The purposes of the direct
filtration process include: compliance with treatment technique regulatory
requirements; targeting impurities; and producing safe and aesthetically
pleasing drinking water.
Compliance with Treatment
Technique Regulatory Requirements
The USEPA (U.S. Environmental
Protection Agency) Surface Water Treatment Rule requires continuous filtration
and disinfection as treatment techniques for all surface waters, including
groundwater sources designated by the VDH as groundwater under the direct
influence of surface water (GUDI).
Turbidity and the Direct Filtration Process
When source water is generally
within the turbidity range of 1 to 2 NTU, it may be a candidate for direct
Pilot tests for
each season of a year are required in order to demonstrate that the filtration
technique will be successful in meeting the turbidity removal requirements
under varying conditions, and that the process will be able to meet minimal
performance efficiency goals.
facilities (direct filtration facilities) are less expensive to construct and
operate than conventional treatment facilities.
Filtration facilities must meet the treatment technique requirements within the
Water Treatment Rule
Enhanced Surface Water Treatment Rule (IESWTR)
Term 1 Enhanced Surface Water Treatment Rule (LT1-ESWTR)
Long Term 2 Enhanced Surface Water
Treatment Rule (LT2-ESWTR).
filter effluent turbidity of all filters must be ≤0.3 NTU in 95% of all
samples monthly, with a minimum of six samples per day.
- The maximum
combined effluent turbidity of any sample must be ≤1 NTU at all times.
- The turbidity
from individual filters must be monitored continuously using online
- If the online
turbidimeters fail to operate correctly, grab sampling for bench turbidity
analysis must be conducted every four hours until the equipment is repaired.
- Repairs must be
completed within five working days.
- The online
turbidimeter must record turbidity at an interval not exceeding every 15
- Individual filter
effluent turbidity measurements require reporting to VDH and could trigger
follow-up activities if any one of the following events should occur:
filterís turbidity exceeds 1.0 NTU in two consecutive measurements taken 15
filterís turbidity exceeds 0.5 NTU in two consecutive measurements taken 15
minutes apart after the first four hours of operation.
If, following a
backwash, the raw water turbidity is less than 1.0 NTU, the filter effluent
turbidity must be no greater than 50% of the raw water turbidity prior to
returning the filter to service from the filter-to-waste period.
- Otherwise, the
filter effluent turbidity must be less than 0.5 NTU prior to returning the
filter to service.
Produce Safe and
Aesthetically Pleasing Drinking Water
The three criteria for the second
purpose of direct filtration, which is to provide safe and aesthetically
pleasing water, include:
of disease-causing organisms
of toxic substances
disagreeable taste, odor, or appearance
Free of Disease-Causing
turbidity requirements discussed earlier are intended to provide and
demonstrate a high assurance that the drinking water delivered to the system is
free of pathogenic microorganisms such as Giardia and Cryptosporidium
(parasitic cysts) and enteric viruses.
- No filter
process can remove these contaminants with 100% assuredness.
- Therefore, all
filtration processes must include an additional disinfection barrier, which is
typically a form of chlorination.
- Cryptosporidium cannot be
inactivated by chlorine.
- Therefore, if
the filters are in compliance with turbidity rules, the filtration process is
given credit for 99% reduction of Cryptosporidium.
- It is assumed
that high quality source waters will be low in Cryptosporidium.
Free of Toxic
are contaminants for which there are Primary Maximum Contaminant Levels (MCL).
- Contaminants may
be organic or inorganic in nature.
- Toxic substances
are generally not present in high quality source waters (where direct
filtration would be used).
- If there is a
risk of organic contaminants in the supply, there may be a process that can
adsorb the contaminants designed into the treatment plant.
- One example of
an adsorption process is the addition of powdered activated carbon (PAC)
upstream of all treatment processes. The PAC is subsequently removed by
- Oxidation and
coagulation are required for the removal of inorganic contaminants.
- Oxidation and
coagulation are always part of the direct filtration process.
- These processes
render the contaminants into precipitated and coagulated particles that can
then be removed by the filters.
Taste, Odor, and Appearance
Tastes and odors
are most often due to metabolites of algae present in the raw water supply.
- These are
organic compounds and may be controlled with existing treatments such as the
addition of PAC or an oxidant.
