NORTHERN WESTCHESTER JOINT
WATER WORKS
FACILITIES DESCRIPTION:
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. |