AP Business & Technology Consultancy (APBTC)

Tunnel Composting

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In the presence of oxygen (air), the biological degradation process is referred to as aerobic composting (in contrast to anaerobic fermentation in the absence of oxygen). The composting process, which takes several weeks to months in the natural environment, can be significantly accelerated through the forced supply of air to the process. Bacteria and fungi, which rely on oxygen and the organic matter as food source, convert the organic matter in a sequence of decomposition stages into the final end products carbon dioxide, water and humus.

Depending on the organic waste composition and the level of ventilation, other - non-desirable - products may be formed such as ammonia, hydrogen sulphur, and methane. In particular lack of fresh air can cause anaerobic conditions in parts of the degrading material (so-called anaerobic spots). The formation of those to some degree toxic products should therefore be avoided.

The decomposition process generates also heat as by-product of the microbiological activity, which leads to temperature increase in the material. The microbiological activity then slowly declines due to depletion of easily available food (so-called easily decomposable, volatile compounds). The decomposition of less easy decomposable material takes more time and ideally runs at a constant temperature of around 50 °C.

Water is generated during the process through various mechanisms. Firstly, the microbacteria produce water as metabolism by-product, secondly, the cell water of the organic matter is released with the destruction of the cell during decomposition.

High temperature in combination with high moisture content stimulates evaporation; some water runs off the material as ‘leachate’ with characteristically high organic loadingIf the material turns too dry, the microbiological activity slows down and can even come to a standstill; moisture is therefore important to maintain biological activity.

Areas of Application:

  • Green & Food Wastes
  • MSW (organic fraction)
  • Biosolids
  • Agricultural Residuals

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Per definition, tunnel composting refers to a static biological process with forced aeration in contrast to dynamic systems, which either batch-wise or continuously agitate or rotate the material for processing. Both systems aim at accelerating the biological process whilst providing a maximum of control.

The tunnel principle applies air as the only medium to control the decomposition process. However, in order to actively stimulate biological activity and manipulate material moisture and temperature progression substantial air volumes (ie high aeration rates) together with an ‘intelligent’ air management are required. This characteristic distinguishes the APBTC tunnel composting system from other in-vessel technologies, which also use air for the process in the one or other form.

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The APBTC tunnel composting plant comprises a number of tunnels - parallel-arranged, fully end ‘garage type’ concrete built structures of approx 20 - 35 m length, 5-8m width and 5-6 m height.

The system is highly modular, and can be extended in stages to accommodate for quantity increase over the life of the plant. A residence time between 7 and 30 days can be selected depending on feedstock material requirements and desired degree of product maturity.

The fully sealed nature of the system protects the surrounding building structure, without risk of corrosion, fogging or excessive condensation, thereby extending the life of the building.

The internal air and process water recycling systems reduce the total air discharge into the deodorisation unit as well as process water volumes which – in most cases can be fully recycled back into the process. The process usually shows a net water demand over the year.

The system is operationally flexible since composting takes place in discrete batches. Different grades of compost can be created simultaneously with different feedstock. With the tunnel system each tunnel load can be treated independently, making it possible to adapt process parameters for optimal composting when deviations in the waste occur.

The objectives modularity and distinct treatment were achieved through the design of an independent air circulation and fan for each tunnel, which provides the flexibility to adjust the airflow according to feedstock characteristic and degradation process progression.

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Each tunnel unit is self-operating and comprises an air ducting system, blowers, process water collection and recycling system, and various process control features (temperature, pressure, fresh air ratio measuring devices etc.).

Each tunnel is equipped with a fan, which blows a mixture of fresh air and recycled air through the trenches into the tunnel. At the same time surplus exhaust air is being discharged to the deodorisation stage.

The applied pressurised aeration results in a much more homogeneous and thorough material aeration profile especially for more critical feedstock. There are virtually no anaerobic zones in the composting matrix. As anaerobic zones are responsible for most of the odours originating from the waste, the process air coming directly out of a tunnel contains very low odour concentrations.

Under optimal process conditions, the micro-biological decomposition process accelerates in the tunnel and delivers a finished product after 20- 25 days, comparable to 12-14 weeks windrow composting.

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The process does not employ machinery or equipment in the corrosive environment of composting thus significantly reducing maintenance and repair costs and extending the life of the plant (in contrast to e.g. systems, which employ agitators).

