Modeling Chemical and Biological Processes of Solid Waste Anaerobic Digestion

Transcription

1 Asheville October 2002 The econd Intercontinental Landfill Research ymposium Decomposition Modeling Modeling Chemical and Biological Processes of olid Waste Anaerobic Digestion Vasily A. Vavilin Water Problems Institute Russian Academy of ciences Moscow

2 MILETONE Anaerobic digestion model No.1 ADM1 Balance between polymer hydrolysis and methanogenesis as the rate-limiting steps Key factors for kinetics waste composition moisture ph particle size temperature microorganisms... Inhibition phenomenon Mass transfer processes timulation of degradation rate leachate recirculation and ph adjustment inoculation

3 Many attempts have been made to apply simple models like first-order equation to describe the rate of solids degradation in landfills but the fit was often poor The main purpose of the presentation is to show the factors potentially affecting decomposition dynamics which should be included into the model

4 There are a number of famous mathematical models of anaerobic digestion systems of a continuous-flow stirred-tank reactor CTR. The IWA Anaerobi? Digestion Model No. 1 is the result of four years collaborative work by the international experts in anaerobic process analysis modeling and simulation

5 cientific and Technical Report No. 13 Anaerobic Digestion Model No. 1 Edited by IWA Task Group for Mathematical Modelling of Anaerobic Wastewater Processes D.J. Batstone J. Keller I. Angelidaki.V. Kalyuzhyni.G. Pavlostathis A. Rozzi W.T.M. anders H. iegrist V.A. Vavilin

6 ANAEROBIC DIGETION MODEL N1 Composite Particulate Material 100% Disintegration Hydrolysis Carbohydrates 10% 30% Inerts 10% 30% 30% 30% Proteins 30% Lipids 30% 1% 29% M 31% AA 30% LCFA 29% Acidogenesis 13% 16% 2% HPr HBu HVa 29% 12% 20% 12% 6% 20% 9% Acetogenesis 9% Acetic 64% H 2 26% Methanogenesis CH 4 90% COD flux for a particulate composite comprised of 10% inerts and 30% each of proteins carbohydrates and lipids in terms of COD. Propionic acid 10% butyric acid 12% and valeric acid 7% are grouped in the figure for simplicity.

7 Time profiles of sorted houshold waste HW degradation D mg/l Propionate mg/l Biomass mg/l Ammonia nitrogen mg/l Biomass mg/l Acidogens Acetogens B Acetogens P Methanogens A Methanogens H Time days Acetate mg/l Butyrate mg/l ph Methane l Methane % 75% 50% 25% 0% Time days ymbols: data alminen & Rintala Curves: the <METHANE>model http: methane.da.ru Rapid increase in VFA concentration low ph efficiently inhhibits the polymer hydrolysis

8 The rates of polymer hydrolysis and methanogenesis as a product of several functions FT FL FI functions describing the temperature dependence mechanism of substrate limitation and inhibition; B biomass concentration of hydrolytic/acidogenic or methanogenic bacteria

9 UBTRATE LIMITATION First-order hydrolysis kinetic as the limiting case of surface- related kinetics Vavilin et al κbh X ρ BH = ρh kh X BH >> 1/ κ; X << KX kh = ρ 1+ κb K + X H X H / K X Monod model for methanogenesis ρ BM = ρ M B K + M

10 The Contois and first-order models lines and experimental data symbols of methane production from the grey waste for the triplicate data Jokela et al Methane production ml Grey Waste Time days First-order The surface - related kinetics as Contois well as the Contois kinetics of polymer hydrolysis fitted to experimental data better than the traditional firstorder kinetics

11 imulations of municipal food waste anaerobic digestion data: Cho et al with the various initial loadings: 1 2gV/l 2 4gV/l 3 10 gv/l 4 50 gv/l Methanogenesis was inhibited at 10gV/l 3; but at 50gV/l 4 both methanogenesis and hydrolysis were inhibited by high VFA concentration. No methane production was observed in this case

12 INHIBITION FUNCTION f m f K f + = 1 1 g m g K g + = 1 1 Dimensionless functions f and g characterise strong or weak inhibition of hydrolysis and methanogenesis by VFA ph where K f K g are the inhibition constants and m f m g are the degree indexes.

