Treatment of olive mill wastewater using ozonation followed by an encapsulated acclimated biomass

Highlights

• An encapsulated biomass was implemented after ozonation to treat olive mill wastewater.

• Ozone pretreatment of tannic acid reduced the subsequent biodegradation time.

• An encapsulated Delftia EROSY biomass degraded up to 1000 mg L⁻¹ tannic acid.

• Ozone pretreatment combined with an encapsulated biomass decreased COD from OMWW by 36%.

• Spectral absorbance can monitor ozonation and biodegradation of OMWW.

Abstract

The environmental impacts caused by Olive Mill Wastewater (OMWW) are a concern for both developing and developed countries. In this study, an ozone pretreatment combined with a fixed biomass bio-treatment using the Small Bioreactor Platform (SBP) capsules technology encasing a pure culture of a phenol-degrading OMWW isolate named Delftia EROSY was implemented to reduce phenolic compounds and organic matter in OMWW.

Up to 90% of tannic acid (TA), a synthetic phenol model, was removed after the ozonation and biological stages. Ozone pretreatment of TA expedites the biological process by decreasing the time needed for the biodegradation of phenols.

Ozonation (ozone dose = 765 mg L⁻¹ O₃) of OMWW demonstrated 20% COD and 61% total phenol removal, with an additional 36% increase in COD removal after the biological step (48 h). Interestingly, our results also showed that spectral absorbance can be used as a tool for monitoring ozonation followed by bio-treatment of OMWW. Absorbance results clearly demonstrate that bio-treatment is necessary for degrading not only phenolic compounds, but also phenol transformation products and the high organic load of the OMWW, following the ozonation step.

Introduction

Olive mill wastewaters (OMWW) are toxic industrial wastewaters due to the presence of toxic compounds and a high load of organic compounds [1]. OMWW is characterized by a dark reddish-black color, mildly acidic pH, high organic content and toxic materials which are composed mainly of sugars, tannins, pectins, polyphenols, polyalcohols and lipids. These compounds are persistent, and thus very difficult to treat by physical and chemical methods or biodegradation [2]. OMWW is characterized by high levels of chemical oxygen demand (COD) (in the range of 80–200 g L⁻¹), biochemical oxygen demand (BOD₅) (in the range of 50–100 g L⁻¹) and recalcitrant phenolic compounds (2–15 g L⁻¹), which are of the main cause for environmental problems arising from the discharge of OMWW. Even though the toxicity of this effluent is well-known, it is still discharged illegally into fresh water ecosystems or dumped on soils without proper treatment [3]. The scale of its environmental impact can be inferred from the fact that 1 m³ of OMWW is equivalent to 100–200 m³ of domestic sewage [4]. The negative effect of OMWW was demonstrated on soil microbial populations [1], on aquatic ecosystems, and even in air [5].

Treatment and disposal of OMWW is currently one of the most complicated environmental problems in the agro-industry [6]. The biotoxic properties of phenols in OMWW constitute a significant inhibitor of the biological processes that take place in common wastewater treatment plants (WWTPs). Municipal WWTPs do not present the desired performance with OMWW discharge. Treatment of OMWW together with municipal wastewater is thus not economically feasible, due to overload of the municipal wastewater that can induce an oxidative stress episode resulting in collapse of the bio-treatment process. Guidelines for managing OMWW through technologies that minimize their environmental impact and lead to a sustainable use of resources are therefore necessary, particularly in light of increasing olive oil production worldwide (2.6 million tons as published in the International Olive Oil Council 2016 newsletter [7]).

Pretreatment of OMWW should be designed to improve the wastewater quality and remove most of its toxicity. In addition to being technically feasible, OMWW treatment processes must be efficient, allow for easy and economical operation and consider the spatial distribution of olive oil production and the seasonality in harvest time. Various waste management practices have been reported in the last two decades, which apply physical, chemical and biological processes as well as their combinations due to the great variety of components found in the OMWW [8]. Some of those treatments include dilution, evaporation, sedimentation, filtration, coagulation-flocculation, adsorption on granular activated carbon, aerobic and anaerobic digestion, and treatments using fungi and bacteria. However, these technologies lead to limited biodegradability levels of organic matter and phenolic compounds [9]. The process efficiency, complexity and costs of installation, operation and energy may vary significantly. High cost is generally the main reason for not adopting efficient OMWW treatment methods [10]. Expensive treatment methods are not profitable, considering the short production period and the small size of most olive mills [11].

Interest in direct oxidation and advanced oxidation processes for the treatment of industrial effluents, as well as for the treatment of OMWW, has been growing in recent years [[12], [13], [14]]. Ozone (O₃) is a powerful oxidizing agent that selectively attacks compounds containing aromatic rings and double bonds. It is thus capable of causing oxidative degradation of many organic compounds such as polyphenols which are present in OMWW. Benitez et al. [15] observed ∼20% COD reduction of OMWW for an initial COD of 10 g L⁻¹ after 2 h of ozonation. This reduction was attributed to the oxidation and breakdown of larger organic compounds into smaller and less polluting ones [15]. COD reduction was ∼20 and 60% when the ozone concentration (240 min ozonation time) increased from 22 to 60 mg L⁻¹, respectively [16]. However, ∼80% phenol removal for the same ozonation time demonstrates the selectivity of ozone towards the toxic fraction of the OMWW (e.g., aromatic rings and double-bond compounds). Ozone pretreatment may thus be a viable procedure for subsequent bio-processes. Ozone is highly soluble in water, and therefore leaves no residuals. This leads to a safer environmental disposal profile.

Physicochemical systems combined with bio-treatments have been reported in the literature to reduce phenolic compounds and organic matter. Ozonation combined with bio-treatment can convert the non-biodegradable and hard-to-biodegrade compounds into readily biodegradable compounds for the bio-treatment, resulting in safer effluent disposal into the environment. For example, Benitez et al. [15] investigated degradation of organic matter by either ozonation or aerobic degradation, and by a combination of the two. Contaminant load, determined by COD and total aromatic and phenolic contents, exhibited 17–28% reduction from the initial COD after the ozonation stage, 76.2% in the aerobic degradation process (HRT = 7 days) and 82.5% in the combined ozonation and aerobic degradation stages. The higher COD reduction obtained in the combined process was associated with the removal of some inhibitory phenol compounds, resulting in decreased toxicity and increased biodegradability. Another study showed that ozone pretreatment followed by anaerobic digestion improved biodegradability, thus enhancing the removal of polyphenolic compounds [17], which is considered the best available technology according to the 96/61/EC directive.

However, microorganisms in a suspension growth state might present some sensitivity to the polyphenols and other ozonated organic transformation products in OMWW, due to direct interactions between the bacteria and toxicants. This is partially prevented in the encapsulated growth state, due to slower diffusion through the capsule’s membrane into the encapsulated biomass. No study has, to date, demonstrated the feasibility of ozone pretreatment followed by an aerobic biological process with encapsulated microorganisms for treating OMWW. The goal of the present study was, therefore, to investigate the ozonation process coupled with the encapsulated biological process for oxidation of phenolic compounds and organic matter from a synthetic phenol model (tannic acid – TA) and on actual OMWW. The ozonation step was designed to enrich the wastewater with oxygen, reduce the toxicity of phenolic compounds and the organic load and increase OMWW biodegradability. Bioaugmentation of an encapsulated phenol-biodegrading bacterial isolate, previously isolated from OMWW, was then conducted and studied for its phenol and organics removal capabilities.