The tunnel aeration floor comprises a concrete slab with a purpose designed aeration duct or vent system integrated into the slab.

Compost material is placed into each tunnel individually and removed after a given composting time link by means of a front-end loader.

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Access to each tunnel is by means of a front-end loader via the front door, which can be lifted automatically. During the composting process, the door is locked hermetically to prevent any odour and leachate from discharge into the environment.

During the composting process there is no need to access the tunnels, thus creating an air tight environment with all process air being collected and either recycled back into the process or cleaned via the biofilter (ie odour free).

In order to keep the operation simple and reliable, we have minimised the reliance on ‘over-engineered’ solutions such as automated, conveyor based feeding systems and alike.

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The decision to incorporate a central process control system with visualisation PC and data recording and processing capabilities were driven by the desire to minimise staffing requirements, to record and document all process data and to gain a maximum of remote control.

The proprietary developed process control system also acquires and records the process data and enables the verification of processing time, temperature progression and other data for each material batches required for local compost quality standard compliance & certification.

The fully automatic operating controlling system which is usually located in the administration office or a separate control room can be also accessed via modem to allow for remote supervision and control of the composting process.

The computer records and stores all instantaneous measurements and cumulative data for each tunnel process run. During the process many different parameters can be shown on the computer screen. The user can create graphic displays to show the desired parameters. This facilitates rapid review, evaluation and adjustments (if necessary) of the composting process.

The constant monitoring of process data, data acquisition (SCADA) including tools such as data evaluation & visual presentation enables the operator to on-going review the process performance and optimise process parameters in terms of energy efficiency, product quality and other environmental criteria.

Regular SCADA software updates provide the operator with the latest developments in order to improve operating monitoring features and control systems over the life of the project.

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For each project, the overall performance requirements for the BioDry process design are a combination of the following criteria:

The natural composting (ie rotting) process always shows a similar temperature pattern, which can be best defined by a sharp temperature increase often beyond 70ºC during the first days due to biological activity built-up and subsequent a gradual temperature decline over several weeks.

The initial temperature peak length depends on nutrient availability, biological activity, the cooling effect through ventilation, condensation and other criteria. Neither the excessively high temperature during the initial composting phase nor a steady decline are desired effects for an efficient process. However as a desired result of the high temperature at the beginning of the process, most of the pathogens are killed, others are inactivated. The same applies for seeds and weeds and insects contained in the material.

The tunnel composting process is therefore designed to incorporate the essentials of the natural progression whilst shortening the composting time from several weeks to several days.

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The tunnel composting process is at the core of a custom designed resource recovery process, which receives organic raw materials from domestic, commercial and other sources in order to produce high quality compost & fertiliser products for different applications.

Pre treatment is crucial to the performance of the system and therefore has to be designed with great care. Depending on feedstock sources and quality this involves storage, shredding, and removal of undesired contamination.

Obligatory for the process is the final thorough mixing in a plug type or equivalent mixer. The prepared raw material can be filled into the tunnel either automatically (via conveyors) or by means of a front-end loader. A multi-stream plant usually employs a combination of shredder, magnet, manual sorting, and mixer as front-end process.

The tunnel composting period can be sub-divided into two or three composting ‘sessions’ which are interrupted by a so-called dynamic interim step. This treatment step involves material unloading, mixing, and reloading into the tunnel for the next session. This option adds flexibility to the operation and widens the spectrum of materials suitable for treatment.

Post treatment involves further curing or maturation depending on the tunnel residence time and final product refinement and storage. Refinement usually includes a trommel screening plant to produce a variety of compost & fertiliser grades and may require further contamination removal by means of wind shifters and alike.

For each project, the overall performance requirements for the tunnel composting process design are a combination of the following criteria:

  • Guaranteed product disinfection (pathogen reduction)
    • Active control of the biological process conditions and acceleration of the degradation process through the ability to adjust.
    • Moisture content
    • Oxygen level
    • Temperature progression
  • Ability of distinct treatment of individual material batches
  • System modularity (to allow for extension and provide contingency)
  • AS 4454 compliance (plant toxicity, NDI, pH, moisture etc.)
  • Active nitrogen immobilisation (increased market value)
  • Reliable environmental performance (zero odour, process water, particle emissions)

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