design aspects of water treatment

DESIGN ASPECTS OF WATER TREATMENT

Bob Clement

Environmental Engineer

EPA Region 8

SLOW 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.

SLOW 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.


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.


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.

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.



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.


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).


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.


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.

DIATOMACEOUS 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.

DIATOMACEOUS 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.

DIATOMACEOUS 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.

DIATOMACEOUS 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.

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.


DIATOMACEOUS 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.

DIATOMACEOUS 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.

DIATOMACEOUS 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.


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.

DIATOMACEOUS 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.

OTHER 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.

BAG 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.

BAG 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.


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 > 1 NTU).


Both filters have been shown to remove at least 2.0 logs of Giardia Lamblia but for crypto:

Bags show mixed results <1 to 3 logs of removal.

Cartridge filters show 3.51 to 3.68 logs of removal. Better removal due to pleats.


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.


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.

BAG AND CARTRIDGE FILTRATION Monitoring of filter integrity may be

needed. States to decide on what type of

integrity tests may be needed.

BAG 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.

MEMBRANES 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)

MEMBRANES

Reverse Osmosis (RO) used for desalination and specific inorganic contaminant removal. Excludes atoms and molecules < 0.001 microns--the ionic range.

MEMBRANES

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.

MEMBRANES

Ultrafiltration (UF) used for organic and protein removal. Excludes molecules greater than 0.005 microns in size--molecular weight cutoff ~10,000.

MEMBRANES

Microfiltration (MF) used for particles, suspended solids, bacteria and cyst removal. Excludes particles and molecules greater than 0.2 microns--the macro molecular range.

MEMBRANES

Filtration Spectrum Overhead

MEMBRANES

Ultrafiltration Rejection Mechanisms Overhead

MEMBRANES

Conventional filtration can remove particles down to 1.0 micron--the micro and macro particule range.

MICROFILTRATION (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.


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.

MICROFILTRATION (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.


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.


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.


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.


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.


A 500 micron screen is usually the only pretreatment needed.

Higher levels of pretreatment are needed towards RO.


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.

MICROFILTRATION (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 < 1 mg/l.• Turbidity 500 NTU to 0.08 - 0.05 NTU.


Removal levels for MF (continued)

• Silt density index (SDI) over 5 to < 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.


Removal levels for MF (continued)• Microorganisms 105 colony forming units

(cfu)/ml to < 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.


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).

MICROFILTRATION (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.

MEMBRANES

SEM Overheads


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.


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).


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.

MICROFILTRATION (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.

MICROFILTRATION (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.


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.


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.


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.


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.

design aspects of water treatment

Documents

slow sand filtration

new sand

normal filtration

filter cleaning

sand depth approaches

filter precoat

functioning biological

redundant ss filters