Section 370.900 Trickling Filters

Section 370

Section 370.900  Trickling

Filters

a)         General

1)         Applicability

Trickling filters may be used for treatment of sewage

amenable to treatment by aerobic biologic processes. Trickling filters shall be

preceded by settling tanks equipped with scum and grease collecting devices, or

other suitable pretreatment facilities.

2)         Design Basis

Filters shall be designed so as to provide the required

reduction in biochemical oxygen demand, ammonia nitrogen, or to properly

condition the sewage for subsequent treatment processes.

3)         Multiple Units

Multiple trickling filter units capable of independent

operation are recommended for all plants and must be provided for those plants

where the design average flow exceeds 100,000 gallons per day.  Plants not

having multiple units shall include other provisions to assure continuity of treatment.

b)         Dosing Equipment

1)         Distribution

A)        All hydraulic factors involving proper distribution of sewage

on the filter should be carefully calculated and submitted with the basis of

design.

B)        The sewage may be distributed over the filter by rotary

distributors or other suitable devices which will permit reasonably uniform

distribution to the surface area.  At design average flow, the deviation from

calculated uniformly distributed volume per square foot of the filter surface

shall not exceed plus or minus 10 percent at any point.

2)         Dosing and Recirculation

A)        Sewage may be applied to the filters by siphons, pumps or by

gravity discharge from preceding treatment units when suitable flow

characteristics have been developed.  Application of the sewage should be

continuous except for low rate filters.  A hydraulic system for recirculation

shall be provided for new facilities and should be considered where existing

trickling filter units are included in treatment plant upgrading

e filters by siphons, pumps or by

gravity discharge from preceding treatment units when suitable flow

characteristics have been developed.  Application of the sewage should be

continuous except for low rate filters.  A hydraulic system for recirculation

shall be provided for new facilities and should be considered where existing

trickling filter units are included in treatment plant upgrading.

B)        The piping system, including dosing equipment and distributor,

shall be designed to provide capacity for the peak hourly flow rate including

recirculation rates determined under subsection (h).

3)         Distributor Head Requirements

For reaction type distributors, a minimum head of 24 inches

between low water level in siphon chamber and center of arms is required.

Similar allowances shall be made in design for added pumping head requirements

where pumping to the reaction type distributor is used.  The design shall

include the head required at the center column for the full range of flows, taking

into account all head losses from the center column back to the dosing facility

at all water levels.  Calculations shall be submitted to justify the basis of

design.

4)         Clearance

A minimum clearance of 6 inches between media and distributor

arms shall be provided.  Refer to subsection (e)(4).

c)         Media

1)         Quality

The media may be crushed rock, slag or specially manufactured

material.  The media shall be durable, resistant to spalling or flaking, and be

relatively insoluble in sewage.  The top 18 inches shall have a loss by the

20-cycle, sodium sulfate soundness test of not more than 10 percent, as

prescribed by ASCE Manual of Engineering Practice, Number 13, the balance to

pass a 10-cycle test using the same criteria.  Slag media shall be free from

iron.  Manufactured media shall be resistant to ultraviolet degradation,

disintegration, erosion, aging, all common acid and alkalies, organic

compounds, and fungus and other biological attack

e soundness test of not more than 10 percent, as

prescribed by ASCE Manual of Engineering Practice, Number 13, the balance to

pass a 10-cycle test using the same criteria.  Slag media shall be free from

iron.  Manufactured media shall be resistant to ultraviolet degradation,

disintegration, erosion, aging, all common acid and alkalies, organic

compounds, and fungus and other biological attack. Such media shall be

structurally capable of supporting a man's weight or a suitable access walkway

shall be provided to allow for distributor maintenance.

2)         Depth

The filter media shall have a minimum depth of 6 feet above

the underdrains.  For rock media filters (subsection (c)(3)(A)), only the top 7

feet of the volume of the filter shall be considered in BOD removal credit

computations. For manufactured media filters see subsection (c)(3)(B).

3)         Size and Grading of Media

A)        Rock, Slag and Similar Media

i)          Rock, slag and similar media shall not contain more than 5

percent by weight of pieces whose longest dimension is 3 times the least

dimension.

ii)         Media shall be free from thin elongated and flat pieces,

dust, clay, sand, or fine material and shall conform to the following size and

grading when mechanically graded over vibrating screen with square openings:

Passing 4 1/2 inch screen – 100% by weight

Retained on 3 inch screen – 95-100% by weight

Passing 2 inch screen – 0-2% by weight

Passing 1 inch screen – 0-1% by weight

B)        Manufactured Media

Suitability of size, space, media configuration and depth

will be evaluated on the basis of experience with installations handling

similar wastes and loadings.  To ensure sufficient void clearance, media with a

specific surface area of no more than 30 square feet per cubic foot may be used

for filters employed for carbonaceous reduction, and media with a specific

surface area of no more than 45 square feet per cubic foot may be used for

second stage ammonia reduction

on the basis of experience with installations handling

similar wastes and loadings.  To ensure sufficient void clearance, media with a

specific surface area of no more than 30 square feet per cubic foot may be used

for filters employed for carbonaceous reduction, and media with a specific

surface area of no more than 45 square feet per cubic foot may be used for

second stage ammonia reduction.  See subsection (c)(1) for quality

requirements.

4)         Handling and Placing of Media

A)        Material delivered to the filter site shall be stored on wood

planks or other approved clean hard surfaced areas.

