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Air pollution control residues

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Description of Overall Problem

Air Pollution Control (APC) residues are the solid output of the flue gas treatment equipment installed on incinerators (this report refers specifically to APC Residues from incinerators handling Municipal Solid Waste - MSW). They comprise the fly ash from incineration (middle and fine grades) together with the reagents (mainly lime and activated carbon) used in the flue gas treatment. Thus, they contain:

  1. Volatile contaminants from the original waste (inc chlorides, metals),
  2. Compounds created in the incineration process (inc dioxins),
  3. Further materials from the flue-gas treatment process (sulphates, together with high alkalinity).

Therefore they are classified as hazardous waste.

Approximately 170,000t/y (Technology Strategy Board 2009) of such residues are produced in the UK - 3-4% of the total waste mass incinerated (Environment Agency, 2002). This tonnage is growing as more waste is incinerated to generate electricity and heat, and to reduce landfill. While increased energy recovery and reduced landfill are worthwhile in themselves, achieving them has created the problem of the hazardous APC residues. In the UK the prevalent destination for these residues has been landfill, but this option is under threat from tightening landfill Waste Acceptance Criteria, and rising landfill taxes, so new solutions are required.

There are various treatment/recovery options available for APC residues. However these raise other concerns, primarily:

  1. Financial and energy cost of treatment
  2. Generation of further effluent
  3. Environmental impact of the treated waste
  4. Quality control of the recovered materials.

Objectives and Scope

The overall goal is to identify cost-effective management options for APC residues, within Waste Acceptance Criteria. The ultimate objectives of implementing such options are shown in Requirements (Appendix 1). For this study, the specific the objectives are:

  1. Briefly analyse the shortcomings of the existing methods of treatment and disposal of APC residue in landfills, along with the barriers in the UK for re-use of APC residues in various industries, such as cement aggregate, asphalt and ceramics.
  2. Propose energy- and cost-effective methods for the treatment of APC residue which reduce the leachability and amount of heavy metal/dioxins present. Also suggest a supplier of technology for each treatment method proposed.
  3. Compare the cost per tonne for each option, including treatment and disposal costs (including current and future landfill taxes), based on the hazardous classification of any remaining waste.
  4. Suggest potential re-use opportunities for materials recovered from the treatment process, indicating potential markets and revenues.

The scope is focused on APC residues from municipal waste incineration. It is assumed that current incineration technology and operating conditions apply, with waste of current composition, resulting in residues of current composition. The objectives have been pursued in the context of current UK and EU regulation. This is explained in terms of the waste management hierarchy in table 1.

Notes of Figure 1:

  1. Further processing leading to recovery may be in or outside the system boundary depending on whether the process is likely to be dedicated to this application. In either case the resulting wastestreams are inside the system boundary
  2. Landfill operations are outside the system boundary, but the long term leaching behaviour of all landfill waste will be considered, even if it meets WAC.

Report structure

This report has been structured to give an overall review of the management options for the Air Pollution Control residue, intended to provide a details of the findings related with work aiming to give recommendations on its treatment.

Chapter 1. Background and scope.

Chapter 2. Introduction to the residues, overview of major management strategies, legislative aspects, and environmental issues

Chapter 3. Details on the residue treatment techniques, operation principles, and development status

Chapter 4. Appraisal on the recovery and utilization techniques, operation principles, and development status.

Chapter 5. Overview of status for available solutions, documentation level, assessment approach for environmental impacts, outline of important aspects for consideration, qualitative and quantitative comparison of each treatment processes.

Chapter 6. Recommendations

System Engineering Management Plan (SEMP) is listed in Appendix 2. This is an outline of system requirements and mechanisms for verifying whether the requirements are met. It will provide an overview to integrate different technical elements of the project. The plan will also describe the activities, processes and tools used to ensure an achievement of the project outcomes to the client and other stakeholders. Press Release is placed in Appendix 3 and this would form a basis of a publicity campaign for the project.


APC residues generation and characteristics

APC residues come from the cleaning process of the gaseous emissions, which are produced during the incineration. Dry and semi-dry scrubber systems are used in the cleaning process and involve the injection of an alkaline material to remove acid gases, particulates and flue gas condensation (Sabbas et al. 2003). Finally, fabric filters in baghouses are used, where the fine particulates, i.e. the APC residues are focalized and removed from the gaseous emissions (Sabbas et al. 2003). It is estimated that APC residues represent 2-5% of the original waste on a wet basis and their production in the UK is approximately 128,000 tonnes per annum (Amutha Rani et al., 2008). In general, APC residues from municipal solid waste incineration (MSWI) consist of fly ash, carbon and lime and contain dioxins and furans (Amutha Rani et al., 2008). They are highly alkaline materials (pH 12.0-12.6) and they comprise significant concentration of heavy metals, salts and micro-pollutants (Sabbas et al. 2003). Depending on the initial waste composition, the incinerator and the air pollution control system, their composition may vary significantly (Amutha Rani et al., 2008). The typical range of APC residues composition is shown in table 2.

The APC residues are characterized as hazardous wastes (190107*, according to the EWC) due to their chemical content and their impact on the environment, primarily by leaching.

