Table 6 Main Difficulties of Some Methods of Chemical Oxidation Application

Oxidation process

Comments

Ozonation

Ozonation enhanced by hydrogen peroxide Ozonation enhanced by UV light

Hydrogen peroxide oxidation enhanced by UV light

Oxidation with Fenton reagent_

Good results are achieved at high pH and low alkalinity Good results are reached at high pH

The formation of scale on UV lamps and inside the pipes due to precipitation of residues; insufficient UV light transmission into leachate; low concentration of carbonate ions required, recommended pH range 5-8 High energy consumption, low transparency for UV light in leachate, recommended pH range 2-4

Sludge precipitation, increase of sulfates concentration, acidic pH required (~3)_

Figure 13 Scheme of photocatalysis with TiO2 (from Ref. 74).

This energy, for TiO2, is equal to 3.2 eV and corresponds with the energy of light of wavelength 400 nm. Therefore UV irradiation of wavelength 300 nm <X< 400 nm, being a part of sunlight, can also excite the electrons of the valence band [74].

The irradiation of energy higher than 3.2 eV creates a pair of electron—"hole": TiO2+Ai^-e-+h+

As a result, free hydroxyl radicals are produced on the surface of the catalyst and/or direct oxidation of organic compounds R takes place according to the following equations:

However, the ejected electrons react with dissolved oxygen, and free radicals are also created:

Further oxidation proceeds with the mechanisms described earlier.

The photocatalysis of leachate is a very promising method. The removal of COD can reach 50% [59] at optimal conditions, but there are also some obstacles. First, the dark color of leachate makes light penetration into the reactor difficult. Therefore, reactors of specific construction are required. The concentrations of inorganic ions and pH also influence photocatalysis efficiency. Cho and colleagues [59] pointed to the acidic pH ranging from 3 to 5 as optimal and explained that at low pH, the carbonates concentration lowered the protection of free radicals against inactivation and promoted organic matter sorption on the catalyst surface.

The presence of ions as chlorides, sulfates, and carbonates in very high concentrations (even a few grams per liter) makes the process slower and less effective. Anions such as chlorides or sulfates react with free radicals and form radicals of lower reactivity than hydroxyl radicals. This results in a decreased process rate. Carbonates are responsible for process inhibition because they are free radical scavengers [59].

28.5 CASE STUDIES OF LANDFILL LEACHATE TREATMENT

In practice, leachate treatment facilities very often consist of combined biological and chemical stages. As an example, the following systems, delivered by WEDECO company, combine a biological stage with ozonation enhanced by UV light, as presented below.

During the period 1991-2002, 40 different installations were constructed with the procedures presented in Table 7. They operate with a flow rate varying from 10,000 to 150,000 m3/year. The COD exists in raw leachate at levels of 2000-4000 mg/L and AOX of 1.5-3.5 mg/L and are reduced to tolerable levels in effluents.

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