Leaves, grasses and other plants, which fall to the floor of a forest, prairie or woodland, slowly decompose and eventually become part of the soil. The nutrients that were in the dead plants then become available to the living plants, closing the cycle of growth and decay.
In contrast, leaves, grass clippings and other yard waste collected in populated areas are a waste management problem. Municipal yard waste can also decompose, as in the forest, returning the nutrient-rich organic residue to the soil. This cycle can be accomplished economically and quickly through a simple composting process.
Yard Waste as Municipal Solid
Waste Since yard waste occurs seasonally and is often raked, bagged or bundled
separately from other household wastes, it can be collected separately. Because
these materials readily decompose, they can be easily recycled by a relatively
uncomplicated composting procedure. The end product of the process is compost, a
valuable organic amendment for soil.
Alternatives for Managing Yard
Waste Composting -- The objective of yard waste
composting as a solid waste treatment process is to reduce the mass and volume
of the waste materials, diverting them from landfills, while converting the yard
waste into a useful finished product for horticultural or agricultural use. The
elimination of weed seeds and any plant disease is also desirable.
Generally, the biggest economic advantage of composting is avoiding disposal
costs charged for landfilling. Additionally, there are usually savings in
hauling distance, waiting time and vehicle maintenance. The compost produced may
be used as a substitute for mulch or topsoil in municipal landscaping, thereby
reducing purchase costs. Compost can also be distributed to residents as a
reward for their co-operation in the composting program or may have some
commercial value for sale as a product.
From an environmental perspective, composting saves valuable landfill space
while adding beneficial humus and nutrients to soil. As a visible,
community-sponsored recycling activity, municipal composting creates a positive
image and may encourage participation in other recycling programs.
Composting can be both an economically and an environmentally sound
alternative for handling yard waste. However, to fully realize these benefits
and to avoid some of the potential problems, care must be exercised in selecting
a site and designing and operating the composting facility.
Landfilling -- With the recognition of the need
for environmental controls at landfills, the cost of this waste management
method has increased dramatically. Siting of new landfills has become extremely
difficult and landfill capacity has been sharply decreasing. Burying yard waste
in landfills uses up this limited capacity unnecessarily.
As the number of remaining landfills continues to decline, the hauling
distance and time will increase for many communities. Landfills also increase
maintenance costs for the trucks which are used on-site: tires, transmissions,
fuel tanks, hydraulic lines and other components endure a lot of wear and tear.
Ironically, once yard waste is placed in landfills, the biodegradability that
makes it suitable for composting becomes problematic, adding to the gas,
leachate, and settling that landfills typically experience. Thus, landfilling of
yard waste is expensive and contributes to environmental problems.
Composting -- the Process Composting is the process which controls decomposition of biodegradable
organic waste by providing an optimal environment for microorganisms. It has
been used for many years for treatment of agricultural wastes and, more
recently, for treating sewage sludge, certain industrial wastes, yard waste and
other elements in the municipal solid waste stream.
Composting -- the Product Incorporating compost into soil increases the soil's nutrient content and
water-holding capacities. It makes soil easier to cultivate, reduces erosion,
and acts as a buffer against extremes in pH, fertilization, and temperature.
Quite simply, composting is the speeding up of a natural biological process.
Much can be learned by actually managing a compost operation, but an
understanding of the underlying scientific principles is useful.
Microorganisms Water Yard waste can be excessively wet, causing oxygen-poor conditions and
lowering the internal temperature of the windrow. Instead of controlled
decomposition, wastes ferment or rot, leading to odour problems. This condition
is often self-correcting, as excess water drains or evaporates from the windrow.
Depending on weather conditions prior to collection, leaves might be
sufficiently moist upon receipt, but this cannot be relied upon in routine
operation. In general, it is better to start with a windrow that is too wet than
to risk dryness.
Oxygen
Municipal waste collected routinely from households includes a
mixture of paper, glass, plastic, metal, yard waste and miscellaneous materials.
Unlike other household wastes, yard waste--grass clippings, leaves, garden
wastes, prunings, tree trimmings and brush--occurs seasonally. In summer and
fall, yard waste may account for 33 to 50 per cent of the residential waste
collected and may comprise 10 to 20 per cent of the annual total.
There are two major alternatives for management of municipal
yard waste: composting and landfilling.
