Association „КЕ&B - UV&P”

VAT.Nr . : BG176245551

O P E R A T I O N A L P R O G R A M M E

E N V I R O N M E N T 2 0 0 7 -2 0 1 3

61 Prek i pat s t r . , Sof ia 1618

Bu lgar ia Te l ./ fax : (+359 2) 857 5197

E-mai l : Biowaste .BG12@gmai l .com

E U R O P E A N U N I O N

E U R O P E A N R E G I O N A L

D E V E L O P M E N T F U N D

W E I N V E S T I N Y O U R

F U T U R E

Project No TA-2011 -KPOS-PP-78 „Technical assistance on waste management” “Development of lega l f ramework on bio -waste management and establ ishment of Qual i ty Assu rance

System for Compost and National Organization of Qual i ty Assurance for the Compost”

Development of Legal Framework on Bio-Waste Management and Es-

tablishment of Quality Assurance System for Compost and National Or-

ganisation of Quality Assurance for the Compost

STAGE IV Development of national technical requirements for in-

stallations treating residual waste fraction (mechanical-

biological treatment (MBT) and incineration) - Guidance

on techniques and technologies to treat residual frac-

tion (best practices)..

Part I

Technical Requirements for Mechanical-Biological

Treatment of Residual Waste

Final Report – 14 June 2013 IV-I_TechRequ-MBT_EN_v0.4

Main Authors:

Wolfgang Müller, Technical University Innsbruck

in cooperation with:

Florian Amlinger, Compost – Consulting & Development

Picture Cover page?

Technical Require-

ments for Mechanical-

Biological Treatment

of Residual Waste

A guideline in good practice

IMPRINT

Edited by: Bulgrian Ministry for Environment and Water Authors: Dr.-Ing. Wolfgang Müller, Institute for Infrastructure - Waste and Resource Management Revision: Floriuan Amlinger, ……………..

yyyyy 2013

Technical Requirements for Mechanical-Biological Treatment of Residual Waste

– a guideline in good practice –

Contents

1 Introduction ........................................................................................ 10

2 Wastes for MBT ................................................................................. 11

3 Objectives of MBT ............................................................................. 12

4 MBT systems and technolgies ........................................................... 14

4.1 MECHANICAL TREATMENT .................................................................................................................... 14

4.2 BIOLOGICAL TREATMENT ...................................................................................................................... 17

4.3 MBT - AEROBIC STABILISATION ........................................................................................................... 18

4.3.1 Option A: Rotting in open Windrows without forced aeration ............................................ 19

4.3.2 Option B: Rotting with Forced Aeration .............................................................................. 20

4.3.3 Conclusions on rotting systems ......................................................................................... 21

4.4 MBT WITH ANAEROBIC DIGESTION (AD) .............................................................................................. 22

4.4.1 Wet digestion systems ....................................................................................................... 23

4.4.2 Dry continuous AD systems ............................................................................................... 24

4.4.3 Dry discontinuous AD systems .......................................................................................... 25

4.5 BIOLOGICAL DRYING ............................................................................................................................ 26

4.6 TECHNICAL PROCESS STEPS ................................................................................................................ 27

5 Outputs from Mechanical Biological Treatment .................................. 30

5.1 STABILIZED ORGANIC WASTE FRACTION FOR DISPOSAL .......................................................................... 30

5.1.1 General explanations ......................................................................................................... 30

5.1.2 Further benefits for the landfill behaviour ........................................................................... 33

5.1.3 Requirements for biological treated waste to be landfilled ................................................. 33

5.1.4 Frequency of analyses ....................................................................................................... 34

5.2 RECYCLABLES FROM MBT ................................................................................................................... 34

5.3 REFUSE DERIVED FUEL (RDF)............................................................................................................. 34

5.4 STABILISED MBT OUTPUT FOR RECLAMATION OF CLOSED LANDFILL SITES OLD MINING AREAS AND

ROAD CONSTRUCTION .......................................................................................................................... 37

5.5 BIOGAS ............................................................................................................................................... 38

6 Technical requirements on the design and operation ......................... 39

6.1 AIR EMISSION CONTROL ....................................................................................................................... 39

6.1.1 Plants with less than 75 tonnes per day capacity .............................................................. 39

6.1.2 Plants with more than 75 tonnes per day capacity ............................................................ 40

6.2 WASTE WATER COLLECTION – GROUNDWATER POLLUTION CONTROL ..................................................... 42

6.3 TREATMENT OF PROCESS WATER ......................................................................................................... 42

– 9 –

6.4 WORKERS HEALTH AND SAFETY ........................................................................................................... 42

6.5 FIRE AND EXPLOSION PROTECTION ....................................................................................................... 42

List of Tables

Table 1. Stability parameter and limits across Europe ................................................................................ 31

Table 2: Recommendations of threshold values to meet the EU Landfill Directive for bio-mechanically treated waste ................................................................................................................. 32

Table 3: Treatment time for the biological treatment to achieve a certain degree of stabilisation .............. 33

Table 4: Number of analyses ....................................................................................................................... 34

Table 5: Heavy metal limit values for Organic Soil Amendment and Stabilised MBT Output (mg/kg

d.m.) ....................................................................................................... Error! Bookmark not defined.

Table 6: Additional requirements for Stabilised MBT Output ......................... Error! Bookmark not defined.

Table 7: Requirements of Compost, Fermentation Product,,Organic Soil Amendment or Stabilised

MBT Output dependent on the possible use ......................................... Error! Bookmark not defined.

Table 8: Limit values for the quality if the exhaust gas after purification in a biofilter ................................. 40

Table 9: Limit values for the quality if the exhaust gas after purification in a biofilter ................................. 41

List of Figures

Figure 1: Hand sorting station ...................................................................................................................... 15

Figure 2: Automatic sorting with optical detection ....................................................................................... 16

Figure 3: Mechanical preparation with automatic sorting of recyclables and RDF ...................................... 17

Figure 4: MBT for stabilisation of the organic rich fraction for landfilling .................................................... 19

Figure 5: Self propelled turner ..................................................................................................................... 20

Figure 6: Forced aeration system with semi-permeable membrane (Gore-system) ................................... 20

Figure 7: Table windrow rotting with automatic gantry mounted turning machine (upper picture) and tunnel rotting (lower picture) ................................................................................................................ 21

Figure 8: MBT with anaerobic digestion ...................................................................................................... 23

Figure 9: Wet digestion system ................................................................................................................... 24

Figure 10: Horizontal, dry digestion system (Kompogas) ............................................................................ 24

Figure 11: Discontinuous (batch) type digestion (Bekon) ............................................................................ 25

Figure 12: MBT-Rennerod (left) and Dresden (right), Germany (both 100,000 Mg/a) ................................ 26

Figure 13: MBT – biological drying .............................................................................................................. 27

Figure 14 Waste Bunker (left) and crane to transport the waste within the MBT (right).............................. 28

Figure 15: Energy recovery options for RDF ............................................................................................... 36

– 10 –

1 Introduction

The EU landfill directive requires a reduction of 65% of biodegradable waste which is landfilled (Art. 5). The major problem with organic waste is that it degrades to the greenhouse gas methane in a landfill. Methane is a greenhouse gas that is 26 times more potent than Carbon Dioxide. Even with a state of the art landfill design incorporating methane capture, substantial amounts of methane will still escape to the atmosphere and contribute to global warming. The reduction can best be achieved with the separation of biodegradable waste at source to produce high quality compost. Where such source separation systems are not yet in place or not efficient enough a pre-treatment of the mixed waste can be applied to meet the

targets of the EU landfill directive. The 2 main approaches for such a treatment: Incineration and Me-

chanical biological Treatment (MBT). MBT is a generic term for the integration of a wide range of waste management processes. MBT is designed to take residual or black bin waste and process it.

This document provides a basic guideline for the treatment of residual waste with mechanical-biological treatment

1. It includes general descriptions of the most common technologies including the techniques

and requirements for the reduction of biodegradable waste.

Furthermore relevant aspects and technical measures for an environmentally sound operation of plants

with a capacity of less than 75 tonnes per day which do not fall under the Industrial Emission Directive (IED) are proposed. The requirements for IED plants are amended in due course .

For the development of the technical requirements and limits documents from Germany and Austria were used as a basis

2.

