The Chemox Process. Ozone Dissolution for RSAG - a case study of our work together

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Benjamin Moore
5 years ago
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1 The Chemox Process Ozone Dissolution for RSAG - a case study of our work together

5 1... The problem There are strict statutory regulations governing the way in which leachate is removed from landfill sites. Such leachate (typically somewhere in the region of 100 cubic metres a day) could, for example, be fed into a river with water flowing at hundreds of cubic metres per second, thus diluting the load of harmful materials until it is below detection levels. But that would mean the total failure of environmental legislation. What is required is an extensive system of leachate treatment at the point of origin resulting in high quality water. A process developed by Air Products can help landfill operators at least partially to come to grips with the increasing dichotomy between stricter environmental legislation on the one hand, and decreasing financial resources on the other. This new development is the CHEMOX process - leachate water treatment in an Impinging Zone Reactor, the CHEMOX-SR.

6 2... The benefits Running costs for the treatment plant, which uses ozone, are reduced by roughly 30 per cent, according to Air Products waste management customer RSAG (Rhine-Sieg-Abfallwirtschafts- Gesellschaft). This opens the way for landfill operators to move away from activated carbon treatment processes. Prices for activated carbon, which works by absorbing the harmful materials, have come down drastically in recent years, making it popular as a purification medium. However, its use means that landfill operators are simply shifting over this problem of disposing of harmful materials to the equally problematic treatment process for activated carbon. The new Air Products ozone-based treatment process wholly satisfies the most stringent regulations at low operating costs. Ozone and hydrogen peroxide are the most environmentally-friendly oxidation agents for waste water treatment. As a result of the reaction process, they are converted into carbon dioxide and oxygen. In addition, the CHEMOX process can easily be integrated into existing treatment plants. The background The CHEMOX-SR reactor was developed by Prof. Dr. Ing. Alfons Vogelpohl and Dr. Edward Gaddis at the Technical University of Clausthal Institute of Thermal Process Engineering. An important resource was provided by Air Products through fluid-dynamic calculations carried out on a computer model of the reactor. Air Products, an international gas company offering a range of environmental solutions, purchased an exclusive licence from the scientists for the further development and application of the CHEMOX-SR reactor. The search for a suitable partner to performance test a pilot plant with a reactor capacity of approximately 250 litres led Air Products to RSAG. The company agreed in the hope of finding a more cost-effective method of water treatment; so they were just as keen to asses the results of the performance test. These tests were carried out at the Siegburg leachate water treatment plant. The Siegburg-based company was so impressed by the efficiency of the experimental plant that they decided to collaborate with Air Products in the construction of a commercial plant.

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8 4... The Venturi Injection, Bubble Column Reactor: a reference plant for state-of-the-art technology In collaborating with Air Products, RSAG was continuing on a course of ongoing modernisation. For some thirty years their landfill had been fitted with a lining. Initially, the leachate which gathered in the lining was treated by means of the conventional combination of biological and activated carbon processes. At the end of 1993, this was replaced by a state-of-the-art ozone plant. Its most important elements are three venturi-bubble-column reactors and a final holding tank. Designed to handle an average flow of up to 70 cubic metres of leachate a day, with peak flows of up to 140 cubic metres, the venturi-bubble-column reactors have a volume of 50 cubic metres each and are constructed from fibreglass-reinforced plastic (upvc). The final holding tank holds 20 cubic metres. The plant functions on the following principle: initially, the leachate is gathered in a 2200 cubic metre balancing tank, to ensure that the plant can operate at a constant throughput. Next, the contaminated leachate is fed into the first biological treatment stage. The biomass created is eliminated in the settling tank. The pre-treated water then passes over a sand gravity filter into the venturi-bubble-column reactors which are linked together in series. Here the leachate is contacted counter-currently with ozone which is produced on site through a silent electrical discharge in a purpose-built generator. Ozone improves the degree to which the contaminants are biodegradable; it destroys compounds which are toxic to the bacteria or compounds which will not biodegrade, such as phenols, mercaptans, amines, cyanide, sulphides etc. through oxidation. Ozone at a concentration of 130 grams per cubic metre of pure oxygen is pumped into the bubblecolumn reactors with the help of a venturi injector. At the end of the reactor series, the ozone which has not reacted (approximately one gram per cubic metre of oxygen) is then converted into oxygen by means of a catalyst. With this process, the operating costs in the late 1990s were between 30 and 300DM (11 and 36 UK pounds) per cubic metre of leachate, depending on the degree of contamination. The ratio of ozone to COD (chemical oxygen demand) - a measure of efficiency - is three to one in this case.

