5 sludge treatment & disposal

Jae K. (Jim) Park

WASTEWATER TREATMENT PLANT DESIGN

11/30/2008 1

Sludge TreatmentSludge TreatmentWhat goes here depends on the method of final disposal:

• Solids concentration• Pathogen reduction

Sludge DisposalSludge Disposal• Land application• Landfill• Incineration

Processedsludge

Raw sludge, grit, cleanings

Influent EffluentWastewater Treatment

Sludge treatment and disposal may account for up to 80% of the total wastewater treatment cost.

11/30/2008 2

Primary Treatment 2º TreatmentPreliminary Disinfection

Jae K. (Jim) Park


11/30/2008 3

Sludge•A mixture of organic and inorganic solids in water•Color : from brown to black

Treated sludge•Incineration, landfilling•Contain low levels of toxic compounds

12-50Dewatered sludge4-10Digested sludge1-5Waste trickling filter sludge0.5-1.5Waste activated sludge

2-7Primary sludge, without thickening

Typical concentration, %

Source

Typical Sludge Concentration

11/30/2008 4

Sludge TypesPrimary sludge

3 to 8% solidsAbout 70% organic material

Secondary sludgeConsists of wasted microorganisms and inert materialsAbout 90% organic materialWAS: 0.5 to 2% solidsTrickling filter sludge: 2-5% solids

Jae K. (Jim) Park


11/30/2008 5

Sludge Types

Tertiary sludgeIf secondary clarifier is used to remove phosphate, this sludge will also contain chemical precipitates (more difficult to treat)Denitrification sludges - similar to WAS sludge

Sources of sludgeSources of sludge

Primary sedimentation tankAeration basin or secondary clarifierScreening and grinderFilter backwash water

11/30/2008 6

Sludge Treatment

Sludge Thicken Condition Dewater SanitaryLandfill

Stabilize Condition Dewater SoilIncorporation

ReductionAsh

Jae K. (Jim) Park


11/30/2008 7

Sludge ThickeningReducing the water in primary and secondary sludges.Methods

Gravity thickening : using an additional clarifier to remove more waterDissolved air flotation (DAF)

Used to concentrate secondary sludgesSludge is pressurized and injected with air releasing into a settling tank (when the pressure is released, the extra air comes our of solution, attaching to the sludge particles as microscopic bubbles) the sludge is floated to the surface in a concentrated form the underflow returns to the head of the plant.

11/30/2008 8

Gravity ThickenerFlotation

Jae K. (Jim) Park


11/30/2008 9

Thickening

FlotationEspecially effective on activated sludgeIncreases solids content from 0.5 - 1% to 3-6%

Gravity thickeningBest with primary sludgeIncreases solids content from 1-3% to 10%

PrimarySludge

Gravity Thickening

SecondarySludge

FlotationFurther processing

11/30/2008 10

Gravity ThickeningAccomplished in circular sedimentation basinsDegree of thickening: 2~5 times the incoming solids conc.Max. achievable solids concentration: < 10%Chemical and waste activated sludges are difficult to thicken under gravity.

10

Jae K. (Jim) Park


11/30/2008 11

Gravity Thickener Design Criteria

85~92

60~8580~9285~98

Solids captured

, %

25~80

10~3535~50

90~144

Solids loading, m3/m2·d

200~1,0002~62~61~4Trickling filter

300~8004~104~60.5~2Combined primary + WAS

200~1,0002~42~40.2~1.5WAS

300~1,00024~335~101~7Primary

Overflow TSS, mg/L

Hydraulic loading, m3/m2·d

Thickened conc.,

%

Inf. solids conc., %Type of sludge

m3/m2·d × 24.57 = gal/ft2 ·dkg/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

• 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.

11/30/2008 12

Gravity Thickener Equipment

Generally circular concrete tanks with bottom sloping toward the center.

Equipment

Rotating bottom scraper armVertical picketsRotating scum-collection mechanism with scum baffle platesOverflow weir

Other configurations

Circular steel tank: Generally cheaper because of simplicity of construction, equipment installation, and operation and maintenance

Rectangular concrete and steel tanks

Jae K. (Jim) Park


11/30/2008 13

Gravity Thickener

11/30/2008 14

Dissolved Air Flotation (DAF)Primarily used to thicken the solids in chemical and WASSeparation 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 solidsDegree of thickening: 2~8 times the incoming solids concentrationMax. solids concentration: 4~5%DAF Variations

Pressurize total or only a small portion of the incoming sludgePressurize the recycled flow from the flotation thickener – preferred because it eliminates the need for high-pressure sludge pumps.