1 I n the Biowaste directive more detailed information can be found which is also relevant for MBT plants.

2 German Ministry of Environment, 2001: Ordinance on Environmentally Compatible Storage of Waste from Human

Settlements and on Biological Waste-Treatment Facilities; (“Abfallablagerungsverordnung”); http://www.bmu.de/files/pdfs/allgemein/application/pdf/ablagerungsverordnung.pdf

Austrian Ministry of Environment, 2002: Guidance for mechanical biological treatment of wastes („Richtlinie für die mechanisch-biologische Behandlung von Abfällen“ http://www.lebensministerium.at/umwelt/abfall-ressourcen/behandlung-verwertung/behandlung-mechanisch/MBA.html

– 11 –

2 Wastes for MBT

This guideline predominantly addresses the treatment of non-hazardous waste, like

mixed household waste or waste which is similar to those of households

Residues from recycling facilities for household waste

Sewage sludge excluding sewage sludge from industrial waste (relevant waste codes table

Since this depends on the specific design and purpose of individual projects a detailed waste list accord-ing to the European waste catalogue should be required by the applicant of a new MBT plant. The compe-tent authority has to decide case by case which wastes might be excluded from the process/technology applied for.

In national MBT regulations and guidelines no waste codes are included.

The guideline is relevant for mechanical and biological treatment with the main aim to stabilize the organic components of the waste prior to landfilling. As part of the process further products might be produced or separated:

Recyclables (plastic, paper, metals, etc.)

Refuse derived fuel (RDF)

Renewable energy (by using anaerobic digestion)

Compost like output for restricted purposes of land application

– 12 –

3 Objectives of MBT

Environmental burdens from waste disposal are mainly related to ongoing uncontrolled anaerobic decom-position of organic material of the disposed waste. Uncontrolled anaerobic decomposition of organics in landfills leads to the following negative effects:

1. Generation of landfill gas, which contains a considerable amount of methane gas; methane gas has a very negative impact on global warming, and methane from landfills is one of the important sources of greenhouse gas emission.

2. Generation of leachate, which is contaminated by the products of decomposition and pollu-tant.

3. Generation of odours, which may negatively affect the well being of the neighbourhood.

4. Settlement of the landfill body, which may lead to stability problems, uncontrolled landfill gas emissions and water seeping into the landfill body.

Some of these negative effects can be reduced by an improved landfill management, i.e:

Choice of location;

Structural measures: such as liners, and

Optimised modes of landfill operation.

In addition, waste can be treated to reduce the organic content efficiently prior to disposal, so less emis-sion will result from the disposed waste. During the past few years, mechanical-biological treatment (MBT) of waste prior to disposal became a well-established alternative or complement to waste incineration in Europe.

Mechanical Biological Treatment (MBT) is a generic term for the integration of a number of waste man-agement processes such as materials recovery facilities (MRF), refuse derived fuel (RDF) production, mechanical separation, sorting, composting and pasteurising. The MBT process is designed to take resid-ual or black bin waste and process it so that valuable recyclable materials can be separated out and the biomass or “compostable” element is separated out and processed through an In Vessel Composting (IVC) or an Anaerobic Digestion (AD) system.

MBT is a residual waste treatment process that involves both

mechanical and

biological treatment.

The first MBT plants were developed with the aim of reducing the environmental impact of landfilling re-

sidual waste. MBT therefore compliments, but does not replace other waste management technol-

ogies such as recycling and composting as part of an in targeted waste management system. A key ad-vantage of MBT is that it can be configured to achieve several different aims. In line with the EU Landfill Directive and national recycling targets, some typical aims of MBT plants include the:

Pre-treatment of waste going to landfill;

Diversion of non-biodegradable and biodegradable MSW going to landfill through the mechan-

ical sorting of MSW into materials for recycling and/or energy recovery as refuse derived fuel

(RDF);

Diversion of biodegradable MSW going to landfill by:

o Reducing the dry mass of BMW prior to landfill;

o Reducing the biodegradability of BMW prior to landfill;

Stabilisation into a compost-like output (CLO) 2 for use on land;

Conversion into a combustible bio gas for energy recovery; and/or

– 13 –

Drying materials to produce a high calorific organic rich fraction for use as RDF. MBT plants

may be configured in a variety of ways to achieve the required recycling, recovery and biode-

gradable municipal waste (BMW) diversion performance. Figure 3 illustrates configurations

for MBT plant and highlights the process steps

– 14 –

4 MBT systems and technolgies

As said, MBT consists of a mechanical and a biological processing step. For biological treatment, there are three main processes applied to the organic fraction of the waste stream:

Aerobic stabilisation;

Anaerobic digestion; and

Biological drying.

What is common to all types is that there is a front end mechanical processing of the waste.

4.1 Mechanical Treatment

The first step of most MBT concepts is the mechanical treatment.

The purpose of mechanical treatment is the separation of

recyclables (plastic, paper, metals, etc.)

refuse derived fuel (RDF)

hazardous waste

contraries which would disturb the following processing steps

preparation of the organic fraction for the biological treatment

To achieve these targets different types of equipment can be used.

As a first step waste bags have to be opened so all waste components are accessible to following separa-tion steps. This is typically done with a coarse shredder or a bag opener.

If a separation of recyclables is envisaged this step should be done with care inorder to avoid too much shredding and soiling of the recyclables.

A typical next step is a screening of the waste in at least 2 fractions:

1. Fine fraction including the organic fraction for biological treatment.

Depending on the biological treatment this might be between <40 mm and <100 mm.

The bigger the mesh size the more material will end up in biological treatment. With 80

mm the split is typically 50 : 50. From this material metals are typically separated.

2. A coarse fraction which contains most of the recyclables material and RDF

If a separation of recyclables is envisaged, the coarse fraction is typically further divided into different streams to make the subsequent separation easier and more efficient.

This could include a further screen, e.g. at 200 – 300 mm and/or a ballistic separation or windsifting.

The separation itself can be done with hand-sorting or automatic sorting.

The separation of metals can be done with magnets for ferrous metals and eddy current for non-ferrous metals.

Batteries constitute a main source of chemical pollution. Until effective systems of collection at the source are implemented, they will continue to constitute a major challenge for material management in MBT plants. Around 90% of batteries are magnetic or slightly magnetic and thus, can be sorted by magnetic separators.

– 15 –

For handsorting the mechanical preparation might be simpler than for automatic sorting. For the handsort-ing a sorting cabin should be provided with adequate air conditioning to protect the sorters health. (see Figure 1).

Figure 1: Hand sorting station

Sorting can also be done by automatic sorting. Figure 2 shows the key elements of a optical sorting instal-lation. The automatic sorting consists of different types of sensors for the detection of the recyclable mate-rials.

Near Infrared detection

Cameras

Laser

X-ray

For the separation typically air jets are used which are controlled by a computer. Once a recyclable waste piece is detected, the relevant air jets are controlled by the computer and blow the recyclable component in a separate chute. If several types of recyclables are separated (e.g. Polyethylene, PET, PVC, etc. ) several separation units are required. To enable an efficient separation the waste has to be spread thinly on an acceleration belt so the detection unit is able to detect the recyclable material. Furthermore there should be no other material on the same place as the recyclable material as it would be also separated and pollute the recyclables fraction. With an appropriate design more than 90 % of the targeted recycling component should be separated. The amount of contraries in the recycling material should be less than 10 % or even less. If high quality of the recyclable material is required a second separation of the separat-ed recyclables might be necessary. This “cleaning” step is often done by hand sorting, as a trained sorting person is more efficient for this purpose.

Each of the different sensor types has its strengths and weaknesses.

Colour-based sorting devices (optical sensors) have been used for over 20 years.

– 16 –

X-ray transmission, evaluation of thermographic images, and electromagnetic measurements, are newer technologies and can achieve potentially better performance because they are independent of the item surface, dirt, or moisture.

Relatively new developments are X-ray systems, image detection, and NIR detection coupled with pneu-matic discharge. These technologies offer novel capabilities for chemically based sorting of waste, in line with the emerging higher requirements for effective material flow management. If their effectiveness can be demonstrated, this could constitute a major breakthrough in waste handling. Promising combinations of NIR with image analysis, using sophisticated cameras, enable separation of materials based on special-ized optical characteristics, such as the surface design.

