A key waste management challenge for any tourist destination with ambitious environmental and climate goals is how to deal with seasonal fluctuations in volume and composition. For the Austrian “climatic spa” town of Zell am See, part of the solution is its novel organic waste and sewage sludge treatment facility. Not only does it cover its own energy needs but it generates revenue by supplying external consumers with biogas and heat.
With its spectacularly scenic location on the shore of Lake See surrounded by the Austrian Alps, it is easy to see why the town of Zell am See together with its neighbouring municipalities in the Salzburg region, is a popular tourist destination. Its key assets, crisp clean mountain air, and crystal clear water, classed as potable, have earned Zell am See the term “climatic spa.”
Whilst the town has a population of just shy of 10 000, the region attracts some 2 million overnight guests annually according to tourism statistics. This seasonally variable influx generates an additional strain on the waste management regime not least the treatment of wastewater and organic fractions.
To deal with the challenge, a new treatment facility, Biogas ZEMKA GmbH, was built adjacent to the existing mechanical biological treatment (MBT) plant sited at the edge of the Hohe Tauren National Park at an altitude of about 800 m above sea level.
On an annual basis, the biogas plant produces around 2 million Nm³ of biogas with a methane content of about 60 percent. In energy terms approximately 14 GWh.
Joint project
Commissioned in early 2015 the EUR 11.6 million biogas project was built by a consortium consisting of BTA International GmbH, a wholly-owned subsidiary of German biogas technology suppliers Agraferm Technologies AG together with the Austrian construction management company Machowetz & Partner Baumanagement GmbH.
The former was responsible for the design, build, and start-up of waste reception and pre-treatment process, anaerobic digestion, biogas treatment, solid-liquid separation, and process water management. The latter took charge of the civil construction and the electrotechnical installations as well as the excess water treatment.
Multi-feedstock reception
The novel plant is characterised by high substrate flexibility and biogas valorisation with a capacity to treat up to 18 000 tonnes per annum of organic waste. This includes waste streams that could not be processed in the existing composting plant due to high water content. The bulk of the feedstock, about 8 000 tonnes, comes from regional household biowaste collection.
The facility receives around 2 500 tonnes of kitchen and food waste from hotels and restaurants, 4 500 tonnes of sewage sludge, 2 000 tonnes of fats, oils, and grease (FOG) as well as around 1 000 tonnes of liquid food processing industry waste.
The material is delivered by truck, tanker, and in various container sizes. As a result, the plant has several reception- and pre-treatment options including container emptying and washing prior to the wet digestion process.
Hydromechanical pre-treatment
Whilst liquid substrates and FOG can be pumped to intermediary suspension storage tanks after screening and mashing, other materials such as household biowaste and catering waste from hotels need pre-treatment for size reduction and to remove contaminants from the fermentable organic fraction.
A core component of the plant is the BTA two-step hydromechanical pre-treatment system.
The wet pre-treatment system enables the very efficient removal of all these impurities in the organic residues yet the very selective removal of the same. This means that our loss of digestible organics through rejects is very low, said Stephan Schulte, BTA International.
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