Current through Register Vol. 41, No. 3, September 23, 2024
A. Conventional
design standards have been established for effluent filtration following unit
operations for equalization, coagulation and chemical clarification. For
conventional design, an equivalent level of pretreatment shall be provided.
Filtration for other wastewater reuse alternatives and the design for nutrient
removal will be evaluated by the department based on an evaluation of
performance data. The owner shall accompany a proposal for nonconventional
filtration design with appropriate pilot plant data or full scale unit
operations data demonstrating acceptable treatment of similar wastewater. The
average BOD5 and suspended solids concentrations applied
to the filter should not exceed twice the required values of filtrate
BOD5 and suspended solids concentrations in accordance
with the issued discharge permit or certificate limitations.
B. General design. Conventional effluent
filtration shall be accomplished at a uniform rate of one to five gallons per
minute per square foot of surface area through filter media consisting of a
specified depth of the following materials, either as a single media, or as an
approved combination of multiple layers:
(i)
sand;
(ii) anthracite;
(iii) mineral aggregate; and
(iv) other filter media considered on a
case-by-case basis.
1. Equipment for the
application of chemicals to the filter influent shall be provided if necessary,
to enhance suspended solids removal and minimize biological growth within the
media.
a. Multiple unit operations for
filtration shall be provided to allow for continuous operation and operational
variability for a system with an average design of 0.04 mgd or
greater.
b. The operating head loss
shall not exceed 90% of the filter media depth.
c. Each filter shall have a means of
individually controlling the filtration rate.
2. The effluent filter walls shall not
protrude into the filter media and the incoming flow shall be uniformly applied
to flooded media, in such a manner as to prevent media displacement. The height
of the filter walls must provide for adequate freeboard above the media surface
to prevent overflows.
3. The filter
shall be covered by a superstructure if determined necessary under local
climatic conditions. There shall be head room or adequate access to permit
visual inspection of the operation as necessary for maintenance.
C. Backwashing. The
source of backwash water upflow to cleanse the filter media shall be
disinfected and may be derived from filtered wastewater effluent, for all
treatment works with an average design flow equal to or greater than 0.1 mgd.
A design uniform backwash upflow minimum rate of 20
gallons per square foot per minute, consistent with wastewater temperatures and
the specific gravity of the filter media, shall be provided by the underdrain
or backwash distribution piping. The backwash rate may be reduced in accordance
with the demonstrated capability of other methods, such as air scour, provided
for cleaning of filter media.
1. The
design backwash flow shall be provided at the required rate by wash water pumps
or by gravity backwash supply storage. Two or more backwash pumps shall be
provided so that the required backwash flow rate is maintained with any single
pump out of service. Duplicate backwash waste pumps, each with a capacity
exceeding the design backwash rate by 20%, shall be provided as necessary to
return backwash to the upstream unit operations.
2. Sufficient backwash flow shall be provided
so that the time of backwash is not less than 15 minutes for treatment works
with design flows of 0.1 mgd or more, at the design rate of wash. A reduced
capacity can be provided if it can be demonstrated that a backwash period of
less than 15 minutes can result in a similar clean media bed headloss and a
similar filter operating period or run time.
3. The backwash control, or valves, as
provided on the main backwash water line, shall be sized so that the design
rate of filter backwash is obtained with the control or valve settings for the
individual filters approximately in a full open position. A means for air
release shall be provided between the backwash pump and the wash water
valve.
4. Air scouring, if
provided, should maintain three to five cubic feet per minute per square foot
of filter area for two to three minutes preceding backwash at the design
rate.
5. The bottom elevation of
the channel or top of the weir shall be located above the maximum level of
expanded media during back washing. In addition:
a. A backwash withdrawal arrangement for
optimizing removal of suspended solids shall be provided.
b. A two-inch filter wall freeboard is to be
provided at the maximum depth of backwash flow above the filter
media.
c. A level top or edge is
required to provide a uniform loading in gpm per foot of channel or weir
length.
d. An arrangement of
collection channels or weirs to provide uniform withdrawal of the backwash
water from across the filter surface shall be provided.
D. Deep bed filters. The deep bed
filter structure shall provide a minimum depth of 8-1/2 feet as measured from
the normal operating wastewater surface to the bottom of the underdrain system.
The structure should provide for a minimum applied wastewater depth of three
feet as measured from the normal operating wastewater surface to the surface of
the filter media.
1. Porous plate and
strainer bottoms are not recommended. The design of manifold type filtrate
collection or underdrain systems shall:
a.
