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 DOI: 10.1177/0734242X12455088 2013 31: 353 originally published online 31 July 2012Waste Manag Res

Andrea Hublin and Bruno ZelicModelling of the whey and cow manure co-digestion process

  

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Introduction

Co-digestion is defined as the anaerobic treatment of a mixture of at least two different waste types with the aim of improving the efficiency of the anaerobic digestion process. Therefore, it is very important to establish the best blend in order to maximize methane production, avoid inhibitory effects and make biogas plants profit-able (Álvarez et al., 2010; Chen et al., 2008; Comino et al., 2010).

The dairy industry generates residues of which whey is the most important wastewater produced with an extremely high organic content (Gannoun et al., 2008; Mockaitis et al., 2006). The strong polluting potential of dairy wastewater is character-ized by high biological oxygen demand (BOD) and chemical oxygen demand (COD) (Demirel et al., 2005). The principal components of whey are lactose, proteins and mineral salts (Vasala et al., 2005). Although whey has a very high biodegrada-bility (close to 99%), it constitutes a difficult substrate to treat due to its high organic content and its low bicarbonate alkalinity (Galegenis et al., 2007).

Livestock manure is a complex substrate containing un- dissolved and dissolved organic matter such as polysaccharides, lipids, proteins and volatile fatty acids (VFA) as well as a high number of inorganic compounds of importance for the environ-ment (Garcia-Ochoa et al., 1999). Dairy manure is one of the most polluting agro-industrial wastewaters. Intensive dairy farm-ing produces large amounts of manure that, when not properly managed due to its high organic content, nitrogen and phospho-rous concentrations can cause numerous environmental prob-lems. Dairy manure, which contains too many suspended solids,

presents low anaerobic biodegradability (Rico et al., 2011). The co-digestion of whey as an easily degradable substrate with dairy manure which presents low anaerobic biodegradable substrate increases specific methane yields when compared to manure-only digestion (Labatut et al., 2011; Ogejo and Li, 2010). The potential of methane production from whey and cow manure co-digestion is achieved by decomposition of organic component. Whey contains easy degradable carbohydrates, whereas cow manure is comprised of lipids which are essential for the forma-tion of long chain fatty acids.

Biogas as a renewable energy source plays an important role in reducing greenhouse gases (GHG) because it is a ‘carbon neu-tral’ fuel (Kothari et al., 2010; Ward et al., 2008). Along with mitigating methane emissions, anaerobic digestion of animal manure has the potential to provide benefits such as pollution control, odour and pathogen level reduction, nutrient recovery and energy production (Amon et al., 2007; Hartmann and Ahring, 2005; Karim et al., 2005; Rico et al., 2011). At the same time, anaerobic digestion of whey offers an excellent solution in terms

Modelling of the whey and cow manure co-digestion process

Andrea Hublin1 and Bruno Zelic2

AbstractThe production of renewable energy, a reduction of waste and prevention of environmental pollution promote the industrial application of anaerobic co-digestion for the treatment of agro-industrial organic waste. In this paper production of biogas/methane was studied by performing a series of laboratory batch experiments using whey and cow manure as substrates. The influence of substrate concentration, temperature and pH on biogas production was analysed. A mathematical model has been developed that describes the co-digestion process. The hydrolysis of proteins, lipids and cellulose has been modelled using first-order kinetics. Fermentation of sugars and amino acids, anaerobic oxidation of long chain fatty acids (LCFA), acetogenesis and methanogenesis have been described using an unstructured model based on Monod kinetic equations taking into account different inhibitory effects. Model applicability was demonstrated by comparing experimental results with the model simulation results.

