| Effluent | |||
|---|---|---|---|
| Overview | Sludge | Key Terms | References |
Sludge
Created in the either primary or secondary treatment of wastewater, sewage sludge contains materials settled out of the raw influent and solids generated in wastewater treatment operations. The different processes in wastewater treatment schemes create various types of sludge (below), and higher degrees of treatment (secondary and tertiary) create a greater volume of sludge that must be dealt with.
Figure 2. Sludge drying beds.
Grit
The material collected in the grit chamber is not sludge, but still requires disposal. Grit is drained of water and usually trucked directly to a landfill (Davis, 2004).
Primary Sludge
Sludge pumped from the primary clarifiers is odiferous and over 2/3 of the solids content is organic material.
Secondary Sludge
The sludge from secondary treatments contains 0.5 - 5.0% solids of which 90% are organic. With no air supply, it becomes anaerobic and virulent. The temperature can reach 200 degrees Fahrenheit due to the organic activity. This category of sludge will contain inorganic chemical precipitates produced to remove phosphorous by the addition of other chemicals.
Tertiary Sludge
Tertiary sludge characteristics depend on the types of treatments applied. Chemical processes used to remove nitrogen and phosphorous create a chemical sludge that is challenging to treat, handle and dispose of.
Sludge Treatment
Wastewater treatment plants use a portion of the sludge to feed the microorganisms in digesters. If the digesters are anaerobic, microorganisms generated methane can be captured and used. The un-digested sludge can be dewatered in sludge drying beds, as shown in Figure 2, or other techniques. Drying beds can take months or years for the sludge to dry enough for disposal.
Sludge Disposal
Sludge disposal techniques include incineration, land spreading and filling, and the use of sludge to make a product, like soil nutrient (Davis, 2004).
- Incineration produces ash and coats air pollution filters with residue, which must then be disposed.
- Land spreading is the application of wastewater solids to crop and forest lands as fertilizer. This sludge must not contain excess heavy metals like lead and mercury that pollute the soil.
- Landfilling refers to the burial of sludge and other solid materials under at least 0.2 meters of soil (Davis, 2004). It is only applicable for dewatered sludge and solids.
- Dedicated land disposal is appropriate for liquid sludges. These sites are set up only for this application, and are designed so that the liquid will not seep into the surrounding environment. Dried sludge can be sent to these sites, but landfilling can be more cost effective.
Regulations for Sludge disposal are set by the EPA or by states with EPA approval.