Figure 2: Automatic sorting with optical detection

Automatic separation of recyclables is typically done for the heavier waste components which have been separated by means of a ballistic separator or wind-sifter. The application of automatic sorting to the light fraction is less efficient as the throughput has to be reduced to ensure that the waste components are placed as single components on the separation belt. Therefore this material is often used for the prepara-tion of RDF. The heavies fraction also has some high calorific material left after separation which can be separated to the RDF fraction. Figure 3 presents the concept for a complex mechanical sorting plant which enables the separation of both different types of recyclables as well as RDF. It is possible to select different types of recyclables for the sensors and thus the plant can be adjusted to the recyclables which are of highest value or volume.

The preparation of the RDF depends on the process for its utilisation. E.g. RDF for cement production should be high in calorific value and low in chlorine. Therefore a separation of PVC might be required which can also be achieved with automatic sorting. Furthermore a maximum particle size would be re-quired of approx. 20 mm. Therefore an appropriate shredder is also needed in this case.

– 17 –

This will be through some form of shredding and additional treatment to separate the organic materials from the non-organic materials. The differences are in the type of the biological treatment (aerobic or an-aerobic) and the treatment target (stabilisation or drying to foster subsequent separation stages).

Figure 3: Mechanical preparation with automatic sorting of recyclables and RDF

More details about RDF are to be found in chapter 5.3.

A detailed overview about RDF/SRF from MBT can be found in Velis and; Longhurst, et al., 20103.This

study presents a lot of information and could serve as a base for a more comprehensive understanding of concepts and design criteria for MBT facilities with special focus on RDF production.

4.2 Biological treatment

For the biological treatment, which aims on efficient biological stabilisation, two basic methods can be used:

Aerobic decomposition or rotting, i. e. decomposition in the presence of oxygen, and

Anaerobic digestion, i. e. decomposition in the absence of oxygen.

3 Velis CA; Longhurst PJ; Drew GH; Smith R; Pollard SJT Production and quality assurance of solid recovered fuels

using mechanical-biological treatment (MBT) of waste: A comprehensive assessment. Critical Reviews in Environ-mental Science and Technology, vol. 40, pp.979-1105. 2010.

– 18 –

4.2.1 MBT - Aerobic Stabilisation

The key target of this approach is to stabilise the waste and hence to reduce the amount of biodegradable municipal waste (BMW) going to landfill. This is based on the requirements of the EU landfill directive and was implemented in different EU member states with different methods to determine the reduction of the biodegradables content in the waste.

For the purpose of BMW diversion from landfill an MBT plant could simply compost all waste without any separation and landfill the residues. This might be a first stage of the development of a waste treatment system and would improve at least the situation with respect to landfill gas emissions and leachate pro-duction.

In addition, this would be a straightforward solution which would not rely on markets for products such as RDF, compost-like outputs or other recycling fractions.

The more common approach is shown in Figure 4. It combines the biological treatment with mechanical processing steps to separate waste streams prior or/and after the biological treatment. The configuration can comprise a wide range of technologies and a wide range of products. This is reflected in the mass flow diagram which shows a fairly high range for the waste fractions that can be separated for recovery.

A common approach is the front-end separation of a RDF fraction which will be utilised in industrial pro-cesses like cement kilns, coal power plants, purpose built combustion facilities (e.g. to feed the energy to an industrial process) or in a mass burn incineration.

In case of a front end separation, the material left after the separation stage is enriched with easily de-gradable components like (residual) food and garden waste and “dirty” paper, like tissues, which are not suitable for recycling. This material is then treated through an aerobic process (rotting, composting) where aerobic (oxygen demanding) micro-organisms transform organic wastes. In the process the bacteria grow and reproduce by using energy and nutrients available in the organic matter. This process yields carbon dioxide, water (vapour) and heat. The time taken for rotting is usually determined by the rate at which the feed can be hydrolysed. Higher temperatures accelerate the hydrolysis stage, but the number and diversi-ty of micro-organisms that can survive these higher temperatures is reduced.

The continuation of the composting process requires the addition of water. The stabilised waste can then be landfilled. An alternative applied in some countries is a compost-like output (CLO) that can be pro-duced through a post-refinement stage. (For the utilisation of CLO please see chapter 5.4 and the re-quirements defined in the Biowaste Ordinance)

At this stage other material, like RDF or aggregates can be separated as well if a market is available and the process is economically viable.

– 19 –

Figure 4: MBT for stabilisation of the organic rich fraction for landfilling

Aerobic Stabilisation is a natural process: Microbes degrade the organic material into carbon dioxide and water. For this process they require oxygen which is supplied with the ambient air. In case of oxygen defi-ciency anaerobic processes occur resulting in increasing rates of methane emissions. Methane is a gas inducing 25 times the global warming effect compared to carbon dioxide. Therefore the entire process of aerobic stabilisation has to be designed and operated in a way that avoids or at least minimises methane emission.

Option A: Rotting in open Windrows without forced aeration

The simplest option for aerobic stabilisation is the open windrow rotting: In this process aeration is achieved by using a windrow turner (Figure 5). In this case windrows are formed in triangular shape with a maximum height of 1,5 to 2,5m. Passive aeration works mainly via convection, oxigen penetrating the pile predominantly in the surface zones from outside. Since the central and bottom part of the heap may be lacking oxygen, frequent turning is required, ideally every day but at minimum once per week

4 ()

4 Further criteria for the process optimisation of open windrow composting is given the National Giudance on Compost-

ing

mechanical processing

composting

RDF (5 - 50%)

Recycables (3 - 25%) (metals, plastics, etc.)

50 - 95%

stabilised waste

25 - 60%

post refinement RDF, recycables

reject

s compost like output (5 - 15%)

Water Carbon dioxid

20 - 30%

Input

100%

– 20 –

Figure 5: Self propelled turner

Option B: Rotting with forced aeration

In order to better control and enhance the rotting process, forced aeration systems can be applied. Typi-cally pipes are mounted into the rotting slab and connected to fans. The fans either blow air through the material or suck it. Aeration systems can be applied to different systems. The simplest system is open windrow rotting similar to the systems previously explained. In order to control the exhaust air emissions, negative aeration systems can be used, as this allows the capture of the exhaust air and sending it to an air treatment system, in most cases a biofilter.

As an alternative, semi-permeable membranes can be combined with positive aeration. The semi-permeable membranes are capable of reducing the odour emissions substantially, while still enabling an efficient aeration (Figure 6).

Figure 6: Forced aeration system with semi-permeable membrane (Gore-system)

More sophisticated rotting systems include windrows in a fully enclosed building. The turning can be achieved using a front-end loader or automatic turning systems.

– 21 –

Figure 7: Table windrow rotting with automatic gantry mounted turning machine (upper picture)

and tunnel rotting (lower picture)

For all rotting systems aiming on effective and high decomposition performance an optimum moisture content in the rotting material is is of utmost importance. This means that the moisture content must be adapted during all process stages to the water holding capacity of the rotting material. To be independent e.g. from heavy rainfall, roofs or semi-permeable covers can be used.

4.2.2 Conclusions on rotting systems

Passively aerated systems require the lowest technical effort and the lowest costs. However, these sys-tems require a longer treatment time to achieve the treatment target and the largest area. As the entire process is not encapsulated, possibilities of emission control are limited. If the process is not performing

– 22 –

properly, particularly when passive aeration is insufficient or completely inhibited in parts of the windrow or the entire windrow, anaerobic conditions will occur. This may affect odour and methane gas emissions.

When implementing forced aeration the treatment time can be reduced and thus less space is required. Most of the forced aeration rotting systems are equipped with an exhaust air collection and treatment sys-tem

5.

4.2.3 MBT with Anaerobic Digestion (AD)

Anaerobic digestion is a biochemical process which takes place in a vessel in the absence of oxygen and results mainly in the formation of a carbon dioxide and methane gas mixture known as "biogas"

Anaerobic digestion is very often referred to as a separate MBT approach. This might be justifiable from the aspect that renewable energy is produced. If looked at with respect to legal requirements for waste treatment, AD is just one component of a MBT strategy.

