Discussion Increase in algal proportion in all the treatments with different inoculums recorded increase in biogas production. However the methane concentration in the biogas was very low when the proportion of algae was more in the feedstock. This might be due to the low C/N ratio of algal feedstock resulting in inhibition of the anaerobic digestion process. The high nitrogen content in algae could also result in accumulation of toxic substances such as free ammonia, which is often an important cause of upset and failure of anaerobic digesters. Addition of rumen inoculum recorded maximum methane concentration and biogas production than control reactors and reactors added with adapted inoculum. Acknowledgements We gratefully acknowledge the funding support provided by USAID and HED to carry out this work under TIES U.S. – Mexico University Partnership Initiative. Introduction The area around Torreon in Coahuila, Mexico, which is known as Comarca Lagunera, is the biggest milk-producing region in Mexico, with an estimated 500,000 cows. It generates one million tons of cattle manure per year and huge amounts of methane emissions which is of great environmental concern. Anaerobic digestion is an attractive waste treatment practice through which both pollution and energy recovery can be achieved (Chen and Cheng, 2008). During this process anaerobic bacteria produce a mixture of methane and CO2 which can be used as a renewable fuel (Burke, 2001). For further information Anaerobic Co-digestion of Dairy Manure and Algal Biomass for Biogas Production Christian Arturo Espino Lopez 1, Senthil Chinnasamy 2, K.C. Das 2 and Nagamani Balagurusamy 1 1 Escuela de Ciencias Biologicas, Universidad Autonoma de Coahuila, Torreon, Coahuila, Mexico 2 Department of Biological and Agricultural Engineering, The University of Georgia, Athens, GA, USA Materials and Methods Cow manure was obtained from a local farm located in Torreon, Coahuila, Mexico. The dried algae (Spirulina platensis) powder produced by Earthrise Nutritionals LLC Farms, USA, was obtained from the Department of Biological and Agricultural Engineering, The University of Georgia, Athens, Georgia, USA. The experiments were carried out in 450 ml serum bottles, sealed with butyl rubber stoppers and tied up with steel wires. The treatment mixtures which were used in the experiment are described in Table 1. In each set of treatment, apart from controls (with out inoculums), additional control treatments were maintained by inoculating rumen and adapted anaerobic cultures in tap water. Table 1 and Table 2 describe various combinations used in each treatment viz., proportion of substrates, volume of water used for making feedstock slurry and the type of inoculum The working volume of reactors was 33% of total volume (450 ml) and the inoculum used was 10% of the working volume. Total solid (TS) was measured by drying the sample at 105˚C to constant weight. Volatile solid (VS) was measured with the residue obtained from TS and it was ignited to constant weight at 550˚C. Methane (CH 4 ) and carbon dioxide (CO 2 ) were measured using a saccharometer filled with concentrated sodium hydroxide solution. Total biogas produced was measured using water displacement technique. Biogas production increased from treatments containing 70% Dairy manure + 30% Algae, to 100% Algae in uninoculated reactors and reactors inoculated with rumen inoculum. Similar trend was observed in the treatments containing 60% Dairy manure + 40% Algae to 100% Algae in the reactors inoculated with adapted inoculum. Inoculation with rumen fluid showed a better biogas production than uninoculated reactors and reactors added with adapted inoculum in most of the treatments, except in the reactors having 90% Dairy manure + 10% Algae, 80% Dairy manure + 20% Algae and 70% Dairy manure + 30% Algae where adapted inoculum performed better. The maximum methane concentration was 74.3%, 68% and 58% for 90% Dairy manure + 10% Algae, 80% Dairy manure + 20% Algae, 70% Dairy manure + 30% Algae fed reactors, respectively. Average methane concentration was 70.9% and 38.2% in 90% Dairy manure + 10% Algae and 10% Dairy manure + 90% Algae treatments respectively in the reactors inoculated with rumen inoculum. Treatments containing 10% Dairy manure + 90% Algae with rumen inoculum resulted in 46.5% decrease in methane concentration. Abstract A preliminary study on anaerobic co-digestion of dairy manure and algae for biogas production was conducted. It was found that the proportion of algae in the treatments increased the biogas production; but it affected the methane concentration in all treatments. The biogas production was maximum in reactors inoculated with rumen fluid followed by adapted inoculum. However, the maximum methane concentration was observed in the following treatments; 90% dairy manure + 10% algae, 80% dairy manure + 20% algae, 70% dairy manure + 30% algae with adapted inoculum. Figure 1. Development of adapted inoculum Figure 2. Batch anaerobic reactors Figure 3. Water displacement system Figure 4. Saccharometer and concentrated NaOH solution for measuring methane concentration Feedstock 100%Dairy manure. 90% Dairy manure + 10% Algae 80% Dairy manure + 20% Algae 70% Dairy manure + 30% Algae 60% Dairy manure + 40% Algae 50% Dairy manure + 50% Algae 40% Dairy manure + 60% Algae 30% Dairy manure + 70% Algae 20% Dairy manure + 80% Algae 10% Dairy manure + 90% Algae 100% Algae Table 1. Different treatments employed in this study DescriptionControl Adapted inoculum Rumen inoculum Total volume 450 ml Work volume 33% Total volume Volume of inoculum 0 10% Work volume Water volume 100% Work volume 90% Work volume Substrate % used 12% Work volume Table 2. Proportions of the different substrates and volumes of water used in this study Results Reactors fed with Increasing proportions of algae in dairy manure enhanced the biogas production in all the treatments. However, a decrease in methane concentration in the biogas was observed in the reactors fed with increasing proportions of algae in dairy manure. Objective T o determine the feasibility of an anaerobic co- digestion of dairy manure and algae in varying combinations to produce biogas and observe the effects of addition of algae on methane production. Figure 5. Biogas and CH 4 production with 90% Dairy manure + 10% Algae and 10% Dairy manure + 90% Algae treatments inoculated with rumen fluid Figure 6. CH 4 concentration in 50% Dairy manure + 50% Algae fed reactors under different treatments (Control, Adapted inoculum and Rumen inoculum) Figure 7. Biogas production in 50% Dairy manure + 50% Algae fed reactors under different treatments (Control, Adapted inoculum and Rumen inoculum). References Burke, D. A. (2001). Dairy waste: Anaerobic digestion handbook. Environmental Energy company, Olympia, WA. Chen, Y., Cheng, J. J. and Creamer, K. S. (2005). Inhibition of anaerobic digestion process: A review. Bioresource Technology., 99, Yen, H. W.,and Brune, D. E. (2007). Anaerobic co- digestion of algal sludge and waste paper to produce methane. Bioresource Technology., 98, Vergara-Fernandez,A., Vargas,G., Alarcon, N., and Velasco, A. (2007). Evaluation of marine algae as a source of biogas in two stage anaerobic reactor system. Biomass and Bioenergy.,doi: /j.biombioe