Friday, January 11, 2008

Fruity Biogas Variant

A variation on the domestic biogas system that uses rotting fruits as the biomass to be digested may serve as a viable alternative to petroleum based cooking fuels.

By: Vanessa Uy

A conventional biogas system that uses cow manure as a biomass or starting material for methane generation is somewhat hard to maintain for everyday domestic use. Why? Because to achieve continuous methane generation, conditions in the biogas digester must be met like the optimum temperature, the absence of oxygen or anaerobic conditions and the steady constant supply of biomass/ organic material to be converted to methane.

In principle, biogas digesters produce methane by mimicking the workings of the stomach of ruminant animals like cows. As of late, herds of farmed cattle are blamed for contributing to global warming since they emit so much methane gas which is more than 4 times as effective a greenhouse gas than carbon dioxide. Since methane can be used as a cooking and heating fuel, it’s better to use it as such than allowing it to be released into the atmosphere.

A novel biogas system being tested in Pune, India that uses rotting fruits and vegetable peels/trimmings might serve to overcome this problem. Developed by a firm called ARTI or Appropriate Rural Technologies Institute, this biogas digester is compact and is ideal for urban applications where cow manure or other animal wastes are scarce. This biogas generating system consists of 3 main parts: the gas/methane output accumulator at the top, the main digester in the center where the rotten fruit or vegetable peelings enters, and lastly the bottom-part where the slurry/effluent of the biogas digester exits. As a bonus, the slurry/effluent can be used as a free organic fertilizer.

The business end of this biogas digester is a type of bacteria that originally dwells in the stomachs of ruminant animals like cattle. A substantial amount of these bacteria is expelled by the cow and can be found in fresh cow manure. The ARTI biogas system only uses the cow dung as a starting material. Since the ruminant stomach bacteria is not fussy about what it is going to digest as long as sugars are present, rotting fruits and vegetables are an almost perfect “biomass meal” for the bacteria. Beside the biogas system’s compact size, it can also be scaled up for increased biogas/methane output in communal situations.

As a viable alternative to purchasing tanks of LPG (liquefied petroleum gas), biogas digesters demands more attention than a sundial. Rotting fruits and vegetables should be mashed up and mixed with water to the same consistency as a cow does to grass or other vegetation that its eating as “food” for the biogas system. This should be done regularly like once a day or the bacterial colony responsible for methane production in the digester will die out.

1 comment:

moo said...

I am the proud owner of new domestic biogas plant in the lines that you have mentioned (completed installation on Thursday, 14 Feb 2008). The agency that has built it has built over 4,000 of them, so I've gone in without much of a due diligence process.

According to Biotech, the Trivandrum-based (Kerala province, India) agency that built mine, the digester wants about 5 kg of waste (vegetable peelings, leftover food, spoilt food, water used for rinsing meat or fish, and even doggie poop) everyday, and will produce 2 hours of clean methane. It doesn't like lemon or orange rind (too acidic, apparently), egg-shells, or onion-skin (can't be digested). My experience (all of 3 days old) is that the gas is soot-less, without smell and seems to have about 70% of the thermal efficiency of LPG. The quantities of input vs. output needs to be verified after a few months of use.

I will be tracking the performance of my biogas plant at my blog