B)        All material shall be rehandled at the filter site and no

material shall be dumped directly into the filter.  Crushed rock, slag and

similar media shall be rescreened or forked at the filter site to remove all

fines.

C)        The material shall be placed by hand to a depth of 12 inches

above the tile underdrains and all material shall be carefully placed so as not

to damage the underdrains.  The remainder of the material may be placed by

means of belt conveyors or equally effective methods approved by the engineer.

D)        Manufactured media shall be handled and placed as recommended

by the manufacturer and approved by the engineer.

E)        Trucks, tractors, or other heavy equipment shall not be driven

over the filter during or after construction.

d)         Underdrainage System

1)         Arrangement

Underdrains with semi-circular inverts or equivalent should

be provided and the underdrainage system shall cover the entire floor of the

filter.  Inlet openings into the underdrains shall have an unsubmerged gross

combined area equal to at least 15 percent of the surface area of the filter.

2)         Slope

The underdrains shall have a minimum slope of 1 percent

ge System

1)         Arrangement

Underdrains with semi-circular inverts or equivalent should

be provided and the underdrainage system shall cover the entire floor of the

filter.  Inlet openings into the underdrains shall have an unsubmerged gross

combined area equal to at least 15 percent of the surface area of the filter.

2)         Slope

The underdrains shall have a minimum slope of 1 percent.

Effluent channels shall be designed to produce a minimum velocity of 2 feet per

second at design average flow of application to the filter and shall have

adequate capacity for the peak hourly flow rate including the required

recirculation flows.

3)         Flushing

Provision should be made for flushing the underdrains.  In

small filters, use of a peripheral head channel with vertical vents is

acceptable for flushing purposes. Inspection facilities should be provided.

4)         Ventilation Requirements for Underdrains

The underdrainage system, effluent channels, and effluent

pipe should be designed to permit free passage of air.  The size of drains,

channels, and pipe should be such that not more than 50 percent of their

cross-sectional area will be submerged under the design hydraulic loading.

Consideration should be given in the design of the effluent channels to the

possibility of increased hydraulic loading.

e)         Special Features

1)         Flooding

Provision shall be made in the design of conventional rock

filter structures so that the media may be flooded.

2)         Maintenance

All distribution devices, underdrains, channels and pipes

shall be designed so that they may be properly maintained, flushed or drained.

3)         Flow Measurement

Devices shall be provided to permit measurement of flow to

the filter, and of recirculated flows.

4)         Protection From Freezing

Trickling filters shall be covered to protect from freezing,

and to maintain operation and treatment efficiencies

n devices, underdrains, channels and pipes

shall be designed so that they may be properly maintained, flushed or drained.

3)         Flow Measurement

Devices shall be provided to permit measurement of flow to

the filter, and of recirculated flows.

4)         Protection From Freezing

Trickling filters shall be covered to protect from freezing,

and to maintain operation and treatment efficiencies.  The filter cover shall

be constructed of appropriate corrosion resistant materials and designed to

allow operator access for maintenance, repair and replacement of the filter

dosing equipment.

5)         Ventilation of Covered Filters

Forced ventilation shall be provided for covered trickling

filters to insure adequate oxygen for process requirements. Windows or simple

louvered mechanisms so arranged to insure air distribution throughout the

enclosure shall be provided.  The ventilation facilities shall be designed to

allow operator control of air flow in accordance with outside temperature.

Design computations showing the adequacy of air flow to satisfy process oxygen

requirements shall be submitted.

f)         Two-Stage Filters

The foregoing standards also apply to second stage filters.

g)         Special Applications

1)         Roughing Filters

In some instances it is desirable to partially reduce the

organic strength of wastewaters.  In such cases trickling filters may be used

for roughing treatment.  Design parameters and contaminant removal efficiencies

will be approved on a case-by-case basis.  Refer to subsections (h)(2) and

standards also apply to second stage filters.

g)         Special Applications

1)         Roughing Filters

In some instances it is desirable to partially reduce the

organic strength of wastewaters.  In such cases trickling filters may be used

for roughing treatment.  Design parameters and contaminant removal efficiencies

will be approved on a case-by-case basis.  Refer to subsections (h)(2) and

(h)(3).

2)         Nitrifying Filters

Trickling filters may, under favorable conditions, be used as

nitrification devices.  Design parameters and contaminant removal efficiencies

will be approved on a case-by-case basis.  Refer to Section 370.1210(d).

h)         Efficiency

1)         Single Stage, Settling Tank – No Recirculation

Expected reduction of BOD of settled normal domestic

wastewater by a single stage filter, packed with crushed rock, slag or similar

material and with subsequent settling, shall be determined from Appendix F,

Figure No. 3. In developing this curve, loading due to recirculated sewage has

not been considered.

2)         Single or Multi-Stage, Settling Tank – Recirculation

Expected BOD removal efficiencies may also be determined by

theoretical and empirical formula if accompanied by detailed explanation,

particularly for roughing filters and for filters with recirculation.  (Refer

to WEF Manual of Practice (MOP) No. 8, "Design of Municipal Wastewater

Treatment Plants", vol. 1 (1992).)

3)         Single or Multi-Stage, No Settling Tank – Recirculation

Filters not followed by a settling tank and discharging into a subsequent

treatment process shall not be credited with BOD removal efficiencies as in

subsections (h)(1) and (h)(2) above.  Expected performance in such cases, including

filters packed with manufactured media, shall be determined from prototype

testing and full-scale plant experience.