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Ingeniería e Investigación

Print version ISSN 0120-5609

Ing. Investig. vol.31 no.1 Bogotá Jan./Apr. 2011


Anaerobic treatment of lactic waste and goat manure

J. Luis Magaña-Ramírez1, Ruia Rubio-Nuñez2, Hugo Jiménez-Islas3, Martín T. Martínez-García4

1 Ph.D. Science, Universidad Politécnica de Valencia, Spain. Research Professor, Universidad de Guanajuato, Mexico.

2 M.Sc. Science in Biomedical Engineering, Instituto Tecnológico de Celaya, Mexico. Professor, Department of Chemistry, Universidad Tecnológica de Salamanca, Mexico.

2Ph.D. Sciences, Universidad Autónoma Metropolitana Iztapalapa, Mexico. Research Professor, Instituto Tecnológico de Celaya, Mexico. .

4 M.Sc. in Business Administration and Mechanical Engineering. Ph.D Candidate Chemical Engineering, Unvdersidad de Guanajuato, Mexico.


Anaerobic digestion was carried out to obtain biogas from lactic waste in combination with goat manure. Waste from lactic products such as cream, cheese and whey was mixed with goat manure using three formulations; the quantity of waste from cream and cheese was maintained, and only the quantity of manure and whey was varied. Methanogenic bacteria obtained from predigestion of goat manure were used as inoculants. Temperature was 35°C and pH 7.0.Biogas methane percentage was determined by gas chromatography. The results showed that the highest methane concentration obtained was 82% with formulation III.

Keywords: anaerobic digestion, lactic waste, goat manure, biogas

Received: September 17th 2009. Accepted: Feuary 7th 2011


Biogas, the result of anaerobic digestion, is when organic materials are decomposed by bacteria in anaerobic conditions. It is a mixture of methane (60%-70%), carbon dioxide (40%–30%) and other trace gases such as hydrogen sulphide, ammonia, nitrogen, hydrogen and organic compounds (Tsai and Lin, 2009). Today, electricity and heat production are direct benefits from aerobic digestion. Other benefits of biogas include odour reduction, organic nitrogen mineralisation, pathogen reduction, greenhouse gas reduction, and improved organic waste handling. The digested residues may also be used as fertilisers (Sahlström, 2003; Mann et al., 2004).

Bioconversion of organic material to methane requires four steps and five distinct groups of microorganisms. The first step is hydrolysis. Organic polymers such as proteins, polysaccharides and fats are hydrolysed to monomers (e.g. sugars, long-chain fatty acids, amino acids) by the action of enzymes produced by fermentative bacteria (Angenent et al., 2004). Monomers are converted to volatile fatty acids (VFAs) and alcohols in fermentation.

Then, in acetogenesis, VFAs are converted to acetate and hydrogen by obligatory hydrogen-producing acetogenic bacteria; these bacteria grow in syntrophic associations with hydrogenotrophic methanogenic bacteria, maintaining hydrogen partial pressure.

Methanogenesis requires acetoclastic methanogenic bacteria to convert acetate to methane and carbon dioxide. In methanogenesis, about 70% of methane is produced by acetoclastic methanogens (Gerardi, 2003; Angenent et al., 2004; Myint et al., 2007). Biogas production can be affected by operational factors such as hydraulic retention time (HRT) and the degree of contact between incoming substrate and bacteria population, pH, temperature, the nature of the substrate, organic loading,chemical organic demand (COD) and carbon/nitrogen ratio (C/N)(Al-Dahhan et al., 2005). Anaerobic digester designs are mainly dependent on hydraulic retention time and type of organic waste. Packed fixed film reactors, up-flow anaerobic sludge blanket (UASB) reactors, horizontal baffled reactors, plug flow reactors, covered lagoons and anaerobic baffled reactors are commonly used (Bouallagui et al., 2003).

Many organic wastes can be used in anaerobic digestion. The dairy industry produces residues in which whey is produced in the greatest volume. Many small dairy industries do not have the technological capability to treat or reuse whey (Mockaitis et al., 2006).Furthermore, due to high salt concentration and poor process stability in dairy waste, the co-digestion of different materials may enhance the anaerobic digestion because better carbo to nutrient balance may be created (El-Mashad and Zhang, 2006).Goat waste has around 2% nitrogen, 3% phosphorous (Orrico et al.,2007) and over 80% methane (Mogami et al.,2006). Goat manure is used in fertilising treatments and for producing methane. Sawdust, rice straw, water hyacinth and other plant materials having 63%–67% methane may be used(Chakraborty et al.,2002).

Biomass energy is very important in Mexico and requires more exhaustive investigation, especially because petroleum(a nonrenewable fuel) is the main source of energy in the state of Guanajuato state. Biomass from crop residues, animal manure, forest and industrial waste, wastewater treatment residues, etc., is easily available. Biomass may be the most suitable alternative for up to 25% of the energy consumed in Guanajuato (CONCYTEG – SDA, 2005).

