sludge treatment

Urban Water Systems 12 Sludge treatment © PK, 2006 - page 1

12 Sludge Treatment

12.1 Overview 12.2 Thickening 12.3 Biological sludge stabilisation 12.4 Volume reduction 12.5 Sludge disposal

Technische Universität Dresden Department of Hydro Science, Institute for Urban Water Management

Peter Krebs

Urban Water Systems


12.1 Overview

12 Sludge treatment


Composition of sludge

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

• 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

• Predominantly water


Goals of sludge treatment Volume reduction

Elimination of pathogenic germs

Stabilisation of organic substances

Recycling of substances

• Thickening • Dewatering

• If used in agriculture as fertiliser or compost

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

• Nutrients, fertiliser • Humus • Biogas


Overview

Thickening

Thickening

Hygienisation

Stabilisation

Dewatering

Drying

Incineration

Pro

cess

wat

er

Biogas

Energy

Agriculture

Disposal site

Atmosphere

Wastewater treatment

Primary, secondary, tertiary sludge

Construction industry

Gujer (1999)


Sludge Treatment Alternatives

Eckenfelder & Santhanam (1981)


12.2 Thickening

12 Sludge treatment


Thickening by Gravity

Gravitative separation, similar to settling tank

Supernatant is introduced to primary clarifier or – if floatables and grease contents are high – to grid chamber

Additional mechanic stirring to enhance flocculation and extraction of water and gas

Thickened sludge is withdrawn from hopper and introduced to sludge treatment

For an efficient thickening process the development of gas bubbles must be prevented


Gravity Thickener

Thickened sludge

Picket fence

Scum scimmerInflow

Sludgeliquor


Thickening by Flotation

Pre treatment: mostly chemical flocculation

Air bubbles attach to solid particles lower specific gravity than water

Slude is placed in contact with air-saturated water (full flow or recycle pressurization)

Floating Sludge bubble composite is collected at the surface

Water is recovered under a scum baffle and removed


Thickening by Flotation


Flotation unit


12.3 Biological sludge stabilisation

12 Sludge treatment


Anaerobic mesophilic sludge stabilisation

Content of digester is mixed Sludge and water obtain a similar residence time

Storage unit Not heated little biological activity

Heated to 33 – 37°C process rates are higher

Digester

Not mixed separation of sludge and process water, which is directed to WWTP

Further thickening

Control of loading to WWTP, app. 10% of N-loading


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


12.4 Volume reduction

12 Sludge treatment


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

11

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

volu

me V S

/VDS


Volume reduction

0

5

10

15

20

25

30

35

40

45

50

1 10 20 30 40 50 60 70 80 90 100dry matter [%]

mas

s [t]

(vol

ume

[m³])

.

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


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)


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


12.5 Sludge disposal

12 Sludge treatment


Use in agriculture

Recycling of nutrients, from stabilised sludge

Problems • Acceptance • Heavy metals • Micro-pollutants, pharmaceuticals, endocrine disruptors

Liquid sludgeDewatered 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 StabilisationDewatering 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

sludge treatment

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