a dynamic simulation of a two-phase anaerobic digestion system for solid wastes

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  • A Dynamic Simulation of a Two-Phase Anaerobic Digestion System for Solid Wastes

    Joan Mata-Alvarez Department of Chemical Engineering, University of Barcelona, Marti i Franques 08028, Barcelona, Spain

    Accepted for publication November 3, 1986

    In this article, a two-phase system for the digestion of wastes with a high solid content is simulated. The solids are charged to the hydrolyzer and then leachate recirculation is activated until biodegradation is nearly complete. Several parameters are tested, namely moisture , l eacha te recirculation flow rate, and hydrolyzer-methanizer volume ratio. The results show that recirculation rate is an important parameter subject to optimization, with optimal values corresponding to hydrolyzer hydraulic retention times below 1 day. The quantity of recirculating water must be the highest possi- ble. As a consequence, the organic load to the methanizer is reduced, making thus possible the use of a smaller methanizer volume.

    INTRODUCTION

    Anaerobic digestion processes are natural biological pro- cesses in which groups of microorganisms in absence of free oxygen cooperate to convert waste organic matter into bio- gas. The biogas contains reduced species such as methane, hydrogen sulfide and the main oxidized species, carbon dioxide. Six distinct steps may be identified in an anaerobic digester,' although usually it is described as a two-phase biological process. In the first phase, complex organic com- pounds are broken down and subsequently metabolized by fermentative bacteria to produce mainly volatile fatty acids (VFA) and carbon dioxide. In the second phase, VFA are converted to acetate and hydrogen. Methanogenic bacteria remove these compounds, together with carbon dioxide, to form methane. The two main microbial groups involved in the anaerobic digestion differ significantly with respect to physiology, nutritional requirements and growth rate.'

    Phase separation in anaerobic digestion appears to be warranted from the kinetic as well as the process control point of view, thus being possible the maintenance of an optimum environment for each phase. Two phase digestion processes can be used advantageously to treat solid wastes, i.e., those with a total solid content (dry matter) over 20%. Included into this group are agricultural and municipal solid wastes. An example of the application of this technology is the digestion processes proposed for agricultural solid

    Biotechnology and Bioengineering, Vol. 30, Pp. 844-851 (1987) 0 1987 John Wiley & Sons, Inc.

    wastes by Colleran et al.3 and Rijkens et aL4 In this approach the solid organic matter is broken down in the first reaction step (first digester) by acid forming anaerobic bacteria, under percolation of recycled process water. The soluble organic matter and VFA are pumped into a specific methane reactor (second digester), containing a high concentration of methanogenic bacteria, which can effectively treat the water, producing biogas. The treated water is recycled to the first digester (see Fig. 1). The procedure to operate such a system is as follows: 1) to load the reactor with the solid waste; 2) to bring water to the hydrolyzer until the desired level is reached (this water could come from another hydro- lyzer or from a buffer tank, and is used repeatedly cycle after cycle); 3) to start the water recirculation; 4) once the di- gestion is completed, to remove the water from the hydro- lyzer (pumping it to another hydrolyzer which could be starting the cycle or, alternatively, to a buffer tank); and 5 ) to unload the digested solids. These digested solids could be dried with the exhaust gas of a cogenerator set - working with the produced biogas-so that they can be packed and sold as soil conditioners. It is important to point out that

    ' C H ~ B1oGAS h t I

    lYDROLYZER 7 VH ( B A T C H

    L O A D E D )

    V O L R = !! V H

    H H R T = !? 9

    M H R T = !!! 9

    Q

    [IVM 2 R -

    LEACHATE RECIRCULATION

    Figure 1. Schematic of the simulated two-phase system for the anaerobic digestion of solid wastes. Hydrolyzer is loaded in a batch fashion whereas methanizer is operated continuously.

    CCC 0006-35921871070844-08$04.00

  • those systems do generate a certain amount of wastewater. Leachate recirculation line must have a drain point after the methanizer in order to keep the salt contents under allowable levels (The volume of water removed during the solids unloading - part of the liquid is inevitably retained - will not probably be enough to achieve this goal). As a con- sequence, it is also necessary to add some make-up water in order to keep the optimum levels of the overall moisture. However, these amounts of water are much smaller than those required to slurry the solids. The problem posed by this wastewater must be balanced with the solid waste removal.

    In this article, a two-phase digestion process similar to the one described above has been simulated to study quali- tatively the effect of several parameters involved in the process. The simplified mathematical model presented fo- cuses on the hydrolytic and methanogenic steps. The model framework consists of the principles of mass conservation law and biochemical reaction kinetics.