This work has focused on methane production using lactic waste and goat manure (Magaña-Ramirez et al., 2006) in different concentrations to determine the highest methane concentration in batch fermentation. The methane was produced using a laboratory- scale bioreactor previously designed and manufactured in the University of Guanajuato, México by some teachers and students. A simple scale-up calculation for estimating the dimensions for a rural digester are proposed, using the volume opera tion and methane concentration obtained in the laboratory-scale experiments.

Materials and Methods


The assays were carried out using laboratory-scale batch fermentation in a stainless steel reactor (Magaña-Ramirez et al., 2006). The reactor had10Ltotal volume, a manual agitator, feed port for load waste, safety valve and a temperature control system. Figure 1 shows a drawing of the anaerobic digestion reactor system and Figure 2 depictsthe physical apparatus. The biodigester was also provided with a bi-metallic thermometer, Bourdon manometer connected to the biogas collector. The bioreactor was set to 35°C and coupled to the gas collector.

Methane concentration was determined by Perkin-Elmer chromatograph model 3920B supplied with a thermo conductivity detector. The standard, or pattern sample, was natural gas provided by PEMEX (Petróleos Mexicanos) having 97% methane purity.


Lactic residues such as cheese, cream and whey from cheese factories in the small dairy industry were used in combination with goat manure. Methanogenic bacteria obtained from previous goat manure digestion were used to inoculate the experimental assays. The formulations used are shown in the Table 1.

Cheese and cream waste were maintained at200 g, the inoculants was also maintained at200 g. Goat manure was increasingly added to formulation II and III. Afterwards, milk whey was added in different proportions to obtain 5 L total volume for all cases. Temperature was maintained at tmesophilicbacterial growth temperature (35°C) and pH was adjusted to7.0. Anaerobic digestion was sustained for 15 days, and manual shaking took place at least once daily.

Results and discussion

The pH was decreased to 4.0 for formulation I and the biogas so produced did not form the typical methane peak. This was most likely caused by milk whey and other lactic residues which had low bicarbonate alkalinity and tended to acidify quickly thereby causing anaerobic process failure. This result was thus discarded. The reactor also exhibited unstable operation with formulation II. pH decreased after seven days to 3.5, supplemental alkalinity was added using calcium bicarbonate, and pH increased to 7.0.Pressure remained at 7 psi (about one psi per day).

The biogas so produced was analysed after15 days and the formation of 22% methane was reported. pH was adjusted in formulation III and maintained at 7.0 for all 15 days. A methane peak was seen and 82% concentration measured. By comparison, other researchers have used salty cheese whey mixed with poultry waste and cattle dung, obtaining 64% and 63%methane content, respectively (Patel and Madamwar, 1996).

Patel et al., (1999) used sweet cheese whey, yielding 72% methane content. Digestion was considered to have performed well if methane content was greater than 70% (Saddoud et al., 2006). When whey concentration was lower, 4 litres and 1,400g of goat manure were used; the organic matter concentration provided by the goat manure in this combination increased alka linity, promoting methane generation.

Rural biodigester dimensioning

Based on the experiments described in this work, an approach to scaled-up biodigester for rural use is presented.


The necessary quantity of biogas for satisfying a rural family´s domestic requirements varies between 1 and 1.5 cubic meters daily, depending on methane content (Monroy and Viniegra, 1990). The operational pressures for a rural biodigester have been calculated to be between 1,200-3,000 Pa and methane content in biogas varies between 50% and 70% (Borroto and Borroto, 1999).

Calculating 1.0 L per day was straightforward using the results for formulation III,82%methane concentration, 1 psi per day working pressure and 5 L organic mixture biogas production.

Evaluation of the volume of required gas

Assuming an ideal gas pattern, the following ratios were taken into account: 1 L per day for basic biogas volume at1 psi pressure and 5 L per day for basic biogas volume at 0.2 psi pressure. Producing 1,500 L biogas every day requires 1,500 L volume organic mixture. Assuming 20% digester volume is for biogas, 1,875 litre volume is needed for a biodigester.

Design parameters for the biodigester

A rural digester can be designed and constructed on the basis of the following parameters: 1.875 m3operational volume, 1,500 L organic mixture volume, 7 psi operational pressure in the biodigester, 0.2 psi biogas pressure at the exit valve, pH 7, at 35°C. Shaking was applied once every day for 2 minutes. This information could be applied for designing a floating bell for a Hindu biodigester or a gasometer for a Chinese biodigester.


This work was aimed at obtaining biogas from lactic waste and validating a previously designed bioreactor. It was shown that the lab biodigester was a safe apparatus for effectively teaching and carrying out research activities related to developing anaerobic digestion. Biogas could be detected by an increase in pressure during anaerobic digestion; methane concentration was measured by chromatographic analysis. Formulation III had the best methane yield (i.e.82%).

This result was used for calculating potential dimensions for rural biodigesters. The methodology is suitable for implementing and scaling-up for designing biodigesters as a source of renewable energy.


The authors wish to acknowledge financial support provided by the University of Guanajuatoás Research and Degree Studies office (Mexico).


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