020-11 Wyo. Code R. § 11-20

Current through April 27, 2019

Section 11-20 - Sludge Handling, Treatment and Disposal

(a) Pumping.

• (i) Design requirements. Sludge pumps shall be provided with a positive suction pressure at the pump impeller, rotor or plunger at dynamic conditions. Discharge pressure shall include static pressure difference and system friction losses based on the higher viscosity of the sludge than water.

• (ii) Piping and valves.
  • (A) Minimum size. Sludge piping and valves shall at least four (4) inches (10.2 cm) in diameter for pressure piping and six inches (15.2 cm) in diameter for gravity pipe. Pump suction and discharge shall not be less than three (3) inches (7.6 cm) in diameter.
  • (B) Minimum velocity. For sludge pipes larger than four (4) inches (10.2 cm) in diameter, the minimum velocity shall be one fps (0.3 m/sec).

(b) Thickening.

• (i) Types.
  • (A) Gravity. Gravity thickening shall only be used for primary sludge, digested primary sludge, lime sludge, or combinations of lime sludge, trickling filter humus and primary sludge.
  • (B) Dissolved air flotation. Dissolved air flotation shall only be used for combination of primary and biological sludges, waste biological sludges, and aluminum and iron salt sludges.

• (ii) Design parameters.
  • (A) Influent solids concentration. The design for influent solids concentrations to gravity or flotation thickeners shall be 5,000 mg/L or less, except tertiary lime sludge.
  • (B) Operating schedule. Sludge thickening facilities shall have the capacity to treat the maximum amount of solids produced. Where intermittent operation is provided, sludge holding tanks ahead of and after the thickening process shall be provided.
  • (C) Solids loading. Solids loadings (solids applied to the thickener) on thickening devices shall be limited to the following maximum values.

    Sludge Type	Solids Loading lb/sq ft/day Gravity	Dissolved Air Flotation	kg/m2/d Gravity	Dissolved Air Flotation
    Primary	24	NA	117.2
    Digested	20	NA	97.6
    primary
    Waste
    activated,
    without	NA	12		58.6
    polymer with polymer		48		234.3
    Primary and	15	--	73.2
    trickling filter
    Anaerobically	NA	NA
    digested
    primary and
    activated
    Primary and	20	NA	97.6
    lime Tertiary lime	60	NA	292.9
    Alum	NA	12		58.6
    *NA - Not allow	ed.

  • (D) Hydraulic loading. Gravity thickeners shall be designed for 400-800 gpd/ sq ft (16.3 m³/m²/d to 32.5 m³/m²/d) of surface area.

• (iii) Number of units. Unless sludge storage capacity for three (3) days is provided, there shall be at least two (2) units of equal capacity provided for sludge thickening.

• (iv) Controls. Controls for gravity and flotation sludge thickening operations shall include provision for influent flow rate control. Centrifuge thickening shall include adjustable manual controls for differential scroll speed, pool depth, and influent flow rate. Where chemical conditioning is required, chemical dosage rate shall have adjustable manual controls.

• (v) Side stream waste characteristics. The flow, organic load, and solids load in the thickener return flow to the plant shall be included in the plant design loadings.

• (vi) Odor control. Provisions shall be made for the continuous chlorination of gravity thickener influent. Any thickening installation for anaerobically digested sludge shall make provisions for enclosing zones where the sludge or decant is exposed to atmosphere, exhausting the zone at an adequate rate to prevent escape of gas, and treating the exhaust air for removal of odor causing agents.

(c) Aerobic digestion.

• (i) Solids retention time. Solids shall be retained in the aerobic digester for thirty (30) days for primary sludge and twenty (20) days for waste sludge from conventional activated sludge systems. Waste activated sludge from extended aeration systems shall be retained for a minimum of ten (10) days.

