Umist research associate Nigel W Brown investigates the removal and destruction of organic pollutants using carbon powder, the process of adsorption and electrochemical regeneration
Umist research associate Nigel W Brown investigates the removal and destruction of organic pollutants using carbon powder, the process of adsorption and electrochemical regenerationIndustries are under increasing pressure to reduce the levels of pollution in their effluent due to increasingly stringent UK and European legislation stemming from heightened public awareness, concern over the long-term environmental and health effects of trace levels of pollutants and improved analytical techniques detecting ever-lower concentrations.
Some of the most difficult effluents to treat are those containing trace quantities of toxic, non-biodegradable or coloured organics. A range of treatment technologies can be used to treat these effluents but adsorption processes are probably the most widely used because very low discharge consents can be achieved.
Activated carbon is the most widely used adsorbent for wastewater treatment and has surface areas of up to 2,000m2/g, ensuring the maximum adsorptive capacity. Once the carbon has been exhausted, it can be disposed of by landfill or incineration, or alternatively it can be regenerated. Regeneration is widely used because it usually represents the most commercially viable and environmentally acceptable option.
Industrially, the most widely used regeneration process is thermal regeneration, however this is a high-energy, high-cost process, which results in material losses of around 10% and, except for large users, off-site regeneration at specialist regenerators. Therefore, research into the development of alternative adsorption processes has been undertaken. Two approaches have been considered by researchers to overcome the problems associated with the use of activated carbon adsorbents. One approach that has been investigated widely is the development of low-cost adsorbents based on carbonaceous waste products that can be used on a once-through basis, eliminating the need for regeneration. However, this merely transfers the pollutant from the liquid phase to the solid phase. The second approach has been to investigate different regeneration processes to eliminate some of the complexities and reduce the processing costs.
For activated carbon these processes include chemical or solvent regeneration, microbial, ultrasonic, wet air oxidation or electrochemical regeneration. Nykin Developments, with the aid of initial funding from a Department of Trade & Industry Smart Award, has developed a process that removes and destroys aqueous organic pollutants using a non-porous, electrically-conducting adsorbent material, Nyex 100. This is a relatively low-cost carbon powder that has been treated by a proprietary process. The use of non-porous materials is an alternative approach to the traditional method of using highly porous materials with massive internal surface areas.
The traditional approach achieves high adsorptive capacities but requires complex and costly regeneration.
The elimination of the internal pores removes the intra-particle diffusion, which is often the rate determining steps in both adsorption and regeneration. This gives an adsorbent material which has high adsorption rates and rapid, easy regeneration at the expense of significantly reduced adsorptive capacity.
The high electrical conductivity of the adsorbent material facilitates its regeneration in an electrochemical cell and results in the electrochemical oxidation of the adsorbed organics. The electrical conductivity of the Nyex 100/electrolyte mix (0.16 ?-1 cm-1) was found to be more than 13 times that of a powdered activated carbon/electrolyte mix (0.012 ?-1 cm-1).
A joint academic/industrial project is currently under way within the chemical engineering department at Umist to investigate this process. Adsorption studies have demonstrated that the rate of adsorption is very quick, with up to 88% of the equilibrium adsorptive capacity being achieved within two minutes.
These studies have shown the adsorbent is very effective in removing trace organic pollutants with atrazine and permethrin being removed to less than 1 and 5ppb respectively - the limits of detection in this work. Industrial effluents investigated include coloured dyehouse and coloured sewage work effluent. In both cases, colour could be eliminated from the wastewaters, with COD being significantly reduced (78%).
Regeneration of the loaded Nyex 100 is achieved in a specially-designed electrochemical cell where anodic treatment of the adsorbent results in the oxidation of the adsorbed organic components.
Laboratory studies have shown that 100% electrochemical regeneration can be achieved in ten minutes by passing a charge of 24C/g adsorbent, however, the efficiency of regeneration depends not only on the adsorbed organic pollutant but also on a range of processing parameters including charge passed, current density, treatment time, electrolyte type and concentration and the adsorbent bed thickness. However, in order for the process to be commercially viable, the adsorbent material must be capable of being recycled many times. For a regeneration charge of 30C/g, approximately 100% regeneration was achieved over ten adsorption/regeneration cycles. These multiple adsorption/regeneration cycles demonstrate there is little or no loss of adsorbent or adsorbent capacity on regeneration, see Figure 1. Figure 2 shows a schematic of a pilot plant used to treat a coloured final effluent from a WwTW. This pilot plant used the three stage process of adsorption (in a continuous stirred tank), adsorbent separation (by gravity sediementation with the aid of an organic flocculent) and regeneration in a batch electrochemical cell. After regeneration the adsorbent is available for immediate re-use. This small-scale pilot plant (100 l/h) was assembled at one of Severn Trent's WwTW, which treats the effluent from a number of local dyehouses.
This plant used a treated effluent as the influent to the pilot plant. During the period of operation of the pilot plant, the final effluent from the WwTW was uncoloured, so to demonstrate the removal of colour the effluent was spiked on two occasions with two different dye solutions, one a proprietary dye and the other an industrial dye solution.
This trial was successful and the discharge consent was achieved throughout the treatment period, even when the effluent was spiked with dyes, see Figure 3. Operational costs for the treatment of this coloured final effluent suggest a treatment cost of 0.26p/m3 (based on the cost of electricity, 4.5p/kWh, organic polymer and electrolyte).
The process is a significant technological advance as there are major advantages over existing treatment processes:
- regeneration results in the destruction of the adsorbed organics rather than merely being a concentration process,
- no sludge or environmentally harmful by-products are produced by the process,
- adsorption and regeneration are both quick processes that can be achieved on-site at room temperature and pressure resulting in low quantities of adsorbent being required,
- while the adsorptive capacity is low, the cost of treatment is dependent on the cost of regeneration and initial calculations suggest this is a lower treatment cost process,
- minimal addition of chemicals and the ability to achieve high water quality standards should make the recycling and re-use of treated water possible.
The operation benefits of such a plant for both industrial and tertiary sewage treatment operation are believed to include minimal manual input and a high degree of automation, immediate treatment on start-up, continuous treatment, limited chemical addition and robustness. It is believed the process will be of benefit in the polishing of treated effluent as well as removing trace organic pollutants and colour.
Initial work suggests red list substances can be rem-oved from water, including organo-metalic compounds. Treatment of wastewaters from the petrochemical and pharmaceutical industries, as well as many others, are likely to be commercially viable. Future work is concentrating in a number of areas:
- development of a continuous single unit treatment process. It is anticipated this approach will enable effluents with higher pollutant concentrations to be treated cost-effectively. Effluents that could be treated in such a cell include dyehouse and textile effluents, concentrates from membrane processes, industrial effluents and landfill leachates,
- evaluation of the treatability of a range of industrial effluents,
- construction of a portable customer demonstration unit and construction of a full-size electrochemical regeneration cell. Further scale-up will be based on a modular approach with more electrochemical cells in series.
Nykin Developments is looking for co-operation and support from other organisations that might benefit from its utilisation. Nykin believes a collaborative approach with other companies will lead to the quicker introduction of the technology, rather than attem-pting to develop it on its own.
The authors would like to acknowledge funding for the project through the EPSRC. Additional resources for the project were provided by Severn Trent Water and Electrode Products Technology.