Biological waste treatment Composting in encapsulated systems (bay, box, tunnel, in-vessel composting)
Key word: biological, waste treatment, composting, encapsulated systems, degrades/converts organic material to CO2 , biowaste
Abstract
Composting in encapsulated systems follows the general objectives of open windrow composting but with the advantage of :
- accelerating the rotting process (and thus increase throughput/save space),
- improving process control and thus product quality, and
- taking better control over emissions and thus reduce potential nuisances
Composting is an aerobic process that by definition requires oxygen and bio-logically degrades/converts organic material to CO2, water and humic substances.
Composting in encapsulated systems means the composting of biodegradable waste in a closed reactor with minimised thermal exchange with the atmosphere and relies on various methods of aeration and mechanical turning to control the process. These mechanical systems are designed to minimize odours and process time by controlling airflow, temperature, and oxygen concentration. Closed systems make it possible to collect gaseous emissions, odors and particulates. The active aeration, watering and mixing functions enable control and optimization of the rotting process, thus considerably accelerating the main biodegradation phase.
Composting in encapsulated systems is more strictly divided into a prerotting and a secondary rotting or maturing stage.
Basic requirements
Input must be source separated biowaste without hazardous components.
A C/N ratio ranging from 25/1 to 30/1 is described as the optimum for a fast composting process, but higher ratios up to 40/1 may be possible. Overloads of nitrogen in the input material must be avoided since almost the entire nitrogen fixed in the organic material is going to be released as ammonium thru micro-biological activities. High concentrations of ammonium at a pH>7 can cause the emission of ammonia. To kill pathogens and weed seeds in the compost material, the process must ensure temperatures of 60°C in minimum for at least one week [22].
Expected results
Output:
- Compost (humus-like product)
- Residues and disturbing components
- liquor and bio gases
Quality requirements for the output:
- Mature compost should meet the following parameters to ensure that it is stable: -a C/N ratio of less than 22 to be safe for agricultural use
-not re-heat over 20 °C upon standing
-reduced volume of raw organic material by at least 60 %.
- Aside from that international compost standards (such as described from the Federal Association for Quality Assurance of Compost in Germany-BGK) should be met
- The liquor collected from composting shall comply with the requirements of Directive 91/271/EEC before being released into surface water or otherwise be suitably treated.
Specific advantages
Allows for a high rate of waste diversion from final disposal and enhances other waste treatment operations by removing organic matter from the waste stream
Quite easy handling and more reliable composting results
Relatively short process duration and less space demanding
Drastically reduced nuisance potential
Well investigated and widely dispersed technology
Generally well accepted
Specific disadvantages
Generally more capital intensive than simple windrow composting
Requires separate collection of biowaste
High quality demands can pose problems to the marketing of the compost
Increased operation demands (required equipment, energy, etc.)
Application details
1. Technical scheme
Biological processes can only treat the biodegradable fraction of the MSW, which means that the waste should be source separate, as much as possible free of disturbing materials and kept away from materials that could release potentially hazardous substances when being in contact with the biowaste [1].
A mechanical pre-tretament before the composting can further improve the quality of the input but not ensure the generation of a waste fraction from mixed household waste that is suitable to meet high standard composting requirements Mechanical pre-treatment can consist of the steps:
- Separation of foreign matter and contaminants
- Size reduction
- Metal separation
as being described for the mechanical biological waste treatment.
Mechanical pre-treatment can also be used to attain the optimum structure and C/N ratio in the composting input by combining various organic wastes. For example, leaves (high in carbon, low in nitrogen) can be blended with food waste (high in nitrogen) to balance the C/N ratio. In this way, emissions of ammonia can be minimized right from the beginning of the rotting.
Composting in encapsulated systems has generally lower demands as regards the conditioning of the input by mechanical pretreatment. Good composting conditions are ensured thru more intensified process control and forced aeration, so that the appropriate particle sizes could be approximately 1 cm and lower [5].
The encapsulated composting systems can be divided into two major categories: plug flow and dynamic. A plug flow system operates on the first-in, firstout principle, whereas a dynamic system mixes the material mechanically throughout the process. Box composting and silos are representative of plug flow systems, while rectangular agitated beds and rotating drums which are characteristic of dynamic systems. The initial investment can be high and handling volumes are typically lower than in windrow composting systems.
