SOLIDS HANDLING PROCESSES
When a wastewater treatment plant is operating properly, most of the solids in the wastewater will be removed at the plant while the water itself will be discharged as effluent to the receiving waters. These solids, known as sludge, must be stabilized and reduced in volume before they can be reused or disposed of economically and safely. The processes used to stabilize sludge and reduce its volume are known as solids handling. Solids handling incorporates four functions; thickening, digestion, dewatering and sludge disposal/ re-use.
The sludge produced by wastewater treatment processes can be highly variable. Sludge from the secondary clarifiers of a treatment process such as extended aeration activated sludge is already partially stabilized while the sludge produced by primary clarifiers is unstable, odorous and full of pathogenic bacteria (raw). Solids must be thoroughly stabilized in order to avoid problems such as odors, human exposure to pathogenic organisms and attraction of vectors (mice, dogs, birds). Although stabilization can be done chemically and thermally, biological digestion is the process most often used to stabilize sludge generated by wastewater treatment plants. We can think of digestion as “breaking down” sludge into simpler components through the action of microorganisms. During digestion, a portion of the sludge is converted to gasses such as CO2, methane and water vapor. This reduces the volume of the sludge and, more importantly, reduces the volatile solids content. It is through a reduction in volatile solids content that the sludge is stabilized during digestion. Digestion can take place under aerobic or anaerobic conditions.
ANAEROBIC SLUDGE DIGESTION
Anaerobic digestion converts wastewater solids such as primary sludge, secondary sludge and scum into a substance that is relatively odor free, dewaterable and capable of being disposed of without causing serious problems. Pathogens are greatly reduced when the process is operated correctly. Anaerobic digestion can also reduce the volatile content of the sludge by 30 – 60% and produce valuable methane gas as a by-product.