Jumat, 22 April 2011

Ozone application for wastewater color removal


Water and wastewater treatment with ozone gas (O3),  became  increasingly popular in  the last twenty years, especially in the industrial applications, gradually and firmly substituting chlorine. Ozone is a powerful oxidizing mean and safer in use in comparison with other oxidizing  means.
In nature, ozone –a colorless gas with suffocating smell– is formed during the exposure of oxygen either in electric discharges of high voltage or ultra-violet radiation. In the atmosphere, where  the aforementioned conditions of its production are met, ozone exists in extremely high quantities.
At sea level, ozone is very difficult to be found and in case it is traced, it is very unstable. Thus the usual concentrations of ozone tracing  are about 0.1 ppm with a life duration of about 30 minutes, converted then to oxygen. As a result from all the aforementioned it is the fact that industrial users produce ozone from gas or from liquid oxygen. Due to short life duration, ozone is used right after its birth.
The smell  that ozone emits makes it identifiable immediately. From sanitary perspective the exposure to 1.000 ppm of ozone for 30 seconds could cause a light irritation while the equal exposure to chlorine gas is often fatal.
In air temperatures of, 30-35 C, the ozone is decomposed into oxygen and hydroxyl radicals , each one of them with an oxidation potential higher then the one of ozone. Even if  teams of free radicals have a life duration much shorter, their action contributes to  the holistic  ozone action.   
Ozone production
Ozone is produced from  atmospheric air which has been under appropriately processed or from liquid oxygen.  Ozone gas is produced by electric evacuation between two electrodes 10 up to 20 kilovolts.
The reaction of ozone production takes place in a suitable reactor which is called ozone generator. The ozone generator is consisted of a horizontal or vertical cylindrical vessel in which there is a specific number of stainless tubes adapted to a solid construction and adhesive to both stable edges of the vessel.
From the external side of the stainless tubes and inside the vessel, there is a flow of cooling water in order to remove the developing temperature during ozone production. This constructive method excludes any contact of  the coolant with high voltage electrodes.
The metallic tubes play the role of cases in which specially graduated glass tubes come into. In the gap which is formed between the internal metallic surface of the tubes and the external glass surface, ozone is produced. In every metallic case a glass tube comes in. The high potential is applied between the metal and the metallic surface of the glass tubes (high voltage electrodes), and produces an electric discharge along the tubes. Under these conditions ozone gas is produced and comes out of the vessel in a specified concentration, depending on the raw material (air or liquid oxygen). In conclusion ozone is produced “on the spot”, with  electric energy and water cooling as unique requirements.
Wastewater color removal
Water is shown colored when visible radiation is absorbed from dissolved materials, or when light is reflected on suspended solids. These two sources of color are the base for the distinction between the pseudo and true color. The pseudo color is due to absorption as well as light reflection. The true color depends exclusively from the kind and quantity of the dissolved substances. Particles  with a  size of 400-800 nm,  that means within the wavelength of visible light, are responsible for light reflection. It is possible with filtering (membrane 0,45 μm) the phenomenon of reflection to be eliminated. It must also be noted that the difference between the pseudo and true color is related to  water’s turbidity.
The units Pt–Co ( USA), or mg Pt-Co / l (Europe) are defined as color measurement units. These units are considered  equivalent. The acceptable limits of color values for the disposal of treated wastewater ranges from 50-100 units Pt-Co, depending on the nature of the receiver (river, sea, lake etc).
True color is created by the presence of compounds that absorb  visible light in wavelengths of 400-800 nm, or from compounds that fluoresce in the 200-400 nm spectrum. These are compounds of poly-aromatic structure, substituted aromatic structure, polyenia, concentrated hetero-circular molecules or perplex ions. It should be noted that  π  bonds absorb into the UV (‹200nm ) spectrum and the existence of conjugate bonds (polyenia) is necessary for the absorption in visible light spectrum. Most compounds  responsible for  color creation contain one or more aromatic rings and start absorbing color at 250 nm.
The synthetic color carriers come mainly from  industrial plants as dye-houses, clothing industries with washing-machines, food and beverage industries, slaughterhouses etc.
Wastewater is processed with ozone after its exit  from the chemical or/and biological treatment plant and the usual dosage varies from 50-150 mg/l, according to the wastewater origin, its temperature, and the degree of its previous process.
The plant for wastewater color removal with ozone includes the following stages:
Air preparation system and ozone production: As it has already been mentioned it is possible to produce from ozone atmospheric air or oxygen. The air preparation system ensures the unceasing supplying of the ozone generator with air completely cleared off from dust, oil and humidity.
The selection of supplying gas is made based on purely financial criteria, even if experience shows that  atmospheric air is preferable, especially in small private industries, as it ensures full  independence in the operation system. 
Ozone-wastewater contact system: The contact system consists of a three-chamber tank, height of 4,5-5m with inside splits that guide the wastewater to a vertical labyrinthine flow. Ozone is supplied to the tanks through diffusers  made of a special porous material of high resistance. These diffusers have the ability to create multi-numbered  and very thin ozone bubbles, with a diameter of 220μm. With their appropriate geometric installation in the bottom of the contact tank, better distribution but also increase in the liquid-gas contact surface to its maximum, is achieved.
The diffuser is used due to the high rate of transport (70 %) and its trivial energy consumption. In a tank of three-chambers, diffusers are installed in depth of 5 m and succeed a transport rate more than 75 %. The wastewater must have a hydraulic retention time greater than 45m.
Residual ozone destruction system: All the residual gases in the contact tank must be subjected in an ozone destruction procedure before they are emitted into the atmosphere due to the corrosive and poisonous nature of ozone. The USA regulations require ozone concentration  below 0,1 ppm before the abduction of these gases.
The ozone destruction units could be catalytic, thermal,  thermo-catalytic or activated carbon. The catalytic units use either manganese dioxide or aluminum coated with palladium and destroy ozone in temperatures around 50 C. The thermal destructive units operate at temperatures around 120 C.
The quality of the ozone treatment effluent in terms of color removal, depends on:
1. the color values of the feed
2. the ozone dosage
3. the wastewater type (in figure 2 a series of data does not descent below 200 Pt-Co units even if an especially high ozone dosage is applied)
4. the wastewater temperature (better results with effluent from the existing treatment whose temperature is much lower than the temperature of wastewater from the equalization tank)
5. the values of the other wastewater characteristics that ozone also affects (better results if BOD5, COD and SS have already been decreased in a previous treatment level)
The best results concerning color removal are achieved if the wastewater has been previously treated in order to lower the values of the other characteristics so that the ozone oxidizing effect is “consumed” only or at least at a maximum proportion in color removal. Additionally the temperature must be below 30 oC in order to achieve the best physical conditions for its solubility.
The above remark certainly concerns the practical usage of ozone technology in wastewater treatment,  as it indicates that  the increase of the ozone dosage  could give good results  even in unprocessed wastewater as long as it has been efficiently cooled.
Wastewater color removal requires an ozone dosage which in most cases fluctuates from 50 to 100 mg/l, for color reduction of 85-92 %. This dosage succeeds simultaneously a COD  reduction about 40 %, while small increases of  BOD5 in the area of 3-7 % have been noticed.
The ozone treatment installation represent a significant construction and purchase cost. On the other hand a conventional treatment scheme using chemical coagulants for color removal, has high operational costs (cost of the coagulants themselves and cost for the produced sludge management requirements). In general and for the same effluent quality, the investment of an ozone installation can be paid off in 3-5 years, depending on the size and other specific details of each case.

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