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The Composting ProcessComposting has been at the forefront of the diversion and processing of organic wastes. This is because it is a relatively simple and robust process. It can be implemented as a small open windrow type facility through to a large facility that uses a sophisticated in-vessel technology. It can be used for a diverse array of organic wastes including food wastes, leaf and yard wastes, biosolids and industrial sludges.

Most simply composting is a managed aerobic (i.e. in the presence of oxygen) microbial process that breaks down organic wastes into compost.

The process is focused on breaking down or decomposing those parts of the waste stream that are most easy to decompose. This includes sugars, starches, fats and proteins. At the end of the process all that is left are the parts of the waste stream that are more resistant to composting. Composting is said to stabilize waste. This means that the resultant compost will continue to break down but at a very slow rate.

A key advantage of the composting process is that its high temperature essentially kills all pathogens and weed seeds that might be found in wastes.

Bacteria, fungi and actinomycetes are the microorganisms responsible for the composting process. While they all play different roles they have essentially the same requirements. Composting is about creating a suitable environment for the microorganisms.

The key process parameters are described in Table 1.

A schematic and simple mass balance of the composting process is shown in Figure 1. The composting process results in the generation of heat, carbon dioxide and water. It results in the production of a stable compost that contains no pathogens or weed seeds.

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Table 1: Key Processing Parameters.

Key parameters What is it? What is the

optimum?

Oxygen Composting microorganisms are aerobic and require oxygen to survive. This is supplied by introducing air into the composting mass.

10-15%

Moisture Microorganisms require moisture to survive.

Moisture is present in wastes. Supplemental moisture can also be added to a composting mass.

50-55%

Carbon to Nitrogen Ratio (C:N)

The mass of carbon and nitrogen in wastes. Carbon rich wastes include leaves and woody materials. Nitrogen rich materials include food wastes, grass and biosolids.

Insufficient nitrogen limits the composting process.

The optimal level is 30 parts carbon to 1 part nitrogen.

It is balanced by blending together a number of feedstocks (i.e. waste types).

30:1

pH A measure of acidity and alkalinity.

The composting process can tolerate a wide range.

6-9

Porosity This is related to void space in the composting mass and is manipulated by adding bulking agents such as wood chips.

1-5 cm

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The composting process is started by blending together different types of wastes. The focus of compost recipe development is to find a blend of wastes with sufficient nutrients and moisture as well as enough porosity so that the composting mass can become and remain aerobic. Nitrogen-rich and carbon-rich wastes are blended together to optimize nutrient levels for the composting microorganisms The carbon-rich wastes also serve to help absorb moisture and provide porosity in the composting mass.

The naturally occurring microorganisms will start to decompose the wastes. This

microbial activity is manifest by the release of energy in the form of heat. The process goes through a number of characteristic temperature phases. It starts off at ambient temperature but then increases rapidly as microbial activity increases. During high rate composting, when microorganisms are decomposing the most easy to break down parts of the waste stream, process temperatures increase. Temperatures up to 70º C are not uncommon, although typically efforts are made to maintain temperatures above 55º C but below 60-65º C. These high temperatures kill human and plant pathogens in the wastes.

After high rate composting, the microorganisms are left with less easy to break down parts of the waste stream to decompose. This results in a reduction of microbial populations and a reduction, over time, of process temperatures. This is referred to as the curing process. This is an essential part of the composting process and important in terms of compost stability and maturity.

The composting process can take anywhere from 6 months to 18 months to complete.

Compost is a soil conditioner and can be used to improve soil quality and health. Its benefits include organic matter, soil porosity, and water holding capacity. It contains relatively small amounts of nutrients such as nitrogen, phosphorus and potassium. It has been shown in some cases to provide disease suppression for some plant diseases.

Composting

Advantages Disadvantages

✚ Reduces mass of waste

✚ Stabilizes Waste

✚ Reduces Pathogens

✚ Produces a product

✖ Generates odours and leachate which must be managed

Wastes Composting MassStart Finish

Start Finish

Finished Compost100% 50%100%

MassVolume 20%

Heat

C02H20

Water

Oxygen H20

Figure 1: Schematic and Mass Balance of Composting Produced

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Adapted from On-Farm Composting Handbook, 1992

Overview of TechnologiesThere are 4 main centralized composting technologies:

1. Static Pile

There is usually some level of feedstock preparation (i.e. size reduction, mixing etc.).

