Thickener Jae K. (Jim) Park, Professor Dept. of Civil and Environmental Engineering University of Wisconsin-Madison

Sludge Management Sources of sludge Primary sedimentation tank Aeration basin or secondary clarifier Screening and grinder Filter backwash water (tertiary treatment for TS removal) Common sludge management processes Thickening Stabilization Dewatering Disposal

Characteristics of Municipal Sludge Constituents Primary sludge WAS Digested sludge pH Total dry solids, % TVS, % dry wt. Particle specific gravity Bulk specific gravity BOD5/TVS COD/TVS Alkalinity, mg/L as CaCO3 Cellulose, % dry wt. Hemicellulose, % dry wt. Lignin, % dry wt. Grease and fat, % dry wt. Protein, % dry wt. Nitrogen, % dry wt. Phosphorus, % dry wt. Heating value, kJ/kg 5.0-6.5 3~8 60~90 1.3~1.5 1.02~1.03 0.5~1.1 1.2~1.6 500~1,500 8~15 2~4 3~7 6~35 20~30 1.5~4 0.8~2.8 15,000~24,000 6.5~7.5 0.5~1 60~80 1.2~1.4 1.0~1.005 - 2~3 200~500 5~10 5~12 32~41 2.5~7 2~7 12,000~16,000 30~60 1.3~1.6 1.03~1.04 2,500~3,500 5~20 15~20 1.6~6 1.4~4 6,000~14,000 1 kJ/kg = 0.43 Btu/lb

Gravity Thickening Accomplished in circular sedimentation basins Degree of thickening: 2~5 times the incoming solids conc. Max. achievable solids concentration: < 10% Chemical and waste activated sludges: difficult to thicken under gravity Bad design Alternative

Gravity Thickener Design Criteria Type of sludge Inf. solids conc., % Thickened conc., % Hydraulic loading, m3/m2·d Solids loading, kg3/m2·d Solids captured, % Overflow TSS, mg/L Primary 1~7 5~10 24~33 90~144 85~98 300~1,000 Trickling filter 1~4 2~6 35~50 80~92 200~1,000 WAS 0.2~1.5 2~4 10~35 60~85 Combined primary + WAS 0.5~2 4~6 4~10 25~80 85~92 300~800 m3/m2·d × 24.57 = gal/ft2 ·d kg/m2 ·d × 0.2048 = 1b/ft2·d Gravity thickener side water depth: 3 m (10 ft) Detention period: 24 hrs Hydraulic loading rate: 10~30 m3/m2·d = 250~740 gpd/ft2·d To achieve the hydraulic loading rate, secondary effluent is often blended with the sludge fed into the thickener. The sludge-blending tank may utilize mechanical mixing or air mixing.

Gravity Thickener Equipment Generally circular concrete tanks with bottom sloping toward the center. Equipment Rotating bottom scraper arm Vertical pickets Rotating scum-collection mechanism with scum baffle plates Overflow weir Other configurations Circular steel tank Generally cheaper because of simplicity of construction, equipment installation, and operation and maintenance Rectangular concrete and steel tanks Conventional sludge - collecting mechanisms with deep trusses or vertical pickets are used to stir the sludge gently, thereby opening up channels for water to escape and promoting densification.

Gravity Thickener

Dissolved Air Flotation (DAF) Primarily used to thicken the solids in chemical and WAS Separation of solids is achieved by introducing fine air bubbles created under pressure of several atmosphere into the liquid, attaching to solids to cause flotation of solids Degree of thickening: 2~8 times the incoming solids concentration Max. solids concentration: 4~5% DAF Variations Pressurize total or only a small portion of the incoming sludge Pressurize the recycled flow from the flotation thickener – preferred because it eliminates the need for high-pressure sludge pumps.

Dissolved Air Flotation (DAF) The released air bubbles become attached to the suspended particles by one of the following mechanisms: Condensation Collision Entrapment

Advantages of DAF Minimal space required Capability to treat a wide variety of organic and inorganic solids and dissolved waste streams Low retention time from wastewater stream to effluent ejection Superior clarification of most waste streams Easy to clean and maintain Higher density sludge with low water content Low installation cost for low flows (For small-scale application, the unit is typically delivered fully prefabricated)

Types of Pressurization Flow Schematic Full Pressurization The entire wastewater flow is injected with air and pressurized for dissolution of the air in the water. This flow then passes into the flotation cell where pressure is relieved. A pump equalization is used to return water to the pump in order to maintain flooded suction during periods of low flow. Partial Pressurization Only part of the wastewater flow is pressurized, and the remainder enters the flotation cell bypassing the pressurization step. A pump equalization return line is also employed in this mode to protect the pressurizing pumps. Recycle Pressurization Raw influent is introduced directly into the flotation cell. A portion (generally 50% of raw flow) of the system effluent is pressurized and recycled to the flotation cell where it is blended with the raw wastewater flow.