Jae K. (Jim) Park


11/30/2008 15

Dissolved Air Flotation (DAF)The released air bubbles become attached to the suspended particles by one of the following mechanisms:1.Condensation2.Collision3.Entrapment

11/30/2008 16

Advantages of DAF

Space requirements are minimal.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.Installation cost is low for low flows. The unit is typically delivered fully prefabricated. Normal concrete pad installation.

Jae K. (Jim) Park


11/30/2008 17

Design ParametersHydraulic Loading: Effective design ranges from 1.0 to 2.5 gpm/ft2, depending upon the application.Solids Load: 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 two percent to eight percent (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.

11/30/2008 18

Air/solids ratio, A/S

sa= solubility of air at the required temperature, mg/L;Sa = solids in incoming sludges, mg/L;f = fraction of air dissolved at pressure P, usually 0.5~0.8P = 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; andQ = sludge flow to the thickener, m3/day.

90~95

80~9590~9885~95

Solids captured

, %

1000~4000

1000~30001000~30001000~4000

Polymer added

(mg/kg)

100~60090~25050~1200.02~0.05Trickling filter

100~60090~25060~1500.02~0.05Combined primary + WAS

100~60060~18050~900.03~0.05WAS

100~60090~25090~2000.04~0.07Primary

TSS in side

stream, mg/L

Hydraulic loading, m3/m2·d

Solids loading

rate, kg/m2·d

Air/solids ratioType of sludge

Jae K. (Jim) Park


11/30/2008 19

Sludge Stabilization

Aerobic DigestionExtension of activated sludgeAccomplished by aeration of sludge then followed by sedimentationSupernatant goes back to head of plant (high in BOD, TKN, total-P)Treated sludge is 3% solids

Anaerobic Digestion2 stage: acid fermentation followed by methane productionAdvantages:

produce methanedo not add oxygen

As with aerobic digestion, supernatant goes to headworks

11/30/2008 20

Sludge Stabilization

Stabilization alters the characteristics of sludge so it can be returned to the environment with a minimum of environmental and health risks.Biological Treatment

1. Aerobic Digestion2. Anaerobic digestion3. Lagoons4. CompostingChemical Treatment

1. Wet combustion2. Lime stabilization3. Chlorination4. Heat stabilization5. Irradiation

Jae K. (Jim) Park


11/30/2008 21

Stabilization

Aerobic DigestionExtension of activated sludgeAccomplished by aeration of sludge then followed by sedimentationSupernatant goes back to head of plant (high in BOD, TKN, total-P)Treated sludge is 3% solids

Anaerobic Digestion2 stage: acid fermentation followed by methane productionAdvantages:

produce methanedo not add oxygen

As with aerobic digestion, supernatant goes to headworks

11/30/2008 22

Stabilization

Anaerobic DigestionAnaerobic DigestionAerobic DigestionAerobic Digestion

Jae K. (Jim) Park


11/30/2008 23

Aerobic Digestion

Aerate until O2 uptake < 2 mg O2/g VS/hrSludge age = 10~20 days at 20°CLoading = 0.02~0.15 lb VS/ft3·dayThermophilic aerobic digestion: 77~122°F (25~50°C); higher removal of biodegradable fraction (up to 80%) at very short detention times (3~4 days)

Primarysedimentation

tankAeration

basinSecondary

clarifier

Aerobicdigester

RAS

Sludge

Decant

11/30/2008 24

Aerobic Digestion - continuedAdvantages: VS reduction similar to anaerobic digestion, lower BOD in supernatant liquor, production of an odorless, humus-like biologically stable end product, recovery of more of the basic fertilizer values in the sludge, relatively easy operation, and lower capital cost

Disadvantages: high power cost, poor mechanical dewatering characteristics of sludge, and sensitive to temp., location, and type of tank material, and loss of CH4recovery potential.

