Nasim Esmaeilirad1*,S Mehdi Borghei2 and Manouchehr Vosoughi2
1Colorado State University, Fort Collins, USA
2Biochemical and Bio-Environment Research Center, Sharif University of Technology, Tehran, Iran
Received: 06 July, 2015; Accepted: 29 July, 2015; Published: 31 July, 2015
Nasim Esmaeilirad, Colorado State University, Fort Collins, USA; E-mail:
Esmaeilirad N, Borghei SM, Vosoughi M (2015) Kinetics of Ethylene Glycol Biodegradation in a Sequencing Moving Bed Biofilm Reactor. J Civ Eng Environ Sci 1(1): 002-007 10.17352/2455-488X.000002
© 2015 Esmaeilirad N, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Treatment of waste water containing ethylene glycol (EG) by implementing a sequence of two Moving Bed Biofilm Reactors (MBBR) were studied. Reactors were operated at different hydraulic retention times (HRT) of 48, 24, 18, and10 hours while EG concentration was in the range of 10 mg/l to 1,150 mg/l. Throughout the experiments the ratio of EG Chemical Oxygen Demand (COD) to total COD was changed from 0.0 to1.0. The maximum removal efficiency of EG was achieved at HRT of 18 hours during the tests and COD removal efficiency varied from 71.7% to 96.7%.
To describe the kinetics of biodegradation in biofilm processes, models based on Monod's equations as well as models suggested by other researchers including Grau and Stover- Kincannon were used. As an outcome of this study, both Grau and Kincannon-Stover models were determined to be the most appropriate models for this reactor. These models gave high correlation coefficients and appeared to be able to predict the reactor performance under different conditions. The kinetic studies showed that biofilm diffusion is the most important parameter in controlling the mass transfer phenomena compared to hydraulic factors in the system.
The moving bed bioreactor (MBBR) has emerged as a compact treatment alternative to conventional activated sludge reactors for the treatment of municipal and industrial wastewater . In an MBBR system the biomass is grown as a thin layer on small plastic carrier elements which move around in the reactor and forms a large quantity of biomass. The accumulation of biomass eliminates the need for sludge recycling. The biofilm reactor was completely mixed and operated continuously. The carrier elements were slightly less dense than water and circulated with a water stream. In an aerobic reactor aeration can cause water circulation. An advantage of this system is that the volume of carrier fill in the reactor can be varied to requirements. The standard carrier fill is 70% of the volume and results in total specific area of 465m2/m3 and an effective specific area of 335m2/m3 
This process was first introduced by Kaldnessin the 1990's. It was then used successfully to treat many industrial waste sites. There are presently more than 400 large-scale wastewater treatment plants in 22 different countries throughout the world based on this process in operation in . During the past decade it has been successfully used for the treatment of many industrial effluents including pulp and paper industry waste , poultry processing wastewater , cheese factory wastes , refinery and slaughter house waste , phenolic wastewater , dairy wastewater , and municipal wastewater [10-17]. Another important application of this system was to upgrade existing treatment plants where activated sludge plants can be readily converted to MBBR plants at little cost.
Because MBBR is efficient in treating high strength waste, has a low foot print, and is operational simplicity, it is fast becoming a preferred means for on-site treatment of industrial effluents. Some petrochemical plants, specifically “Olefin” production plants, have to treat large quantities of wastewater containing ethylene glycol where low space and treated effluent standards are important objectives.
Modelling and simulation of reactors is an important tool for design and operation of MBBR plants . In contrast to activated sludge plants, which have been widely modelled using the ASM model family , the modelling of MBBR systems remains very challenging to process engineers. Mathematical models for biological reactors have found little use in engineering practice because the applications are too complex to model easily. Although many studies regarding the performance and application of MBBR have been published so far, very little attempt has been made to describe the kinetics and modelling of this reactor type.
In this research, the organic removal rate in a new MBBR system (using Kaldnes type suspended media) was studied and different mathematical models, which could describe the behaviour of the reactor, were tested. The objective was to find a model which could closely follow the experimental results and could describe the kinetics of the system.
Mathematical models describing the biofilm processes, especially biological filters and Rotating Biological Contactors (RBC), have been proposed in the past [20-26]. Kincannon and Stover  proposed a design concept for RBC's based on total organic loading rate and established a kinetic model for such a reactor. Experiments and research carried out on moving bed biofilm reactors indicate that models based on Monod kinetics developed by Stover-Kincannon could be useful models to describe the process and accurately predict results.
The primary difference between the two models is that in the Kincannon-Stover model, the substrate utilization rate is expressed as a function of the organic loading rate, which is considered to be the most important parameter influencing the behaviour of the reactor.
In a complete midex system the substrate concentration rate of change in, assuming that first order kinetics prevail, can be expressed as follows:
Under steady state conditions, the rate of change in substrate concentration is negligible and Equation 1 can be rearranged and reduced to:
The slope k1 can be obtained by plotting
versus S in Equation (2), 1.2 Stover-Kincannon model. In the Stover-Kincannon model the substrate utilization rate is expressed as a function of the organic loading rate by the monomolecular kinetic for biofilm reactors including rotating biological contactors and biological filters. However, due to the difficulties in measuring the active surface area which supports the biofilm growth, the effective volume of the reactor is used in the version of the Stover-Kincannon model originally suggested by Borghei and Hosseyni  for Moving Bed Biofilm Reactor:
Where dS/dt, the rate of substrate utilization is defined in Equation4: (3)
Eq. (5) is obtained from linearizing Eq. (4) as follows:
The general equation of a second-order kinetic model used by Optaken , Grau et al.  is illustrated in Eq. (6)
If Eq. (6) is integrated and then linearized, Eq. (7) will be obtained:
If the second term of the right part of Eq. (7) is accepted as a constant, Eq. (8) will be obtained:
(S0-S)/S0 expresses the substrate removal efficiency and is symbolized as E. Therefore, the last equation can be written as follows:
Materials and Methods
The pilot scale plant
The pilot scale plant incorporated one reactor which had a volume of 30m3 and one secondary settling tank of 4 m3. The reactor was filled with Kaldnes carrier elements (K1). The Kaldnes carrier elements are made of polyethylene (density 0.95 g/cm3) and shaped like small cylinders (about 10mm in diameter) with a cross inside (Figure 1). The effective specific growth area is about 500m2/m3 at 100% filling grade . The reactor filling grade was 40% which provide 200m2/m3. The reactor was aerated using membrane diffusers. In order to retain the carrier in the reactor, a sieve with 6 mm opening was placed at the reactor outlet. Effluent from the secondary settling tank was recycled to the reactor every one to two days depending on HRT.