three stages and to carefully monitor time-to-temperature data to better control the overall raw-material-to-finished-goods cycle****. Eventually, the pile cools again in the third phase, where mesophilic microbes dominate again until pile stabilization or compost maturity.
During the three phases microbes are important contributors, but not the only active organisms. Fungi, protozoa, and rotifers are also active in breaking down the organic matter in the pile.

C:N Ratio

The carbon to nitrogen (C:N) ratio is critical to an effective microbiological environment for breaking down the feed stock particles and to the value of the mature compost. The C:N ratio can be controlled by the mix of feed stocks. Wood chips contain lots of carbon, while green waste, manure, or chicken litter contain higher amounts of nitrogen. Roughly, a 3:1 ratio is typical with three units of high carbon materials to one unit of high nitrogen materials. Some feed stocks vary widely in their carbon and nitrogen contents, but the requirements of the microbes remain relatively constant.

Odor Control

The C:N ratio, amount of moisture, and aeration are key factors in controlling objectionable odors. Most of the unpleasant odors associated with composting come from anaerobic microbes, which thrive in low-oxygen environments. Many of these odors are from a class of chemicals called mercaptans. A common, household example of a mercaptan is the stinging, pungent odor of a peeled onion. Other mercaptans, along with organic sulfides, amines, volatile fatty acids, and terpenes produce most typical objectionable odors of composting. Turning the piles frequently introduces the oxygen needed to restrict anaerobic microbes, thus reducing the odors*****. Mature, stable compost does not produce objectionable odors.

Economic Benefits of Good Processes in Compost Production

Using the best processes in compost production can
• Reduce the time to produce a finished product
• Keep odors and methane generation at a minimum
• Result in a more uniform product
• Maximize profits by controlling costs
• Enhance the value of the finished compost.

The First Step: Choosing a High-Speed Grinder

When candidate organic materials for size reduction are heavily contaminated with metals, rocks, or dirt, then slow-speed single or double-shaft shredders are sometimes selected because these machines are good at a primary reduction. However, production rates may be low, and a high-speed grinder may still be required for a secondary reduction stage to meet size specifications and production goals.
To reduce organic materials without contaminants, such as virgin wood or agricultural crops, chippers can be an acceptable choice, if the feed stock is clean enough that the cutting knives don’t need to be sharpened or replaced too often.

Uncrushables in the Mix

While clean materials without contaminants can be chipped or cut, mixed woody debris that may contain contaminants such as stones or metal are often most economically reduced using high-speed horizontal or tub grinders. Most feed stocks have a very high probability of containing some anomalous materials, such as a splitting wedge that accidently fell into the pile, or a tool left in the back of a pickup and then swept into the pile of green waste. Such things unavoidably happen frequently.
High production grinders can often achieve the required size reduction in a single pass, but must be capable of encountering uncrushables with a minimum of downtime and damage. The range of feed materials, potential damage from impacting uncrushable objects in the feed, wear rates, safety, and the size of available operating sites all help determine the choices among high-speed grinders. When all these factors are weighed, high-speed horizontal grinders are the safest, most effective, economical size reduction machines.
Among the variety of horizontal grinders available, each having different capabilities, two major types of horizontal grinders can be distinguished. Some brands have down-turning rotors while Peterson and others utilize up-turning rotors. The direction the rotor turns and other key design differences determine operating costs and the ranges of materials where these size reduction machines are most economical.

Do I Want A Down-Turning Rotor?

An important factor in both types is feed rate compared to rotor-turning rate or bit speed. Typical feed rates of 30-60 feet per minute at a rotor speed of 840 rpm result in feed rates per bit revolution of about 1/2 inch (12 mm).
A typical revolution of the bit advances 1/2 inch (12 mm) into the feeding material, where the next revolution of the bit strikes the material again. With a down-turning rotor, the tendency is to repeat that strike-to-feed rate, and in theory, reduce the entire load of feed materials to a size determined by the strike-to-feed rate. Each strike occurs the same way at the same distance, absorbing the same amount of energy, and resulting in uniformly sized finished product... in theory.
If a horizontal grinder with down-turning rotor uses a sharp cutting bit in clean material, the result can be very productive. Unfortunately, keeping the bits sharp is a high-cost activity if any contaminated material is in the feedstock. Cemented carbide bits (the usual way to attach a sharp edge or point to a bit) are very brittle and readily fracture in high impact environments.
Down-turning rotors with blunt carbide overlay bits cut or strike the feed material between the bits and the anvil. Material is smashed and fragmented into smaller pieces against an anvil mounted at floor level. If the material is friable, many fines – usually too many – result. With a down-turning rotor using blunt tips, it is difficult to avoid making too many fines. In addition, production output is limited by the high energy required to fragment the material into such small pieces.

