sludge mngment, anaerobic treatment and aquatic plant systems

8/7/2019 Sludge mngment, anaerobic treatment and aquatic plant systems

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SLUDGE MANAGEMENT

Organic matterin ww

Carbon

dioxide

New sludge

mere transfer of pollution : water phase

In biological wastewater treatment plants ,

Solid phase


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Sludge management highly required for aerobic wwt systems

Typical solids content of sludge

Sludge sourceSpecific

gravity

Dry solids ( lb/1000 gal )

Range Typical

Primary settler 1.02 0.9 1.4 1.25

Activated sludge 1.01 0.6 0.8 0.7

Trickling filter 1.03 0.7 1.0 0.8

Aerated lagoon 1.01 0.7 1.0 0.8


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Processes in sludge management

sludge

thickening

stabilization

conditioning dewatering reduction

to sanitary

landfill

used as soil

conditioner


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ANAEROBIC TREATMENT OF WASTEWATER

consists of a series of microbiological processes that convert

organic compounds to methane.

anaerobic process

Organic matter CH4 + CO2 + H2 + NH3 + H2S + cell growthOver-all

equation :


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Group 1: Hydrolytic Bacteria

complex organic molecules soluble monomer molecules

relatively slow & can be limiting !

Group 2: Fermentative Acidogenic Bacteria

monomers organic acids, alcohols & ketones

fast growing group !

generation time = a few hours

Process Microbiology


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Group 3: Acetogenic Bacteria

organic acids, alcohols & ketones acetate, H2 & CO2

grows

faster than

the methane-

forming group !

CH3CH2OH + H2O CH3COOH + 2H2

ethanol acetic acid

CH3CH2COOH + 2H2O CH3COOH + CO2 + 3H2

propionic

acid

acetic acid

CH3CH2CH2COOH + 2H2O 2CH3COOH + 2H2

butyric acid acetic acid


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Group 4: Methanogenic Bacteria

Hydrogenotrophic methanogens:

grow very slowly !

CO2 + 4H2 CH4 + 2H2O

methane

Acetotrophic methanogens:

CH3COOH CH4 + 2H2O

methane

generation time = a few days


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Requires no oxygen, thus, low treatment costs

Produces lower amount of sludge

(3-20 times less than aerobic process)

Produces useful gas, methane

low nutrient requirement

low disposal costs

excess sludge is quite stable

Advantages of Anaerobic Treatment

Suitable for high-strength industrial wastes

Applicable at temperatures from 10 to 65 oC


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Disadvantages of Anaerobic Treatment

Start-up of the process requires long period

High sulfate concentrations can cause odor problems

Anaerobic treatment is a pre-treatment method


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Anaerobic Digester


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Two main strategies developedTwo main strategies developed

Anaerobic Digesters

1. Biomass Recycle (Anaerobic Contact)

analogous to aerobic activated sludge systems

biomass washed out of the system is separated and

returned to the digester

separate SRT from HRT - biomass retention time

becomes longer


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BIOGAS

SLUDGE RECYCLE

INFLUENT

EFFLUENT

Anaerobic contact digester


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allows operation at higher organic loading rates - smaller

digester volumes required lower capital costs forconstruction

allows reduction of the HRT to 6-12 days (1/2 to 1/4 ofdigester volume)

can achieve 60-95% COD removal

used mainly for food processing wastewaters with asignificant content of suspended solids ( wastewater from

starch production, meat processing, abbatoir, distilleries,green vegetable canning )


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2. Retained Biomass systems

second generation ofIWW AD designs

AC systems rarely operated below 6 day HRT - becauseww being treated usually contains insoluble organicpolymers, so that hydrolysis is the rate limiting step

but most IWWhave very low SS content and in most cases,BOD or COD is contributed by soluble, low molecularweight organics that are readily biodegradable. So useof long HRT is not necessary and is obviously very costly


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alternative designs were developed that allowed further

reduction of the HRTs and these 2nd generation

digesters are the most important in terms of modern

IWW treatment

idea is to retain biomass inside the digester

independent of the ww flow - allows HRT to be much

reduced

significant reduction in reactor volume achieved


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1. Fixed-Film Systems

2. Granular Sludge-based Systems

Two Main Types of Retained-Biomass DigestersTwo Main Types of Retained-Biomass Digesters

Anaerobic filter/fixed film systems

strategy is to provide an inert surface for bacterial

adhesion (biofilm formation)


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Well-settling nature

of granules allows

them to be retainedin the reactor


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Scanning electron micrograph of mesophilic sludge granule

at low magnification (Sekiguchi et al., 1999).


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The spaghetti theory of granulation

I) disperse methanogens (filamentous Methanosaeta);

II) floccule formation via entanglement;

III) pellet formation ("spaghetti balls"); and

IV) mature granules, with attachment of other anaerobic

microorganisms onto the pellet.


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Different Configurations of Anaerobic Digesters


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Integration of anaerobic treatment in the WWTF

Levels/degree of wastewater treatment :

primary

treatment

secondary

treatment

tertiary

treatment

anaerobic

treatment

aerobic

treatment

anaerobic

treatment

Natural

treatment

OR


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Plant parts Functions

Roots and/or stems inwater column

1. uptake pollutants.2. provide surfaces on which bacteria grow.

3. act as media for filtration and adsorption of solids

Stems and/or leaves

at or above water

surface

1. attenuate sunlight; thus can prevent

algae growth.

2. reduce effects of wind on water.3. reduce transfer of gases and heat between

atmosphere and water.

4. transport oxygen from leaves to roots

Plant functions in aquatic treatment systems

AQUATIC TREATMENT SYSTEMS

AND WETLANDS


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Reedbed Systems

Free water surface

reedbed (FWS)

cattails


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Subsurface reedbed

(SF)


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Constructed

wetlands


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Parameter Subsurface Flow

(SF)

Free Water Surface

(FWS)

Water flow rate 12 mL/min 30 mL/min

Residence time 5 days 2 days

Bed depth 25 cm 21 cm

Free water depth 5.7 cm 9 cm

Bed slope (inclination) 2.41 % 0 %

Bed materials (substrate) Soil (silk loam) & pebble Soil (silk loam) only

Bed Porosity and K value

pebble layer :

porosity = 41.22 % ;

K = 9.1 cm2/cm/sec

overallporosity = 14 %

n.a.

Initial number of plants 35 80

sludge mngment, anaerobic treatment and aquatic plant systems

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