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Last update: 27/06/2017

 

Dry digestion synergies

Synergies of combining thermal waste treatment and organic dry digestion

 

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No two kinds of waste are the same.

Due to their composition, residual and organic waste require different processes when it comes to their treatment. In particular separately collected organic waste holds the potential for producing high-quality products such as biogas, compost and liquid fertilizer. Modern disposal concepts are characterized by the fact that every residue is treated with the most suitable process so that the individual potentials for the recovery of raw materials are ideally exploited.

Implementing thermal waste treatment and dry digestion plants in the same location offers many potential possibilities for synergies. Various material and energy flows can be united in a meaningful way, which in turn results in additional benefits for both types of plants.

Necessary infrastructure such as weighing devices, road systems, supply facilities, disposal systems and staff may be used for the benefit of both plant types. Frequently, residual waste and separately collected organic waste is already delivered in separate batches.

Placing two plants in close proximity to each other promotes advantageous joint material and energy use. Exhaust air flows, waste water and waste generated when operating dry digestion plants can be disposed of cost-effectively and efficiently in thermal waste treatment plants. What is more, thermal waste treatment plants ensure that digestion plants are reliably supplied with the electricity and heat required for daily operation. In return, use of district heating networks or other heat consumers existing at thermal waste treatment plant sites can be increased by converting gas into electricity in combined heat and power plants without generating additional costs.

Exploiting synergies efficiently

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One plant, where these potentials are already used, is located in the German city of Augsburg.

The Augsburg thermal waste treatment plant treats 225,000 Mg of waste per year. The plant's three combustion lines have a throughput of 12 Mg/h each and are equipped with MARTIN horizontal grates. The energy released during combustion is used to generate electricity and fed into the district heating network.

Since 2013, a dry digestion plant has been operated on the Augsburg thermal waste treatment plant premises. The dry digestion plant can treat 75,000 Mg/a of separately collected organic and garden waste. Every year, the Thöni plug flow digester produces 35 million kWh of natural biogas. The untreated biogas is processed and supplied to the natural gas grid as biomethane. In addition to biomethane, the plant produces high-grade compost and liquid fertilizer.

Employing the operating staff in both plants makes it possible to continuously monitor and support the thermal waste treatment and dry digestion plants around the clock.

The thermal waste treatment plant supplies electricity to the dry digestion plant and produces the heat required to operate the digester. The thermal waste treatment plant's continuous operation and emergency power supply make sure that the dry digestion plant can be operated without interruptions.

Waste water occurring in the dry digestion plant, condensates from the composting hall, undesired materials, screen overflow and residues from compost screening are combusted in the thermal waste treatment plant. The residues' consistent properties make them additionally useful in adjusting the heating value.

Exhaust air from the areas of the dry digestion plant that produce emissions is disposed of in the thermal waste treatment plant via the combustion air. The exhaust air is collected and supplied to the thermal waste treatment plant's refuse pit by means of an underground pipe. Climate-damaging gases, for example methane or dinitrogen oxide, are safely destroyed in this process and odour emissions are effectively reduced.

Combining thermal waste treatment plants and dry digestion plants in one location facilitates the implementation of modern residue recovery centres, which not only ensure up-to-date disposal but also focus on an ideal recovery of recyclables.