Monday, July 23, 2018
Ozone application for wastewater color removal
Ozone application for wastewater color removal
by Kostas Delimpasis
6/1/2001
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 ?m) 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 ? 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 220?m. 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.
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