Regulatory Framework


Regulations and legislation on waste management in the UK have evolved considerably over the years as a result of identification of new pollutants, public health and environmental concerns, economics and technological advancement (Pocklington, 1997 and McDougall et al, 2001). This assertion suggests that legislation and regulations play a major role in ensuring sustainable waste management. In addition, the establishment of legislation on waste management shows the radically changing perception of humans and communities towards the environmental impact of human activities (Pocklington, 1997). Today, regulations and legislation provide a framework for efficient handling of hazardous wastes such as APC residue. Amutha Rani et al (2008) observed that sustainable management of APC residues depend on the implementation of UK and EU waste management legislation.

The Existing regulatory and legislative framework for managing APC residues in the UK

About 80% of the environmental legislation in the UK have their origins in the European Commission laws (Pocklington, 1997). The existing legislative and regulatory framework for APC waste management in the UK and EU include:

  1. Waste Incineration Directive
  2. Integrated Pollution Control Directive
  3. Landfill Directive and ensuing waste acceptance criteria/procedure
  4. EA guidance on the classification of hazardous waste
  5. Water Framework Directive

However, the discussion on the regulatory and legislative framework for this project focuses mainly on the UK Landfill directive and EA guidance on classification of hazardous wastes. These subjects are pertinent within the boundary of this project more so as Landfill disposal is common in the UK. Also IPPC directive is discussed briefly to highlight the roles public participation and deployment of best available techniques in meeting our objectives.

The key objectives of these legislation and regulation are to:

  1. Reduce the amount of APC residue generated and improving the quality of exhaust gas (McDougall et al, 2001)
  2. Reduce the amount of APC going to Landfill (EA Guidance on landfill, 2006)
  3. Prevent environmental impact (ESA 2004)
  4. Reduce the risk of human harm (US National Research Council 2000, ESA 2004)

This diagram illustrates the relationships between the established regulatory framework and stages in the APC management process. There is no specific legislation covering recovery or reclassification of APC residue in the UK (ESA 2004). Quina et al (2008) also points out that legislation for recycling APC has not yet been established in the UK.

The Integrated Pollution Control Directive: Directive 2008/1/EC concerning integrated pollution prevention and control

This Directive aims at establishing means to prevent or reduce emissions into air, water and land (IPPC, 2008). Hence this directive is crucial as it suggests various methods of incineration and treatment that could reduce the impact of APC residues on the environment during incineration, treatment or landfill. The IPPC Directive is based on four principles namely:

  1. Best Available Technique (BAT)
  2. Integrated waste management
  3. Flexibility
  4. Public participation

The BAT refers to the most effective methods of operation that would reduce environmental impact and enhance results such as making residues from incineration less hazardous. In the BAT, optimizing resources and harnessing or saving energy are prioritized (Gargulas N. and Mentzis A, 2007). Also, the BAT is flexible and no terms are imposed since it recognizes that different conditions apply in different cases. The Best Available Techniques Reference (BREF) is a reference document on technical input needed to determine the BAT to be adopted. This BREF contains technical information on available means of treating APC residues such as sintering, vitrification, stabilization and solidification. This project has considered the BATs to APC treatment and these methods are discussed in chapter 3. However, there are no BATs available for landfills. Notwithstanding, Landfill operators and APC treatment plants require permits issued by the Environment Agency with public support to ensure that there are no health or environmental impacts as a result of their activities (Macleod C. et al 2006 and IPPC 2008).

The role of the public is crucial in this directive. Article 15 of the Directive, gives the public full privileges to participate in decision making processes leading to the issuance of permits for installation of plants, and for carrying out technical and administrative changes. This aspect is very important especially in the proper project planning and execution (see SEMP). Therefore the installations of APC treatment facilities and the method involved are tailored to meet public requirements as well as legislative requirements. All hazards inherent in operating APC treatment facilities shall be made known to the public in accordance to this directive. Also the outcome of compliance tests on treatment facilities with regard to environmental impact shall be made public (IPPC 2008). Thus, it can be argued that since the public are key stakeholders in this project, good public perception is needed in accordance with the IPPC directive to ensure sustainable management of APC residues.

Environment Agency guidance on classification of hazardous waste

The essence of this guidance is to distinguish different kinds of wastes based on their physical and chemical properties which include their toxicity or hazardous nature.

The Hazardous Waste Directive (HWD), council directive 91/689/EC and the Revised European Waste Catalogue (EWC) form the regulatory framework for this guidance. The HWD aims at defining "hazardous wastes to ensure the correct management and regulation of such waste" (EA Hazardous Waste 2008, pg 5). This directive identifies 14 hazardous properties of wastes, thus hazardous wastes are classified H1 - H14 according to their hazardous properties (EA, Hazardous waste 2008). The EWC code is derived from the industry and process producing the waste, and the type of waste (EA hazardous waste 2008).

APC residues are categorized as hazardous wastes with absolute entry (Rani et al 2008 and ESA 2004). Under the European Waste Catalogue (EWC), APC residues fall under the category of wastes from incinerators (waste management facilities) that have a generic code '19'. The specific code for solid wastes generated from gas treatment such as APC residues is 19 01 07.