Targeting impurities is the
third, and final, purpose of the direct filtration process. The key constituents
in the source water that are targeted by an ďabbreviatedĒ (filtration) process
- These are silts
and clays that occur naturally in the watershed.
- They are washed
into the supply sources during precipitation.
- These are
extremely small (<1.0 μm) particles that cannot settle naturally
because of the electrostatic repulsive forces of particle surface charges.
- A large
component of the colloidal mass may be due to organic materials.
- These can be
harbored in the suspended solids and, therefore, shielded from chemical
- These are
naturally-occurring minerals and organic matter.
Characteristics of Appropriate Source Waters
Direct filtration is used for
source waters with the following general water quality characteristics.
generally less than 2 NTU.
- Higher values
can be treated for short periods, but very short filter runs may result.
Generally the limit for raw turbidity is less than 25 NTU.
- True color is
less than 20 c.u.
- True color is an
indicator of colloidal solids, typically consisting of naturally-occurring
- These likely
require a higher coagulant dose than suspended solids to coagulate.
- The combination
of the colloidal particles and the typically higher coagulant doses generally
required to treat colloids results in a high solids loading onto the filters.
- A short filter
run will result.
- Algal blooms are
less than 2000 asu/mL.
- This level is
- Above this level,
filter run times may be impacted negatively.
- Growth of algae
in natural bodies of water is most successfully controlled by adding copper
- Iron is less
than 0.3 mg/L (average).
- Manganese is
less than 0.05 mg/L (average).
- Coagulant demand
is below 15 mg/L.
Pilot Testing of
To verify that a source water can
be treated with direct filtration, pilot testing should be performed prior to
the design of the facility. This demonstrates the ability of the process to perform
in accordance with all drinking water regulations.
The abbreviated direct filtration
process does not include the following processes that are components of the
conventional filtration process:
OF DIRECT FILTRATION
Direct filtration can be
accomplished through different types of media.
Earth (D.E.) Filtration
- Slow Sand
- Bag or Cartridge
The objectives of any filter
process are to:
effluent with the highest quality possible.
the production of filtered water by encouraging long filter run times.
efficient backwash processes with low backwash water requirements.
The optimum dose for direct
filtration is the lowest dose that achieves filtrate quality and production
goals. The purpose of adding a coagulant aid is to increase floc strength and
or full scale filter runs must be performed in order to determine the best
coagulant dose for optimum filter performance. That dose is dependent on solids
concentrations (usually measured as turbidity), and by the concentration of
soluble organics (measured by true color or total organic carbon (TOC).
The most commonly used chemicals
to achieve coagulation in this type of process are:
is the most pH-dependent coagulant.
may require supplementation of raw water alkalinity.
reacts much more slowly under cold water conditions.
may result in a significant degradation in direct filtration performance.
most efficient filter performance is achieved when the alum requirement is
approximately 5 mg/L or less.
alum must be stored in a dry place.
chloride or ferric sulfate
chloride and ferric sulfate have slower reaction times under cold water
conditions, although not as slow as alumís reaction time.
chemical can contribute to a less than desirable filter run time.
overdose of either coagulant may result in the passage of iron compounds into
chloride may prove to be particularly beneficial as a coagulant for direct
has a faster reaction time under cold water conditions than the coagulants
is particularly valuable for plants that typically have short flocculation
contributes to longer filter run times due to a lower solids-loading
contribution from the coagulant.
requires minimal or no pre-alkalinity addition.
frequently develops a more filterable floc than with alum or ferric salts.
Any one of the following filter
bed configurations might be found in a direct filtration plant:
media filter beds not usually found in treatment plants, but the deepest beds
are generally mono media beds.
are inexpensive, but difficult.
or GAC would be the media used in this filter bed configuration.
- Deep bed filters are generally mono.
L.A. Aqueduct Plant is a deep mono media filter bed plant.
filters there consist of large anthracite media (ES = 1.5 mm), which are six
feet in depth and have a design loading rate of 13.5 gpm/sf.
filters typically consist of a lower layer of silica sand, 8 to 12 inches in
depth, with a layer of anthracite or GAC, 18 to 30 inches
in depth, that serves as the filter cap.
media filters typically consist of a bottom layer of garnet sand, 2 to 3 inches
deep, with a middle layer of silica sand (6 to 9 inches), and a top layer of
either anthracite or GAC, which is 18 to inches in depth.