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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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 2
12.1 Overview
12 Sludge treatment
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 3
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 4
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 5
Overview
Thickening
Thickening
Hygienisation
Stabilisation
Dewatering
Drying
Incineration
Pro
cess
wa
ter
Biogas
Energy
Agriculture
Disposal site
Atmosphere
Wastewater treatment
Primary, secondary, tertiary sludge
Construction industry
Gujer (1999)
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 6
Sludge Treatment Alternatives
Eckenfelder & Santhanam (1981)
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 7
12.2 Thickening
12 Sludge treatment
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 8
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Thickened sludge is withdrawn from hopper and introduced to sludge treatment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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 9
Gravity Thickener
Thickened sludge
Picket fence
Scum scimmerInflow
Sludgeliquor
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 10
Dimensioning of gravity thickeners surface
Solids overflow rate
Th
inThWASThTSS A
XQq ,
,
Typical values for solids overflow rate qTSS,Th and concentration of thickened sludge XTh
qTSS,Th XTh
Primary sludge
Primary and secondary sludge
80 – 120
50 - 70 50 - 100
80 - 150
qTSS,Th Specific solids overflow rate (kg TSS / (m2 d))
QWAS Inflow to thickener (m3/d)
XTh,in Solids concentration in thickeners inlet (kg TSS / m3)
ATh Surface of thickener (m3)
Secondary sludge 25 - 30 20 - 35
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 11
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 12
Thickening by Flotation
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 13
Flotation unit
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 14
12.3 Biological sludge stabilisation
12 Sludge treatment
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 15
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 16
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%
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 17
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%
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 18
• 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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 19
12.4 Volume reduction
12 Sludge treatment
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 20
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
vo
lum
e V
S/V
DS
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 21
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
] (vo
lum
e [m
³])
.
Thickening Dewatering Drying
Dry matter
Water
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 22
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 23
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)
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 24
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 25
12.5 Sludge disposal
12 Sludge treatment
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 26
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
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 27
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!
Urban Water Systems 12 Sludge treatment © PK, 2006 - page 28
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