Algae oil augmented biogas digester

For those unaware, a biogas digester is a low energy device which is fed manure, and produces biogas and compost. The biogas is a mix of mostly CH4 and CO2, with traces of N2, H2, H2S and O2. The compost has about the same proportions of nutrients as the original manure, but in a form that's more absorbable by plants, and furthermore is nearly sterile -- almost all the pathogens that were in the manure are destroyed.

Using manure alone produces relatively low gas yields -- the main benefit of using the digester in such a situation is the improved compost.

Adding fats and grease to the digester can vastly boost production of biogas... adding 20% fats can boost gas production by a factor of 5. However, for a digester that's built close to livestock, it might not be economical to buy and transport fats and grease for this purpose.

The solution: Build, near the livestock, a small algae farm -- use a species of algae which produces lots of oil. Then, without separating the oil from the algae, move the algae into the biodigester. The algae get digested, oil and all, and biogas is released.

Most large biodigesters feed their gas directly to diesel generators, where are generally nearby. If this is the case, then the exhaust from the diesel engines can be fed back to the algae, providing a source of sterile CO2.

[edit] And if we're lucky, there will be enough waste heat from the generator to sterilize a portion of the biodigester effluent, allowing it to be used as fertilizer for the algae.

[edit2] Alternatively, if the biogas from the digester is upgraded to biomethane (and sold to the local utility company) instead of being used to generate electricity directly, then the upgrader will produce a stream of pure, sterile, CO2; naturally, this can be fed to the algae.