Mechanical-biological waste treatment/stabilisation

Mechanical-biological waste treatment/stabilisation

Key word: mechanical-biological, waste treatment/stabilization, environmental, treated biologically, wet house-hold waste, biodegradable, biogas.

Abstract

Mechanical biological treatment comprises a combination of mechanical and biological processes that further treat mixed residual waste before disposal. The aim of this process combination is to minimise the environmental impacts of end disposal and to gain some further value from the waste through the recovery of recyclables and, in some cases, energy. The possible process configurations are numerous although consisting always of mechanical processes and a core biological treatment. With rising environmental standards and higher recycling requirements, integrated systems have been developed that combine the two technology stages as an integrated entity and include treated biologically and odour control facets within a closed cycle [2].

In “splitting”(or mechanical-biological treatment-MBT), a derived fraction of material is treated biologically. The core biological process used in such system can be anaerobic digestion or composting or elements of both technologies (as in some of the newer process schemes). When anaerobic digestion is used, the process is usually configured to optimise biogas production. When composting is the core technology to biologically treat the derived waste material, no biogas is produced and the rottening process used to convert the mixed waste into stabilised matter for landfill disposal. The typical process configuration is the following:

In “stabilisation” (or mechanical-biological stabilisation-MBS) the entire waste is subjected to biological treatment with subsequent splitting of the mass of stabilised material for recycling, refuse derived fuel (RDF) and land-filling. Main objective of the treatment is to generate a refuse-derived fuel type material which can be used for energy recovery and as fuel replacement [1].

Application objective

Mechanical biological waste treatment is applied on mixed waste with the aim to achieve [8]:

- a stabilisation and reduction of the risk potential together with a significant weight and volume loss thru biological decomposition which could count towards the diversion of biodegradable waste from landfill, and in conjunction therewith;

- the processing of the waste in order to generate separate material streams and improve suitability for subsequent treatment processes;

- the recovery of recyclable materials.

Specific advantages

• reduces the volume and reaction potential of waste that must go to landfills and therewith the landfill void space taken, and the emissions of gas, leachate, vermin and odour at the landfill site.

• combines material specific treatment and material recovery

• generates various material fractions for further use

• allows for energy recovery (from biological processes and/or generated RDF material)

• simple and little capital intensive installations can be possible

Specific disadvantages

• no complete mineralisation of the waste is being achieved requiring further disposal measures

• comparatively low exploitation of energy content

Application details

Technical scheme

The core technology used in any mechanical biological waste treatment is the biological process. Biological processes can only treat the biodegradable fraction of the MSW, however. Depending on the end disposal option and the quality of material they require, mechanical processes of different intensity do either precede the biological treatment stage in order to separate the non-biodegradable (recyclable/combustible) from the biodegradable material, or they follow the biological treatment and further process the output so that the use as fuel replacement and further material recycling become possible [2].

Mechanical treatment

Mechanical treatment usually consists of several mechanical operations adapted to or applied to change the physical properties and composition of the waste input in order to facilitate the further processing and possibly recover valuable material from it.

The minimum technical requirements for an efficient treatment comprise of installations for:

- storage and feeding of the plant

- removal of disturbing material and contaminants

- size reduction

1. Storage and feeding installations

The delivered waste is mainly stored in flat bunkers, sometimes in deep bun-kers. In flat bunkers, the bulky disturbing materials can be separated roughly by grabbing with wheel loaders oder special gripper equipment. Apart from that, the delivered waste can be controlled without any problem, if necessary any problematic deliveries can be rejected from treatment.

Separate storing of different fractions (e.g. dry commercial waste, bulky waste, and wet household waste) is very easy in flat bunkers. Flat bunkers are cheaper than deep bunkers, but need more area.

In deep bunkers the delivered waste can easily be mixed. On the other hand it is difficult to sort disturbing material out of bulky and commercial waste in these bunkers. Deep bunkers are specially suited to store wet household waste. All dry waste should be stored in flat bunkers.

Flat bunkers in this case are more suitable for combined mechanical-biological waste treatment processes.

2. Separation of foreign matter and contaminants

In case a flat bunker is available the disturbing bulky materials can be separated quite easily with a special gripper equipment (e.g. grap dredger) or wheel loader.

