Short CommunicationFenton’s pre-treatment of mature landfill leachate
Introduction
Although landfill leachates have been proved to be toxic and recalcitrant, landfilling still remains one of the main methods for municipal and industrial solid waste disposal. There are many factors affecting the quality and the quantity of such leachates, i.e., seasonal weather variation, landfilling technique, piling and compaction method, waste type and composition, structure of the landfill, etc. In particular, the composition of landfill leachates varies greatly depending on the age of the landfill (Baig et al., 1999). Accordingly, several treatment technologies are used in practice (Haapea et al., 2002).
To remove the bulk of pollutants, biological treatments are usually preferred over physico-chemical (Im et al., 2001). However, good performances are obtained with biological processes only treating “young” biodegradable leachates. In the case of “old” leachates, instead, COD (chemical oxygen demand) maximum allowable concentration (MAC) for direct or indirect discharge cannot be met because of the occurrence of pollutants that inhibit biomass activity and/or are recalcitrant to biological treatments. In such instances, MAC values are usually achieved by more expensive physico-chemical treatments such as flocculation–precipitation, adsorption on activated carbon, evaporation, chemical oxidation, incineration. Among them, growing interest has been focused on advanced oxidation processes, AOP, (Huang et al., 1993; Schroder, 1996) which, exploiting the strong oxidation potential of hydroxyl radicals (HO), can achieve two alternative goals: (i) the reduction of the COD content of wastewater up to the desired MAC value through the mineralization of recalcitrant pollutants (i.e., their transformation into CO2); (ii) the enhancement of the biodegradability of treated effluents with the aim of making their subsequent biological treatment possible.
In general, AOP are defined as oxidation processes which generate hydroxyl radicals in sufficient quantity to affect water and wastewater treatment (Huang et al., 1993). The hydroxyl radical is one of the most reactive free radicals and one of the strongest oxidants (HO + H+ + e− ⇒ H2O; E0=2.33 mV). Many systems can be classified as AOP and most of them use a combination of: two oxidants (e.g., O3 plus H2O2); catalyst plus oxidant (e.g., Fe2+ + H2O2); oxidant plus irradiation (e.g., H2O2 plus UV); oxidant plus photo-catalyst (e.g., H2O2 plus TiO2 plus hν); oxidant plus ultrasounds (e.g., H2O2 plus ultrasounds). One common feature of such systems is the high demand of electrical energy for devices such as ozonizers, UV lamps, ultrasounds, and this results in rather high treatment costs. The only exception is the Fenton’s process. In such a process, in fact, under acidic condition, a Fe2+/H2O2 mixture produces OH radicals in a very cost-effective way. The major advantages of the Fenton’s reagent (Fe2+ + H2O2 + H+) are: (i) both iron and hydrogen peroxide are cheap and non-toxic; (ii) there is no mass transfer limitation due to its homogeneous catalytic nature; (iii) there is no form of energy involved as catalyst; (iiii) the process is technologically simple.
Because of these features, Fenton’s process has been applied in many areas (Prousek, 1995) including that of recalcitrant wastewater and/or landfill leachates treatment (Koyama and Nakamura, 1994; Gau and Chang, 1996; Tang and Huang, 1996; Bae et al., 1997; Kim et al., 1997; Steensen, 1997; Kwong et al., 1999; Rivas et al., 2001; Zhu et al., 2001). However, at least in the case of landfill leachates, Fenton’s process has been used mainly as a post-treatment to achieve desired COD MAC values rather than as a pre-treatment in order to permit a subsequent economical biological treatment (Chamarro et al., 2001).
On the basis of the above considerations, the investigation described in the present work was carried out specifically to check the effectiveness of the Fenton’s process for pre-treating a municipal landfill leachate with the aim of improving its overall biodegradability up to a value compatible with subsequent aerobic biological treatment. Measuring biodegradability as the ratio between the biochemical oxygen demand measured after 5 days (BOD5) and COD, such a value must be ⩾0.5.
Section snippets
Fenton’s treatment procedure
Fenton’s treatment of landfill leachate was carried out at ambient temperature according to the following sequential steps. (1) Leachate sample was put in a beaker and magnetically stirred; its pH was adjusted to fixed values by H2SO4 95–97% (w/w). (2) The scheduled Fe2+ dosage was achieved by adding the necessary amount of solid FeSO4 · 7H2O. (3) A known volume of 35% (w/w) H2O2 solution was added in a single step. (4) After fixed reaction time (2 h), before carrying out BOD tests, 3 g l−1 of
Results and discussion
The composition of the investigated leachate is reported in Table 1. Taking into account the low concentration of heavy metals, the pH value (8.2), the low value of the BOD5/COD ratio [(2300/10540)=0.2] and the high contents of NH4-N (5210 mg l−1) and alkalinity (21 470 mg l−1), the leachate was classified as “old” and non-biodegradable (Baig et al., 1999).
Referring to the Fenton process, it is well known that higher hydrogen peroxide to substrate ratios result in more extensive substrate
Conclusions
An investigation aimed at checking the effectiveness of Fenton’s reagent (Fe2+ + H2O2 + H+) for the enhancement of the biodegradability of real old municipal landfill leachate has been carried out at lab scale at ambient temperature. The investigation has led to the following results:
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The maximum amount of COD that could be removed by the Fenton’s pre-treatment was about 60% of the initial value (i.e., 10 540 mg l−1). Such a maximum removal was achieved using reagent dosages as high as 10 000 mg l−1 of H
Acknowledgements
The authors thank Mr. Michele Cammarota and Mr. Giuseppe Labellarte for their technical assistance during the investigation.
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