Title: DESIGN ASPECTS OF WATER TREATMENT
1DESIGN ASPECTS OF WATER TREATMENT
- Bob Clement
- Environmental Engineer
- EPA Region 8
2SLOW SAND FILTRATION(SS)
- An alternate BAT for complying with the SWTR is
SS. SS is a biological process that requires
sufficient natural organic matter (NOM) to
provide a nutrient supply to the biological mat.
3SLOW SAND FILTRATION (SS)
- SS requires influent water that does not exceed
the following parameters - Turbidity of less than 10 NTU.
- Color of less than 30 units.
- Algae of less than 5 mg per cubic meter of
chlorophyll A.
4SLOW SAND FILTRATION (SS)
- SS is 50 to 100 times slower than normal
filtration. - SS requires smaller sand particles (smaller pore
spaces), effective size 0.25 to 0.35 mm, with a
uniformity coefficient of 2 to 3. - Start-up of a SS may take as long as 6 months to
develop the initial biological mat.
5SLOW SAND FILTRATION (SS)
- SS filters perform poorly for 1 to 2 days after
filter cleaning, called the ripening period.
The ripening period is the time required by the
filter after a cleaning to become a functioning
biological filter. This poor water quality
requires a filter- to-waste cycle.
6SLOW SAND FILTRATION (SS)
- Because of the length of time required for
cleaning and ripening, redundant SS filters are
needed. - The biggest enemy to a biological mat is the lack
of moisture. Therefore, a SS filter must always
be submerged.
7SLOW SAND FILTRATION (SS)
- Initial headloss is about 0.2 feet, maximum
headloss should be no more than 5 feet to avoid
air binding and uneven flow of water through the
filter medium. - SS filters should be enclosed in a building so
that they can be cleaned in the winter months and
avoid ice buildup.
8SLOW SAND FILTRATION (SS)
- The housing should also be light free to
eliminate algae growth. Regardless of the type
of filtration technology used, design should
consider ways to minimize algae growth (e. g.,
sed basins housed with no outside light).
9SLOW SAND FILTRATION (SS)
- The normal length of time between cleanings is 20
to 90 days. Cleaning involves scraping manually
1 to 2 inches and discarding the sand. Another
method of cleaning is called harrowing and uses a
very low backwash rate while manually turning the
media. New sand should be added when sand depth
approaches 24 inches, approximately every 10
years.
10SLOW SAND FILTRATION (SS)
- No chemical pretreatment is done for SS. SS has
been successfully used in South America treating
waters with greater than 1000 NTUs when roughing
filters are used. - Capital costs may be higher, but operational
costs are lower.
11DIATOMACEOUS EARTH (DE) FILTRATION
- DE is composed of siliceous skeletons of
microscopic plants called diatoms. Their
skeletons are irregular in shape therefore
particles interlace and overlay in a random
strawpile pattern which makes it very effective
for Giardia and crypto removal.
12DIATOMACEOUS EARTH (DE) FILTRATION
- Difficulty in maintaining a perfect film of DE of
at least 0.3 cm (1/8 in) thick has discouraged
widespread use of DE except in waters with low
turbidity and low bacteria counts. - The minimum amount of filter precoat should be
0.2 lb/ft2 and the minimum thickness of precoat
should be 0.5 to enhance cyst removal.
13DIATOMACEOUS EARTH (DE) FILTRATION
- The use of a alum (1 to 2 by weight) or cationic
polymer (1 mg per gram of DE) to coat the body
feed improves removal of viruses, bacteria and
turbidity but not necessarily Giardia.
14DIATOMACEOUS EARTH (DE) FILTRATION
- Continuous body feed is required because the
filter cake is subject to cracking. Also, if
there is no body feed there will be a rapid
increase in headloss due to buildup on the
surface.
15DIATOMACEOUS EARTH (DE) FILTRATION
- Interruptions of flow cause the filter cake to
fall off the septum, therefore, precoating should
be done any time there are operating
interruptions to reduce the potential for passage
of pathogens.
16DIATOMACEOUS EARTH (DE) FILTRATION
- Body feed rates must be adjusted for effective
turbidity removal. Filter runs range from 2 to 4
days depending on the rate of body feed and DE
media size.
17DIATOMACEOUS EARTH (DE) FILTRATION
- An EPA study showed greater than 3.0 log removal
for Giardia for all grades of DE. Whereas the
percent reduction in TC bacteria, HPC, and
turbidity were strongly influenced by the grades
of DE used.
18DIATOMACEOUS EARTH (DE) FILTRATION
- For example the coarsest grades of DE will remove
95 percent of cyst size particles, 20-30 percent
of coliform bacteria, 40-70 percent of HPC and
12-16 percent of the turbidity.
19DIATOMACEOUS EARTH (DE) FILTRATION
- The use of the finest grades of DE or alum
coating on the coarse grades will increase the
effectiveness of the process to 3 logs bacteria
removal and 98 percent removal for turbidity.