Wastes resulting from the treatment of the APC residues such as the partially stabilized APC residue, the vitrified material etc are also categorized as hazardous with absolute entry (EA hazardous waste 2008). However, when tests confirm that the constituents of treated waste have become less or non hazardous, they can be reclassified as hazardous wastes with mirror entry or non-hazardous as the case may be(EA Hazardous waste 2008). Solidified and partly stabilized wastes are coded 19 03 06 and 19 06 04 respectively with absolute entries, while vitrified wastes from flue gas treatment are assigned the code 19 04 02.

The Landfill Directive 1999/31/EC of 26 April 1999 on the landfill of waste

The primary objectives of the landfill directive are:

  1. To reduce waste going to landfill
  2. the prevent or minimize environmental impact as a result of waste disposal

Stringent measures and standards have been set to reduce the burden and reliance on landfill (EA guidance on landfill, 2006).

Landfills are classified into three categories: non-hazardous, inert and hazardous landfills (Landfill (England and Wales) Regulations, 2002)

For APC residues, a key requirement in the Directive prior to landfill is to perform tests to determine its long term and short term leaching behaviour and to carry out treatment to reduce its toxicity (EA guidance on landfill, 2006). This project suggests that the need for treatment of APC residues augments the case for exploring potentials for recovery or reclassification of the residues.

Article 6 c (iii) of the Directive reflects our aim to make APC residues at least "stable non-reactive" hazardous wastes (SNRHW) or completely non-hazardous through efficient treatment techniques such as vitrification, washing, stabilization and plasma technology. SNRHW are known to have low leaching potentials (EA guidance on landfill, 2006). Treated APC residues with leaching behaviour equivalent to those of non-hazardous waste can be disposed at designated non-hazardous landfill subject to meeting the relevant waste acceptance criteria (EA guidance on landfill, 2006 and Landfill (England and Wales) Regulation, 2002).

APC residues must meet the waste acceptance criteria (WAC) for a designated landfill after treatment. Schedule 1 of the Landfill (England and Wales) (Amendment) Regulation 2004, stipulates the procedure and criteria for disposing or accepting waste at landfills.

Waste Acceptance Criteria and Procedure

The WAC is elaborately designed to deal with the technical requirement of wastes such as APC residues designated for landfills in the UK (England and Wales). It also aims at controlling the disposal of wastes into landfill which is a common practice in the UK. Technically, the WAC ensures that the numerical leaching characteristics of APC residue are determined prior to disposal at landfill (EA Guidance on Landfill 2006). Thus, after their mandatory treatment, APC residues must meet the relevant waste acceptance criteria before they are accepted into landfill. The leaching characteristics include: the elements and compounds in APC residue and their leaching properties (in mg/kg or L/S) and the hazardous nature of the APC residue (EA Guidance on Landfill, 2006). The Environment agency is responsible towards ensuring that the criteria for particular landfills are met. Preceding the WAC is the Waste acceptance procedure.

The Waste acceptance procedure for APC requires basic characterization, compliance testing and on-site verification (EA Guidance on Landfill 2006). The basic characterization is done to determine the physical and chemical characteristics of the waste (EA Guidance on Landfill 2006). Incineration plants are responsible for carrying out the basic characterization of the APC residues since they produce the APC while the landfill operator ensures that compliance testing and on-site verification are done (EA Guidance 2006). Approved tests are defined in schedule1 part 2 of the landfill regulation 2004 and they include tests for determining treated APC composition and leaching behaviour. The compliance tests are carried out to verify if leaching limit levels predicted in the basic characterization are credible.

Besides exploring opportunities for reuse of APC residues and recovery of valuable materials from the residues, this project also aims to treat APC residues to meet at least the waste acceptance criteria for SNRHW to enable disposal at a non-hazardous landfill. Amendment 14 of the Landfill (England and Wales) regulation 2004 states the criteria for disposing SNRHW in the non-hazardous landfill.


There is no specific legislation on the reuse of wastes such as the APC residues in the UK. Notwithstanding, the UK without incineration network (www.ukwin.org.uk) tagged the use of fly ash and APC residues for construction works as "irresponsible". Thus it can be suggested that "re-use" is implied in this clause as well. However, if it is well proven that APC residues can be managed sustainably without any long or short term environmental repercussions, it will pave way for debates to strengthen the existing regulatory frame and also re-focus the views of environmental activists toward the prospects in the residues.

ESA report (2004) argues the provision of regulatory certainty by the government is necessary in enhancing investment towards sustainable management of APC residues. The report also suggests that investments will focus on reliable APC treatment technologies. There are several scientific developments for managing APC residues pioneered by waste management companies such as Techtronic in the UK.

Environmental problems and management strategies

Most of the APC residues (around 88%) (Environmental Agency, 2002) produced in the UK, are disposed of into landfills. During their disposal or any kind of utilization or handling, a number of environmental impacts can be caused.

Dust and Gas emissions

Dust emissions are represented as a potential risk, due to the size of the APC residues particles (0.001-1 mm) (Sabbas et al., 2003). Despite the easily dispersion of these fine particles, a survey by the Environmental Agency at a number of landfill sites in the UK testifies that their concentrations are within the recommended air quality objectives (Environmental Agency, 2002).

Gas production is another potential environmental impact related to the disposal of APC residues. Gas is produced by metallic aluminum hydration (Sabbas et al., 2003) and because of that, some explosions have been reported (Sabbas et al., 2003). However, the production of gas is significantly lower compared to the production of the municipal solid waste landfills, due to their low biodegradable content of the APC residues.

