Composting Process
• 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
Overview of Technologies
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.
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 specialized windrow
turner.
• This is a common method for composting,
particularly for leaf and yard wastes
3) Enclosed Channel
• channels.
• Mixing of organic waste is provided with a specialized
automated turner.
• 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.).
4) In-Vessel
• composting takes place in a purpose built container
or tunnel.
• one or a number of containers.
• 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.)
Mechanical Biological
Treatment Process
• Mechanical Biological Treatment (MBT) is a generic
term for a range of processes that may be used to treat
residual waste using a combination of mechanical
separation and biological treatment.
• This commonly comprises three stages:
1. Mechanical Stage – mechanical size reduction of waste
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.
MBT Processes Schematic
4 MBT technologies are :
1) MBT with Aerobic Composting
2) MBT with Biostabilization
• 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 produces a ‘biogas’ which is rich in methane
and can be used to generate energy.
4) MBT with Refuse-Derived Fuel (RDF) Production
• 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.
Anaerobic Digestion Process
• The AD process typically includes the following stages:
1. Pre-processing – removing recyclables and/or contaminant fractions
prior to digestion, typically 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. 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
AD technologies can be categorized
into three system components.
1. 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.
• This process requires the introduction of
significant quantities of diluting water.
• The process is also challenged by the
precipitation of the heavier fraction of the
waste to the bottom of the reactor
• 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.
• 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.
3. Mesophilic and Thermophilic Systems
• Regulating the temperature inside the
digestion reactor.