Minimize loss of head in the manifold and baffles.
b. Assure even distribution of wash water and
a uniform rate of filtration over the entire area of the filter.
c. Provide the ratio of the area of the
underdrain orifices to the entire surface area of the filter media at about
0.003.
d. Provide the total
cross-sectional area of the laterals at about twice the area of the final
openings.
e. Provide a manifold
which has a minimum cross sectional area that is 1-1/2 times the total area of
the laterals.
2. Surface
wash means shall be provided unless other means of media agitation are
available during backwash. Disinfected, filtered water or wastewater effluent
shall be used as surface wash waters. Revolving type surface washers or an
equivalent system shall be provided. All rotary surface wash devices shall be
designed with:
a. Provisions for minimum wash
water pressures of 40 psi.
b.
Provisions for adequate surface wash water to provide 0.5 to 1.0 gallon per
minute per square foot of filter area.
3. Deep bed filters shall be supplied with:
a. A loss of head gauge.
b. A rate of flow gauge.
c. A rate of flow controller of either the
direct acting, indirect acting, constant rate, or declining rate
types.
d. If necessary, continuous
effluent turbidity monitoring.
e. A
rate of flow indicator on the main backwash water line, located so that it can
be easily read by the operator during the backwashing process.
E. Rapid rate filters.
The conventional design rapid rate of filtration shall not exceed five gallons
per minute per square foot of filter surface area. The selected filtration rate
shall be based upon the degree of treatment required and filter effluent
quality requirements.
1. A filtration media
sieve analysis shall be provided by the design consultant. The media shall be
clean silica sand having (i) a depth of not less than 27 inches and generally
not more than 30 inches after cleaning and scraping and (ii) an effective size
of 0.35 millimeters to 0.5 millimeters, depending upon the quality of the
applied wastewater, and (iii) a uniformity coefficient not greater than
1.6.
2. A sieve analysis for
supporting media shall be provided for the design. A three-inch layer of
torpedo sand shall be used as the supporting media for the filter sand. Such
torpedo sand shall have (i) an effective size of 0.8 millimeters to 2.0
millimeters and (ii) a uniformity coefficient not greater than 1.7.
3. A sieve analysis of anthracite media shall
be provided for the design, if used. Clean crushed anthracite or a combination
of sand and anthracite may be considered on the basis of experimental or
operational data specific to the project design. Such media shall have (i) an
effective size from 0.45 millimeters to 0.8 millimeters and (ii) a uniformity
coefficient not greater than 1.7.
4. Gravel used as a supporting media shall
consist of hard rounded particles and shall not include flat or elongated
particles. The coarsest gravel shall be 2-1/2 inches in size when the gravel
rests directly on the strainer system and must extend above the top of the
perforated laterals or strainer nozzles. Not less than four layers of gravel
shall be provided in accordance with the following size and depth distribution:
| SIZE |
DEPTH |
| 2-1/2 to 1-1/2 inches |
5 to 8 inches |
| 1-1/2 to 3/4 inches |
3 to 5 inches |
| 3/4 to 1/2 inch |
3 to 5 inches |
| 1/2 to 3/16 inch |
2 to 3 inches |
| 3/16 to 3/32 inch |
2 to 3 inches |
Reduction of gravel depth may be considered upon
application to the department and where proprietary filter bottoms are
proposed.
F.
High rate gravity filters. The highest average filtration rate shall not exceed
six gallons per minute per square foot unless the department can verify that a
higher rate meets treatment needs based on evaluation of pilot plant studies or
operational data. The selected filter rate shall be based upon the filter
effluent quality requirements.
The media provided for high rate filtration shall consist
of anthracite, silica sand or other suitable sand. Since certain manufacturers
are presently utilizing multiple media and homogeneous media that are
proprietary in nature, minimum standards are not established for filter media
depth, effective size and uniformity coefficient of filter media, or the
specific gravity of that media.
G. Shallow bed filters. The shallow bed
filtration rate should not exceed 1-1/4 gallons per minute per square foot and
shall not exceed two gallons per minute per square foot of filter area at
average design flow.
1. Chlorination prior to
shallow bed filtration shall be sufficient to maintain a chlorine residual of
one mg/l through the filter for a system with average design flow of 0.1 mgd or
greater.
2. Multiple unit
operations shall be provided to allow for continuous operability and
operational variability.
