kinetic study of the anaerobic digestion of the solid fraction of piggery slurries

$: Kinetic study of the anaerobic digestion of the solid fraction of piggery slurries$
Short Communication

Kinetic study of the anaerobic digestion of the solidfraction of piggery slurries

A. RodrõÂ guez Andara*, J.M. Lomas Esteban

Departamento de Ingenieria Quimica y del Medio Ambiente, Escuela de Ingenieria TeÂcnica Industrial, Universidad del PaõÂs Vasco,

C/ Nieves Cano 12, 01006, Vitoria, Spain

Received 16 February 1999; received in revised form 4 May 1999; accepted 2 June 1999

Abstract

This work deals with the determination of a kinetic model in an anaerobic digestion process. It is applied to theorganic stabilization of the solid fraction of piggery slurry. The substratum results after ®ltering the farm

wastewaters through a 1 mm mesh hydraulic sieve. Two pilot scale reactors were used, one digester was working inlayered way Ð without agitation Ð and other was mixed with a stirrer. The experiments were carried out in adiscontinuous process, 60 days hydraulic retention time, in the mesophilic interval (358C). Average in¯uent

concentration was 68 and 97 g lÿ1 COD respectively for the non-stirred and stirred reactor, whereas the achievedreduction was 61 and 65%. Organic loading rates were 0.80 and 1.45 K VS mÿ3 dÿ1 . Speci®c biogas output wasmore e�cient in the stirred reactor than in the strati®ed one. Several kinetic models were tried out in order to

represent the methane production. A ®rst order kinetic model applied in two stages was ®nally adapted for bothreactors. The ®rst stage presented the microbial growing as the limiting step, whereas the second stage was limitedby the substratum availability. The e�ect of mixing on the kinetic parameters was analyzed. Signi®cant di�erenceswere attained in the coe�cients, thus K was 0.048 and 0.75 dÿ1 respectively for the non-stirred and stirred

reactor. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Pig; Swine; Wastewater; Manure; Slurry; Anaerobic digestion; Kinetic model

1. Introduction

A previous separation of the liquid and solidfractions is a physical operation commonly usedto deal with the pig slurries. The main aim is tofacilitate their transportation through the diges-

ters, tanks or lagoons. Moreover, an eliminationof most of the suspended particles is required inseveral kinds of anaerobic reactors. These bio-logical systems usually operate under limitationsin total solids concentration, in order to processthe organic matter. As a consequence, the solidfraction is often scattered directly on the ground,without a proper treatment, which gives rise tocontamination. So, more improvements should

Biomass and Bioenergy 17 (1999) 435±443

0961-9534/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0961-9534(99 )00059-8

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* Corresponding author.

be done about organic stabilization of this sub-stratum before the ®nal disposal.

The standard separation of the solid fractionconsists of speci®c hydraulic sieves or centrifuges.Although the solids content related to the totale�uent volume is small (approximately 5%), dis-posal of this waste is a problem in intensivefarms. Studies on the degradation of this fractionof the slurry are scarce. Some research was madeby GoÂ recki et al. [1], with previous centrifugationof a piggery slurry before the anaerobic diges-tion. Several works related to the digestion of thesolid fraction were presented by Cechi et al. [2],Iniguez et al. [3], Pescod et al. [4], and Zadraziland Puniya et al. [5]. RodrõÂ guez [6] presented stu-dies on the e�ect of the particle size for pig slurryin an anaerobic process, often di�cult to bedegraded.

Anaerobic digestion is commonly applied inorganic stabilization of piggery manure.Hydrolysis and liquefaction of insoluble organiccomplexes are key steps in the process. Parkin

and Owen [7] proposed that this phase allows theorganic small size particles to go through the cellmembrane, supplying energy resources to thebacteria. The hydrolysis stage is considered thelimitating step of the anaerobic process for thissubstratum.

Besides temperature, hydraulic retention time,etc., reactor mixing is a main factor in the an-aerobic process. Mixing improves the e�ciencyof the reactor, avoiding strati®cation of the sub-stratum and temperature gradients. It also dis-perses the products of the bacteria metabolismand the eventual toxics in the residual liquor.

The objective of the present pilot plant study isto determine a reliable kinetic model for thedigestion of the mentioned substrata. The studyof the rate of reaction is fundamental in order togeneralize the results. Mixing has notable in¯u-ence in the determination of the kinetic modelsand the value of the constants. So, two similardigesters, stirred and non-stirred, are consideredhere. Moreover, the kinetic model of the anaero-

Fig. 1. Hydraulic 1 mm mesh ®lter for the solid fraction separation.

