Expanded
Summary
Performance and process mechanisms of a high-rate
direct filtration plant targeting 0.1 ntu
Yoav Bar ka y- A r be l , S am i r Hatukai , T e hi l a Ash e r i, D a l it V a ize l - Oh a y o n , a n d M e n a h e m R e bh u n
http://dx.doi.org/10.5942/jawwa.2012.104.0156
The central filtration plant (CFP) of Israel’s National
Water Carrier treats 1.7 × 106 m3/d of surface water in
direct filtration mode at a maximum filtration rate of 20
m/h through anthracite media with a depth of 2 m. The
CFP produces filtered water of < 0.2 ntu, thereby
receiving accreditation for 3-log removal credit for
Cryptosporidium oocysts. Precise dosing of chemicals in
the pretreatment stage—particularly preoxidation with
chlorine dioxide (ClO2)—was found to be a critical
factor in achieving low filtered water turbidities and
particle counts as well as effective removal of algae and
parasitic cysts and oocysts. Although these findings were
for the conditions at this specific site, the effect of
preoxidation with ClO2 is considered applicable to other
water types. This article describes the results of pilotplant studies that were conducted to develop design
criteria and performance of the full-scale CFP. The
authors explain and discuss the processes and
mechanisms involved in achieving low filtered water
turbidities and high algae and parasite removals.
The CFP at the Eshkol site of Israel’s National Water
Carrier treats surface water originating from Lake Kinneret (Sea of Galilee) to supply high-quality filtered water
to millions of people throughout the country. With a
maximum capacity of 1.7 × 106 m3/d (440 mgd), the CFP
is considered one of the largest filtration plants in the
world. The main objective of the CFP is to reduce filtered
water turbidity to < 0.2 ntu (targeting 0.1 ntu), thus
complying with the requirement of 3-log removal credit
for Cryptosporidium oocysts.
The CFP design was based on the findings of large-scale
pilot-plant filtration studies. Throughout two years of
extensive investigation, major design criteria for the
granular filtration process were taken into consideration,
including rapid and slow mixing, coagulant and flocculant type and dosage, various granular media, filtration
rate, and other parameters. In addition, the studies were
designed to take into account seasonal variation of raw
water qualities.
300–400 s–1, residence time of < 1 min), followed by a
slow-mixing (flocculation) stage (velocity gradient of
20–90 s–1, residence time of > 7 min).
A distinctive feature of this filtration plant is its use of
ClO2 as an effective preoxidant before the filtration process. Ozone is generally used as a preoxidant. However,
because Lake Kinneret water has a high bromide content
of 2 mg/L (one of the highest bromide concentrations in
fresh lake water in the world), ozone could not be used
for this purpose because of bromate formation, which is
limited to 0.01 mg/L in drinking water because of its
carcinogenicity. ClO2 at low doses of 0.2–0.5 mg/L
enables the CFP to achieve its high targets in turbidity
reduction (0.1 ntu) and particle and algae removal while
maintaining long filtration cycles. Because ClO2 is less
costly than ozone, its effective use has significant implications for any water, but particularly for those waters that
are high in bromide.
The chemical doses used in the CFP in order to achieve
optimal filtration are ClO2 at 0.4–0.5 mg/L, alum at 3–4
mg/L as aluminum sulfate [(Al2SO4)3] · 18H2O, cationic
polymer (40 weight percent) at 0.4 mg/L as product, and
coagulation pH adjustment with sulfuric acid to 7.
The CFP includes two identical symmetrical modules,
each consisting of 12 filters for a total of 24 filters. Each
filter has two filtration cells with a filtration area of 80.5
m2 per cell; the total filtration area of the CFP is 3,864 m2.
Turbidity removal. Turbidity is the most important quality parameter for filtered water and is regulated by drinking water standards. The filtered water turbidity determine the credit for Giardia and Cryptosporidium
re­­movals awarded for the filtration process. Results
showed that at all times filtered water turbidities were
CFP OPERATION
Raw water from the Eshkol reservoir flows via two
parallel closed conduits to the CFP. Chemicals are directly
injected into the conduits for preoxidation and pH adjustment (for optimal coagulation). In each module, the
water enters two rapid mix chambers in series and then
proceeds to 10 parallel slow mix chambers, each divided
into two units in series.
Filtration. The CFP works in the direct filtration mode
at a maximum filtration rate of 20 m/h using 2-m-deep
beds of anthracite monomedia (effective size of 1.5 mm,
uniformity coefficient of ≤ 1.4). The filtration process is
preceded by a rapid-mixing stage (velocity gradient of
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significantly lower than those required by regulations and
health authorities. Turbidities were close to 0.1 ntu most
of the time and below 0.2 ntu 95% of the time. This
excellent level of plant performance was achieved despite
variable water quality in the operational reservoir (primarily during algal blooms). For example, Rhizosolenia,
a species of diatom algae, produced a bloom in the Eshkol
operational reservoir several times each year. Results
confirmed pilot-study findings that deep-bed granular
filters working at high filtration rates in the direct filtration mode can produce water of extremely low turbidities. These low turbidities yield a credit for 3-log removal
of Cryptosporidium and Giardia, which meets both
Israeli standards and US Environmental Protection
Agency Long-Term 2 Enhanced Surface Water Treatment
Rule requirements.