Backyard and municipal
yard waste composting are complementary activities. Municipalities should
encourage backyard composting as a part of their overall yard waste management
program. Participation in backyard composting and mulching will depend in part
on the public's understanding of the cost and problems associated with
landfilling yard waste. Co-operation on the part of residents will depend on
effective public information efforts.
Composting: the Process and Product
When biodegradable
organic materials containing sufficient moisture and nutrients are placed in a
long pile (a windrow), a natural process of decomposition will occur.
Microorganisms, mainly bacteria and fungi, begin to grow rapidly on the organic
materials, using them as a food source and decomposing them. Because the
microbes are not 100 per cent efficient, some of the chemical energy stored in
the organic materials is wasted and released as heat. Thus, the material heats
up through the intense metabolic activity of the microorganism. A large enough
windrow will act as an insulator, retaining heat and leading to an increase in
temperature. As the readily biodegradable food supply is exhausted,
decomposition is mostly completed, growth and heat generation slow down, and the
windrow cools.
As composting
progresses, the original material becomes less recognizable, although certain
components, such as twigs and small branches, persist longer than others. The
material darkens, acquires a granular texture, increases in water-holding
capacity, and eventually develops the pleasant odour characteristic of freshly
turned soil. Compost bears little resemblance to the original starting material.
Underlying Scientific Principles
Microorganisms found
on leaves, grass clippings and other wastes initiate the composting process. As
composting progresses, new types of microbes, insects, and earthworms, among
others, migrate to the windrow and continue the decomposition process.
Inoculants, which introduce the proper microorganisms, are commercially
available, but experience shows that additives need not be purchased for yard
waste composting. Once a composting operation is established, finished compost
from a previous year can be used to inoculate new windrows. In fact, this method
of inoculation is highly recommended.
Water is essential for biological
functions and composting is no exception. Initial dryness will slow the
composting of leaves and should be avoided. To discourage dryness, windrows
might contain grass clippings mixed with leaves. Since a triangle-shaped windrow
composed mostly of leaves sheds water somewhat like a shingle roof, application
of water to the surface does not effectively wet the interior material. Adding
water before windrow formation ensures adequate moisture throughout the windrow.
An initial moisture content of at least 50 per cent (wet weight basis) is
recommended. We have found that the addition of water on a regular basis is
essential on the prairies because of the dry climate. Therefore, locating your
compost site near a good water source is essential. A useful rule of thumb is to
keep materials about as wet as a damp sponge.
Composting is basically an aerobic
process (requiring oxygen), although anaerobic (without oxygen) conditions also
may occur. A 5 to 15 per cent oxygen content is ideal. Most of the heat produced
in composting results from the consumption of oxygen by microorganisms and
production of carbon dioxide and water. Because the windrow must be sufficiently
porous to allow oxygen to enter and carbon dioxide to escape, materials should
be placed loosely in the windrows, and compaction should be avoided.
| Waste Type | C/N Ratio |
| Grass clippings (fresh) | 20:1 |
| Leaves (fresh) | 40-80:1 |
| Weeds (fresh) | 25:1 |
| Leaves and weeds (dry) | 90:1 |
| Hay (dry) | 40:1 |
| Straw (dry) | 100:1 |
| Sawdust | 500:1 |
| Wood | 700:1 |
| Manure | 20:1 |
Temperature Temperature is a powerful determinant of the rate of decomposition. Properly
decomposing wastes will experience a significant rise in temperature over time,
followed by a gradual decrease until decomposition is nearly complete. The
temperature increase is caused by increased biological activity as decay
organisms grow and multiply. Temperatures of actively composting windrows can
reach 70oC or more, but generally range between
45-55oC. Internal temperatures should reach at least
45oC for several days to ensure that pathogens, insect
eggs and larvae, weed seeds and other undesirable organisms are killed or
rendered ineffective. Precise control and monitoring of temperature is usually
not essential for yard waste composting. However, if the windrow does not heat
up, decomposition will proceed slowly, or not at all toward the outside of the
pile.
Windrow Size and Turning Different methods for large-scale yard waste composting may be considered.
The method most appropriate for a given application will depend mainly on the
site selected, although the availability of equipment and labour are also
factors.
