Air pollution control residues
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Published: Tue, 25 Apr 2017
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:
- Volatile contaminants from the original waste (inc chlorides, metals),
- Compounds created in the incineration process (inc dioxins),
- 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:
- Financial and energy cost of treatment
- Generation of further effluent
- Environmental impact of the treated waste
- 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:
- 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.
- 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.
- 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.
- 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:
- 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
- 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.
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.
MANAGEMENT AND REGULATORY FRAMEWORK FOR APC RESIDUE
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.
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:
- Waste Incineration Directive
- Integrated Pollution Control Directive
- Landfill Directive and ensuing waste acceptance criteria/procedure
- EA guidance on the classification of hazardous waste
- 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:
- Reduce the amount of APC residue generated and improving the quality of exhaust gas (McDougall et al, 2001)
- Reduce the amount of APC going to Landfill (EA Guidance on landfill, 2006)
- Prevent environmental impact (ESA 2004)
- 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:
- Best Available Technique (BAT)
- Integrated waste management
- 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:
- To reduce waste going to landfill
- 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.
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.
- And non-hazardous waste deposited in the same cell.
- Either TOC or LOI must be used for hazardous wastes.
- UK PAH limit values are under development.
- Following the recent consultation exercise the UK Govt may review the limit values in tow years time (2006).
- 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.
- The values for TDS can be used instead of the values for Cl and SO4.
- 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
- 3M HNO3 L/S = 5
- 0.1 M EDTA with pH adjustment L/S = 5
- 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 resid
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