KeywordsCo-digestion, whey, cow manure, batch reactor, modelling

1 EKONERG – Energy and Environmental Protection Institute, Ltd, Zagreb, Croatia

2 University of Zagreb, Faculty of Chemical Engineering and Technology, Zagreb, Croatia

Corresponding author:Bruno Zelic , University of Zagreb, Faculty of Chemical Engineering and Technology, Marulicev trg 19, HR-10000 Zagreb, Croatia. Email: bzelic@fkit.hr

455088WMR31410.1177/0734242X12455088Waste Management & ResearchHublin and Zelić2013

Original Article

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354 Waste Management & Research 31(4)

of both energy saving and pollution control (Ergüder et al., 2001; Saddoud et al., 2007). The advantages of this process are the profitability of the plant and the convenience in realizing an anaerobic digestion plant to produce biogas which becomes fea-sible when installation costs are amortized within a period of 4–7 years. This is possible if one has benefited from Green Certificates and the sale of electrical energy at favourable prices (Tricase and Lombardi, 2009).

The kinetic modelling of the anaerobic degradation of com-plex wastes is needed for a better understanding of the perfor-mance of the system. It is essential for the rational design and operation of biological waste-treatment systems to predict the system stability, effluent quality and waste stabilization (Angelidaki et al., 1993). Anaerobic degradation of complex organic material has been described as a sequential process that involves the steps of hydrolysis, acidogenesis, acetogenesis and methanogenesis. During the anaerobic digestion of complex organic matter the hydrolysis is the first and often rate-limiting step (Angelidaki and Sanders, 2004). However, over a wide range of operating conditions, the limiting step is not always the same. It may depend on wastewater characteristics, hydraulic loading, temperature, etc. (Speece, 1983). For efficient methane production it is important to have a balance between the reaction rates of the different steps involved in the anaerobic digestion of complex organic material. Although the hydrolysis of particulate organic material has been considered the rate-limiting step in anaerobic digestion, acetogenesis or methanogenesis might be the rate-limiting stages in complex waste (Vavilin et al., 2008).

The ‘limiting step hypothesis’ leads to simple and readily usable models. Such models, however, do not describe the digester behaviour very well, especially under transient operating conditions (Lyberatos and Skiadas, 1999). On the other hand, the IWA Anaerobic Digestion Model No1 (ADM1) includes multiple steps describing biochemical as well as physico-chemical pro-cesses. The biochemical steps include disintegration from homo-geneous particulates to carbohydrates, proteins and lipids; extracellular hydrolysis of these particulate substrates to sugars, amino acids and LCFA, respectively; acitogenesis from sugars and amino acids to VFA and hydrogen; acetogenesis of LCFA and VFA to acetic acid; and separate methanogenesis steps from acetic acid and hydrogen/CO2. The physico-chemical equations describe ion association and dissociation, and gas-liquid transfer (Batstone et al., 2002). The structured ADM1 model was tested to simulate the behaviour of a bioreactor for the anaerobic co-digestion of waste activated sludge and biowaste. ADM1 showed acceptable simulating results and is considered a powerful tool for predicting the behaviour of anaerobic digesters (Derbal et al., 2009; Parker, 2005).

The aim of this study was the development of a kinetic model for anaerobic co-digestion of whey and cow manure, able to describe methane generation in the short (several days) and long (several weeks) term period. For this aim a model based on the work of Galegenis et al. (2007) was applied to simulate methane generation. The model takes into consideration the specific

composition of the feed, as distinguished to proteins, lipids and carbohydrates. Model applicability is reflected in prediction of methane generation for different substrate concentrations.

Materials and methodsWhey and cow manure

The whey and cow manure used in this study were obtained from the Milk Plant and the Cow Farm located near the city of Osijek in Eastern Croatia. The fresh whey and dairy manure were stored at 4 °C to avoid a chemical composition modification.

Experimental set-up

Experimental studies were performed in 1 dm3 glass batch anaer-obic reactors at set temperatures of 35 or 55 °C. Agitation was provided by an agitator (0.3 Hz). The reactors were fed from the upper part. Air immersion into the reactor was avoided by block-ing the feeding upper part. Gas production was measured daily using a liquid displacement device. Experiments were performed using a series of batch reactors; the numbers depended on the duration of the co-digestion process conducted. The substrate samples were taken at the beginning of the experiment from each reactor in the series. Three reactors in each series had the same conditions, each day the process was stopped in all three reactors with the same conditions and samples were taken from these reactors. The corresponding reaction mixture from each reactor was analysed.

