From Open-Air Composting to Efficient Anaerobic Digestion

by Jim McMahon (Biomass Magazine) After 35 years of open-air composting, Zurich’s recently-opened anaerobic digestion facility can process 25,000 tons of organic municipal solid waste annually, producing biogas for power generation and high-grade organic fertilizer. -- Since 1978, the city of Zurich, Switzerland has collected green waste through its recycling and waste management unit, ERZ Entsorgung+Recycling Zurich, and processed it into compost at its Werdholzli facility in western Zurich, adjacent to the city’s wastewater treatment plant along the Limmat River. The source-segregated organic municipal solid waste (MSW) is collected from the city’s 400,000 residents, businesses and from surrounding communities. It includes waste from households such as tree and lawn cuttings, and organic kitchen waste like fruits, vegetables and foodstuffs, as well as organic wastes from restaurants and food processors. Until 2013, up to 25,000 tons of organic waste was collected annually and open-air composted at the facility, which created ongoing concerns from local urban residents because of odors.

In 2011, Biogas Zürich AG was commissioned by the Zurich City Council, with the mandate to decommission the open-air compost yard at Werdholzli, and recycle the organic waste collected from the city and outlying communities of Zurich. This would be done by constructing an enclosed anaerobic digestion process to convert the organic waste into organic high-grade fertilizer, and produce environmentally-friendly biogas to supply the city with carbon-neutral energy, in support of the 2000-Watt Society vision.

Engineering a Solution
“The plant for organic waste digestion opened in 2013 to provide environmentally friendly organic waste management, which includes production of high-quality compost, fertilizer and the recovery of biogas as an energy source,” says Helmut Vetter, managing director, Biogas Zürich AG. “The objective was to contribute to the sustainable development of the region.”

Biogas can be directly combusted, such as in a combined-heat-and-power plant, thereby generating environmentally friendly electricity and heat, which is typically fed into the public power supply or local heating network. Alternatively, the biogas can also be processed and upgraded for use as CO2-neutral vehicle fuel, or a natural gas replacement and fed into the gas grid.

Uniquely to other processes, the digester creates an extremely efficient microorganism environment, which separates and optimizes the different steps of biomass degradation throughout the process, making for very stable microbiology inside the reactor. “The process recirculates approximately one-third of the digestate, rich in thermophilic microorganisms, from the output and back upfront into the digester to activate and accelerate the anaerobic digestion process of the fresh material fed into the digester,” Burri adds. “This allows a perfect adjustment of the hydrolysis and acidogenesis rate in the digester feeding section and facilitates high bioprocessing efficiency within the system.

“Conversely, with conventional anaerobic digestion, the key steps of hydrolysis, acidogenesis, acetogenesis and methanogenesis are completely mixed,” Burri continues. “Each step takes place at the same time and at the same place in the digester. This does not permit optimum conditions for the microorganisms to digest the organic material.”

Containment of Odors
A vital component of this project was containing the release of odors into the surrounding environment. The design of the plant ensured odors would be adequately contained within the facility. The digester itself is a completely closed system, as the process operates under anaerobic conditions. Therefore, no emissions are released into the surrounding environment by the digestion plant. To prevent the air from penetrating into the environment both the treatment hall and the composting hall are kept within enclosed structures and slightly underpressurized. All air from the buildings is treated by an acid scrubber and then led though a biofilter.

Biogas
The raw biogas from both digestion plants (biowaste and sludge digesters) is transported by pipeline to an upgrading station. By removal of trace gases hydrogen sulfide and CO2, the biogas is transformed into biomethane, which is injected into the natural gas grid. The upgrading is achieved by an amine scrubber technology, which guarantees a very low methane slip (0.1 %) and low electricity demand. “Utilizing a single upgrading facility for the two biogas plants brings a substantial financial advantage by economy of scale,” Vetter explains. “The biogas transformation system is therefore not only environmentally friendly, but also a financially viable solution.” READ MORE