Wastewater Treatment Process

There are three categories of municipal collection systems; storm sewers, sanitary sewers and combined sewers. Systems that convey stormwater runoff and other drainage directly to surface waters while excluding sanitary wastes are considered storm sewers. Systems that receive wastewater from residential, commercial or industrial sources along with relatively small amounts of groundwater infiltration or stormwater inflow are considered sanitary sewers. Sewers that convey both sanitary wastes and stormwater are referred to as combined sewers. The operation and maintenance of storm sewers are the responsibility of individual municipalities and are, therefore, beyond the scope of the BCUA’s water pollution control responsibilities. Combined sewers are present in Fort Lee, Hackensack, and Ridgefield Park. While combined sewers are also a municipal responsibility, the BCUA treatment plant does receive discharge from combined sewers.

The BCUA operates a system of gravity sewer lines, pumping stations and forcemains that receive the discharge of wastewater from the individual municipal collection systems and transports the wastewater to the treatment plant in Little Ferry. A gravity sewer is sloped downward so that the wastewater flows toward the treatment plant. When the topography is such that the construction of gravity sewers creates very deep sewer lines, a pumping station is built to lift the sewage to a level that will allow it to once again flow by gravity. Typically, pumped sewage is discharged into a forcemain, which is a pressurized line that eventually feeds into a gravity sewer.

The BCUA does not own or operate the local collection systems. Traditionally, each Bergen County municipality independently constructed its own sanitary sewer system and sewage treatment plant when necessary for public health reasons. As such, many of the municipal collection systems predate the formation of the BCUA in 1947. Generally, the materials and methods used for sewer construction were greatly improved after 1950. Pipes with better and fewer joints were available and became the standard for the industry, making sewers more impermeable. Older systems allow groundwater to enter the sewers, which is referred to as infiltration. It was also standard practice in the past to connect stormwater conveyance systems, such as roof leaders and sump pumps, to the sanitary sewers so that local flooding problems could be alleviated. This is known as inflow. Infiltration and inflow present engineering and regulatory challenges for the BCUA and for many municipalities within the BCUA service area.

The BCUA began constructing its collection system in 1948. The system consists of the trunk and intercepting sewers which convey the wastewater flow from the municipal collection systems to the treatment plant in Little Ferry. Each subsequent construction phase connected additional municipalities to the system. With the completion of the most recent expansion in 1992, the collection system now encompasses approximately 108 miles of sewer lines and nine pumping stations serving all or part of 46 municipalities in Bergen County.

Three separate trunk sewer systems collect and transport wastewater to the BCUA treatment plant. The first trunk sewer constructed by the BCUA was the Overpeck Trunk Sewer which extends from Little Ferry to Tenafly. Interceptor sewers were also constructed which allowed 12 municipalities in the Overpeck Valley to abandon their sewage treatment plants and discharge wastewater to the BCUA. This construction was completed in 1951. The next expansion of the service area occurred through construction of the Hackensack Valley Trunk Sewer. This second stage, completed in 1964, extended from Little Ferry to Westwood and added 16 municipalities to the system. The service area was also expanded to the southwest through the construction of the Southwest Trunk Sewer which extends from Little Ferry to Hasbrouck Heights. This third trunk sewer system was completed in 1972. Additionally, two major subsystems were completed in 1976 extending service to the Pascack Valley and Northern Valley areas of Bergen County. Both of these subsystems discharge to the Hackensack Valley Trunk Sewer.

Measuring the amount of wastewater produced by each municipality is important for both operating and billing purposes. To accomplish this task, the BCUA has constructed 166 metering chambers throughout the service area. The typical BCUA metering chamber is an underground concrete vault that contains a metering device. Metering data is transported via telephone lines to the BCUA’s User Charge Department automatically.

The nine BCUA pumping stations were constructed on the outer reaches of the service area except for the largest station which is located in Harrington Park and serves the Northern Valley region. All BCUA pumping stations are designed to run automatically and do not require personnel on a 24-hour per day basis. Each station contains its own electrical generating facilities in the event that normal power is lost. The pumping stations require periodic cleaning of the wet wells to remove the buildup of grit and other materials that are not removed by the pumps.

The Northern Valley extension of the BCUA collection system circumnavigates the Oradell Reservoir owned by United Water Inc. Five of the nine BCUA pumping stations are located within the reservoir watershed. Since the discharge of wastewater into the reservoir has the potential to contaminate a significant portion of the drinking water supply for Bergen County, the pumping stations, forcemains, and other parts of the collection system located in this region receive proportionally greater attention due to the impact that sewage overflow would have on this system.

The three main collection system trunk sewers merge at the Water Pollution Control Facility in Little Ferry in a 40-foot deep chamber. Before further processing, the wastewater entering the plant must be screened to remove rags, glass, rocks, and other large debris. Screening is performed in two separate screening chambers that may be operated independently. The screens consists of vertical steel bars spaced to catch debris of a certain size. Mechanized rakes continuously scrape the screens to remove the debris and deposit the material into hoppers that press the liquid from the material. The screenings are then disposed of as a solid waste. The quantity of screenings increases dramatically during wet weather induced high flows.