The experimental procedure

Experiments were performed in order to investigate the best co-digestion process conditions for biogas production. The total vol-ume of the reaction mixture (whey and cow manure) in all experiments was 0.5 dm3. Experiments were performed in tripli-cate and in the 95% confidence range the results showed no sig-nificant differences. Standard deviation and confidence intervals were calculated for each experimental point.

The influence of whey ratio on specific biogas production in a mixture with cow manure was analysed at 35 and 55 °C, for dif-ferent initial pH values and for different concentrations of sup-plemental bicarbonate (NaHCO3) in experiments carried out over 12 days.

According to the results of the experiments with different ini-tial whey content (5, 10 and 15 v/v, pH 3.5), additional experi-ments were performed with addition of alkalinity to the mixtures with different initial content of whey and cow manure at 35 and 55 °C. The best fitting results were achieved for an initial whey content of 10% v/v. The following experiments with alkalinity addition at 35 and 55 °C were performed with the initial whey content of 10% v/v. The initial pH value of whey in these experi-ments was adjusted within the range 6.5 to 7.5 by adding NaHCO3. Furthermore, the effect of NaHCO3 addition on the batch co-digestion process was investigated for different initial

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Hublin and Zelic 355

concentrations of NaHCO3 (5, 10 and 15 g dm−3). The pH, total solids (TS), removal efficiencies of COD, biogas productivity and biogas compositions were measured daily.

In order to validate the optimized conditions for co-digestion of whey and cow manure in the one-stage batch process, the experiments were performed within 45 days, at 55 °C, with 10% v/v of whey in the initial mixture and 5 g dm−3 NaHCO3 was added as a supplemental bicarbonate. The pH, TS, volatile solids (VS), organic carbon, total nitrogen, biogas productivity and biogas compositions were measured daily. The removal effi-ciency of COD was measured at specific intervals during the 45-day period.

Analytical methods

The pH measurements were performed by a pH digital electrode (Methrom). TS were determined by drying at 105 °C to constant weight. TS samples were ignited to constant weight at 550 °C for VS determination. Oxidation in the potassium dichromate diges-tion solution was used for determination of organic carbon. The Kjeldahl method was used for determination of total nitrogen. COD determination was based on potassium dichromate as an oxidizing agent (under acidic conditions by addition of sulfuric acid) and measuring of absorbance at 605 nm on a HACH DR/2000 spectrophotometer for colorimetric measurements (APHA, 1998; ISO 14235, 1998). Biogas composition (methane, CH4; nitrogen, N2 and carbon dioxide, CO2) was determined by gas chromatography (Varian GC 3900) using 10 m of the capil-lary column CP-PoraPLOT Q and helium as the carrier gas.

Kinetic modelling

The kinetic model based on the work of Galegenis et al. (2007) was applied to simulate the methane generation. To model the system, the reaction scheme, rate equations and kinetic constants for anaerobic co-digestion of whey and cow manure in the one-stage batch processes were chosen from published models on anaerobic digestion (Costello et al., 1991; Gujer and Zehnder, 1983; Kalyuzhnyi and Fedorovich, 1998; O’Rourke, 1968). Aspects from each published model were included (Ristow and Hansford, 2001).

The model takes into consideration the specific composition of the feed, as allocated to proteins, lipids and carbohydrates. The model regards six biological processes namely hydrolysis (of proteins, lipids and cellulose), fermentation (of sugars and amino acids), anaerobic oxidation (of LCFA), acetogenesis, acetoclastic methanogenesis and hydrogenotrophic methanogenesis. The reaction scheme proposed by Gujer and Zehnder (1983) was cho-sen for description of the anaerobic digestion of whey and cow manure, from hydrolysis to methane production.

