SECTION 217.250. Sludge Dewatering

(a) The engineering report must include a justification for the proposed sludge dewatering units, including design calculations, results from any pilot studies, all assumptions, and appropriate references.

(b) The design of a dewatering unit must be based on mass balance principles.

(c) General Requirements.

(1) Centrate or Filtrate Recycle.

(A) The drainage from beds and centrate or filtrate from dewatering units must be returned to the headworks of the wastewater treatment facility for treatment.

(B) The design of a treatment unit downstream from a dewatering unit must be based on the organic load from the centrate or filtrate recycle.

(2) Sludge with Industrial Waste Contributions. A dewatering system must prevent the release of any constituent (such as a free metal, an organic toxin, or a strong reducing or oxidizing compound) that adversely impacts human health, safety, or welfare, water quality, or compliance with the associated wastewater permit.

(3) Redundancy.

(A) A mechanical dewatering system must have at least two dewatering units, unless the engineering report justifies adequate storage or an alternative means of sludge handling.

(B) Mechanical dewatering units must be able to dewater the average daily sludge flow with the largest dewatering unit out of service.

(4) Storage Requirements.

(A) A mechanical dewatering system must have separate storage if the equipment will not operate on a continuous basis and the wastewater treatment facility has no digesters with built-in short-term storage.

(B) In-line storage of stabilized or unstabilized sludge must not interfere with any treatment unit.

(C) The separate sludge storage from a primary digester must be aerated and mixed to prevent nuisance odor conditions.

(5) Sampling Points. A dewatering system must have sampling stations before and after each dewatering unit and must allow periodic evaluation of the dewatering process.

(6) Maintenance. Each dewatering system unit must have a bypass to allow for maintenance, repair, and replacement. The engineer must specify where the bypass flow will be routed in the engineering report.

(d) Sludge Conditioning.

(1) The design and location of a chemical addition point must consider interactions of the chemical with other chemicals and processes used in the wastewater treatment facility.

(2) A dewatering system must provide adequate mixing time for the reaction between an additive and the sludge. Any subsequent handling must eliminate floc shearing.

(3) The engineering report must include a pilot plant or full-size performance data used to determine the characteristics and design dosage of any sludge additive.

(4) The engineering report must justify the in-stream flocculation and coagulation system design by including comparable performance data or pilot plant data.

(5) The engineering report must include whether the mixers require conditioning tanks.

(6) The engineering report must include calculations for a range of detention times.

(7) Solution storage capacity, at maximum chemical demand, must be based on:

(A) the amount of chemical needed per shift for continuous processes; or

(B) the amount of chemical needed for a full batch for intermittent and batch processes.

(8) Solution storage capacity may be reduced if the specific chemical or additive selected is adversely affected by storage.

(9) The engineering report must justify any storage volume reduction and any other method used to ensure a continuous supply of chemicals by accounting for chemical use through a full operating day or a full batch.

(e) Sludge Drying Beds.

(1) The size of sludge drying beds must be based on data from a similar wastewater treatment facility in the same geographical area with the same influent sludge characteristics.

(2) If the data required by paragraph (1) of this subsection is not available, or if the executive director determines that the data is not appropriate for a proposed wastewater treatment facility, the design of sludge drying beds must be based on the following:

(A) Open Beds.

(i) A sludge drying bed system must have at least two sludge drying beds.

(ii) The engineering report must include the calculation of the minimum surface area for a sludge drying bed using the values in the following figure for an area of the state with less than 45 inches annual average rainfall or less than 50% annual average relative humidity, as determined by data from the nearest National Oceanic and Atmospheric Administration's weather station that has at least ten years of data. The entire period of record for the weather station must be used.

Attached Graphic

(iii) Another method of sludge dewatering is required in lieu of a sludge drying bed in an area of the state that experiences either 45 or more inches of average annual rainfall or 50% or greater annual average relative humidity, as determined by data from the nearest National Oceanic and Atmospheric Administration's weather station that has at least ten years of data. The entire period of record for the weather station must be used.

(iv) A sludge drying bed system must:

(I) dewater sludge during normal operations;

(II) provide accelerated sludge dewatering during abnormally wet conditions;

(III) store accumulated sludge during periods of extended high humidity and rainfall;

(IV) use an alternative dewatering method to dewater the sludge during periods of extended high humidity and rainfall; and

(V) prevent the unauthorized discharge of solids from the sludge drying beds.

(v) The engineering report must justify the use of innovative or non-conforming sludge drying beds in high rainfall, high relative humidity areas of the state, as described in clause (iii) of this subparagraph.

