Tag Archives: Biogas

Anaerobic Digestion

pH – It’s All About Balance

Posted on by Elliot Volpe
31
Aug

The pH balance in the anaerobic digestion process is critical. If the pH level is unstable, it can lead to underwhelming biogas yields, and often leads to odour from the process. There is a sweet spot for the pH levels to enable sufficient yield.

In an experiment conducted on grass silage and its biogas yield, dependent on process variables, it showed there is desirable combinations in the systems. These optimum points are largely down to the temperature and pH levels in the digester.

A mesophilic process normally sees the temperature set between 30-38˚c. In this experiment in particular the temperature was set at 35˚c. The pH levels were tested at 3 different levels. These were: 6.5, 6.8 & 7.2

The established, best combination for a mesophilic process was a pH of 6.8 at 35˚c. This combination saw a biogas yield of over 60%.

Multiple pH levels and temperatures were then trialed to find the best overall yield. The highest yield from the experiment as a whole, was achieved from having a temperature of 45˚c, at a pH of 6.5. It is thought that the reason for this is that the pH levels and temperature allowed for rapid growth of the bacteria that produce the biogas.

Continuous monitoring of the pH level in the digester is essential. It can be the difference between producing high amounts of biogas or unsatisfactory amounts. Less gas produced, equates to a less profitable process.

More on pH in the process

 

Reference – Sibiya, Noxolo & MUZENDA, EDISON & Tesfagiorgis, Habtom. (2014). Effect of Temperature and pH on the Anaerobic Digestion of Grass Silage. 

Anaerobic Digestion

How to Achieve 23% More Biogas Yield

Posted on by Elliot Volpe
27
Aug

Biogas yield is one of the main priorities in any Anaerobic digestion process. The more gas your digester can produce, the more gas you can use to generate electricity, or feed back into the grid. In short, more gas produced = more money made.

Sisal fibre was used in an experiment to explore the link between substrate degradation and biogas yield in a process. This was to see how the industry can get the most from their AD system.

The report shows that the sisal fibre degraded in the digester went from 31% to 70% in the experiment. Biogas yield from this particular experiment increased 23% m3 CH4/kg.

How was this done? Maceration.

It has been proven that maceration of the feedstock can lead to quicker degradation of the substrate, in turn, producing a higher gas yield from the process. In this experiment, the original substrate used was raw and untreated. The substrate was then macerated to 2mm to see the difference in process outputs.

Of course, macerating feedstock to 2mm in an AD system may not always be practical. However, this experiment proves the importance of macerating feedstock to get the most out of your process.

More on Biogas Yield

Reference – Performance Improvement for Sisal Waste Anaerobic Biodegradation by Digester Redesign and Feed Size Reduction (scirp.org)

Anaerobic Digestion

What Is Anaerobic Digestion?

Posted on by Elliot Volpe
13
Aug

What Is Anaerobic Digestion?

The process of anaerobic digestion dates as far back as the 19th century. The process itself has evolved over time from open lagoon systems, through to creation of closed tanks to get optimum yield from the process.

So, what is it? Anaerobic digestion is the breakdown of material in the absence of air. Feedstock is fed into the digester tanks and left to sit there at a set temperature. Over time the material goes through different stages and gets broken down by microorganisms, with the result being a methane rich biogas.

Once the reaction has taken place, the methane can then be used to power a combined heat and power (CHP) unit, which often is used to generate electricity to allow sites to be self-sufficient. Any unused electricity is sold back to the grid. Some sites take the methane and export it straight back into the grid as soon as it is extracted from the process.

As a bonus, the digestate that is left in the system once the reaction has occurred is a nutrient rich fertiliser, which farmers can then spread on their land.

How Does AD Cut Emissions?

Anaerobic digestion is a renewable energy system, as the feedstock can be replenished at no harm to the environment.  Often, cow manure is used for the feedstock, which is the first stage of the process. Then, at the end of the process the nutrient rich digestate is then spread onto the fields in which the cows eat the grass from. This means it is seen as almost net zero carbon emissions, making it a popular choice in the drive for renewable and clean energy.

In some cases, food waste is used as a feedstock. This means that the food waste that would usually decompose and emit harmful gases into the atmosphere is now being used as the input to a system that can then be turned into energy.