Both mesophilic and thermophilic anaerobic digesters are currently being utilized to treat sludge derived from more than typical municipal sewerage sources. Wastewater treatment plants are accepting septage and sludge from food waste and industrial contributors routinely today. Receiving these other sources of waste which are extremely high in volatile solids is a source of significant income for the utilities. However, high volatile solids loading into the digesters can cause out-of-balance biochemical conditions in the digesters. High volatile fatty acid to alkalinity ratios and low methane production are but a few of the repercussions of these imbalanced conditions. These process upsets result in high acetic, propionic and other intermediate fatty acids. These exposures, when sufficiently prolonged, can cause degradation to the polymers in some protective coating systems commonly used successfully in the past.
This paper will present evidence of this type of attack based on both field experience and laboratory testing data. Recommendations will be made for future material selection criteria and testing to avoid premature lining failures in anaerobic digesters in the future.
The reader is likely familiar with the Activated Sludge Process used by the majority of wastewater treatment plants to convert dissolved organic solids to suspended solids so those solids can be settled out of solution as sludge or biosolids. This is accomplished in conventional treatment plants consisting of primary clarifiers, some form of aeration such as ambient aeration basins, pure oxygen reactors, or trickling filters. The wastewater is mixed with large quantities of microorganisms and air. The microorganisms utilize the organic waste matter for food and the oxygen in the air to burn a portion of this food to form carbon dioxide and water. This provides energy to the microorganisms. Because the microorganisms take-on so much energy from this oxidation, they grow rapidly. This results in a large percentage of the organic cells to be converted into different cellular forms. This portion of the converted waste is not stabilized, but takes on the form of settleable solids or sludge. This sludge then presents a disposal challenge as it contains pathogens and other organic substances which are reactive and unstable. The activated sludge process obtains its name from the activated sludge called return activated sludge or RAS (taken from the secondary clarifiers and pumped into the aeration tanks) to make sludge collectible so it can be handled while the water is made cleaner. Activated means active with bacteria.