"No Tech" Composting Communities with large tracts of rural land have the option of transferring
yard waste to an area where it will slowly decompose over many years (usually
five or more), as happens in a forest or grassland. This method is not actually
composting since the rate of decomposition is not controlled, but it is an
economical alternative to consider if land is available which is remote,
well-drained, and meets the standards of Saskatchewan Environment and Resource
Management (SERM). Yard waste can be dumped in unformed piles, but it must not
be piled in bags. Further, it is important to monitor incoming loads to ensure
that the area does not become a dumping ground for other wastes. Although this
method does not compost yard waste, it must still be conducted in a
nuisance-free manner (i.e., no problems with run-off or odour). Communities
planning this approach to yard waste disposal should notify SERM.
Windrow Composting Since windrow composting is appropriate for most Saskatchewan communities, a
detailed description of this process is outlined in the next section.
Static Piles with Forced Aeration An ideal rate of decomposition requires near-optimal levels of temperature
and oxygen. This also minimizes odours because the putrescible (odour-causing)
materials are quickly decomposed, and anaerobic conditions are minimized. Rather
than turning the windrows, air may be blown or drawn through the piles by using
perforated pipes and either a forced-draft fan or an induced-draft fan. Forced
air systems primarily have been used in sewage sludge composting projects in
which the compost is fairly moist and homogeneous. Forced air systems require
careful initial processing to ensure that materials are properly mixed and have
a suitable moisture content and porosity. Careful site preparation is also
necessary. Once implemented, a successful forced air system may result in
reduced costs and other benefits.
The forced air approach is advantageous because large windrows can be formed
initially, using less space while avoiding extensive anaerobic conditions
because of good aeration. Therefore, serious odours and slowed decomposition do
not occur. Another benefit of forced aeration is that, as a result of rapid
decomposition, the compost process can be completed within a year.
A disadvantage of forced air composting for yard waste is that these systems
do not work well with grass clippings, which tend to clog the air holes of the
piping. As well, the requirement for electric power at the composting site is a
drawback.
In-vessel Composting A successful municipal composting operation will ultimately be customized to
the community's resources and needs. While the basic principles of windrow
composting remain the same, the size of the windrows, their placement on site,
the schedule of turning the windrows, and the length of the composting cycle can
vary greatly. The two approaches described below have been successfully
implemented by municipalities. Variations of the basic windrow method are
already used by many communities. The second method described was developed at
the community compost site in Saskatoon, run by Services for Seniors. Municipal
planners are encouraged to use these descriptions as a guide to various methods
that can be combined and expanded upon, according to the unique needs of the
individual community.
Basic Windrow Method -- Minimal Effort Materials brought to the site are formed immediately into large windrows
using a front-end loader. (Recommended windrow sizes are discussed later). Once
each year the windrow is turned and reformed. An additional windrow is
constructed with the new materials each year. After three years, the material in
a windrow is usually sufficiently stabilized to be used as compost. Inclusion of
grass clippings in the windrow will shorten composting time by providing
additional nitrogen and moisture.
With such minimal technology, ideal conditions for rapid composting are not
achieved. Much of the windrow remains anaerobic for a full year between
turnings. Odour can be expected prior to the first turning, and serious odours
will probably be released during the first turn - by the second turning, odours
have usually diminished. Because of these odours, an extensive buffer zone is
required: a distance of one kilometre or more to sensitive neighbouring land
uses is recommended.
The obvious advantage of the basic windrow method is that it is extremely
inexpensive. It requires the use of a front-end loader only a few hours each
year. Because of the large buffer zone needed, a disadvantage of this approach
is the extensive total area required, although only a small portion is actually
used for the composting. This method might be feasible in a wooded area, so that
only a small clearing would be required, or at an isolated industrial site,
public works yard, landfill or nearby farm field.
The Prairie Method Balancing the need for oxygen flow with the need for a high enough
temperature is a factor when determining the size of the windrow. The simplest
way to achieve the ideal temperature range for composting is to build windrows
large enough to conserve sufficient heat, but not so large as to overheat or
cause compaction. On the other hand, smaller windrows ensure adequate oxygen
flow. Unfortunately, no single windrow size completely reconciles these
conflicting goals. The desired conditions can be approached by starting with
moderately sized windrows (three to four metres high by four to five metres
wide), then combining two windrows after the first burst of microbial activity,
which lasts approximately one month. To avoid anaerobic conditions, windrows
should be no higher than six metres. However, Saskatchewan's colder climate
requires somewhat wider windrows of three to four metres high by seven to eight
metres wide.
