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Journal of Bioscience and BioengineeringVOL. xx No. xx, 1e5, 2014

Chemically pretreating slaughterhouse solid waste to increase the efficiency ofanaerobic digestion

Cyntia R. Flores-Juarez, Adrián Rodríguez-García,* Jesús Cárdenas-Mijangos, Leticia Montoya-Herrera,Luis A. Godinez Mora-Tovar, Erika Bustos-Bustos, Francisco Rodríguez-Valadez, and

Juan Manríquez-Rocha

Environmental Electrochemistry, CIDETEQ, S. C., P.O. Box 064, C.P. 76703, Parque Tecnológico Querétaro S/N, Sanfandila, Pedro Escobedo, Querétaro, Mexico

Received 13 September 2013; accepted 24 March 2014Available online xxx

* CorrespondE-mail

(A. Rodríguez-

1389-1723/$http://dx.doi

Please citeanaerobic d

The combined effect of temperature and pretreatment of the substrate on the anaerobic treatment of the organicfraction of slaughterhouse solid waste was studied. The goal of the study was to evaluate the effect of pretreating thewaste on the efficiency of anaerobic digestion. The effect was analyzed at two temperature ranges (the psychrophilic andthe mesophilic ranges), in order to evaluate the effect of temperature on the performance of the anaerobic digestionprocess for this residue. The experiments were performed in 6 L batch reactors for 30 days. Two temperature ranges werestudied: the psychrophilic range (at room temperature, 18�C average) and the mesophilic range (at 37�C). The waste waspretreated with NaOH before the anaerobic treatment. The result of pretreating with NaOH was a 194% increase in thesoluble chemical oxygen demand (COD) with a dose of 0.6 g NaOH per g of volatile suspended solids (VSS). In addition,the soluble chemical oxygen demand/total chemical oxygen demand ratio (sCOD/tCOD) increased from 0.31 to 0.7. Forthe anaerobic treatment, better results were observed in the mesophilic range, achieving 70.7%, 47% and 47.2% removalefficiencies for tCOD, total solids (TS), and volatile solids (VS), respectively.

� 2014, The Society for Biotechnology, Japan. All rights reserved.

[Key words: Anaerobic digestion; Chemical hydrolysis; Mesophilic conditions; Chemical pretreatment; Psychrophilic conditions; Slaughterhousewaste]

Problems related to the environmental contamination are someof the most important research topics due to their major impact onthe sustainability and the continuity of plant, animal and humanlife. For example, increasing emissions of greenhouse gases likecarbon dioxide (CO2), methane (CH4), and chlorofluorocarbons(CFCs) constitute the main cause of global warming, and areasonable proportion of these emissions are related to the un-controlled degradation of organic matter contained in theincreasing amount of human produced wastes. Interestingly,anaerobic digestion is still the most attractive option for solid sta-bilization and volatile solid removal of organic matter contained insolid wastes (1).

Solid waste from slaughterhouses is especially problematic,because it consists primarily of ruminal and stomach content,viscera and blood. Due to its elevated concentration of biodegrad-able organic matter, this waste can be efficiently treated byanaerobic digestion (1), although the high content of lignocellulosicand refractory material causes the process to be relatively slow. Forthis reason, a good approach is to pretreat the solid waste whichshortens the hydrolysis phase (2,3).

A variety of pretreatments focused on both the degradation ofthe organic fraction of organic wastes and increase the production

ing author. Tel.: þ52 442 2116046; fax: þ52 442 2116001.addresses: arodriguez@cideteq.mx, adrian_r_g@hotmail.comGarcía).