Other disturbing materials are separated at the feeding units or on the conveyor belt. In case of the dry bulky and commercial waste, a manual separation of disturbing materials in aerated cabins is acceptable. Due to potential health risks for the workers, such procedures are not aplicable to wet house-hold waste. Here separation by a grab dredger is the only suitable solution.

3. Size reduction

Following the screening of the waste input, size reduction of the overflow may be necessary for adjusting the particle sizes. Aside from generating a more homogenous waste mix, this will help to increase the reaction surface and make certain (packed) materials accessible to further processing at all.

Because this is the most energy consuming step in the mechanical treatment process, size reduction may in certain cases be only useful for (bulky) parts of waste. Bulky and commercial waste must at least be pre-shredded before feeding it into the subsequent processes.

For the pre-shredding (up to a size between 250-500 mm) shearing units (rotor and in some cases Guillotine shearing units), shredder and screw crusher are used.

4. Metal separation

Large-sized metal parts should be separated at the storage area, whereas small parts can remain in the waste. The small iron parts may be separated by over-head magnets if the waste is sufficiently distributed after screening and/or shredding or while passing them on a conveyor belt. Because iron can easily be separated and utilised, metal separation should always be part of the process design.

4.1. Separation of non-ferrous metals

Also possible is the separation of non-ferrous metal, preferrably from the material flow < 80 mm. Usable non-ferrous metals can be sold for high prices.

4.2. Separation of the overflow by screening

If essential amounts of plastics and wood are in the waste, these should be separated together with paper/cardboard in a sieving drum. Screening at a size of about 100 to 150 mm usually generates a high calorific fraction (pa-per/cardboard, plastics and wood) in the screening overflow. In the screening pass flow the biodegradable waste material is concentrated. Sieving drums are not actually suitable for bulky waste like wooden planks etc. These parts should be pre-shredded or completely removed beforehand.

If the screening overflow will be used to generate a refuse derived fuel (RDF) then it need to be shredded and possibly conditioned and compacted in sub-sequent operations.

5. Separation of light and heavy fractions by classification

A classification, for example with the help of an air classifier, to separate the high calorific fraction is less important than the screening, although with air classifying glass and stones may be discharged.

6. Separation by sorting

If the dry waste (especially when it is made up from commercial, C&D and bulky waste) contains a high amount of recyclable materials, a manual separation may be appropriate. Sorting stages are often attached to the screening operations. Air classification is a valuable support here as it helps segregating the waste mix and generates a separate fraction (RDF-fraction) out of it.

7. Baling press

If combustible material (RDF) is to be derived from the dry waste, this can be done after metal separation and shredding. For a better storing and transportation of this material (mostly consisting of plastics and paper) a baling press is often being used for compacting.

8. Further comminution

In order to use the high calorific fraction as fuel, for example in cement kilns or other thermal processes, a further comminution is often necessary. At best this can be achieved in a high-speed crusher. Such installations can chop the material into pieces of 60-80 mm in size. Is still a further comminution necessary, the waste has to be pelletised first which is technically very expensive.

Biological treatment

Different technologies can be applied for the biological treatment stage. Usually these are either intensive rotting/composting or anaerobic digestion methods. They are detailed in separate factsheets.

Only major adjustments of these methods towards the relaisation of a me-chanical-biological waste treatment will be explained here.

As MBT-schemes are concerned, they are as follows [2]:

Rotting method

As in composting, static and dynamic methods can be used for the rotting of waste. Static techniques are the simplest methods for rotting and hence more often applied for biological waste stabilisation than in composting. In this case the material is not turned during the process of biological degradation. For this the homogenized waste is piled into simple rotting heaps, triangle-shaped or flat-top windrows. A biofilter which consists of wood chips or already stabilised organic material or com-post is spread on the top of each pile. In the heaps the biologically active material partly decomposes to CO₂, water and humic substances, water evaporates and leaves an biologically inactive substrat.

Rotting without turning the material and without technical aid for aeration and irrigation is only used for the passively aerated biological post rotting (open air post rotting on dumping sites).

