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Article 16 of the European Union Water Framework Directive (2000/60/EC) set out a "Strategy against pollution of water". The first stage in the strategy was a list of priority substances which would become Annex X of the Directive. Once the list of priority substances has been adopted, the Commission will propose community-wide water quality standards and emission controls for those priority substances (European Commission, 2001). The 32 substances or groups of substances on the proposed list of priority substances include selected chemicals, plant protection products, biocides, metals and other groups, such as polyaromatic hydrocarbons. Finally, Decision No 2455/2001/EC of the European Parliament and of the Council of 20 November 2001 was made by establishing the list of priority substances in the field of water policy and amending Directive 2000/60/EC. Several non-biodegradable chlorinated solvents (NBCS) are included in these substances: 1,2-dichloroethane, dichloromethane and trichloromethane (chloroform). In the same context, the EU IPPC Directive (96/61/EC) required the development of technologies and management practices for specific industrial sectors (see Annex I of the Directive) for the minimisation of pollution and for the development of water recycling. Due to the lack of available on-site treatment technologies, a large number of industrial activities do not treat such wastewater adequately. As consequence, simple, available low-cost technologies are in great demand.
Advanced Oxidation Processes (AOPs) have been proposed as an alternative for the treatment of biorecalcitrant wastewater. Many studies have concentrated on this goal, pointing out that these processes, while making use of different reacting systems are all characterised by the same chemical feature: production of OH radicals (·OH). Hydroxyl radicals can be generated with a semiconductor which absorbs UV radiation when it is in contact with water. Whenever different semiconductor materials have been tested under comparable conditions for the degradation of the same compounds, TiO2 has generally been demonstrated to be the most active. OH radicals can also be produced by the Fenton reagent (addition of H2O2 to Fe2+ salts). Photo-Fenton combines Fenton and UV-VIS light. The photolysis of Fe3+ complexes enables Fe2+ regeneration. Under these conditions iron can be considered a real catalyst. Both catalytic systems are of special interest because solar light can be used.
TiO2 has often been used for the treatment of NBCS, but photo-Fenton has not been applied as often. In any case, most of this research has been done in small, air-tight and very often cooled (to avoid NBCS volatilisation) laboratory photoreactors illuminated artificially. Results have been successful, but there is one aspect not very often addressed in these articles. The future application of photoreactors for the treatment of wastewater containing NBCS must be done in large systems, where cooling would not be economically feasible and air-tightness very often impossible, more so in a solar photoreactor. To work under air-tight conditions it would be necessary to: (i) inject oxygen (or air) to maintain its concentration in the reactor (for organic carbon mineralisation), (ii) continuously purge the CO2 produced and (iii) increase the plant construction cost (tanks, tube connections, photoreactor, etc) to prevent gas leaks. It must therefore be demonstrated that an AOP is suitable for application to volatile or semi-volatile compounds (dissolved in water) without exorbitant increase in plant and operating costs. For this reason, the main objective of this work was to demonstrate that NBCS treatment is feasible in an open-air pilot-plant photoreactor illuminated by sunlight.
In CADOX Project, it has been demonstrated that both photo-Fenton and TiO2 photocatalytic treatment of wastewater containing NBCS is possible, but if an unsealed reactor (or photoreactor) is to be used, operating parameters must be very carefully selected. Anaerobic conditions are the only way to guarantee that NBCS will not be released into the atmosphere during TiO2 treatment. But, under these conditions the NBCS are not mineralised and other toxic compounds could be formed. In this context, a photo-Fenton treatment with an iron concentration of around 56 mg L-1 could be a suitable treatment. Special care must be taken in selecting reactor operating temperature (which promotes both NBCS volatilisation and reaction rate) and initial hydrogen peroxide concentration. Besides, chloride determination is proposed as key parameter for controlling NBCS photocatalytic treatment.
The coupling between Photo-Fenton and ozonation has been used as Advanced Oxidation Proces for the degradation of the following biorecalcitrant pesticides: alachlor, atrazine, chlorfenvinfos, diuron, isoproturon and pentachlorophenol.These organic compounds are considered Priority Hazardous Substances by the Water Framework Directive of the European Comission.Also,Femam (a-methilfenilglicinamide, C9H12N2O, Deretil Co.), T000824 (C11H10Cl2N2O, Janssen Pharmaceutica) and R113675( C17H21NO3, Janssen Pharmaceutica)have been used for testing degradation efficiency.
These organic compounds are considered Priority Hazardous Substances by the Water Framework Directive of the European Comission. The degradation process of the different pesticides, that occurs through oxidation of the organic molecules by means of their reaction with generated OH radical, follows a first-order kinetics. The Photo-Fenton/ozone coupling together with the traditional ozone+UV, have been used to investigate TOC reduction of the different organics aqueous solutions. In all studied cases, the best results of pesticide mineralization are obtained when PhFO is applied.
