Though algal biomass appears to be a good substrate for the production of biogas, there are certain restrictions due to anaerobic digestion of algal biomass. It has been reported by Sialve et al. (2009) that there are two major problems of microbial flora of anaerobic absorption: sodium ion toxicity and ammonia inhibition.

Toxicity of ammonia mainly arises because of excessive C/N ratio of the algal biomass. Free ammonia present during the harvesting procedure barely disturbs the acidogens and mostly hinders acetoclastic methanogens. The range of obstructive concentration of ammonia varies widely (1.7-14 g/L) because there are several other factors contributing to inhibition of ammonia. These factors are nature of the feed, inoculum, operating and functioning conditions, i. e., pH and the presence of antago­nistic ions like sodium ion, calcium ion, and magnesium ion (Angelidaki and Ahring 1993; Chen et al. 2008; Koster and Lettinga 1988; Sialve et al. 2009). Furthermore ammonia when released in the course of hydrolysis of amino acids causes an upsurge in both alkalinity and pH of digester liquid. At elevated concentration of ammonia and alkaline pH, acetate ion, which is the basic substrate for the methanogens, gets trans­formed into ammonium acetate or ammonium bicarbonate leading to the reduction of accessible acetate to methanogens (Shanmugam and Horan 2008, 2009). This reduc­tion of acetate ion and subsequent less development of methanogens due to high ammonia released during algal biomass ingestion can be the main reason of low meth­ane in the headspace biogas (Shanmugam and Horan 2009).

13.4 Conclusions

Algal biomass can prove to be the most promising source of bioenergy to cope with high energy demands. Algal biomass can generate improved quantities of biogas and biodiesel as compared to traditional substitutes. With the latest and more advance technologies, it can successfully replace the conventional feedstock. If the internal and external factors are carefully controlled such as increased water, CO2, light, and sufficient space, the algal feedstock can generate maximum biomass and in turn maximum bioenergy output. The biofuels generated through algal biomass is more environment-friendly with minimum contribution towards global warming. Among different methods of algae cultivation, the most successful is the mixotro — phic production, which possesses both photoheterotrophic and photoautotrophic capabilities. Open and close pond systems are used equally with certain benefits and limitations, however, if the problems associated with production of algal biomass such as cost, temperature maintenance, salinity control, and contamination are elim­inated, algal biomass can be grown to its maximum. Harvesting of algal biomass after mass cultivation plays a vital part in shaping the process budget of algal biofu­els. The harvesting of macroalgae biomass is a simple process as compared to the microalgae harvesting. Due to the diluted nature of algal culture cells and small size, the operating expense of dewatering and harvesting of algal biomass is ele­vated. To resolve this challenge, a number of procedures including chemical as well as mechanical, can be performed which includes centrifugation, flotation, floccula­tion, filtration, screening and gravity sedimentation, and electrophoresis with ben­eficial output. Optimized culture technologies are the key elements to regulate the effective cost of the production of algal biomass for the purpose of biogas produc­tion. With the selection of those algal strains that are rich in oil, there is a great potential of algal biomass in biodiesel and biogas production.