3.4.1 Total and Volatile Solids

Total Solids (TS) and Volatile Solids (VS) can be described in two ways: as a percentage or as a mass per unit volume. When expressed as a percentage, it tells us the proportion of solid material in a given volume of liquid or slurry. For example, if the TS is 5%, it means that 5% of the volume is made up of solids.

On the other hand, TS can also be measured as a mass per unit volume, typically in milligrams per litre (mg/L). This measurement tells us the weight of solids present in a specific volume of liquid. For instance, if the TS is 100 mg/L, it means that there are 100 milligrams of solid material in every litre of the liquid.

Low levels of TS can pose a challenge for anaerobic digestion. This is because it means there is less organic material in a given volume, which increases the required capacity of the digesters. To address this issue, it is advisable to use water-saving flush systems in toilets that are connected to biogas plants.

Both ways of expressing TS provide valuable information about the concentration of solid material in a liquid sample, allowing us to assess the composition and characteristics of the substance at hand.

On the other hand, the upper limit for TS content can lead to challenges related to fluidity and viscosity. Highly solid-rich slurries tend to have increased viscosity, which can result in higher frictional resistance within the pipes, leading to potential clogging and blockages. Further, transporting sludge with high TS content may require additional energy. Generally, sludge with TS content below 10% would not require pumping, as it has sufficient fluidity for gravity flow.

The increased concentration of solids also implies a higher concentration of organic matter per volume of the digester. It is relevant because each digester type has a specific capacity to process organic matter, which is expressed as the organic loading rate. This capacity defines the maximum amount of organic matter that can be effectively treated in the digester. The organic load and its significance will be further discussed in the chapter on the organic loading rate.

Volatile solids measure the overall organic matter content of feedstock and are often used to assess specific biogas yield and biogas production. These measurements are essential in evaluating the effectiveness of the anaerobic digestion (AD) process for sewage sludge. By analysing the absolute volume of biogas produced and the biogas yield, one can gain valuable insights into the efficiency and performance of the AD system.