Leaching production

The major environmental impact is the leaching production of APC residues The leaching behaviour of the elements present in APC residues is the main source of environmental concern. Leachates can cause pollution of soil, groundwater and surface water bodies. The leaching behavior of the APC residues is very complex and depends on a lot of parameters. The pH and the liquid to solid (L/S) ratio of the residues that will occur in the landfill site are important factors which affect determine their leaching behaviorbehaviour as well as the availability of the elements which are contained in the APC residues.

The pH depends on the characteristics of the leaching fluid and the waste, i.e. APC residues, and is the key factor of many elements' leachability. Leaching of most major elements (e.g. Al, Ca, S, Mg) and heavy metals (e.g. Cd, Pb, Zn) are strongly pH-dependent (Astrup et al., 2006). This dependency of the pH causes a significant difficulty on the prediction of the leaching behaviorbehaviour. Generally, APC residues carry on their pH in alkaline values for a long time (many thousands of years) (Astrup et al., 2006). However, their pH decreases as the time passes and the APC residues are washed by the infiltrating water (the neutralize capacity decreases) (Astrup et al., 2006). Thus, the prediction of the landfills pH and thereby the leaching behaviorbehaviour of the residues in over a long term period is complex.

The L/S ratio represents 'the amount of the leachate that comes in contact with a given amount of APC residues' (Sabbas et al. 2003 pp what page?) and depends on the characteristics of the APC residues and the climatic conditions, the hydrology and the hydrogeology of the area (Sabbas et al. 2003). Usually, as the time of disposal passes the value of the L/S ratio becomes higher for a particular application site. Due to this contact the properties of the waste as well as the leaching behaviorbehaviour of the waste change. Thus, the value of this ratio is a very important parameter for the leachate content.

The availability for leaching is a parameter, which characterizes the particular waste and represents a fraction of the total content of contaminants in the waste itself (Sabbas et al. 2003). The typical values of the availability for the APC residues are shown in table 2 and they can provide a theoretical estimation of the maximum release of a contaminant in a period of 1000 to 10000 years (Sabbas et al. 2003).

The prediction of the leaching behaviour and the evaluation of the environmental impact of APC residues are based on leaching tests. Leaching values for the APC residues arising from leaching test are summarized in table 4.

The first leachate from APC residues is usually characterized from soluble salts (e.g. chlorides, hydroxides of calcium, sodium and potassium) and trace element such as Pb and Mo (Sabbas et al., 2003). Contrary to the high solubility of this elements, the solubility of toxic organic compounds is believed to be not high due to their hydrophobic nature and their low concentration in APC residues (from properly operated MSWI plants) (Sabbas et al., 2003).

Long term leachate concentrations are usually lower than the initial or they may remain atto the same level. The only exceptions are the elements Al and Zn, which concentrations in the leachate are increase d inover a long term period (Astrup et al., 2006).

As it is explained above the leaching behaviour of the APC residues depends on the environmental conditions and changes during the time of the disposal. Thus, an analytical prediction of the long term leaching behaviour is very difficult and it should be based on a combination of 'information on leaching principles, leaching tests, field measurements, simulation of mineral changes and speciation' (Sabbas et al., 2003 page number pls). Due to the complexity of the long term leaching behaviour, the data available in literature are limited.

Management of APC residues

In the UK the disposal of any waste to landfill is regulated (see regulations section). Generally, the landfills are classified as suitable for hazardous, non-hazardous or inter wastes and, for each of these types of landfill, particular leaching limit values (Waste acceptance criteria, WAC) are defined and should be achieved for any waste are to be landfilled. Table 5 shows the leaching limit values (WAC) for the three types of landfill sites and if they are compared with the values in table 4, it becomes obvious that APC residues cannot be landfilled without a prior treatment.

  1. And non-hazardous waste deposited in the same cell.
  2. Either TOC or LOI must be used for hazardous wastes.
  3. UK PAH limit values are under development.
  4. Following the recent consultation exercise the UK Govt may review the limit values in tow years time (2006).
  5. If an inert waste does not meet the SO4 at L/S 10 limit, alternative limit values of 1500 mg/l SO4 at C2 (initial eluate from the percolation test) prEN 14,405 and 6000 mg/kg SO4 at L/S10 (either from percolation test or bach test BS EN 12457-3), can be used to demonstrate compliance with the acceptance criteria for inert wastes.
  6. The values for TDS can be used instead of the values for Cl and SO4.
  7. Or DOC at pH 7.5-8.0 and L/S 10 can be determined on pr EN 14429 (pH dependent test) eluates.

Disposal to landfill (Amutha Rani et al., 2008)

APC residues are mixed with wastewater to form a solidified product. During this treatment the residues react with the CO2 from the atmosphere reducing the pH to values between 8 and 9. This mixing also eliminates the dispersion of the APC residues particles. After this treatment, the APC residues reach the WAC and they are landfilled into monofill cells at a hazardous waste landfill. This process is used by a treatment plant in GloucesstershireGloucestershire, from which most of the APC residues treated by this method in the UK are coming.