e see front matter � 2014, The Society for Biotechnology, Japan..org/10.1016/j.jbiosc.2014.03.013

this article in press as: Flores-Juarez, C. R., et al., Chemicallyigestion, J. Biosci. Bioeng., (2014), http://dx.doi.org/10.1016/j.j

of methane have been developed. Studies have been reported onmechanical, chemical and thermal primarily pre-treatments. Liaoet al. (4) reported the positive effect of pretreatment of solideliquidseparation by screening prior to anaerobic digestion. Regardingchemical pretreatments, it has been mainly evaluated the use ofalkali, polyacrylamide and acid such asHCl andH2SO4 (5,6). In termsof thermal treatments, the best results were obtained with tem-peratures between 100�C and 170�C (7). Gonzalez-Fernandez et al.,(8) conducted a comparative study to determine the effect betweenthe methods of mechanical and chemical pretreatment with eitherHCl or NaOH in pigmanure. The best results were obtainedwith thealkaline pre-treatment, enhancing the biodegradability of the res-idue by 81.6% compared to 69.4% for the residue obtained pre-treated with HCl. Heat treatments are still the most efficient option,however, the thermophilic pretreatment is more sensitive to envi-ronmental changes than the mesophilic pretreatment (9).

A variety of alkaline pretreatments have been reported for thesolubilization of biopolymers (proteins, polysaccharides, nucleicacids and lipids). For instance, a comparative study of the effect ofpretreating with two alkalis, NaOH and CaO, on the degradation ofresidual sludgewas carried out, obtaining better results with NaOH,using a dose of 0.5 g NaOH per g of volatile suspended solids (VSS)(10). Lopez et al. (11), on the other hand, studied the effect of theaddition of Ca(OH)2 to municipal solid wastes and determined thatthe optimum alkali dose was 62 mg Ca(OH)2/L with a contact timeof 6 h. They obtained an 11.5% of solubilization of chemical oxygendemand (COD).

All rights reserved.

pretreating slaughterhouse solid waste to increase the efficiency ofbiosc.2014.03.013

TABLE 2. Operational parameters for the digesters at the beginning of theexperiment.

Operational conditions Test at 18�C Test at 37�C

Volume (L) 6 6HRT (days) 30 30Solid load (kg TS/m3 d) 1.42 1.42Volumetric organic loading (Kg

COD/m3 d)1.72 1.72

Temperature (�C) Room T 37

sCO

D (

mg/

L)

0.40

0.50

0.60

0.70

0.80

/tC

OD

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

g NaOH/g VSS

a

b

2 FLORES-JUAREZ ET AL. J. BIOSCI. BIOENG.,

Our research was focused on facilitating the treatment ofslaughterhouses waste, which is extremely difficult to degradebecause of its high organic content. Based on the studies citedabove, we decided to try to accelerate the anaerobic digestionprocess by solubilizing the organic matter with an alkaline pre-treatment of the waste, which increases the hydrolysis of thesubstrate, thereby accelerating the first stage of anaerobic digestionprocess. In this work, we achieved a 28% of solubilization in threedays, increasing it to 7460e9500 mg/L for an organic load of 50 g/Kg. Concerning the effect of temperature on COD removal forslaughterhouse waste, we performed literature search, failing tofind published information evaluating the COD removal of bothpsychrophilic and mesophilic conditions. Then we tried to accel-erate themethanogenic phase of the anaerobic digestion process byincreasing the operating temperature, testing the treatment of thepretreated waste by anaerobic digestion in both the psychrophilicrange (at room temperature) and the mesophilic range, at 37�C. Inorder to evaluate the effect of pretreatment study on the produc-tivity of biogas through anaerobic digestion, the same tests wereperformed on a substrate without pretreatment (control).

MATERIALS AND METHODS

Substrate The weight percentage for each component of the slaughterhousewaste studied was determined according to a previous work (12), and was 80%ruminal content, 11% blood and viscera, and 9% manure. Additional wastecharacterization experiments were carried out according to standard methods(13), adapted to waste with both a high organic load and high solid load. Thesedata are presented in Table 1.

Pretreatment Chemical pretreatment was carried out by adding NaOH tothe waste with the goal of increasing the hydrolysis rate as well as the solublefraction of COD. In order to quantify the effect adding the alkali, doses of NaOHwereadded ranging from 0.1 to 0.7 g per g of VSS. A contact period of 24 h between theresidue and the NaOH was defined and constant agitation conditions were main-tained during this time using a magnetic stirrer working at 150 rpm. The pre-treatment experiments were performed at room temperature (18�C average). Thesolubilization rate was calculated from the relationship (sCOD/tCOD) where sCODand tCOD correspond to soluble and total chemical oxygen demand, respectively.

