Lecture 7. Sludge Management

The Islamic University of Gaza- Environmental

Engineering Department

Wastewater Management- EENV 6306

Husam Al-Najar

Qinf P.S A.T S.S Qeff

sludge sludge

Qthick

Qthick

Qdew

1 St Thickener

Sludge

Sludge

digester

Sludge

Dewatering

2nd Thickener

Quantity Source Sludge Type

It is all the solids collected in the grit

removal chamber, because grit can be

easily drained and is relatively stable

in terms of biological activity, it dose

not need treatment and is generally

disposed directly to landfills.

Grit sludge

The quantity of primary sludge is

calculated using the following

equation:

Where: Mp kg/d

ss= suspended solids in the

influent, kg/m3

E = efficiency of primary

sedimentation tank

Qin = influent flow to primary

sedimentation tank, m3/d.

Primary sludge is the sludge taken

from the bottom of the primary settling

tanks. It contains from 3-8% solids

.This sludge is composed of settleable

raw solids. Almost 7% of the primary

sludge is organic.

Primary or raw

sludge

Px = XV ÷ Øc kg/d

Qwaste = Px ÷ Xr m3/d

This sludge consists of

microorganisms and inert materials

that has been wasted from the

secondary treatment processes. Thus

9% of this sludge is organic this

sludge contains from 0.8 to 2% solids .

Secondary Sludge

sludge treatment:-

The main goal of sludge treatment is to convert it to an inert solid

product that can be safely disposed in the environment or reused for

useful purposes.

The main sludge treatment steps are-

a. Sludge thickening.

b. Sludge stabilization.

c. Sludge dewatering.

In the following section, a detailed discussion of these treatment steps is

given.

Sludge thickening:-

The main goal of sludge thickening is to reduce the sludge volume by

removing as much as possible of the water content of sludge, leading to the

increase of the solids content of sludge. Thickening is achieved by the

following three methods:-

a) Gravity thickening:-

Gravity thickening is achieved in circular tanks similar to sedimentation

tanks. Sludge is allowed to settle and compact. The thickened sludge is

withdrawn from the bottom of the thickener. To improve thickening some

chemicals are added, this process is called sludge conditioning. Sludge

conditioning is discussed below.

b) Flotation thickeners:-

For sludges with low weights, gravity thickeners are not efficient. In

this case, the sludge floats on the surface and than removed by scum

removal arms.

c) Mechanical thickening:-

Mechanical thickeners include centrifuges, rotary drams and filter belts.

The centrifuges and rotary drums use the centrifugal force to separate

solids from liquids.

Filter belts are squeezing machines in which the sludge is inserted

between two moving belts resulting in the separation of solids from

liquids.

Sludge conditioning:-

Sludge conditioning is a pretreatment step to increase the efficiency of

sludge thickening and dewatering of sludge. The two most common

methods of sludge conditioning are:-

a) Chemical conditioning:-

Addition of lime Ca (OH)2.

Addition of ferric chloride Fe cl3

Addition of alum AL2 (SO4)3.

Addition of organic polymers.

These chemicals are coagulants as they neutralize and destabilize the

electrically charged particles in the water and allow them to settle

efficiently.

Organic polymers replace Fe cl3 and AL2 (SO4)3 salts in order to

overcome the major problems involved with these inorganic

chemicals.

Advantages of organic polymers over inorganic chemicals are:-

Dosage is around 10 times lower than that of inorganic

chemicals (typical dosage is 5-10 kg/ tonsludge)

Cost saving obtained in capital cost and running cost.

Treated sludge dose not contain inorganic chemical such as

Fe3+

and AL3+

.

b) Heat treatment:-

Heat treatment is a conditioning process that involves heating of sludge

for short periods of time under pressure. By heat treatment, the structure

of sludge flocks is altered to liberate more water from particles. The high

capital cost of equipment limit the use of this method to small treatment

plants only.

sludge stabilization:-

Sludge stabilization is the process in which the biodegradable organic

matter is converted to non-biodegradable (or inert) matter.