The most common approach is the use of AD as part of the stabilisation process of the organic fraction.. AD in such a context would then be used as the first stage of the biological treatment which focuses on the anaerobically easily degradable waste components. The "biogas" produced during digestion is used to provide internal electrical power generation and heating requirements. Surplus electrical power (and heat) can be sold as renewable energy.

The digestate is usually dewatered and treated aerobically (composted; often referred to as “maturation”). The purpose of the second stage is to further stabilise the waste, reduce the mass and reduce the odour potential of the material.

Figure 8 shows such an approach. The flow chart looks very similar to the “stabilisation” approach. There is a significant impact in terms of process technology involved and the investment costs of such an ap-proach are higher. On the other hand, revenues from the biogas utilisation via CHP can be generated which might offset the higher investment costs.

An alternative to the approach of dewatering and further maturation is the direct use of the digestate as a liquid fertiliser/soil conditioner. Digestate from ixed waste streams, based on the requirements of the Bio-waste Ordinance No. xx from yy.zz.2013 is restricted to reclamation layers on closed landfill and mining sites..

5 Waste air treatment and with biofilter and scrubber systems are described in the National Giudance on Composting

– 23 –

Figure 8: MBT with anaerobic digestion

There is a wide variety of AD technologies available which are summarised below.

In general, continuous and discontinuous processes can be distinguished. The continuous systems can be either fully mixed systems which are usually equivalent to wet AD systems, or they are designed as upright or horizontal plug flow reactors.

Wet digestion systems

Reactors equipped with a mixing agitator are regarded the “classical wet digestion” (Figure 9). The diges-tate is like a slurry but feedstock can be solid or liquid when added to the digester and mixed in the di-gester to create the required moisture content. If needed additional process water is added. The reactor is mixed by mechanical devices or hydraulically. To be capable of being mixed thoroughly the material has to be fairly wet (semi-liquid, pasteuse).

To enable the treatment of mixed household waste a thorough mechanical treatment is required to sepa-rate contraries from the waste material which would disturb the anaerobic process. This is mainly mineral components (glass, sand, stones). Such material causes high wearing in the pumps and dewatering ma-chines of the plants. Furthermore they will settle in the digestion tank and reduce the volume of the tank. Typically wet separation processes are used to both, separate coarse material and the inerts. These pro-cesses use the properties of the waste components in terms of swimming or sinking often combined with hydro-cyclones. The main purpose of these processes is the separation of inerts which would disturb the further process (filling of the digester; wearing of the pipes, pumps etc. )

mechanical

processing

AD

RDF (5 - 50%)

Recycables (3 - 25%)

(metals, plastics, etc.)

50 - 95%

stabilised waste

25 - 60%

post refinement RDF, recycables

rejects

compost like output (5 - 15%)

70 - 120 m³ Biogas/t

Input

100%

maturation

– 24 –

An appropriate front-end preparation of the waste is crucial for a successful operation of a wet digestion. Mixed waste cannot be treated in simple anaerobic digestion systems which are designed for the treat-ment of slurries or energy crops.

Figure 9: Wet digestion system

Dry continuous AD systems

In dry, continuous AD systems, the material moves through the reactor without being mixed with fresher or older material to a large extent. The digestate is either in a pourable stage or also pasty (Figure 10). For the dry continuous process the waste also requires a mechanical-frontend preparation. This typically con-sists of a screening at approx. 50 mm. Only material smaller than 50 mm is suitable for the digestion pro-cess. Further steps might be included, e.g. ballistic separation to reduce the content of inert material which contributes to higher wearing in the process. Typical retention times in the digestor are 3 weeks. Afterwards the digestate is dewatered: the solids are then further matured. The liquids contain high con-tents of nutrients which would suggest an utilisation as liquid fertilizer. But the liquids also contain contam-inants which might prevent the land application.

Figure 10: Horizontal, dry digestion system (Kompogas)

– 25 –

Dry discontinuous AD systems

The discontinuous systems are batch processes. Usually the fermentation units are designed as batch batteries of a certain number of containers in order to reach a quasi-continuous process with almost con-sistent gas production. For the batch systems the material has to be sufficiently solid to allow it to be han-dled with a front-end loader (Figure 11).

Figure 11: Discontinuous (batch) type digestion (Bekon)

– 26 –

Most of the batch systems claim that one of their advantages is that there is no mechanical preparation required at all. As the material has to establish a good structure to enable the percolation of the liquids through the material a shredding might even be counterproductive. If the material does not have a good structure, bulking agents may need to be added, e.g. drier waste, shredded bulky waste or shredded wood waste. The anaerobic digestion systems operate in one of two distinct temperature regimes for optimum

growth rates of anaerobic microbes – mesophile (35 to 40°C) or thermophile (45 to 60°C) temperatures. Under thermophile conditions the process is faster, i.e. more biogas is produced in the same time and hence the retention time can be reduced in comparison to mesophilic conditions. In addition, the high temperatures support the sanitisation even though the typical requirements of the Animal By Products Regulation (EC) No. 1069/2009,(e.g. 70°C for one hour) might have still not been met. One disadvantage of the thermophile conditions is the higher energy demand for heating the material.

4.2.4 Biological Drying

“Biological Drying” is another fundamentally different MBT approach. The scope of this approach is to use heat produced in the aerobic process to drive off water from the waste and hence dry the waste.

Unlike other aerobic treatment options, the biological drying of waste is not aiming for a maximum degra-dation of the organic matter, but for a short-term drying process of 1 to 2 weeks. Once the waste is dried, it can more easily be sorted for recyclables like plastics, paper, textiles, wood and fine organic matter can easily be separated. Sorting is typically done by a combination of mechanical separation steps (screening, wind-sifting, ballistic separation) and hand-sorting or automatic sorting (optical, x-ray) to separate the re-cyclables with a high purity, like PET, PE etc. In addition to the recyclables a RDF material is also pro-duced. The RDF typically also includes the dried organic fraction. The amount and quality of RDF from dried waste is better than RDF from separation of fresh waste. The remaining residues to be landfilled consist mainly of non-organic (inert) material such as sand, stones and glass. The amount of landfill mate-rial is typically around 20 % subject to the waste composition of the fresh waste. A first separation and recycling of glass and inerts is technically also possible but often is too expensive.

The most well-known technology suppliers/developers of this approach are “Herhof” (Germany, now owned by the Greek civil construction company “Helector”) and “Ecodeco (Italy)” but there are other tech-nologies available which can be used for biological drying.

The waste is shredded and placed in enclosed bio-drying boxes for a pre-determined period. Air is forced through the waste creating optimum conditions for microbial respiration, and hence drying of the waste. Air passed through the boxes is re-circulated, which significantly reduced the volume of exhaust air.

There are also a few examples of existing facilities where no biological system is used for the drying pro-

cess but a physical drying is used instead of using gas or oil to produce the heat for evaporating the moisture from the waste.

Figure 12: MBT-Rennerod (left) and Dresden (right), Germany (both 100,000 Mg/a)

– 27 –

Technical Process steps

Figure 13 shows a generic process flow and mass balance for a. biological drying MBT.

Figure 13: MBT – biological drying

After the waste has been delivered to the facility, it is dumped into a deep bunker with a storage capacity of several days. By means of an automated crane, the waste is then delivered to a pre shredding unit to open the sacks and to ensure a maximum particle size for the downstream processes of biological drying and mechanical sorting (Figure 14).

Water Carbon dioxid

shredding

composting

rejects to landfill

approx. 20%

mechanical processing

RDF (40 - 50%)

metals (2 - 4%)

Input

100%

Recycables

?

20 - 30%

70 - 80%

– 28 –

Figure 14 Waste Bunker (left) and crane to transport the waste within the MBT (right)

After the pre shredding, a second automated crane system passes the waste to the biological drying area consisting of a number of drying boxes made of reinforced concrete. The boxes are equipped with an airtight lid system. Each of the drying boxes has an effective volume of approximately 600 m³ and can take approximately 300 tons of waste. During the filling process, the filled level of the box is automatically monitored by the crane system. Once the box has been filled, the crane lifts the lid and closes the box, rendering it air-tight. Due to the fully automated operation, no manual activities are required in the bunker and decomposition hall.

