SOLID/
SLUDGE
MANAGEMENT
SOLID AND BIOSOLIDS
Term biosolid: WW solid are organic
product that can be used beneficially
after treatment (WEF 1998)
Form:liquid or
semisolid
liquid (0.2512% solid)
The largest
waste (in
volume)
Term sludge: used only
before achieve
beneficial use criteria
SOLIDS AND
BIOSOLIDS
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:
Wp  f . SS
 Where
 Wp = raw primary sludge solids (g/d)
 f = fraction of SS removed in primary settling
 SS = suspended solids in unsettled ww (g/d)
CHARACTERISTICS AND QUANTITIES OF
WASTE SLUDGE
 To estimates the secondary treatment solids generated:
Ws  K . BOD
 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
CHARACTERISTICS AND QUANTITIES OF
WASTE SLUDGE
 Volume of wet sludge, V (Liter)
W
W
V

 s   100  p 

 

 100   100 
 Where
 W = weight of dry solids, kg
 s = solids content
 p = water content
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
Volume reduction
Elimination of
pathogenic germs
Stabilization of
organic substances
Recycling of
substances
• Thickening
• Dewatering
• If used in agriculture as fertiliser or
compost
• Gas production
• Reduction of dry content and odour
• Improvement of dewatering
• Nutrients, fertilizer
• Humus
• Biogas
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
Anaerobic
digestion
Composting
Aerobic
digestion
Alkaline
Stabilization
STABILIZATION
Gravity Thickener
Inflow
Scum scimmer
Sludge
liquor
Picket fence
Thickened sludge
Thickening by Flotation
Flotation unit
Processes in digester
Anaerobic degradation
2 C5H7NO2  8 H2O  5 CH4  3 CO2  2 NH4  2 HCO3
Degradation of organic substances of app. 50%
Biogas production:
63% CH4 (Methane)
35% CO2
2% other gases (N2, H2, H2S)
 electricity and heating
Organic nitrogen is converged to NH4+
 N-loading of WWTP
Characteristic values of digester
Mean residence time of sludge
Small units, badly mixed
Medium size units with mixing
Large plants with mixing
Biogas production related to degradation of organic
substances
Degradation of organic substances
< 30 d
20 d
12 – 16 d
0.9 m3 / kg VSSdegr.
40 – 55%
Simultaneous aerobic sludge stabilisation
• No primary clarifier  no primary sludge
• High sludge age SRT, app. 25 d
• Activated sludge tank is larger than that combined with an anaerobic sludge
stabilisation
• No biogas production
• 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 yield
Low 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 rate
High rate: 0.75-1.5 kg COD/kg VSS-day
Low rate: 0.15-0.75 kg COD/kg VSS-day
Start-up time
Long start-up: 1-2 months for mesophilic
: 2-3 months for thermophilic
Short start-up: 1-2 weeks
Comparison between anaerobic and aerobic processes
Anaerobic
Aerobic
SRT
Longer SRT is essential to retain the slow
growing methanogens within the reactor
SRT of 4-10 days is enough for the
activated sludge process
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 nutrientremoval processes
Environmental factors
The process is highly susceptible to
changes in environmental conditions
The process is more robust to
changing environmental conditions
ANAEROBIC SLUDGE DIGESTION
Sludge
Digestion:
Anaerobic
DEWATERING
DRYING
Volume reduction
Water content in stabilised sludge > 95% !
 Reduction of water content and volume
Sludge volume
VS  VDS  VW  VDS  WVS

1
VS 
VDS
1  W
non-linear
relation!
VW

VS
25
relative volume VS /VDS

W
With water content
20
15
10
5
0
0,0
0,2
0,4
0,6
Water content W
0,8
1,0
Volume reduction
.
50
Thickening
Dewatering
Drying
45
40
mass [t] (volume [m³])
35
30
25
20
15
10
Water
5
Dry matter
0
1
12 Sludge treatment
10
20
30
40
50
60
dry matter [%]
70
80
Urban Water Systems
90
© PK, 2006
- page 39
100
Dewatering
Conditioning with flocculation agents (poly-electrolytes) for efficient dewatering
W
DS
Centrifuge
> 0.7
< 0.3
Batch-wise
Hydraulic pressure
through plates in
water-tight chambers
> 0.6
≤ 0.4
continuous
Pressed between two
filter belts around
staggered rollers
> 0.7
≤ 0.3
Unit
Operation
Method
Decanter
Continuous
Chamber filter
press
(large plants)
Belt filter press
(small plants)
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
Dimensioning  W  0.55 (Imhoff, 1990)
Plant type
Specific surface
Only mechanical treatment
13 PE/m2
Trickling filter
6 PE/m2
Activated sludge plant
4 PE/m2
Filling the drying bed
with sludge
Starting the drying process
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/REUSE
Use in agriculture
 Recycling of nutrients, from stabilised sludge
*
Sludge treatment
Fertiliser*
Liquid sludge
Dewatered sludge
Dried sludge
P- and N-fertiliser
P-fertiliser, N as storage product
P-fertiliser
Limit re. over-fertilisation
Problems
• Acceptance
• Heavy metals
• Micro-pollutants, pharmaceuticals, endocrine disruptors
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)
• 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!
Co- incineration
• In coal power station
• In solid waste incinerators
• In cement production, ash is bounded to cement
Fluidized bed sludge incineration
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