Section 110.19. Trickling filters.

 

(1)   Applicability.

(a) Surface water discharge. New trickling filters shall be used in conjunction with other treatment units which, in combination, will produce an acceptable level of treatment as defined in s. NR 110.15 (2) (a) . Existing trickling filters may be used as a treatment unit in plant expansion if the effluent quality requirements of s. NR 110.15 (2) (a) are met.

(b) Land disposal. Trickling filter treatment systems are an acceptable means of treatment prior to land disposal of effluent.

(2)   Design report. A design report must be submitted in accordance with s. NR 110.15 (1) . The report shall show the empirical equations and the assumptions used for designing the trickling filter and the additional treatment units.

(3)   Design requirements.

(a) Recirculation. Recirculation shall be provided for intermediate and high-rate filters to increase treatment efficiency and to provide wetting of the biological growth. The recirculation rate shall be variable. The recirculation rate to average influent flow ratio should not exceed 4:1.

(b) Dosing cycle. The interval between dosing cycles may not exceed one hour.

(c) Flooding. Filter structures should be designed to allow flooding of the filter.

(d) Primary treatment. Trickling filters shall be preceded by primary treatment facilities.

(4)   Design loading. Hydraulic and organic loadings to trickling filters may not exceed the values given in Table 4. Higher loadings may be approved if justified by pilot studies or if manufactured media is used. Higher loadings may also be used if the trickling filter is intended to act only as a roughing or polishing treatment unit.

Table 4 - See PDF for table

(5)   Hydraulic features.

(a) Dosing equipment.

1. Sewage shall be distributed over the filter by rotary distributors or other suitable devices which will permit uniform distribution to the filter surface area.

2. Sewage shall be applied to the filters by siphons, pumps or by gravity discharge from preceding treatment units when suitable flow characteristics have been developed. The dosing rate shall be large enough to insure rotation of the distributor arms.

3. A minimum clearance of 15 centimeters (6 inches) between media and distributor arms shall be provided.

(b) Underdrainage system.

1. An underdrainage system which covers the entire floor of the filter shall be provided. Inlet openings into the underdrains shall have an unsubmerged gross combined area equal to at least 15% of the surface area of the filter.

2. The underdrains shall have a minimum slope of 1%. Effluent channels shall be designed to produce a minimum velocity of 60 centimeters per second (2 feet per second) at the average design flow of application to the filter including recirculated flows.

(6)   Media.

(a) Type. The media shall be crushed rock, slag or material specially manufactured for use as media in trickling filters.

(b) Quality.

1. Rock and slag media shall be durable, resistant to spalling or flaking, and be relatively insoluble in sewage. Slag media may not contain iron.

2. Manufactured media shall be resistant to ultraviolet degradation, disintegration, erosion, aging, all common acids and alkalies, organic compounds, and fungus or biological attack. Either the media shall be structurally capable of supporting a person's weight or a suitable access walkway shall be provided to allow for distributor maintenance.

(c) Size and grading of rock, slag and similar media.

1. Rock, slag and similar media may not contain more than 5% by weight of pieces whose longest dimension is 3 times the least dimension. They shall be free from thin elongated and flat pieces, dust, clay, sand, or fine material and shall conform to the following size and gradings when mechanically graded over a vibrating screen with square openings:

a. Passing 11.4 centimeter (4 1/2-inch) screen — 100% by weight;

b. Retained on 7.6 centimeter (3-inch) screen — 95-100% by weight;

c. Passing 5.1 centimeter (2-inch) screen — 0.2% by weight;

d. Passing 2.5 centimeter (1-inch) screen — 0.1% by weight;

2. The department may approve other rock media gradations provided the gradations are consistent with accepted published engineering practices.

(7)   Structural features.

(a) Mercury seals. Mercury seals may not be used on trickling filter distributors. Mercury seals shall be removed from existing filters during renovation. Mercury seals removed from existing filters during renovation or destruction of the filter shall be disposed in a location and manner approved by the department.

(b) Depth of media. Rock or slag filter media, or loose synthetic media, shall have a minimum depth of 1.5 meters (5 feet) above the underdrains. Synthetic corrugated filter media should have a minimum depth of 3 meters (10 feet) to provide adequate contact time with the wastewater. Rock or slag filter media depths may not exceed 3 meters (10 feet) and synthetic filter media depths may not exceed 9.1 meters (30 feet) except where special construction is justified through pilot studies.

(c) Covers. Covers shall be provided on all filters to prevent icing and freezing and to increase the treatment efficiency of the filter during winter conditions.

(d) Ventilation.

1. The underdrainage system, effluent channels, and effluent pipe shall be designed to permit free passage of air. The size of drains, channels and pipe shall be such that not more than 50% of their cross-sectional area will be submerged under the maximum daily design hydraulic loading. The design of the effluent channels should consider the probability of increased hydraulic loading.

2. Filter covers shall be designed to allow adequate ventilation to maintain the filter in an aerobic state at all times.

(e) Maintenance. All distribution devices, underdrains, channels and pipes shall be installed so that they may be properly maintained, flushed and drained.

(f) Flow measurement. Devices shall be provided to permit measurement of flow to the filter, including the amount of recirculated flow.

History: Cr. Register, November, 1974, No. 227 , eff. 12-1-74; r. and recr. Register, February, 1983, No. 326 , eff. 3-1-83; CR 09-123 : am. (5) (b) 2. and (6) (b) 1. Register July 2010 No. 655 , eff. 8-1-10.