On the other hand,from biotoxicity experiments, it is concluded that the use of this Photo-Fenton/ozone coupling as Advanced Oxidation Process, the aqueous pesticide solutions become detoxyfied except in the case of atrazine and alachlor aqueous solutions for which no detoxification is achieved at the experimental conditions used in the work, at least after 2 and 3 h of treatment, respectively. With relation to Deretil and Janssen productsDetoxification for atrazine, alachlor and Janssen and Deretil products biodetoxification has not been successful for treatment times lower than about 180 min.
The experimental conditions that lead to an optimisation of the ozone treatment of effluents containing recalcitrant organic substances such as the CADOX target compounds, i.e., alachlor, atrazine, chlorfenvinfos, diuron, isoproturon and pentachlorophenol,have been obtained. Particularly, it have been performed some experimental tests using aqueous solutions containing a mixture of the organic compounds previously mentioned and in the presence of T-824, R113675 (both, Janssen products)and the very high soluble organic compound femam (Deretil), as a model solution to carry out experimental research. The oxidation reactor used was a bubble column operating at atmospheric pressure. The liquid requiring treatment was re-circulated via a peristaltic pump in the column, while gas containing the ozone was continuously injected into the bottom of the column through a porous diffuser (semi-batch reactor). The tests were carried out at ambient temperature.
From the ozonation experiments, it is observed the establishment of two different kinetics. The first corresponding at the yellow coloration and discoloration of the effluent during the first twenty minutes, and the second corresponding at an other step of oxidation. In the first step, after a contact time of 20 minutes and an ozone rate of 900 mg/l, the treated effluent is turned from yellow to colourless. On the other hand, it has been observed that the COD is reduced as the pH of the effluent decreases until a value closed to 3. This acid pH obtained after ozonation is the suitable pH to apply a PhotoFenton procedure for decreasing even more the TOC of the effluent.A decrease of TOC with treatment time is observed, but is much lower than the observed for DCO, this leads to a reduction of the DCO/TOC ratio being an indication that the effluent becomes more oxidized with increasing ozonation time.
In view of the characteristics of the effluent and with relation to the design of the pilot plant, it is suggested the use of a Venturi injection system in a column by means of a diffuser. The diameter and height of the ozone reactor will depend on the gas volume flow and the concentration of ozone needed.
A coupled solar photocatalytic-biological system has been developed to enhance the biodegradability and complete mineralization of a biorecalcitrant industrial compound, methylphenylglycine, dissolved in seawater at 500 mg L-1. The pollutant was completely degraded by a solar photo-Fenton treatment plant made up of Compound Parabolic Collector (CPC) units. The catalyst concentration employed was 20 mg L-1 of Fe2+ and the H2O2 concentration was kept in the range of 200-500 mg L-1. A Zahn-Wellens (Z-W) test applied to photo-treated samples demonstrated that intermediates produced within a short time of starting the photo-Fenton process were biodegradable.
Consequently, the photocatalytic and biological processes were combined. Biodegradable compounds generated during the preliminary oxidative process were biologically mineralised in an aerobic Immobilised Biomass Reactor (IBR), filled with propylene Pall Ring supports colonized by activated sludge. Almost total mineralization (90% overall Total Organic Carbon removed) was attained in the combined treatment system. Moreover, nitrification and denitrification phenomena were also observed.
This result indicates that a combined solar photocatalytic-biological process is an effective approach for the treatment of biorecalcitrant pollutants present in seawater. A preliminary study estimated the cost per m3 of effluent treated between 7 and 10 (30% and 70 % capital and operational costs, respectively).
SolarCadox is based on the Photo-FENTON process, a combination of a well-known reaction discovered in 1894 with the most recent advances of photochemistry. The Fenton reagent is the iron II-salt-dependent decomposition of hydrogen peroxide generating the highly reactive hydroxyl radical, which attacks and oxidises organic matter. During the reaction the iron II ion is oxidised to iron III, which is not useful for a latter reaction. The coupling with a UV-VIS light (Ultraviolet and Visible) allows regeneration of iron II in a reaction that also produce hydroxyl radicals. The SolarCadox is fully solar driven so it does not waste energy through lamps or electronic materials for creating the photons, which drive oxidation reactions.
The installation of SolarCadox treatment plants comprises three fundamental items:
- Previous feasibility studies
- Engineering
- Installing plant in turn key operation concept
Previous feasibility studies:
The PSA developed a procedure for testing any wastewater and determine if its suitable for the SolarCadox technology. All this information was gathered in a Handbook which contains standard procedures and parameter limits to assess about collector area and final point of the degradation reaction. PSA also has skilled personnel, a complete analytical lab and solar and biological pilot plants. So PSA will be in charge of these previous tests.
