Jim McMahon
Nestled in the steep, sunny slopes of the northern Italian region of Trentino-Alto Adige is the small alpine municipality of Faedo. This quaint village of 500 residents, not far from the borders of Austria and Switzerland in the Italian Alps, is known for the production of apples, grapes and other fruits and vegetables. But Faedo is also known for having Italy’s first liquid digestate-free dry fermentation plant for processing of biowaste into high-grade fertilizer and biogas. In light of the specific regional, geological and topographic features of the area, the use of liquid digestate as a fertilizer is prohibited. However, farmers can use dry, fresh compost for fertilizer and soil amendment.
To supply this fertilizer, organic waste is collected from the surrounding area, consisting primarily of food and kitchen waste. The above-average organic waste and a low green waste fraction means this substrate is particularly moist and lacking in structure, but at the same time extremely rich in energy. Integrating Hitachi Zosen Inova’s Kompogas® dry anaerobic digestion technology with Cesaro Mac Import’s II Girasole® tunnel composting system, this substrate is converted into solid fresh, high-grade compost, and biogas. The process is performed without residual process water, and with no resultant liquid digestate.
Optimizing Compost Quality and Biogas Production
The plant, which opened in 2012, is owned and operated by Bioenergia Trentino SRL. It is a role model for other similar plants, in that it combines Kompogas dry anaerobic digestion with Cesaro Mac Import’s II Girasole tunnel composting, in a two-phase integrated process. Raw substrate is input on the front end of the system, and high-grade dry compost is produced after being processed through the two systems.
“The Kompogas section of the plant is particularly interesting in that it processes the organic solid waste at a thermophilic temperature of 131° F (55° C),” said Markus Burri, Process Engineer with HZI. “The higher processing temperature ensures that spores and bacteria are eliminated, completely sanitizing the organic matter during processing. And also, the higher temperature permits the biogas potential to be fully exploited by the time the material comes out of the digester, in effect, extracting a higher volume of carbon-neutral biogas.”
Phase 1: Kompogas® Continuous Dry Anaerobic Digestion
The Kompogas continuous dry anaerobic digestion process is based on the dry digestion of solid biowastes in an oxygen-free environment. Anaerobic digestion is a multi-step biological process with four fundamental steps that include hydrolysis, acidogenesis, acetogenesis and methanogenesis. Throughout this entire process, large organic polymers that make up biomass are broken down into smaller molecules by microorganisms. Upon completion of the anaerobic digestion process, the biomass is converted into biogas, as well as into solid digestate.
Biogas is composed primarily of methane (CH4), carbon dioxide (CO2), with trace amounts of hydrogen sulfide (H2S), and ammonia (NH3). Typically, biogas is saturated with water vapor and may have trace amounts of hydrogen (H2), nitrogen (N2), and oxygen (O2). Biogas can be directly combusted – such as in a combined heat and power (CHP) 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.
Digestate is the remaining solid residual from the dry anaerobic digestion process. The anaerobic digestion process recovers and recycles the nutrients contained in this organic material. From the produced digestate, top-grade solid compost is produced. “TheKompogas process uses a horizontal plug-flow digester,” added Burri. “The organic material is transported inside the digester, with the material moving horizontally through the system by feeding on the inlet side and discharging on the outlet side. A slowly-turning agitator ensures that the substrate is optimally mixed within the digester, and the biogas bubbles are permitted to vent for high-yield formation of methane. This facilitates the biological strength of the Kompogas anaerobic environment, enabling it to make maximum use of the organic waste’s energy potential.”
Uniquely different from other anaerobic digestion processes, the Kompogas digester creates an extremely efficient microorganism environment, which separates and optimizes the different steps of biomass degradation throughout the process, much better than conventional anaerobic digestion, 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. This allows a perfect adjustment of the hydrolysis/acidogensis rate in the digester feeding section, and facilitates high bioprocessing efficiency within the system. Conversely, with other fully-mixed anaerobic digestion, the key steps of hydrolysis, acidogenesis, acetogenesis and methanogenesis are completely mixed. 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.