Storage in salt mines

In this disposal method the APC residues are loaded in sealed capsules and pitted 170m below the surface (Amutha Rani et al., 2008). The disposal in salt mines can take place for a long term. They are characterized as 'well isolated, very dry, with stable atmosphere and natural gas-impermeable salt layers' (Clement, 2000). Salt mine for this purpose is located in Cheshire, England, where a major percentage of the APC residues, produced in the UK, are stored (Amutha Rani et al., 2008).

Use in waste acid treatment (Amutha Rani et al., 2008)

Due to the mixing of waste acid (usually HCl) and APC residues, the lime content of the APC residues is convertedsed into less hazardous components (CaCl2) and the concentrations of Zn and Pb are reduced. Furthermore, the pH is at high levels, preventing the salts release. Thus, the final mixture from this process is non-hazardous and it is described as sludge from a physico/chemical treatment; it is classified as EWC code 190206 and can be disposed of in non-hazardous landfills.


Ash Washing Process


The objective of Ash washing process is to extract a number of minerals from the APC residue obtained after Municipal Solid Waste incineration and thereby diminish the leachability of various compounds remaining in the residue. The process also aims to improve the quality of the residue obtained for further re-use applications or to reduce the overall content of waste going to the landfill. According to Quina et al (2001), ash washing, acid leaching, electro-chemical process and thermal treatment are some of the most widely used methods for extracting metal values from the APC residues.

The separation techniques studied in this section are ash washing with MgSO4, bioleaching using Asphergillus niger fungi and leaching using extracting agents. Each process has different prerequisites, operation time and cost, objectives and risks associated with them.

Ash Washing With MgSO4:

Chimenos et al (2005)

The process aims to apply the optimum parameters for washing APC residue by utilising minimum energy and water. This process uses multi-stage washing process to diminish the leaching of chloride and sulphate salts present in APC residue and thereby ensuring that the amount of harmful substance present in wastewater is reduced. The wastewater produced is recycled and re-used in the process using employing a rapid spray evaporation technique which runs on the waste heat produced from pumps, turbines and incineration furnace. Figure 3a showsrepresent the overall process diagram of operation.

The research conducted by Zhang et al (2008) shows that the leachability of the heavy metals and chlorides present in APC residue depends on its pH level. The pH of the solution, when MgSO4 is added during the washing process, may be controlled by the formation of gypsum as shown in Eq(1).

Ca(OH)2 + MgSO4 CaSO4 + Mg(OH)2.......................(1)


Q.Wang et al (2009)

This process is considered to be a biohydrometallurgical approach to extract heavy metals from APC residue. It is considered to be a 'green technology' because of it makes use of the natural ability of microorganisms to break down solid compounds into soluble and extractable form by enzymatic oxidation or reduction. The process uses the acids secreted by Aspergillus niger fungi such as oxalic acid, citric acid and gluconic acids to extract the heavy metals present. Water-washing was is used as a pre-treatment before the bioleaching process to reduce the bio-leaching period from 30 to 20 days and to extract the maximum amount of chloride and sulphate salts. Figure 4 shows an overall process diagram for the bioleaching process. Bioleaching is a low cost and low energy consumption approach.

Leaching Using Extracting Agents

Fedje et al (2010)

This process uses leaching agents other than water for extracting heavy metals like Zn and Pb. The efficiency of the extraction agent depends on heavy metals of interest, the concentration of the extracting solution, the pH and the liquid/Solid ratio used. The goal of the process is obtain a solution in which the concentrations are high enough to enable further separation or recovery.

The leaching media used for this process are

  1. 3M HNO3 L/S = 5
  2. 0.1 M EDTA with pH adjustment L/S = 5
  3. 3M NH4NO3 L/S = 5

The choice of these leaching media was based on their ability to form a complex with metal ions. Table 6 compares the efficiency of the aforementioned leaching agent in extracting the heavy metals from APC residue. Figure 5 represents the overall process diagram.

The most widespread leaching method used for APC residues is acidic leaching using strong mineral acids such as HCl and H2SO4. However, due to the high alkalinity of APC residues, large amounts of acids are needed which results in trouble with storage and handling.

Moreover, the reaction of APC residue with mineral acids is very exothermic and it has a high rate of discharge of anionic concentration into wastewater.


The washing process is very efficient in removing approximately 90-100% of the water soluble chloride salts along with some of the sulphate salts present in APC residue with a few exceptions such as Pb, Cu, Hg, Sr and Ca. These heavy metals have to be removed using leaching media or bioleaching. Unfortunately, the washing process does not affect the percentage of dioxins present in APC residues. These dioxins can be broken down into simpler compounds by heating the APC residue to a temperature of 800-900^(o)C during the drying of ash after washing process.

Moreover, the washing process reduces the total volume of APC residue by 20-30% and increases its density. This is due to the fact that these soluble chloride salts are responsible for holding together some of the particles in APC residues.

The main disadvantage of the washing process is its demand for large quantities of water. However, this problem canit is suggested that this problem can be overcome by recycling and re-using this water within the incineration plant using employing a AquaSonics RSETM (Rapid Spray Evaporation) or other water purification system. The advantage of using the Rapid Spray Evaporation technique is that it can be run using the waste heat from incineration operation and from heat generated from a Combined Heat and Power (CHP) plant

Moreover, the extracted metals from the washing process can be sold as mineral ore for extracting the chlorides and other metals such as K and Na. These extracted chloride salts can be used for manufacture of chlorine which is used in garments, paper and other industries.