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10 6... The Chemox Process: 30 per cent improvement in consumption and increased operational reliability At 1.8 kilograms of ozone per kilogram COD, the plant installed by Air Products immediately achieved a considerably improved ratio. In the stainless steel CHEMOX-SR reactor, two jets of leachate, previously enriched with ozone, impinge on each other creating an area of high turbulence. The combination of high turbulence and a partial internal recycle resulting from the geometry of the reactor ensures that the gas remains inside the reactor for a longer time. This results in the ozone being more thoroughly mixed with the liquid than it is in the bubble column. This zone of high turbulence results in a high coefficient of mass transfer whilst requiring only a low amount of energy, thereby considerably improving the reaction efficiency. At the same time, the dynamics of the turbulent flows suppress any excessive build-up of foam. With all these favourable performance characteristics, the new reactor, with a volume of only 20 cubic metres, is considerably smaller than a bubble column reactor. Other processes which increase the solubility of ozone in water by means of increased pressure also achieve a similarly high mass transfer. However, they require energy to drive pumps to increase the pressure; the tanks must be designed in accordance with pressure vessel legislation and the inherent level of safety is not as high, due to an increased risk of gas leaks: at a concentration as low as five grams of ozone per cubic metre of air, a single intake of breath can be deadly. The CHEMOX process operates at close to ambient pressure. The improved ozone to COD ratio reduces operating costs significantly. The manager in charge of the plant, Manfred Vetter, states that the CHEMOX process produces a 30 per cent saving in ozone. This reduction in ozone requirement also frees up capacity for the treatment of greater volumes of leachate or leachate from third parties. In addition, the energy consumption required for the introduction of ozone is cut roughly by half, to less than two kilowatts for each kilogram of ozone required.

11 7... A risk-free investment for RSAG The areas of application for these technologies lie mainly in the field of waste water which will not biodegrade, have relatively low flow rates and have a high concentration of contaminants: such is the view taken by Air Products. Leachate waters are severely contaminated with extremely stable humic substances as a result of the rotting of waste materials, and these humic matters are almost impossible to treat biologically. Nevertheless, the path leading to the installation of the CHEMOX-SR reactor at RSAG was less than smooth: after all, a new plant had been completed only a few years previously. However, after extremely promising trials carried out in the 250 litre laboratory plant, Air Products reached an agreement with RSAG for the purchase of a full-scale reactor with a capacity of 20 cubic metres, on condition that the target of 1.8 kilograms of ozone per kilogram COD would be achieved with the full scale plant. Otherwise, Air Products would be obliged to take the plant back at no expense to the customer. The reactor was subsequently put into operation and proved its efficiency: the full-scale reactor confirmed the results achieved with the experimental plant; the leachate from the landfill was treated in accordance with the performance criteria set. Due to the additional treatment of third party leachate and the resulting increased carbonate content of the water - carbonate being a deciding factor in determining the amount of ozone needed - it was not possible to achieve the performance target under peak-load conditions. Nevertheless, the performance achieved was regarded as satisfactory, as the existing plant with its bubble column reactors also required increased amounts of ozone under similar conditions. The fact that the targets were not achieved when operating at peak load does not diminish the actual success of the reactor, which is paying off for RSAG in more than just reduced ozone consumption. In the late 1990s, the company received a contribution of 50,000 DM from RWE Energie AG towards the upkeep of the plant. This was a prize awarded under a newly-instituted RWE competition titled KesS (customer energy saving service) for the introduction of this new energy-saving technology.

12 A growing market With its CHEMOX process, Air Products is addressing a growing market. In the UK alone, there are several hundred waste dumps which need to be equipped with a water treatment plant. However, the general water treatment market is even larger. A report published in the 1990s by the market research consultancy, Frost & Sullivan, indicates that the European market for water and waste-water treatment plants is expected to increase to 5.52 billion dollars in So the trend must be leading towards the development of more efficient, more reliable and less maintenance-intensive plants - as represented by the Air Products CHEMOX process. According to the information provided by the study, the largest market sector is that of the biological sewage treatment works. It is of significance for the new Air Products process that the customers of private waste disposal companies are reducing their production levels of industrial waste water within the context of ecologically-optimised processes. However, the lower volumes of waste water thus generated contain even higher levels of contaminates. In many cases, it is not possible to remove these materials with conventional treatment processes. It is in situations like this that the CHEMOX technology can come into its own.

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