With this approach it is possible to produce a thoroughly decomposed
(finished) yard waste compost in six to eight months. If windrows are initially
formed before freeze up, the compost will be ready for use in spring, the time
of peak demand for the product. Slight odour may be a by-product early in the
composting cycle, but this is not usually detectable beyond 50 metres from the
windrows. After 10-13 weeks, large curing piles can be formed around the
perimeter of the site, freeing the original area to accept new materials.
1. Site Preparation The yearly site preparation should include regrading and road maintenance as
well as removal of refuse and debris from the previous year's operation. This
step will require a few days work at most. Site preparation should be scheduled
any time after the active site has been cleared of the previous year's
materials, through formation of curing piles, but before the new collection
season begins.
2. Receiving and Sorting Windrow formation must take place as soon as possible after yard waste is
received. If freshly dumped waste is allowed to sit for more than a few days in
the staging area, odour problems may develop. Some minimal supervision may be
required to prevent dumping in undesired locations. Also, a record of the amount
of waste delivered should be kept: a daily tabulation of the number of loads for
each individual truck of known capacity may be an effective accounting method.
At most yard waste composting facilities, the waste is delivered in bulk.
However, some sites may find it necessary to accept at least a portion of their
capacity in plastic bags. These bags pose considerable extra problems but can be
handled successfully. The bags should be dumped in a separate portion of the
staging area where, in a very labour intensive process, they must then be slit
open and emptied. (Some American operations use community labour service -- a
Canadian equivalent might be the Fine Options Program). Any trash must be
separated and disposed of along with the bags.
A difficulty with collecting bagged leaves is the odour released from some of
the bags upon opening. One alternative is to open and dump the bags directly
into the hopper of the collection vehicle during the collection process. While
this practise slows the collection crews, it prevents double handling of the
bags and provides for greater control over feedstock quality. Programs which use
a drop-off site for residents should require participants to unbag their own
wastes in order to take plastic bags back home for reuse. Drop-off sites should
have some method for monitoring incoming loads to avoid contamination of
windrows with other wastes.
3. Wetting The need to add water can be reduced or eliminated by forming windrows with
flat or slightly concave tops. This will facilitate the capture of precipitation
and induce seeping of moisture down through the windrow.
4. Forming Windrows Initially, the windrows can be three to four metres high by three to four
metres wide. Any convenient length can be used. (Note: windrows that will be
left over winter should be three to four metres high by 10-15 metres wide to
help retain heat in Saskatchewan's cold climate.) To conserve space, two
windrows can be formed side by side, with only one metre between. Remember to
allow sufficient aisle space between pairs of windrows for the loader to
maneuver when turning the piles.
Neatly formed windrows with well maintained aisles give a professional
appearance to the facility, while messy windrows give the impression of a
"garbage dump." Care should be taken so that equipment, especially the loader,
does not drive up on the windrows, compacting them. Loosely piled windrows are
required in order to maintain adequate air penetration.
5. Monitoring Decomposition Compost windrows should be monitored closely to ensure that decomposition
proceeds properly and does not create a nuisance. Monitoring should consist of
daily temperature readings at several points in each windrow and bi-weekly
inspections for moisture content, physical appearance and internal windrow
odours.
Daily temperature readings are especially good detectors of problems before
they become major. Sharp drops in temperature, for example, may indicate
decreasing moisture content or the onset of anaerobic conditions. Failure of
temperatures to rise to 50-75oC shortly after windrow
construction may indicate a poor carbon/nitrogen ratio or improper moisture
content. In both these cases a physical inspection of the windrows should reveal
the source of the problem. An anaerobic pocket, for instance, might be tightly
packed, emit unpleasant odours, be very wet, and possibly have a green or shiny
black appearance. Prompt aeration would prevent the problem from becoming a
major nuisance. (Further troubleshooting is discussed later).
While thermometers are available for monitoring temperatures, experienced
compost program operators often find that physical inspection of windrows is
adequate to determine if the material has sufficiently heated up.
6. Turning Windrows Additional turnings throughout the summer increase the composting rate and
product quality. Turning at least once every week to 10 days is recommended to
maintain high temperatures.
7. Combining Windrows 8. Curing At this time, therefore, the windrows can be moved and formed into a large
curing pile on the perimeter of the site. This operation provides additional
turning and mixing of the compostable material. To conserve space, the curing
pile may be as large as desired, taking care not to compact when forming.
