Weapons of mass organic destruction
Narinder Bains of Shipley Europe describes a new method for destroying organic contaminants and recycling wastewater from process effluent streamsEthylenediaminetetra-acetic acid (EDTA) and other organic compounds are widely employed in plating processes used by the Printed Circuit Board (PCB), metal finishing and plating on plastics industry. These materials can pose problems with downstream wastewater treatment, and metals and water recycling processes, due to their ability to complex heavy metal ions and their low biodegradabilities. Conventional treatment methods, such as carbon adsorption, air stripping and reverse osmosis (RO) can create secondary waste problems and are normally applied as 'end of pipe' treatments (ref. 1). The development of new technology to address these problems would be welcomed.
Many of the chemical processes used in the fabrication of PCB and metal finishing products include various organic species, which whilst being essential to the process, are undesirable from an effluent and waste treatment perspective. Some of these organics can inhibit the recovery of metal ions whilst others are potentially harmful and must be removed before the effluent can meet the demands of increasingly strict legislation and regulation. In particular, electroless copper plating, is a good example and it represents a key activity within the overall PCB manufacturing process. This activity uses process baths which contain various organic species such as EDTA, which can be present in concentrations of up to 40g/litre.
EDTA resolubilises metals and forms complexes which are difficult to destroy and thus its presence is undesirable from an effluent treatment perspective in this type of operation because it can prevent metal ions such as copper from being reduced to acceptable levels. Rinse waters following the plating operation become contaminated with EDTA and thus treatment using conventional wastewater treatment systems is much more difficult. In order to circumvent this problem many smaller companies need to have their waste tankered away, which is both inefficient and expensive.
Restrictive legislation on the discharge of chelating agents and general organics is becoming ever more stringent and new treatment methods are needed to help industry comply with these regulations and ideally to reduce costs. As well as being environmentally desirable, savings in water consumption, metal recovery and waste disposal costs would significantly improve the cost competitiveness of the UK and European PCB industry (ref. 2) with similar benefits to the metal finishing industry. Other industries that generate this type of waste would also benefit from new technology.
A project to address these issues has been underway for the past two years with research work being carried out by C-Tech Innovation and Shipley Europe. The ROCWAT project, detailed here, is a Cooperative Research 'CRAFT' Project funded by the European Community under the Energy, Environment and Sustainable Development Programme. It was undertaken to develop an innovative technique for the in-situ destruction of chelating agents and other persistent and toxic organics. The techniques would be deployed as integral parts of PCB and metal finishing manufacturing processes and effluent stream treatment.
The techniques used in this project, advanced oxidation and electrochemical oxidation, directly oxidise organic components without, it is anticipated, the production of unwanted by-products and secondary waste. The oxidation processes will ultimately result in the conversion of organic pollutants into benign final products such as carbon dioxide and water. In this work the two techniques are used in series: the electrochemical method being used initially to reduce initial higher levels of organics. The advanced oxidation technique is then used to further reduce the concentrations of organic compounds.Electrochemical oxidation
Electrochemical oxidation is an established technique, which has an extensive range of applications including chemicals manufacture, surface finishing, effluent treatment and water treatment (ref. 3). In principle, the technique can also be exploited as a new waste treatment method using electron transfer for the partial or complete oxidation of organic compounds in aqueous waste streams without the addition of chemical reagents. Oxidation can occur by direct electron transfer at the anode, or via the formation of highly reactive species, such as hydroxyl radicals, which are produced at the anode surface. Following demonstration in feasibility studies there is growing industrial interest in the technique. In small-scale laboratory work, diamond coatings have demonstrated high catalytic activity for oxidation of organic compounds and excellent chemical stability.
They have not yet achieved industrial status, being available only in small quantities and at high cost. This cost is decreasing with development however, and there is a reasonable prospect that prices will fall to a level which is economically justified by their greater durability. One of the partners in this project is a supplier of specialised anode coatings (ref. 4). The electrochemical technique has been found to be more effective at higher concentrations. Under these conditions the conductivity is greater, which is necessary for minimising power consumption, while the greater availability of the solute at the anode prevents the process being limited by mass transfer.Advanced oxidation
Advanced oxidation techniques are based on the use of ozone and/or hydrogen peroxide in combination with UV radiation. In the case of UV and ozone, UV radiation at 254nm is readily absorbed by ozone resulting in the formation of highly reactive hydroxyl radicals (.OH), which are more powerful oxidising agents than either ozone or hydrogen peroxide. These are capable of converting virtually all organic compounds to carbon dioxide, water and similar species. Hydroxyl radicals are a short-lived species formed when ozone in the presence of UV radiation (254nm) forms oxygen (O2) and an excited oxygen atom (O*). The excited oxygen combines with water to form both hydrogen peroxide and hydroxyl radicals. Hydrogen peroxide subsequently forms hydroxyl radicals in the presence of UV radiation. The basic equations leading to the generation of hydroxyl radicals by this route are shown below:
The oxidation rates achieved with hydroxyl radicals are much greater than those attainable from conventional oxidants such as ozone, hydrogen peroxide and chloride. In some cases, reaction rates achieved using hydroxyl radicals are 106-109 times larger than the rates achieved using ozone alone (ref. 5-9).Methodology/experimental
Although it was anticipated that electrochemical oxidation would be complementary to advanced oxidation, the tests were initially conducted in parallel, on the same solutions. The initial focus was on EDTA, but destruction of a range of other key organics was also evaluated. Eventually, real industrial effluent solutions were also tested. For electrochemical oxidation, a laboratory scale Dished Electrode Cell (DEM) was used. This has a recirculatory system and was developed in order to explore practicalities and indicate scaling parameters. All advanced oxidation work was carried out using a purpose-built laboratory scale recirculation system employing a commercially sourced ozone generator and a short-wavelength UV reactor vessel.