• (ii) Mixing and aeration requirements. Aeration requirements shall include the oxygen requirements for BOD stabilization, nitrification of ammonia nitrogen in the sludge, and nitrification of organic nitrogen in raw sewage solids and biological solids. A minimum dissolved oxygen of 2 mg/l shall be maintained. Minimum aeration requirements shall be:

  Sludge	CFM/1,000 lb solids/day	m3/min/1,000 kg/d
  Extended Aeration	300	18.7
  Conventional Activated Sludge	800	50.0
  Primary Sludge	2,100	131.0

  The aerobic digester aeration shall be provided with nonclog diffused aeration. Mechanical surface aerators shall not be allowed. Aeration provisions shall be a minimum of 30 cfm/1,000 cu ft (30 m³/min/1,000 m³) of volume.

• (iii) Number of digesters. Where aerobic digesters are used, two (2) or more shall be provided for treatment plants having an average design capacity of 100,000 gpd or more. Multiple aerobic digesters shall be arranged to permit either parallel or series operation.

• (iv) Supernatant removal and disposal. Supernatant shall be returned prior to the influent of the biological treatment process.

(d) Anaerobic digestion.

• (i) Sludge characteristics. The minimum sludge concentration for feed to anaerobic digesters is four percent.

• (ii) Number of digesters. Two or more digesters shall be provided for treatment plants having an average design capacity of 100,000 gpd (378.54 m³/d) or more.

• (iii) Design requirements.
  • (A) Temperature. Primary anaerobic digesters shall be heated to provide a minimum temperature of 95°F (35°C). Controls shall maintain the digester temperature within ±5°F (±2° C).
  • (B) Mixing equipment. Digester mixing shall, as a minimum, provide control of scum accumulation at the gas/liquid interface. Mixing that is designed for increasing the effectiveness of the digester and thereby reducing detention time shall mix the entire tank contents. Mixing devices and their application rate that will be considered to provide high-rate digestion are:

    Volume	Per 1,000 cf	Per 1,000 m 3
    Slow speed turbine mixers	0.25 hp	6.7 kw
    Draft tube mechanical mixers	0.40 hp	10.5 kw
    External pumps and jet nozzles	500 gpm	66.7 m3/m
    Gas mixing applied at bottom of digester	10 cfm	10 m3/m

    Less mixing may be provided; however, longer solids retention times than described below shall be required.

  • (C) Solids retention time. The minimum solids retention time for heated, primary digesters are:

    Unmixed	Completely mixed
    30 days	10 days

    Solids retention time shall be the same as liquid retention time in the primary digester where waste activated sludge is an aerobically digested.

  • (D) Volatile solids loading. As an alternative design basis to solids retention time, heated primary digesters may be designed for the following maximum volatile solids loading:

    Unmixed

    0.1 lb/ft³/day (1.6 kg/m³/d)

    Completely mixed

    0.3 lb ft³/day (4.8 kg/m³/d)

• (iv) Sludge piping.
  • (A) Inlet. Except in completely mixed digesters, multiple inlets shall be provided. The piping shall provide the opportunity to heat undigested sludge prior to entering the digester.
  • (B) Sludge withdrawal. Except in completely mixed digesters, multiple withdrawal pipes shall be provided. One or more withdrawal pipes shall be from the digester floor.
  • (C) Supernatant withdrawal. The design basis for facilities using digesters for waste activated sludge shall assume no supernatant withdrawal. Piping for supernatant withdrawal may be provided. A minimum of three (3) supernatant withdrawal levels shall be provided otherwise.

• (v) Gas system. All portions of the gas system, including the space above the tank liquor, storage facilities, and piping shall be designed to be under greater than atmospheric pressure at all times.
  • (A) Piping. Gas piping shall be 2.5 inches (6.4 cm) diameter or greater. Piping from the digester shall be provided with a flame trap. Piping shall slope to condensate traps. Float controlled condensate traps are not permitted.
  • (B) Safety equipment. All necessary safety equipment shall be included. Pressure and vacuum relief valves, flame traps and other safety equipment shall be provided. Gas safety equipment and gas compressors shall be housed in a separate room with an exterior entrance.
  • (C) Metering. A gas meter with bypass shall be provided for measurement of total gas production.

