The sewer collection system in Concord delivers wastewater from residential, commercial, and industrial properties to the Hall Street Wastewater Treatment Plant or the Penacook Wastewater Treatment Plant. Concord General Services’ Wastewater Treatment Division takes special care in assuring that the wastewater receives a rigorous cleaning process before leaving the treatment facility.
Wastewater treatment plants use a biological process to remove organic matter from wastewater before it is discharged into the natural environment. The organic matter is then processed to be returned to land as natural nutrients.
Hall Street Wastewater Treatment Plant
1. Preliminary Treatment
Wastewater enters the wet well of the Hall Street Wastewater Treatment Plant at the influent building and is pumped to the main influent channel in the influent building. The wastewater flows through bar screens that remove course material from the waste stream, including rags, sticks, and large plastic items. The wastewater then flows into an aerated grit tank. These large tanks slow the speed of the water and allow for heavy inorganic material in the wastewater to settle to the bottom of the tank to be removed for solids processing.
2. Primary Clarification
There are two 742,000 gallon primary clarification tanks at the Hall Street Wastewater Treatment Plant. Wastewater enters the primary clarifier and allows for solids to settle to the bottom of the tank for 4-6 hours. The solids are collected by scrapers at the bottom of tank that push the sludge material to a collection pit at the center of the clarifier. The sludge is pumped from this pit to a sludge holding tank for processing.
3. Sludge Processing
Sludge is transferred to processing tanks where it is heated and mixed to create stabilized biosolids that are recycled for beneficial purposes, such as fertilizing and conditioning agricultural land.
4. Secondary Treatment
As the sludge is carried away in one direction, the wastewater flows from the primary clarifier to the intermediate pump station wet well for secondary treatment. The wastewater is mixed with return activated sludge (sludge post secondary clarification) and pumped to one of two biotowers. Wastewater is constantly pumped over the plastic media material in the biotowers to cause a biofilm to grow. The biofilm consists of bacterial organisms that use the organic material within the wastewater as a food source. This biological process naturally removes organic material from the wastewater. The wastewater then flows to one of two aeration basins. Oxygen levels are increased to encourage growth of the beneficial bacteria to assist in the natural removal of the remaining organic materials.
5. Secondary Clarification
The Hall Street Wastewater Treatment Plant has three secondary clarifiers, only two are typically in service. Wastewater flows into the secondary clarifiers where the bacteria material is separated from the treated wastewater by settling for 8 to 12 hours. The settled bacteria is either returned to the intermediate wet well (during secondary treatment) and mixed with the primary clarifier effluent discharge, or removed from the system.
The treated water flows from the secondary clarifiers to the chlorination building. Chlorine (sodium hypochlorite) is added to the treated water for disinfection. The water travels through a series of maze-like structures beneath the building to allow for adequate contact time for the chlorine to kill most of the harmful bacteria. Since chlorine can be harmful to aquatic life, the chlorinated effluent water is neutralized using sodium bisulfite.
Penacook Wastewater Treatment Plant
1. Preliminary Treatment
Wastewater from Penacook enters the Penacook Wastewater Treatment Plant at the 25 foot-deep wet well. The wastewater is pumped from the wet well and flows through a mechanical step screen to remove any course material that is in the waste stream, including rags, sticks, and large plastic items.
2. Sequencing Batch Reactor
The Penacook Wastewater Treatment Plant is equipped with two sequencing batch reactors (SBR). One of the SBRs is accepting in-flow at all times. The SBR process treats the wastewater through a series of stages and utilizes naturally occurring bacteria to breakdown the organic material in the wastewater. The SBR then enters a settling stage that allows the bacteria and inorganic material to settle to the bottom of the tank. The clear treated water is decanted from the top of the tank and flows to a flow equalization tank.
3. Flow Equalization
The SBR process treats water in batches. In order to produce a constant effluent flow, the treated water is discharged from the SBRs into one of two flow equalization tanks. These tanks store the treated water and allow for the plant’s effluent pumps to maintain a fairly consistent flow of treated water to flow from the plant. The consistent flow rate aids in chemical dosing rates, allows for required chemical contact time, and minimizes chemical usage.
The treated water is pumped from the flow equalization tank and chlorine (sodium hypochlorite) is added to the treated water. The chlorinated water is then discharged to one of two chlorine contact tanks to allow for adequate contact time for the chlorine to kill harmful bacteria.