PRIMARY - SECONDARY - TERTIARY TREATMENT OF SEWAGE INFLUENT
P. 9256 Federal Register / Vol. 58, No. 32 / Friday, February 19, 1993 / Rules and Regulations
Municipal wastewater treatment works may use one or more levels of treatment (i.e., primary, secondary, or
tertiary) to clean this wastewater. Each level of treatment provides both greater wastewater cleanup and greater
amounts of sewage sludge.
Primary treatment processes remove the solids that settle out of the wastewater by gravity. This generates 2,500
to 3,500 liters of sludge per million liters of wastewater treated. Primary sludge contains 3 to 7 percent solids, 60 to
80 percent of which is organic matter. The water content of primary sludge can easily be reduced by thickening or
by removing water.
Secondary treatment produces a sludge generated by biological treatment processes. Biological treatment
processes (e.g., activated sludge systems, trickling filters, and other attached growth systems) utilize microbes to
break down and convert the organic substances in the wastewater to microbial residue. These processes remove
up to 90 percent of the organic matter in the wastewater and produce a sludge that typically contains from 0.5 to 2
percent solids. These solids are generally more difficult to de-water than primary sludges. The organic content of
the solids ranges from 50 to 60 percent. Secondary treatment processes increase the volume of sludge generated
over primary treatment by 15,000 to 20,000 liters of sludge per million liters of wastewater treated.
Advanced wastewater treatment processes, such as chemical precipitation and filtration, produce an advanced
and tertiary sludge. Chemical precipitation uses chemicals to remove organics and nutrients and to separate the
solids from the wastewater. Characteristics of these sludges vary, depending upon the type of advanced treatment
process used and the type of wastewater entering the treatment process. Since these sludges typically contain
considerable amounts of added chemicals, the solids content will vary from 0.2 to 1.5 percent, while the organic
content of the solids will be in the 35 to 50 percent range. Tertiary treatment increases the volume of sludge
generated over secondary treatment by another 10,000 liters of sludge per million liters of wastewater treated.
Unprocessed sewage sludge contains from 93 to 99.5 percent water, as well as the solids and dissolved
substances that were present in the wastewater or that were added or cultured by the wastewater treatment
process. While virtually all sewage sludge contains nutrients (e.g., nitrogen, phosphorus) and significant numbers
of pathogens (e.g., bacteria, viruses, protozoa, and eggs of parasitic worms), some sludges also contain more
than trace amounts of organic Chemicals (e.g., chloroform) and inorganic chemicals (e.g., iron). These pollutants
come from domestic wastewater, from the discharge of industrial wastewater to municipal sewers, and from the.
runoff from parking lots and lawns and fields where fertilizers and pesticides were incorrectly applied.