A. Rodrguez Andara, J.M. Lomas Esteban / Biomass and Bioenergy 17 (1999) 435±443436

bic process is a previous step to design a diges-tion plant at commercial scale, which is the ®nalaim of this project.

2. Materials and methods

2.1. Equipment

The experiments were accomplished in a pilotplant, placed at Artxa Farm estate, located inVillarreal (Basque Country, Spain). Averagefarming operations consisted of feeding about6000 animals, mostly swine and suckling piglets.Common volume of e�uents from the farm werearound 30 m3 dÿ1. The wastewaters were pouredin a discontinuous way to an adjacent 12000 m3

lagoon, where a spontaneous anaerobic digestionwas achieved, i.e., COD elimination was almost20%. Final dumping was through a pipe to anearby river stream, with an obvious harmfule�ect on the downstream waters.

A 3 m3 sewer provided with a sludge pumpand a stirrer was built in order to collect the slur-ries for the reactors from the farm sewage. It

provided a certain homogenization prior to thetreatment. The slurry was conditioned before theintroduction to the digester through a solidsseparator. The ®lter was designed for this kind ofresidue, built of stainless steel and operatedunder gravity. It consisted of a 1 mm meshinclined sieve, removing most of the suspendedsolids (Fig. 1).

The liquid fraction was treated in a Contactand DSFF anaerobic digesters, related by Lomaset al. [8], whereas the ``solid'' or concentratedfraction of the manure was the substratum forthe present experiment. Two pilot scale anaerobicreactors were designed and built for this particu-lar substratum. They were made in an ovoidshape, with di�erent volume and characteristics(Fig. 2). The net volume of the mixed digesterwas 245 l, and was provided with a helicoid stir-rer to mix the substratum. The net volume of thestrati®ed reactor was 565 l, and the substratumload was arranged in layers. Both digesters werebuilt of galvanized iron. Walls were thermallyisolated with 80 mm thick Vitro®lm ®bre glassand covered with a 0.8 mm aluminium sheet. Theinner surfaces were treated with Derekane 411 for

Fig. 2. Mixed (left) and strati®ed (right) digesters.

A. Rodrguez Andara, J.M. Lomas Esteban / Biomass and Bioenergy 17 (1999) 435±443 437

chemical protection. An internal heating systemwith a copper coil, by means of hot water ¯ow,was installed.

The long duration of the fermentation in dis-continuous culturing facilitated the register ofmany parameter data. The experiments were per-formed into the mesophilic range (around 358C),and the temperature was usually kept into theselected interval. The process was regulated ana-lyzing the characteristic physico-chemical par-ameters of in¯uents and e�uents. Automaticdevices for the operational control were installed,both at the plant and the farm sewage. Volumeof the biogas was monitored with an industrialgas meter, Sacofgas brand, mod. E2/160.

Composition was recorded with a gas chromato-graph mod. Shimadzu 6C-8A equipped with agraphic integrator model Chromatopac C-R6A.Electronic probes for the temperature and pHmeasurement with continuous recording wereused. An ultrasound ¯ow meter Ð Flowgefbrand Ð was installed to monitor load rate.Density, volatile acidity, total alkalinity, totaland soluble COD, total and volatile solids havealso been determined, using conventionalmethods [9].

3. Results

3.1. Substratum characterization

Table 1 displays the average values of thecharacteristic parameters of the substrata. Thedisparity of the in¯uent data for both reactorswas related with the di�erent periods of exper-imentation. Some characteristic parameters pre-sented great alterations, i.e., total COD, totalsolids or volatile solids. Moreover, the acid±basefeatures of the in¯uents displayed scatteredvalues, a�ecting the digestion. This diversity inload characteristics led to a knowledge of thedigester e�ciency in actual feeding conditions,quite di�erent from the previous experiences atthe laboratory, when a uniform substratum wasutilized [6].

Table 1

Substratum characterization of the reactors

Parameter Stirred reactor Strati®ed reactor

pH 7.54 7.86

Temperature (8C) 35 35

Density (kg/l) 1.005 1.003

Total solids (g/l) 110.0 64.2

Volatile solids (g/l) 87 48.1

% Volatile solids 79.1 75.5

Total CDO (g/l) 97 68

Alkalinity (g/l, CaCO3) 10.3 15

Volatile fatty acids (g/l. HAc) 8.16 9.90

Total nitrogen (g/l, N) 4.37 4.92

Ammoniacal nitrogen (g/l, N) 2.49 3.22

Total phosphorus (g/l, P) 0.30 0.27

Fig. 3. Biogas production vs t. Mixed and strati®ed reactors.