This is a pile or windrow of composting material that receives minimal turning (i.e. 1-4 turnings) and aeration. It takes longer to produce a finished compost product than with other technologies.

Mechanical aeration (i.e. forced air) may be added to a static pile system in which case it becomes an Aerated Static Pile. This will reduce the processing time required. An aerated static pile system may be enclosed in a building.

A finished product can typically be produced in 12-18 months.

2. Open Windrow

There is usually some level of feedstock preparation (i.e. size reduction, mixing etc.).

Large piles or windrows of composting materials can be composted outdoors on a paved or unpaved surface. Aeration and mixing is provided with a pay loader or specialized windrow turner. This is a common method for composting, particularly for leaf and yard wastes. Food wastes can be composted outdoors although additional care must be taken to ensure the process is properly managed.

If windrows are constructed indoors then it becomes an enclosed windrow system. Additional aeration (i.e. mechanical) may be provided at these types of facilities. Off-gases generated as a result of the composting process may be collected and treated using a biofilter (i.e. a variety of porous organic materials including a formulation of all or some of the following: wood chips, peat, compost).

A finished product can typically be produced in 6-12 months

Figure 2: Windrow Composting Using a Windrow Turner

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3. Enclosed Channel

This refers to composting systems in which composting takes place in channels.

Mixing of organic waste is provided with a specialized automated turner, which typically straddles the concrete walled channels on rails or wheels. Additional aeration is provided via a mechanical aeration system.

All enclosed channel systems will include some level of feedstock preparation (i.e. size reduction, mixing etc.).

Materials typically reside in enclosed channel systems from 7-28 days

These systems typically employ automated temperature gathering equipment (e.g. thermocouples, programmable logic controller, desk-top computer).

These systems typically include a mechanical off-gas removal system and odour abatement infrastructure (e.g. biofilter).

After discharge from the channel compost is cured in a separate area, often employing a windrow type technology.

A finished product can be made in 3-6 months.

Figure 3: Enclosed Channel

Adapted from On-Farm Composting Handbook, 1992

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4. In-Vessel

This refers to composting systems in which composting takes place in a purpose built container or tunnel. A composting system employing this system will have one or a number of containers. This is a modular system so additional containers (i.e. to increase processing capacity) can be added as required. Aeration is provided via a mechanical aeration system.

All in-vessel systems will include some level of feedstock preparation (i.e. size reduction, mixing etc.).

Materials typically reside in in-vessel systems from 3-14 days

These systems typically employ sophisticated automated temperature gathering equipment (e.g. thermocouples, programmable logic controller, desk-top computer).

These systems typically include a mechanical off-gas removal system and odour abatement infrastructure (e.g. biofilter).

After discharge from a composting container compost is cured in a separate area, often employing a windrow type technology.

A finished product can be made in 3-6 months.

Table 2 describes some of the advantages and disadvantages the various composting technologies.

Table 2 Advantages and Disadvantages of the Various Composting Technologies

Technology Advantages Disadvantages

Static Pile ✚ low cost ✖ larger site required

✖ less control over potential nuisances

✖ slow process (12-18 months)

Open Windrow (with specialized turner)

✚ medium cost ✖ larger site required

✖ less control over potential nuisances

✖ slow process (6-12 months)

Enclosed Channel ✚ rapid active composting period

✚ multiple turnovers of site footprint

✚ good control over potential nuisances

✖ high cost – capital and operating

In-Vessel ✚ rapid active composting

✚ multiple turnovers of site footprint

✚ superior control over potential nuisances

✖ highest cost – capital and operating

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Mechanical Biological Treatment ProcessMechanical Biological Treatment (MBT) is a generic term for a range of processes that may be used to treat residual waste (i.e. post curbside collection of source separated recyclables and organics) using a combination of mechanical separation and biological treatment. This commonly comprises three stages:

1. Mechanical Stage – mechanical size reduction of waste (e.g. shredding and trommelling) and removal of some recyclable material;

2. Biological Stage – waste is either digested or composted, usually in an enclosed system; and

3. Biostabilization Stage – material separation or ‘splitting’ to segregate different output streams for different purposes.

