Real municipal wastewater treatment using simultaneous pre and post-ozonation combined biological attached growth reactor: Energy consumption assessment

Highlights

• Treatment of real municipal wastewater was investigated in the combined ozonation and biological process.

• Complete ammonium removal and 94 % COD removal were obtained in the combined treatment.

• Total coliforms reduced to 0.51 MPN/mL in the final effluent.

• The ozonation process was the dominant energy consumption source.

Abstract

In this study, simultaneous pre and post-ozonation processes combined biological attached growth reactor were used in order to investigate the removal of COD, ammonium and total coliforms from real municipal wastewater. The biological process was carried out in a fixed bed aerobic bioreactor with an aeration rate of 6 L/min and hydraulic retention time of 5 h. The ozonation process was performed simultaneously in pre-ozonation and post-ozonation reactors, with 1 g/h ozone mass flow within 30 min every 8 h. The results showed that complete ammonium removal and 94 % COD removal were obtained. Also, total coliforms in the final effluent were measured to be about 0.51 MPN/mL. The analysis of energy consumption showed that the ozonation process was the dominant energy consumption source. The combination of fixed bed bioreactor and ozonation process showed a promising technology for the treatment of real municipal wastewater.

Introduction

The crisis of supply of safe water is a vital question in the recent century [1]. Treatment and disinfection of reusable water are one of the major pathways for preparation of potable and non-potable water [2].

The ultimate aim of wastewater treatment is to mineralize, that is the transformation of organic molecules into relatively harmless and inorganic substances [3]. Carbon, nitrogen, phosphorus, sulfur, and halogen compounds constitute the most part of sewages. To achieve mineralization, it is desirable to convert carbon to carbon dioxide, nitrogen to nitrates, phosphorus to phosphates or phosphoric acids, sulfur to sulfates and halogens to halogen acids [4].

Pharmaceutical materials, pesticides, personal care products (PCP), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbon (PAHs) are the most important sources of resistant pollutants that are discharged into the surface waters [5]. Detection of this pollutants in the effluent of the sewage treatment plants indicated the inadequate performance of conventional wastewater treatment processes [6].

The discharge of effluents containing organic pollutants to surface waters causes eutrophication phenomenon and mortality of aquatic species [7,8]. Biological treatment methods, as a cost-effective and efficient processes, are capable of treating a wide range of contaminants, and they are the first selection in wastewater treatment [9]. However, the efficiency of these processes is not desirable in critical conditions such as high organic loading rate and the existence of toxic pollutants or non-biodegradable compounds [10,11].

The combination of the biological and chemical processes are a promising solution for the treatment of wastewaters containing resistant and toxic contaminants [12,13]. In the last two decades, advanced oxidation processes have been used successfully for treatment of resistant wastewaters such as pharmaceutical, textile, petrochemical industry, leather industry, etc [14,15]. Advanced oxidation processes rely on the in-situ production of active oxidizing agents, such as free OH radicals with 2.80 eV redox potential [4]. Ozone with 2.07 eV redox potential is one of the strongest oxidants for disinfecting water, oxidizing organic components and decomposing a wide range of pollutants into simple materials [16,17]. Ozone also destroys organic contaminants by the way of direct oxidation with pollutants and, indirect reactions through free OH radical generation [18]. In practice, both direct and indirect mechanisms will take place. One kind of reaction will dominate, depending on various factors, such as temperature, pH and chemical composition of the wastewater [19]. Another advantage of ozone is that it destroys bacteria such as Escherichia coli and other groups of coliforms and disinfects final effluent [20]. Also, ozone decomposes to molecular oxygen and does not impress any by-products [21]. It was also reported that ozonation could reduce COD as well [22].

Several studies have been reported the combination of ozonation with biological processes as pre-treatment (before biological process) or post-treatment (after biological treatment) [[21], [22], [23], [24]]. Degradation of complex and resistant compounds in pre-treatment ozonation lead to enhancement in biodegradability of wastewater, which increases BOD/COD ratio. In post-treatment process, the effluent from ozonation should be recycle into the biological process to enhance the efficiency of treatment [25]. In fact, the biological process before ozonation will remove biodegradable compounds and, accordingly, the ozonation process can be done with lower loading rate.

Extensive agricultural activity around Tabriz city has led to the entrance of agricultural pollutants such as fertilizers and pesticides into the wastewater treatment plant of Tabriz [7]. Activated sludge process as conventional treatment method causes to remain resistant materials in treated water [26]. In the recent decades, fixed bed biological systems were introduced as a viable candidate to replace conventional wastewater treatment. High sludge retention time in the fixed bed bioreactor makes it more resistant against toxic pollutants and it is possible to achieve higher removal efficiency compared with other biological processes [27,28]. Also, as these systems have a small footprint, they are appropriate for uses in areas with space limitations [29].

In the present study, the combination of ozonation and fixed bed biological attached growth reactor for municipal wastewater treatment was studied. In the first stage, the experiments were performed with synthetic wastewater in a biological reactor without ozonation process. In the second stage, simultaneous combination of pre and post-ozonation with biological reactor were evaluated on the removal of COD, ammonium and total coliforms from real wastewater. Furthermore, the system energy efficiency was evaluated based on the required electrical energy.