Water water everywhere

In a drive to improve water quality, a project within the School of Chemical Engineering and Analytical Sciences at the University of Manchester has been developing a new process for removing and destroying aqueous toxic and non-biodegradable organic compounds.

The quantity and quality of water is becoming an issue of major international concern and it has been suggested that if the twentieth century was the century of oil, then the twenty-first century will be the century of water. Unlike many parts of the world, the UK has traditionally been a country where water has been in sufficient supply. However, even here, water is becoming increasingly expensive and ensuring the security of water is of increasing concern. The possibility of more droughts in the UK, particularly in the South-East of the country, is increasing the pressure on industry to ensure a secure supply of water.  In addition, heightened public awareness, notably concern over long-term environmental and health effects, and improved analytical techniques (capable of detecting extremely low pollution levels) continue to drive stringent legislation, particularly for discharges from effluent streams. Some of the most difficult effluents to treat are those containing trace quantities of coloured, toxic and non-biodegradable organic contaminants.

The problems with security of supply can be minimised by controlling water usage on site.  Nigel Brown has worked within the field of water management and wastewater treatment for over twenty years and has seen the benefits that can be obtained from managing water.  He uses a staged approach to managing water and has shown that water management projects can often give short payback periods. 

In order to ensure the quality of water a number of treatment process are available, however all are expensive, use toxic or hazardous compounds, generate a further waste stream or are still under development (See Table 1). 

 Technique  Disadvantages
Activated carbon   Limited capacity that requires disposal or regeneration
Disposal – Landfill – organics only transferred from liquid to solid phase. Not considered best practice. Transportation to suitable sites.
Disposal – Incineration – problems with generation of dioxins and furans. Transportation to suitable sites. High cost.
Regeneration – More economically viable and environmental friendly than disposal. Requires off-site transportation to specialist regenerators. Loss of 5 – 10% of activated carbon. High energy/high cost process. Large volumes of secondary effluent produced.
Coagulation and flocculation Generates voluminous sludge requiring both treatment and disposal.
Merely transfers organic pollutants from liquid to solid phases
Chemical oxidation   Involves the addition of expensive and hazardous chemicals (e.g. chlorine, ozone).
Membrane process  Expensive to purchase and operate.
Produce a concentrate stream that requires further treatment.
Advanced Oxidation Processes Based on the production of hydroxyl radicals.
Expensive and still undergoing development.
Mass transport limitations mean that treating dilute and trace organic levels are difficult.


Table 1 – Existing treatment techniques

To address the problems of processes to improve water quality, an academic/industrial project within the School of Chemical Engineering and Analytical Sciences at the University of Manchester (UoM) has been developing a new process for removing and destroying aqueous toxic and non-biodegradable organic compounds. This work has been based on a novel, non-porous, highly electrically conducting carbon based adsorbent material, Nyex. Nyex is the first adsorbent specifically designed for electrochemical regeneration and is capable of simple, quick and cheap electrochemical regeneration.  Initial work on the process involved batch adsorption, followed by solid/liquid separation and batch electrochemical regeneration. This work demonstrated that the adsorption and regeneration steps in this process are very rapid (a few minutes), with low pollutant levels (< 0.5 ppb atrazine), high regeneration efficiencies and repeated adsorbent recycling (over 15 cycles with no loss in performance) being achieved. The regeneration recovered 100% of the adsorptive capacity by passing a charge of 25 C g-1 through loaded adsorbent in a simple divided electrochemical cell.  Industrial effluent treatment tests, including dyehouse effluent, waste cutting oils and coloured final effluent from a sewage works, have all been successful to date, although the bulk removal of biodegradable organic matter is most economically achieved using biological processes. The use of the process for the tertiary treatment of a final effluent to remove colour has been demonstrated with operating costs in the order of 0.3 p m-3, compared with a cost for alternative techniques of 2-6 p m-3. The electrical cost of regenerating the Nyex electrochemically is around £3 per tonne compared with thermal regeneration costs for activated carbon of around £400 per tonne. It demonstrated that this process has a number of major technological advantages over existing processes:
• Pollutants are adsorbed and destroyed, with no sludge produced.
• Fast on-site regeneration results in low quantities of on-site adsorbent.
• Regeneration at room temperature and pressure with low cost.
• Addition of chemicals is minimised or avoided.
• Re-use of treated water possible.

In addition to this novel adsorbent, an innovative continuous and simultaneous adsorption/regeneration unit has been developed. The design is elegant and robust with no internal moving parts as hydrodynamic control is achieved through air sparging (See box 1). The first prototype device for continuous adsorption and regeneration was commissioned in March 2006 using the removal of colour from a single dye, crystal violet, in tap water. Since March, the unit has been used to exhibit the capability of the technology to a number of industrial companies, based on the removal of colour from single dye solutions. More recently, a number of industrial effluent samples have been tested and these initial trials have achieved positive results. The success of a laboratory prototype has led to two patent applications.  

