This is the basic process on how our Delaware No. 1 Water Pollution Control Facility Works.
The Sewer Collection System
The CCMUA maintains the regional sewer collection system, 135 miles (215 km) of underground pipes, while each municipality maintains its own internal collection system piping. Individual connections to the regional collection system are not allowed. New individual users must connect to their municipal sewer system. The municipal collection systems were built, before the CCMUA existed, to come to a central location at the municipal treatment plant. The regional system makes use of this centralization by connecting to each municipal system at the site of the former municipal plant. Camden is an exception to this pattern. The Camden City collection system feeds directly into the CCMUA’s main plant (which used to be the city of Camden’s plant). Pipe diameters vary from 16 inches (40 cm) – about the length of a man’s forearm and hand – to 96 inches or 8 feet (2.5 meters) – the height of an ordinary room, or more than enough for a basketball player to stand up in! The system was designed to deal with flow projected for a fully built up County. Additional capacity was included to deal with daily and seasonal variation. People use more water early in the morning, just after waking. Average projected use was multiplied by a “peak factor” to allow enough capacity. The regional collection system, designed, built, and maintained by the CCMUA, is strictly a “sanitary sewer” system – it collects only water discharged into the sewer by homes and businesses. Storm sewers, which collect runoff from rainstorms and melting snow, are kept separate. Most of the system is gravity propelled. (Water runs downhill!) This is the cheapest, most reliable power source for the collection system. Where required by topography (that is, a need to travel uphill), twenty-seven pumping stations raise the wastewater to higher ground to continue flowing to the treatment plant. Corrosion is always a concern with sewer pipes. Corrosion tends to happen because the organic material in the sewage gives off hydrogen sulfide. The hydrogen sulfide reacts with water to form sulfuric acid. The acid eats away at the pipe, eventually forcing repair or replacement. To minimize this problem, the CCMUA’s system includes chemical feed stations at strategic points. Hydrogen peroxide, sodium hypochlorite (bleach), and other oxidizing chemicals are added to the sewage to prevent the formation of sulfides which may eventually lead to corrosion of the sewer pipes if untreated.
The Junction Chamber
Three underground pipes bring influent to the junction chamber, where the three flows mix before entering the Preliminary Treatment Facility. One of the two main pipes brings sanitary sewage from the main regional collection system. The second brings sewage from the Camden City collection system. A smaller pipe brings sewage from North Camden and Pennsauken.
Preliminary Treatment Facility
In the preliminary treatment facility, the wastewater flows through three mechanically cleaned bar screens which catch debris, rags, and large solid objects. After this stage, centrifugal pumps lift the wastewater 35 feet (a little more than ten meters) to begin its trip through the gravity-fed treatment tanks. Next the water enters three round grit chambers, where small stones and sand settle out. Removing sand and larger solid objects helps extend the life of equipment. The grit and screenings are pumped to grit separators and washers where they are cleaned before being trucked to sanitary landfills.
Primary Sedimentation Tanks
Wastewater takes 12 to 15 hours to flow through the primary sedimentation tanks. During this slow passage, solids settle to the bottom of the tanks. These solids, called “primary sludge,” are skimmed off the bottom and held for further processing. Oil and grease scum is skimmed off the top of the primary sedimentation tanks into scum wells. Delaware No. 1 Water Pollution Control Facility has ten primary sedimentation tanks, measuring about 186 feet by 50 feet (57 x 15.24 meters) with an average side water depth of ten feet (3 meters). The primary sedimentation tanks remove 50-60% of the solid contaminants in the wastewater. When the plant was run by the city of Camden, this was all the treatment provided before discharging the wastewater into the Delaware River. The CCMUA purchased the plant and upgraded it to produce cleaner water.
Aeration Tanks
In the aeration tanks, “primary wastewater” coming from the primary sedimentation tanks is mixed with oxygen and secondary sludge – a mixture of microbes and bacteria coming from the final sedimentation tanks down the line. The wastewater spends four hours in the aeration tanks. These tanks are designed to be “bacteria heaven”, with a combination of food (the organic solids in the wastewater) and oxygen. Under these circumstances, bacteria grow and consume the organic solids very quickly. There are eight aeration tanks at Delaware No. 1. Each tank is about 220 feet by 55 feet (67 x 16.76 meters), with an average liquid depth of 15 feet (4.6 meters). Mechanical surface aerators are used to mix the activated sludge, the primary wastewater, and the oxygen.
Final Sedimentation Tanks
In the aeration tanks, the bacteria grow and consume solids. Here in the final sedimentation tanks, these bacteria are allowed to settle to the bottom forming secondary sludge. Secondary sludge is more liquid than primary sludge. It is composed of small particles. About 30% of the secondary sludge is recycled to the aeration tanks. The rest is removed for further processing. Delaware No. 1 WPCF has eight final sedimentation tanks. Each is about 270 feet long by 78 feet wide (82 x 24 meters).
Chlorine Contact Tanks
The final stage of treatment takes place in the chlorine contact tanks. To kill any remaining bacteria, the water is mixed with a solution of sodium hypochlorite. (This is the same chemical found in household chlorine bleach.)
Outfall
The treated wastewater is discharged through an outfall pipe into the Delaware River. It has spent about one full day passing through the treatment plant.
Sludge
The main product produced at the CCMUA’s treatment plant is clean water. But like many manufacturing processes, wastewater treatment creates by-products which must be dealt with. The main byproduct of wastewater treatment is sludge.
What is Sludge?
Sludge is a mixture of solid wastes and bacteria removed from the wastewater at various stages of the treatment process. It can be categorized as “primary sludge” and “secondary sludge.” Primary sludge is about 4% solids and 96% water. It consists of the material that settles out of wastewater in the primary sedimentation tanks, before bacterial digestion takes place. Secondary or activated sludge is much more liquid – about 1% solids and 99% water. Secondary sludge consists of bacteria and organic materials on which the bacteria feed. About 30% of the secondary sludge produced is returned to the aeration tanks to assist with the biological process of sewage treatment. The remaining 70% must be disposed of.
Removing Water
There are various ways to dispose of sludge, but in all cases the first goal is to reduce the water content. Removing water decreases the volume of sludge. This makes storage and transportation much cheaper. If the sludge will be dried, removing water first also cuts costs because there is less water to boil off. The CCMUA has four sludge storage tanks at the Delaware No. 1 Water Pollution Control Facility. These cylindrical tanks are about 69 feet (21 meters) in diameter at the base, and about 32 feet (9.75 meters) high from the base to the peak of the dome on top. At Delaware No. 1 WPCF, sludge is next processed in the Thickening and Dewatering Building. Secondary or waste activated sludge is first thickened in one of three gravity belt thickeners. This thickened waste activated sludge is then mixed with sludge that has been removed from the primary tanks. This mixture of primary and thickened waste activated sludge is then dewatered on seven belt filter presses. Belt presses remove water by essentially squeezing it out of the sludge. Sludge entering the belt presses is 96-99% liquid and has the consistency of dirty water. By the time it leaves the belt presses, the sludge is only 75% water and has the consistency of mud. This is “dewatered sludge cake.” Water removed from the sludge by the belt presses is returned to the beginning of the treatment process.
Disposal
The dewatered sludge is next processed in one of three sludge dryers. The dried sludge is either used as a fuel, replacing coal, in a cement kiln or beneficially reused as landfill cover.