(B) Gravel Media Beds. A gravel media bed must be laid in two or more layers. The gravel around the underdrains must be properly sized to allow drainage. The gravel around the underdrains must be at least 12 inches deep, extending at least 6.0 inches above the top of the underdrains. The top layer of a gravel media bed must be at least 3.0 inches thick and must consist of gravel 1/8 inch to 1/4 inch in size.

(C) Sand Media Beds. A sand media bed must consist of at least 12 inches of sand with a uniformity coefficient of less than 4.0 and an effective grain size of at least 0.3 millimeters but not more than 75 millimeters above the top of the underdrain.

(D) Underdrains.

(i) The underdrains must be at least 4.0 inches in diameter and a slope of at least 1.0% to the drain.

(ii) The underdrains must not be spaced more than 20 feet apart.

(iii) The engineering report must specify where the flow from the underdrains will be routed.

(E) Decanting. A sludge drying bed may have a method of decanting supernatant installed on the perimeter of the bed. The decanted liquid from a sludge drying bed must be returned to the headworks of the wastewater treatment facility or to the beginning of the secondary treatment process.

(F) Walls.

(i) The interior walls of a sludge drying bed must be watertight and extend 12 inches to 24 inches above and at least 6 inches below the bed surface.

(ii) The exterior walls of a sludge drying bed must be watertight and extend 12 inches to 24 inches above the bed surface or ground elevation, whichever is higher.

(G) Sludge Removal.

(i) A sludge drying bed system must be arranged to facilitate sludge removal.

(ii) The sludge drying beds must have concrete pads for vehicle support tracks on 20 foot centers for all percolation type sludge beds.

(H) Sludge Influent.

(i) A sludge pipe to the sludge drying beds must terminate at least 12 inches above the surface of the media and be arranged so that the pipe drains to a sump that pumps to the headworks of the wastewater treatment facility or the influent lift station.

(ii) A sludge discharge point must have a concrete splash plate.

(I) Drying Bed Bottom.

(i) The bottom of a sludge drying bed must consist of a layer of clayey subsoil having a thickness of a least 1.0 foot and a permeability of less than 1 × 10^-7 centimeters per second.

(ii) An impermeable concrete pad must be installed over a liner in locations where the groundwater table is within 4.0 feet of the bottom of the liner.

(3) Innovative or Non-Conforming Sludge Drying Beds. The executive director will review any vacuum assisted sludge drying beds or other variations to the gravity drying bed concept as innovative and non-conforming technologies subject to §217.7(b)(2) of this title (relating to Types of Plans and Specifications Approvals).

(4) Rotary Vacuum Filtration.

(A) Filtration Rate. The engineering report must justify the value calculated for the rates of filtering for various types of sludge with proper conditioning, using Table J.4. in Figure: 30 TAC §217.250(e)(4)(A).

Attached Graphic

(B) Duplicate Equipment. Unless two treatment trains are provided, a feed pump, a vacuum pump, and a filtrate pump must be provided in duplicate to allow equipment alternation. Spare filter fabric must be kept at the wastewater treatment facility, except when metal coil filters are used.

(C) Filter Equipment. Parts that will get wet must be constructed of corrosion-resistant material. Drum and agitator assemblies must be equipped with variable-speed drives, and provisions must be made for adjusting the liquid level in the filtration system.

(D) Pumps.

(i) A vacuum pump with a capacity of at least 1.5 cubic feet per minute per square foot must be provided for metal-covered drums.

(ii) A dry-type vacuum pump must have a vacuum receiver.

(iii) A filtrate pump must have adequate capacity to pump the maximum amount of liquid to be removed from the sludge.

(iv) Each filter must be fed by a separate feed pump to ensure a proper feed rate.

(5) Centrifugal Dewatering.

(A) The engineering report must justify the sizing and design of a centrifugation system. A centrifuge design must be based on performance data from a similar centrifuge when available. If no performance data is available, the results of a pilot or full-scale test must be used.

(B) Selection of a material for a scroll must include consideration of the amount of grit expected in the sludge.

(C) A centrifugation system must include adequate sludge storage, based on the disposal process.

(D) Unless two treatment trains are provided, a centrifugation system must have the following spare equipment, including necessary connecting pipes and electrical controls:

(i) drive motor;

(ii) gear assembly; and

(iii) feed pump.

(E) A feed pump must have a variable speed drive.

(F) Each centrifuge must have a separate feed system.

(G) A centrifuge must be equipped for variable scroll speed and pool depth.

(H) A centirugation system must have a crane or monorail for equipment removal and maintenance.