Engineering and
Plant in turn key concept are under ECOSYSTEM responsibility taking in account each client distinctive situation regarding building situation, wastewater generation and storage tanks, solar irradiation and local weather characteristics.
Photo-Fenton treatment with 20 mg L-1 of Fe2+ was efficient enough, and no catalyst separation was required in the combined system AOP-biotreatment, as the concentration was low enough to ensure non-inhibitory effects on the activated sludge.
Evaluation of the combined photocatalytic-biological system developed has demonstrated photo-Fenton pre-treatment completely removed the pollutants and enhanced its biodegradability, producing a biocompatible effluent, which was completely mineralized by the biological system in an Immobilised Biomass Reactor. The combined system was able to totally mineralise 95% of initial TOC of over 400 mg L-1.
The beneficial effects of this two-steps field treatment has therefore been confirmed at pilot scale. Photo-Fenton under sunlight using CPC reactors was able to remove the biorecalcitrant compound and produce biocompatible intermediates required for further biological treatment. These results indicate that a combined solar photocatalytic-biological process is an effective approach for the treatment of biorecalcitrant pollutants present in water.
Solar collectors are the key component in any solar installation. After the identification of constructive, structural, mechanical and optical problems of different non-concentrating solar collectors systems, Compound Parabolic Concentrators (CPC) were selected as the best option for photocatalytic processes. CPC non-imaging concentrators, extensively employed for evacuated tubes, are static collectors with a reflective surface following an involute around a cylindrical reactor tube and have been found to provide the best optics for non-concentration systems.
The static behaviour of CPC solar collectors are a good technological solution in large volume of water treatment systems which adds to its capability to capture UV sunlight, coming with the diffuse as well as the direct beam; as the UV radiation is not absorbed by water, when clouds are present, an important portion of solar UV radiation reaches the earth surface as diffuse light.
This means that the Solar Detoxification process could works with solar light even in cloudy days. CPC design was optimized by performing a set of optical calculations, using tray-tracing tools, to stress the optical behaviour of this collector type. These calculations support the final design of the UV collection system and help on decisions taken for the design and constructive solutions selected, initially to collector�s prototypes and later to the final system design.
Final CPC reflectors, made of electropolished anodized aluminium, were constructed based on the following data design:
Acceptance angle 90º Truncation angle 90º
Concentration ratio 1.0 Optical gap 1.4mm
Ext. absorber radius 16.0mm Int.absorber radius14.6mm
The CPC reflector is made in high reflective anodised aluminium and collector frame is constructed with galvanised sheet frame and with 16 parallel tubes of 1.5 m length. Tubular reactor tubes are made of borosilicate glass with low iron content to enhance the transmissivity in the region of 300-400 nm. Each tube has at its end an appropriate connector for the tube of next adjacent collector. A complete module is formed by a series of collectors connected in a row. Collectors are designed to constitute long linear modules and treatment plants would be formed by parallel rows of modules with East-West orientation. Collectors and modules are also designed with a small structural tilt (1%) in the same E-W orientation as a way to dry-out and to avoid the accumulation of rain water on the CPC troughs. Fig. 2 shows the supporting structure of collectors, with different inclination angle capability.
System design is completely modular. Collectors are connected in series using HDPE quick connections between reactor glass tubes absorbers. Water flows simultaneously by all parallel tubes and modules have no limit in the number of collectors components. Two manifolds at the module extremes serve to water input and output.
In order the well design the capacity of the ozone production needed in the final ozone plant, the main parameter to take in consideration was not the ratio g O3/g COT reduced, but the ozone dosage versus the COT reduction.
In such approach, we may consider with the effluent we will test on the final plant, that we will need A mg O3 per litre to remove it (usually around 3 g O3/g COT). In such conditions, and taking into account an ozone transfer of around 50% and with an effluent flow of B g/h, we will need 3xB g O3/h.
If we would let us the possibility to over dosage the ozone quantity introduced in the final plant, we could be able to reach a final COT which has given good results after the biological treatment. So, this over dosage will correspond to a max ozone capacity of 4xB O3/h.
From such analyses, we have designed for the final ozone pilot plant, an ozone generator able to produce 4.5xB g O3/h. After this first conclusion, we had to ask us the following question: Which type of feed gas do we have to use to produce that ozone quantity? In fact, there are two possibilities to produce ozone: From dry air or from dry oxygen. From dry air, we may produce ozone with concentrations between 20 to 50 gO3/Nm3 and from dry Oxygen, we may produce ozone with concentrations between 100 up to 200 g O3/Nm3.
How to select the best feed gas (air or oxygen): different parameters enter in consideration:
- Aim of the ozone treatment: oxidation, disinfection?
