BioCycle September 2010, Vol. 51, No. 9, p. 50
The core concepts of a continuous stirred reactor and an upflow anaerobic sludge blanket are combined to process about 105,000 metric tons/year at a facility near Venice, Italy.
Gertrud Aichberger and Klaus Ruhmer

DUE to favorable legislation and a subsidy structure all across Europe, many anaerobic digestion (AD) plants have been built there over the years. The majority was designed for use of manure or biosolids. In today’s market, however, modern AD plants have to handle more complex substrates (feedstock) and varying volumes of these streams. As a result, demands placed on AD technology in terms of reliability, stability and robustness are significant and ever increasing.
The United States produces several hundred million tons of MSW annually. About 20 percent of the MSW volume would be suitable for anaerobic digestion, yielding enough biogas to supply over 2 million homes with renewable energy. In addition to MSW, incredible amounts of industrial organic waste streams, many high in biomethane potential, are available, e.g., from food processing operations, breweries, meat processors, agricultural operations, ethanol plants, etc.
As the waste streams being treated via anaerobic digestion get more complex, impurities and the varying content of lipids, proteins and carbohydrates can cause problems. As a result, a pretreatment step involving removal of impurities and particle size reduction may be needed to allow stable digestion performance. One method of analyzing substrates and the treatment options is to measure the organic content via their COD concentration – the total quantity of oxygen required for oxidation to carbon dioxide and water.
Different types of AD technologies are suitable for different COD concentrations. For example, UASB (upflow anaerobic sludge blanket), EGSB (expanded granular sludge blanket) or IC (internal circulation) digesters are especially suited for low COD concentrations at less than 25 g O2/l in the substrate.
CSTR (completely stirred tank reactor) or PFTR (plug flow tank reactor) systems are used to treat substrates with high levels of fats, lipids and particles and therefore, with high COD concentration. However, these systems operate at very low loading rates to ensure complete anaerobic digestion.
The COD loading rate is the daily quantity of organic matter, expressed in COD, which can be fed into the system per cubic meter of digester volume. The most common unit is kg COD/m3/d (Chemical Oxygen Demand per cubic meter and day). Finding a way to combine the volume capabilities of the UASB approach with the high COD capability of the CSTR reactor design has always been “the holy grail” of the AD community.

LARGE-SCALE DIGESTER OPERATION
A large-scale organic waste processing facility about 20 miles from Venice, Italy receives well over 300,000 metric tons of organic waste from a densely populated area. Over half of the waste goes directly to composting; about 140,000 metric tons are processed by the enbasys BioGas process line and digester system – a High Load Hybrid Reactor technology (ENBAFERM). This is the first industrial-scale biogas plant based on this technology, which combines the core concepts of CSTR and UASB fermenters into a two-phase anaerobic digester system, accommodating hydrolysis/acidification and methanogenesis in two separate process steps.
The facility, operated by SESA, began operating in 2005. It has a sophisticated waste receiving hall with truck washing, various waste receiving bunkers, food waste depackaging equipment (centrifugal separation) and maceration. Front-loaders are used to move waste. Operators have been trained to inspect the waste streams and make decisions regarding further processing. The building is kept at a negative pressure and all exchanged air is treated with biofilters prior to release. After pretreatment of the waste feedstock (substrate) including separation of nondigestable materials such as glass, metal and plastic, approximately 105,000 metric tons are actually being digested.
The enbasys BioGas system in Italy consists of two 2,900 m3 fermenters operating in parallel. Based on the processed waste volume, the loading rate ranges from 10 to 13 kg COD/m3/d. The system automatically regulates its feed-in and pumping rates based on composition of the loads coming in. Seasonally, the feedstock streams vary quite a bit.
The hydrolysis phase is relatively short, and can also be used as a buffer. At the SESA plant, the feedstock pretreatment plant operates Monday through Saturday, 12 hours/ day. The hydrolysis tank ensures a constant feeding rate of the digesters even during those times when the feedstock pretreatment is not operating.

BIOGAS GENERATION
The produced biogas – 1450 N m3/h – is used to run a combined heat and power (CHP) plant rated at approximately 3.4 MW. It consists of three GE Jenbacher 1.4MW co-gen units running at all times. A fourth unit is installed for redundancy purposes and to carry the load during routine maintenance tasks on the other gas engines (e.g. oil changes, etc.). Prior to the generators, the biogas is desulpherized and dried to meet the quality specifications for the gas engines. Thermal energy generated by the co-gen units is utilized within the facility and for the local district heating system.
Last but not least, the plant also incorporates an effluent treatment module. Solids are mechanically separated from water using centrifugal separators. The water is aerobically treated via enbasys’ ENBAAIR technology, a combination of biological and membrane based processes (ultrafiltration, reverse osmosis). The cleaned and desalinated water is subsequently reused or discharged depending on current water demand at the facility. Solids are transferred to the composting operation and ultimately turned into valuable by-products.
Just like in many other areas around the globe, lack of landfill space and waste diversion away from landfills are the biggest drivers for these types of facilities in Europe. In recent years, Italy has adopted strict laws related to waste issues. Tipping fees vary but often lay between Euro 40-60 (US$55-80). In addition, feed-in tariffs for electricity generated from renewable sources are established well above current power retail values. This combination of tipping fees and feed-in tariffs made the SESA plant very profitable, leading to an accelerated payback of the original investment.

Gertrud Aichberger is Project Manager at enbasys gmbh, a subsidiary of BDI-BioEnergy based in Graz/Grambach, Austria (Gertrud.aichberger@enbasys.com). Klaus Ruhmer is Business Development Manager at BDI-BioEnergy & enbasys (Klaus.ruhmer@bdi-bioenergy.com). For additional details on the anaerobic digestion/composting facility profiled in this article, contact Mr. Ruhmer.