The main purpose of this process is to produce a sludge that will not

undergo any further decomposition when disposed to the environment (i.e

stable sludge).

If sludge is disposed without stabilization it will create bad odor and health

hazards.

The most common method used for sludge stabilization is the anaerobic

digestion.

This process is achieved in a closed anaerobic tank called digester.

The digester shown is a cylindrical tank with a conical shaped bottom. It is

an anaerobic completely mixed reactor.

The cover of the digester is a floating cover to give flexibility for gas

accumulation and to prevent explosion.

The sludge in the digester is heated to 35oC to give the best digestion

efficiency.

The digested organic matter is converted into gases such as methane (CH4),

carbondioxide (CO2) and (H2O).

The produced methane is collected and used for energy production

*CO2 + 4 H2 CH4 + 2 H2O

*CH3COOH CH4 + CO2

Sludge transformed to H2

and CH3COOH by

fermentation

Sludge de-watering:-

Dewatering is a physical process used to reduce to water content of sludge

after stabilization. Several methods are used for dewatering as follows:-

A) Natural dewatering:-

Natural dewatering depends on evaporation and percolation. Sludge

drying beds are the most common example of natural dewatering.

Evaporation

Sludge

Fine sand

Gravel

Perforated pipe

Sludge drying beds

B) Mechanical dewatering:-

Mechanical dewatering methods are divided to three main categories:-

a) Vacuum filtration:-

Water is removed under applied vacuum through a porous media that

retains solids and allow water to pass.

• Cylindrical drum covered by mesh

or fabric - rotates into partially

submerged vat containing

conditioned sludge

• Apply vacuum to pull out water

• Achieves 15-30 % solid content

Vacuum Filtration

b) Pressure filtration:-

Water is removed by applying presume (squeezing). Belt filter

process is the most common example on this method.

Dewatered sludge

Sludge disposal Squeezed water

Sludge

• Belt Filter Press

– Forces out water by essentially squeezing water between two

moving filter belts

– Apply pressure to pull out water

– Achieves ~19 % solid content

– Lower energy consumption than vacuum filters

– Does not have problems with sludge pickup like vacuum filters

Volume Reduction

• Incineration

– Complete evaporation of water from sludge

– Requires fuel

– Solid material is inert

– Exhaust air must be treated prior to discharge

Sewage Sludge Land Application

Experiment

Nutrient elements N, P, K, Ca …..etc

Micronutrients Fe, Zn, Mn, ……….etc

Heavy metals (toxic elements) Cd, Pb, Tl, Hg

Mineral elements and ecology

Sludge and soil from the bottom of effluent pool

Sludge Soil

Elements

(mg/l) Settled Limits* Under layer soil Limits *

Cd <0.01 39 <0.01 2.0

Co <0.01 - <0.01 -

Hg <0.001 17 <0.001 1.0

Pb 0.15 300 <0.01 150

Zn <0.01 2800 <0.01 130

Cr 0.2 1200 0.4 -

Ni <0.01 420 <0.01 35-50

Potential toxic elements concentration in soil and sludge from

the bottom of the lake from Beitlahia and the EPA limitations

in the sludge and soil.

Heavy

metal

soil

content

(mg/kg)

Sewage sludge

content (mg/kg ds)

Current EC soil limit

Soil quality

limit

(mg/kg)

Min. No. of

years to soil

limit at 1

tds/dunum

Zn 60 600 300 120

Cu 20 90 140 400

Ni 15 20 75 900

Cd 0.8 3 3 220

Pb 30 50 300 1620

Hg 0.1 2 1.5 210

Cr 30 70 - -

Minimum period to achieve EC soil limit values with Gaza Sludge

Without Sludge

With Sludge

Proposed fertilizer recommendations for application

of sewage sludge and bio-compost

(per ha; mean values)

Limits Dry M.

(t)

Organic M.

