The WWTP provides preliminary, primary, and secondary treatment of municipal wastewater. The first process is preliminary treatment. This step consists of screening which removes rags, sticks, and other large items from the wastewater stream by means of a bar rack. The next step is grit removal, as the wastewater enters the chamber the flow decreases to a rate of 2 feet per second (fps) which causes sand and other inorganic materials to settle out of the stream. The air from the grit blowers keeps organic materials, such as food wastes and human waste, in suspension where it progresses to the next treatment process: primary treatment. The grit is pumped out of the chamber then is removed by a mechanical classifier and disposed of in a landfill.
Primary treatment occurs in three circular 125 ft. primary clarifiers. The wastewater from the grit chamber enters the primary clarifiers where it has a residence time of approximately 2 hours. During this process, solid materials settle due to gravity. The settled solids are collected by sweeping mechanisms at the bottom of the tanks where they are pumped to the gravity thickeners, which are part of the sludge handling process. Approximately 50 to 60% of the suspended solids are removed during primary clarification. From here, the wastewater begins the secondary treatment phase.
During secondary treatment, two processes occur. The first process is the activated sludge process. This occurs within aeration tanks where bacteria are grown and cultured. The bacteria use oxygen and feed on the remaining suspended solids and dissolved organic matter. Air is introduced to the aeration tanks to assure sufficient oxygen is available to allow the bacteria to survive. From here the wastewater enters the secondary clarifiers. Like the primary clarifiers, there are three circular 145 ft. secondary clarifiers. The secondary clarification process allows the bacteria from the aeration basins to settle out via gravity. The solids from the secondary clarifiers are either returned to the aeration basin and/or transferred to the thickening centrifuges. At this point the wastewater is disinfected prior to discharge.
During the disinfection process, the wastewater is chlorinated and dechlorinated. Sodium hypochlorite is added to the wastewater and travels in a maze-like pattern in the chlorine contact chamber to allow for a minimum of one-hour contact time between the chlorine and the wastewater. Because chlorine can be harmful to aquatic life, the wastewater is dechlorinated prior to final discharge. Sodium bisulfite is added to the wastewater and a minimum of two-minutes contact is necessary to allow the chlorine to be neutralized into harmless salts. At this point the water becomes plant effluent and is discharged to the Merrimack River.
The biosolids collected in the primary and secondary clarifiers are transferred to the sludge handling process, which consists of thickening, dewatering, and eventual incineration in the Fluidized Bed Incinerator (FBI). The primary sludge is thickened in the gravity thickeners. The gravity thickeners consist of three tanks 50 ft. in diameter. By means of gravity, further solid/liquid separation occurs to a point where the solids content in the thickeners is approximately 4% to 6% solids.
The waste activated sludge is sent to one of three thickening centrifuges. The sludge is thickened to approximately 3% to 4% solids. The thickened waste activated sludge and the thickened primary sludge are pumped to an inline mixer where they are blended. The blended thickened sludge is then pumped to one of three dewatering centrifuges for dewatering. A screw mechanism within the center of the spinning centrifuge moves the sludge as solids are separated from liquid. The sludge has a solids content of approximately 24% to 26% when it exits the centrifuge. At this point the sludge is sufficiently dewatered and it is sent to a Sludge Silo for storage. The stored dewatered sludge is then sent to the FBI for incineration once the level in the silo is 75% of its capacity. If the incinerator is not in operation the sludge is then sent to sludge trailers for off-site disposal. In 2023, the facility generated 4,424 dry metric tons of biosolids.