Mechanical biological treatment of urban waste

Mecánico-biológica installation will prepare the waste according to the type of waste to deal with, its composition and amounts, applicable legislation and the requirements of the client, in addition to the features of the finalist treatment. With all this, the installation must adapt to the fulfilment of the hierarchy of waste management urban giving priority to the options of selection of materials for recycling and other types of valuations. This is achieved through:

• The maximum recovery of functional fractions (composting or stabilization and other biological processes).
• The maximum recovery of metals and metal compounds (selection of ferric and aluminium).
• The maximum recovery of other inorganic materials (selection of glass, plastics, briks, etc.) both the recyclable and those aimed at the preparation of re-usable fuel heat.

According to the provisions of Directive 2008/98/EC, article 3, the valuation is any operation whose main result is that the waste has a useful purpose by replacing other materials which otherwise would have been used to fulfil a particular function. On the other hand, according to this directive, the removal is any operation other than the recovery, even when the operation has as a secondary consequence the use of substances or energy.

The following operations for recovery of the waste according to Directive 2008/98/EC, annex II are:

• R1 main as fuel or otherwise use to produce energy.
• R2 recovery or regeneration of solvents.
• R3 recycling or recovery of organic substances which are not used as solvents (including composting and other biological transformation processes).
• R4 recycling or recovery of metals and metal compounds.
• R5 recycling or recovery of other inorganic materials.
• R6 Regeneration of acids or bases.
• R7 recovery of components used for pollution abatement.
• R8 recovery of components from catalysts.
• R9 regeneration or other new employment of oils.
• R10 treatment of soils to produce a benefit to the pertains tura or an improvement ecological thereof.
• R11 use of wastes obtained from any of the numbered s operacione from R1 to R10.
• R12 Exchange of wastes for submission to any of the operations listed from R1 to R11.
• R13 storage of waste pending any of the operac ions numbered from R1 to R12.

Directive 2008/98/EC, article 4, States the waste hierarchy that serves as the order of priorities in the legislation and policy on the prevention and management of waste:

1. Prevention

2. Preparing for re-use.

3 Recycling.

4. Another type of recovery, e.g. energy recovery.

5 Elimination.

The R1 (main use as a fuel or any other way of producing energy), are included in incineration facilities for the treatment of solid urban waste where their energy efficiency is equal to or greater a: 0.60 (in the case of installations in operation and permitted in accordance with Community legislation applicable before January 1, 2009) or 0.65 (the case) (for installations permitted after 31 December 2008) by applying the following formula:

Energy efficiency = (Ep-(Ef + Ei) /(0,97_x_(Ew_+_Ef)).)

EP is the annual energy produced as heat or electricity, which is calculated by multiplying the energy in the form of electricity by 2.6 and heat produced for commercial use by 1.1 (GJ/year). EF is the annual contribution of energy to the system from fuels contributing to the production of steam (GJ/year), Ew is the annual energy contained in the treated waste calculated using the net calorific power of the waste (GJ/year) and Ei is the annual energy imported excluding Ew and Ef (GJ/year).

There are also other options of recovery as the R3 recycling or recovery of organic substances which are not used as solvents (including composting and other biological transformation processes). This includes gasification and pyrolysis using components such as chemicals.

The R5 (recycling or recovery of other inorganic materials) includes the cleaning of the floor which would result in the recovery of the soil and recycling of inorganic building materials.

The R12 (Exchange of wastes for submission to any of the operations listed from R1 to R11) is when there is no proper R code, may be included here provided for initial operations to the recovery, including pretreatment, such asamong others, disassembly, classification, crushing, compaction, the pelletising, drying, fragmentation, conditioning, repackaging, separation, the combination or mixture, prior to any of the listed operations R1 to R11.

The main advantages of the system are:

(1) Environmental and social sustainability

(2) Give value to the residue until now not usable as a biofuel or electricity.

(3) Heat recovery.

(4) Installation of landfill at a minimum, reducing the need for landfill sites to maximize.

(5) Other waste of difficult treatment production

(6) Savings of CO2 emissions and gas default greenhouse for the production of renewable energy and landfill disposal.

The waste can be separated according to different factors. One of them is the size separation, separation of the flow of waste into 2 or 3 fractions of different sizes is achieved through the use of Rotary sieve (drum). There are three types of fractions: the fine fraction (fraction wet and professional that will be subject to a subsequent biological process), the intermediate fraction (large number of containers that were subsequently triarán as PET, HDPE, briks, boats and metal cans), and the coarse fraction (mainly other recoverable larger such as paper, cardboard, film and others).

Another type of separation can be by form and density through the use of multiple screens driven by twin-crank shaft system solidarity (or a single shale if ramp) and with the regulation of its angle of inclination is achieved the separation of the flow of waste in several fractions according to its form and behavior physical. They can also incorporate pneumatic aspiration.