Considering the nutrients factor in the afore-mentioned characterization, the phosphorus:ni-trogen coe�cients were around 1:15 and 1:18 forthe stirred and strati®ed reactors respectively.Results from literature are between 1:5 and 1:7,in order to supply adequate nutrients for themicroorganisms, since Lema et al. [10]. The ratiosindicated that the e�uent left the plant contain-ing a high content of nitrogen. In connectionwith the coe�cients COD/N/P, related to the cel-lular output of the bacteria, they were about 323/15/1 and 251/18/1 for the stirred and strati®edreactors respectively. According to the sameauthors, these relations vary widely with the kindof substratum: For a sour in¯uent, capable onlyto carry out the methanegenic step, relations arearound 2660/7/1. Meanwhile, proportions areabout 443/7/1 for a complex substratum, whichis closer to the present case.

3.2. Biogas production and composition

The hydraulic retention time was kept around60 days. Biogas ¯ow was measured daily duringa period of working stability (Fig. 3). Greatdi�erences were found comparing biogas yield inboth reactors. So, production increased notablyafter one week in the stirred digester. The lowoutput during the starting period can be attribu-ted to the adaptation of the biomass to the reac-tor conditions. Afterwards gas production rate

increased, reaching a maximum at around 30days, and from then on clearly decreased. On theother hand, the biogas output in the strati®eddigester was smaller and more uniform along theexperiment. These results can be partiallyassigned to the mixing, because the substrataconditions were di�erent, although the nutrientsremoval led to a more e�cient process.

With regard to the biogas composition, onlysigni®cant concentration of three componentswas found in the analysis: methane, carbon diox-ide and nitrogen. Nevertheless little hydrogenand sulphur were detected, which are usualgasses from the fermentation of this substance.Gas composition was quite similar in both reac-tors. Methane concentration rose after 10 days,stabilized between 45 and 50% around 40 days,and decreased later. These values were smallercompared to the literature references, but similarto some previous experiences for this substratum,as Lapp et al. [11] and Montalvo [12]. On theother hand, the high nitrogen concentration inthe beginning was attributed to the initial air intothe reactors.

4. Kinetic study

4.1. Approach

Several common kinetic models were tried to

Fig. 4. Determination of B0. Mixed and strati®ed reactors.


adapt to the global anaerobic process in discon-tinuous load, i.e., Monod or Chen andHashimoto [13]. Nevertheless, the constantsobtained from those models were not reliable,because those systems didn't ®t the process ofthis particular substratum. The results from the®rst order model were acceptable in some cases,whereas di�cult to be adapted in other stages ofthe production. This led to divide the range ofthe experiments in two stages, which wereadjusted to simple empirical models, with a mini-mum amount of parameters. In consequence, atwo-stage model was tried. The ®rst model wasapplied from the early methane production untilreaching the maximum rate. The second modelwas applied along the period from that pointuntil the ending of the fermentation.

Evaluation of the constant B0, speci®c methaneproduction at in®nite hydraulic retention time,was required to evaluate both phases, due to thedi�culty to know the temporal variation of S.Fig. 4 displays B (m3 CH4/kg VS0.) vs the HRTinverse. B trends to B0 when HRT trends to in®-nite, which is equivalent to a batch feeding sys-tem.

Ordinate B0 values resulted as follows:

Stirred reactor 0:165 m3 CH4=kg VS0

Stratified reactor 0:176 m3 CH4=kg VS0

4.2. Two stages model

4.2.1. Development of the model for the ®rst stageThe organic matter, coming from the solid

substratum, is usually easily to digest. Then, thehypothesis for the ®rst stage model considers thatthe limitation factor for the gas production is themicrobial growing rate, because of the low initialconcentration of the microorganisms. The endo-genic respiration is negligible relative to thegrowing factor, due to excessive nutritional con-ditions. Thus, the variation of the cellular massvs time in a discontinuous reactor can beexpressed as follows:

dX=dt � mX �1�

in which X= microorganisms concentration;m=speci®c cellular growing rate.

Integrating Eq. (1), the next expression isachieved for the microorganisms concentration:

X � X0 exp �mt� �2�The relation of substratum transformation intocellular mass is de®ned by the output equation:

dX=dt � ÿY�dS=dt� �3�in which S= soluble substratum concentration;Y = cellular output constant.

Combining Eq. (1), (2) and (3), and integrat-ing, the result is:

S0 ÿ S � ÿ�X0=Y �� exp�mt� ÿ 1� �4�where S0=initial substratum concentration;X0=initial microorganisms concentration.

Other reference used in this model is themethane to substratum equation of proportionfrom Chen and Hashimoto [13).

