Adding AD To Existing Composting Site Footprint

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Courtesy of BioCycle Magazine

About an hour’s drive west of Munich in southern Germany, the City of Augsburg had been served by an indoor composting facility since 1994. It was operated by Abfallverwertung Augsburg (AVA), three quarters of which was initially owned by the City of Augsburg and the districts of Aichach-Friedberg and Augsburg. Until recently, the remaining quarter was owned by a private company but as of 2015, the facility is under full public ownership.

The enclosed composting operation utilized a travelling bucket-type turner (Wendelin Komposting from Bühler). However, it could only handle yard trimmings and was limited in its potential to be adapted to an input stream including food waste. Overall the facility was a substantial net energy consumer. In 2012, the AVA Supervisory Board passed a resolution to modernize the site for the future by building a new facility for both anaerobic digestion and composting. The decision was driven by the need to receive wetter materials and reduce energy consumption — or better still, produce energy.

Because of site limitations, the new facility had to be built within the existing footprint of the old composting site with a minimum break in service, and was expected to use as much of the existing facility as possible. All of the old technology was stripped out and the buildings remodelled to optimize process flow and meet engineering requirements of a combined AD and composting process. At the same time the new site had to allow for future expansion and increases in tonnage. The contract for redevelopment of the site and the AD and composting technologies was awarded to Thöni Industriebetriebe GmbH for the installation of a thermophilic plug flow “dry” digester and in- vessel composting (IVC) system.

Construction of the new facility started in February 2013 while the composting unit was still receiving waste. Composting operations and construction continued in parallel until May 2014 and then there was a four-month break in waste reception. Commissioning of the AD plant started at the beginning of October and by December 2014 the site was in full operation once again.

The process flow is illustrated in Figure 1. Yard trimmings and comingled food waste come from the City of Augsburg and the districts of Augsburg and Aichach-Friedberg. They can be mixed with landscape waste, which is delivered by private traders and parks maintenance teams to the site. The materials are first treated in the digesters before composting and therefore undergo a two-step hygienization process — first in the thermophilic AD and second in the enclosed composting operation. The site is licensed to take up to 61,000 tons/year. The facility uses a double Thöni TTV horizontal plug flow digester configuration in which two digesters are structurally linked but are operated totally independently. Each unit has an active volume capacity of 1,650 cubic meters (58,000 cubic feet). The process operates in the thermophilic range, maximizing process rate, hygiene and biogas yield.

Incoming biowaste is shredded, followed by separation into fine and coarse fractions using a linked star screen. The coarse fraction (> 80 mm; about 3 inches) mainly consists of plastic films, which although discouraged, are still a significant issue. This fraction is delivered to the neighboring waste incineration plant for combustion. In turn, the incinerator provides heat for the AD and composting site. The incinerator receives garbage and material otherwise unsuitable for reprocessing or recycling.

The fine fraction (< 80 mm) is passed through a magnetic separator and conveyed to fully automated Thöni storage/feeders (3 x 200 cubic meters). Packaged food waste is not accepted at the plant so there is no need for a separate depackaging line. Overall, the pretreatment equipment is kept deliberately simple to maximize throughput and minimize stoppage time.

The computer control system links to a series of load cells and level sensors to monitor the availability of input material in the feed train so that the AD system can be continuously charged 24/7 every day of the year. The feeders dose the digestible material into a premixer where it is blended with press water produced by the Thöni screw presses at the discharge end of the digesters. This high-solids viscous mix with a dry solids content of about 39 percent is then force-fed into the digester using hydraulic pumps. Each has a longitudinal paddle shaft that slowly rotates. This ensures the digestion process is gentle without end-to-end mixing so that the biochemical digestion steps can take place sequentially as the mass moves slowly from the input to the outlet end.

The digesters, which are located outside, are large static vessels, approximately 115 feet in length that sit on a steel base within an insulated concrete cradle. The entire digester is then insulated and clad to match local buildings as required by planners. This design allows heat distribution and monitoring systems to be installed in the cradle zone below the digester body, guaranteeing a frost-free area without the need of a separate building. The internal area of the digester incorporates floor- and cross-heating zones linked to the computer-controlled heating system. This maintains the thermophilic process at around 54 °C (129.2°F), accelerating bioconversion on the one hand while hygienically treating the biological waste within the digesters on the other.

Read the full article in BioCycle Magazine

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