Hydrolysis reactions were assumed to follow first-order kinet-ics (Equations (1) to (3)), whereas the Monod equation was applied to all other reactions (Equations (4) to (8)). A competitive acetic acid inhibition term was included for the fermentation reaction (Equation (4)), while a non-competitive acetic acid

inhibition term was included for the acetogenic reaction (Equation (6)) (Costello et al., 1991). To determine the growth of the vari-ous groups of bacteria, the model proposed by Kalyuzhnyi and Fedorovich (1998) was used as a basis, since it was developed for

the degradation of a mixture of propionic acid and acetic acid. Arrhenius-type temperature dependence was considered at the optimization of the model parameters. The kinetic models (rate equations) for key components of the reaction system are pre-sented in Equations (1) to (8).

r k cC H CC= ⋅

r k cP H PP= ⋅

r k cL H LL= ⋅

µ µS MAXS AA

S A iA

SS

S

= ⋅⋅

⋅ +( ) +c c

K c K c1 /

µ µLCFA MAXLCFA

S LCFALCFA

LCFA

= ⋅+

c

K c

µ µPP MAXPP

S A iA

PPPP

PP

= ⋅⋅ +( ) +

c

K c K c1 /

µ µA MAXA

S AA

A

= ⋅+

c

K c

µ µH MAXH

S HH

H

2 2

2

2 2

= ⋅+

c

K c

In order to describe reactor concept and macro-kinetic behaviour in the reactor a mass balance over the reactor was conducted. Mass balances for key components of the reaction system are presented in Equations (9) to (18).

d

dC

C

c

tr= −

d

dS

C S S

c

tr c= − ⋅µ

d

dP

P

c

tr= −

d

dAA

P AA AA

c

tr c= − ⋅µ

d

dL

L

c

tr= -

d

dLCFA

L LCFA LCFA

c

tr c= − ⋅µ

d

dPP

S S S PP PP

c

tc Y c= ⋅ ⋅ − ⋅µ µ

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

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356 Waste Management & Research 31(4)

d

dA

S S S LCFA LCFA LCFA

PP PP PP A A

c

tc Y c Y

c Y c

= ⋅ ⋅ + ⋅ ⋅

+ ⋅ ⋅ − ⋅

µ µ

µ µ

d

dH

S S S LCFA LCFA LCFA

PP PP PP H H

c

tc Y c Y

c Y c

2

2 2

= ⋅ ⋅ + ⋅ ⋅

+ ⋅ ⋅ − ⋅

µ µ

µ µ

d

dCH

A A A H H H

c

tc Y c Y4

2 2 2= ⋅ ⋅ + ⋅ ⋅µ µ

where the subscripts used are C for cellulose, P for proteins, L for lipids, S for sugars, AA for amino acids, LCFA for long chain fatty acids, PP for propionic acid, A for acetic acid, H2 for hydro-gen, and CH4 for methane; KH is the hydrolysis constant, KS is the saturation concentration (half-velocity concentration), Ki is the inhibition constant, µMAX is the maximum reaction velocity, and Y is the biomass yield coefficient.

A software package SCIENTIST (Micromath®) was used to inte-grate the rate equations and mass balances to simulate the process.

Results and discussionOptimization of conditions for co-digestion of whey and cow manure

The chemical composition of whey and cow manure (Table 1) indicates that these substrates may be suitable for co-digestion although there are slight differences from literature data. The whey showed a lower pH value and TS than the values which were reported by Kavacik and Topaloglu (2010) and Venetsaneas et al. (2009). Comparable values for pH and TS of cow manure were obtained in relation to the study of Kaparaju et al. (2008) and Tang et al. (2008). The C/N ratio of whey was higher whereas the C/N ratio of cow manure was lower than the values which were reported by Kavacik and Topaloglu (2010). The COD val-ues of whey and cow manure were lower than the values reported by Venetsaneas et al. (2009) and Comino et al. (2010).