Washing process can always be supplemented by methods such as stabilisation and thermal treatment methods (vitrification and sintering). The advantage of such systems will be discussed in the following chapters.



Stabilisation aims to reduce the mobility of the metals such that the risk of leaching will accordingly be lessened in the landfill. [Quina et al. 2008] Stabilisation is not effective when there are soluble salts that APC residues often contain hence occurrence of undesirable leaching over time. [Quina et al. 2008]

Chemical Stabilization

Chemical stabilization involves the binding and immobilizing of the hazardous metal ions in the pollutants with "stabilizing agent" to make it thermodynamically stable. (Ecke et al. 2000) The process often comprises of several sub-processes, beginning with water extraction, then followed by chemical reaction and completed with dewatering. (ISWA 2008) (cf. Figure 6)


Solidification helps to reduce the hazardous nature through capsulation, with the purposeconcept of reducing thee surface area and permeability for contact. Solidification is also treated as a stabilization process, as activities of the metal ions will greatly be reduced. (ISWA 2008). In consequence, solidification creates a physical barrier and this helps to minimize the impact of heavy metals on the environment. (Quina et al. 2008). (cf. Figure 7)


Solidification/Stabilization (S/S) was developed basing on the combination of solidification and stabilization. It was commonly used because of their enhanced efficiency. The leachability toxicity level from the S/S process was found to be lower than solidification or stabilization on its own. S/S begins with the use of solidification transforming harmful substance into solid with the use of cement or pozzolanic materials to avoid hazardous chemical contact with the natural environment. Chemicals such as sodium silicate or soluble phosphates would be used to change the contaminants into less harmful form, since the mobile hazardous ions had been "stabilized". (Ecke et al. 2000) Both processes aim to decrease the rate of dissolution of the metal in landfill and it had to be closely monitored to comply with the current standards for timely rectification if required. However, the efficiency of interrupting the mobile metal ions depends on the method used so as the property of the cement and asphalt used for solidification. (cf. Figure 7)


The main problem of APC residue is about the high levels of soluble chloride salts, which is one of the reason of causing leaching, and possible leading to soil, ground, surface water contamination. (Dimech et al. 2009)

Therefore, chloride leaching data, price of cement (CEMI) and the cement content (CEMI) are presented in Figure 9 to study the effect of cement on chloride leaching in relation to the use of cement for changing the properties of APC residue. The result from the study is going to be used for investigating the possibilities of using APC residue for other purposes rather than sending it to landfill.

As noted from the findings, the concentration of chloride decreases while the price of the cement increases. Since the price of the cement correlates with the cost of the treatment, that implies, in order to achieve an effective removal of chloride, the treatment cost will be expensive, which is not a viable solution. Furthermore, the concentration drops rapidly at the beginning without any addition of cement, but the APC residue is not stabilized. Following on, it is not practical to pay a difference of £90 (£150 - £60) to obtain an insignificant reduction in 20 and 50 wt% of CEMI/APC residue (cf. Figure 94). Moreover, the volume of the APC contained cement will take up much area in the landfill and this is not desirable. (Lampris et al. 2009 in Gaynor 2010)

As a result, it is not feasible and economic to use cement to manage the APC residue. Therefore, this helps to eliminate two methods, S/S and solidification for treatment of this residue.

Thermal Treatment


The literature (Quina et al, 2007; Amutha Rani et al, 2007 (2)) identifies 3 types of thermal treatment:

  1. Sintering
  2. is defined as "the bonding of adjacent surfaces of particles in a mass of powder or a compact by heating. Sintering strengthens a powder mass and normally produces densification and, in powdered metals, recrystallization." (About.com:metals, 2010) In this case there are examples where this is carried out under pressure, others without. Sintering is discussed as a direct means of treating the APC residues, and as a secondary process after vitrification.

  3. Vitrification
  4. is defined as "the process of converting a material into a glassy amorphous solid that is free from crystalline structure. This can be achieved through the addition of heat or introduction of an additive. Vitrification occurs at the glass transition temperature which is lower than the melting point." (About.com:chemistry, 2010) In this case this is achieved in a furnace - either fuel-fired or electric (including plasma-arc), and requires the addition of "flux" (glass cullet, silica and alumina, or bottom ash) to achieve the correct glass properties. Vitrification may directly treat APC residues, or may require a pre-washing stage to remove salts, particularly Chloride.

  5. Melting (or Fusion)
  6. is similar to vitrification but without the addition of flux, resulting in a less homogeneous material. There is discussion of melting processes used in Japan (Quina et al, 2007; Amutha Rani et al, 2007 (2); Jung and Osako, 2009) and the potential for metal recovery. In Japan these processes are usually applied to combined bottom and fly ash, and in some cases to the raw MSW itself, so they are not directly comparable to APC residues in the UK. Advice from Tetronics (Tetronics, 2010), who operate such processes in Japan and (through a sister company) in recovering metal from automotive exhaust catalysts in the UK, is that metal recovery is unlikely to be economic in the UK. This option is not developed further.

Common Features of Thermal Processes

  1. Production of an inert material
  2. The principal output of all these processes is an inert material in which the remaining metals are bound into the structure and leaching levels are very low. The mass of this material is typically around the mass of the incoming APC residue, but the material is much denser (3-4 times). This is at least an inert material for landfill, and has the potential for recovery as a building material, replacing aggregate. Other higher value recovery opportunities (use in ceramics, metal recovery) are less well proven.

