The anaerobic digestion is a collection of processes by which the bacteria break down the organic matter in the absence of oxygen. This process involves four stages:
The overall process can be described by a simple chemical reaction were the glucose becomes carbon dioxide and methane:
C6H12O6 –> 3CO2 + 3CH4
One of the most important things is to have the biological process under control. A small problem of temperature, pH, acids, overfeeding, underfeeding can cause gas production to be reduced.
To avoid these problems we take daily samples and test for the important parameters such as FOS/TAC, organic acids, ammonia, pH, temperature, phosphates, TIC, dry matter.
With the FOS/TAC analysis we can measure the amount of volatile organic acids and the total inorganic carbon. The ratio between these will give us an indication on whether we are overfeeding or underfeeding the system. Each plant will operate with a different ratio, it takes time to establish the working paramaters. Once you have these it enables the plant operator to fine tune the daily feed requirements.
The pH is another crucial parameter. Methane production occurs in a small range. The biogas production and the methane concentration will decrease if the system is out of the optimum range; 7.4 to 8.0 in our system.
Our plant operates in the mesophilic temperature range which is between 37 and 46 C. If from one day to the other the temperature changes rapidly the performance of the bacteria will be affected and the gas production will be affected.
The Hach Lange technology this gives us a big range of digestate analysis. The boxes and cuvettes are colour-coded for fast and easy parameter and range recognition.
Hydrogen sulphide is a colourless, flammable, extremely hazardous gas with a “rotten egg” smell, one of the consequences of biogas production. To keep the amount of hydrogen sulphide to as few parts per million, we add ferrous chloride to our system. Since we started adding ferrous chloride, the H2S has been reduced from 400 ppm to 10-40 ppm.
The dry matter content is another important parameter to analyse and to keep under control. Every day the plant is fed with 300 tons of silage to achieve the amount of gas required. Some plants work with a high dry matter content and others, like our plant, work with a lower dry matter content (6.5-7.5%). To achieve a consistant dry matter we use a separator to split the liquid from the solids and recirculate the liquid fraction back through our hydroliser. A high dry matter content can cause problems inside the digesters because the mixers in the tanks need more power to move the digestate. Using the Moist xLAB device we can measure the dry matter content in the liquid and also in the silage which is delived to the plant.
Other paramaters tested daily are those of ammonium, total nitrogen, total organic carbon, organic acids, COD and phosphates.