Ostellato composting plant

The production process of the Ostellato composting plant can be broken down into five main stages:

• delivery of the waste;
• separation of the organic and dry waste fractions through unseparated municipal waste selection;
• production of the stabilised organic fraction (SOF) from the organic fraction of municipal solid waste (OFMSW);
• production of compost from the decomposable organic fractions (DOF) and the lignocellulose fraction;
• storage of products and non-reusable fractions.

1 - Waste delivery. All vehicles entering the site must first pass through the weighing station, where the incoming flows are recorded and the deliverer´s authorisation to enter the site is checked, along with the relevant transport documents and permits. The waste is then unloaded in the designated storage areas and, before treatment, is divided into categories of unseparated municipal waste, decomposable organic fraction (DOF) and lignocellulose fraction.

2 - Pre-selection of municipal waste. Unseparated municipal waste unloaded into the storage pit of the selection plant is picked up by a bucket crane, which transfers it to the selection plant. It is then ground in a suitable machine operating on an automatic cycle which transports it, on a conveyor belt, to the screening stage. This stage is performed by an automatic rotating screen, which selects the waste on the basis of its size. This initial selection produces two types of waste: a dry fraction and a wet fraction or OFMSW (Organic Fraction of Municipal Solid Waste). The dry fraction first undergoes a deferrisation process, which recovers any metal contained in it. It is then either transported to a waste-to-energy plant or, using a special automatic machine, pressed and bound in plastic film to produce rotobales of the selected material.
The rotobales are then transported to a suitable external storage area while awaiting disposal.
The wet fraction (OFMSW) is transported to the biostabilisation area for subsequent treatment.

3 - Biostabilisation and production of SOF. In the biostabilisation stage, OFMSW delivered to Herambiente from other plants is added to the previously separated wet fraction.
The incoming material is stored in piles inside a storage facility to allow it to ferment and mature. Fermentation takes around 21 days. The hygienisation temperature is automatically maintained at a constant temperature of approximately 55°C for three days.
During the maturation process, the piles of OFMSW are progressively turned in a controlled sequence, according to the day on which the waste arrived. The entire maturation process takes at least 21 days from the day of arrival; during this time, the leachate produced is collected and placed in external storage tanks.
At the end of the turning/maturation process, the waste is conveyed by a tractor loader to a suitable screening machine (rotating screen), which separates the Stabilised Organic Fraction (SOF) from the oversieve or non-reusable fractions. Both products are then stored in designated areas to await recovery/disposal.
In conformity with legal requirements, the final destination of these products is authorised landfills. SOF, in particular, is used as a daily covering for the layers of waste in the landfills.

4 - Production of high-quality compost

• Grinding of the lignocellulose fraction and mixing. The lignocellulose fraction generated by the maintenance of public and private gardens and parks, stored in one of the facility´s external areas, is ground and sent to the new compost production plant to be mixed with decomposable organic fraction (DOF). The DOF obtained from separated waste collection - another type of compostable waste - is then mixed with the lignocellulose fraction in order to obtain a homogeneous matrix suitable for producing compost. Mixing concludes with the transport of the mixture, via wheel loader, to the biocell tunnel.
• Maturation in biocells and intermediate screening of the compost. The plant has 10 static biocells in which an active microbiological process is triggered that brings the entire mass to temperatures of above 55°C for at least three days. The mixture remains here for at least 14 days, during which time the production of foul smells is controlled through a system that blows forced air through the tunnels, thus helping to avoid the anaerobic conditions that would encourage the synthesis of odorous compounds. An aspiration system inside the biocell channels the exhausted air to the biofilter for purification. The leachates produced during this stage of intensive maturation are collected in temporary storage tanks and then re-used for humidification (spraying) of the compost contained in the biocells.
• Maturing in piles and final screening of the compost. The material is stored in a maturation facility, where a loader places it in trapezoidal piles. The maturation stage lasts 60-80 days, during which the piles are periodically turned mechanically. The composting process lasts for a minimum of 90 days. At the end of the maturation period, and before leaving the plant, the compost is refined through a vibrating screen (with a 6 or 8 mm mesh). Its conformity is then evaluated through specific laboratory tests before it is distributed. The non-reusable fraction resulting from the screening process is either re-inserted into the production cycle with the lignocellulose fraction or sent for disposal/recovery.

5 - Storage of products and non-reusable fractions
The two different types of product (SOF and compost), as well as the by-products or waste (non-reusable fraction and rotobales) generated by the processes described above, are stored in appropriate dedicated areas.

In particular, the Stabilised Organic Fraction is accumulated by means of a tractor loader in a covered area located in the eastern part of the SOF production plant, while the compost is transferred and stored in a suitable external area pending delivery to customers.