Anaerobic digestion After the slaughterhouse waste was pre-treated,increasing its soluble COD value, we processed this pre-treated waste by anaerobicdigestion at two different temperature ranges: psychrophilic (room temperature,18�C average) and mesophilic (37�C), to study the effect of the temperature on theremoval of COD, total solids (TS) and volatile solids (VS).

A series of experiments were performed in two Upflow Anaerobic SludgeBlanket (UASB) laboratory scale digesters, (reactors 1 and 2), each having a workingvolume of 6 L. The first one was operated at room temperature (18�C average) andthe other operated at a controlled temperature of 37�C. The experiments wereperformed in batchmode, having a total duration of 30 days. The hydraulic retentiontime (HRT) for both experiments was kept constant for 30 days resulting in a solidloading of 1.42 g TS/L day and an organic load of 1.72 g tCOD/L day. Mixing wasmaintained by recirculating the biogas produced inside the reactors. Table 2 showsthe operational conditions at the beginning of the experiments.

Inoculum An anaerobic seed sludge collected from an operating UASBreactor was used to inoculate the digesters. The seed sludge had total suspendedsolids (TSS) and VSS concentration of 83.4 and 62.4 g/L respectively. 1 L of anaerobicseed was added with a specific methanogenic activity of 1.32 g COD per g of VSS perday.

Analytical methods The feed and effluent samples were analyzed for tCOD,sCOD, TS, VS, TSS, VSS, pH, volatile fatty acids (VFA), partial alkalinity (PA), totalalkalinity (TA), and ammoniacal nitrogen (NH4eN) according to standard methods(13). Continuous biogas production was measured through a system based on

TABLE 1. Waste characterization parameters.

Parameter Value

pH 4.8 � 0.24Total COD (g/Kg) 215.90 � 8.81Total Kjeldahl Nitrogen (g/Kg) 7.01 � 0.10Total solids (g/Kg) 179.10 � 11.73Volatile total solids (g/Kg) 149.10 � 7.98Total suspended solids (g/Kg) 156.10 � 3.40Volatile suspended solids (g/Kg) 129.70 � 8.00Alkalinity (mg CaCO3/L) <0.5

Please cite this article in press as: Flores-Juarez, C. R., et al., Chemicallyanaerobic digestion, J. Biosci. Bioeng., (2014), http://dx.doi.org/10.1016/j.j

volumetric displacement using a 1 M NaOH. This solution was replaced when thepH value fell below 12.

RESULTS AND DISCUSSION

Waste pretreatment NaOH pre-treatment of the waste un-der study was carried out as described in the experimental sectionusing different concentrations of alkali. As shown in Fig. 1a, thesCOD values increased in proportion to the concentration ofNaOH, and reached 194% of the initial value, as expected.

Since the minimal dose of NaOH that produced the maximumchange in sCOD was 0.6 g NaOH per g of VSS, this value of alkaliconcentration was set for the forthcoming experiments.

Consistent with this assumption, soluble COD increased from13,526 mg/L for the waste without pretreatment, to 39,798 mg/L

0.00

0.10

0.20

0.30

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7g NaOH/g VSS

sCO

D

FIG. 1. (a) COD solubilization and (b) sCOD/tCOD ratio after the pretreatment withNaOH.

pretreating slaughterhouse solid waste to increase the efficiency ofbiosc.2014.03.013

05

404550

0 3 6 9 12 15 18 21 24 27 30Operation period, days

18°C

37°C

0

10

20

30

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0 3 6 9 12 15 18 21 24 27 30

TS

(g/K

g)

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18°C

37°C

VS

(g/K

g)

35302520151 0

a

b

FIG. 3. (a) VS and (b) TS removal for the experiments at 18�C and 37�C.

VOL. xx, 2014 INCREASE EFFICIENCY ON SLAUGHTERHOUSE WASTE TREATMENT 3

with a dose of 0.6 g NaOH per g of VSS. After that concentrationwasreached, the soluble COD remained constant.

