Download Sludge Handling, Treatment and Biosolids

SLUDGE HANDLING AND TREATMENT AND BIOSOLIDS
Sludges can be defined as the residual streams that are generated in
water and wastewater treatment that are high in solids content
The term biosolids has a specific meaning and refers to “An organic
semi-solid wastewater product that remains after solids are stabilized
biologically or chemically and are suitable for beneficial reuse”
1.) Sources of Sludges in Wastewater Treatment
2.) Typical Sludge Characteristics
Parameter
Total Solids (%)
Volatile Solids (% of TS)
Nitrogen (N, % of TS)
Phosphorous (P2O5, % of
TS)
Alkalinity (mg/L as
CaCO3)
Energy Content (kJ/kg
TS)
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Primary Sludge
Range
Typical
5-9
6
60-80
65
1.5-4
2.5
0.8-2.8
1.6
Waste Activated
0.8-1.2
60-90
2.4-5
2.8-11
500-1500
600
580-1100
2300029000
25000
19000-23000
3.) Objectives of sludge handling and treatment processes
- Reduce cost of disposal/reuse
- Produce a product that can be reused/disposed
These objectives can be achieved by:
Volume reduction
Mass reduction
Vector Attraction Reduction
Pathogen Reduction
Energy generation?
4.) Sludge Treatment Processes
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A typical process flow sheet
5.) Sludge Mass-Volume Relationships
Sludge consists of a slurry of water and solids. The solids consist of
a mixture of organic (volatile) and inorganic (fixed) materials
Consider the solids:
Define:
Vw
Vf
Vv
Ms
Mf
Mv
Ss
Sf
Sv
w
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Now consider the slurry
Define:
S
Sw
fs
The relationship between the volume and the mass of the sludge:
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The relationship between the volume and the mass of solids:
An example:
6.) Thickening
Sludges from wastewater treatment are relatively dilute and variable
in consistency
- relatively large volumes are generated and without
thickening will result in increased:
o dimensions of subsequent processes
o chemical consumption
o heating requirements for digestion
o pumping and transportation costs
Thickening processes are employed to increase the solids
concentrations and reduce the sludge volume
- i.e. an increase in sludge concentration from 0.8 to 4% TS
results in a 5-fold reduction in volume
Thickening can be achieved in:
- wastewater settling processes
- digestion processes (more on this later)
Focus on separate processes that tend to be used in larger plants
(Qw/w > 4000 m3/day)
Some technologies (Centrifuges and Gravity Belt Thickeners) require
the use of polymer addition to enhance thickening process
- more on this when discussing Dewatering Processes
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6.1 Gravity Thickening
Similar to conventional settling processes
Sludge is stirred gently:
- opens channels in sludge to allow escape of water
Volume is provided for sludge storage
Supernatant is typically returned to the wastewater processes for
treatment
Design is typically based on solids loading rates
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Maximum hydraulic loading rates:
Primary sludge: 15-31 m3/day
Waste Activated Sludge: 4-8 m3/day
Mixed: 6-12 m3/day
An example:
6.2 Centrifugal Thickening
Used for thickening and dewatering (more on this later)
- typically only applied to thickening of WAS at larger plants
(Qw/w > 17,000 m3/day)
- can be conceptualized as settling of sludge under centrifugal
forces rather than gravitational forces
- “Solid bowl” configuration most common
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Sludge is introduced into spinning cylinder
- concentrates at the wall
- Augur (Scroll) that spins at a slightly different speed moves
thickened sludge to end of cylinder
- centrate discharged at opposite end
Characterized in terms of:
- solids concentration of thickened sludge
- solids recovery (capture)
6.3 Gravity-Belt Thickening
Sludge is distributed onto a permeable moving “belt”
- water drains through belt as sludge moves towards
discharge
Sized on the basis of hydraulic loading rates:
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Solids loading rates are typically 200-600 kg/m•hr
7.) Anaerobic Digestion
Biodegradation of organic compounds in the absence of dissolved
oxygen, nitrate and sulphate
Results in the generation of biogas that contains primarily methane
(CH4) and carbon dioxide (CO2)
7.1 Microbiology and Biochemistry of Anaerobic Processes
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The methanogenic bacteria are the most crucial step and are often
the rate-limiting step
The activity of these bacteria is sensitive to temperature and pH
Successful operation of anaerobic processes is dependent upon
maintaining a balance between the acid producing (acidogenic)
bacteria and the methanogenic bacteria
Disruption of methanogens

Build up of organic acids

Reduction in pH

Further reduction in methanogenic activity
pH Sensitivity
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Temperature Sensitivity
7.2 Biogas Production
In anaerobic processes a substantial fraction of the organic material
entering a process is converted to biogas
In practice, the following estimates are often employed for biogas
production
350 L CH4/kg COD removed
500 L Biogas/kg COD removed
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Typically assume that COD in sludges is primarily particulate (Volatile
Solids)
Assume 1.42 kg COD/kg VS
Biogas generation on basis of VS destruction becomes:
500 L CH4/kg VS removed
700 L Biogas/kg VS removed
7.3 Design of municipal sludge digesters
Usually designed as either standard rate or high rate:
7.3.1 Standard Rate
Intermittent feeding and withdrawal
HRT = 30-60 days
Organic loading = 0.5 - 1.6 kg VS/m3-d
An old technology employed in treatment plants with wastewater
flows  3500 m3/day
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7.3.2 High Rate
First stage
- mixed
- heated (mesophilic 35-370 ; thermophilic 55o)
- CSTR w/o recycle
- HRT = 10 - 15 days
- Organic loading: 1.6 - 4.8 kg VS/m3-d
Second stage
- Operated primarily for solid-liquid separation
- HRT = 20 - 30 days
In large modern wastewater treatment plants the secondary digester
is replaced with a dewatering device:
- centrifuge
- belt press
7.4 Ontario MOE Design Guidelines
2-Stage Configuration
Stage 1
Maximum Solids Loading
Minimum Hydraulic Retention Time
Minimum Temperature
Mixing
Gas
Mechanical
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1.6 kg VS/m3-d
15 days
35oC
5 W/m3
6.5 W/m3
Stage 2
Volume
Other components
i.)
Mixing
a. Mechanical
b. Liquid recycle
c. Offgas recycle
ii.)
Heating
iii.) Gas Collection and Handling
7.5 Performance
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V2 = 2 V1
8.) Dewatering and Conditioning
Dewatering is performed to:
- reduce costs for hauling offsite
- improving handling characteristics
- reduce moisture content prior to incineration
- reduce requirement for amendments in composting
- Required prior to landfilling
8.1 Centrifuges
Concepts are similar to thickening application
- subtle differences in centrifuge design
- different loading rates, solids concentrations and recoveries
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8.2 Belt-Filter Press
Combines gravity drainage with mechanically applied pressure
Typically sized on the basis of solids loading
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Experience has shown that the dewaterability of sludges can be
improved through the addition of chemicals
- known as “conditioning”
- most surfaces in sludges are negatively charged and hence
they repel each other with water filling the gaps
- conditioning chemicals coagulate the solids by “bridging”
between particles
- releases adsorbed water
- can increase the rate of dewatering and the solids capture
Chemical conditioners include:
- ferric chloride
- lime
- alum
- organic polymers
Inorganic conditioning agents can substantially increase the mass of
solids that need to be handled (i.e. 20-30%)
Typically need to determine:
- type of conditioner
- dosage (usually kg/tonne solids)
Not all sludges are equal
Usually determined with testing
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