Biogas Digesters are an important technology that allow farmers to convert waste (either manure or kitchen scraps) into methane. This is accomplished through anaerobic respiration. The gas that is made can be harnessed for cooking or running a generator (see Technical note #44http://edn.link/tn44. A by-product of the process is the biogas effluent, a nutrient-rich slurry that is expelled every time the digester is fed.
On the ECHO campus in Florida, two digesters are in use. One digester is fed with cow manure. Another digester is fed with kitchen scraps. We tested the nutrient contents of the two types of effluent. The results are displayed below, compared to the numbers that could be expected from a compost tea:
*Each value is the estimated quantity available to plants in the first year, reported in g/1000L which is equivalent to ppm.
The manure-fed biogas digester produced effluent with high levels of phosphorous and potassium, but lower nitrogen than the kitchen waste biogas digester. However, the sodium content of manure-fed digester is higher than the kitchen-waste digester. Both types of biogas effluent are significantly higher than the compost tea results.
The next step is to measure how well the effluent acts a fertilizer. A field trial with corn is in progress. The experiment will compare the performance of the kitchen-waste effluent against a commercial soluble fertilizer. Each area will be treated weekly with the chosen fertilizer. We will test for biomass, chlorophyll count, and plant height.
Thank you for the excellent summary of the nutrient composition of various bio-gas fermentation schemes. I await the results of continued experiments with both gas production and effluent nutrient composition.
Let me add a note to potential promoters of this technology: Naturally, there is a tendency to urge the adoption of the most technically efficient method for producing gas and fertilizer. However, efficiency must be paired with social norms, which means that the most efficient method may not necessarily be the one that is best suited to the context of the adopting community. I have often made the mistake of promoting the most efficient, but least acceptable method for a particular community, resulting in eventual abandonment of the technique. Often, the surest way to determine which method is most apropos to your context is through informal discussions with farmers. You don’t want to short-cut this vital step.
Do you know of any research conducted in a more arid climate? I am currently working on a project that is attempting to install a bio-digester in Chad, Africa. We are currently in the research phase and are wondering what would be the best source to feed the digester and use of the byproducts after the biogas has been collected. So far, we have discovered that there is limited food supply so we were interested in trying to fuel it with manure. Do any of you have any advice?
Sydney,
Thanks so much for your question and sorry for such a late response. I hope this can still be of help. There is a lot of potential to have a productive biogas system using manure. Cow manure is generally the preferred feedstock, because it is often available, you can get a large quantity from just a few animals, and it easily mixes with water. Other manures can be used (such as goat/sheep/rabbit), but because they are pelletized they need to be mechanically broken up, so they are not often recommended. I am not familiar with any research comparing the differences of the gas or the effluent produced by different manures. I have heard pig manure is effective, it easy to collect because pigs are often contained, and it produces decent biogas. I do not have much experience with pig manure, but maybe if some else has they could comment with some recommendations.
There are systems that use human waste. There is quite a bit of research on this ranging from large water treatment facilities to household digesters. There could potentially be cultural implications for working with human waste. I am not sure of your cultural context, but that would be a factor to consider.
Your goals for gas production and feed source will ultimately depend on what your system will look like. How much gas do you need? How much manure do you have available? How accessible is it? What resources do you have available? There are many different designs depending on resources available and your context. I personally do know of research done about digesters in arid regions. Again, if some one else does, please feel free to comment.
We have been continuing to monitor NPK values of the effluent of our two biogas systems. These charts are comparing the difference of values during our coldest and warmest month. You can see a rather large variation in the different seasons. Some thoughts regarding these results:
-The values do not seem significant enough to use as a main fertility source, but keep in mind it is diluted in water. It is comparable to fertigation (injection of fertilizer into irrigation system), not pelletized fertilizer. It would be interesting to compare values with commercial fertigation when diluted in water.
-Depending on your amount produced, your amount needed, and a limited fertility source, biogas effluent will most likely not be your main source of fertility, but a supplement fertilizer.