“A specially developed heating system regulates the temperature during processing at 131° F (55° C) during a retention period of approximately 14 days,” continued Burri. “Because the digestate is completely sanitized, the system does not require any upstream pasteurization, as is the case with other anaerobic digestion solutions.”
Kompogas Process Steps
Material Reception, Pre-Treatment and Feeding – The organic waste and green waste delivered to the plant is tipped in the reception bunker. All handling of organic materials takes place in closed and ventilated rooms, thereby minimizing odor emissions to acceptable levels. Automatic roll doors allow trucks to enter the facility and close immediately upon safe entry. From here, the material is fed into the processing area using a fully automated crane. The material is pre-processed through a shredder and followed by a star screen that removes contaminants such as plastic, paper, ferromagnetic particles and other non-organic items. The material is screened to pieces of approximately 2 inches in size. The pre-treated material is then fed to the digester via belt conveyors. One of the belt conveyor is equipped with a scale to monitor the amount of material fed into the digester.
Continuous Digestion – The continuously-fed, horizontal PF1300 plug-flow, dual-digester has a capacity of processing 44,000 tons of waste annually. It is double-walled concrete construction, with a water- and gas-tight steel line, and inner dimensions of 130 feet length x 28 feet diameter. A slowly-turning agitator, structurally designed to withstand the highest torque loads and enable smooth restarts after short interruptions, mixes and degases the substrate optimally at low-drive power with plow-shaped blades to prevent sedimentation.
Phase 2: Cesaro Mac Import’s II Girasole®
Tunnel Composting Process Steps
Mixing – Unlike standard methods which use a dewatering press to separate the digestate into solid and liquid fractions following fermentation in the digester, in this process the digestate is mixed with shredded green waste and coarsely-structured sieved fraction from the composting process.
Composting Tunnel – The mixture then undergoes an intense two-week, closed-tunnel composting process. This aerobic treatment substantially increases the dry material content by evaporating water due to self-heating of the biological activity. The inflated floor in the tunnel facilitates air penetration into the digestate material. The tunnel reutilizes the air inside the plant through recycling, reducing the total amount of air to be treated for release to environment. Leachates are collected in the floor’s openings. Nozzles on the ceiling spray the recycled leachates on the digestate material. Spraying occurs during well-defined phases of the process. The tunnel system’s parameters are constantly monitored and measured. Probes and sensors measure pressure, temperature and humidity to adjust the flow of the process.
Intermediate Screening – The material loses approximately 25 percent of the weight at the end of the composting tunnel cycle. Screening removes bulky material from the flow that is recycled in the mixing phase. The intermediate screening saves storage space and facilitates later stages.
Maturation – The maturation process is completed via an aerated floor. The material matures in 25 – 35 days. The volume of material is substantially reduced due to previous steps.
Final Refining – Final screening fine combs the material, making the end product an excellent compost and soil for use in agriculture – like for use in the surrounding orchards and vineyards, horticulture, landscaping and home use.
Biogas Pre-Treatment and Power Generation
“Raw biogas from the digester is first desulfurized and then dewatered to an acceptable level for subsequent combined heat and power (CHP) biogas utilization,” explained Burri. “The biogas is analyzed for its content of methane, carbon dioxide, oxygen and hydrogen sulfide.”
The pre-treated biogas is then led to a CHP unit. The Kompogas plant is designed to produce 4,000,000 Nm3/a biogas annually, enough to generate 1 MWel continuously. However, due to the high energy potential of the feedstock, the biogas production is exceeding expectations. Consequently, the plant is reviewing possible installation of a biogas upgrade solution.
Odor Containment
The plant’s process and composting buildings are always maintained under negative pressure (suction). The system contains waste odors to the inside of the buildings, providing an odor-free environment for the surrounding area. Ventilation ducting ensures adequate air circulation inside the buildings, while the waste air is treated by a scrubber and bio-filters.