4 Methods to Maintain and Improve Biogas Yield from Anaerobic Digestion Processes

Anaerobic digestion processes convert the organic fraction of municipal wastes such as green (garden / yard) waste, food waste and non-recyclable paper waste into biogas (methane) and if PAS110 is complied with, a high quality compost - but costs can be high and the economic viability can be marginal. Agricultural waste, undigested sludge and animal manure may also be processed separately or along with municipal wastes. The typical biogas yield has been quoted as being about 5,000 scf (standard cubic feet) per ton of organic waste, but that varies greatly  between different plants.

The typical composition of biogas from AD processes comprises 60% methane and 40% carbon dioxide. In the region of 20% of the biogas output may be available in excess of plant operating load, which is then usually used to generate electricity and steam for some form of CHP use, either within the facility, or via delivery of the otherwise wasted spare heat to an external user. The residue consist of a high quality compost like material, stabilized and hygienically safe. Pathogens as well as seeds are killed very effectively because of the combination of anaerobic and high temperature conditions of the waste processing, combined with the retention time in the digester. Additional pasteurization is applied where low temperature AD processes are used.

So, in the anaerobic digestion process there lies many advantages, but how do you maximise the biogas yield and thereby ensure profitability for an Anaerobic Digestion Plant?

Here are 4 methods we found being suggested when we did a web search:

Ensure Mixers are Installed and Operating Correctly

It is commonly accepted mixers (or other methods of mixing such as biogas sparging) play a vital role in keeping biogas yield at an optimum level. Mixers and agitators homogenize the digester substrate thereby conveying fresh nutrients to the biogas-bacteria which do all the "digesting" which produces the methane gas. This, by providing a constant supply of essential food to the biomass, makes the anaerobic digestion process more effective and delivers higher methane per unit of feed, and more reliably.

Mixers are an important factor when it comes to run a stable anaerobic digester process. Mixers installed wrongly or in the worst case are broken down lead to formation of heavy bottom sludges, and lighter particles to float and form a solid crust. Methane production goes down due to insufficient nutrient supply in such circumstances and the economics of the complete system suffer. Separation and layer formation within the digester lead to depleted nutrient areas and a change in the microbial species in the population. This can cause sever odor problems and at times a complete failing of the digester.

Reduction in Particle Size for Improved Biogas Yield

The degradation and biogas production potential of all fibrous feedstocks can be significantly increased by pre-treatment for reduction of particle size. In one report which is available on the web, batch-wise anaerobic digestion of sisal fibre waste was carried out in 1 litre digesters with fibre sizes ranging from 2 to 100 mm, at an ambient temperature of 33 °C. The sediment from a stabilisation pond at a sisal production plant was used as starter seed. The proportion of total fibre degradation increased from 31% to 70% for the 2 mm fibres, compared to untreated sisal fibres. In addition, the results atre said by the reporter to have confirmed that methane yield was inversely proportional to particle size. specifically, the methane yield is said to have increased by 23% when the fibres were cut-down to 2 mm size and was 0.22 m3 CH4/kg volatile solids, compared with 0.18 m3 CH4/kg volatile solids which was all that was achieved for untreated fibres.

AD Process Co-digestion for Optimization for Improved Biogas Yield

In this paper, a batch of experiments under the condition of 8% mass fraction of total solid were conducted in a self-manufactured anaerobic fermentation reactor, with pig dung, cattle dung, and wheat straw as fermentation materials, and the substrate of constant temperature fermentation pool as the inoculation substance. The biogas yield, fermentation duration, and optimal temperature were determined. It was shown that the cumulative biogas yield of mixed anaerobic fermentation of pig dung and wheat straw was 2.4 times higher than that of the fermentation of pig dung alone, but no significant difference was observed between the cumulative biogas yields of the mixed fermentation of cattle dung and wheat straw and the fermentation of cattle dung alone. The optimal fermentation temperature for the mixed anaerobic fermentation was above 30 degrees C, and the fermentation duration was about 60 days. The fermentation duration was not always shortened by increasing temperature, and it would be not feasible to only use temperature to determine the duration of anaerobic fermentation.

Enzymes, Probiotics and Micro-elements as an Additive for Improved Biogas Yield Increases

The mixture of enzymes, probiotics and micro-elements is sometimes used as an additive for biogas yield increase. Additives cost additional money, and need to be continually dosed, however the manufacturers of ZorgEnzym claim that their additive allows them to achieve considerable yield improvements at a low cost. The main advantages of successful enzyme application is claimed to be an increase of biogas yield from 15 to 40% without any changes in plant design. They also say that the stability of the process is better and they see an increase of methane content in the biogas. An further advantage cited for the ZorgEnzym product is that it is claimed to be possible to get almost all of the biogas out of the main digester stage, without needing a post-digester stage.

Achieving a consistently good or excellent biogas yield is essential for all biogas plants to repay the investment placed in them.

In fact these are just a few of the methods we describe in our eBook “How to Increase Biogas Digester Gas Yields€”. But more importantly the eBook provides solutions rather than just suggested methods, as in the article above.

For more information on How to Solve Biogas Plant Yield Problems click here.