The sCOD/tCOD ratio on the other hand, was determined to be0.31 before the chemical pretreatment. This suggests that the wastebiodegrades slowly, primarily due to the cellulose and lignin ma-terials present in the ruminal content (14,15). After pretreatment, itis interesting to note that a maximum sCOD/tCOD ratio of 0.7 wasobtained when a dose of 0.6 g NaOH per g of volatile suspendedsolids (VSS) was employed (Fig. 1b).

Performance of the psychrophilic (18�C average) andmesophilic (37�C) digester For the decrease of the solubleCOD, Fig. 2 shows the results obtained for soluble COD as a functionof the digestion period. The experiments in these two reactors werecarried out using slaughterhouse waste pre-treated with NaOH, asdescribed above. At the beginning, the sCOD for both digesters wasaround of 13,000 mg/L. After 30 days, concentrations of 8670 and4000 mg/L were obtained with corresponding efficiencies of 35and 71% for experiment 1 (at room temperature, 18�C average)and 2 (at 37�C), respectively.

It is noteworthy that in both experiments no increase in thesCOD concentration was observed in the initial days as commonlyoccurs due to the solubilization of the organic compounds (16). Thisphenomenon can be explained by the fact that the residue was pre-treated, so that the organic material is completely solubilized. Thisspeeds up the process of anaerobic digestion, as the first step (hy-drolysis and solubilization of matter) occurred during pre-treat-ment. These results are consistent with the expected pre-treatmenteffect, which was the subject of the first part of this work.

About the behavior of solids during the experiment, for experi-ment at 18�C, the concentration of TSdecreased from52.1 to33g/Kg,a 34% reduction. In contrast, for experiment at 37�C, the TSconcentration decreased from 53 to 28 g/Kg, a removal of 47%. Forthe VS, the removals were 34% and 48% for experiments at 18�C and37�C (Fig. 3).

The anaerobic digestion process is favored by high temperature(usually between 20�C and 40�C), because the rate of growth of themicroorganisms responsible for the anaerobic process increaseswith temperature (17). The mesophilic range is most commonlyused, although the thermophilic has certain advantages, including:speed, sanitation residue, removal of larvae, weed seeds, pathogens(18), and increased particle hydrolysis (19). However, the thermo-philic range can be more unstable, especially due to the increasedtoxicity of certain compounds at high temperatures, such asammoniacal nitrogen (19,20) or long chain fatty acids (21). Theseresults clearly demonstrate the positive effect of anaerobic diges-tion in the mesophilic temperature range and agree with those

Operation period, days

0

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4000

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12000

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16000

0 3 6 9 12 15 18 21 24 27 30

sCO

D (

mg/

L)

18°C

37°C

FIG. 2. Soluble COD removal for the experiments at 18�C and 37�C.

Please cite this article in press as: Flores-Juarez, C. R., et al., Chemicallyanaerobic digestion, J. Biosci. Bioeng., (2014), http://dx.doi.org/10.1016/j.j

obtained by several authors (16,17) where there have been com-parisons between processes in psychrophilic and mesophilic rangeand greater efficiencies are obtained at temperatures close to35e37�C, range being considered optimum for these microorgan-isms as both specific growth rate of the microorganisms and thedegradation of substrates, biological and enzymatic activitieschanges are temperature-dependent (22,23).

Regarding the control parameters, in general, the results ob-tained in both experiments were acceptable. The high values ofpartial and total alkalinity obtained due to carbonates and bi-carbonates indicate an adequate buffer capacity. Jenkins et al. (24)proposed monitoring the evolution of the reactor by the alka-linity ratio, alpha (a), defined as:

a ¼ partial alkalinity ðPAÞ total alkalinity ðTAÞ= (1)

As the value of a approaches one, the system is more stable andcan process a greater organic load. In this work, the alpha valueswere generally above 0.5. Thus, there was an appropriate balancebetween the VFA produced and the bicarbonates available to reactwith the acid, thus avoiding an abrupt pH reduction, as explainedpreviously (25). For NH3eN, the values obtained at the end of theexperiment were within the range considered to be non-inhibitingfor methanogenic bacteria, although it is not possible to define theconcentration that can cause an adverse effect, because the mi-croorganisms can adapt to environments containing toxic com-pounds (26). Studies were performed in order to determine theammonia concentration for the inhibition of the anaerobic process.Hashimoto (20), found signs of inhibition from concentrations of2.5 g/L of ammonia nitrogen in reactors without acclimatization, inmesophilic and thermophilic processes.

pretreating slaughterhouse solid waste to increase the efficiency ofbiosc.2014.03.013

TABLE 3. Control parameters and biogas yield for the control, test at 18�C and 37�Cafter 30 days.