(t)

CaO

(kg)

Other

nutrients

Sewage

sludge

30 kg P

0.75 -1.5

0.4 -0.8

0 - 150

30-75

kg N

10-20

kg K

Bio-

compost

100 kg N

7.5

2.5

220

20 kg P

80 kg K

long term application of sewage sludge to a Loess derived

Luvisol in South-West Germany as the following:

I) control without sludge or mineral fertilizer

II) mineral fertilizer application without sludge

application

III) 5 ton DM sludge ha-1 a-1

IV) 5 ton sludge aerobic stabilised

V) 15 ton DM sludge ha-1 a-1

VI) 30 ton DM sludge ha-1 a-1

Each treatment has 4 replicates

Input of mineral nutrients and heavy metals with sewage sludge (5 t/ha.a)

and removal by crop plants

Input Removal

-----------kg ha-1a-1------------

Removal1)

(%)

N

P

K2)

Ca

Mg

Zn

297

110

235

265

39

13

199

26

175

54

2.9

0.3

67

24

74

23

74

2.5

-------------g ha-1a-1-------------

Cu

Cd

Pb

Ni

Cr

1701

86

1167

253

933

56

1.5

27

14

8

3.3

1.7

2.3

5.5

0.9

1) Removal rate in % of input with sewage sludge

2) K input with sewage sludge and mineral fertilizer

Accumulation of heavy metals in the soil

Heavy metals accumulation in the soil

Mobile (µg kg-1 soil) (NH4NO3) extraction and total (mg kg-1 soil) of heavy metal content in 0 -

30 cm soil layer of sewage sludge field experiment 1995

Treatment

Cd

Zn

Cu

Ni

total

mobile

total

mobile

total

mobile

total

Control

0.16

±0.04

< 1.0

82 ±2.04

< 30

21 ±2.04

23 ±2.4

24 ±1.22

NPK

0.25

±0.04

1.8 ±0.1

95 ±4.08

39 ±0

23 ±1.22

28 ±4.7

23 ±0.82

5 t KS ha-1 a-1

0.35

±0.04

1.3 ±0.4

123 ±2.04

50 ±12

29 ±2.45

26 ±9.4

25 ±2.04

5 t KS ha-1 a-1

anerobic stab.

0.38

±0.10

2.3 ±0.7

122 ±1.63

101

±16

29 ±1.63

33 ±4.7

24 ±0.41

15 t KS ha-1 a-1

0.71

±0.07

4.2 ±1.0

182 ±6.94

282

±15

41 ±0.41

46 ±1.9

26 ±1.22

30 t KS ha-1 a-1

1.36

±0.11

6.0 ±0.1

270 ±0.41

418

±34

59 ±2.86

71 ±4.2

26 ±0.82

After 10 years termination of sewage sludge application

Intensive agriculture with the following crop rotation:

• Summer wheat

• Potato

• Maize

Yield of internsive Agriculture

Yield of summer wheat, potato and maize. Data represent means ± SD of 4 replications

Treatment

Summer wheat yield

Potato

Maize

Grain

(t DM ha-1)

straw

(t DM ha-1)

(t DM ha-1)

(shoot+grains)

(t DM ha-1)

Control

5.61.0

4.91.0

5.90.5

14.31.6

NPK

7.40.4

6.60.6

6.81.1

17.41.3

5 ton

6.50.3

5.70.3

6.70.9

17.70.6

5 ton aerob.

6.01.2

5.61.2

6.70.5

18.11.8

15 ton

6.00.7

5.30.4

7.10.5

17.21.1

30 ton

6.00.6

5.60.6

8.00.6

17.91.1

Heavy metals concentration in summer wheat.

Treatment

Cd (µg/kg d m)

Cu (mg/kg d m)

Ni (mg/kg d m)

Zn (mg/kg d m)

Grains

Shoots

Grains

Shoots

Grains

Shoots

Grains

Shoots

Control

361

645

5.60.

2

2.20.2

0.200.0

1.60.3

32.82.0

11.42.8

NPK

5114

10113

5.60.

