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

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. Overview of 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.