Dynamical modeling of substrate and biomass effluents in up-flow anaerobic sludge blanket (UASB) biogas reactor

International Journal of Industrial Chemistry https://doi.org/10.1007/s40090-019-00194-w RESEARCH Dynamical modeling of substrate and biomass effluents in up‑flow anaerobic sludge blanket (UASB) biogas reactor Gunawan Nugroho1 · Surya A. Santoso1 Received: 12 October 2017 / Accepted: 12 August 2019 © The Author(s) 2019 Abstract Organic liquid waste from food production industry is inevitable. High chemical oxygen demand (COD) contents in organic liquid waste could disrupt the water ecosystem. On the other hand, COD contents can be reduced and utilized to produce biogas by UASB reactor. However, there is a problem in operating UASB reactor, namely the high biomass content in methanogenic granule form, which is washed out with the effluent. The influent flow rate affects biomass content and the suitable flow rate is important for the particular UASB reactor. To investigate the matter, the estimation of Monod parameters is determined to study the kinetics of substrate (COD) and biomass (active methanogenic granule). In this work, simulations of lumped and distributed models are performed to observe the behavior of substrate and biomass inside the reactor. It is concluded that the suitable influent flow rate for UASB reactor is 150–175 m3/h, and the washed out biomass content is relatively low (from 0.001393 to 0.4919 kg/m3). The steady-state condition is achieved from 2027 to 2533 days, with high COD removal. Keywords Anaerobic process · Biomass content · Chemical oxygen demand · UASB reactor Introduction There are several parameters to determine water quality, i.e., COD, BOD (Biological Oxygen demand), DO (Dissolved Oxygen) and total amount of solute. High COD content on organic liquid waste could disrupt water environment ecosystem, because high COD content water tend to have low oxygen content. After COD is degraded aerobically, it will produce carbon dioxide and sediment, and for anaerobic processes, methane gas is released and would deplete the Earth’s atmosphere [1]. UASB reactor (Upflow Anaerobic Sludge Blanket) is a type of widely used biogas reactors to treat organic liquid waste with high efficiency of 70–90% [2]. During the biogas production process, the UASB reactor utilizes methanogenic * Gunawan Nugroho ; Surya A. Santoso ; 1 Department of Engineering Physics, Institut Teknologi Sepuluh Nopember, Kampus ITS, Sukolilo, Surabaya 60111, Indonesia bacteria which form granules as a medium to decompose COD into methane and carbon dioxide. The methane gas is then collected and used as fuel for various industrial purposes. One of the 10 UASB reactors in Indonesia is able to produce as much as 30,000 N m3 methane gas per day from pineapple and tapioca liquid waste. The produced biogas is then used to heat cassava for tapioca production process and as fuel of combined heat and power plant [3]. Yet, the considerable amount of biomass content in the granule form is released along with effluent flow. The clean waste should contain the treated liquid waste (with low COD content) and inactive biomass. In this case, the biomass that is released with effluent is composed by the dead or inactive bacteria, which is lifted upward due to gas composition within the granule [4]. The high released biomass contents in effluent flow are often formed by the short hydraulic retention time (HRT) or high influent flow rate [5], the overcapacity of biomass inside the reactor [6], or the operation during the start-up of granular reactors which would unintentionally resulted in the reduction of process performance [7]. As a hypothesis, it is estimated that the amount of wasted biomass is caused by too high influent flow rate. In the 13 Vol.:(0123456789) International Journal of Industrial Chemistry previous study by Bolle et al., an experimental method was conducted to determine the relationship between flow rate and reactor height to avoid short circuit flow which resulted in non-treated substrate in liquid waste [8], but the effect on biomass concentration of effluent has not been explained yet. In this research, the UASB reactor was modeled as multilevel CSTR. The Monod parameters for the kinetics of substrate and biomass were determined from the available data. The simplified model was investigated firstly for observing the behavior of substrate and biomass in the reactor, and then the research was extended by implementing distributed models to take the height of the reactor into account. concentration in the influent is assumed to be zero. Then, the above equations become: 0= Q (−Xe ) + (𝜇X − Kd X) Vb (3) 0= 𝜇X Q (S0 − Se ) − Vb Y (4) Simplifying mass conservation equation by cell retention V X 𝜇 S time, 𝜃c = QXb and specific growth rate, 𝜇 = Kmax−Se to produce e the following equation: This research requires data for influent substrate concentration (S0 ), effluent substrate concentration (Se ), effluent biomass concentration (Xe ), biomass volume inside the reactor (Vb ), and influent flow rate (Q). The data were collected in 2 years (January 2015–December 2016). The method of simplified linear regression model (SLRM) was applied to extract the Monod parameters. The important information of bacterial specific growth rate, saturation coefficient, bacterial decay coefficient, and yield coefficient can be obtained by utilizing the kinetics models since microbial growth is also an autocatalytic reaction. Estimation of bacterial reaction kinetics with simple linear regression method e X= QY𝜃c (S0 − Se ) Vb + Kd 𝜃c Vb (5) Se = KS (1 + Kd 𝜃c ) 𝜃c (𝜇max − Kd ) − 1 (6) Methodology Data parameters of biogas plant S Moreover, Eq. (5) is rearranged to form a simple linear regression equation for determining Monod parameters, as: Q(S0 − Se ) Y 1 1 + = × Vb X Y 𝜃c Kd (7) Equation (7) can be redefined as follows: (8) The value of Monod constant, KS and maximum specific growth rate, 𝜇max is obtained by substituting Eq. (6) into Eq. (5), which is: yd = 𝛽1 × xd + 𝛽0 K Vb X 1 1 1 + = S × Q(S0 − Se ) Y 𝜇max Se 𝜇max (9) Or can be written as: (10) Based on the laboratory data, 𝜇max is varied with time which can be approximated by a logarithmic function. After the values of these parameters (Y , Kd , and KS) are known, statistical analysis is conducted by calculating the mean value of the maximum specific growth rate as: yS = 𝛽4 × xS + 𝛽3 By utilizing kinetic models, the important information of bacteria can be obtained, such as bacterial specific growth rate, saturation coefficient, bacterial decay coefficient, and yield coefficient. To obtain the kinetic model parameters of the UASB reactor, a simple linear regression method can be applied as also implemented by Matangue et al. [9] and Bhunia and Ghangrekar [10]. For a UASB reactor without biomass recycling, the growth rate of biomass and the substrate change in the system can be expressed as in the following equation: Q dX (X − Xe ) + (𝜇X − Kd X) = dt Vb 0 (1) 𝜇X Q dS = (S − Se ) − dt Vb 0 Y (2) Under (...truncated)