The biological drying process is a strictly aerobic degradation process using a computer controlled forced aeration system and aims to reduce the moisture content in a short time based on the exothermal biologi-cal decomposition.. Due to an automatic control system adjusted to the requirements of the biological conversion process, the easily degradable organic substances are converted into heat during a six to sev-en day aerobic biodegradation process. This heat is used to evaporate the humidity and thus to dry the waste. No external heat is required for the drying process.

Due to the individual control of each drying box and an even air supply, it is possible to guarantee an even and efficient drying process. The relevant data such as heat quantity, temperature curve, and CO2 dis-charge are entered into the process control as is the air permeability of the waste. In an optimum biocon-version process, the mass is reduced by up to 30% in 6 days. The dried waste then has only a residual moisture content of less than 15%. The reduced moisture contents leads to a highly efficient mechanical sorting (quality and quantity wise) by means of different screening and wind shifting steps in order to sepa-rate combustible parts of the waste such as plastics, wood, textiles and organics (RDF) from inerts such as sand, stones, glass and ceramics. Depending on the moisture content and the composition of the treated waste, the amount of separated RDF ranges between 40 to 50 % by weight.

Unlike MSW which cannot be stored for a while without producing odour and leachate emissions, RDF becomes biologically stable and storable (e.g. by baling) after the biological drying process. Hence RDF can be utilised when it is needed, e.g. stored in summer for winter.

After the drying process, magnets and eddy current separators can separate metals. Compared to a sep-aration of metals out of untreated waste, the quality of these metal fractions is much better and those frac-tions can be sold directly to the recycling industry.

Another aspect is that compared to raw waste, full separation of metals out of MSW leads to a significant reduction of the content of heavy metals in the RDF. The reduction rate is around 80 to 90%. This is why such types of RDF easily meet the requirements set by the cement industry for secondary fuels.

After separation of combustibles and metals, the remaining fraction mainly consists of inert material such as sand, stones, glass etc. Compared to the treated waste the amount of this fraction is only at around 15 – 20 % by weight. Due to the previous separation of combustibles and organics, this remaining fraction

– 29 –

can easily be landfilled. Aside from the high mass reduction also the emission potential of this fraction is significantly less compared to untreated waste.

The exhaust air arising from the drying and sorting processes contains significant quantities of dust (from mechanical sorting) as well as steam, CO2 and odorous substances (from biological drying). In this pro-cess air is extracted and partly reused for the biological drying process. An Exhaust Air Treatment System consisting of a dust removal unit followed by a humidifier/bio-filter system cleans the main air volume. All waste processing parts of the plant are enclosed and process air is extracted at every point of emission in order to minimise plant internal emission, which could affect workers.

– 30 –

5 Outputs from Mechanical Biological Treatment

5.1 Stabilized organic waste fraction for disposal

5.1.1 General explanations

During biological treatment the organic matter is decomposed and hence reduced. Depending on the pro-cess parameters applied and the waste composition, a mass loss from 25 – 45 % due to biological de-composition and water loss can be achieved. Since the easily biodegradable components have been de-composed or transformed into humus complexes the remaining organic substance is stabilized and thus the landfill gas potential is reduced to a great extent.

In extensive research, predominantly in Germany, but also in Austria, Italy and other countries it has been demonstrated that several parameters may be used to determine the biodegradable content of waste.

At the end biological tests measuring the aerobic (respiration) or anaerobic (gas formation) decomposition have been selected in individual countries and implemented in national regulations or guidelines. In vari-ous inter-laboratory tests, the reliability and accuracy of these parameters have been confirmed. These tests measure the oxygen demand of the treated waste (respiration activity, AT4; DR4) or the gas for-mation (GB21; BMP100) on laboratory scale. The lower the oxygen demand or gas formation rate the more stabilized is the waste.

– 31 –

Table 1. Stability parameter and limits across Europe

Country Parameter Limits Method/regulation

Germany Static respiration index “AT4”

Gas formation test “GB21”

< 5 mg O2/g dm

< 20 Nl/kg dm

Fixed in German landfill ordinance[

6]

Austria Static respiration index “AT4”

Gas formation test “GB21” or “GS21”

< 7 mg/g O2 dm

< 20 Nl/kg dm

Fixed in Austrian landfill ordinance[

7]

Italy Dynamic respiration index (Adani method) DRI [

8]

< 1,000 mg O2/(kg VS x h)

Regional requirements

England and Wales

Change of biodegradability in from beginning to end of a treatment process, biodegradability parame-ters:

- Biological methane potential in 100 days “BMP 100”

- Dynamic respiration index “DR4”

No limits but determi-nation of the reduction of the gas potential in a treatment plant

UK Envrionment Agen-cy guidance[

9]

Scotland Change of organic content from beginning to end of a treatment process

Assessment parameter proposed:

- LOI (loss on ignition)

Alternative approaches are possible

Equivalent to Eng-land/Wales

Scottish guidance [10

]

EU Static respiration index “AT4”

Dynamic respiration index (Adani method) DRI

< 10 mg O2/g dm

< 1,000 mg O2/(kg VS x h)

2nd

Working Document of a draft EU Biowaste Directive 2001, with-drawn [

11]

Whilst in other European countries parameters to assess the organic content in waste have not yet been implemented in the national regulations, the parameters and limits proposed in the 2nd draft Working Document on an EU Biowaste Directive in 2001 are often used on a regional level.

6 German Ministry of Environment, 2001: Ordinance on Environmentally Compatible Storage of Waste from Human

Settlements and on Biological Waste-Treatment Facilities; 20 February 2001; http://www.bmu.de/files/pdfs/allgemein/application/pdf/ablagerungsverordnung.pdf

7 Verordnung des Bundesministers für Umwelt über die Ablagerung von Abfällen (Deponieverordnung); modified

23.01.2004 StF: BGBl. Nr. 49/2004; http://ris1.bka.gv.at/authentic/index.aspx?page=doc&docnr=1

8 Riffutti e combustibili rcavati da rifiuti, Determinazione della stabiliata biologica mediante I´indeice di Respirazione

Dinamico (IRD); UNI/TS 11184, ottobre 2006; www.uni.com

9 Environment Agency (2005): Guidance on monitoring MBT and other pre-treatment processes for the landfill allow-

ances schemes (England and Wales); http://www.environment-agency.gov.uk/commondata/acrobat/the_final_outputs_1096040.pdf

10 Landfill Allowance Scheme (Scotland) Regulations 2005: SEPA Guidance on Operational Procedures; http://www.scotland.gov.uk/Publications/2005/06/08111144/11463

11 EUROPEAN COMMISSION; Working document; Biological Treatment of Biowaste, 2nd draft; http://www.compost.it/www/pubblicazioni_on_line/biod.pdf

– 32 –

The limits applied in Germany and Austria are somewhat stricter than proposed in this Working Docu-ment.

This is because the limits have been derived from an existing technical guideline in Germany (“TASI”; TA Siedlungsabfall), where limits for loss on ignition (<5%) and TOC (<3%) were specified. In a court case it has been successfully demonstrated that the 3% TOC could be fully degradable organic material similar to sugar. From one tonne of waste with 3% fully degradable organic substances, about 55 m³ of landfill gas could be produced in a landfill. This sets the benchmark for stabilised waste. It can then be demonstrated from repeated landfill simulation tests with biologically stabilised waste that waste with a respiration rate AT4 of 5 mg O2/g dm shows a gas potential of usually less than 55 m³ landfill gas. Furthermore the gas potential of waste with an AT4 <5 mg O2/g dm is reduced by over 90% compared to fresh, untreated waste.

As a consequence it has been confirmed by EU that MBT treated waste can be classified as non-degradable if the limits in the German and Austrian regulations are achieved.

If assuming that the 65% reduction requirement in the EU landfill directive refers to a reduction of landfill gas production, then the limits set in Germany and Austria exceed the EU landfill directive requirements. A 65% reduction of the landfill gas production corresponds more closely with the limits set in the 2nd draft EU biowaste directive, i.e. < 10 mg O2/g dm

England and Wales developed a different approach. This does not suggest a limit value but a calculation of the actual diverted mass of biodegradables from landfill. The gas formation test BMP100 is carried out with the fresh waste prior to and the processed output after the biological treatment ). The reduction of the biogas potential in the process is defined as reduction of biodegradables.