After the influent passes through the screens, the flow enters two pumping stations which lift the wastewater from the screening chamber to approximately 10 feet above ground level so that it may flow through the treatment process by gravity. The pumps in each pumping station are separated from the wet wells so that maintenance can be performed without removing the pumps. The electric motors which power the pumps sized are up to 500 horsepower. The two pumping stations were constructed separately. The older station has seven pumps with a combined capacity of 230 mgd and the newer station has five pumps with a combined capacity of 200 mgd. The treatment plant can pass a maximum flow of approximately 190 mgd before bypassing may be necessary to protect the surrounding areas from basement flooding, and the treatment plant capital equipment from damage. Three turbo generators, 2,500 kW each, are continuously on standby to provide power in case of a utility company emergency.

After the wastewater is pumped to 10 feet above ground level, it flows into a discharge channel that leads to the grit collector. Grit consists of sandy materials and other particulates that readily settle from the wastewater. Although some grit may be discharged to the sewer system by users, most grit is washed into the system along with groundwater infiltration. Since grit is inorganic, it cannot be removed in the biological treatment processes. If it is not removed prior to biological treatment, it accumulates in the process units, particularly the sludge digesters, and tends to cause excessive wear on the equipment. The grit is allowed to settle in a grit tank by slowing the velocity of the wastewater flow to approximately one foot per second. The inorganic grit settles at this velocity, but the organic material requiring further treatment does not. The grit is removed from the tanks and washed to remove residual organic material. As with screenings, the grit is disposed of as a solid waste. The BCUA currently has four units ranging from 35’ to 45’ in diameter.

The treatment performed prior to this step of the process is intended to remove materials that could damage equipment and impair the downstream processes. Primary settling represents the first step of treatment intended to abate water pollution. Primary settling is merely a physical separation of solids from the wastewater. After grit removal, the wastewater flows into the primary settling tanks where the flow velocity is further reduced and the suspended material is allowed to settle to the bottom of the tanks. Approximately 50% of the suspended solids and 35% of the BOD are removed in this unit process. The settled material, referred to as primary sludge, is pushed by automatic sludge collection equipment into a hopper from which the sludge is pumped to the sludge thickeners. The treatment of this sludge is discussed separately. As part of the primary treatment, floatable materials such as oil and grease that rise to the surface of the tanks are skimmed and commingled with screenings and grit for removal to an out-of-state landfill. Lime is used for odor control, if necessary. There are currently sixteen tanks with two bays each, ranging in length from 100’ to 115’ long and 29’ to 33’ in width. Each tank is 7.5’ deep.

Because of the high BOD loads that remain in the wastewater following primary treatment, further treatment must be provided before the effluent may be discharged to the Hackensack River. The BCUA employs a biological treatment system known as the activated sludge process to achieve secondary treatment. During this process, the wastewater flows into an aerated and agitated tank containing a complex mixture of bacteria, fungi, protozoans, and other microorganisms which are referred to collectively as the biomass. The dissolved and suspended organic matter in the wastewater serves as a food source for the biomass which the organisms use to grow and reproduce.

Sufficient air must be provided to supply the biomass with the oxygen necessary for respiration. If too little air is introduced into the aeration tanks, the biomass will use anaerobic respiration to metabolize the organic matter, producing foul odors and poor effluent quality. The BCUA introduces air into the process tanks using one or two of its five 45,000 cfm capacity blowers.

The BCUA uses a variation of the activated sludge process known as contact stabilization. In the first step of this process, the wastewater is brought into contact with the biomass for a short period of time, in which the biomass absorbs the soluble BOD. The biomass is then settled and introduced into a stabilization tank where it is aerated for a longer period of time. In this step, the organic material is fully oxidized and the volume of the biomass increases.

After the wastewater and biomass have been aerated for a sufficient period to allow the soluble BOD to be incorporated into the cells of the biomass, the mixture flows to the final, or secondary settling tanks. Since these tanks are not aerated or agitated, the biomass is allowed to settle. The remaining effluent, which by this point in the process appears quite clear, is ready to be disinfected and discharged to the Hackensack River. The settled biomass is either reintroduced into a contact tank to serve as the inoculum for the process, or is wasted. Wasting the biomass, which is now designated as secondary sludge, refers to the removal of this material from the treatment process for final disposal. The wasted secondary sludge is pumped to the gravity thickener tanks and/or thickening centrifuge.

The BCUA utilizes thirteen aeration tanks, typically six as stabilization and seven as contact tanks. The aeration tanks are 31’ wide, 300’ long, and 15’ deep. There are sixteen secondary sedimentation tanks 37’ wide, 170’ long, and 12’ deep.

After final settling, the supernatant from the secondary settling tanks flows into the chlorine contact tanks for disinfection. Disinfection by sodium hypochlorite solution is intended to kill or inactivate the pathogenic bacteria, viruses and protozoan cysts commonly found in wastewater. Disinfection is the critical step necessary to insure that waterborne diseases are not spread through the discharge of treated sewage. The BCUA’s NJPDES Permit contains fecal coliform limits of 200/100 ml 30-day average and 400/100 ml during a seven day average.

Chlorination by chlorine or sodium hypochlorite is one of the most commonly used methods for the destruction of pathogenic organisms in wastewater. However, residual chlorine compounds in wastewater can also be toxic to aquatic life forms. Therefore the BCUA utilizes sodium bisulfite to reduce the level of chlorine produced oxidants (CPOs) in the effluent to a maximum of 0.1 mg/l.

Sodium hypochlorite and sodium bisulfite solution is delivered in bulk in tank trucks and stored in four fiber glass reinforced 8,000 gallon capacity tanks. The solutions are fed by positive displacement metering pumps that are controlled by a chlorine residual signal.

Just prior to discharge and measured by parshall flume, the flow is sampled at each of the BCUA’s four outfall channels.