Process descriptionC5H7NO2 + 7O2 → 5CO2 + NO3

- + 3H2O + H+

pH drop7.14 lb of alkalinity as CaCO3 lost/lb ammonia oxidized

Primary sludge: direct oxidation of organic matter Biological sludge: endogenous oxidation of the cell tissue

Jae K. (Jim) Park


11/30/2008 25

Aerobic Digestion - continued

Conventional Aerobic DigestionDigester liquid temperatures: dependent on weather conditions and thus can fluctuate extensively → use concrete instead of steel tanks, place tanks below grade instead of above grade, provide insulation, or use subsurface instead of surface aerationDesign at the lowest expected liquid operating temperature and provide the max. oxygen requirements at the max. expected liquid operating temperature.Tank volume

where Y = fraction of influent BOD5 consisting of primary sludge

Kd = reaction rate constant, 1/day; andfv = volatile fraction of digester suspended solids.

)1/θfX(K)YS(XQV

cvd

iii

++

=

11/30/2008 26

Aerobic Digestion - continued

Conventional Aerobic Digestion – continued

Oxygen requirementsBiological sludge: 2.3 lb O2/lb of cellsPrimary sludge: 1.6~1.9 lb O2/lb destroyedDO > 1 mg/L under all operating conditionsEnergy requirements for mixingMechanical aerators: 0.75~1.5 hp/103 ft3Diffused-air mixing: 20~40 ft3/103 ft3·minSolids loading: 0.1~0.3 lb VS/ft3·minVS reduction: 40~50%Process operationpH may drop to ~5.5 at long HRT → filamentous bulkingProvide decanting facilities so as to use to thicken the digested sludge solids before discharge to subsequent operations. Consider operator control and visibility of the decanting operation.

Jae K. (Jim) Park


11/30/2008 27

Anaerobic DigestionAnaerobic Digestion

1. Fermentative organisms (50% of viable organisms in anaerobic digester)

Cellulose Acetate, alcohols, Strepto cocciLipids other organic acids, EnterobacteriaProteins H2, CO2, NH3, HS- Chlostridia

2. Acetogenic bacteria (acid formers)Simple organics (except acetate)

(fatty acids, sol. organics)3. Methanogenic bacteria (methane formers)

Get energy from forming CH4Need low redox potentialO2 toxicNeed temp.↑

}→

→ Acetate

11/30/2008 28

Anaerobic digestion of sludge decrease the volatile organics by 40-50% and reduce the numbers of pathogenic organisms in sludges.

Accomplished by holding the sludge in closed tanks for periods of 10 to 90 days.

Old process : unmixed, unheated, long detention time (30-90 days)

Recent process : complete mixing, heating (35-45℃), detention time 10-20 days

Anaerobic Digestion

Jae K. (Jim) Park


11/30/2008 29

Complete mix, or high-rate, anaerobic digester

Heater

Digestion Separation

Digestergas

Digestergas

Gas storage Gas storage

mixer Supernatant

Scum

Settled,Digestedsludge

11/30/2008 30

Advantages

High degree of waste stabilization at high organic loading rates Very little sludge production (< 5% of biodegradable organic matter being converted to cell material) (10% of aerobic sludge production) Easy dewatering of the excess sludge Low nutrient requirement (10% of aerobic process requirement)No aeration equipmentMethane production – very low energy input (if the methane gas is used to heat the digester)

Jae K. (Jim) Park


11/30/2008 31

Disadvantages

Low bacterial yield prolonged periods of biomass build-up, requiring longer start-up period (8 to 12 weeks).

Temperature, pH, toxic sensitiveInherent process instabilityHigh capital costsComplex operation requiring skilled operators

11/30/2008 32

Anaerobic Digestion - TheoryComplex organics

CH4

10%

13%

60% 15%

Intermediates Propionate

100%

20%

5%

Acetate H2

Fermentation & hydrolysis

72% 28%

Methanogenic phase Methanogenic phase

Acetogenic phase 2%50%

Rate limiting stepsConversion of propionic and acetic acid to CH4Hydrolysis of organic solids (cellulose)

Jae K. (Jim) Park


11/30/2008 33

Anaerobic Digestion: Reactions

CH3COOH → CO2 + CH4CH4 + 2O2 → CO2 + 2H2O

CH3COOH + 2O2→ 2CO2 + 2H2O

* CH4 has high energy

11/30/2008 34

Anaerobic Contact Process

Recycle increases biomass levels and sludge age, thereby separating sludge age from HRT.Difficult to obtain efficient settling of the biomass.A degasifier is required to aid in solids settling.

Jae K. (Jim) Park


Egg Shape Digester

35

11/30/2008 36

Mixing DeviceGas injection

Unconfined: collect gas at the top of the digesters, compress the gas, and then discharge the gas through a pattern of bottom diffusers or through a series of radially placed top-mounted lances...