(I) A centrifuge system must provide access for wash down of the interior of each centrifuge.

(6) Plate and Frame Presses.

(A) Sizing.

(i) The design of a plate and frame press must be based on performance data developed from a plate and frame press of similar size with similar operational characteristics. If no performance data is available, the results of pilot scale tests or full-scale tests must be used.

(ii) The design of a plate and frame press may be based on appropriate scale-up factors for full size designs if pilot scale testing is done in lieu of full-scale testing.

(iii) The engineering report must justify the size of a plate and frame press.

(B) Duplicate Equipment and Spare Parts. Unless multiple units are provided, a plate and frame press system must include the following spare equipment:

(i) one duplicate feed pump;

(ii) two extra plates, or at least one extra plate for every ten plates required for startup, whichever is greater;

(iii) one complete filter fabric set;

(iv) one closure drive system;

(v) one air compressor; and

(vi) one washwater booster pump.

(C) Operational Requirements.

(i) The filter feed pumps must be able to handle initial high volume flow, low pressure filling, and sustained periods of operating at 100 pounds per square inch to 225 pounds per square inch.

(ii) A plate and frame press system may include an integral pressure vessel to produce initial high volume flow.

(iii) A plate and frame press system may use operating pressures less than 225 pounds per square inch if the engineering report justifies the design operating pressure using actual performance data from a plate and frame press that processes similar sludge.

(iv) A plate and frame press system may include provisions for sludge-cake breaking to protect or enhance down line processes.

(D) Maintenance.

(i) A plate and frame press system must have a crane or monorail capable of removing the plates.

(ii) A plate and frame press system must have a high-pressure water or acid wash system to clean the filter.

(7) Belt Presses.

(A) Sizing.

(i) Actual performance data developed from a wastewater treatment facility with similar operational characteristics must be used to size a belt press system. If pilot plant testing is performed in lieu of full-scale testing, appropriate scale-up factors must be used to develop a full-scale design.

(ii) A belt press system must have a duplicate belt press, or another method of sludge processing or disposal that has been approved in writing by the executive director, if the design flow exceeds 4.0 million gallons per day.

(iii) The engineering report must include all data used to size a belt press system.

(B) Duplicate Equipment and Spare Parts. Unless multiple belt press units are provided, a belt press system must have the following spare equipment:

(i) one duplicate feed pump;

(ii) one washwater booster pump;

(iii) one complete set of belts;

(iv) one set of bearings for each type of press bearing;

(v) one set of tensioning equipment;

(vi) one set of tracking sensors;

(vii) one set of wash nozzles;

(viii) one doctor blade; and

(ix) conditioning or flocculation drive equipment.

(C) Conditioning. The engineering report must include the polymer selection methodology, account for sludge variability, and document the anticipated sludge loading to the belt press.

(D) Sludge Feed.

(i) The sludge must be fed at a relatively constant rate that does not interfere with polymer addition and belt press operation.

(ii) The engineering report must include the range in feed rate variability.

(iii) A belt press system may include grinders ahead of a flocculation system.

(iv) The sludge feed must provide a method for uniform sludge dispersion on a belt.

(v) A belt press system must use thickening of the feed sludge, unless the engineering report justifies separate thickening or dual purpose thickening.

(E) Filter Press Belts.

(i) A belt must have a variable speed drive.

(ii) A belt press system must have belt tracking and tensioning equipment.

(iii) The engineering report must justify the weave, material, width, and thickness of the belts.

(F) Filter Press Rollers.

(i) The rollers must have a protective finish.

(ii) The maximum roller deflection and operating tension of the belt must be included in the engineering report to justify equipment selection.

(iii) The roller bearings must be watertight and rated for a life of 100,000 hours.

(G) Spray Wash System.

(i) A belt press system must use high-pressure wash water for each belt.

(ii) The design of a spray wash system must specify the operating pressure at the point of washwater discharge.

(iii) A spray wash system must allow cleaning without interfering with the spray wash system operation.

(iv) The engineering report must justify the nozzle and nozzle cleaning system selection.

(v) A belt press system must have replaceable spray nozzles and spray curtains.

(H) Maintenance Requirements.

(i) A belt press system must have drip trays under the belt press and under the thickener when gravity belt thickening is employed.

(ii) The side and floor of a belt press must have adequate clearance for maintenance and removal of the dewatered sludge.

(iii) An electrical panel or other component subject to corrosion must be protected from splashes and corrosive gases, or be located outside of the belt press area.

(iv) A doctor blade clearance must be adjustable.

Source Note: The provisions of this §217.250 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254