The in-vivo levels of organics likely to be encountered in typical PCB industry effluent plant solutions were believed to be in the region of 100-1,000ppm. Therefore, all initial evaluations have targeted 1g/litre (1,000ppm) initial concentrations of organic components with the exception of 'real' effluent samples, which can vary depending on the supplier and production demand. The experimental programme investigated the destruction of a range of target organic compounds. The organic compounds selected for investigation were EDTA, ethanolamine and thiourea amongst others, as they currently pose major problems to wastewater treatment, water recycling and metal recovery within the PCB and metal finishing industries.
Chemical oxygen demand (COD) determination was chosen as the common analytical method for measuring the concentration in the test solutions during and after treatment. It allows comparison of the destruction efficiencies of the various organics on a common and comparable basis across the different techniques. The COD is a standard measure for quantifying the organic content of a solution.Results Electrochemical oxidation
The experiments were coordinated with those using the advanced oxidation technique, based on a standard concentration of 1g/litre. Later, samples of real effluent were tested during field trials. Initial experiments concentrated heavily on EDTA, which was recognised as a priority material by the industry.
In the pumped cell, reduction of COD level was at an average of 500mg COD/h, using a diamond-coated anode (see Figure 3). Electrochemical oxidation was also effective on all of the other tested organics. Using a beaker cell with a diamond anode, the time taken to achieve a 90 per cent reduction in COD level ranged from 2-6 hours (Table 1). The figures in the table are typical average values over the period in which the majority of the destruction (about 90 per cent) takes place. The electrochemical oxidation method also proved effective on samples of real effluent from PCB manufacturing plant.Advanced oxidation
Experiments were carried out on the destruction of 1g/litre organic species. The destruction rates are shown in Table 2. Figure 5 shows a comparison of destruction for different organic species using the UV/ozone system. The destruction rates reported relate to the initial treatment of specific organics and do not take into account the destruction of impurities and breakdown products and conditions were not optimised.
COD analysis taken at hourly intervals was used to give a more accurate account of overall organic destruction rates including impurities and breakdown products. COD measurements showed, predictably, lower destruction rates (see Table 2). This is due to the formation of by-products from the continuous breakdown of the specific organics during treatment. Figure 6 shows a comparison of COD destruction for different organic species using the UV/ozone system. Table 2 shows the overall destruction rates for individual organic compounds, COD destruction rates and the overall treatment efficiencies.
The combined electrochemical and advanced oxidation method also proved effective on samples of spent developer solution which contained persistent polymeric resins. Developer solutions are difficult to treat because of the high stability of the polymers to degradation. Figure 7 shows the destruction of organics in a developer solution from a PCB manufacturing plant.Conclusions
The results of experiments carried out to date have demonstrated the capabilities of using a combination of UV and ozone as a method for successfully destroying a variety of organic materials found at relatively low-levels in effluent from process chemistries and potentially larger levels of organics from spent chemistries. Results have shown varying destruction rates and treatment efficiencies for the different organic compounds. Treatment efficiency is generally greater for higher concentrations.
All of the major chemicals identified as being typical of the types found in effluent from the PCB and general metal finishing industries can be treated by both oxidation techniques. These include thiourea and EDTA, which have been identified as being particularly important, and in the case of thiourea, difficult to treat by other means. In separate related tests cyanide bearing solutions have also been treated effectively.
The breakdown of EDTA is complex and the initial destruction is only the first step in the overall COD reduction process. Electrochemical oxidation is particularly effective at higher concentrations. It becomes more limited by mass transfer as the effluent becomes more dilute. Advanced oxidation does not suffer from this problem and remains effective at low and high concentrations. It scales directly with concentration. Therefore, the two techniques are highly complementary.
The ultimate goal of the project is to deliver a novel treatment system capable of treating real effluent produced in manufacturing plants. Shipley Europe and C-Tech have built pilot treatment systems for field trials. Field trials were conducted at the project partners' manufacturing sites. Figures 8 and 9 show the demonstrator systems in operation at a PCB manufacturing facility.Further work
This work represents a study into the capabilities of two oxidation techniques for destroying organic compounds in aqueous solution. There are several areas where additional investigations will need to be undertaken.
Although the destruction of organic compounds using UV/ozone has been clearly demonstrated, it has not yet been possible to gain insight into the reaction mechanisms. It would be useful to identify the species formed during the treatment process. This could be achieved using analytical techniques such as GC/MS on samples taken during a suitably timed destruction run.
Commercial, economic viability and exploitation are also a key part of the project programme. The project consortium is currently implementing a plan to take the developed technology forward for commercial exploitation.References
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