The disadvantage of this approach and also of the proposed tests is their complexity:

The BMP100 test (bio-methane potential test) takes 100 days to present a result. This can cause prob-lems especially during commissioning of new plants as it delays the determination of whether a plant is performing successfully or not.

One general criticism of biological tests sometimes expressed is that there may be toxic substances in the waste which could inhibit biological activity during the test and hence show a lower biodegradability than in reality. Whilst this might be relevant for untreated waste it is less relevant for waste that comes from a biological treatment plant because if there was a toxic component in the raw waste it would have had an impact in the biological process and hence would have been detected earlier. Nevertheless, an additional non-biological test could be introduced in a landfill regulation to mitigate this risk. In the German landfill ordinance the total organic carbon has been specified in addition to the biological tests for this purpose.

In order to comply with the EU Landfill Directive, which requires the operator to reduce the environmental impact of landfills with respect to landfill gas emissions and leachate quality by 65 %, the following threshold values for the reduction rates of BMW have to be met when using MBT:

Table 2: Recommendations of threshold values to meet the EU Landfill Directive for bio-

mechanically treated waste

On the basis of investigations in operational MBT´s following treatment periods are required to achieve the corresponding stability level. This applies to MBT with good technical standard (e.g. forced aeration, enclosed composting systems). More time has to be allowed for simpler technological solutions.

Threshold values to meet EU Landfill

Directive

Respiration activity (within 4 days) < 10 mg /g DM

Total organic carbon in eluate (TOCeluate)

< 700 mg/l

– 33 –

Table 3: Processing time for the biological treatment to achieve a certain degree of

biological stability

Respiration activity

< 5 / mgO2/g DM

(German limits)

Respiration activity

< 10 mg/g DM

(EC limit)

Aerobic treatment (Rotting)

8 – 16 weeks 5 – 8 weeks

Anaerobic digestion (AD) + composting

2-3 weeks AD

4 – 10 weeks rotting

2-3 weeks AD

2 – 5 weeks rotting

5.1.2 Further benefits for the landfill behaviour

In addition to the mass reduction and stabilisation of the organic matter, the structure of the waste is changed. As a consequence the compaction rate of the disposed pre-treated residual waste improves due to homogenization and reduction of particle size and change in material properties. Generally, the com-paction density for untreated waste can be estimated at 0,6 – 1,0 Mg/m

3 (depending on waste composition

and landfill equipment). For pre-treated residual waste a higher compaction density up to 1,2 Mg/m3 is

achievable depending on the MBT process and compaction technique. Additionally, daily coverage of pre-treated disposed waste using soil is not necessary, since the fine fraction has properties similar to soil. All the mentioned factors contribute considerably to extension of the life span of landfills and the reduction of required airspace respectively. Generally, the lifespan of landfills can be at least doubled, depending on the MBT process and landfill operation technique.

Furthermore the disposal of pre-treated residual waste leads to reduction of leachate generation due to higher compaction of the disposed waste and subsequently decreased permeability. Due to the decompo-sition and stabilisation of organics, the quality of the leachate improves considerably, reducing the amount of Total Organic Carbon (TOC), Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) by up to 90%.

5.1.3 Requirements for biological treated waste to be landfilled

Waste from MBT treatment can only be landfilled if following criteria are met:

Respiration rate AT4 < 10 mg/d dm

Total organic Carbon in Eluate TOCeluate < 700 mg/l

Mixing of different waste types with each other, or with other materials, in order to achieve compliance with the stabilisation criteria above for the relevant landfill class shall not be permitted. The requirements for a sanitary landfill in Bulgaria still apply.

With a higher degree of stabilisation of the organics a gas collection system in the landfill is no longer required but the landfill has to be covered with a landfill oxidation layer. The limits for this case are:

Respiration rate: AT4 < 5 mg/d dm

Total organic Carbon in Eluate TOCeluate < 350 mg/l

– 34 –

As part of the mechanical biological treatment recyclable material, waste components with high calorific value and pollutant-containing fractions should not be landfilled but be separated as part of the treatment. The competent authority might impose additional requirement to enforce this requirement.

5.1.4 Frequency of analyses

For the process control and internal measurements of AT4 and TOCeluate at the plant have to be conduct-ed at least once a month. More frequent analyses can be requested in case of inhomogenous wastes, varying types of waste and changes in the operation of the plant.

External independent Analyses of AT4 and TOCeluate by certified laboratories have to be done depending on the capacity of the plant:

Table 4: Number of analyses

Capacity output stabilized organic

fraction (tonnes per year)

Number of analyses per year

<10.000 4

10.000 - 20.000 6

20.000 - 50.000 9

> 50.000 12

The analyses has to be done according following methods

AT4 (Austrian Norm ÖNORM S 2027-4:2012 06 01)12

TOCeluate (EN 1484)13

Equivalent methods according state of the art are possible but the equivalence has to be demonstrated and confirmed by the competent authority.

5.2 Recyclables from MBT

Recyclables derived from the various MBT processes are typically of a lower quality than those derived from a separate collection system from households or at recycling centres and therefore have a lower potential for high value markets. However, these plants may enhance overall recycling levels and enable recovery of certain constituent items that may otherwise not be collected efficiently in household systems (e.g. batteries, etc.)

During the mechanical step of the MBT, various types of recyclables can be prepared for recovery (see chapter 4.1). Criteria to be considered for successfully producing recyclables during MBT include:

Costs for recovery and revenues from marketing

Social aspects (i.e. possible competition to informal sector activities and micro-enterprises).

5.3 Refuse Derived Fuel (RDF) and Solid Recovered Fuel (SRF)

MBT output fractions intended as secondary fuels are referred to as solid waste fuels, secondary fuels, substitute fuels, or alternative fuels.

12 ÖNORM S 2027-4: Evaluation of waste from mechanical-biological treatment ― Part 4: Stability parameters ― Res-

piration activity (AT4)

13 EN 1484: Guidelines for the determination of total organic carbon (TOC) and dissolved organic carbon (DOC)

– 35 –

In the absence of a legal definition or universally accepted term, the two most established terms relevant to thermally recoverable waste fractions are RDF and SRF. Many other partially overlapping terms exist . Conventionally, RDF refers to a combustible, high-calorific value (CV) waste fraction (e.g., paper, card-board, wood, and plastic) produced by the mechanical treatment of municipal or similar commer-cial/industrial waste.

Refuse derived fuels cover a wide range of waste materials which have been processed to mainly to achieve a high calorific value. Waste derived fuels include residues from MSW recycling, industrial/trade waste, sewage sludge, industrial hazardous waste, biomass waste, etc.. The percentage of each type of component varies subject to both waste composition and applied biological and/or mechanical treatment.

The term SRF is as per the mandate given by the European Commission to the Technical Committee (TC343) of the CEN. The standard EN 15359:2011 SRF – Specifications and classes prepared by the CEN defines solid recovered fuel as solid fuel prepared from non-hazardous waste to be utilised for ener-gy recovery in incineration or co-incineration plants and meeting the classification and specification re-quirements laid down in prEN15359. “Prepared” here means processed, homogenised and upgraded to a quality that can be traded amongst producers and users. Scope of this EN says, amongst other notes, that SRF are produced from non-hazardous waste and that untreated municipal waste is not included in the scope of the document. It then goes onto classify SRF into chosen fuel characteristics to be used for trad-ing and for information or permitting authorities etc. The classification is based on net calorific value (eco-nomic), % chlorine (technical) and mercury content (environmental).

Annex A Part 1 of the standard also lists parameters that are obligatory to specify. Annex A Part 2 lists non-obligatory properties to be specified and this list includes compositional information on the weight percentage of main fractions of wood, paper, plastics, rubber, textiles etc. The EN also gives a template that can be filled in on the fuel preparation techniques used. This information would give the end-user valuable information on how to store, transport and handle the fuel. A wide range of possible preparation techniques are listed in the EN including sorting (manual as well as mechanical), biological treatment, crushing, grinding, shredding, separation, screening, washing, drying, cooking, homogenisation, compact-ing etc.

Almost every MBT plant configuration is capable of separating an RDF/SRF product.

For MBT plants a main distinction can be made between the specific plant configurations.

Those in which production of SRF is their principal objective that employ an initial bio-drying step coupled with downstream extensive mechanical processing and

those where RDF is a by-product of only mechanical pretreatment, with the aim to optimally sepa-rate the organic fraction for subsequent biological treatment to stabilize the waste prior to land-filling.