Combining several windrows into a single pile exposes a relatively small surface
area to drying and freezing conditions. Further weed and pathogen destruction is
achieved at the temperatures reached within the large, well-insulated curing
pile. This material should be well stabilized by the following spring but may be
left in place longer if convenient.
9. Shredding or Screening The major advantage of using a shredder is that it yields a more uniform and
debris-free final product. It also can be used to mix finished compost with
soil. Disadvantages include the labour and equipment requirements, the need to
dispose of rejects, and, of course, the capital cost of the specialized machine.
One way to reduce costs is to share a single unit among several sites or
communities. This is feasible since the specialized equipment is needed only for
a month or two per year, making scheduling flexible.
Grass Clippings The best alternative for grass clippings is to not bag them at all. Turf
specialists recommend mowing frequently enough so that the short clippings
filter through the growing grass and return their nutrients to the soil.
Contrary to popular belief, this practice does not contribute to thatch. If the
clippings are collected, they can be incorporated in moderate amounts in back
yard composting piles or used as a garden mulch.
Since grass clippings are still green when collected, they are relatively
higher in nitrogen, moisture content, and readily degradable organic material
than leaves collected in autumn. They therefore decompose more rapidly, have a
higher oxygen demand, and quickly go anaerobic. Since grass clippings often have
a strong smell by the time they are delivered to a composting site, it is
especially important to properly implement (and strictly enforce) the odour
control measures discussed. Additional precautions, such as enlarging the buffer
zone, also may be necessary.
Grass clippings should be incorporated into a leaf windrow before the end of
the delivery day. A 50:40 ratio of leaves to grass clippings provides an optimal
carbon to nitrogen ratio. (Until experience is gained, a higher ratio of leaves
to grass can be used to reduce odour problems). Thorough mixing is essential:
this can be done with a front-end loader combining 20-30 bucketfuls of material
at a time, then forming a windrow with the mixture. Once the material has been
mixed in this way, no further odour problem is expected. The partially composted
leaves act as a bulking agent to improve penetration of oxygen to the grass
clippings. The grass in turn speeds the decomposition of the leaves by providing
needed nitrogen. The end result is a higher quality compost product ready within
a shorter period of time. However, these benefits must be balanced against the
increased potential for odour problems. Green weeds can be considered identical
to grass clipping for the purposes of composting, but large quantities of
diseased plant wastes should be excluded from compost windrows.
A common concern about using grass clippings for composting is the potential
inclusion of lawn chemicals in the finished product. While there is some debate
on this topic, University of Saskatchewan horticulturists indicate that most
commonly used lawn pesticides degrade in six to eight weeks. Therefore, compost
made from yard waste will be free of significant quantities of lawn chemicals.
Further, concerns about the toxicity of materials used in agriculture usually
centre on the uptake of heavy metals by plants grown in treated soil. Finished
compost made from yard waste typically contains extremely low levels of heavy
metals.
Woody Materials Tree trunks and large branches can be cut and given away or sold as firewood,
while smaller diameter woody materials can be chipped to produce a useful mulch.
Communities can use wood chips as mulch or bedding for municipal landscaping,
park pathways, and school playgrounds. Residents will appreciate free wood chips
for use in their own yards. As well, wood chips are valuable at the compost site
to form roads and all-weather work surfaces. Accidental inclusion of small
quantities of wood chips into the windrow will not adversely affect the quality
of the final product.
Pine Needles Site selection for a municipal yard waste composting program is an extremely
important decision that should be made only after careful consideration. Siting
the facility should take into account factors such as proximity to residences
and streams, prevailing winds, traffic patterns, and travel distance and its
effect on collection equipment and labour costs.
Area Requirements Buffer Zone Location Slope and Grading Percolation Water Supply Security On-Site Roads Safety Considerations One relatively new concern with leaf composting is the release of spores of
the fungus "A Spergillus fumigatus". These spores can produce an allergic
response in some individuals and can, in a few cases, cause infection in
individuals with a weakened immune system, who are taking antibiotics or adrenal
cortical hormones, or have a punctured eardrum. Workers having such conditions
should not be assigned to the composting operation (as well as any other tasks
putting them at similar elevated risk), unless a health specialist is consulted.
Public Acceptance A plan for the distribution of the end product should be developed early in
the setup of the composting facility. Centralized composting systems require
that the yard waste be transported to the processing site and that the finished
product be transported off site to provide room for the processing of the next
batch.