Particle counts. Although particle counts are not regulated by drinking water standards, they are used in CFP
operations as an effective tool for maintaining control over
the filtration process. Counts of particles (≥ 2 µm in size)
were reduced from raw water levels of 10,000–15,000
particles/mL to the filtered water target value of 200–400
particles/mL. Particle counts of < 50 particles/mL could be
achieved in the plant by increasing ClO2 and flocculant
doses, but this resulted in shorter filter run cycles.
Algae removal. The degree of algae removal in the CFP
is considered to be an additional indicator of parasitic
cyst removal because algae are biological “living” particles and many of them are similar in size to Cryptosporidium oocysts and Giardia cysts.
In the operational reservoir water, algal counts varied
between 100 and 10,000 asu/mL, depending on the
season; in the filtered water, algal counts varied between
~ 5 and 100 asu/mL, which approaches 3-log removals.
In addition to serving as an indirect indicator of cyst
removal, the elimination of algae has beneficial effects
for the water supply systems. It prevents accumulation
of algal biomass in local (municipality) water reservoirs
(which had caused odor in water supplied to consumers
before the CFP was commissioned) and reduces cleaning
and maintenance of reservoirs, resulting in significant
cost savings.
Parasite removal. Cryptosporidium oocysts raw water
concentrations were between 0 and 3 per 10 L, and Giardia cysts raw water concentrations were 0.4 per 10 L. In
filtered water, no Cryptosporidium oocysts or Giardia
cysts were found in the monthly tests during proper
operation of the CFP.
Preoxidation importance. Correlation between the ClO2
dose (specifically, the ClO2 residual concentration in the
prefiltered water) and counts of particles ≥ 2 µm in size
was found to be an important factor for achieving filtered
water quality targets. When the ClO2 dose was reduced
(either intentionally to study the effect or because of operational problems), a corresponding increase in particle
counts was observed in the filtered water. A similar effect
38
was observed in the algal counts. As with particle counts,
algal counts in filtered water proved sensitive to the ClO2
dose, i.e., the degree of preoxidation. Analysis of the results
in relation to the CFP operation showed that the presence
of Cryptosporidium oocysts in the filtered water coincided
with accidental malfunctioning or interruptions in ClO2
dosing. These findings demonstrated that preoxidation had
a significant influence on the efficacy of flocculation and
filtration processes with regard to particle counts and algae
and oocyst removals. During the interruption of the ClO2
dosing, dosing of all other chemicals was not changed, and
no change in pH was observed.
Operational performance. Most filter cycle durations were
30–40 h and never less than 24 h. Runs were terminated
on reaching either the limiting head loss of 2.5 m or a run
time exceeding 72 h. In most cases unit filter run volume
was 300–400 m3/m2 and never below 240 m3/m2, demonstrating excellent production levels. Backwash water consumption was on average 1.7%, and 93% of this backwash
water was treated and returned to the operational reservoir.
Conclusion
During its five years of operation, the CFP has demonstrated that direct filtration with proper chemical pretreatment and high-rate filtration (20 m/h) through a
monomedia bed of anthracite (effective grain size of
1.5–1.7 mm) at a depth of 2 m enabled the facility to
meet its main water quality goals. These included filtered
water turbidity of 0.1 ntu crediting 3-log removal of
Cryptosporidium oocysts, 2- to 3-log reduction of counts
of particles ≥ 2 μm in size, 3-log removal of algae, and
zero counts of Cryptosporidium oocysts.
ClO2 as used in the CFP has proven an effective preoxidant, and the dose of 0.2–0.5 mg/L has been shown
to be an essential parameter in the production of high
water quality. Because ClO2 is less expensive than ozone,
the effective preoxidation that it delivered at low doses
in the CFP makes ClO2 an attractive choice for water
treatment plants worldwide.
As demonstrated by CFP operations, in treating raw
water of relatively low turbidity (~ 1.0 ntu), the slowmixing stage (flocculation) is essential to attaining filtered
water turbidity targeting 0.1 ntu. In addition, achieving
high and long-lasting quality in the filtered water depends
largely on proper, well-controlled pretreatment, including
exact and optimal chemical dosing as well as rapid- and
slow-mixing retention times.
The operation of the full-scale CFP corresponded well
to the pilot-plant operational and design parameters,
confirming the importance of a well-designed and -operated pilot plant with credible process design parameters.
Corresponding author: Yoav Barkay-Arbel is a water
treatment process engineer at the Eshkol site of
Mekorot, Israel’s national water company, POB 610,
Nazareth Illit, 17105, Israel; [email protected].
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