20DIATOMACEOUS EARTH (DE) FILTRATION
- Systems in Wyoming have shown as high as six logs
of microorganism removal, whereas others have
shown negative log removal for particles which
might be the media passing the septum.
21OTHER FILTRATION TECHNOLOGIES
- These include cartridge, bag, membranes, and
other types of filters. - You must be able to prove to the state that they
will meet state regulatory requirements. These
may include studies on performance for turbidity
removal, Giardia, crypto and virus removal
through pilot studies.
22BAG AND CARTRIDGE FILTRATION
- Units are compact.
- Operates by physically straining the water -- to
1.0 micron. - Made of a variety of material compositions
depending on manufacturer. - Pilot testing necessary.
23BAG AND CARTRIDGE FILTRATION
- Depending on the raw water quality different
levels of pretreatment are needed - Sand or multimedia filters.
- Pre-bag or cart. of 10 microns or larger.
- Final bag or cart. of 2 microns or less.
- Minimal pretreatment for GWUDISW.
24BAG AND CARTRIDGE FILTRATION
- Units can accommodate flows up to 50 gpm.
- As the turbidity inc the life of the filters dec
(e.g., bags will last only a few hours with
turbidity gt 1 NTU).
25BAG AND CARTRIDGE FILTRATION
- Both filters have been shown to remove at least
2.0 logs of Giardia Lamblia but for crypto - Bags show mixed results lt1 to 3 logs of removal.
- Cartridge filters show 3.51 to 3.68 logs of
removal. Better removal due to pleats.
26BAG AND CARTRIDGE FILTRATION
- In an MS-2 Bacteriophage challenge study no virus
removal was achieved. Therefore, there must be
enough disinfection contact time to exceed 4.0
logs of inactivation of viruses for both filters.
27BAG AND CARTRIDGE FILTRATION
- Factors causing variability in performance
- The seal between the housing and filters is
subject to leaks especially when different
manufacturers housings and filters are used. - Products use nominal pore size (average) rather
than absolute pore size. 2 um or less absolute
should be used.
28BAG AND CARTRIDGE FILTRATION
- Monitoring of filter integrity may be needed.
- States to decide on what type of integrity tests
may be needed.
29BAG AND CARTRIDGE FILTRATION
- For a conventional or direct filtration plant
that is on the borderline of compliance
installing bag/cart filtration takes the pressure
off by increasing the turbidity level to 1 NTU
and increases public health protection by
applying two physical removal technologies in
series. Check with State Drinking Water
programs.
30MEMBRANES
- Many investigations in the last decade have shown
that membrane filtration are very powerful
treatment processes. Membranes have been utilized
commercially for over 25 years. There are four
membrane technology groups - Reverse Osmosis (RO)
- Nanofiltration (NF)
- Ultrafiltration (UF)
- Microfiltration (MF)
31MEMBRANES
- Reverse Osmosis (RO) used for desalination and
specific inorganic contaminant removal. Excludes
atoms and molecules lt 0.001 microns--the ionic
range.
32MEMBRANES
- Nanofiltration (NF) used for softening and
natural organic matter removal (best technology
for meeting the DBP rule). Excludes molecules
greater than 0.001 microns in size--multivalent
ion range.
33MEMBRANES
- Ultrafiltration (UF) used for organic and protein
removal. Excludes molecules greater than 0.005
microns in size--molecular weight cutoff 10,000.
34MEMBRANES
- Microfiltration (MF) used for particles,
suspended solids, bacteria and cyst removal.
Excludes particles and molecules greater than 0.2
microns--the macro molecular range.
35MEMBRANES
- Filtration Spectrum Overhead
36MEMBRANES
- Ultrafiltration Rejection Mechanisms Overhead
37MEMBRANES
- Conventional filtration can remove particles down
to 1.0 micron--the micro and macro particule
range.
38MICROFILTRATION (MF)
- MF is a physical separation (sieving) process and
removes all particles greater than 0.2 microns (1
x 10-6 meters). Excludes molecules greater in
size than 200,000 molecular weight cutoff.
39MICROFILTRATION (MF)
- MF is easy to operate and produces greater than 6
logs of removal for protozoans. - With Programmable Logic Controllers they can be
left unattended with only periodic monitoring and
data logging.
40MICROFILTRATION (MF)
- The advantage is that filter quality is achieved
irrespective of changes in turbidity,
microorganism burden, algae blooms, pH,
temperature, or operator interaction. - Conventional treatment is cumbersome and is
operator intensive compared to microfiltration.
41MICROFILTRATION (MF)
- Membrane systems lose operational performance
such as increasing pressure differentials across
the membrane and shortening of the cleaning
frequency, instead of compromising finished water
quality.
42MICROFILTRATION (MF)
- The biggest concern is failure of the membrane
since it is a single barrier, whereas filtration
is multi-barrier. Consider bag filtration as a
backup barrier for a failed membrane.
