Anaerobic Digestion Model to Enhance Treatment of Brewery Wastewater for Biogas Production Using UASB Reactor

Abstract

Biogas produced from an upflow anaerobic sludge blanket (UASB) reactor is a clean and an environmentally friendly by-product that could be used to meet partial energy needs. In this study, a modified methane generation model (MMGM) was developed on the basis of mass balance principles to predict and increase methane production rate in a UASB reactor during anaerobic fermentation of brewery wastewater. Model coefficients were determined using the data collected from a full-scale reactor. The results showed that the composition of wastewater and operational conditions of the reactor strongly influence the kinetics of the digestion process. Simulation of the reactor process using the model was used to predict the effect of organic loading rate and temperature on methane production with an optimum methane production at 29 °C and 8.26 g COD/L/day. Methane production rate increased from 0.29 to 1.46 L CH₄/g COD, when the loading rate was increased from 2.0 to 8.26 g COD/L/day. The results showed the applicability of MMGM to predict usable methane component of biogas produced during anaerobic digestion of brewery wastewater. This study would help industries to predict and increase the generation of renewable energy by improving methane production from a UASB reactor. To the best of our knowledge, MMGM is the first reported developed model that could serve as a predictive tool for brewery wastewater treatment plant available in the literature.

Appendices

Appendix 1

1.1 The Microbial Mass Balance

The microbial mass balance of an UASB reactor (Fig. 8) was described as follows by Ghaly et al. [20]:

Fig. 8

Schematic diagram of a single compartment of an upflow anaerobic sludge blanket reactor (See abbreviations for definition of symbols)

$$ \begin{array}{l}\mathrm{Microbial}\ \mathrm{change}\ \mathrm{rate}=\mathrm{Microbial}\ \mathrm{input}\ \mathrm{rate}+\mathrm{Microbial}\ \mathrm{growth}\ \mathrm{rate}-\mathrm{Microbial}\ \mathrm{death}\ \mathrm{rate}-\\ {}\mathrm{Microbial}\ \mathrm{output}\ \mathrm{rate}\end{array} $$

(A.1)

The microbial growth rates in a batch experiment have traditionally been measured, in which a single species of microorganisms passes through a logarithmic growth phase during the conversion of the organic substrate. The microbial growth rate, dX/dt, is described by

$$ \frac{dX}{dt} = \mu X, $$

(A.2)

which can be written as

$$ \frac{dX}{dt}={\mathrm{QX}}_{\mathrm{i}} + \mu {X}_{\mathrm{r}}V - {K}_{\mathrm{d}}{X}_{\mathrm{r}} - {\mathrm{QX}}_{\mathrm{e}}. $$

(A.3)

During steady-state conditions, the biomass concentration in the influent is negligible (X _i ≈ 0), compared to the biomass concentration in the reactor. In addition, X _r is equal to X _e due to perfect mixing in a completely mixed reactor. The rate of substrate removal from the reactor is therefore neglected. In steady-state conditions, dX/dt = 0 and Eq. (A.3) can be rearranged to obtain Eq. (A.4). Thus,

$$ \begin{array}{l}Q{X}_i \approx \kern-1em 0\hfill \\ {}0={X}_e\left(\mu V-{K}_dV-Q\right)\hfill \\ {}\mathrm{Q}=\mathrm{V}\left(\mu -{\mathrm{K}}_{\mathrm{d}}\right)\hfill \end{array} $$

(A.4)

Eq. (A.4) can be rewritten as

$$ \frac{Q}{V}=\mu - {K}_d. $$

(A.5)

The hydraulic retention time, θ_h, is defined as V/Q. The inverse of θ _h can be substituted into Eq (A.4) as

$$ \mu - {K}_{\mathrm{d}}=\frac{1}{\theta_h} $$

(A.6)

As shown in Eq. (A.6), the net specific growth rate is μ − K _d.