One of the most important characteristics of RDF is its increased calorific value compared to raw waste. For example, biological drying followed by a separation of the inert matter (stones, glass, and ceramics) leads to an increase in calorific value by around 80%.

Alongside, in comparison to the incineration of unsorted waste (mass burning) the energy recovery of RDF is less harmful due to a significant decrease of the heavy metal content achieved by the precedent sepa-ration of pollutants such as metals, batteries and E-scrap.

Transforming waste into RDF requires certain technical standards for mechanical and/ or biological waste processing in order to comply with the technical, environmental and administrative specifications of the proposed industrial processes.

This is not referring to a legally binding standard but it refers to the fact that different RDF utilisation pro-cesses require different properties (=“standards“) of RDF. This can vary quite a lot and can be dependent on the individual plant concept and technology. It is therefore typically specified by the operator of the RDF utilization plant and the RDF producer has to design the plant to comply with the requirements.

In terms of its energy content, RDF, e.g. derived from MBT, can provide an energy equivalent compared to solid fossil fuels such as wood or different types of coal.

However, the calorific value of RDF basically depends on both parameters: composition of the waste and the type of treatment / sorting of the raw waste and can thus vary within certain limits.

Quality defined RDF can be used as a substitute for fossil fuel in designated industrial plants, such as cement kilns or coal fired power plants, as most of its chemical parameters are comparable to coal. As RDF represents only a certain percentage of the total fuel amount, this kind of RDF utilisation is called co-

– 36 –

incineration or co-firing. Based on the increasing demand for RDF in Europe, during the past ten years co-incineration has become a common practice for most of the European cement industries.

Alternatively, RDF can be used in designated industrial power stations in order to produce electricity and/or process steam for industrial purposes such as the production of paper or chemical goods. A num-ber of such plants have been built in Europe over the past years due to the reasons described before.

The following figure illustrates the options for RDF utilisation:

Figure 15: Energy recovery options for RDF

Both, physical and chemical composition of RDF is of major importance with regard to the dedicated com-bustion process. Hence, the design of the RDF separation process has to meet the requirements as set by its final end user.

Whilst substituting fossil fuels in industrial combustion processes such as cement kilns, coal fired power plants or tailored industrial power stations, RDF can be regarded as a “secondary fuel” with regard to pri-mary fuels such as coal, oil and gas.

Regardless of its application in mono-incineration or co-incineration systems, RDF/SRF must fulfil general quality requirements in order to be efficiently utilized.

The RDF/SRF output should be produced to a specification in accordance with commercial agreements with the end-user, in addition to national and international quality assurance and control (QA/QC) proce-dures.

From the perspective of an MBT plant operator, this translates into three objectives. The first is to achieve a high yield of the RDF/SRF product.

Secondly the operator seeks to raise the heating value, compared with the plant input; and thirdly to re-duce the chemical (e.g., volatile trace elements of concern, such as Hg) and physical contamination (stones, glass, porcelain, ceramic, concrete, Fe and non-Fe metals) of the RDF/SRF fraction.

In order to achieve high recovery rates for the RDF/SRF fraction, effective concentration of combustible particles, such as plastics (excluding long-lasting plastic products), papers and cardboard, packaging composites, textiles, and wood, is needed.

In the case of bio-drying, inclusion of the biomass fraction is attempted.

Incorporation of the biogenic content into the RDF/SRF can be highly desirable. It concurrently serves the main target of diverting biodegradable waste away from landfill and results in a secondary fuel high in biogenic content, which qualifies for the production of renewable energy.

RDF

- Coal fired power stations

- Cement kilns

Biomass related carbon credits

Power plants

for industrial energy supply

– 37 –

Achieving a high calorific value (CV) is crucial for the marketability of RDF/SRF. CV of the bio-dried output is already been increased by reducing the moisture content. Mechanical processing can further improve the CV by separating out the incombustible mineral fraction, which largely constitutes of dry recyclables such as Fe and non-Fe metals, and secondary aggregates (stones, sand, glass, ceramics, porcelain, etc.).

One relevant aspect of RDF utilisation is the price advantage, compared to fossil fuels, along with its price calculability on a long term basis. Unlike fossil fuels which belong to more or less volatile world market prices, RDF is derived from waste which normally belongs to long term treatment/disposal contracts. Moreover, RDF, depending on the treatment process, contains a certain amount of renewable organics and thus benefits from an emerging demand for carbon credits.

Prices are very much dependent on local conditions. ItThey are is composed of an technical and market component and also the individual contract conditions. In Germany, for instance might be able to pay almost nothing very low gate fees may be found even for low quality RDF at the “spot market” if there is a high demand by WtE plants on the spot market whereas for longer term contracts requested for the same material a payment of up to 80 € or even more is required.

Cement kiln require high quality SRF. In some cases the cement kiln even pay up to 15 € per ton. In gen-eral prices are currently close to zero.

5.4 Stabilised MBT Output for reclamation of closed Landfill sites old mining areas and road construction

The primary goal of MBT is to minimise the environmental burdens of waste disposal by effective stabilisa-tion. Additionally, in MBT processes, recyclables could be recovered, Refuse Derived Fuels (RDF) and biogas for heat and/or power generation can be produced. The terms MBT and composting are often used together, as both processes are using very similar techniques. However, the two processes have to be distinguished in terms of input material and proper utilisation of the output. The main objective of compost-ing and/or digestion of organic material is to obtain high-quality compost, fertiliser and soil conditioner. This requires “pure” organic material as input, such as source separated bio-waste. As MBT aims for effi-cient reduction of the organic material in residual waste, the input is mixed municipal waste, which is pre-treated prior to disposal. A utilisation of the pre-treated waste (i.e. as soil conditioner) may be possible; however, it is subject to certain restrictions. If “Compost Like Output” or “Stabilised MBT Output” is pro-duced from mixed solid wastes in a MBT the quality criteria and requirements for proper land application have to be met according to the Biowaste ordinance.

– 38 –

5.5 Biogas

An MBT plant that uses Anaerobic Digestion (AD) as its biological process will produce biogas. During AD, the biodegradable material is converted into methane (CH4 ) and carbon dioxide (together known as bio-gas), and water, through microbial fermentation in the absence of oxygen leaving a partially stabilised wet organic mixture known as a digestate. Biogas can be used in a number of ways. It can be used as a natu-ral gas substitute (distributed into the natural gas supply) or converted into fuel for use in vehicles and engines. More commonly it is used to fuel boilers to produce heat (hot water and steam), or to fuel gen-erators in combined heat and power (CHP) applications to generate electricity, as well as heat. More de-tails about biogas and biogas utilisation are explained in the guideline National technical requireents for Anaerobic digestion plants

– 39 –

6 Technical requirements on the design and operation

6.1 Air Emission control

6.1.1 Plants with less than 75 tonnes per day capacity

For plants with less than 75 tonnes per day capacity the Industrial Emissions Directive (IED) does not apply. Nonetheless the plants should be designed in a way that emissions to the environment are mini-mized.

Assuming that predominantly mixed household waste is treated in the MBT the content of hazardous components should be low. Investigations in various MBT´s confirmed that the major part of emissions are intermediate products from the biodegradation of the organic matter, like methane, ammonia, alcohols, organic acids, etc. Compared to these substances the amount of toxic chemical substance (e.g. aromatic hydrocarbons) are fairly low.

What is common to all biological treatment concepts is the formation of odorous substances. These sub-stances are typically not toxic and dangerous but can cause nuisance in the neighbourhood.

In terms of environmental impact the formation of methane and release to the environment might become a relevant issue. Methane release can happen both in rotting and anaerobic digestion processes.

To avoid problems with odour and to minimize methane emissions following aspect have to be taken into account:

Anaerobic digestion:

Methane is the valuable component of biogas which can be converted into energy. This mainly applies to the digester itself and storage tanks, e.g. for liquid digestate. To avoid methane release to the environ-ment all parts of the digestion plant where methane is produced have to be closed and the biogas has to be captured and utilised. Furthermore methane can be produced in if the digestate is dewatered after the digestion for a subsequent aerobic maturation (rotting). It has to be ensured by technical and organisa-tional measures that the anaerobic process is stopped immediately and transformed into an aerobic pro-cess. This can be done by forced aeration in combination, if needed, with mixing with bulking agents to ensure the necessary pore space for proper aeration.

