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.
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).
Sludge pumped from the primary clarifiers is odiferous and over 2/3 of the solids content is organic material.
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 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.
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 techniques include incineration, land spreading and filling, and the use of sludge to make a product, like soil nutrient (Davis, 2004).
Regulations for Sludge disposal are set by the EPA or by states with EPA approval.