Parameter Control Test at 18�C Test at 37�C

VFA (mg/L) 1555 � 23.48 556 � 9.40 555.5 � 10.33PA (mg CaCO3/L) 2952 � 123.10 4790 � 159.51 5833.3 � 99.75TA (mg CaCO3/L) 4604 � 258.74 6685 � 162.45 6388.8 � 322.00Alpha ratio 0.64 � 0.070 0.71 � 0.041 0.91 � 0.053VFA/TA 0.33 � 0.058 0.08 � 0.03 0.09 � 0.054NH3eN (mg/L) 1638.34 � 82.88 1550 � 96.10 2156.08 � 128.29Accumulated biogas (L) 13.7 24.8 36.5Biogas yield

(L/g SV removed)0.25 0.28 0.3

Biogas yield (L/Kg waste) 8.55 15.5 22.8

4 FLORES-JUAREZ ET AL. J. BIOSCI. BIOENG.,

Poggi-Varaldo et al. (27) studied the inhibition by ammonia inmesophilic solid waste, testing COD/N ratios of 90, 80, 65 and 50within the reactor and evaluating its behavior depending on theremoval efficiency of VS, methane production and stability. Fromthese experiments, it was determined that the reduction of mi-crobial activity occurred when the COD/N ratio was 50. Gallert et al.(19) determined that ammonia inhibition in mesophilic and ther-mophilic reactors starts from a total ammonia nitrogen concen-tration of 3 and 3.5 g/L. Krylova et al. (28) conducted studies withpoultry manure and found that from a concentration of 2.8 g/L ofammoniacal nitrogen, biogas production decreased by 50%. Work-ing with microorganisms adapted to high concentrations ofammonia nitrogen, Lü et al. (29) found that at concentrations ashigh as 8 g/L of total ammonia nitrogen is possible to carry out theanaerobic digestion of fish waste. However, at a concentration of16 g/L of total ammonia nitrogen, the hydrolysis rate decreasesmarkedly. Table 3 shows the measured values for the operationparameters of the digestion period for both experiments.

Concerning the biogas production, is well known that one of theadvantages of anaerobic processes is the ability to recover thebiogas which is produced. Fig. 4 shows the accumulated productionof biogas during the experiment. In experiment at 18�C biogasproduction started on the 6th day and reached its highest pro-duction from day 9 to 24. Reactor of the experiment at 37�C pro-duced a high amount of biogas from the startup of the reactor untilday 24. After day 25 biogas production ceased entirely. The totalaccumulated biogas production for the experiments at 18�C and37�C were 24,800 mL and 36,495 mL, respectively, and the biogasyields were 0.28 and 0.3 L per g of removed VS.

30

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Operation period, days

18°C

37°C

Cum

ulat

ive

bio

gas,

mL

FIG. 4. Biogas production during the operation period.

Please cite this article in press as: Flores-Juarez, C. R., et al., Chemicallyanaerobic digestion, J. Biosci. Bioeng., (2014), http://dx.doi.org/10.1016/j.j

Based on the biogas produced in this part of the experiment,Table 3 also shows the biogas yield obtained.