1

2.40.1

0.210.0

1.20.3

38.54.8

11.41.4

5 Ton

558

9121

5.80.

2

2.50.2

0.360.1

1.40.4

43.63.3

19.75.8

5 ton areob. 608 819 5.90.

2

2.20.1 0.270.1 1.20.3 41.81.9 11.90.1

15 Ton

8821

14821

5.40.

8

2.40.1

0.300.0

1.10.2

49.19.9

21.72.6

30 Ton

1244

21032

6.10.

1

2.60.1

0.230.0

1.30.2

60.73.0

29.45.0

Heavy metals concentration in Maize grains and shoots .

Treatment

Cd (µg/kg d m)

Cu (mg/kg d m)

Ni (mg/kg d m)

Zn (mg/kg d m)

Grains

Shoots

Grains

Shoots

Grains

Shoots

Grains

Shoots

Control

13.32.5

59.94.6

2.30.2

5.30.2

0.60.1

1.00.1

23.93.2

18.62.0

NPK

11.92.2

11224

2.20.0

5.80.0

0.50.0

0.80.1

21.22.9

22.13.4

5 Ton

16.23.2

12625

2.80.2

5.70.4

0.60.1

0.70.1

29.16.2

24.02.5

5 ton areob.

11.62.7

11413

2.50.4

5.90.8

0.50.1

0.90.1

24.82.3

24.11.0

15 Ton

13.74.1

17623

2.71.0

5.40.4

0.60.1

0.70.1

26.42.5

28.13.1

30 Ton

19.03.9

26527

3.00.9

5.80.5

0.60.2

0.70.1

31.55.1

40.31.8

Leaching of heavy metals into the ground water

(preferential flow)

Installation of „monitor boxes“ filled with exchangeable

resin

Cd accumulation in the top (0-30 cm) and subsoil (30-60 cm) after 18

years application of sewage sludge at different annual rates

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

0 NPK 5t 15t 30t 5t

0-30cm

30-60cm

----------Sewage sludge-------

-----

Similar results for Zn accumulation in the top soil

mg Cd /kg soil

Limit

(legislation)

7.26.56.05.55.0

Yield

Tl conc.

Tl cont.

0.4 1.0 1.1 1.1 1.2

23.3 18.6 16.4 14.7 8.9

9.3 18.6 17.5 15.6 10.2

Yield, Tl concentration and content of 3 kale plant cultivated in 80 g L1

soil at different pH values and harvested after 30 days growth periods

Soil

pH

Rhizosphere Effect

Effect of rhizosphere pH on Cd uptake in rye grass

(Qi Tang Wu et al., Cr. Acad.Sci.309, 215-220, 1989)

Cd concentration in shoot

(mg kg-1 DM)

N-Form

Rhizosphere

pH

1.

Cutting

2.

Cutting

3.

Cutting

NO3-

6.8

6.5

5.5

4.2

NH4NO3

6.3

9.2

8.2

7.6

NH4+

5.5

12.4

12.8

12.2

Protection of environment and food chain

Legislative measures

eg. Regulation for application of sewage sludge and

municipal wastes compost to agricultural land

Limits for maximum concentrations of individual heavy

metals in

• Sewage sludge, bio-compost

• Soils for application (depending on soil pH)

Avoidance of a relevant contamination of soils and

food chain

Recent legislation for the use of sewage

sludge in Germany relative to heavy metals

input (for example: Cd total content)

Restrictions:

Application of sewage sludge is forbidden on:

- Soils (> 5% clay content; pH >6): > 1.5 mg Cd kg-1 soil

- Sandy soil (< 5% clay content) with pH 5-6: > 1.0 mg Cd kg-1

soil

- Acid soils (pH < 5)

The agricultural use is forbidden:

- of sewage sludge with a Cd content > 10 mg kg-1 soil or

- on sandy soil (< 5% clay content with pH 5-6) sewage

sludge with a Cd content > 5 mg kg-1 D.M.