For all of the fermentation conditions studied, the specific volume of biogas produced, V′ and average biogas production rate, r (Table 2) showed dependence on temperature and pH. Maximal specific biogas volume (V′, dm3 dm−3) produced during the 12 days and the average biogas production rate (r, dm3 dm−3 day−1) were achieved in the thermophilic experiments with 10% v/v of whey in the initial mixture where the initial pH value of whey was adjusted to 7.5 by the addition of 3.5 g dm−3 NaHCO3 (Table 2). Comparable results of biogas productivity were obtained in relation to the study of Venetsaneas et al. (2009).

Maximal CH4 content in a biogas mixture (72.0%) was achieved in experiments performed at 55°C with 10% v/v of whey where the initial pH value of whey was adjusted to 7.5. The initial pH of the whey and cow manure mixture was 6.8 and it showed a slight increase to 7.4 after 12 days in all of the experiments with different whey contents at mesophilic conditions. In the

thermophilic experiments only the lowest whey content (5% v/v) provided optimal pH conditions for methanogenesis (pH 7.1) after 12 days, whereas the higher whey content (10 and 15% v/v) affected the decline of pH below 5.8, which was too low for biogas production.

Investigation of the influence of different bicarbonate initial con-centrations (5, 10 and 15 g dm−3 NaHCO3) on biogas production in thermophilic experiments with 10% v/v of whey in the initial mix-ture showed that supplemental alkalinity enhances the production of biogas and methane content in a biogas mixture. The maximum spe-cific biogas volume produced during the 12 days were achieved in experiments with the addition of 5 and 10 g dm−3 NaHCO3 where pH values ranged between 7.1 and 8.1, which is in line with the opti-mal process conditions and in accordance with literature data (Göblös et al., 2008; Ward et al., 2008; Yadvika et al., 2004).

Maximal CH4 content in a biogas mixture was achieved in the experiment with 5 g dm−3 NaHCO3 during the whole fermenta-tion process (from 0.1% at the beginning of the co-digestion pro-cess to nearly 80% after 12 days). These results were comparable to the results of other process conditions, which showed lower CH4 content during the whole fermentation process.

Optimal conditions for co-digestion of whey and cow manure were determined to be at a temperature of 55 °C, 10% v/v of whey in the initial mixture and 5 g dm−3 NaHCO3 as a supple-mental bicarbonate. Although CH4 is the most significant con-stituent of biogas regarding energy capacity, one could conclude that defined conditions are preferable for efficient co-digestion of whey and cow manure.

In order to validate the optimized conditions for co-digestion of whey and cow manure in the one-stage batch process, the experi-ments were performed within 45 days. The specific biogas produc-tion increased from 0.10 to 20.7 dm3 dm−3 (Figure 1(a)). Maximal biogas production of 21.8 dm3 dm−3 occurred on the 44th day. Maximal CH4 content (78.7%) in a biogas mixture was achieved on the 19th day. Good CH4 content in a biogas mixture was realized from the 10th to the 45th day (from 67.3 to 71.8%) (Figure 1(b)).

As can be seen from Figure 1(b) the increase in CH4 content dur-ing the co-digestion process influences the decrease of N2 as well as CO2 content in a biogas mixture. At the beginning of the co-digestion process, when the CH4 content in a biogas mixture were negligible, the N2 content were significant, due to the high nitrogen content in

Table 1. The composition of whey and cow manure.

Whey Cow manure

pH 3.53 7.11Total solids, TS (%) 4.69 6.4Volatile solids, VS (%) 90.85 85.34Organic carbon (%) 40.95 42.76Nitrogen (%) 4.68 5.61C/N (g g−1) 8.74 7.62Chemical oxygen demand, COD (mg dm−3)

47 950 43 900

The percentages of volatile solids, organic carbon and nitrogen are rates in solid matter.

