The Northern Westchester Joint Water Works (NWJWW) utilizes two (2) conventional surface water treatment plants to produce an average of 7.46 (seasonal range 3.84-11.47 MGD) million gallons of water per day (MGD).  With a combined maximum capacity of 15 MGD, NWJWW has sufficient capacity for future growth. 

The Amawalk Water Treatment Plant, the older of the two plants,(built in 1975), is located in the Town of Somers, and utilizes the Amawalk Reservoir as its source water. In 2010 the plant averaged 3.08mgd, and had a range of 1.15-5.72mgd.  The plant was originally built and operated by the County of Westchester and was transferred to the NWJWW during the formation of the Joint Water Works in 2002. 

Amawalk is a convention surface water treatment facility, which uses the multi-barrier approach to water treatment.  Six (6) separate processes are employed to take raw reservoir water and make it safe for human consumption.  These processes are as follows:

1)      Pre-oxidation

2)      Coagulation

3)      Flocculation

4)      Sedimentation

5)      Filtration

6)      Post-Filtration chemical addition


The 1st step is pre-oxidation, with Potassium Permanganate (KMnO4) and dissolved oxygen (aeration), of inorganic metals (iron and manganese) associated with deep water reservoirs.   Due to natural biological processes and the rock composition beneath the reservoir, the facility experiences seasonal spikes in iron and manganese concentrations.  To combat this, potassium permanganate is added just prior to the aeration basin to aid in the oxidation and precipitation of these dissolved metals.  There is no heath risk associated with these metals; however, if they go unchecked elevated turbidities and unpleasant tastes may be experienced within the distribution systems.

 Coagulation is the next stage of treatment, in which a highly charged inorganic coagulant (Poly Aluminum Chloride [PACL]) is added to the water.  Particles which make up turbidity tend to negatively charged, and like magnets- similar charges repel each other.  It is this charge repulsion that keeps particulate suspended in the water column and requires a positively charged coagulant to be added.  Therefore coagulation is the process which neutralizes the negatively charged particles commonly found in raw untreated water. This is an instantaneous reaction, involving a lot of mixing energy, and is immediately followed by Flocculation.  

Flocculation is the process by which small neutrally charged particles are gently mixed together, resulting in the formation of particles large enough to settle out.   During the flocculation process, a  non-ionic organic polymer is added to aid in the conglomeration of particles.   This flocculate aid serves two purposes; 1) bridges small particles together and 2) adds density to the particle to assist in the settling process in the clarifiers


Upon exiting the inner draft tube, the water enters the sedimentation basins.  These tanks are designed so that influent water is slowed, allowing the particles formed during coagulation/flocculation to settle out.  The settled solids generated during this process are collected, dewatered, and trucked offsite for disposal. 

The supernatant is collected and conveyed to the filtration galley via a 24” main.   Amawalk has 6 filters.  Filters 1-4 contain three types of media; anthracite coal on sand on gravel.  Filters 5 & 6 contain the same amount of both support gravel and sand, but are capped with 30” of granular activated carbon (GAC).  Filters 5& 6 are primarily used in the summer and fall to assist in the removal of taste and odor causing compounds.

Filtration is a physical process in which particles larger then the filter pore size can not physically pass through.  A properly operated filter has a nominal pore size of roughly 0.45 microns, which is smaller, then most bacteria, but larger then most viruses.   Turbidity is the unit of measurement used to monitor the filtration process.  The current regulations required the filter effluent turbidity to be less then or equal to (≤) 0.30ntu 95% of the time.  To ensure compliance with this regulation we are required to provide continuous turbidity monitoring, recording, and reporting.

The last stage of treatment is post-filtration chemical addition.  All chemicals used are NSF certified and approved by the NY Department of Health for use in the potable water.    The chemicals used and function they serve is listed below:

1)      Calcium Hypochlorite (CaOCl)- Source of chlorine.  Required to disinfect the water and carry a residual into the distribution system.  Residual in the distribution system is essential to prevent microbial re-growth.  Target residual entering the distribution system is 1.5 mg/l  +/- 0.2 mg/l.

2)      Ortho Phosphoric Acid (H3PO4)-Source of phosphate.  Added to water to assist is corrosion control.  Due to the long contact times of some of the distribution systems serviced, mix of piping materials, and brass components within homes lead and copper leaching into the water has become an issue.  To combat this phosphorous, as phosphate, is added to the water to sequester and precipitate out any dissolved metals.

3)      Sodium Hydroxide (NaOH)-Used in conjunction with the phosphoric acid to aid in corrosion control.  NaOH is a strong base and is also used to adjust the pH to a range of 7.5 +/- 0.2.  pH is the measurement of concentration of acids and bases in solution.  

4)      Hydrofluorosilicic acid (H2SiF6)-Source of fluoride. Used to fluoridate water supply.  The chemical is dosed to achieve a fluoride residual of 0.7mg/l.


 The Catskill Water Treatment Facility is the newest of N.W.J.W.W. treatment plants (built in 2000), is located in the town of Cortlandt Manor, and uses the NYC Catskill aqueduct as a source.   The facility has a maximum rated capacity of 7.5 MGD.  In 2010 the plant averaged 4.38mgd with a range from 2.69-5.75mgd.  

The facility utilizes a Supervisor Control and Data Acquisition (SCADA) system for operational control, data acquisition, and reporting purposes.  Although the Catskill Plant utilizes conventional coagulation and flocculation, it differs from the Amawalk facility by employing Dissolved Air Floatation (DAF) rather then sedimentation for solids removal.  DAF technology is relatively new and is typically used on high quality source water, which the Catskill aqueduct is. 

Using compressors and air saturators, 10% of the total plant flow is super saturated with air and mixed with the flocculated water.  The pressure drop, from this blending result in a column of continuously rising “micro” air bubbles. These bubbles entrap flocculated particles and cause them to rise to the surface. The result is a separation of the bulk particles from the “clear” water.  The floating mass of particles is continuously removed from the surface, by a mechanical scrapper, thickened using gravity, and trucked offsite for disposal.

This process takes place within the filter, so as the particles rise to the surface the supernatant is drawn through the mixed media filter by gravity.   The Catskill Facility has four (4) treatment trains each with a mixed media filter made up of sand and anthracite coal.  

The last stage of treatment is post-filtration chemical addition.  All chemicals used are NSF certified and approved by the NY Department of Health for use in the potable water.    The chemicals used and function they serve is listed below:

1)      Sodium hypochlorite (NaOCl) is added to the filtered effluent water prior to the clear well to achieve required Contact Time (CT) for microbial inactivation.

2)      Caustic soda (NaOH) is added to adjust the finished water pH to 7.7 +/- 0.2. 

3)      Phosphoric acid (H3PO4) is added to achieve a 0.7 mg/l residual of phosphate (PO43-) for corrosion control. 

4)      Hydrofluorosilicic acid (H2SiF6)-Source of fluoride. Used ONLY for the Yorktown system to fluoridate water supply.  The chemical is dosed to achieve a fluoride residual of 0.7mg/l as F-


Water is then pumped to a 3 million gallon (MG) storage tank on site.  From this point, water is either pumped into the Town of Cortlandt’s distribution system, the Town of Yorktown’s distribution system, or flows by gravity to Montrose improvement District. 

Emergency interconnects exist between the NWJWW and the City of Peekskill for moving both raw and finished waters in the event the situation arises.  There is also a 24” transmission main that connects Amawalk and Catskill treatment facilities to allow greater flexibility in daily operations.      




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