• (vi) Heating equipment. Sludge and digester contents shall be heated with an external heat exchanger. Where sludge is heated using digester gas, an auxiliary fuel supply shall be provided. Boilers using digester gas shall be designed to minimize corrosion and to facilitate burner replacement. All digester gas that is not beneficially used shall be incinerated in a waste gas burner.

• (vii) Access. The roof of the digester and the top sidewall shall be provided with sealed access hatches.

• (viii) Sampling. One and one-half inches (3.8 cm) or larger sampling ports shall be provided for inlet sludge, effluent sludge, supernatant and digester contents.

• (ix) Supernatant disposal. Supernatant from secondary digesters or from subsequent thickening or dewatering facilities for digested sludge shall be treated independently or returned immediately preceding the biological process. Supernatant shall not be returned to the primary clarifier.

(e) Dewatering.

• (i) Mechanical dewatering. Where provided, mechanical dewatering facilities shall include storage tanks for liquid sludge and shall provide for reliable use.

• (ii) Drying beds.
  • (A) Gravity. Drying beds may be strictly evaporation or evaporation -percolation. Evaporation-percolation beds shall be provided with graded gravel and sand beds over perforated underdrain pipe. Evaporation beds shall be designed for the application of 1.5 feet (0.46 m) of sludge per year. Evaporation - percolation beds shall be designed for the application of four feet (1.2 m) of sludge per year. Storage of sludge in the beds or in separate basins shall provide 180 days of capacity. Percolate shall be returned to the plant ahead of the biological treatment process.
  • (B) Vacuum. The bed area for vacuum assisted open drying beds shall be based on the application of no more than forty (40) feet (12.2 m) of liquid per year. If the beds are housed, the bed area shall be based on the application of eighty (80) feet (24.4 m) per year. Where beds are not housed, sludge storage shall be provided for 180 days of capacity. Polymer conditioning, chemical feed, chemical storage and facilities for mixing the polymer with the sludge shall be provided. Vacuum pumps, sump pumps, chemical feed equipment and motor control equipment shall be housed.

• (iii) Filtrate disposal. Filtrate, centrate or underdrain liquid shall be returned to a point upstream of the biological treatment process. Centrate or filtrate shall not be returned upstream of the primary clarifier.

(f) Disposal.

• (i) Degree of stabilization.
  • (A) Land application. Sludges shall be stabilized. Sludges that are to be used on public lands that are accessed by the public (parks, golf courses, cemeteries) or sludges that are to be made available to the public shall be composted or stabilized and stored for a period of at least one (1) year. Sludges that are to be incorporated into the land shall be stabilized.

    "Stabilized sludge" shall have reduced organic content and reduced pathogenic content. Stabilized sludge shall have less than 60 lb of BOD₅ per 1,000 lb (60 kg/1,000 kg) of dry weight sludge solids.

  • (B) Landfill. Sludge processed for incorporation into a landfill shall be (1) a solid or semisolid material that will not release water upon standing, and (2) has been subjected to anaerobic or aerobic digestion, or chemically treated with lime to a pH of 12.0 or chemically treated with chlorine to a free chlorine residual. Waiver of this requirement must be obtained from the Solid Waste Management Section of the Department of Environmental Quality.

• (ii) Storage. Sludge storage shall be provided in lined earthen lagoons or structural tanks. The lagoon lining shall be designed to protect the groundwater pursuant to the requirements of Chapter 8 of the Water Quality Divisions rules and regulations. Sludge storage volume shall be sufficiently large to provide for independent operation of the sludge dewatering or disposal facilities from preceding liquid or sludge processes.

020-11 Wyo. Code R. § 11-20

Amended, Eff. 6/29/2018.