Down-turning Rotor illustration 
Figure 4. Down-turning Rotor
A down-turning rotor machine is therefore not best suited for reducing mixed materials that contain any stones or hard metals because blunt tips make too many fines and sharp tips dull or break too quickly.

Up-Turning Rotors

Up-turning rotors reduce materials using a different, multi-stage grinding process. An upturning rotor tends to draw material into the grinding chamber. The up-turning bits typically first break off longer fragments of the feed stock because they are not cutting against an anvil, although the material is pressed down toward the feed conveyor and held against the rotor by the feed roll/compression roll. So the material is often first broken by the upward force of the rotor against the downward force of the roll, occurring well before the material is trapped against the anvil. See Figure 5.
Much of the material turns or rotates in space in this grinding area so that during the secondary reduction it is fractured parallel to the wood grain, where the wood is weakest. Shorter pieces can spin and most pieces are struck several times by several bits during the passage from feed conveyor floor level to the anvil, placed above the rotor. Each particle’s reduction process is well on its way before reaching the anvil.
Secondary reduction occurs above the rotor and against the anvil. During this phase a fully trapped piece of material is battered and fragmented between bits and anvil similarly to a down-turning rotor, but with a higher probability that a partly trapped piece can still rotate in space and fragment along the grain, a more efficient use of rotor energy.
Up-turning rotor illustration
Figure 5. Multi-step Size Reduction Process with Up-Turning Rotor
The final reduction sizing occurs as the material is forced through the grates at Stage 3, but because the earlier two stages are more efficient in an up-turning rotor, adequate sizing can be achieved while using larger grate openings.
The combination results in significant overall throughput rate increases and higher yield. Where a down-turning rotor requires a 4-inch opening grate to maximize its sizing function, an up-turning rotor can use a 6-inch grate and still achieve better sizing.
Grinding with an up-turning rotor causes various failure modes in the wood as it is reduced. However, a significant part of the reduction comes from the wood failing in tension perpendicular to the grain or from rolling shear, both of which tend to separate the wood fiber.
Both down-turning and up-turning rotors incur wear at the bits, anvils, and grates, but some designs are more effective, efficient, and economical.
The up-turning rotor with multi-step fragmentation grinding typically is less energy-consuming yet more productive, with greater quantities of through-put while producing fewer fines than a down-turning rotor, and with wear rates that prove cost effective compared to the material through-put rates. Total cost of operation shows up-turning rotors to be most economical. Peterson has refined this up-turning grinding process with a variety of additional innovations.

Innovations: How a Peterson Grinder Does a Better Job

Impact Release Systems

In the Peterson horizontal grinders made since 1999 the anvil housing assembly, containing anvil and first grate, releases as a unit when sufficiently impacted, permitting un-crushable objects to be ejected from the grinding chamber and passed to the discharge area and, thereby, minimizing damage to the bits, anvil, rotor and grates. This type of release is only possible with an up-turning rotor, where the releasing mechanism has room to pivot away from the material path.
See Figure 6. The latching detent impact release system keeps the anvil housing firmly latched in place until an un-crushable object strikes the anvil or first grate with enough kinetic energy to force the latch past the spring-loaded detent. When that happens, the anvil housing opens upward, carrying the first grate with it while pivoting around a shaft. Depending on the amount of force from the impact, the housing may open far enough to contact and compress the rubber anvil stops.
When the un-crushable object passes onto the discharge conveyor, the weight of the anvil housing, plus possibly the rebounding momentum from the energy of the impacting object, cause the housing to rapidly swing downwards, pivoting to its closed position where it re-latches in its original position.
This impact release system keeps the housing completely closed until forced open, then re-latches to a completely closed position again, unless the un-crushable object obstructs the housing. If the object clears, grinding can continue.
When the housing opens, a switch indicates housing movement, sending its signal to the micro-controller unit. Program coding is set to initiate reactions to housing movement, including slowing the engine to help prevent mechanical damage from the un-crushable object, and to alert the operator that product sizing may be compromised.
Impact release system illustration
Figure 6. Impact Release Systems, Applied to Up-turning Rotor Designs
This system is an effective tool for maintaining consistency where strictly controlled maximum particle sizing may be required, such as for compost, mulch, or bio-fuels. The latching detent impact release successfully controls particle sizing while minimizing the risks of damage to the machine or major downtime if an occasional un-crushable object forces the anvil housing to release.
The floating anvil system, sometimes called an air-bag system, works without latching or detaining the anvil housing to keep it from moving. Instead, a pressurized air bellows limits anvil housing movement, keeping it in place or nearly in place most of the time. The anvil and first grate can “float” against an adjustable air spring.
Objects with little kinetic energy do not open the housing at all but are subjected to successive blows from the hammering bits until reduced in size enough to pass the grates. The housing can open a little, a lot (to the limits of the pivot), or not at all. This configuration is typically used with a half second grate removed, which permits un-crushable materials to more readily pass. The low pressure required to open the anvil minimizes impact damage to the bits from hard objects with a resulting increase in bit life.