The yields obtained per g of removed VS and per Kg of waste arecomparable to the biogas yields obtained by other authors. Karimet al. (30), worked with pig manure in a digester operating at atemperature of 35�C, and obtained a biogas yield of 0.68 L per g ofremoved VS. Studies with other substrates have shown similaryields to those obtained in this study. Zhang et al. (31), analyzedbiogas production from the co-digestion of activated sludge andkitchen residuals. The process was carried out at temperature of37�C, and the yield was about 0.31 L per g of removed VS. Likewise,studies carried outwith foodwastes (1) in batch reactors obtained abiogas yield of 0.17 L per g of VS. Regarding pretreated biomass,Lopez-Torres et al. (32) evaluated the production of biogas frommunicipal solid waste, comparing the effect of pretreatment withCa(OH)2, and obtained for non-pretreated residue a methane yieldof 0.055 m3 CH4/kg VS while for the residue chemically pretreatedCa(OH)2 a value of 0.150 m3 CH4/kg VS was obtained. Del Borghiet al. (33) obtained values of methane production of 0.39m3 CH4/kgVS for municipal solid waste mixed with activated sludge pre-treated with NaOH and specific hydrolytic bacteria. Similarly,Chulhwan et al. (34) obtained values of 0.52m3 CH4/kg VS for wasteactivated sludge thermochemically pretreated with NaOH. There-fore, the results of this study are within the range of typical biogasyields from diverse solid wastes.

In this work we considered the possibility of using anaerobicdigestion as a method for the treatment of solid slaughterhousewaste, since this technique allows simultaneous stabilization of thewaste by reducing the organic load, the elimination of pathogens,coliform bacteria, Helminth eggs and so on, and brings an addi-tional benefit due to the production of biogas, which can beexploited as an alternative source of energy.

Batch tests showed the technical feasibility of anaerobic diges-tion for the treatment of this waste, besides allowing the evaluationof the methanogenic potential of the inoculum and substrate used.Based on the results of this study the following conclusions havebeen reached.

Pretreatment with 0.6 g NaOH per g of VSS increased the solu-bilization of the organic compounds. Starting from a sCOD/tCODratio of 0.31 for thewastewithout pretreatment, increased the ratioto 0.7.

Based on the comparative study of anaerobic digestion of pre-treated solid waste in the psychrophilic and mesophilic tempera-ture ranges, higher efficiency and more stable operation wasachieved when the process was carried out at 37�C with a solidsconcentration of 50 g/Kg, resulting in removal rates of 58%, 39.6%and 45% for the COD, TS and VS, respectively.

Regarding the production of biogas, the best results were ob-tained working at mesophilic conditions. This is reflected in anincrease of 47% of biogas produced, and 7% in yield.

From the pretreatment of waste with NaOH, we can concludethat the fecal coliform count is within norms, not only because ofthe HRT and temperature of the reactor, but also because of theNaOH, since anaerobic digestion performed in the mesophilictemperature range, at a pH value of approximately 12 for a period of2 h or more, inactivates the pathogenic microorganisms, creatingsludge type B or C (30,35e37).

The results of this study show that the impact of the limitingstep of the anaerobic digestion process is reduced by pre-treat-ment, and thus increased the efficiency of the anaerobic treatment.

In conclusion, anaerobic digestion for the treatment of thiswaste has many advantages over other processes. It eliminatesorganic matter, produces a renewable fuel (biogas), and generates afully stabilized product that can be used as fertilizer or soilamendment. Indeed, this is a process that in some cases couldreplace itself, a wholly integrated system of treatment.

pretreating slaughterhouse solid waste to increase the efficiency ofbiosc.2014.03.013

VOL. xx, 2014 INCREASE EFFICIENCY ON SLAUGHTERHOUSE WASTE TREATMENT 5

This is part of the experimental work required to analyze thiswaste treatment process, identifying and optimizing the variablesthat influence the behavior of the anaerobic reactors used, likeNaOH pretreatment and temperature. More detailed studies areunderway to determine the kinetics of the reactions that take placeand to create a dynamic model of the waste treatment process ofanaerobic digestion of solid waste from slaughterhouses. Poten-tially this could operate as an automated system, where the vari-ables are controlled automatically.

ACKNOWLEDGMENTS

The authors wish to acknowledge the Ibero-American Programfor Science, Technology and Development (CYTED), which hasfacilitated this work.

References

1. Forster-Carneiro, T., Pérez, M., and Romero, L. I.: Influence of total solid andinoculum contents on performance of anaerobic reactors treating food waste,Bioresour. Technol., 99, 6994e7002 (2008).

