SOLID/ SLUDGE

MANAGEMENT

SOLID AND BIOSOLIDS

SOLIDS AND BIOSOLIDSThe largest

waste (in volume)

Form:liquid or semisolid liquid (0.25-12% solid)

Term biosolid: WW solid are organic product that can be used beneficially

after treatment (WEF 1998)

Term sludge: used only before achieve

beneficial use criteria

Term solid: if uncertain

criteria

SOLIDS SOURCES

SCHEMATICS DIAGRAM OF CONVENTIONAL ACTIVATED

SLUDGE

CHARACTERISTICS AND QUANTITIES OF WASTE SLUDGE

To estimates the raw solids removed by plain sedimentation:

Where Wp = raw primary sludge solids (g/d) f = fraction of SS removed in primary settling SS = suspended solids in unsettled ww (g/d)

SS .W fp

To estimates the secondary treatment solids generated:

Where Ws = biological sludge solids (g/d) K = fraction of applied BOD that appears as excess biological solid.

K depend on F/M ratio or bacteria growth rate. BOD = BOD in applied ww after primary sedimentation(g/d)

Total sludge solids production in conventional treatment plant with primary and secondary treatment, Wps = Wp+Ws

BOD .W Ks

CHARACTERISTICS AND QUANTITIES OF WASTE SLUDGE

Volume of wet sludge, V (Liter)

Where W = weight of dry solids, kg s = solids content p = water content

100100

100p

WsW

V

CHARACTERISTICS AND QUANTITIES OF WASTE SLUDGE

EXAMPLE

A ww with 200 mg/L BOD and 220 mg/L SS is processed in trickling filter plant. K=0.34. Assuming 50% SS removal and 35% BOD removal from primary clarifier:

i) Estimates the quantities of sludge solids from primary and secondary biological filtration per cubic meter of ww treated.

ii) Calculate volume of wet sludge from final clarifier and combined with solid content 5%.

SLUDGE COMPOSITION Predominantly water

Micro-organisms

Viruses, pathogens, germs in general

Organic particles, heavily bio-degradable

Organic compounds, inert, adsorpted to sludge flocs

Heavy metals

Micro-pollutants, pharmaceuticals, endocrine disrupters

All non-degraded compounds extracted from wastewater are found in the sludge

SLUDGE CHARACTERISTICS

SLUDGE TREATMENT GOAL• Thickening • Dewatering

Volume reduction

• If used in agriculture as fertiliser or compost Elimination of

pathogenic germs

• Gas production • Reduction of dry content and odour• Improvement of dewatering

Stabilization of organic substances

• Nutrients, fertilizer • Humus • Biogas

Recycling of substances

SLUDGE TREATMENT ALTERNATIVES

Eckenfelder & Santhanam (1981)

SLUDGE PROCESSING

THICKENING

SLUDGE THICKENING

Used to increase solid content of sludge by removing removing a portion of liquid fraction

Usually accomplished by physical means; e.g settling, flotation, centrifugation, gravity belt and rotary drum.

Why is it important? 1. Beneficial to the subsequent treatment processes

from the following stand points: Capasity of tanks and equipment required Quantity of chemical required for sludge conditioning Amount of heat (in digester) and fuel required for heat

drying and incineration

SLUDGE THICKENING

2. Cost reduction

-small pipe size and pumping cost

3. Liquid sludge can be transported easily

In designing thickening facility, it is important to:

Provide adequate capasity to meet peak demand Prevent septicity, with its attendant odor problem.

Gravity belt thickener

Schematic diagram : CENTRIFUGAL THICKENING

EXAMPLE

Estimate the sludge volume reduction when the sludge is thickened from 4% to 7% solids concentration. The daily sludge production is 100 m3.

Solution:

1. Calculate amount of dry sludge produced

2. Calculate volume in 7% solid content

3. Calculate percentage of sludge volume reduction

Ans: 42.9%

EXAMPLE:

Primary sludge containing trickling filter humus is gravity thickened in circular tank with 3.6m dia. and 3 m side water depth. The applied sludge is 10 m3/d with 4.5% solids and the thickened sludge withdrawn is 5 m3/d at 7.5% solids. The blanket of consolidating sludge in the tank has a depth of 1 m. For odour control, 170 m3/d of treated wastewater is pumped to the tank along with sludge to increase overflow rate. Calculate:

Solid loading

% solid captured

Overflow rate

Solid retention time

STABILIZATION

STABILIZATION Stabilization process of sludges for volume reduction, production

of usable gas (methane), and improving the dewaterability of sludge

Solids and biosolids (sludge produced from primary or secondary treatment) are stabilized to:

- reduce pathogens

- eliminate offensive odors

- inhibit, reduce, or eliminate the potential for

putrefaction (decay, decompose of organic matters).

Therefore, stabilization involves the reduction of volatile content and addition of chemicals to solid and biosolid and…

Not suitable for survival of microb

STABILIZATION

Aerobic digestion

Anaerobic digestion Composting

Alkaline Stabilization

STABILIZATION

Gravity Thickener

Thickened sludge

Picket fence

Scum scimmerInflow

Sludgeliquor

Thickening by Flotation

Flotation unit

Processes in digester

Biogas production: 63% CH4 (Methane) 35% CO2 2% other gases (N2, H2, H2S)

electricity and heating

Anaerobic degradation

34242275 HCO2NH2CO3CH5OH8NOHC2

Organic nitrogen is converged to NH4+

N-loading of WWTP

Degradation of organic substances of app. 50%

Characteristic values of digester

Mean residence time of sludge

Small units, badly mixed

Medium size units with mixing

Large plants with mixing

< 30 d

20 d

12 – 16 d

Biogas production related to degradation of organic substances 0.9 m3 / kg VSSdegr.

