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 #44 http://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.
- Values from the compost tea on page 10 were averaged to obtain the numbers in the chart above.