 
Laboratory scale adsorption and regeneration equipment

Its technological advantages indicate that the process may be applicable to a wide range of applications including: tertiary treatment of sewage, effluent polishing, groundwater treatment, colour removal, industrial effluent treatment including the recycling of process water and potable water treatment (removal of pesticides). It has the benefit that it can be viewed as a piece of process plant rather than being a unit for effluent treatment. Additionally it is a technology that appears able to address future legislation regarding the removal of endocrine disrupting compounds (EDCs).

EDCs are compounds which affect the endocrine (hormone) system of animals, for example causing feminisation of fish.  The presence of such hormonally active compounds in sewage and industrial effluents has been highlighted by recent research. Research in this field includes the formation of CREDO (Cluster of Research into Endocrine Disruption in Europe) through the 5th European Framework Programme, involving 63 laboratories in Europe with a total budget of approximately 20 million euros. The need to develop and prove the economic viability of suitable processes to eliminate these compounds is demonstrated by the investment being made by the water companies under the current Asset Management Programme (AMP4). Under the auspices of the Environment Agency (EA) and DEFRA, the UK water companies are investigating EDC removals at 17 Sewage Treatment Works across the UK, including the installation of a number of pilot plants to demonstrate the cost and effectiveness of a range of existing processes, e.g. Granular Activated Carbon (GAC), in removing these wastes.  The total project cost is in the region of £26 Million. No technology currently available has been demonstrated to be cost-effective in removing these compounds. Future EU legislation is very likely to demand the removal of EDCs generating a potentially enormous market. Previous work at Manchester has demonstrated that the industrial EDC Bis-Phenol A is removed using the adsorption/regeneration process and initial trials at Loughborough University on the use of Nyex to remove 17β-oestradiol have shown that it can be removed to below detectable levels (< 10 ppt).

Whilst these developments are of particular relevance to the water industry, many of the identified EDCS are industrial chemicals and there is increasing EU pressure for the harmful effects of these compounds to be removed from effluent discharges. Industry’s problems are likely to increase as the US EPA is now assessing an estimated 87,000 compounds for endocrine disrupting capability.

A new company, ElectroClean Technology Ltd (ECT) is currently being established through UMIP Ltd (University of Manchester Intellectual Property) to exploit this highly-promising, proprietary technology for cost-effective removal of such impurities without secondary waste generation.  This spin-out company from the University of Manchester will aim to develop a pilot plant capable of on-site demonstration of the technology. ECT will be investigating further applications of the technology in the treatment of drinking water and by combining the unit with a solar panel. This would allow the unit to have a low carbon footprint and to be used in areas away from conventional power sources. It would provide a flexible unit that could be used to improve water quality throughout the world.

In summary, ECT’s technology, based on over 9 years’ research and development, utilises a novel, carbon-based adsorbent material (Nyex) to eliminate problems associated with existing processes. Nyex adsorbs organic pollutants very rapidly (~ 2-3 minutes). Regeneration is achieved electrochemically using a continuous process, during which pollutants are destroyed and very low pollutant levels (<ppb) achieved in discharged effluent. There is no sludge or secondary waste produced, eliminating requirements for offsite transport of waste for thermal regeneration or for additional treatment of waste-streams associated with existing technologies. Finally, for the first time effluent treatment and absorbent regeneration occur simultaneously within a single unit, with no moving parts and 100% recovery of the absorbent capacity. The process was recently awarded the biennial Royal Society of Chemistry’s Process Technology Group Award for Innovation 2006.

Unit operation

 
The unit operates continuously with adsorption and regeneration occurring in different zones of an electrochemical cell.  Adsorption occurs in two side zones, using an air-lift pump principle. Air is injected at the bottom of the air-lift zone, reducing the density within this zone. This causes the water within the zone to rise resulting in water and adsorbent from the bottom of the regeneration zone being drawn into the adsorption zones. The effluent to be treated is injected into the bottom of the air-lift region and the bubbles cause significant mixing. As the adsorbent material has no internal surface area, adsorption is rapid. At the top of this zone the effluent and solid adsorbent overflow into the settlement zone. Here the high density, non-porous solid particles settle rapidly forming a bed of particles. The treated effluent is allowed to over-flow out of the unit at the top. The adsorbent bed is drawn down by the action of the air-lift pump, thus recycling the adsorbent.  This drags the bed of adsorbent particles through the regeneration zone of the unit. This regeneration zone has an anode on one side and a membrane on the other with the counter electrode (cathode) placed behind the membrane. A current is applied resulting in the electrochemical regeneration of the adsorbent.

By Dr Nigel Willis Brown. Nigel is currently developing this process at the University of Manchester.  Prior to this work he was an independent environmental consultant specialising in water management, wastewater treatment and R&D into clean technologies.


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