A third selection is the automatic of metals. Through the use of a powerful electromagnet will attract ferrous elements found in the flow of waste until a served tape to extract them from its zone of influence for its recovery. Through a rotor magnetic located at the end of a tape transport and equipped with permanent magnets of high remanence creates a magnetic field of high frequency which induces eddy currents in conductive metallic materials (aluminium). These, in turn, create a magnetic field opposite to the rotor by what are repelled. In the automatic selection of other materials, the residue enters an accelerator tape where infrared sensor detects the material selected according to the wavelength of the light it reflects. The associated computer detects the position of the material and depending on the speed of the active tape one of the blowers of compressed air which are at the end of it, separating it from the rest.

Another type of selection is the manual selection of recoverable which is achieved through the intervention of workers placed at foot of tape to recover materials in a manner completely manual. The recovered materials are unions (usually in trojes under cockpit) being waiting for prior to its marketing packaging.

In this section we find different types of composting. One of them is the composting in parva or plateau (aerobic fermentation of organic fraction of municipal waste in parvas or plateaus both abroad and on ship deck or closed). Controls the level of oxygen, temperature and humidity turned and regular irrigation of the material with special vehicles (volteadoras). Another type of composting is that applied in a tunnel (aerobic fermentation of organic fraction of municipal waste in closed tunnels). The levels of oxygen, temperature and humidity are permanently controlled by a computer system that activates the irrigation or drive or aspiration of air through the mass of the residue. This system is a static process where material introduced in the interior of the tunnel doesn't move until the end of the composting process.

Composting is also used in trenches, where oxygen levels, temperature and humidity are monitored permanently by means of a computer system that activates the drive or aspiration of air through the mass of the residue. The system is completed with the perfecting and irrigation periodic mass of waste by means of a special vehicle (palletized). The loading and unloading of the trenches can be automatic.

Composting through reactor of vertical axis (aerobic fermentation of organic fraction of municipal waste in reactor closed with automated tipping) get that oxygen levels, temperature and humidity controlled permanently by means of a computer system that activates the aspiration of air through the mass of waste. The system is completed with the swing and irrigation periodic mass of waste by means of a bridge crane equipped with car with helical screws that are transversely moving product. The loading and unloading of the reactor is also automated.

On the other hand there composting through a reactor with horizontal axis. In this case, the levels of oxygen, temperature and humidity are permanently controlled by a computer system that activates the air discharge through the mass of the residue. The system is completed with the swing and irrigation periodic mass of waste by means of a bridge crane equipped with cart with rotopala which is longitudinally moving product.

The biomethanisation is an anaerobic process of decomposition of organic matter which gets a gaseous fraction called biogas, which can be used for the production of electricity, heat and electricity combined, or as fuel for vehicles; and a digested material, which is usually a solid of doughy consistency, with a high content of nutrients for plants.

The biomethanisation can occur in various ways, among them are:

• Dry way: anaerobic digestion of organic fraction of municipal waste in reactor for the production of biogas with a high content of methane (55-60%). The solids in the digester content is high (30-40%). The movement of the waste inside the digester can be with mechanical action or through the recirculation of the own biogas.
• Wet: anaerobic digestion of organic fraction of municipal waste in reactor for the production of biogas with a high content of methane (55-60%). The solids in the digester content is relatively low (6-10%) for what we need to prepare a suspension by adding water prior to digestion.
• Dry pretreatment of the MOR: simple mechanical treatments which selected the organic fraction of the organic waste (cribado+separación of metals) are only valid for composting, not for Biomethanization processes with it are also numerous non-biodegradable materials such as sand, glass, rubble, small plastic, hard shells of fruits, etc. Thus the theoretical organic fraction obtained, as intern product by the aforementioned sieve, actually contains between 25 and 40% of items not suitable for the biomethanisation. These not only 'improper' elements are hardly biodegradable, they hinder the overall functioning of the facility producing traffic jams, not controlled tank of sand, glass and other aggregates, as well as segregation of layers of unwanted floating elements by reducing the effective capacity of the biomethanisation.
• Wet pre-treatment of the MOR: in the case of the biometanizaciones wet process can increase the efficiency of the pretreatment of dry with a removal of improper system added to the system of preparation of the suspension (hidropulper). Thus, the concentration of improper of the material which we eventually send to digestion decreases significantly.

In the biosecado processes of aerobic fermentation of the organic fraction have also another utility, the reduce the humidity of the residue that take advantage of the aerobic fermentation (exothermic reaction with excess of heat). Thus, by controlling the humidity and through the use of a forced air suction system is achieved, as well as the stabilization of organic matter contained in the waste (by aerobic fermentation), the reduction of its level of humidity below 20 %, and therefore its weight reduction produce a material of high PCI which can be valued as a fuel. This system can be used with fractions mixed without mechanical treatment of selection.

This information was obtained from the last edition of Reinnova, International Congress of innovation in the management and treatment of municipal waste, which took place in Sabadell last 1 and July 2, 2009, where Carlos Guijarro Castrotechnical director of Sufi, stated in a Conference the mechanical biological treatment of urban waste.