The principal objectives for which MBT is currently utilized include:

●● Reducing quantities of waste (mass and volume) to landfill;

●● Stabilization of waste prior to landfill;

●● Production of Refuse Derived Fuel (RDF) for secondary energy production;

●● Collection of biogas for energy production; and

●● Composting of the resulting digestate.

Although different technologies may be used, with varying levels of complexity and different outputs, they all have similar characteristics. The figure below provides a generic schematic for a MBT facility.

MBT Processes Schematic

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Overview of Technologies Four MBT technologies are presented below:

1. MBT with Aerobic Composting

This process begins with mechanical separation/sorting of waste, followed by biological processing of organic materials via aerobic composting. Aerobic composting is a controlled aerobic microbiological process that decomposes organic matter into carbon dioxide, water, minerals and stabilized organic matter.

2. MBT with Biostabilization

Biostabilization is a method in which waste is subjected to a process similar to composting to reduce the biodegradability of the waste, while also removing recyclable materials. Biostabilization occurs through aerobic composting, and it requires oxygen and typically the addition of moisture. The output might either be sent to landfill or used in land restoration or remediation projects.

3. MBT with Anaerobic Digestion

This process begins with mechanically removing recyclables and/or contaminant fractions prior to digestion. AD is a process of biologically degrading materials in the absence of oxygen. This produces a ‘biogas’ which is rich in methane and can be used to generate energy.

4. MBT with Refuse-Derived Fuel (RDF) Production

A bio-drying operation is similar in its operation to biostabilization. Bio-drying is a dry stabilization process that produces a light, high calorific fraction to use as an RDF. The main goal of bio-drying is to drive off moisture in the waste through heat from aerobic degradation and airflow through the material.

Materials can then be separated relatively easily from the dried output, so recyclables (metals and inerts) are more typically removed at this point. The remainder is typically screened into a larger fraction used to produce RDF and organic fines which may be composted.

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Source: Department for Environment Food and Rural Affairs. (2007) “Mechanical Biological Treatment of Municipal Solid Waste.” Enviros Consulting Limited. < http://archive.defra.gov.uk/environment/waste/residual/newtech/documents/mbt.pdf> [Accessed on September 16, 2011]

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Technology Advantages Disadvantages

MBT (general) ✚ Increase capture rates of recyclables

✚ Reduce volume of residual waste

✚ Reduce biodegrability of waste, therefore reducing methane and leachate production

✚ Plants can be constructed for small or large scale operations

✚ Stabilization reduces odour and dust

✚ Less public opposition than incineration

✖ Markets for output can be limited

✖ Potentially high operating costs

✖ Significant volumes of stabilized residue may require disposal at a landfill

MBT with Aerobic Composting

✚ Lower capital costs even at small scales, in contrast to other MBT systems

✚ Modular and flexible

✚ Final compost product can be used for remediation and/or other agro-environmental uses depending on quality

✖ No energy recovery

MBT with Biostabilization ✚ Stabilization of waste reduces negative effects of landfills such as odour and dust

✚ Increase capture rates of recyclables

✚ Output can include compost for land restoration and remediation

✖ Retention time at facility can be long

✖ No energy recovery

MBT with Anaerobic Digestion

✚ Enable high energy recovery (biogas production)

✚ Agro-environmental uses of compost and digestate

✚ Greenhouse gas reduction by the processing of waste

✖ Less flexible than aerobic processes

✖ Higher investment costs, compared to traditional composting

✖ Dependent on waste composition

MBT with RDF Production ✚ RDF for alternative fuel ✖ RDF production does not have a well-established market

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Advantages and Disadvantages of Various MBT Systems

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Anaerobic Digestion ProcessAnaerobic digestion (AD) is a treatment process that biologically degrades materials in the absence of oxygen. This produces a ‘biogas’ which is rich in methane (CH4) and can be used to generate energy, either through a generator, or by upgrading the gas to the point when it can be used as a in-vehicle transport (compressed natural gas), or injected into the gas distribution network.

The AD process typically includes the following stages:

1. Pre-processing – removing recyclables and/or contaminant fractions prior to digestion, typically using trommels or other forms of screening to separate the small sized fraction for digestion, as well as recovering recyclable materials;

2. Digestion – the feedstock enters the anaerobic digester or bioreactor for treatment and processing. In this step organic materials are converted by fermentation into biogas and digestate;

3. Energy Production – processing and/or cleaning the biogas (e.g. methane) to produce energy; and

4. Stabilization – composting and curing the solid digestate prior to its use or disposal in a landfill (usually between 60 to 120 days).

