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 / Primary Sludge / Waste Activated
Range / Typical
Total Solids (%) / 5-9 / 6 / 0.8-1.2
Volatile Solids (% of TS) / 60-80 / 65 / 60-90
Nitrogen (N, % of TS) / 1.5-4 / 2.5 / 2.4-5
Phosphorous (P2O5, % of TS) / 0.8-2.8 / 1.6 / 2.8-11
Alkalinity (mg/L as CaCO3) / 500-1500 / 600 / 580-1100
Energy Content (kJ/kg TS) / 23000-29000 / 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

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

Now consider the slurry

Define:S

Sw

fs

The relationship between the volume and the mass of the sludge:

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:

  • dimensions of subsequent processes
  • chemical consumption
  • heating requirements for digestion
  • 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

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

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/w17,000 m3/day)

- can be conceptualized as settling of sludge under centrifugal forces rather than gravitational forces

- “Solid bowl” configuration most common

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:

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

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

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

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

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 Loading1.6 kg VS/m3-d

Minimum Hydraulic Retention Time15 days

Minimum Temperature35oC

Mixing

Gas5 W/m3

Mechanical6.5 W/m3

Stage 2

VolumeV2 = 2 V1

Other components

i.)Mixing

  1. Mechanical
  2. Liquid recycle
  3. Offgas recycle

ii.)Heating

iii.)Gas Collection and Handling

7.5 Performance

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

8.2 Belt-Filter Press

Combines gravity drainage with mechanically applied pressure

Typically sized on the basis of solids loading

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

Wayne Parker