Rotting (aerobic process):

Methane production and release in the aerobic process can happen if the process is not designed and operated in an appropriate way. Relevant aspects for good conditions are

Enough air space in the waste material (add bulking agents if needed)

For windrow composting: The same principles apply as for biowaste composting. These are

explained in detail in the guideline National Technical Requirements for composting plants

(size of windrows, mixing of material, turning frequncy, moisture content)

For in-vessel composting: These technologies work with forced aeration. Typically the exhaust

air is captured and cleaned in a bio-filter. Design criteria and operation of biofilters are speci-

fied in the Austrian ÖWAV-Regelblatt (technical sheet) 51314

,

For good management of in-vessel composting system please refer to the guideline National

Technical Requirements for composting plants.

14 This ÖWAV technical sheet 513 „Operation and Maintenance of biofilters“ will be provided in Bulgarian language on the MoEW web page

– 40 –

Odour emissions cannot always be prevented, even under the best management conditions. The Odour emission to be expected depend on the type of technology used and the capacity of the plant. To avoid problems with sensitive receptors the location of the MBT plant have to be carefully selected. As part of the selection process odour distribution modelling should be conducted to determine the distance needed to sensitive receptors to avoid that they are affected by the plant.

6.1.2 Plants with more than 75 tonnes per day capacity

For plants with more than 75 tonnes per day capacity the Industrial Emissions Directive (IED) does apply. The same general requirements in terms of good operational practise as specified for plants with a ca-pacity less than 75 tonnes per day also apply for the plants with a capacity bigger than 75 tonnes per day.

As a general rule a minimum distance of 1.000 m to sensitive receptors applies. The technical standards for plants to be built with less than 1.000 mm distance have to be assessed site specific with the compe-tent authority.

Mechanical treatment

The waste reception and mechanical processing has to be done in an enclosed hall. Air has to be extract-ed from the mechanical treatment building. This air can be used for the aeration of the biological treatment of has to be send to an air treatment system. As a minimum a dust filter has to be used for the air purifica-tion. The dust removal has to achieve a level of less than 10 mg/m³ cleaned air.

Transportation and storage systems have to be designed to avoid dust emissions from the waste.

For the collection of dry products with dust production sealed container have to be used.

To minimize dust from traffic all roads have to be paved an regularly cleaned. To avoid the transfer of waste from the plant a wheel washing installation should be in place.

Biological treatment

It has been found that the major part of the air emissions are released during the first stage of biological treatment process.

The design of an aerobic biological process has to enable an entire capture of the exhaust air of this stage. The exhaust air of this stage has to be captured and cleaned. This stage lasts at least 4 weeks or until a respiration activity of less than 20 mg O2/g dm is achieved.

For the solid residues from an anaerobic treatment the aerobic post-treatment has to be done at least 2 weeks in an invessel composting system and a sufficient aeration has to be applied to ensure that the process has been converted to an aerobic process and no anaerobic process gases are released.

After this first stage the exhaust air from the process might be released to the environment if there are no other reasons to require a further capture and cleaning, e.g. too high odour immission at the nearest sen-sitive receptor.

The air treatment system has to achieve following limits in the cleaned air stream which is released to the environment:

Table 5: Limit values for the quality if the exhaust gas after purification in a biofilter

Odour < 500 oU/Nm³

(TOC) Total Organic Carbon < 50 mg/Nm³ and 500 g/t waste input

Ammoniac < 20 mg/Nm³

Dust < 10 mg/Nm³

oU … odour units

For the air purification an efficient biofilter including scrubber system is seen as necessary as a minimum.

– 41 –

Air emission measurement

The emission requirements as specified above have to be analyzed by a competent person or institution. In the first year of the operation the paramters have to be analysed every 4 month. In the following years the frequency of the measurement has to be done depending on the capacity of the plant:

Table 6: Limit values for the quality if the exhaust gas after purification in a biofilter

Capacity (to/a) Odour (n/a) TOC (n/a) Ammonia (n/a) Dust (n/a)

< 30.000 1/2 2/1 2/1 2/1

30 – 50.000 1/1 3/1 3/1 3/1

50 – 100.000 2/1 4/1 4/1 4/1

> 100.000 2/1 5/1 5/1 5/1

n/a = number of analyses per year

The evaluation and assessment of the emission analyses have to be done according to generally accept-ed methods which have to be confirmed by the approval authority.

Odour minimisation

Unfortunately, odour emissions cannot always be prevented. Effective operational management can help control the formation of odours. This includes:

a. processing incoming feedstock as soon as possible

b. ensuring proper stabilisation of the biomass within the retention time in enclosed build-ings, so as to ensure only odourless materials are present in the open curing stage

c. avoiding an early refining step to reduce the particle size too far, which would hinder the diffusion of air through the material that still has to complete its biochemical transfor-mation (a smaller particle size could cause the aerobic digestion to lose structure and make anaerobic decomposition more likely)

d. preventing the formation of leakage puddles (e.g. ensuring proper slopes to paved sur-faces)

e. avoiding the external stockpiling of coarse rejects from pre-process screening steps, as these would also contain a certain percentage of fermentable materials

f. withdrawing the exhaust air from the odorous sections of the process (tipping, deep bunkers storage of input fermentable materials, pre-treatment, early process steps. Sometimes also the curing section can be enclosed and exhaust air treated)

g. designing the withdrawal system to prevent any loss of exhaust air from windows, doors, etc.

h. fitting the facility with properly dimensioned odour abatement systems

i. ensuring proper maintenance of odour abatement technologies is carried out

j. using surfactant reagents

k. providing enclosed leachate collection/storage tank(s), to minimise odour emissions while holding liquor prior to recirculation and/or off-site disposal

l. providing stored leachate treatment, such as aeration, to prevent septic conditions caus-ing odour

m. providing odour abatement, to control emissions from specific sources, such as odour masking atomisers

n. designing the enclosed buildings in such a way so as to have a negative air pressure, to prevent odour emissions from doorways.

More details about the minimisation of odour formation in the biological treatment are explained in the guideline National Technical Requirements for Composting Plants.

– 42 –

Bio-aerosols

Depending on the nature of an individual facility, the health effects of MBT facilities might be expected to be comparable to those of composting facilities, such as those related to bio-aerosol emissions. Bio-aerosols are normally found in higher concentrations at facilities where large amounts of organic matter are processed.

Bio-aerosols may comprise of complex mixtures of micro-organisms transported in the air. They are common in rural environments and may arise from a wide variety of activities including agriculture. Some bio-aerosols can cause health problems, notably Aspergillus fumigatus , but also some other fungal spores and bacteria.

One source of bio-aerosols is composting operations and similar waste treatment processes.

Raised levels of community exposure to bio-aerosol may arise within 250m downwind of a composting facility and under rare circumstances at distances of up to 0.5 km

Available evidence suggests that communities located more than 250m away from composting facilities are unlikely to be exposed to harmful levels of bio-aerosols; however they may experience odours associ-ated with the process as these can travel much further.

Bio-aerosol emissions can be mitigated by conducting operations that may give rise to higher quantities of bio-aerosols (such as screening and shredding) within an enclosed building. A risk assessments shall be undertaken on sites where there are sensitive receptors nearby.

6.2 Waste water collection – groundwater pollution control

In order to prevent the pollution of ground water, all areas of the plant where waste is transported or treat-ed have to be paved water tight. Further measures might be necessary in particular parts of the plant, e.g. retention basins, double layer containers, etc. All water tight paved areas have to enable a capture and save removal of polluted water. A system to control the water tightness of the pavement might be neces-sary in certain circumstances.

6.3 Treatment of process water

All polluted liquids have to be reused in the process (watering of the rotting material) or treated in an ap-propriate waste water treatment plant. For waste water a sufficient storage capacity has to be in place to enable undisturbed operation of the plant.

6.4 Workers health and safety

MBT design and operation has to be done in compliance with the relevant Bulgarian health and safety regulation.

6.5 Fire and explosion protection

MBT design and operation has to be done in compliance with the relevant fire and explosion prevention regulations