Confined: collect gas at the top of the digesters, compress gas and then discharge through confined tubes...

Mechanical stirring: low speed turbine or mixers, suitable for fixed or floating covers

Mechanical pumping: propeller-type pumps mounted in internal or external draft tubes or axial flow or centrifugal pumps and piping installed externally, suitable for fixed covers

Jae K. (Jim) Park


Mixer Types

Unconfined gas injectionsystems

Confined gas injectionsystems

Mechanical stirringsystems 37

11/30/2008 38

Digester HeatingTo raise the incoming sludge to digestion tank temperatures, to compensate for the heat losses through the walls, floor, and roof of the digester, and to make up the losses that might occur in the piping between the source of the heat and the tank.Internal or external heat exchangersHeat requirements

q = U A ∆Twhere

q = heat loss, BTU/h (W); U = overall coefficient of heat transfer, BTU/ft2·hr·°F (W/m2·°C); A = cross-sectional area through which the heat loss is occurring, ft2 (m2); and ∆T = temperature drop across the surface in question, °F (°C).

Jae K. (Jim) Park


11/30/2008 39

Sludge Conditioning

Chemical Conditioning

Add lime, ferric chloride, or alumCan also add polymersChemicals are added just prior to de-watering stage

Heat TreatmentHigh temperatures (175-230 oC)High pressures (10 to 20 atmospheres)Advantages

bound water is released and sludge is easily dewatered

Disadvantagescomplex processhighly concentrated liquid stream

11/30/2008 40

Heating sludge under pressure to temperatures in the range of 200-300℃ for a few minutes can effectively sterilize it and convert it to a form that is easily dewatered.Disadvantages

Its high energy requirementThe production of a high-strength return liquid form the dewatering process

Heat treatment is used only in a few large POTWs

Heat Treatment

Jae K. (Jim) Park


11/30/2008 41

Dewatering & Drying

Sludge Drying BedsMost popular methodSimpleLow maintenanceEffected by climate

FiltrationApply vacuum to pull out waterForce out water by essentially squeezing water between two moving filter belts

11/30/2008 42

Dewatering and Drying

By applying the sludge to sand drying bedsConsist of a layer of sand with an underdrain systemThe sludge is pumped onto the bedThe sun and wind dry the material furtherUsed at smaller POTWs

By using mechanical dewatering equipmentUsed medium size plants, larger POTWsThe sludge is applied to a metal, cloth, or systheticrubber surfaceThis equipment is automated but and experienced, well-trained operator is required.

Jae K. (Jim) Park


11/30/2008 43

De-watering

Sludge Drying Beds Vacuum Filtration

11/30/2008 44

Filter PressDewatering 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 ppmsuspended 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.

Jae K. (Jim) Park


11/30/2008 45

45

11/30/2008 46

Centrifuges

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 rotatingcentrifuge, it is thrown out against the bowl wall. The denser materials are separated first, and hug the interior wall of the rotating machine.

Jae K. (Jim) Park


11/30/2008 47

Centrifuges - continued

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.

11/30/2008 48

Sludge Volume Reduction

IncinerationComplete evaporation of water from sludgeRequires fuelSolid material is inertExhaust air must be treated prior to discharge

Wet OxidationTreated sludge is wetRequires energySolid material is inertExhaust air must be treated prior to discharge

Jae K. (Jim) Park


11/30/2008 49

Volume Reduction: Fluidized Bed Incineration

11/30/2008 50

Ultimate disposal the return of the material to the environment.LandfillingLand application & Land Spreading:

• gardens• agricultural land• forest land• golf courses and other public recreational areas

IncinerationOther methods

Care must be taken in applying sludge to land, so that excessiveconcentrations of heavy metals or other toxic materials do not accumulate in the soilIncineration : lager municipalities

Maximum volume reducing, detoxification, and energy recoveryCapital and operating costs are highThere are environmental effects (air discharges, scrubber sludgegeneration), operation problems, and the continuing need for trained operating personnel.

Sludge Ultimate Disposal

Jae K. (Jim) Park


11/30/2008 51

Wastewater Sludge and Biosolids

11/30/2008 52


Jae K. (Jim) Park


11/30/2008 53


11/30/2008 54


+ Biosolids =+ Biosolids =

Jae K. (Jim) Park


11/30/2008 55

5 sludge treatment & disposal

Documents

anaerobic

inert exhaust

dissolved

activated

methane production

air mixing

anaerobic

produce methane