13 INHIBITION FUNCTION FOR MFW methanogenesis is inhibited by VFA much stronger than hydrolysis harp and smooth inhibition functions correspond to hydrolysis and methanogenesis respectively

14 The effects of mass transfer and acid inhibition are cruicial for anaerobic digestion AD of solid waste.. A balance between the rates of polymer hydrolysis and methanogenesis is extremely important Distributed mathematical models describing dynamics of biochemical processes in reactor space should be developed to analize the effects of mass transfer on the rate of anaerobic digestion

15 implified kinetic model of solid waste anaerobic digestion Product Inhibition ubstrate Inhibition Polymer W Hydrolysis & Acidogenesis VFA Biomass B CH 4 P Methanogenesis

16 patial Reaction Zones Hydrolysis/acidogenesis Methanogenic area expansion diffusion VFA advection CH 4 Initial Methanogenic Biomass VFA diffusion VFA advection

17 The conditions of a mass transfer-based acceleration of methane production in the reactor Θ when intensity of VFA utilization in methanogenic area is sufficient for the complete digestion of incoming acid flow J + R < Θ H X T dx R M X T Θ Θ dx where R H is the rate of VFA production by hydrolysis; R M is the rate of VFA consumption by methanogens; J T is the incoming VFA flow through boundaries of T due to diffusion and advection.

18 An increase of the initial hydrolysis rate and a decrease of the initial methanogenesis rate above and below the critical values respectively causes an inhibition first of methanogenesis and then of hydrolysis

19 BATCH REACTOR WITH LEACHATE RECIRCULATION Z vertical coordinate; L reactor depth; q volumetric liquid flow rate per unit surface area

20 1-D DITRIBUTED MODEL with saturated water flow = + = + + = + + = = 1 Z N u Z N N D Z T N K B g m Y A T P B d k K B g m Y Z B q Z B B D Z T B K B g m f W k Z q Z D Z T f W k T W υ υ α υ χ W solid waste; VFA; B biomass; N sodium; dp/dt methane production rate; T time; D D B D N diffusion coefficients of VFA biomass and sodium respectively

21 BOUNDARY CONDITION with ph adjustment 0 T Z = q D 0 T L T L T Z = 0 VFA B0 T = Z q α B0 T B L T D B B L T Z = 0 Biomass 23 1 N0 T = L T [ H ]/ K d N L T Z = 0 odium

22 Leachate recirculation and ph adjustment during MW degradation data: Barlaz et al waste biomass and VFA values averaged over the reactor volume D =1*10-4 L 2 /day D B =1*10-5 L 2 /day D N =1*10-4 L 2 /day q = 1.0 L/day ph=7.0 adjusted An increase in VFA concentration effectively inhibits methanogenesis as well as hydrolysis/acidogenesis

23 Time sequence of distributions of waste VFA biomass and sodium concentrations through reactor depth Z at the low leachate recirculation rate equence Days Waste degraded more rapidly at the top of the reactor than the bottom because leachate recirculation with ph adjustment prevents inhibition of methanogenesis and hydrolysis by non-ionized VFA mostly at the reactor top

24 Concentrations of waste VFA and biomass averaged over the reactor volume and methane production rate ph adjusted: Run1: 7.0 Run2: 5.0 Without ph adjustment or with ph adjusted to 5.0 a strong inhibition of methanogenesis takes place

25 The case without a and with b leachate recirculation and neutralization for old MW Jokela et al a b Leachate recirculation have no much effect on methane production because low VFA concentration did not inhibit methanogenesis significantly

26 1-D DITRIBUTED MODEL with unsaturated water flow W solid waste; VFA; B biomass; moisture; dp/dt methane production rate; T time; D diffusion coefficient of VFA; K D hydraulic conductivity and water diffusivity = + = + = + + = = / / 1 / / / / 2 2 κ ρ ρ ρ χ f W k Z K Z D Z T K B m g Y A T P B k d B K B m g Y T B B K B m g f W k Z K Z D T f W k T W

27 imulated landfill column leachate generation during MW degradation data: Pohland et al Water consumption during hydrolysis/acidogenesis was taken into account to describe the leachate volume drop

28 Time sequence of distributions of moisture waste VFA in leachate and methanogenic biomass concentrations through reactor depth Z equence Days Initially water passes through the column. At the same time a stronger inhibition occurs at the bottom of the reactor rather than the top

29 CONCLUION answers on some questions Waste composition and physical character of placed waste effect polymer hydrolysis rate; methanogenic biomass concentration is responsible for methanogenesis rate. A balance of these processes is extremely important to keep a non-acidified leachate On the basis of distributed models of solid waste anaerobic digestion including diffusion and advection of VFA an expansion of the initial methanogenic areas to the total reactor volume was obtained. However such a model requires a lot of parameter values to be determined Advanced decomposition modeling depends on improved understanding of the key processes. The generic model including mass-transfer processes should be developed

30 ACKNOWLEDGEMENT This work was supported by the Copernicus-2 Program ICA2-CT