2. Navia, R., Soto, M., Vidal, G., Bornhardt, C., and Diez, M. C.: Alkaline pre-treatment of kraft mill sludge to improve its anaerobic digestion, Bull. Environ.Contam. Toxicol., 69, 869e876 (2002).

3. Park, C., Lee, C., Kim, S., Chen, Y., and Chase, H. A.: Upgrading of anaerobicdigestion by incorporating two different hydrolysis processes, J. Biosci. Bioeng.,100, 164e167 (2005).

4. Liao, P. H., Lo, K. V., and Chieng, S. T.: Effect of liquidesolids separation onbiogas production from dairy manure, Energy Agric., 3, 61e69 (1984).

5. Liao, W., Liu, Y., Liu, C., Wen, Z., and Chen, S.: Acid hydrolysis of fibers fromdairy manure, Bioresour. Technol., 97, 1687e1695 (2006).

6. Campos, E., Almirall, M., Mtnez-Almela, J., Palatsi, J., and Flotats, X.: Feasi-bility study of the anaerobic digestion of dewatered pig slurry by jeans ofpolyacryalmide, Bioresour. Technol., 99, 387e395 (2008).

7. Mladenovska, Z., Hartmann, H., Kvist, T., Sales-Cruz, M., Gani, R., andAhring, B. K.: Thermal pre-treatment of the solid fraction of manure: impact onthe biogas reactor performance and microbial community, Water Sci. Technol.,53, 59e67 (2006).

8. González-Fernández, C., León-Cofreces, C., and García-Encina, P. A.: Differentpretreatments for increasing the anaerobic biodegradability in swine manure,Bioresour. Technol., 99, 8710e8714 (2008).

9. Kim, J. K., Oh, B. R., Chun, Y. N., and Kim, S. W.: Effects of temperature andhydraulic retention time on anaerobic digestion of food waste, J. Biosci. Bioeng.,102, 328e332 (2006).

10. Vigueras, S.: Anaerobic digestion in one and two steps of chemically pretreatedresidual sludge. Master thesis. Autonomous Metropolitan University, Mexico.(2002) (in Spanish).

11. Lopez, M. and Espinosa, M.: Effect of alkaline pretreatment on anaerobicdigestion of solid wastes, Waste Manag., 28, 2229e2234 (2008).

12. Alvarez, R.: Anaerobic biogas production-exploitation of anaerobic processwaste. Monograph. Research Institute of Chemical Process. Universidad Mayorde San Andres, Bolivia. (2004) (in Spanish).

13. APHA: Standard methods for the examination of water and wastewater, 19thed. American Public Health Association, Washington, DC (1995).

14. Budiyono, B., Widiasa, I. N., Johari, S., and Sunarso, S.: Study on slaughter-house wastes potency and characteristic for biogas production, Int. J. WasteResour., 1, 4e7 (2011).

15. Sayed, S., Van der Zanden, J., Wijffels, R., and Lettinga, G.: Anaerobicdegradation of the various fractions of slaughterhouse wastewater, Biol.Wastes, 23, 117e142 (1988).

Please cite this article in press as: Flores-Juarez, C. R., et al., Chemicallyanaerobic digestion, J. Biosci. Bioeng., (2014), http://dx.doi.org/10.1016/j.j

16. Appels, L., Degrève, J., Van der Bruggen, B., Van Impe, J., and Dewil, R.: In-fluence of low temperature thermal pre-treatment on sludge solubilisation,heavy metal release and anaerobic digestion, Bioresour. Technol., 101,5743e5748 (2010).

17. Van Lier, J. B., Hulsbeek, J., Stams, A. J., and Lettinga, G.: Temperature sus-ceptibility of thermophilic methanogenic sludge: implication for reactor start-up and operation, Bioresour. Technol., 43, 227e235 (1993).

18. Ahring, B. K.: Methanogenesis in thermophilic biogas reactor, Antonie VanLeeuwenhoek, 67, 91e102 (1995).

19. Gallert, C., Bauer, S., and Winter, J.: Effect of ammonia on the anaerobicdegradation of protein by a mesophilic and thermophilic biowaste population,Appl. Microbiol. Biotechnol., 50, 495e501 (1998).

