Sludge Processing

Sludge Processing

The solids content of processed raw water is much less than the wastewater solids stream.  There are also less environmental concerns with the processing and disposal of solids accumulated at a water treatment plant.  The two primary sources of waste solids are from the sedimentation tank, where chemical coagulation or softening generate solids that are a mix of the process chemicals and the suspended solids from the raw water; backwash water from the media filter is the second source.

The solid materials generated are highly variable in composition due to the variety of chemicals utilized to treat the raw water and the raw water source itself.  As there are a multiple ways of treating water, there are also many ways of processing or discharging the waste materials generated in water treatment.  Many water treatment plants will not treat or process their waste stream at all, but pump them to a wastewater treatment facility for disposal.  Other facilities will separate the solids content as economically as feasible and dispose of the residuals in lagoons, where the water is allowed to evaporate from the retained solids.  Dried solids are then removed for disposal at a landfill site.

Where there is insufficient land area for lagoons, the water treatment facility must remove as much water from its wastes as possible to make it economically feasible to transport and dispose of the solid wastes at a landfill site.  Because of the unique characteristics of each plant’s waste stream, there isn’t a standard process method utilized.  Each plant will have its individual process method to deal with the waste stream.

The typical sludge processing steps include:

1. Sludge Thickening
2. Dewatering
3. Disposal

Sludge Thickening

Settled solids from the sedimentation tank are pumped to a gravity sludge thickener or clarifier where solids are allowed to settle out of the liquid.  In many facilities, thickening agents or polymers are added to facilitate the solids separation process.  The supernatant or process water flows into weirs at the top of the clarifier and are pumped back to the beginning of water processing, where it is reprocessed.  The thickened sludge is either pumped out to a lagoon for drying or to an additional dewatering process to remove additional water prior to transporting the solids or sludge to landfill.

Dewatering

Dewatering is the final stage prior to sludge disposal. As in a wastewater treatment plant, the goal is to economically remove as much liquid as possible from the sludge prior to disposal.  The two most common methods of dewatering in a water treatment plant are centrifugation and pressure filtration. Polymers can be added to the process to enhance dewatering capabilities.

Supernatant that is removed in the dewatering process is returned to the beginning of the treatment plant for reprocessing.

Disposal

Sludge is generally disposed as landfill. It can also be incinerated and the remaining ash disposed as landfill.  Economics and environmental regulations will be the primary drivers in what disposal method an individual water treatment plant uses.