Further Improvements

Now further enhancements have been incorporated to extend the economic application of Peterson grinders.

Impact Cushion System

The pivot shaft for the feed roll and anvil housings are mounted in impact-absorbing mounts that reduce peak loads into the shaft and supporting structures. Shear pins at each end of the shaft further protect the pivot shaft and supporting structures against damage from impacts.

Larger Grate Area with Interchangeable Grates

Peterson grinders have a large screen area with multiple grates for combining grate opening sizes to match your feed materials. Separate grates and access from both top and side mean less down-time in changing grates.

Grate Shear Pin Protection

Grate retainers are also held in place with shear-pin-protected supports. If an un-crushable object is encountered, the pins shear to minimize rotor damage.

Optional Rotor Speeds

The rotor speed can be changed on the 67 series grinders with optional sheaves. Incremental adjustments to rotor or tip speed can help adapt a high-speed grinder to varying feed material conditions.

Adaptive Feed Advantages

Peterson’s Adaptive Feed System helps keep production at peak volumes by automatically adapting the feed rate to the materials in the hopper. Sensors combine data inputs to the micro-controller unit allowing analysis of the load at the grinder face. Lower loads result in faster feed rates, while higher loads can slow the feed rate to keep from choking or stalling the rotor. The result is an amazingly steady throughput rate at high volumes.

Wear Parts

Wear parts constitute one of the highest variable operating costs for a grinder, and are a key factor in meeting product specifications. On Peterson grinders, wear resistant plates line the rotor’s side walls.
Carbide-coated bits, cast carbide bits, and bits with brazed-on carbide inserts are available for Peterson grinders, depending on application.

Peterson Product Support

Peterson has dedicated resources to address customer support needs, including on-site service, a growing network of parts distribution centers, and telephone-based troubleshooting and parts ordering. Comprehensive product information, including parts books, operation and service manuals, and schematics are available on-line or in print. Both dealer and direct sales networks support active, working machines throughout the USA, Canada, Japan, Australia, Europe and other overseas operations.


Whether your end products are mulch or compost in bulk form or packaged for retail, Peterson horizontal grinders can contribute value to your bottom line profits. The combination of features and innovations makes Peterson horizontal grinders ideal machines for most size reduction jobs. The grinders work well in mixed green waste, in C&D, scrap board, land-clearing, woodlot thinning, and other materials that are primarily organic, but contain some degree of contaminants.
Peterson can provide estimated owning and operating costs for our grinders in various materials. The costs can be used in a discounted cash flow analysis to determine an internal rate of return (ROI) and payback from grinding. Key variables in this payback analysis are:
• The types and quantities of feed materials
• Types and uses of ground material
• Associated processes for packaging or bulk delivery
• The cost of capital.

Other Useful Information Resources

General info: US Composting Council: see, or Midwest Biosystems: see
Compost Quality:, or
For more information call your Peterson representative. Locate them using the contacts and dealers tabs at the top of this web page, and you can reach us at 800.269.6520.
* In 2004, the mulch and soil industry adopted standards prohibiting the use of CCA-treated wood in consumer mulch and soil products. For a useful reference see Wood that has been painted or coated is also usually separated from mulch and compost feed stocks.
** See Wood Properties, by Jerrold E. Winandy, USDA-Forest Service, Forest Products Laboratory, Wisconsin, page 557, Volume 4 of Encyclopedia of Agricultural Science, Chas. J. Arntzen, editor, published 1994. Winandy describes various failure modes among the many properties of wood.
*** See Compost Microorganisms by Nancy Trautmann and Elaina Olynciw.
**** 40 CFR Part 503, (EPA) Standards for the Use and Disposal of Sewage Sludge, states that Class A biosolids composting requires 55 °C for at least 15 days with 5 turns for windrow composting. See Biosolids Technology Fact Sheet at
***** See Comprehensive Compost Odor Response Project produced by San Diego State University, March, 2007, Contractor’s Report to the Board, page 27, “What Conditions Lead to the Formation of Odorous Compounds?”.