While the nutrient content of yard waste compost is too low for it to be
considered a fertilizer, it is a valuable soil conditioner and organic amendment
which improves the physical, chemical and biological properties of the soil.
Most Saskatchewan soils will benefit considerably from the increase in organic
matter which compost can provide. Demand for finished compost is dependent on a
number of interrelated factors: product quality, assured supply, cost, and
consumer education.
Product quality involves product appearance, uniform consistency, level of
impurities (foreign objects such as stones, sticks, glass, plastics, metals),
and chemical composition (nutrients and heavy metals). To a large extent,
product appearance is dependent on the level of impurities found in the incoming
yard waste and the level of processing done to it. Grass and leaves that are
free of brush and other impurities can be processed into a good quality compost
using minimal technology with an adequate composting period.
Shredding and screening can be used to improve appearance, remove impurities
and improve consistency, as well as reduce the length of the processing period.
Assured supply is essential if finished compost is to be used on a regular basis
by agencies and institutions.
Price is primarily a function of supply and demand, although it also reflects
the costs embodied in the processing and marketing of the product. There is the
attitude that "you get what you pay for," so if a product is free it may be
considered to have no value. Poorly processed compost has little value compared
to its principal competition (black dirt, peat and similar soil amendments). The
closer finished compost approaches the quality, uniformity and consistency of
the competition, the higher its relative value.
Consumer education is the final component of demand. As with any other
product, the quality, availability and price of compost mean little if it is
unknown to the consumer. Advertising through various media is needed to raise
consumer awareness about composting in general.
Municipal Use
The temperature of the
compost windrow reflects the balance between heat generation and heat loss to
the surrounding air. The rate of heat generation is a function of factors such
as temperature, moisture level, oxygen, and nutrients. The rate of heat loss is
a function of factors such as ambient temperature, wind speed, and windrow size
and shape.
Control over
temperature and oxygen content can be achieved to some extent through windrow
size and turning operations. The need for oxygen must be balanced with the need
for heat conservation. Adequate oxygenation requires small windrows to minimize
the distance to the windrow interior that the air must penetrate. In contrast,
the need for heat conservation, especially in winter, requires large windrows
which provide greater insulation. However, windrows that are too large may
result in anaerobic (airless) conditions. These requirements can be reconciled
in part through management of windrow size and by turning. It is essential to
turn the pile at regular intervals to maintain optimum oxygen levels in the
pile. However, turning the pile releases substantial heat into the air. So, the
art of managing windrows is to turn often enough to keep oxygen levels high but
not so often as to cause excessive cooling of the pile.
Alternative Composting Methods
As mentioned earlier,
communities with a farmer, nursery or landscaper nearby who will accept
municipal yard waste for either composting or direct land application will have
the easiest and cheapest option available.
In rural Saskatchewan,
siting of a "no tech" composting area is often possible, but in more highly
populated areas, where composting space is limited, a faster process is
necessary. This involves closer regulation of moisture content, oxygenation, and
temperature, commonly accomplished with the basic windrow method. Yard waste is
formed into windrows which are periodically turned with a front-end loader to
promote faster decomposition, a process typically occurring over a period of 12
months.
If
limited space is available and completion of composting in less than one year is
desired, a higher level of technology is required. Simply turning the windrows
more frequently (for example, once per week) will produce a finished product in
under six months. While costly specialized windrow turning machines may be used
to increase turning efficiency, this actually requires more space because the
starting windrow size is limited by the machine's working height and width.
In-vessel systems have
been successfully used for composting sewage sludge and municipal solid waste,
but this technology has not been directly applied to yard waste composting.
During in-vessel composting, incoming material is often shredded, then mixed
with a bulking agent or nitrogen source, regularly turned in a digester designed
to foster rapid growth of microorganisms, and then windrowed to "cook" for a
short period, usually a few weeks. Although this method greatly reduces
composting time, the high capital equipment and operating expenses make it an
unlikely choice for composting yard waste.
Two Versions of the Windrow Method
If an
area that is well-isolated from sensitive neighbouring land uses is available, a
very low-cost approach to yard waste composting is possible. This method differs
from the "no tech" approach only in that the yard waste is formed into windrows
and periodically turned. Even this minimal effort will produce finished compost
faster than the "no tech" approach.