3. The
filter media shall consist of a series of up to eight inch filter increments
having a minimum total media depth of 11 inches. The sand media shall have an
effective size in the range of 0.40 mm to 0.65 mm and a uniformity coefficient
of 1.5 or less.
4. Filter inlets
shall consist of ports located throughout the length of the filter.
5. The filter underdrainage system shall be
provided along the entire length of the filter so that filter effluent is
uniformly withdrawn without clogging of the outlet openings provided for
collection and backwash.
6.
Duplicate backwash pumps, each capable of providing the required backwash flow,
shall be provided.
7. Facilities
shall be provided for addition of filter aid to strengthen floc prior to
filtration.
8. A skimmer shall be
provided for each filter.
H. Pressure filtration. Pressure filter rates
shall be consistent with those set forth in gravity filtration. Pressure filter
media shall be consistent with that set forth in gravity filtration.
1. For pressure filter operation. The design
should provide for:
a. Pressure gauges on the
inlet and outlet pipes of each filter to determine loss of head.
b. A conveniently located meter or flow
indicator with appropriate information to monitor each filter.
c. The means for filtration and backwashing
of each filter individually, using a minimally complex arrangement of
piping.
d. Flow indicators and
controls convenient and accessible for operating the control valves while
reading the flow indicators.
e. An
air release valve on the highest point of each filter.
2. The top of the wastewater collection
channel or weir shall be established at least 18 inches above the surface of
the media.
3. An underdrain system
to uniformly and efficiently collect filtered wastewater and that distributes
the backwash water at a uniform rate, not less than 15 gallons per minute per
square foot of filter area, shall be provided. A means to observe the wash
water during backwashing should be established.
4. Minimum sidewall heights of five feet
shall be provided for each filter. A corresponding reduction in sidewall height
is acceptable where proprietary bottoms permit reduction of the gravel
depth.
5. An accessible manhole
should be provided as required to facilitate inspections and repairs.
I. Traveling bridge. This type of
filter is normally equipped with a shallow bed divided into cells with a
continuously operated reciprocating cell-by-cell traveling backwash system.
This filter system shall comply with applicable design criteria set forth for
shallow bed filters. Use of these filters will be evaluated by the department
on a case-by-case basis.
J.
Microstraining. Microstraining involves the passing of treated effluent through
a horizontally mounted, rotating drum with a filtering fabric fixed to its
periphery by a porous screen. Microstrainer equipment is typically used to
improve treatment of biologically treated wastewater which has received
secondary clarification. Thus, biological attached growth can accumulate on the
filter fabric. Means to control such biological growth shall be addressed in
the design.
1. The most common screen opening
(aperture) sizes are 23, 35 and 60 microns, but other sizes may be available.
Normally, the larger sizes are used in cases when only the coarser solids are
desired to be removed. The type of mesh weave, when considered in conjunction
with aperture size, greatly affects the hydraulic capacity of a microstrainer.
Screen size selection must be based on the particle type and size to be
removed.
2. Screens are made from a
variety of woven metals and nonmetals, with stainless steel being the most
commonly used material. Nonmetallic filter cloths are especially suitable for
those applications where the presence of corrosive chemicals would be harmful
to metallic cloths. Chlorination immediately ahead of microstraining units
employing metallic cloths should be avoided.
3. The area of the submerged portion of the
screening fabric helps to govern the hydraulic capacity. Normal submergence is
2/3 to 3/4 of the drum diameter. The speed of rotation of the drum should be
based on particle type size to be removed. Decreasing the speed of rotation
causes increased removal efficiencies but has the effect of increasing the head
loss through the filter fabric and decreasing the hydraulic capacity of the
unit. The design rotational speed should be about seven rpm.
4. The backwash system should be designed to
serve the dual function of applying energy in the form of pressurized washwater
spray to the screen to dislodge retained particles and to collect and transport
the solids-laden washwater away from the microstrainer. The backwash system
shall be designed to minimize splash-over (solids-laden backwash spray water
that falls short or long of the washwater collector rather than into the
collector as intended). The microstrainer design shall provide for solids
retained on the screen which fall back into the drum pool. Backwashing shall be
continuous. Backwash water requirements should be based on particle type and
size to be removed. The volume of wash water required shall be determined on an
individual basis. The normal source of backwash water is the microstrainer
effluent collector. Normally only one-half of the backwash water volume
actually penetrates the screen; the rest, called a splashback, flows into the
effluent section. The backup system should minimize splashback. Increasing the
backwash flow and pressure has the tendency to decrease the headloss through
the screen. Up to 25% of the total throughput volume may be required for
backwash purposes, but averages of 1.0% to 5.0% are typical. Adequate backwash
waste storage and treatment facilities should be provided to dispose of the
removed materials within the design limitations of other system
components.