43MICROFILTRATION (MF)
- MF is compact, the building and area needed for
installation is small. - MF reduces the dosage of chlorine needed due to
the reductions of microorganisms and chlorine
demand.
44MICROFILTRATION (MF)
- MF with a molecular weight cutoff of 200 can
remove DBP precursors greater than 90. - MF can achieve a 10 reduction of Disinfection
Byproduct (DBP) Precursors. - MF used in conjuncture with coagulants can obtain
DBP removals similar to a conventional plant.
45MICROFILTRATION (MF)
- A 500 micron screen is usually the only
pretreatment needed. - Higher levels of pretreatment are needed towards
RO.
46MICROFILTRATION (MF)
- For RO and NF systems to operate economically,
suspended solids, microorganisms, and colloids
have to be removed before these technologies can
effectively remove dissolved contaminants.
47MICROFILTRATION (MF)
- Removal levels for microfiltration
- Acceptable range of raw water pH 2-14.
- pH adjustments are not required for scaling
control, since MF does not remove uncomplexed
dissolved ions. - Suspended solids 200 mg/l to lt 1 mg/l.
- Turbidity 500 NTU to 0.08 - 0.05 NTU.
48MICROFILTRATION (MF)
- Removal levels for MF (continued)
- Silt density index (SDI) over 5 to lt 1.0. An SDI
of less than 1.0 means that the fouling rate
potential is low. MF is recognized as the most
appropriate technology for pretreatment for RO.
Fouling susceptibly increases towards RO.
49MICROFILTRATION (MF)
- Removal levels for MF (continued)
- Microorganisms 105 colony forming units (cfu)/ml
to lt 1 cfu/ml. Bacteria are typically greater
than 0.2 microns in size. This includes algae
removal. - Crypto Giardia 106 cysts/100ml to none
detected. Size exclusion is the major mechanism
of removal, and is an absolute barrier as long as
the membrane is intact.
50MICROFILTRATION (MF)
- Removal levels for MF (continued)
- Viruses 103 plaque forming unit (pfu)/100ml to
101 pfu/100ml. - Viruses are usually smaller in size than 0.2
microns (MS2 phage is 0.027 microns).
51MICROFILTRATION (MF)
- Removal levels for MF (continued)
- The mechanism of removal appears to be related to
three factors physical sieving/adsorption, cake
layer formation and changes in the fouling state
of the membrane. - The highest log removal was attributable to
fouling. The remaining virus removal, to 4 log
removal/inactivation, is achieved through
disinfection.
52MEMBRANES
53MEMBRANES
54MICROFILTRATION (MF)
- MF is a low pressure membrane (20-35 psi). High
pressure RO membranes can require pressures of
greater than 300 psi. - Recovery for MF is 90. Recovery decreases
towards RO and the waste streams increase
significantly towards RO. - Range of flow 0.6 to 22 MGD.
55MICROFILTRATION (MF)
- For the Town of Winchester a 1.0 MGD MF plant was
estimated to cost 1.5 M. If financed at 8
interest over a 20-year period, the annual dept
would be 152,820. Therefore, the capital costs
were 0.42 capital per 1000 gallons. The
operating costs were 0.165 operation per 1000
gallons and included power for pumps and
compressors, chlorine, membrane replacement
(22,500 per year) and cleaning chemicals (4000
per year).
56MICROFILTRATION (MF)
- Membrane life for MF is 3-5 years.
- Backwash volume for MF is 6 for low turbidity
up to 12 for high turbidity. Gas backwash is
very efficient in removing foulants.
57MICROFILTRATION (MF) MAINTENANCE
- Cleaning is usually done with a 2 mixture of a
caustic detergent every 30 days and takes less
than 3 hours to complete. - The cleaning solution is recovered and reused.
58MICROFILTRATION (MF) MAINTENANCE
- A citric acid cleaning following the caustic has
been found to be effective in cleaning membranes
with high hardness and/or iron. - If no pretreatment chemicals are used, the spent
cleaning fluid is the only waste stream requiring
special attention.
59MICROFILTRATION (MF)
- Automatic membrane integrity tests are based on
the principle that air pressure must overcome the
capillary resistance before an intact membrane
leaks. An integral module will exhibit little,
if any, decay over the test period.
60MICROFILTRATION (MF)
- The hollow tube configuration is the most widely
used format for membrane construction due to its
bi-directional strength which makes backwashing
possible. - The hollow tube maximizes the available
filtration surface area within the smallest
physical area. Materials of construction for
membranes can be polymeric or ceramic.
61MICROFILTRATION (MF)
- There must be redundancy of units in case one of
the units fails, or is being cleaning, or is
undergoing membrane replacement. - One company has installed 44 MF systems
nationwide.
62MICROFILTRATION (MF)
- The largest MF facility is the 5 MGD plant at San
Jose California. It is an unmanned plant. - The installation is successful but is
mechanically complex with 100 automatic valves
and more than 7,000 connections that require
o-rings to achieve a tight water seal.