The completion of the composting process lasts only 2 to 5 weeks for the prerotting, plus 7 to 26 weeks of secondary rotting [22].
Bay composting
In rotting bays the advantages of a closed system are combined with windrow composting methods. In fully automated rotting bays, the organic materials are piled into tabular heaps, force-ventilated, and automatically turned by a turning unit. During the turning process, the material is watered as necessary. The exhaust from the heap is collected and treated in a closed system. In the course of rotting, the waste therefore 'wanders' from the input end to the output end of the bay. From there, it is forwarded to curing in order to become a mature compost forget recomposted [22].
Tunnel reactor
The tunnel reactor works similar to an enclosed bay system. The rotting takes place in fully enclosed tunnels with a walking floor system. The waste is continuously moved through the tunnel where it is aerated and watered with respect to the achieved rotting grade. Exhaust air can be collected and treated.
Rotting boxes/ Rotting containers
The rotting boxes are made out of reinforced concrete or steel. They are operated in a batch mode with a stationary or a driveable perforated bottom. The boxes are supplied with air from the perforated bottom, and exhaust air is sucked from the top of the rotting container for further treatment. Similar to tunnel composting, intensive rotting is completed after 8 to 10 days. The technology is simple and more durable. Composting in rotting boxes requires a thorough mechanical pre-treatment, however. The rotting material tends to dry easily (this effect is specially used for biological stabilisation in the frame of mechancial biological treatment schemes) and thus moisturization is usually necessary for the composting process.
In-vessel composting/composting drums
In-vessel systems using perforated barrels or drums are simple to use and easy to turn. These drums are highly suitable for the pre-rotting as a good homogenization and mechanical disintegration can take place. However several moving parts on it lead to high wear. As such, the drums should be preferrably be used for relatively short time pre-rotting. They require minimal labour, are not weather sensitive, and can be used in urban and public areas. Drum composting is specially suitable for small-scale applications.
2. Quantity aspects
Mass balance:
Input:
- 100% biowaste
Output:
- 2-3% screening residues from input
- 1-2% screening residues from finished compost
- 35-40% finished compost product
(remaining 55-60% equals to the loss of weight as result of the de-composition process evaporated water, gas emissions) [10].
3. Scale of application
The capacity of composting installations is varying vastly. The mean plant size in Germany is one with 14,000 t/a throughput. Minimum throughput can go as low as 300 Mg/a, whereas the upper range of throughput is at 120,000 Mg/a. Tunnel composting ususally has a higher throughput than container composting. Tunnel composting can become economically viable with an input from about 3,000 t per annum.
Single rotting boxes may have capacities between 50 and 250m³.
4. Interoperability
Composting can be a preceeding measure to waste disposal operations but most preferably should be part of an integrated waste management concept which comprise of separate collection and various activities for material recovery and recycling.
Composting in encapsulated systems and windrow composting are often done in combination. Whereas encapsulated systems are best suited for the pre-rotting of the waste material, open windrow composting can be well applied for secondary rotting and maturing.
Operational benchmarks. Resource consumption
The aerobic decomposition generates 0.6-0.8 g water and 25.1 kJ thermal energy per-gram of organic matter.
Intensive rotting systems have an overall energy demand between 15 to 35 kWh/Mg and those with a differentiated control system between 20 to 65 kWh/Mg. Mechanical pre-treatment contributes in average with about one third (10 kWh/Mg) to this [1].
Significant emissions of CO2 and other (greenhouse) gases occur during the biological treatment, however, unlike in incineration or with untreated waste on landfills carbon is to a larger extent also bound for a long term in the stabilised organic material and wont thus get released into the atmosphere.
The demand on labour force depends largely from the capacity of the installation. The demand of an average plant size in Germany is about 10 persons (1 foreman, 6-8 personnel for operations/maintenance, 1 for the receipt/sale).
Integrating a mechanical pre-treatment, especially with manual sorting naturally requires a larger workforce.
The minimum space needed depend from the planned capacity but is generally lower than compared to windrow composting. The specific space requirement for intensive processes amounts to about 0.20.3 m²/Mg*a.
Aftercare measures must be applied on the residues from screening, exhaust air and leachate.
Composting residues may also be used as
- landfill cover
- bio filter
- site remediation and horticultural applications
Useful websites on Biological waste treatment
22. www.juniper.co.uk
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