    MODEL SETUP

    The system is composed of a hydrolyzer of volume VH and a methanizer of volume VM (see Fig. 1). The volume ratio VM/VH will be denoted by V,,,. Leachate is re- circulated with a flow rate Q from the hydrolyzer to the methanizer and from the latter to the former. In this sim- plified model neither liquid drain nor make up water is considered. The waste in the hydrolyzer has an initial total solid content of TSo, an initial total volatile solids of VSo, and an initial biodegradable volatile solids content of BVS,,.

    In order to keep the moisture level above critical values, a recirculating water volume Wv (L) is added per gram of

    substrate added to the hydrolyzer. If in an overall system a moisture level H (%) is desired, Wv will be:

    TSO/(100 - H ) - 1 1000

    W" =

    (where a density of 1 kg/L is assumed for the liquid).

    Hydrolysis

    The hydrolysis essentially is the conversion of the bio- degradable volatile solids to volatile fatty acids, that is, hydrolysis and acidification steps are considered together. As a first approach it is assumed that all the biodegradable matter is converted into acids. The fraction being converted into C 0 2 , H2, and cellular matter is not differentiated. The refinement of considering two methanogenic cultures, i.e., including also the homoacetogenic bacteria, would compli- cate the model resolution, affecting only the absolute re- sults, but not the relative ones.

    Kinetics of the hydrolysis step are considered first order,IS2 since the rate of disappearance of the biodegradable volatile solids is proportional to their concentration. This concen- tration is expressed as milligrams of BVS to milligrams of initial sample weight. Table I presents the kinetic equation.

    It has also been assumed that the rate of hydrolysis is pH dependent. According to different authors in the literature, a pH of ca. 6 is optimal for hydr~lysis .~-~ Based on these data, the hydrolysis first-order constant has been expressed as a pH function (Table I). Table I1 presents the numerical values assumed for the rate constant at pH 6.8-9 The pH is expressed in relation to the VFA concentration. The formula has been based on our own experimental data using straw and cow manure as a substrate:

    Table I. Two-phase digester simulation, with kinetic models used for the hydrolytic and meth- anogenic step in the hydrolyzer and in the methanizer. First-order kinetic constant and maximum substrate removal rate have been assumed pH dependent, with maximum values at pH 6.0 and 7.5, respectively.

    Hydrolytic Step

    Rate equation

    First-order kinetic constant as a pH function kk = k,(-0.5pH; + 6.1pHh - 17.6)

    Methanogenic Step

    Substrate removal rate

    Yield equation

    Maximum substrate removal rate as a pH function

    Maximum substrate removal rate as pH function

    Hydrolyzer Methanizer

    Steady state assumed (X,,, = 1.5 x lo4 mg/L) rrh = Y r , - kdXk

    Hydrolyzer

    k , = kd(-0.501pH2 + 7.319pHh - 25.701) Methanizer

    k,. = kmo(-0.501pH2 + 7.139pHm - 25.701)

    MATA-ALVAREZ: A TWO-PHASE ANAEROBIC DIGESTION SYSTEM 845

  • Table 11. Two-phase digester simulation, with kinetic constant values used for the hydrolytic and methanogenic step in accordance with the literature (see references in text). Initial values of microorganism and solid concentrations used in the program runs.

    Hydrolysis

    km = 3 day-'

    Methanization

    kd = kmmo = 7 mg VFA/mg microorganisms day K, = 400 mg/L

    k, = 0.02 day-' Y = 0.04 mg/mg day

    Initial Conditions

    TSo = 90% VSo = 80%

    BVS, = 35%

    X, = 10 mg/L X, = X, = 1.5 x lo4 mg/L

    pH = -log,," + 10.23)]

    where S applies for the VFA concentration in the methanizer or in the hydrolyzer, that is, for Se or S,.

    Methanization

    Methanization can take place in both the hydrolyzer and methanizer, provided that the VFA concentration is low enough. Kinetics for methanization have been assumed to follow a Monod equation.'.' In the methanizer the model is simplified considering the amount of active microorganism concentration constant. This hypothesis is acceptable pro- vided that the methanizer is a high rate digester in which the microorganisms are attached to some kind of support.

    The kinetic equations are shown in Table I, for both the hydrolyzer and methanizer. The maximum substrate re- moval rate constants have been assumed as a function of the digester pH. These functions present a

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