  3. Energy intensity
  4. Operating temperatures required vary between 800 and 1600oC, some with additional heat treatment, and all with the prolonged heating/cooling regimes that such high-temperature industrial plant typically requires. So all require significant energy input - for example plasma-arc treatment uses (in steady state operation) around 13MWh per tonne of APC residue (Tetronics, 2010). The energy is an important economic consideration, not only as a high absolute cost, but as a potentially increasing cost in the future, depending on carbon taxes/trading etc. Where this is electrical energy it may be advantageous to co-locate the treatment process with the incinerator, taking advantage of the electricity generated on site in the context of Renewable Obligation Certificates (ROCs), although that simply reduces the power available for sale so there is still a net requirement for additional power generation. Energy needs to be considered beyond the economic effect, in terms of CO2 emissions.

  5. Destruction of Dioxins / Furans
  6. While correct incineration operation should reduce these carcinogens to xxxxxxx, any remaining traces in the APC residues will be destroyed by the high temperatures in the thermal processes.

  7. Residual Waste
  8. All these processes generate their own flue gases, containing volatile components of the APC residues, requiring flue gas cleaning, and resulting in a small quantity of residual waste. Limited specific information is available from the literature, and further trials are required in this area:

    • The literature reports the need to clean the flue gas, and nowhere indicates any difficulty in doing this to meet relevant gaseous emission standards, so it is assumed that the flue gases can be effectively cleaned, i.e. gaseous emissions are then not an issue.
    • The literature generally does not quantify or characterise the solid waste. For this report it is assumed that it represents a similar percentage of the APC residue as the APC is of the original waste (around 3%), and that it is similarly hazardous to the APC residues, but clearly it will not be identical, and is likely to have higher levels of the volatile metals (Pb and Zn). In the economic calculations it is assumed that this has to undergo hazardous landfill disposal. Possible mitigation would include recycling through the treatment process (although this is not discussed in the literature), and treatment with phosphate (Quina et al 2007).

    An advantage of electric thermal treatment over fuel-fired is the much lower gas flow rate (because there is no need for a combustion air flow), reducing the quantity residue carried over.

Discussion of Process Options

Five possible processes are shown in figures 10 - 14 : (Diagrams to follow)

Figure 10 Fuel-fired sintering (Lee et al, 1999), with the option of further heat-treatment (Boccaccini et al 1995). This reports lab-scale work and does not quantify mass-balance, economics, or waste streams. Recovery would be from the regenerator pebbles, a more difficult process than the wet scrubbers and bag filters proposed elsewhere. This is not considered further.

Figure 11 Washing followed by Sintering (Dimech, 2008). This reports lab-scale work with mass balance information. It does not consider waste streams (including the liquid effluent from washing). There are a large number of process steps, including crushing and pressing. Further discussion to follow after discussion with David Deegan.

Figure 12 Plasma-arc vitrification (Amutha Rani et al, 2007 (1)). This reports pilot-scale work done in collaboration between Tetronics Ltd and Imperial College London. This option was explored further in discussion with Tetronics (Teronics, 2010). Unlike other references to vitrification, this allows thermal treatment without pre-washing, with the Cl extracted as (saleable) HCl in a proprietary process step. This is the only option with cost information available - £100/t of APC residues (Gomez et al, 2009), comprising £60/t operating cost, £40/t amortisation of capital. From this a capital cost of around £10-15M is estimated for a 30Kt/y (see appendix x). The client has further information on the economics of this option from Tetronics, given under confidentiality agreement. Further discussion to follow after discussion with David Deegan.

Figure 13 Plasma-arc vitrification followed by sintering (Roether et al 2009). Further discussion to follow after discussion with David Deegan

Figure 14 Washing followed by plasma-arc vitrification. Further discussion to follow after discussion with Anup and David Deegan



APC residues are the by-product from MSW incineration. APC residue is classified as hazardous waste due to its high content of heavy metals and soluble salts. Special treatments are always required for APC residues. However, the raise of APC residues is consequent on the increase of MSW. It leads to a gradual increase in landfill tax. Thus, sending the APC residues to landfill is not an ideal method. Utilization and recovery would be an alternative method to handle the massive amount of APC residues. Although appropriate utilization and recovery can minimize the environmental impacts of APC residues, there are a limited number of utilization and recovery solutions available at present.


The present elements of APC residues may be recovered and used again by suitable recovery methods. Thos The recovered elements can be used in other industrial processes. The aim of recovery is to produce a material which can substitute a similar raw material and used in a similar way.

  1. Salts
  2. Evaporation can be used to recover salts from treated waste water from wet scrubbers. A highly concentrated salt solution or re-crystallized salt can be produced through this process. Those plants without any permission for discharge of waste water can perform with this process. Salt can also be directly recovered from washed APC residues. Salts can be also extracted by acidic solution.

  3. Metals
  4. Acidic extraction is used for dissolution of solid materials. Most of the heavy metals present in APC residues are much soluble at low pH solution which results in improved heavy metals removal. Nevertheless, the quality of the recovered metals is varied due to the composition of solution used. Further treatments may be required for upgrading the quality of metals.

