Plant and Equipment

BACKGROUND

The Town of York has been a summer resort area for many years, and has seen moderate growth in both the year-round and seasonal populations. Since the mid-1980s these populations have increased substantially.

The original secondary treatment plant was constructed at the current site in 1975. It was designed to treat an average daily flow of 1.6 million gallons per day (mgd) and a peak flow of 4.5 mgd. The treatment facility was designed to serve York for approximately 20 years; however, due to the increased population growth of the 1980's, flows to the facility had reached design levels by 1990.

In 1990, the York Sewer District, which owns and operates the treatment facility, obtained the services of Wright-Pierce Engineers to design an upgraded facility at the existing site. In 1994, the new facility went on line, providing treatment for an average flow of 3.0 mgd and a peak flow of 7.5 mgd.

NEW FACILITY

In order for the York Sewer District to provide treatment services to their current users, as well as provide capacity for growth within the Town of York, many components of the treatment facility were replaced with larger, more efficient systems. A new Process Building was also constructed to house the new equipment.

Like the treatment plant constructed in 1975, the new plant incorporates a secondary treatment process to treat the incoming waste water. Biological activity breaks down the various components in the incoming waste system such that over 90% of the biochemical oxygen demand (BOD) and 90% of the total suspended solids (TSS) are removed.

New facilities include: a headworks, where large items in the waste stream are removed by a screening device, and grit is removed by a degritting system; aeration basins, where mixed liquor and oxygen are combined with the waste stream to promote the biological treatment process; clarifiers, where the mixed liquor settles to the bottom of the tank and clean effluent is discharged from the top; and the chlorine contact tank, where liquid chlorine is introduced to kill the bacteria and sodium bisulfite is introduced to remove excess chlorine. In addition, excess mixed liquor (sludge) wasted from the process is dewatered and lime stabilized prior to being applied to farm land as a natural soil amendment. Support equipment for these processes, including several pumps and air blowers, are housed in the Control Building and the new Process Building.

To keep the treatment system operating properly, a Supervisory Control and Data Acquisition (SCADA) system was included in the design. This SCADA system is operated by a computer interface. The SCADA system automatically adjusts equipment speeds to maintain preset levels; provides feedback to the operator as to how the process is operating; provides alarms for equipment malfunction or pending malfunctions; and automatically stores data from the various instruments used to control the process for end of month reporting to the Department of Environmental Protection.

FUNDING

The design and construction of the treatment facility was funded by a loan from the State Revolving Loan Fund (SRF). The SRF will loan money for facilities such as this at an interest rate 2% below the current state bond bank level, provided all applicable state and federal regulations are followed. The York Sewer District borrowed approximately $6.8 million for design and construction of the facilities.

PROCESS FLOW DIAGRAM - See Process steps below:



A. Headworks
In the headworks wastewater receives preliminary treatment by mechanical screening and grit removal prior to flowing to the aeration basins. Preliminary treatment is provided to protect the downstream piping and equipment from damage and plugging, and to reduce the quantity of grit collected in the aeration basins.

B. Aerations Basins
From the headworks wastewater flows to the existing aeration basins where bacteria provide secondary treatment by breaking down wastes and converting it to cell mass (sludge), water and CO2. This biological treatment requires air which is introduced into the aeration blowers and fine bubble ceramic diffusers.

C. Clarifiers
From the aerations tanks the wastewater flows to two new 70 foot diameter clarifiers each covered by an aluminum dome. In the clarifiers, sludge settles to the bottom of the tank and clarified wastewater flows out to the chlorine contact tank for disinfection. A portion of the settled sludge is pumped back to the aeration tanks to maintain the bacterial population and a portion is wasted to the sludge holding tanks.

D. Disinfection System
From the clarifiers, treated wastewater (effluent) flows tot he chlorine contact tank where liquid chlorine is introduced to disinfect the effluent. Before the disinfected effluent is discharged to the ocean in York Harbor, sodium bisulfite is added to remove any residual chlorine that was not used during the disinfection stage.

E. Sludge Pumps

1. Return Pumps
The return sludge pumps return activated sludge from the clarifiers tot he aeration basins. The sludge can be returned to the headworks, the aeration flow splitter box or directly to the aeration tanks.

2. Waste Pumps (existing)
The waste sludge pumps waste excess sludge generated during the biological treatment of the wastewater and pump the sludge tot he sludge holding tanks.

3. Belt filter Press Feed Pumps
The belt filter press feed pumps feed the sludge stored in the sludge holding tanks to the belt filter presses which dewater the sludge prior to disposal.

F. Dewatering System
The dewatering system is comprised of two belt filter presses, polymer feed, and a lime stabilization system. This system reduces the volume of sludge that must be disposed of.

G. Plant Water System
The plant water system provides washwater to the belt filter press, to hydrants located around the treatment plant and to a foam control system located in the aeration tanks.

H. Influent Pumps
The majority of the wastewater flow coming to the treatment facility if pumped to the plan by the Long Beach Pump Station. The gravity portion of the flow enters the treatment facility at the influent pump station where it is pumped to the headworks. In addition, belt filter press filtrate and spray wash water flows to the influent pump station.

I. Sludge Holding System
Excess sludge generated during the biological treatment of the wastewater is wasted to the sludge holding tanks where the sludge is stored until it is dewatered by the belt filter presses. The sludge is aerated while it is stored to prevent odors and to further stabilize the sludge.