Overview
The process of composting municipal organic waste and non hazardous special waste is carried out in the plant. The facility is part of the Centro Integrato Rifiuti (Integrated Waste Centre): a real recovery hub, which also includes a selection system for the recovery of material from the dry fraction of sorted waste. The Voltana composting plant uses an anaerobic aerobic process that recovers waste and produces quality compost, as well as energy from renewable sources. Not only energy from waste but also from the sun. In fact, a photovoltaic system for the production of electricity from 338 kWp is active in the plant.
Forms
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Treatment capacity
Total maximum quantity of waste allowed for recovery operations (R3) is equal to 80,000 tonnes per year consisting of lignocellulosic waste and organic wet waste (the latter up to a maximum of 50,000 tonnes per year). -
Tipologia di rifiuti ammessi
Rifiuti organici a bassa putrescibilità (scarti verdi e lignocellulosici, ecc.) -
Types of waste accepted
The plant treats the following types of organic waste: Slowly decomposable organic waste (green and lignocellulosic waste, etc.) Easily decomposable organic waste (humid organic fraction from separate waste collection, vegetable waste from agro industrial activities, sludge)
The plant features three main treatment lines: production line for High quality Compost (or composted mixed soil improver); production line for wood biomass; The High quality Compost and Biostabilised material production line consists of the following sections and related processes: section for receiving easily decomposable organic waste in dedicated storage boxes; section for grinding the incoming organic waste; section for mixing the ground, easily decomposable organic fraction with digestate extracted from the cell (roughly 50%) and any lignocellulosic fraction (builder). In this section, the cell is opened and the mixture (digestate and builder) is formed; approximately 50% of the extracted digestate is then sent to the aerobic bio oxidation area, while the remaining part is used as inoculum for subsequent loading of the digester; section for anaerobic digestion consisting of 10 cells (digesters), where the digestion process lasts 25 days and produces biogas, which is conveyed to the internal combustion engines generating electricity; section for accelerated aerobic stabilisation in which the oxidation process lasts at least 14 days; section for secondary maturation in which the process lasts approximately 7 days. Overall, the aerobic stabilisation and secondary maturation process must last for at least 21 days; section for final screening and storage of the compost produced; section for treatment of the exhausted air captured from the waste storage and treatment buildings by means of 2 biofilters covering an overall surface of approximately 800 m2. The proposed technology is based on a batch type, single stage dry anaerobic degradation process (not continuous). In this type of process, the anaerobic stages of biological biomass degradation and biogas production (hydrolysis and acidification, acetogenesis and methanogenesis) all occur within the same fermenter. The digestion process takes place under controlled mesophilic conditions and is carried out at a temperature of approximately 37 45gradiC.
The batch plants are operated via a discontinuous process that consists of cyclically loading the inoculated substrate, activating the process for a given time (≥25 days) and emptying the fermenter. The mixture to be treated is loaded into the digester using a wheel loader.
During the dry anaerobic digestion process, the constant humidity of the substrate, needed to execute the digestion process, is guaranteed by use of the leachate generated by the process itself, stored in a dedicated tank, and sprayed over the fermenting mass. The proper temperature is guaranteed by using the heat generated by the energy recovery section. This promotes the most favourable conditions for the development and growth of the bacterial strains required for the digestion process. Each single digester consists of a concrete bio tunnel of appropriate size, closed by a gas tight hatch. The mixture undergoes the digestion process in a gas tight environment under anaerobic conditions, with no need for any further mixing.
The biogas produced by the anaerobic digestion process is conveyed towards two 499 kWe cogeneration units for the combined generation of electricity and heat.
Even though this is a discontinuous process, by exploiting multiple digesters in series that are loaded and emptied at precise intervals, it ensures continuity of the digestion treatment and the constant production of biogas, and thus of both thermal and electrical energy.
In the subsequent aerobic stabilisation stage, the mixture output from the anaerobic digestion section is arranged in piles on aeration tracks. The at least 14 day oxidative microbiological process starts on these tracks.
The aerobic environment is ensured by the addition of oxygen through a forced ventilation system located under the piles.
During this fundamental stage, characterised by the bio oxidation of microorganisms such as bacteria, fungi and actinomycetes, degradation of the waste takes place, with simultaneous production of carbon dioxide, water and heat. Because of the poor thermal conductivity of the biomass, the heat accumulated inside it reaches and exceeds a temperature of 55gradiC, which ensures that the material is completely hygienised as the microorganisms harmful to humans and plants disappear.
As the enzymatically oxidisable compounds are exhausted, microbial activity and heat generation decrease, and a "humified" fraction is obtained, namely compost.
Finally, the material taken from the accelerated bio oxidation section is transferred and arranged in piles for the secondary maturation stage, thereby completing the aerobic stabilisation cycle, which lasts at least 21 days in total (accelerated bio oxidation and secondary maturation).
Finally, the material taken from the secondary maturation section is conveyed to the screening and refining section, where the coarse fractions are separated and the composted mixed soil improver (high quality compost) used in agriculture is produced.
The product is stored in a dedicated area pending its distribution on the market. Wastes and by products of refining, such as biostabilised material (non conforming compost), are transferred to suitable authorised facilities for recycling or disposal.
The Wood Biomass production line consists of the following sections and related processes: section for receiving, storing and grinding lignocellulosic waste; section for treatment, deferrisation and screening; section for storing the wood biomass produced. The wood biomass production line yields the following products: biomass fuel to be transferred to suitable facilities, materials for the production of panels, materials for the production of pellets, materials for mulching, materials for biofilters.