In some American states,
yard waste has been banned from landfills for a number of years and it is
mandatory for each county to have a yard waste composting program. Several
states have published manuals on yard waste composting from which the following
recommendations have been adapted. Ultimately, all yard waste can be
successfully composted, but it might be advisable to begin with select sources
of materials, adding new materials as experience is gained and the needs of the
community are better understood. The inclusion of grass clippings and other yard
waste in the composting system will increase the quality of the finished product
as well as the speed of the process.
Prior to each collection season, the
site must be readied to allow truck access and front-end loader operation.
Delivery of the collected waste is the one aspect of the operation with little
scheduling flexibility. Once leaves are collected, they must be formed into
windrows promptly, (sections 2-4 below). Therefore, it is critical to avoid
operational bottlenecks, where, for instance, an area becomes so muddy that
trucks get stuck trying to drop off their loads.
It is recommended that trucks dump
their loads of leaves in a staging area, rather than trying to form windrows
directly. Although a staging area involves additional labour, its use is
justified for several reasons:
Due to the normally dry climate of the prairies,
wetting of the yard waste is required during much of the collection season.
Adequate wetting can only be achieved prior to or during windrow formation, or
when windrows have been opened up for turning or other purposes. Because most of
the water applied to the outside of a windrow will be shed by the waste, the
water should be sprayed on the yard waste as it is broken apart by the loader in
the staging area, and/or as it is placed in the windrows. Approximately 80-90
litres of water will be required on average per cubic metre of yard waste
collected. Less formally, the rule of thumb is that it should be possible to
squeeze a few drops of water from a fistful of the leaves.
Once the yard waste has been dropped in
the staging area, the front-end loader can be used to break apart and spread the
compacted materials to aid wetting. The front-end loader can then be used to
place the uncompacted waste in windrows.
In the spring, each windrow should be
turned as early as is practical (March or April). Turning mixes the material,
redistributes the moisture in the windrow, oxygenates the interior, and exposes
the cool exterior to the hotter internal temperatures. This results in an
increased rate of decomposition and improved destruction of any pathogens and
weed seeds. For adequate oxygenation, maximum mixing and fluffing is desired
during turning. Additional water may be added at this time if the material is
too dry, although every effort should be made to provide sufficient water
initially.
After approximately one month, much of
the initial oxygen demand of the pile will have been exerted and the windrows
reduced to about half their original size through decomposition and
self-compaction. At this point, two windrows can be combined to form a single
one of about the same size as each of the initial windrows. Combining the
windrows will help conserve heat during colder weather. Portions of the centre
of the new, combined windrow may go anaerobic temporarily, but significant
odours and acidification are not expected because much of the readily degradable
material has already been consumed by the microorganisms. Combining should be
done by moving and turning both windrows, rather than placing one on top of the
other, thereby achieving the maximum degree of mixing and fluffing.
Using the Prairie Method of composting, much of
the product will not be completely stabilized by the end of the summer, yet the
composting area must be cleared to allow for site preparation for the next
year's collection. This is not a problem since the material is now well
decomposed, has little oxygen demand, and is unlikely to produce odours.
Once composting is complete
(post-curing), shredding or screening is a final optional step to improve the
physical quality and appearance of the finished compost. This process screens
any uncomposted leaves, branches, rocks, plastic, and other extraneous materials
and breaks up clumps. Organic "rejects" may be composted for an additional
period, then shredded to minimize the amount requiring disposal. Shredding and
screening is quite labour intensive. Leaf compost can be processed at only about
half the rated capacity of the equipment. Because shredding or screening will
proceed more rapidly if the compost is not too damp, moist material can be
spread out for a day or two to dry.
Grass Clippings and Wood Wastes
Grass clippings represent a
significant seasonal waste management problem. In some Saskatchewan communities
they may account for nearly one-half of the total municipal refuse load during
peak grass-growing periods.
Wood tends to decompose very
slowly, making composting impractical in most cases. These woody materials
should not be intentionally incorporated into yard waste composting windrows,
although small amounts of incidental branches and twigs pose little problem.
Pine needles can be
successfully composted if they are mixed with grass and leaves. However, since
pine needles decompose very slowly, the formation of windrows containing almost
exclusively pine needles should be avoided. Christmas trees should be treated as
woody materials.