(18)

(17)

(16)

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Hublin and Zelic 357

cow manure. The effectiveness of the co-digestion process is usually represented in terms of the biogas production rate. Comparable results for biogas production were obtained in relation to the study of Kavacik and Topaloglu (2010) in experiments which were performed in a specially designed anaerobic reactor at different hydraulic reten-tion times, total solid matter and temperatures, in which the biogas production rate from co-digestion of cheese whey with manure at a hydraulic retention time of 10 days at 34 °C and 8% total solids mat-ter was 1.21 dm3 dm−3 day−1. The average biogas production rate in this study at the 15th day amounted to 1.01 dm3 dm−3 day−1.

Model validation

Hydrolysis reactions were assumed to follow first order kinetics, while the Monod equation, taking into account different inhibi-tory effects, was applied to all other reactions.

The model takes into consideration the specific composi-tion of the feed, as distinguished to proteins, lipids and car-bohydrates. The CH4 potential of manure and whey comes from the digestion of the organic components, which are mainly carbohydrates, proteins and lipids. According to the literature, manure has a higher content of lipids than whey, whereas whey has a higher content of easily biodegradable carbohydrates (Amon et al., 2007; Biswas et al., 2007; Galegenis et al., 2007). Under these assumptions, the input values for carbohydrates, proteins and lipids concentrations of the whey/cow manure mixtures were included in the

model. Input values for the mixture containing 10% v/v of whey and 90% v/v of cow manure were taken from Labatut et al. (2011).

Figure 2 shows a comparison between the measured and predicted methane profiles in the batch reactors. The model predicts that methane is generated as a result of the simultane-ous performance of all phases of anaerobic digestion. A corre-lation between the experimental data and the model prediction results could be observed in the methanogenesis phase. However, in the case of a slightly delayed response of the methane generation during the hydrolysis phase, which may be justified by the time required for adaptation of the micro-organism (not included in the model), the model over-predicts methane generation.

With regard to the shapes of the curves obtained from co-digestion experiments, an extended lag phase was observed dur-ing the hydrolysis phase. This result may indicate unfavourable conditions for anaerobic microflora. Different environmental conditions can lead to different populations of organisms or dif-ferent metabolic conditions of the micro-organisms. However, once the organisms had acclimatized, the digestion proceeded at a high rate (Cuetos et al., 2011; Lopez and Borzacconi, 2010). Moreover, cow manure and whey originate from a wide range of operations, involving different animal breeds, ages, diets, as well as management practices, which may have resulted in inhibitory effects such as pH inhibition, ammonia toxicity, high volatile acid concentration, among others.

Table 2. Specific biogas volume produced during 12 days and the average biogas production rate for different process conditions.

Process conditions mesophilic (35°C) thermophilic (55°C)

V′(dm3 dm−3) r (dm3 dm−3 day−1) V′(dm3 dm−3) r (dm3 dm−3 day−1)

0% whey 6.49 0.54 3.11 0.265% whey, initial pH 3.5 4.88 0.41 1.93 0.1610% whey, initial pH 3.5 2.50 0.21 1.82 0.1515% whey, initial pH 3.5 1.16 0.10 1.08 0.0910% whey, initial pH 6.5 2.12 0.18 2.15 0.1810% whey, initial pH 7.5 6.40 0.53 12.08 1.01

0

5

10

15

20

25

V'

(dm

³ dm

¯³)

(a)

0

20

40

60

80

100

Bio

gas

com

pos

ition

(%)

(b)

t (days)

453525151050 20 30 40

t (days)

453525151050 20 30 40

Figure 1. Biogas production and composition during 45 days of co-digestion (■ specific volume of biogas produced; • CH4, %; Δ CO2, %; ♦ N2, %).

