Elsevier

Process Biochemistry

Volume 39, Issue 11, 30 July 2004, Pages 1653-1660
Process Biochemistry

Integrated aerobic biological treatment and chemical oxidation with Fenton’s reagent for the processing of green table olive wastewater

https://doi.org/10.1016/S0032-9592(03)00308-XGet rights and content

Abstract

Table olive processing wastewater (TOPW) is unsuitable for disposal at municipal or industrial wastewater treatment plants due to its high organic and phenol content. Aerobic biological treatment using an Aspergillus niger strain in a bubble column bioreactor in combination with chemical oxidation was studied for the management of TOPW to a quality which corresponds to the input standards for wastewater treatment plants (COD<1200 ppm, BOD<500 ppm). After 2 days of biological treatment COD was reduced by 70%, while the total and simple phenolic compounds were decreased by 41 and 85%, respectively. In the chemical treatment step, the effect of different H2O2 concentrations on the patterns of COD and phenol reduction was studied. The main effect of the chemical oxidation step was the elimination of persistent phenolic compounds during the biological treatment of total phenolic compounds. Coagulation with CaO significantly improved the efficiency of the process.

Introduction

Table olive processing wastewater (TOPW) causes an important local environmental problem, as it is characterised by seasonal peaks, very high organic load and high concentrations of phenolic compounds, which are known to cause toxic effects to living organisms. In Mediterranean countries, this type of wastewater is usually discharged untreated to small streams or directly to the sea. In the best cases, it is transported to evaporation ponds, where malodours are a common nuisance, while the risk of polluting surface or ground waters is not always avoided [1], [2], [3]. For green olive preparation, the characteristics of the wastewater which arises from the several treatment stages (cleaning, debittering using NaOH, washing after debittering, fermentation brines and general use water) fluctuate as follows—pH: 3.6–13.2; suspended solids: 0.03–0.4; dissolved solids: 0.2–80; BOD5: 0.1–6.6; COD: 0.3–16.2; chloride: 0.0–48.5 and sodium chloride 0.0–80.0 g/l [1]. The stages of debittering and subsequent washing constitute the largest and most heavily polluted fraction of the wastewater, seasonally produced from September to November.

In the last few years, as environmental regulations and enforcement have become more stringent, a growing interest in the development of new treatment methods for this type of wastewater has emerged. Biological treatment methods have been recognized as overall economical and effective processes [3], [4], [5], [6]. However, the presence at high concentration of aromatic, phenolic and polyphenolic compounds, which are toxic to many microorganisms (especially those found in municipal wastewater treatment plants), inhibits the efficiency of biodegradation processes [3], [7].

In order to facilitate the degradation of toxic or non-biodegradable organic substances, many researchers have proposed combined methods comprising of chemical and biological treatment steps. A common approach refers to the oxidation of the wastewater using a strong oxidative agent, such as ozone [8], Fenton’s reagent, a mixture of hydrogen peroxide and ferrous or ferric iron [9], [10], a combination of UV radiation and hydrogen peroxide as well as photo-Fenton [7]. These methods are based on the creation of very reactive oxidizing free radicals, especially hydroxyl radicals.

In Fenton’s reaction, the ferrous and/or ferric cation decomposes catalytically hydrogen peroxide to generate powerful oxidizing agents, capable of degrading a number of organic and inorganic substances. Fenton’s oxidation is a complicated system that involves a large number of reactions, including redox reactions, complexation, precipitation equilibrium, etc. Also, there is an uncertainty in the nature of oxidizing species generated during the process (formation of radical species or generation of aquo- or organocomplexes of high valence iron). A model of Fenton’s reagent processing of brines from the table olive industry, a wastewater relatively similar to that resulting from the olive debittering stage, which was studied in this work, is described by Rivas et al. [10].

In this study, wastewater from the debittering process of green table olives and the subsequent washings were treated aerobically using Aspergillus niger [11]. The Aspergillus genus has been used successfully for the bioremediation of olive oil mill wastewater, which is also characterized by high organic and phenolics content and satisfactory removal efficiencies have already been reported [12], [13], [14]. Fenton’s reagent was used as a secondary chemical treatment step for the oxidation of the recalcitrant organic compounds or metabolites of those that could not be oxidized biologically. The use of chemical oxidation as a secondary treatment process offers the advantage of reducing the amount of the required oxidants and improves the economic feasibility of the treatment process.

Section snippets

Wastewater

Fresh washing and debittering wastewater (TOPW, pH 12.1–12.5, conductivity 24–44.3 mS/cm) was obtained from the industrial plant of the Agroindustrial Cooperation of Stylida (Lamia, Central Greece). Before biological treatment, the pH was adjusted to 4.5–4.8 using an average of 5.06 ml/l conc. H2SO4. COD and conductivity of TOPW after pH correction ranged from 6.50 to 13.55 g/l and 12.5 to 22.2 mS/cm, respectively.

Inoculum

A strain of A. niger isolated from undiluted TOPW was used for the biological

Aerobic biological treatment of green table olive wastewater

Fig. 1 shows the COD, pH and VSS evolution during the initial 3 day batch culture and the subsequent continuous culture, during which the hydraulic retention time of the wastewater was 2 days. After 1 day of batch operation, the COD removal was 56% and reached 71 and 74% on the second and the third day, respectively. During the continuous operation phase, the average organic loading rate was 5.4 g [COD]/l per day with a standard deviation of 1.1 g/l per day and the mean COD removal was

Conclusion

The aerobic biological treatment of TOPW with A. niger constitutes an effective method for the reduction of the organic and phenolic load of this type of wastewater. In this study, the biological treatment stage yielded a 70% COD reduction, 41% total phenolic reduction and 85% simple phenolic reduction, with a hydraulic retention time of 2 days. Biological treatment of industrial wastewaters of special composition, such as TOPW, requires a prolonged acclimatization period for the biomass, even

Acknowledgements

This work was supported by a grant from the Greek General Secretariat of Research and Technology (EPET II, 98 VIA-08).

References (27)

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