Wastewater – Philadelphia Water Department

PWD manages and treats Philadelphia wastewater, which includes the upkeep of more than 3,000 miles of sewer. We use a system of combined sewers in approximately half of the city’s neighborhoods and a separate sewer system in the other half.

Getting it Clean: Wastewater Treatment

Combined Sewer System

In areas with combined sewers, a single pipe carries both stormwater from streets, houses, and businesses as well as wastewater from houses and businesses to a wastewater treatment plant. Many of the older sections of our city are served by a combined sewer system.

Separate Sewer System

In areas with separate sewers, one pipe carries stormwater to the city’s streams while another carries wastewater to a wastewater treatment plant.

Wastewater Treatment Process

Preliminary Treatment

Wastewater flows through several sets of racks and screens where debris is removed. The trash is collected from the racks and screens and is hauled to a landfill off-site.

Next, wastewater flows through basins that allows heavy particles, called grit, to settle. Grit is composed of materials such as sand and gravel that enter the plant through storm sewers.

Primary Treatment

Primary treatment physically removes many of the solids suspended in the wastewater as it flows slowly through primary sedimentation tanks. Solids that are heavier than water sink to the bottom, while scum and grease float to the top. The solids on the bottom are scraped out and pumped to digesters, while the scum and grease are pumped to concentration tanks and then landfilled off-site.

Secondary Treatment

Secondary treatment uses biological processes to remove organic materials – materials from living organisms – still dissolved or suspended in the wastewater after primary treatment. The wastewater is combined with microorganisms, or “microbes,” that decompose sewage in nature. The wastewater, microbes, and air are mixed together in aeration tanks. This creates an environment where microorganisms can “eat” the pollution. The wastewater from the aeration tanks then flows to the final sedimentation tanks where remaining, finer solids settle out.

Just before reaching the river, the wastewater is mixed with enough chlorine to kill any remaining disease-causing organisms. The EPA requires 85% removal of suspended solids from wastewater. The treated water that leaves the plant – called effluent – is even cleaner than that.