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358 Waste Management & Research 31(4)

Model-based prediction of optimal initial whey/manure ratio

The biodegradability characteristics of substrates and produc-tion of inhibitory intermediate products will mainly control the kinetics of the different steps of anaerobic digestion (Labatut et al., 2011). As previously commented, cow manure is a slowly-degradable substrate due to its composition, which consists of approximately 60% lignocellulose, whereas whey is mostly composed of easily-degradable sugars. Figure 3 depicts the simulated cumulative methane production of the substrate containing a mixture of whey and cow manure (10/90 v/v), native whey and native cow manure. The steep methane production pattern of the substrate containing native whey indicates a high degradation rate with regard to the lag phase which was observed in another two substrates used for simulation experiments.

Although the model does not include all components of the complex system, it can be used to predict methane generation for different substrate concentrations in both short (several days) and long (several weeks) term periods. Even though the increase of whey in a mixture indicates an increase of carbohy-drates as well as proteins, the methane generation does not fol-low this trend. The simulated data of the 45-days co-digestion process for different whey/cow manure ratios (10/90 v/v; 25/75 v/v; 45/55 v/v, respectively) did not differ significantly (15.36; 14.98; 14.52 dm3 kg−1, respectively). This may be due to the fact that the model includes different inhibitory effects. In practice, larger amounts of carbohydrates favour the growth of acidogenic bacteria (Frigon et al., 2009), which in turn pro-duces VFA. The presence of these acids could inhibit the growth of methanogenic bacteria causing subsequent lower methane generation. An increase of the initial protein concen-tration could cause a decrease of methane concentration prob-ably due to competitive formation of ammonia through the acetogenic step (Biswas et al., 2007). As ammonia is known to inhibit the aceticlastic step, the formation of methane could be decreased.

Conclusion

This study shows that a combined treatment of whey and cow manure offers the possibility of treating this kind of waste more efficiently, because of the co-digestion of whey as an easy degra-dable substrate with dairy manure which presents low anaerobic biodegradable substrate that increases specific methane yields when compared with manure-only digestion.

Optimal conditions for maximal biogas production and meth-ane content in a biogas mixture in a one-stage batch process are 55 °C, 10% v/v of whey in the initial mixture, with the addition of 5 g dm−3 NaHCO3 for alkalinity control.

A mathematical model has been developed that describes the co-digestion process in the short (several days) and long (several weeks) term period. The model predicts that methane is generated as a result of the simultaneous performance of all phases of anaero-bic digestion. A correlation was found between experimental data and model prediction results for the methanogenesis phase, whereas methane generation was over-predicted during the hydrol-ysis phase. Model applicability is reflected in the prediction of methane generation for different initial substrate concentrations.

It is planned that this laboratory-scale study of a whey and cow manure co-digestion process will be verified by a pilot-scale study which will be conducted under defined optimal conditions for maximal biogas production. This technology can solve two complex problems. On one side is the methane production, on the other it allows efficient treatment of waste materials. By applying the results of the pilot plant to the full scale production cycle it will be possible to obtain an efficient conversion from biodegrad-able organic waste to electricity production.

AcknowledgmentThe authors gratefully acknowledge the financial support from EKONERG - Energy and Environmental Protection Institute, Ltd.

FundingThis study was supported by EKONERG - Energy and Environmental Protection Institute, Ltd. [Project No: I-14-0090/10].

0

5

10

15

20

25

V'

(dm

³ kg¯

¹)

(b)

0

5

10

15

20

25

V'

(dm

³ kg¯

¹)

t (days)

(a)

t (days)

453525151050 20 30 4012106 8420

Figure 2. Methane generation in thermophilic batch experiment (a) experiment with addition of 3.5 g dm−3 NaHCO3 (b) experiment with addition of 5 g dm−3 NaHCO3 ( experimental data; — simulated data).

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Hublin and Zelic 359

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Figure 3. Simulation of the methane generation in batch experiment using different substrates; — mixture of whey and cow manure (10/90 v/v); – – – native whey; ...... native cow manure).

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