- Kinetic of reaction between ozone and COT reduction?
- Quantity of effluent flow to ozonate?
When we treat industrial waste effluents, the need of ozone is often important (which is the case in this application), and the effluent flow is, as a general rule, small. In our application, ozone is used as a strong oxidant, the kinetic of reaction is very low and the effluent flow will be also very low (one or two hundred liters).
In such conditions and in order to optimise the ratio ozone gas flow / water effluent flow, we will produce ozone from dry oxygen which will allow us to produce ozone with a low gas flow and high concentrations. It is the solution which has been retained for the final ozone plant and the ozone generator proposed will be able to produce an ozone concentration of 11,7% w/w.
About ozone contactor and ozone transfer, we have to take into consideration that the kinetics of oxidation of these wastewaters is usually low and the ozone transfer is bad with a lot of loose of ozone in the off gas. From these conclusions, we have decided to inject ozone in two contact columns in series.
As the effluent will contain a lot of salts and the ozone concentrations will be high, the different components of this pilot will be manufactured in Stainless Steel. Fron the ozone generator ozone gas produced from oxygen will be splitted in two directions : (i) one to an hyroinjector located upstream the bottom of the first ozone contact column, (ii) the other one to the porous diffusers located on the bottom of the second contact column. Each contact column will utilise a different mode of ozone transfer :
In the first contact column, we will have the possibility from a raw water pump to introduce a small quantity of ozone in an hydroinjector. Always in this first contact column, and in order to improve the contact between effluent and ozone gas, we have installed a recirculation pump which will suck from a second hydroinjector the ozone off gas leaving the second ozone contact column.
In the second contact column, we will use conventional ozone transfer by using porous diffusers made in Stainless Steel (classical porous diffusers in PVC are not compatible with the high ozone concentrations produced from ozone generator fed from dry oxygen).
In such a way, the ozone transfer will be optimized and the loose of ozone reduced to it maximum. The ozone contained in the off gas leaving the first contact column, will be destructed by a thermo catalytic ozone destructor.
With this pilot, we will be able to:
- Treat different effluent flows,
- Test different ozone concentrations and different ozone rates.
- Introduce different ozone quantities in order to feed the biological step with a pretreated effluent containing organic matters more biodegradable.
Ozone is an oxidant which may have a good efficiency on biorecalcitrant compounds oxidation but due to the high concentration of Femac and the associated high dose of ozone we have been obliged to introduce in the effluent, the ozone investment cost is high. The main possibility to reduce this investment costs, is to increase the ozone transfer in order to save a big part of the ozone loose.
Coupled with a biological treatment, ozone stays one of the cleanest technologies, which produces no sludges and no harmful products. In this case, biotreatment costs should be also taken into account.
The inability of conventional biological wastewater treatments to effectively remove many toxic pollutants shows that new treatment systems are needed. Rigorous pollution control and legislation in many countries has resulted in an intensive search for new and more efficient water treatment technologies. The adoption of the Water Framework Directive provides a policy tool that enables this essential resource to be protected. Among other measures, surface water deterioration must be prevented and bodies of water enhanced and restored, good chemical and ecological status of such water must be achieved and pollution from discharges and emissions of hazardous substances reduced by 2015.
In the near future, Advanced Oxidation Processes (AOPs) may become the most widely used water treatment technologies for organic pollutants not treatable by conventional techniques due to their high chemical stability and/or low biodegradability. The use of AOPs for WW treatment has been studied extensively, but UV radiation generation by lamps or ozone production is expensive. Photo-Fenton has been demonstrated to be effective for treating wastewater containing pollutants at concentrations of > 10 mg L-1, as the reaction rate is usually much higher and separation of iron is very often not necessary. Despite its obvious potential for the detoxification of polluted water, there has been very little commercial or industrial use of the solar photocatalysis technology so far.
The proposed technology could be applicable to different organic hazardous contaminants, such as pesticides, solvents, detergents and a variety of industrial chemicals, which are capable of substantial contamination of the environment due to their toxicity and persistency.
In this context, treatment of industrial waste water seems to be one of the most promising fields of application of solar detoxification. There is no general rule at all, each case being completely different. Consequently, preliminary research is always required to assess potential pollutant treatments and optimize the best option for any specific problem, on a nearly case-by-case basis. Solar photocatalytic degradation technology might be feasible for the treatment of waste water containing hazardous contaminants at medium or low pollutant concentrations when biological treatment is impossible. The technology is dependent on the energy flux, as is the associated investment contingent on the collector surface. In general, the types of compounds which have been degraded include alkanes, haloalkanes, aliphatic alcohols, carboxylic acids, alkenes, aromatics, haloaromatics, polymers, surfactants, herbicides, pesticides and dyes.
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