Design Parameters Air/solids ratio, A/S (mL air/mg solids) sa= solubility of air at the required temperature, mL/L; Sa = solids in incoming sludges, mg/L; f = fraction of air dissolved at pressure P, usually 0.5~0.8 P = pressure in atmosphere = (p + 101.35)/101.35 (SI units) = (p + 14.7)/14.7 (US customary units); p = gauge pressure, kPa (lb/in2); q = recycle flow or a portion of incoming flow pressurized, m3/d; and Q = sludge flow to the thickener, m3/day. Type of sludge Air/solids ratio Solids loading rate, kg/m2·d Hydraulic loading, m3/m2·d Polymer added (mg/kg) Solids captured, % TSS in side stream, mg/L Primary 0.04~0.07 90~200 90~250 1000~4000 85~95 100~600 Trickling filter 0.02~0.05 50~120 1000~3000 90~98 WAS 0.03~0.05 50~90 60~180 80~95 Combined primary + WAS 60~150 90~95

Design Parameters Hydraulic Loading: Effective design ranges from 1.0 to 2.5 gpm/ft2day, depending upon the application. Solids Loading: Design points for solid loadings range from 0.5 to 3.5 lbs/hr/ft2, depending on the application and the type of solids involved. It should be noted that any chemical additives used to promote coagulation and flocculation are generally included as solids determining the surface loading since the chemicals used are removed with the float from the system. Air-to-Solid Ratio: Generally, air is injected in a range of 2 to 8% by volume. Depending upon the type of solids and application, the air-to-solids ratio ranges from 0.020 to 0.1.

Gravity Belt Thickener

Belt Filter Press The first section is a gravity belt thickener and can operate independently by diverting the slurry to discharge with the optional pivoting chute.  The second stage is a filter press, which applies a mechanical pressure. 

Gravity Thickener (1)

Gravity Thickener (2)

Sludge Blending Tank

Sludge Metering System

Gravity Thickener(3) Section view Layout

Centrifuges (1) Centrifugation is the process of separating solids from liquids by the use of centrifugal force. The centrifuge is a cylindrical drum that rotates to develop the separating force. When the slurry enters the interior of a rotating centrifuge, it is thrown out against the bowl wall. The denser materials are separated first, and hug the interior wall of the rotating machine.

Centrifuges (2) A helical screw conveyor fits inside the bowl. Rotating at a slightly lower rate that the bowl, it conveys solids from the zone of settling to the dewatering beach, where it is discharged to screw conveyors located below. The main bowl is turned by an electric motor while the screw conveyor inside is controlled or slowed down using a hydraulic backdrive. Prior to entering the centrifuge, sludge would have been conditioned with polymer. Larger and heavier particles are most easily captured by the centrifuge. Fine particles that cannot be settled separately must be agglomerated by chemicals (polymer) to a size that will settle.

Centrifuge - continued

Filter Press (1) Dewatering is accomplished by pumping sludge into chamber (A) surrounded by filter cloths (B). As pumping pressure is increased, the filtrate is forced through the accumulated filter cake (C) and cloth, leaving the chambers full of solid filter cake. The chambers in HEI presses are formed by two recessed plates held together under hydraulic pressure. The hydraulic ram (D) moves the follower (E) against the stack of filter plates (F) closing the press. The ram continues to apply pressure of sufficient force to counteract the high internal compaction pressures. The head stock (G) and tail stock (H) are held in place by specially engineered side rail supports bars (I). The filtrate passes through the filter cloth and is directed by channels in the plates and drain ports (J) to the head stock for discharge. The filtrate typically contains less than 15 ppm suspended solids. The filter cake is easily removed by simply reversing the hydraulic ram, thus opening the press. The lightweight plates may then be moved apart permitting the compacted cake to fall from the chamber.

Filter Press (2)