Systems of mechanical-biological treatment of MSW

Objectives of the system proposed

The proposed plant is designed to treat, dispose of and recover unsorted municipal solid waste for a total amount of 280 t / day (approximately 84,000 t / y), corresponding to a basin of about 210,000 inhabitants.

Sorting and selection operations on the waste “as it is” from the waste do allow to:

• Sort out all the valuable and recyclable materials
• separate the dry fraction, having high calorific value, to be sent to the production of RDF
• dispose of the organic portion, having recovered all the resources still available.

Overview of the system

The municipal waste, unsorted, is supplied to the system and stored in a specific area, waiting to be sent to treatment.

Bags opening and primary size reduction

A handling system provides the conveying and loading of waste into a primary shredder, in order to open the bags and reduce the size of the substance to be sent to the next steps.

Separation by size

Shredded waste is collected by an inclined belt conveyor. After magnetic separation of ferrous metals, is fed to a machine providing its screening/sieving – in alternative, a rotating drum or a star mesh sieve Ø 80 mm, in order to sort the material according to its size and, at least  partially. based on the type of material (in fact, primary shredding leaves intact the long fibers of plastics, textiles and paper that will therefore not pass through the holes of the sieve).
Since most of the organic fraction has particle sizes below 80 mm, this stage of size separation can has also the result of splitting the incoming waste in its organic and inorganic fractions.

The inorganic fraction and the production of RDF

The screen “overs”, mainly consisting of inorganic compounds such as plastics and textiles, having a high calorific value, are sent by conveyor belts to a RDF production line
The overscreens are characterized by a LHV in the range between 3,300 and 3,600 kcal / kg, a moisture content of 20-30% and a density of 150-300 kg / m³.

On the average, the “overs” are composed by:

• Paper and cardboard
• Plastics
• Poly laminated packaging films
• Textiles
• 10-15% of organic matter – contaminating the dry matrix.

The “overs” is composed of materials leaving a large volume of inter-particle voids, which allows a consistent ventilation, facilitating the onset of oxidation processes by anaerobic bacteria finding nourishment in the residual organic contaminants. Therefore, the “overs” must be stabilized and transformed into RDF as quickly as possible.

The “overs” from the end of the screen are downloaded directly onto an inclined conveyor that, after further magnetic separation, collects and delivers it to the feeding hoppers of the secondary crushers.
After the secondary size reduction, this finely ground material is collected by another inclined conveyor and transferred, with a belt conveyors based distribution systems, in order to load the bunkers of the uniquely designed PROMECO densifiers/agglomerators.

From the bunker of the densifiers/agglomerators - equipped with a level probe - the material falls on a vibrating table acting as feeder for the extrusion chamber of the machine.
Inside the extruder the material is homogenized and disrupted by the  high friction produced by  two counter-rotating screws; subsequently it is forced by pressure through an extrusion matrix and expelled with the desired shape.
Immediately after the extrusion matrix is a stabilization rail , followed by an cutting system  determining the length of the RDF briquettes.
Briquettes are then collected by a lifting belt and sent to a gasifier for the production of gaseous fuel.

The organic waste: biogas production from anaerobic digestion processes

The screen “unders” are characterized by a low calorific value, due to the high moisture content (approximately 40-60%), and a specific weight of 500-600 kg / m³.

On average, the “unders” are composed by:

• Organic material
• Paper, cardboard, wood and textiles
• Plastics
• Glass and Aggregates
• Metals
• Hazardous waste and pollutants present in MSW (batteries, syringes, etc..)

Pre-treatment of organic
The “unders” are sent to a pre-treatment system. Different solutions are possible, mainly depending on the anaeronic digestion system to feed (wet, semi-wet, dry). In this case it is assumed the use of a system hydropulper – multisorter, ideal to feed a wet  AD system.

The dimensioning of the system is made based on the mass of the dry weight of “unders”,  considering anyway a wide variability (± 30%) of the fractions composing the waste stream in input.

Through a system of distribution belts, the “unders” are sent to a unit of magnetic separation, to intercept any metals still present in the stream.
The material is then loaded in a hydropulper, where the selection of different wet matrices present in the waste and their separation into distinct flows take place.
In the hydropulper, consisting of a mechanical reactor, the “unders” are diluted with water and subjected to strong mechanical action. Hydropulper allows to achieve two results: the separation by sedimentation of the heavy fraction of the waste, and the mechanical mixing and dissolution in the a liquid solution of the organic fraction.
Materials sinking on the bottom of the tank (glass, metals, aggregates), are removed through a sump which, by a system of valves, allows to divert this stream from the process

The material remained in suspension is discharged into a multisorter, allowing a further separation of the components present in the waste. The light fraction (plastics, textiles, wood) is separated and extracted by flotation and intercepted by a worm, while other  foreign materials are collected on the bottom of the machine.

This process allows to get a main stream consisting of organic material in suspension, and three distinct waste streams (heavy, light and fine fractions), characterized by a sufficiently homogeneous composition, which simplifies disposal.

In fact:

• The heavy fraction and the fine residues can be dump to a landfill for inert materials, because they have a organic content lower than the limit of 10% expected for this class of special waste
• The light fraction, after being pressed to release the fluid trapped in it, is sent to the RDF production line extruders, as it mostly composed by plastic, with high calorific value.

Anaerobic digestion and energy production from biogas
The organic suspension is pumped in the anaerobic digester, where methanization reactions and the recovery of biogas formed during this process take place.

Biogas has a high methane content (50-60%) and, after a stage of desulphurization can be used as fuel in boilers to produce heat, or feed internal combustion engine for the production electricity and heat in cogeneration.

The sludges leaving the digester is sent to one or more centrifuges to be dehydrated. The purpose of this treatment is to separate the water in the stream, in order to obtain two results:

• Recover, almost completely, the process water for its reuse in the hydropulper, as this machine do implement a mechanical operation, and thus does not have any need of clean water. To enable this solution, the clarified water coming from the centrifuges is accumulated in a storage tank.
• Get a sludge with a content of about 20% dry matter, in order to reduce the volumes of material to handle and have a product with has the characteristics required to be sent to a composting plant.

Production of quality compost
The dried organic material is mixed with shredded green waste and undergoes a pretreatment stage using the PROMECO Extruder, which could give the matrix a structure suitable to optimize the process of biological oxidation. The mixture then undergoes an accelerated fermentation process in biocells.
In order to produce a compost mix another line, based on PROMECO Extruders, is installed to process ligno-cellulosic material, and produce a fiber with characteristics allowing its use as a substitute for peat.
At the end of the minimum period required by law, the compost is refined, mixed with peat substitute (and / or other suitable substrate) and then packaged as compost, for sale.