Facility Siting and Preparation
A minimum of one
hectare per 1600-2100 cubic meters (one acre per 3000-4000 cubic yards) of yard
waste is required for the actual composting operation in addition to a buffer
zone. Volume reduction and the time required to complete the composting process
are variables which influence the area required for an adequate site.
A buffer zone between the
site activities and neighbouring areas will minimize possible odour, noise, dust
and visual impacts. There are no hard and fast rules on the size of the buffer
zone needed for composting but it is prudent to provide at least 50 meters
between the composting operation and the property line. According to SERM, a
minimum of 500 meters must be allowed between the compost site and any sensitive
neighbouring land uses, such as residences, restaurants, hotels/motels, schools,
churches or public buildings. The buffer zone may include a berm, consisting in
part of finished compost, to serve as a visual barrier, help control vehicle
access, and reduce noise levels off-site. A landscaping plan will enhance the
appearance of the facility.
A centralized area will reduce transportation time
and costs, although such sites are not always available, or practical. If
residents are to bring materials to the site, the location should be convenient
to encourage their participation. Often an area at the local landfill serves as
an ideal site for composting, keeping in mind that access on uncrowded,
non-residential, hard surface roads is preferable.
Steep slopes on site
are unsatisfactory because of problems with erosion, vehicle access, and
equipment operation. However, a gentle slope of about 2 per cent is desirable to
prevent ponding of rainwater and runoff. Initial site preparation usually
requires grading; yearly maintenance should include regrading where necessary.
Windrows should run up and down rather than across slopes to allow runoff to
move between windrows instead of through them. Care should be taken so that
leachate does not run off into nearby streams or lakes.
An ideal composting site
will have moderate soil percolation, so that ponding will not be a problem.
Where percolation is poor, or where an impervious surface is used, particular
care must be taken to prevent ponding. An impervious surface such as a paved
site offers advantages in terms of vehicle access, equipment operation, and
groundwater protection, but these advantages must be weighed against the loss of
direct contact between composting materials and soil microorganisms, as well as
the difficulties in managing runoff.
Water is essential for yard
waste composting. Windrows should be formed with flat or slightly indented tops
to maximize the collection and infiltration of precipitation. Water can also be
supplied by using a hose from a fire hydrant or by pumping from a nearby lake,
stream or well. For smaller operations, use of a water truck is practical,
depending on the amount of yard waste composted.
Vehicle access to the site must be
controlled to prevent illegal dumping of materials as well as vandalism. A gate
across the entrance road is the minimum precaution. In some cases the entire
site will have to be fenced, although pre-existing features such as streams,
trees and embankments may provide partial security: an ideal site will have
natural barriers on three sides. Since permanent chain link fencing can
interfere with site operations, replaceable snow fencing is suggested, while a
berm consisting of earth and finished compost can serve in place of a fence at
other points.
Because of heavy truck
traffic during the collection period, a limited road network within the site
(paved, gravel, or covered with wood chips) may be desirable to improve
all-weather access. A circular traffic flow pattern is advantageous at heavily
used sites. The purpose of the on- site roads is to facilitate drop-off of
materials and to prevent trucks from getting stuck in muddy conditions, so an
extensive road network is not required. (If gravel roads are used, contamination
of windrows with gravel should be avoided).
Safety measures common
to any heavy machine operation should be exercised. Road layout should be
designed with safety in mind, restricting public access unless citizens are to
drop off materials. A ready supply of water and delivery capacity, initial
wetting of leaves, and aisles between windrows are good fire precautions.
Normally, a windrow of yard waste will burn poorly since the interior is wet.
Thus, while vandals may be able to ignite the dry surface leaves, a major fire
is unlikely.
An acceptable
site location is important in gaining public support for the project. Composting
operations will be noisy at times due to the truck traffic and heavy equipment
involved. Odours, flies, dust and other nuisances may occur if the operation is
mismanaged, or if equipment problems cause the operation to be temporarily
suspended. Consequently, the site should be located in non-residential and non-
recreational areas. Communities often locate composting operations at existing
or closed waste handling facilities such as landfills, transfer stations and
sewage treatment plants. Public acceptance will be enhanced by an effective
public information campaign.
Use of Finished Compost
Use of compost in
municipal applications will result in a cost savings to offset some of the
expenses of collecting and composting yard waste. Some specific applications for
compost are:
Compost also may
be blended with poor soils to produce a good quality topsoil.