B0 ÿ B=B0 � S=S0 �5�Replacing Eq. (5) in Eq. (4), the next expressionis attained:

B=B0 � �X0=Y S0��exp�mt� ÿ 1� �6�in which B and B0 are already de®ned.

According to other studies quoted by Luengo[14], the value of m for similar waste is about0.4±0.8 daysÿ1 or shorter. When methane pro-duction is high, the 1 term is negligible in Eq.(6). In consequence, the simpli®ed equation is:

B=B0 � �X0=Y S0� exp�mt� �7�expression easy to linear conversion vs time, withthe result:

ln�B=B0� � ln�X0=Y S0� � mt �8�The speci®c cellular growing rate can be calcu-lated from the slope value with a linear corre-lation of ln (B/B0) vs t. Once X0 and S0 areknown, the intersection of the straight line withthe vertical axis leads to the value of the outputconstant.


4.2.2. Development of the model for the secondstage

A ®rst order kinetic model was applied for theadjustment of the second stage, which yields thegreater amount of biogas. The next equation isthe expression of the model applied to the sub-stratum S:

dS=dt � ÿkS �9�

in which k = kinetic coe�cient (dÿ1).Using the Eq. (5), and de®ning B ' variable as:

B 0 � �B0 ÿ B �=B0 �10�

the next expression is achieved:

B 0 � S=S0 �11�

Dividing Eq. (9) by S0, it results:

�1=S0�dS=dt � ÿk S=S0 �12�

Combining Eq. (11) and (12), it leads to:

dB 0=dt � ÿk B 0 �13�

Separating variables and integrating the formerequation in which B '=1, if B = 0, according toEq. (10) when t = 0, then:

ln B 0 �between B 0 and 1� � ÿk

t�between t and 0��14�

Thus:

ln B 0 � ÿk t �15�

Exponentialling the two terms in Eq. (15)and undoing the change of variable in Eq. (11),the expression of the considered model isattained:

�B0 ÿ B �=B0 � exp�ÿk t� �16�

from which

ln��B0 ÿ B �=B0� � ÿk t �17�

The Eq. (17) is adjusted to the B results in thepresented experiments. Once known B0, theequations of straight lines are accomplished inthe linearization of ln (B/B0) vs t, as in the ®rst

Table 2

Kinetic constants for the two stages

First stage

m (dayÿ1) Y Regression coe�cient

Strati®ed reactor 0.0903 0.4227 0.957

Mixed reactor 0.1712 0.5835 0.981

Second stage

K (dayÿ1) Regression coe�cient

Strati®ed reactor 0.0479 0.961

Mixed reactor 0.0745 0.992

Fig. 5. Adjustment of the two stages model. Strati®ed reactor.


stage. The value of K coe�cient is calculatedfrom the slope of the linear equation.

4.3. Calculation of the kinetic constants

Table 2 displays the values of the kinetic coe�-cients calculated for each stage. The speci®cgrowing rate (m ) determines the cellular outputor the microbial mass generation rate. Referringto Y constant, substratum a�nity by the mi-crobial mass, the attained value is lower than thementioned data by Luengo [14] and Martinez[15]. This result can be due to the diversity of theslurry characteristics. As a consequence, anappropriate selection of HRT for the anaerobicdigestion is fundamental in this system. The reac-tor is more in¯uenced by the accumulation ofacids when there is a low a�nity for the substra-tum, due to overloads. These results lead tosmall sludge yield compared to other related an-aerobic processes.

4.4. Adjustment of the two stages model

Once attained, the kinetic model was adjustedto the reactor data obtained. Figs. 5 and 6 dis-play the experimental values and the adjustmentof B, according to Eq. (8) and (17). As a result, acoincidence of both series of values for the diges-ters was found.

Concord can be evaluated for the stirred reac-tor as follows:

Xt�60t�0�BExperimental ÿ BAdjusted�2 � 0:000335

whereas for the strati®ed reactor results:

Xt�60t�0�BExperimental ÿ BAdjusted�2 � 0:001692

Once achieved the results for this experiment, thetwo stages model can be accepted with fairadjustment to the data.

5. Conclusions

The speci®c production of methane in the an-aerobic digestion of the solid fraction of screenedpiggery waste can be adjusted dividing the pro-cess in two stages. The separation of those stagesis considered at the time of maximum speci®cmethane production rate. Then a ®rst order kin-etic model is applied in each period. The mi-crobial growing is the limiting factor in the ®rststage, whereas the nutrient availability is the lim-iting factor in the second stage. This two stagemodel suitably ®ts the presented experimentalresults.

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Fig. 6. Adjustment of the two stages model. Stirred reactor.


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kinetic study of the anaerobic digestion of the solid fraction of piggery slurries

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