20. Hashimoto, A. G.: Ammonia inhibition of methanogenesis from cattle wastes,Agric. Wastes, 17, 241e261 (1986).

21. Hwu, C.-S. and Lettinga, G.: Acute toxicity of oleate to acetate-utilizingmethanogens in mesophilic and thermophilic anaerobic sludges, EnzymeMicrob. Technol., 21, 297e301 (1997).

22. Youn, J.: Comparative performance between temperature phased and con-ventional mesophilic two-phased processes in terms of anaerobically producedbioenergy from food waste, Waste Manag. Res., 23, 32e38 (2005).

23. Connaughton, S., Collins, G., and O’Flaherty, V.: Psychrophilic and mesophilicanaerobic digestion of brewery effluent: a comparative study, Water Res., 40,2503e2510 (2006).

24. Jenkins, S. R., Morgan, J. M., and Sawyer, C. L.: Measuring anaerobic sludgedigestion and growth by a simple alkalimetric titration, J. Water Pollut., 55,448e453 (1983).

25. Rojas, O.: Start up and operation of UASB systems. Course manual. Universityof Valle, CVC and University of Wageningen, Cali, Colombia (1987).

26. Chen, Y., Cheng, J. J., and Creamer, K. S.: Inhibition of anaerobic digestionprocess: a review, Bioresour. Technol., 99, 4044e4064 (2008).

27. Poggi-Varaldo, H. M., Rodríguez-Vázquez, R., Fernández-Villagómez, G., andEsparza-García, F.: Inhibition of mesophilic solid substrate anaerobic digestionby ammonia nitrogen, Appl. Microbiol. Biotechnol., 47, 284e291 (1997).

28. Krylova, N. I., Khabiboulline, R. E., Naumova, R. P., and Nagle, M. A.: Theinfluence of ammonium and methods for removal during the anaerobictreatment of poultry manure, J. Chem. Technol. Biotechnol., 70, 99e105(1997).

29. Lü, F., He, P. J., Shao, L. M., and Lee, D. J.: Effects of ammonia on hydrolysis ofproteins and lipids from fish residues, Appl. Microbiol. Biotechnol., 75,1201e1208 (2007).

30. Karim, K., Hoffmann, R., Klasson, T., and Al-Dahhan, M. H.: Anaerobicdigestion of animal waste: waste strength versus impact of mixing, Bioresour.Technol., 96, 1771e1781 (2005).

31. Zhang, P., Zeng, G., Zhang, G., Li, Y., Zhang, B., and Fan, M.: Anaerobic co-digestion of biosolids and organic fraction of municipal solid waste bysequencing batch process, Fuel Process Technol., 89, 485e489 (2008).

32. Lopez-Torres, M. and Espinosa-Llorens, M.d. C.: Effect of alkaline pretreat-ment on anaerobic digestion of solid wastes, Waste Manag., 8, 2229e2234(2008).

33. Del Borghi, A., Converti, A., Palazzi, E., and Del Borghi, M.: Hydrolysis andthermophilic anaerobic digestion of sewage sludge, Bioprocess Eng., 20,553e566 (1999).

34. Chulhwan, P., Chunyeon, L., Sangyong, K., Yu, Ch, and Howard, C. H.:Upgrading of anaerobic digestion by incorporating two different hydrolysisprocesses, J. Biosci. Bioeng., 100, 164e167 (2005).

35. Godfree, A. and Farrel, J.: Processes for managing pathogens, J. Environ. Qual.,34, 105e113 (2005).

36. Watanabe, H., Kitamura, T., and Ozaki, M.: Inactivation of pathogenic bacteriaunder mesophilic and thermophilic conditions, Water Sci. Technol., 36(6e7),25e32 (1997).

37. Willis, J., Farrell, J., Murthy, S., and Schafer, P.: Pathogen inactivation inthermophilic anaerobic digestion, pp. 367e380, in: Proceedings of the WaterEnvironment Federation, Residuals and Biosolids Management 2007. WaterEnvironment Federation, Alexandria, VA (2007).

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