Performance analysis of four-stage rotating biological contactor in nitrification and COD removal from petroleum refinery wastewater

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Highlights

  • The maximum NH3-N and COD removal efficiency were achieved at 99.07 and 85.76 %.

  • Flow rate and interaction between discs rotational speed have a large impact on removal efficiency (p-value < 0.0001).

  • The most ammonia removal occurred in the first stage at the lowest HLR (0.005 m3/m2d).

  • The optimum operating conditions were determined with low error (0.12 %) by the RSM.

  • The CCD revealed that predicted data were in agreement with observed ones (R2 = 0.999).

Abstract

The petroleum refining process generates large quantities of wastewater that discharge of untreated wastewater into water bodies results in environmental and human health effects due to the release of toxic containments. In this study, the biological removal of ammonia from petroleum refinery wastewater in a bench-scale rotating biological contactor (RBC) under continuous operation was investigated. The experiments were conducted base on a central composite design to evaluate the effect of four significant independent variables on the performance of the system. Response surface methodology was applied to minimize the number of runs and investigating the interactive effects of variables on ammonia and chemical oxygen demand (COD) removals. The RBC was operated at different hydraulic loading rates of 0.005, 0.008, and 0.016 m3/m2d. The optimum conditions were the temperature of 32.19 °C, influent ammonia concentration of 20.54 mg/l, the flow rate of 5.57 mL/min, and discs rotational speed of 4.58 rpm for ammonia and COD removal efficiency simultaneously. The maximum removal efficiency of ammonia and COD were obtained at 99.07 % and 85.76 % respectively. Finally, our results suggest that RBC may be considered as a promising method for petroleum refinery wastewater treatment, especially in simultaneously COD and ammonia removal.

Introduction

Petroleum refineries generate large amounts of priority pollutants that after recovery of oil in American petroleum institute (API) separators and removal of oil in dissolved air flotation (DAF) units, and the activated sludge process (ASP) is typically the biological treatment of this wastewater. Due to some problems with the suspended growth process, it can be replaced by fixed-film biological reactors such as the rotating biological contactor [[1], [2], [3]]. According to the United States, Environmental Protection Agency (USEPA) and World Health Organization (WHO), petroleum refining wastewater (PRW) have to be sufficiently treated for quality to meet the established regulations and they limited ammonia concentration to 2.5 mg/l in drinking water [4,5].

PRW with high ammonia content is generally difficult to treat effectively by conventional physical and chemical methods. Therefore, there is a great need to improve the existing biological treatment processes [[6], [7], [8]]. One of the most important methods used for ammonia removal from industrial wastewater is the biological methods using nitrification and denitrification processes [9]. Nitrification is a two-step microbial process in which two groups of autotrophic nitrifying bacteria are involved. In the first step, there is the conversion of ammonium nitrogen to nitrite by Nitrosomonas-like bacteria (Eq. (1)) and subsequently, in the second step there is the conversion of nitrite to nitrate by Nitrobacteria-like bacteria (Eq. (2)) [10,11]:NH3+O2NO2-+3H++2e-No2-+H2ONO3-+2H++2e-

The rotating biological contactor is attached growth fixed biofilm reactor based on biomass that develops on the surface of a series of circular discs mounted onto a horizontal shaft with approximately 35–45 % of the discs submerged in the wastewater. Also, the rotating disc surface alternately partially immersed in the wastewater and comes into contact between air-water allows the transfer of oxygen and adequate an active thin layer of microorganisms development [[12], [13], [14], [15], [16]]. RBCs are widely used in industrial and municipal wastewater treatments by exploiting the advantages of both fixed film and suspended growth systems. RBC offers operational simplicity, low costs of operation and maintenance, low energy requirement, high biomass concentration, little sloughing of biomass, short hydraulic retention time (HRT), resistance towards shock loadings, and capability for handling toxic pollutants [[17], [18], [19], [20], [21], [22]].

RBC represents a viable means for nitrification and denitrification processes since the dominant microorganisms on biofilm were very stable and also grew very well on the discs. Nitrifying bacteria in RBC were the major organisms responsible for decreasing the total Kjeldahl nitrogen (TKN) level in the industrial wastewater treatment [[23], [24], [25]]. Previous studies have reported on the successful application of RBCs to the treatment of wastewaters containing ammonia [[26], [27], [28], [29], [30], [31], [32], [33], [34]].

To overcome the current limitations of ASP and based on the above background, the main objective of this work was to evaluate the effectiveness of a four-stage RBC for the simultaneous removal of COD and ammonia of PRW under variable hydraulic loading rate (HLR). Also, the effect of temperature, influent ammonia concentration, flow rate, and discs rotational speed on removal efficiency of ammonia and COD were evaluated. Finally, design expert software was applied for design, mathematical modeling, and optimization.

Section snippets

Experimental set-up

A bench-scale four-stage RBC was fabricated with a thick iron sheet that was coated with fiberglass [35]. The schematic experimental setup is shown in Fig. 1. The total volume of RBC was 16 L and a working volume of 8 L. The dimensions of the RBC reactor were 52 cm length, 28 cm width, and 11 cm depth which consisted of four-stages. The parameters of the experimental setup of the RBC reactor are shown in Table 1. Each stage was separated by fixed baffle plates that comprised one hole drilled

Regression models and statistical analysis

Regression models and statistical testing in this paper, correlations between the responses and independent variables were obtained by the following second-order model with a least square method [45]:Y=B0+j=1kBjxj+i<jBijXiXj+j=1kBjjXj2+Where Y is the response, B0 is a constant coefficient, Bj, Bij, and Bjj are the coefficients for linear, quadratic, and interaction effects, respectively. Xi and Xj are the coded levels for the independent variables; k is the number of independent variables

Conclusions

In this investigation, the performance of a four-stage RBC for continuous treatment of real wastewaters was evaluated. The mineral salts medium was used to enrich nitrifying bacteria for the oxidation of ammonia in refinery wastewater. Also, In the first stage, the maximum ammonia and COD removals efficiency occurred at the lowest and medium HLR. The increase of HLR due to the reduction of HRT seemed to have a negative effect on ammonia removal. Likewise, a negative correlation was observed

CRediT authorship contribution statement

Vahab Ghalehkhondabi: Methodology, Software, Validation, Writing - review & editing. Alireza Fazlali: Conceptualization, Supervision, Funding acquisition. Behrooz Fallah: Formal analysis, Investigation, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors gratefully acknowledge the support provided by Shazand Petroleum Refinery Company (Arak, Iran).

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