To use the technique for the main rotting, an actively aerated method with control of water content and oxygen supply should be adopted.

The following drawing shows the chimney draught method.

Another static method is that of rotting boxes and containers.

The rotting boxes are made out of reinforced concrete or steel. They have a driveable perforated bottom. They are operated in a batch mode. The boxes are supplied with air from the perforated bottom, and exhaust air is then sucked on top of the rotting material for further treatment. The intensive rot-ting is completed after 8 to 10 days. The technology is simple and more durable. However, the rotting boxes require a thorough mechanical pre-treatment. Also the rotting material tends to dry easily. As of now rotting boxes have mainly been used for separated biowaste or biological drying.

As dynamic or quasi-dynamic methods, rotting drums, tunnel reactors and windrow techniques with regular turning can be applied.

Rotting drums are highly suitable for pre-rotting. A good homogenizing and mechanical disintegration takes place in the rotting drums. However several moving parts on the drums can lead to high wear. Rotting drums hence should be used for relatively short time pre-rotting processes only.

The methods are in more detail described in the fact sheets on composting.

Intensive rotting technologies are the choice to implement a MBS-scheme. They are applied on the entire input stream in order to biologically dry and sterilise the material and to produce in this way an output which is largely suitable for thermal treatment and combustion processes. Given the unsorted input and the high rate of emissions and leachate it produces in the early phase of treatment, fully encapsulated systems (such as shown in Pict.b) are used for the rottening process. The systems use the biological properties of the waste for drying purpose. The systems are filled almost completely with the unsorted but homogenised waste, leachates and exhaust air emissions are collected and the latter cleaned by biofilters [7].

The calorific composition of the output material will be relatively high as both the liquid content is being reduced thru biological degradation and non-combustible materials (e.g. metals and inert materials) are separated after-wards. The calorific value of the so derived RDF type material can range between 12-16MJ/kg dependent on waste input and achieved moisture levels. There is much scope for different ways of recovering the energy from this material. These range from RDF-type combustion or co-combustion (in power plants or cement kilns) through to gasification. Most suitable is a co-incineration in an industrial plant which can readily handle fuels with higher calorifc values, unlike energy from waste incinerators.

When anaerobic digestion is incorporated into the MBT, the process is usually configured to optimise biogas production. However, in some instances the technology has been configured to optimise the production of biogas and RDF. In the anaerobic digestion process biological degradation takes place in closed reactors without air supply. A difference can be made between wet and dry processes.

Both process schemes are described in the factsheet on anaerobic digestion.

The following specifics for digestion technologies as biological stage of a mechanical-biological treatment apply.

Advantages of the dry process are:

- lower water demand

- because of the higher dry matter content sedimenting components are better integrated in the digestion material than in the wet processes.

The typical problems of the treatment of residual waste by anaerobic digestion can be minimized with the following technical solutions:

- use of biogas nozzles instead of agitators to circulate the feedstock in the digesting vessel for minimizing surface scums (avoidance of wrapping on agitators),

- prior segregation (e.g. ballistic classifier) and discharge of heavy components (sedimenting materials) and light materials (e.g. textiles and foils to avoid wrapping, occlusions and surface scum),

- adjusting to a dry matter content of 20-40 % before digestion (eliminating a demixing in the vessel), or

- a washing of the fine fraction that has been obtained after the mechanical pre-treatment to remove light materials, sand and other abrasive materials such as glas from the feedstock. The remaining material which consists mainly of biodegradable substances can be digested then in a wet process.

Digestion processes are completed after about 18-21 days and the remains dewatered in a press. The solid matter can then be further cured by composting and deposited at landfills whereas the waste water has to undergo further treatment. Due to the high COD, expensive methods for the treatment of the waste water have to be employed, however.

Conclusions

Mechanical-biological waste treatment in the basic application leads to waste which is much less active in landfill and reduces the long term management needed. This has strong implications for the sustainable management of land-fill sites and their potential environmental impacts. Environmental impact studies suggest that landfilling of stabilised biowaste generates only 10% of the landfill gas and 10% of the leachate which would be generated by the untreated waste. These are benefits which are to be taken into account in addition to the possibility to meet with this technology the primary targets laid down in the Landfill Directive.