1.2 Substrate Mass Balance and Effluent Substrate Concentration

The rate of substrate balance in the UASB reactor can be expressed using Eq. (A.7)

$$ \left[\mathrm{Substrate}\ \mathrm{change}\ \mathrm{rate}\right] = \left[\mathrm{Substrate}\ \mathrm{input}\ \mathrm{rate}\right]-\left[\mathrm{Substrate}\ \mathrm{utilization}\ \mathrm{rate}\right]-\left[\mathrm{Substrate}\ \mathrm{output}\ \mathrm{rate}\right] $$

(A.7)

Mathematically, Eq. (A.7) can be written as

$$ V\frac{dS}{dt}=Q{S}_{\mathrm{i}} - \left(\mu - {K}_d\right)V\ \frac{X_r}{Y} - Q{S}_{\mathrm{e}}\ . $$

(A.8)

At steady state, Eq. (A.8) was divided by V, and Q/V was substituted for θ_h. At equilibrium the substrate balance of a working system was obtained as

$$ \frac{\ {S}_{\mathrm{i}} - {S}_{\mathrm{e}}}{\theta_h}=\left(\mu - {K}_{\mathrm{d}}\right)\frac{X_{\mathrm{r}}}{Y}. $$

(A.9)

Thus, under perfect mixing of the reactor content (X_r = X_e), the microbial mass concentration in the effluent can be written as Eq. (A.10). This gives the concentration of microorganism in the effluent as

$$ {X}_{\mathrm{e}}=\frac{Y\ \left({S}_{\mathrm{i}} - {S}_{\mathrm{e}}\right)}{\theta_h\left(\mu - {K}_{\mathrm{d}}\right)}, $$

(A.10)

where (S _i –S  _e)/θ _h is the rate of substrate utilization. Contois [15] defined the relationship between limiting substrate concentration and specific growth rate for effluent substrate concentration as

$$ \mu =\frac{\mu_{\max }{S}_r}{b{X}_r + {S}_r}. $$

(A.11)

Under perfect mixing (S _e = S _r and X _e = X _r), the association between the rate-limiting substrate concentration and specific growth rate can be expressed as

$$ \mu =\frac{\mu_{\max }{S}_{\mathrm{e}}}{{\mathrm{bX}}_{\mathrm{e}}+{S}_{\mathrm{e}}}. $$

(A.12)

Equations derived from the combination and rearrangement of Eqs. (A.6), (A.10) and (A.12) are:

$$ {S}_{\mathrm{e}}=\frac{S_{\mathrm{i}}K}{\frac{\mu_{\max }{\theta}_h}{K_{\mathrm{d}}{\theta}_h+1}+\left(K-1\right)} $$

(A.13)

$$ \frac{S_{\mathrm{e}}}{S_{\mathrm{i}}}=\frac{K}{\frac{\mu_{\max }{\theta}_h}{K_{\mathrm{d}}{\theta}_h+1}+\left(K-1\right)}, $$

(A.14)

where Eq. (A.14) shows that the influent substrate concentration is inversely proportional to the substrate concentration in the final effluent.

1.3 Biogas Production

In the reactor, the biodegradable COD is proportional to (B _o − B). B_o is directly proportional to the biodegradable COD loading rate [48]. Therefore, from Eq. (A.14), the methane yield (B) is described by

$$ \frac{B_o-B}{B_o}=\frac{K}{\frac{\mu_{\max }{\theta}_h}{\mu_m{\theta}_h+1}+\left(K-1\right)}. $$

(A.15)

Methane production per gram of substrate (COD) added, B is described by

$$ B={B}_o\left[1-\frac{K}{\frac{\mu_m{\theta}_h}{K_{\mathrm{d}}{\theta}_h+1}+\left(K-1\right)}\right]. $$

(A.16)

Since B is equal to the volume of methane produced per unit of COD added, the volumetric methane production rate, Y _v is equal to B, multiplied by the organic loading rate, S _i/θ _h . The equations describing the theoretical methane output rate per unit of reactor volume therefore are written as Eqs. (A.17) and (A.18):

$$ {Y}_{\mathrm{v}}=\frac{B{S}_{\mathrm{i}}}{\theta_h} $$

(A.17)

$$ {Y}_{\mathrm{v}}=\frac{B_o{S}_{\mathrm{i}}}{\theta_h}\left[1-\frac{K}{\frac{\mu_{\max }{\theta}_h}{K_{\mathrm{d}}{\theta}_h+1}+\left(K-1\right)}\right] $$

(A.18)

Appendix 2

Table 6 Predicted methane yield at different hydraulic retention times

Appendix 3

Table 7 The trend between observed and predicted volumetric methane production rates at different organic loading rates using the newly developed model (Fig. 6a)

Appendix 4

Table 8 The scatter plot of predicted vs observed volumetric methane production rates at lower organic loading rates (Fig. 6b)

Appendix 5

Table 9 Nonlinear regression for the predicted volumetric methane production rates at different temperatures using the newly developed model (Fig. 7)