Degradation of organic substances 40 – 55%

• Activated sludge tank is larger than that combined with an anaerobic sludge stabilisation

• No biogas production

• High sludge age SRT, app. 25 d

Simultaneous aerobic sludge stabilisation

• No primary clarifier no primary sludge

• Possibly combined with storage or thickener unit

• Stable and simple operation

Comparison between anaerobic and aerobic processes

Anaerobic Aerobic

Organic loading rate

High loading rates:10-40 kg COD/m3-day Low loading rates:0.5-1.5 kg COD/m3-day(for high rate reactors, e.g. AF,UASB, E/FBR) (for activated sludge process)

Biomass yieldLow biomass yield:0.05-0.15 kg VSS/kg COD High biomass yield:0.35-0.45 kg VSS/kg COD

(biomass yield is not constant but depends on types of substrates metabolized)

(biomass yield is fairly constant irrespective of types of substrates metabolized)

Specific substrate utilization rateHigh rate: 0.75-1.5 kg COD/kg VSS-day Low rate: 0.15-0.75 kg COD/kg VSS-day

Start-up timeLong start-up: 1-2 months for mesophilic

: 2-3 months for thermophilicShort start-up: 1-2 weeks

Anaerobic Aerobic

SRT

Longer SRT is essential to retain the slowgrowing methanogens within the reactor

Microbiology

Anaerobic processes involve multi-step chemical conversions and a diverse group of microorganisms degrade the organic matter in a sequential order

Aerobic process is mainly a one-species phenomenon, except for nutrient-removal processes

Environmental factors

The process is highly susceptible to changes in environmental conditions

SRT of 4-10 days is enough for the activated sludge process

The process is more robust to changing environmental conditions

Comparison between anaerobic and aerobic processes

ANAEROBIC SLUDGE DIGESTION

SludgeDigestion:Anaerobic

DEWATERING

DRYING

Volume reduction

Water content in stabilised sludge > 95% !

Reduction of water content and volume

Sludge volume

SWDSWDSS VVVVV With water content

S

WW V

V

DSW

S VV

1

1

non-linear relation!

0

5

10

15

20

25

0,0 0,2 0,4 0,6 0,8 1,0

Water content W

rela

tive

vo

lum

e VS

/VDS

12 Sludge treatment Urban Water Systems © PK, 2006 - page 39

Volume reduction

0

5

10

15

20

25

30

35

40

45

50

1 10 20 30 40 50 60 70 80 90 100

dry matter [%]

ma

ss [t] (

volu

me

[m

³])

.

Thickening Dewatering Drying

Dry matter

Water

Dewatering

Conditioning with flocculation agents (poly-electrolytes) for efficient dewatering

Decanter Continuous

Chamber filter press (large plants)

Batch-wise

Belt filter press (small plants)

continuous

Centrifuge

Hydraulic pressure through plates in water-tight chambers

Unit Operation Method

Pressed between two filter belts around staggered rollers

W DS

> 0.7 < 0.3

> 0.6 ≤ 0.4

> 0.7 ≤ 0.3

CENTRIFUGE

FILTER PRESS

FILTER PRESS

Drying bed

• Thin sludge layer (< 20 cm)

• Sand layer as drainage and filter layer

• Sludge is first dewatered by drainage

then air-dried through evaporation

• Applicable for small plants

Plant type Specific surface

Only mechanical treatment 13 PE/m2

Trickling filter 6 PE/m2

Activated sludge plant 4 PE/m2

Dimensioning W 0.55 (Imhoff, 1990)

Starting the drying process

Filling the drying bed with sludge

Drying

Vaporisation of water content

Partial drying W 0.3 – 0.4

Full drying W down to < 0.1

Contact drying over heated areas

Drying by convection through hot air counter-current inlet app. 600°C, outlet app. 300°C (Imhoff, 1999)

For large plants

Disposal is critical: fire, dust explosion

In granulate form as fertiliser

SLUDGE DISPOSAL/R

EUSE

Use in agriculture

Recycling of nutrients, from stabilised sludge

Problems

• Acceptance

• Heavy metals

• Micro-pollutants, pharmaceuticals, endocrine disruptors

Liquid sludge

Dewatered sludge

Dried sludge

P- and N-fertiliser

P-fertiliser, N as storage product

P-fertiliser

Sludge treatment Fertiliser*

* Limit re. over-fertilisation

Composting

Aerobic biological degradation of organic substances

Prerequisites Stabilisation

Dewatering

Hygienisation

Approach

• Structure means: straw, wood, saw dust, wood chips

• Mixture app. 1:1

• Water content app. 0,65

Requirements are more demanding than for sludge use as fertiliser!

Incineration Use of energy content, but not of nutrients

Mono incineration (sludge exclusively)

Co- incineration

• In solid waste incinerators

• In cement production, ash is bounded to cement

• Calorific value of sludge high enough no biogas use before, no stabilisation

• Water content not minimised (no full drying)

• Fluidised bed incinerator, incineration at 800 – 950°C in fluidised sand bed

• Expensive!

• In coal power station

Fluidized bed sludge incineration

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