    Besides, appropriate control of those melting processes of APC residues may help to recover specific metal phases which depend on use of temperatures. (Astrup et al., 2008)


"Utilization is characterized by substitution of materials in products or applications to which the residues can contribute with useful properties." (Astrup et al., 2008 pp. x) The dominant principals of utilizing APC residues are (i) extending existing landfill capacity (and thus reducing disposal costs) and (ii) the ability to create from APC residues value-added products that conform to regulatory requirements for management and use (such as substitution for natural aggregates). The composition of APC residues has the similar essential components for cement. It is commonly used to substitute cement in concrete. APC resides are also suggested to be utilized as fill material or aggregates. The high contents of soluble salts and potential for hydrogen generation may lead to the difficulties on substituting APC residues directly for cement.

  1. Cement
  2. Substituting APC residues for cement in concrete for construction industries has been widely used, such as shore protection blocks, artificial reefs, etc. Compare with adding cement into APC residues for solidification, substituting APC residues for cement is relatively difficult. The strength development and settling time are directly affected by the amount of substitutes. (Geiker et al., 2006) Metallic Al in APC residues can generate hydrogen under moist conditions. (Astrup et al., 2006) As a result, cracks and disintegration of concrete with APC residues may happen. APC residues are also used for backfilling of mines to avoid collapse. It is done in German salt mines.

  3. Filler materials
  4. Filler materials can be used in embankments, highway ramps, noise barriers, etc. However, the pozzolanic properties of APC residues results in up taking water can cause hardening the material. It is now generally not accepted due to the changing of environmental aspects.

  5. Asphalt (need more details)
  6. Utilizing APC residues in bituminous structures can stabilize the residues and minimize leaching. (Ali et al., 1996; Sawada et al., 2001)

  7. Glass-ceramic
  8. Glass-ceramic characteristics materials can be formed blending washed APC residues with soda lime glass and waste electrostatic precipitator dust containing boric oxide. It can produce a dense glass-ceramic material with a hardness of 4.5GPa. It can be used in high value construction products due to the high density and hardness. (Dimech et al., 2008)

  9. eutralization
  10. APC residues are alkaline in nature which can act as a neutralization capacity of acidic waste materials. In Norway, it is utilized on the acid waste from the titanium industry (NOAH, 2003). The remaining products after neutralization are then landfilled. This utilization is also carried out in the United Kingdom (Veolia, 2007).


APC residues require treatments before disposal which varies the disposal costs since the treatment process costs varies with economics. Table 1x shows the total disposal costs varied in different countries. One of the key factors for utilization is the relative cost of disposal against utilization.

In Canada

In commercial applications, soluble phosphates are used as a stabilization process. The total disposal costs for untreated APC residues are typically €71.25/tone and €77.25/tone for treated APC residues. However, the treatments for APC residues may cost €49.5/tone.


APC residues are classified as hazardous waste which requires special treatments before landfill. Most heavy metals are naturally stable except they are being utilized. Recovery and utilization are more ideal to manage those APC residues in order to minimize the environmental impact. However, they are an ecological and economic challenge. Due to the variable chemical and physical characteristics of APC residues from different MSW incineration plants, they affect the acceptance of APC residues in re-use applications.

The recovered salts and metals are able to generate revenue for the MSW incineration plants. However, the content of both recovered materials highly depend on the treatment processes. Economic changes day by day, there is no exact number available for the costs of recovery and utilization. It is hard to predict the total costs of recovery and utilization.


APC Air Pollution Control

BAT Best Available Technique

BREF Best Available Technique Reference Document

CEMI Portland cement, Cement type I

EDTA Ethylene Di-amine Tetra Acetate

EWC European Waste Catalogue

H2SO4 Sulphuric acid

HCl Hydrochloric acid

HNO3 Nitric acid

L/S ratio liquid to solid ratio (L/mg)

LOS Loss of Size

MSW Municipal Solid Waste

NH4NO3 Ammonium nitrate

RSE Rapid Spray Technique

S/S Solidification/Stabilization

SEMP Systems Engineering Management Plan

WAC Waste Acceptance Criteria


Figure 1 Overall System Diagram Showing System Boundary.

Figure 2 The regulatory framework in APC residues management

Figure 3 Process diagram for ash washing with MgSO4

Figure 4 Process diagram for bioleaching with water washing pre-treatment

Figure 5 Process diagram for leaching with extracting agents

Figure 6 Process Diagram for Stabilization

Figure 7 Process Diagram for Solidification

Figure 8 Process Diagram for Solidification/Stabilization

Figure 9 Chloride Leaching vs. Cement Content vs. Price cf. Table 1

Figure 10 Fuel-fired sintering

Figure 11 Washing followed by Sintering

Figure 12 Plasma-arc vitrification

Figure 13 Plasma-arc vitrification followed by sintering

Figure 14 Washing followed by plasma-arc vitrification


Table 1 Scope of Project, shown in terms of the Waste Management Hierarchy

Table 2 Chemical content of APC residues

Table 3 Availability of APC residues

Table 4 Range of concentrations typically leached using the L/S= 10 EU compliance leaching test for granular wastes BS EN 12457-3

Table 5 Landfill waste acceptance criteria (WAC) for granular wastes

Table 6 Fraction of APC residue component released after 24h leaching.


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