(215v) Dynamic Modelling and Simulation of Anaerobic Digester For High Organic Strength Waste

High organic strength waste such as wastewater generated from various industrial activities and sludge generated from industrial and municipal wastewater treatment plant requires treatment prior to disposal in order to reduce environmental pollution load and also to comply with the disposal norms. Waste containing organicimpurities in nature must be treated biologically using life forms such as microorganisms, plants, etc. Anaerobic digestion is the most suitable treatment method for treating high organic strength waste as compared to other waste treatment methods such as aerobic and advanced treatment methods, because energy in the form of methane is recovered and also treated sludge has good soil conditioning values. Complete mix anaerobic digester is more suitable for treating high organic strength waste as compared to other because of proper heat transfer amd also proper mixing for maintaining uniformity throughout the reactor. Complete mix reactors are resistant to shock loading and also withstand high organic loading rates. Anaerobic digester suffers stability problem due to the accumulation of the volatile fatty acids and drop in pH. Once the failure of digester takes place it requires long time and efforts to restart it again. Dynamic modelling and simulation are useful tools for predicting the process stability by studying the behaviour under transient conditions and are also helpful in understanding the process operations.

In the present work an attempt has been made to develop simplified dynamic mathematical model for the anaerobic digestion of sludge.The simplified mathematical model for sludge digestion is based on the some important models described below.  The inhibitory function developed by Andrews (1969) given below was an important investigation considering the inhibitory effect of unionized volatile fatty acids on methanogen bacteria. The unionized volatile fatty acids act as growth limiting substrate at lower concentration and cause inhibition at higher concentration for microorganisms utilizing them as substrate.

The inhibition model was given as

where,  

 µ  =   û / (1+ (K_s/HS) +( HS/K_i))

 µ         Specific growth rate , day^-1

 û         Maximum  specific growth rate in the absence of inhibition , day^-1

 HS      Unionized substrate concentration, mg/L

 K_s      Saturation constant, mg/L

  K_i      Inhibition constant, mg/L

The model was restricted to fixed pH and only methanogenesis step was considered. Andrews et al. (1971) removed the limitation of fixed pH  by considering the interaction between liquid, gas and biological phases and pH variation took place between 6-8. The buffering system was carbon dioxide-bicarbonate is assumed.         This model was also restricted to methanogenesis step. Hill et al. (1977) developed model for animal waste digestion based on two microbial culture acidogens and methanogens. They considered the inhibition caused by unionized volatile acids and unionized ammonia on growth kinetics of the methanogens.

The inhibition model was given as

Where,

µ = û / ( 1+ (K_s/VA) + (VA/K_ia) + (NH₃/K_i2))

µ        Specific growth rate , day^-1

û        Maximum  specific growth rate in the absence of inhibition , day^-1

VA      Unionized substrate concentration, mg/L

 K_s     Saturation constant, mg/L

 K_ia     Inhibition coefficient of acids, mg/L

  K_i2   Inhibition coefficient of ammonia, mg/L

NH₃   Concentration of unionized ammonia, mg/L

Havlik et al. (1986) developed model for digestion of   complex organic substrates and also stated that the path of methane generation was either from acetate and via carbon dioxide reduction by hydrogen. The model developed by Moletta et al. (1986) was based on easily fermentable organics such as pea bleaching wastewater and synthetic substrate containing sucrose and organic acid. In the developed model biomass and metabolite production rates were described by distinct relations. No pH variation and no buffering system were assumed in this model. The dynamic Model developed by Mendoza et al.(1997) for anaerobic digestion of sewage sludge describes digester behaviour under non-ideal mixing conditions. He considered no buffering system in the developed model and the growth rate of microorganisms was assumed to be dependent on Monod Kinetics. Hydrolysis and death of microorganisms were described by first order reactions.

In 1997, an International anaerobic modelling task group was established in Japan as a common platform for the establishment of anaerobic digestion model for complex organic wastes.

The kinetic model developed in the present work describes the anaerobic digestion of sludges considering the hydrolysis of particulate material to soluble compounds by the extracellular enzymes produced by acidogens, fermentation / acidogenesis of soluble organics to volatile fatty acids by the acidformers and methanogenesis of volatile fatty acids to methane and carbon dioxide by methanogenic bacteria. In most of the cases first step in anaerobic digestion of waste depends on the nature of the organic waste. Sometimes fermentation may be the first step if most of the impurities are soluble in nature. The growth and decay of acidogens and methanogens are assumed to depend on Andrews’s inhibitory function and also the inhibitory effect of unionized ammonia on methanogens. Hydrolysis of particulate organic compound is described by first order reaction. The microbial kinetic model expressions are linked to the complete mix anaerobic digester. Carbon dioxide and bicarbonate with ammonium ion is considered as the buffering system. Computer simulation of developed mathematical model helps to predict the dynamic behaviour of the developed model under batch mode, transient conditions, steady state conditions and also helpful in improving design.

The developed modified dynamic mathematical model  describes the process operations in more quantitative way which is helpful in designing a better control system to enhance stability, improving performance of anaerobic digester and  also optimizing process performance.

Keywords: anaerobic digester, organic waste, sludge, modelling and computer simulation

References:

1. Andrews, J.F., A dynamic model of the anaerobic digestion process. J. Sanitary Engineering Div., Proc. Am. Soc. Civil Eng. (1969) 95,SAI, 95.

2. Andrews, J.F. and Graef, S.P., Dynamic modelling and simulation of the anaerobic digestion process. Advances in Chemistry, American Chemical Society (1971),126-162.

3. Havlik, I., Votruba, J. and Sobotka, M., Mathematical modelling of the anaerobic digestion process: Application of dynamic mass-energy balance. Folia Microbiol (1986),31,56-68.

4. Hill, D.T. and Barth, C.L., A dynamic model for simulation of animal waste digestion. Journal of Water Pollution Control Fed.(1977),10,2129-2143.

5. Mendoza, R.B., and Sharratt, P.N., Modelling the effects of imperfect mixing on the performance of anaerobic reactors for sewage sludge treatment. J. Chemical Technol. Biotechnol.(1998),71,121-130.

6. Moletta, R., Verrier, D. and Albagnac, G., Dynamic modelling of anaerobic digestion. Water Resources (1986),20,427-434.