5. The most suitable
pressure differential through the screen shall be determined on an individual
basis. Usual pressure differential under normal operating conditions is 12 to
18 inches. The pressure applied to the screen affects the flow rate through the
screen. The low pressure requirement is one of the microstrainer's advantages.
The secondary effluent should not be pumped, but allowed to flow by gravity to
the microstrainer unit to minimize the shear force imparted to the fragile
biological floc.
6. Hydraulic
capacity of the microstrainer is affected by the rate of clogging of the
screening fabric. The accumulation or build-up of attached bio-mass on the
screen over time must be prevented. The use of ultraviolet light may reduce the
rate of such accumulation. Microstrainers shall not be utilized to treat
wastewaters containing high grease and oil concentrations, due to their
clogging effects. Iron and manganese buildups also tend to clog the screen.
Periodically, the screen must be taken out of service and cleaned.
Microstraining units shall be provided in sufficient numbers and capacities to
maintain 100% operability of the microstraining process. Automatic control of
drum speed and backwash pressure based on head loss through the screen shall be
utilized to help overcome this sensitivity problem.
7. Pilot plant studies can be conducted to
determine the applicability and design of the microstraining unit to the
specific wastewater to be treated. The hydraulic capacity of a microstrainer is
determined by the following: head applied, concentration of solids, size of
solids, nature of solids, rate of clogging, drum rotational speed, drum
submergence, mesh weave and aperture size. These factors are interrelated such
that a change in any one of them will cause a change in some or all of the
remaining factors.
K.
Nonfixed beds and upflow. Continuously backwashed and other nonfixed bed
filters are considered as nonconventional technology. Conventional design
standards may be established through evaluation of performance data as provided
for in this chapter.
L. Membrane,
ultra and micro. Filtration of treated effluent through membranes and other
media involving molecular sized removal is considered nonconventional
technology. Application of this technology will be considered based on
evaluation of performance data as provided for in this chapter.
M. Carbon adsorption. Carbon adsorption
involves the interphase accumulation or concentration of dissolved substances
at a surface or solid-liquid interface by an adsorption process. Activated
carbon, which is generally a wood or coal char developed from extreme heat, can
be used in powdered form (PAC) or granular form (GAC). Generally, carbon
adsorption is used as the polishing process to remove dissolved organic
material remaining in a wastewater treated to a secondary or advanced level.
Activated carbon adsorption can also be used for dechlorination.
1. Parameters with general application to
design of carbon adsorption units are carbon properties, contact time,
hydraulic loading, carbon particle size, pH, temperature and wastewater
composition, including concentrations of suspended solids and other
pollutants.
2. The adsorption
characteristics of the type of carbon to be used shall be established. Such
characteristics may be established using jar test analyses of various activated
carbons in reaction with the waste to be treated. Adsorption isotherms for each
form of carbon proposed for use shall be determined. The source and
availability of replacement carbon, as designed, shall be addressed.
3. Pilot plant studies shall be performed
upon the selected carbon using the wastewater to be adsorbed, where industrial
and domestic wastes are present to determine: breakpoint, exhaustion rate,
contact time to achieve effluent standards; and if applicable, the backwash
frequency, pressure drop through the fixed bed columns, and the carbon
regeneration capacity required. Where strictly domestic waste is to be treated,
data from similar full scale unit operations or pilot plant data will be
acceptable.
4. Where carbon
regeneration is provided, carbon loss due to transportation between the columns
and regeneration furnace in the range of five to 10 percent total carbon usage
shall be considered normal for design. The rate at which carbon will lose
adsorption capacity with each regeneration should be established.
5. If fixed-bed GAC carbon columns must be
backwashed to remove solids entrapped in the carbon material, then backwash
facilities shall provide for expansion of the bed by at least 30%.
6. Carbon adsorption unit operations may be
provided in parallel or series. Sufficient capacity shall be provided to allow
for continuous operability of the carbon adsorption process.
7. Nonfixed bed carbon adsorption unit
operations may be operated in the upflow or downflow mode. Duplicate pumping
units shall be provided for such unit operations.
8. Carbon adsorption unit operations should
provide for purging with chlorine or other oxidants as necessary for odor and
bio-mass control.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
