What is a POTW?
POTW: Publicly Owned Treatment Works
In short
A POTW (publicly owned treatment works) is a sewage treatment system owned by a state, municipality or other public body. Under the Clean Water Act and 40 CFR 403.3 the term covers the treatment plant and the sewers, pipes and pump stations that convey wastewater to it. A plant that a company owns to treat its own wastewater is not a POTW.
Klarifi tracks 11,774 POTWs across 50 states, treating a combined 15,264 million gallons per day for about 145 million residents, each with its NPDES permit, flow and compliance record.
What counts as a POTW
The definition comes from section 212 of the Clean Water Act and the EPA’s pretreatment regulations. It covers any device or system used to store, treat, recycle or reclaim municipal sewage or liquid industrial waste, and sewers only if they convey wastewater to the treatment plant. The owner can be a city, town, county, sanitary district, regional authority, tribe or state. A plant run by a private contractor for a public owner is still a POTW.
The EPA counts roughly 16,000 POTWs. They serve about four in five US residents; the rest rely on septic systems.
The rules a POTW works under
Every POTW that discharges to surface water needs an NPDES permit. The federal secondary treatment standard (40 CFR 133) is the floor: a 30-day average of no more than 30 mg/L of BOD and 30 mg/L of total suspended solids, at least 85 percent removal of both, and a pH between 6 and 9. States add limits for ammonia, nutrients, bacteria, metals or toxics where the receiving water needs them.
A POTW with a design flow above 5 MGD that receives industrial wastewater which could pass through or interfere with treatment must run an approved pretreatment programme: it permits its significant industrial users, sets local limits, and inspects and samples them. Regulators can require the same of smaller POTWs. Sewage sludge is regulated separately, under 40 CFR 503.
POTW, WWTP, WRF: which term to use
POTW is the legal term, used in permits, regulations and enforcement documents. WWTP (wastewater treatment plant) and STP (sewage treatment plant) describe the facility itself, whoever owns it. WRF and WRRF (water reclamation facility, water resource recovery facility) are the names many utilities now prefer. In EPA data the same plant appears under its NPDES permit number, which is the reliable way to look one up.
Find POTWs by state
The Klarifi facility directory lists the POTWs in each state with average and design flow, permit number and expiry, treatment technology and a three-year violation count, and ranks the largest plants nationally and by state.