The Simulation and Experimental Study of COD Removal from Rubber Industrial Wastewater using Anaerobic Fixed Bed Reactors

Industrial wastewater from natural rubber factories has a strong odor due to its high organic content of mostly protein compounds. An anaerobic fixed bed reactor (AFBR) can be used in treating rubber wastewater due to the reactor’s ease of operation and short residence time. However, the reactor's design parameters need to be modeled and tested for use in a laboratory before it can be used industrially. This study aims to examine the effect of immobilization media and the presence of trace element, Fe(II), in AFBR for the treatment of natural rubber wastewater. The reactor was operated in two modes: batch and semicontinuous modes. The removal of soluble chemical oxygen demand (sCOD) and methane production during the process were monitored, and the kinetics of COD decomposition were simulated. A high reaction rate constant and high COD removal rate of more than 90% was observed when the reactor was operated using immobilized media and Fe(II).

Anaerobic fix bed reactor; Biogas; Immobilized media; Rubber wastewater

Natural rubber industries produce wastewater with the following characteristics: a pH of 4.2–4.8, Chemical Oxygen Demand (COD) of 2,000–6,000 mg/L, Biological Oxygen Demand (BOD5) of 1,000–3,500 mg/L, 250–400 mg/L of suspended solids, and a Total Kjeldahl Nitrogen (TKN) of 250–700 mg/L (Vijayaraghavan et al., 2008). The disposal of these effluents into public water bodies can cause severe pollution problems. However, in some small-scale rubber processing factories, the effluent is discharged into surroundings without sufficient treatment.

    In general, Indonesia's rubber industries adopt conventional lagoon wastewater treatment systems that require extensive areas of land due to prolonged natural degradation processes. The low digestion efficiency of the anaerobic pond digestion method warrants several reaction stages to produce biogas, which requires an extensive residence time. Moreover, uncontrolled waste accumulations release methane into the atmosphere, and this accelerates global warming (Ariyanto et al., 2017). Many efforts have been made to improve conventional lagoon digester performance; for instance, high flow rate anaerobic reactors, such as anaerobic fixed bed reactors (AFBRs) and up-flow anaerobic sludge blankets (UASBs), have been developed. 

In general, an AFBR is easier to operate than a UASB, even though it has a lower COD removal rate (Sarono et al., 2016). An AFBR consists of a vertical column packed with a solid material with a high surface area, which wastewater is passed through; thus, it is also called an anaerobic filter reactor (AFR). Various packing media materials for supporting biofilm formation have been studied, such as zeolite rocks, pebbles, plastic rings, granulated activated carbon, wooden blocks, and rubber sheets (Loupasaki and Diamadopoulus, 2013). A packed bed serves as a support for biofilm; it is attached on the surface or retained within the pore spaces of the biofilm for faster growth. Joung et al. (2009) confirmed that the anaerobic filter with packed bed (honeycomb type) immobilization media from polystyrene had a 79% COD removal efficiency. Jo et al. (2016) used a downflow anaerobic filter packed with blast furnace slag (BFS) grains to treat cheese whey, and there was an 80% COD removal efficiency and a high loading rate (OLR) with an increasing methane production rate.

An optimum substrate for supporting microorganism growth should be rich in nutrients containing various elements, such as energy sources, electron acceptors, cell buildings blocks, and micronutrients (Schurer and Jarvis, 2009). In terms of microelements, Takashima et al. (2011) suggested that there is a minimum concentration of metallic elements needed to affect biogas production. For instance, the minimum concentrations of Fe, Ni, Co, and Zn are 3.5, 0.40, 0.45, and 2.0 mg/L, respectively. These micronutrients, together with sulfide, are essential for methanogen bacteria to convert acetic acids into methane (Gerardi, 2003). As stated earlier, the amount of Fe required is larger than the needed amounts of the other metals. The significant requirement of Fe ions shows that these ions become various building blocks of cell structure and microorganism metabolism systems.

In addition to being a nutrient, iron has been proven to be effective in controlling H₂S formation and increasing the microbial population in the solution (Choi et al., 2018) and in anaerobic reactors with packed media (Purnomo et al., 2017; Mawaddah et al., 2019). Qiang et al. (2012) demonstrated the importance of the Fe ion over other ions during enzyme synthesis in the growth of methanogenic bacteria. A current review noted three major roles of Fe ions: for synthesizing cellular components, such as metalloenzymes, for controlling major microbial reactions, such as methanogenesis, and improving the granulation process in anaerobic digesters for the successful operation of high flowrate digesters and preventing the accumulation of methanogen inhibitors (sulfides) in digesters (Baek, 2019).

    The results showed that bio balls in packed bed reactors increased sCOD removal compared to those in hollow tube reactors. The addition of Fe accelerates the rate of VFA conversion to methane. Reactors with bio balls and added Fe(II) are optimal, shown by their COD removal rates of 90.7%, which was the highest measured rate. The combination of packed media and Fe addition will provide a robust AFR due to fast degradation performance and the ability to withstand the organic loading fluctuation. This effort can increase the performance of AFBR to a level that is similar to that of the more expensive UASB reactor. This finding is supported by the simulation result of the high value of rate constants for AFBRs with added Fe(II). Other essential trace metal additives should be studied in the future, along with the possibility of metal recovery or recycling to reduce the operational cost and avoid the environmental impact of excess ions.

    This study was supported by Rekognisi Tugas Akhir (RTA) grant UGM no. 488/UN1.P.III/DIT-LIT/PT/2020 and Penelitian Terapan Unggulan Perguruan Tinggi (PTUPT) no. 1860/UN1/DITLIT/DIT-LIT/PT/2020 Ministry of Research and Higher Education grants. We would like to express sincere gratitude to Dr. Mohd. Razif bin Harun from UPM for his contribution during the preparation of this manuscript.

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