There are a variety of AD systems used for the treatment of residual wastes. The treatment component of these processes varies according to:

●● The temperature of operation - mesophilic (~ 37˚C) or thermophilic (57˚C to 70˚C);

●● The solids content of the waste in the reactor; and

●● Whether the treatment process is single- or multi-step.

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Source: European Commission. (2010). “European Biogas Initiative to Improve the Yield of Agricultural Biogas Plants,” EU-Agro-Biogas. <http://ec.europa.eu/energy/renewables/bioenergy/bioenergy_anaerobic_en.htm> [Accessed on September 16, 2011]

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As with most systems, anaerobic digesters can be developed in all shapes and sizes. The figure below provides a generic schematic for an AD Facility.

Source: Federation of Canadian Municipalities (2004) “Solid Waste as a Resource – Review of Waste Technologies,” Federation of Canadian Municipalities, Ottawa.

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Overview of Technologies AD technologies can be categorized into three system components.

1. Single Stage, Multi-Stage and Batch Systems

Anaerobic digesters can comprise single-stage, multi-stage and batch systems. In single stage digesters, all of the biochemical reactions take place simultaneously in a single reactor, while in two or multi-stage systems, the systems take place sequentially in at least two separate reactors. In a batch system, the reactor is loaded once and discharged until the anaerobic process is completed.

2. Wet and Dry Systems

In wet systems, the incoming waste is pulped or slurried to less than 15 % total solids in water, so that a classic mix reactor may be used. In the case of a residual municipal solid waste (MSW) feedstock, this process requires the introduction of significant quantities of diluting water and may involve substantial pre-treatment to provide the required consistency for AD. The process is also challenged by the precipitation of the heavier fraction of the waste to the bottom of the reactor. The process is well suited to materials with a high as-received water and volatile solids content (i.e. organic waste streams).

In dry systems, the fermenting mass has a solid content in the range 20-40%, such that only very dry incoming wastes (>50% total solids) require the introduction of any process water. The biggest challenge is in the handling of dry waste, which is undertaken using conveyor belts, screws and powerful pumps. That said, the rewards of a dry system are much higher biogas yields due to the higher biomass content, plus a simpler reactor design and cheaper pre-treatment stage. These systems are typically much better suited to residual MSW feedstocks.

3. Mesophilic and Thermophilic Systems

Regulating the temperature inside the digestion reactor is important to the chemical reaction process and microbe development. Mesophilic and thermophilic designs return typically the same results, but the operating costs of thermophilic systems and consequences of a fall in reactor temperature are considerably higher in contrast to mesophillic systems. However, mesophilic AD plants generally require a retention time of 12 to 25 days, whereas, a thermophilic reactor can achieve the same result in approximately 6 days.

Source: Ontario Ministry of Agriculture, Food and Rural Affairs. (2007) “Anaerobic Digestion Basics.” Queen’s Printer for Ontario. < http://www.omafra.gov.on.ca/english/engineer/facts/07-057.htm#4> [Accessed on September 16, 2011]

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Advantages and Disadvantages of Various Digester Technology

Technology Advantages Disadvantages

Single Stage, Wet Systems ✚ Less expensive material handling equipment

✚ Derived from well developed wastewater technology

✖ Substantial pre-treatment required

✖ High consumption of water and heat

✖ Precipitation of the heavier fraction of the waste to the bottom of the reactor can reduce gas yields

Single Stage, Dry Systems ✚ Higher biomass content (higher gas yields)

✚ Low water usage and heat requirement

✚ Less expensive pre-treatment stage and simpler design in contrast to other AD technologies

✖ Expensive waste handling equipment required

✖ Challenges associated with conveying dry waste

✖ Not appropriate for a wet waste stream

Multi-Stage Systems ✚ Operational flexibility

✚ Higher throughput, smaller footprint

✚ Higher loading rate

✚ Can tolerate fluctuations in loading rate and feed composition

✖ Larger capital investment

✖ More complex design and material handling

Batch Systems ✚ Low cost

✚ Simplified material handling

✚ Reduced pre-sorting and treatment requirements

✖ Variable gas production

✖ Less complete degradation of organics