Physical and Biological Properties

Raw primary sludge particle size distribution is: greater than 7 mm (5 to 20%), 1 to 7 mm (9 to 33%), and smaller than 1 mm (50 to 88%), of which about 45% is less than 0.2 mm. In activated sludge, the approximate distribution is: 90% below 0.2 mm, 8% between 0.2 and 1 mm, 1.6% between 1 and 3 mm, and 0.4% over 3 mm. The organic part of the sludge decays more rapidly, with an increase in the quantity of finely dispersed and colloidal particles and bound water resulting in a decrease in the separation of water from the sludge and poor dewaterability.

The density of primary sludge is 1.0 to 1.03 g/cm3, and the density of activated sludge is about 1.0 g/cm3. The density of dry sludge solids is 1.2 to 1.4 g/ cm3. Primary sludge at solids concentrations above 5% and activated sludge at solids concentrations above 3% are non-Newtonian, which means that head losses in piping are not proportional to the velocity and viscosity. They are also thixotropic, which means that they become less viscous when mixed.

Thermophysical characteristics of sludge are shown in Table 2.5. The specific heat of a mixture of primary and thickened activated sludge is 3.5 to 4.7 x 103/kg-K. The heat value of combustion of sludge dry solids equals 16.7

TABLE 2.5 Thermophysical Characteristics of Sludge

Temperature

Thermal

Specific

Conductivity

Conductivity

Heat

Type of Sludge

(108 m2/s)

(W/m • K)

(kJ/kg • K)

Raw primary and waste

0.4-0.6

3.5-4.7

activated sludge

Vacuum filter dewatered

10.9-14.3

0.2-0.5

2.1-3.0

Centrifuge dewatered

8.5-12.1

0.1-0.3

2.0-2.4

Thermally dried

14.0-21.6

0.1-0.3

1.7-2.2

to 18.4 MJ/kg. Sludge burns at a temperature of 430 to 500°C (800 to 930°F); however, to eliminate odors, the temperature needs to be raised to 800 to 850°C (1470 to 1560°F). In the process of thickened activated sludge digestion, 15 MJ of heat is produced per kilogram of volatile suspended solids.

Dewatering is the process of natural or mechanical removal of water from sludge. Water may be present in sludge as free water or bound to the particles physically or chemically. The greater the bound water present in sludge, the more the energy or reagents it takes to condition the sludge for removal of the bound water. The separation of water from sludge depends on the size of solid particles; the smaller the particles, the poorer the water separation from sludge. Therefore, any sludge treatment process that reduces the size of suspended solids particles has a negative effect on the conditioning and dewa-tering of sludge. The chemical composition of sludge also exerts a significance influence on its treatment and dewaterability. Compounds of iron, aluminum, chromium, and copper, as well as acids and alkalis, improve the process of precipitation and dewatering and reduce the consumption of chemical reagents for conditioning of sludge before dewatering. Oils, fats, and nitrogen compounds intensify the anaerobic sludge digestion but interfere with the conditioning and dewatering processes. The dewaterability of sludge can be evaluated by measuring its specific resistance, which is determined in a Buchner funnel test by measuring the volume of filtrate collected from sludge and the time it takes to filter. Specific resistance varies depending on the type of sludge and its characteristics; the values are shown in Table 2.6 for various types of sludge.

TABLE 2.6 Specific Resistance of Sludge

Moisture

Specific Resistance

Type of Sludge

Primary sludge from municipal wastewater treatment plants with substantial industrial wastewater contributions from: Machine and metallurgical plants Synthetic rubber plants Textile plants Various industrial plants

91-95

92-95

95-97

93-96

96-98 93-96

50-300 200-400 300-700 300-1,000 400-8,000 400-2,000

Thickened activated sludge Digested primary sludge Digested mixture of primary and thickened activated sludges under: Mesophilic conditions Thermophilic conditions

96-98 96-98 96-97

800-6,800 4,000-10,000 2,400-4,000

Aerobically digested mixture of primary and activated sludges, thickened

TABLE 2.7 Levels of Indicator Bacteria and Pathogens"

Range of Levels

Average Level

Reported

Reported

Agent

(number/g)bc

(number/g) b-c

Total coliform

1.1 x 10^3.4 x 109

6.4 x 108

Fecal coliform

ND-6.8 x 108

9.5 x 106

Fecal streptococci

1.4 x 104-4.8 x 108

2.1 x 106

Salmonella sp.

ND-1.7 x 107

7.9 x 102

Shingella sp.

ND

ND

Pseudomonas aeruginosa

1.5 x 101-9.4 x 104

5.7 x 103

Enteric virus

5.9-9.0 x 103

3.6 x 102

Parasite ova/cysts

ND-1.4 x 103

1.3 x 102

Source: Lue-Hing et al., 1998.

" Values are for raw primary, secondary, and mixed sludge. b Dry weight basis. c ND, none detected.

Source: Lue-Hing et al., 1998.

" Values are for raw primary, secondary, and mixed sludge. b Dry weight basis. c ND, none detected.

Levels of indicator bacteria and pathogens in raw primary, activated, and mixed sludges are shown in Table 2.7. The diversity of microflora makes it difficult to enumerate the total population. Primary sedimentation and activated sludge treatment of wastewater are very efficient in removing microorganisms from wastewater and transporting them to the sludge. Primary sedimentation reduces microorganisms in sewage by 30 to 70%. After activated sludge treatment, the reduction of microorganisms reaches 90 to 99%.

The types of bacteria in activated sludge are mostly floc-forming, but the sludge also contains filamentous microorganisms. An excessive amount of filamentous organisms can cause sludge bulking in secondary clarifiers. Sludge bulking is a condition in the secondary clarifier where the flocs do not compact or settle well, causing large amounts of flocs to discharge with the clarifier effluent.

2.4 MASS BALANCE

A good approach to estimating solids production is to prepare a material mass balance for the entire wastewater treatment plant. A material balance is prepared for the key components of flow, BOD, and TSS, and in facilities where nutrients are removed, nitrogen and phosphorus should be included. A mass balance is typically computed for average dry weather flow and concentrations. However, if higher flows and concentrations are likely to be sustained for a long period of time, such as in communities with seasonal fluctuations in population, it is important to compute the balance for maximum conditions (usually, for maximum monthly average conditions) and to design the sludge-

handling facilities for higher values to avoid shock loading to those facilities. Recycle streams from sludge-processing facilities, such as thickeners, digesters, and dewatering systems, must also be included in the mass balance.

Example 2.1 demonstrates the computation of mass balance for a waste-water treatment plant. Table 2.8 shows the solids concentrations and solids capture efficiencies for the most commonly used sludge-processing units. Table 2.9 shows the BOD and TSS concentrations in the recycle flows from the various processes. There are wide variations in some of these values. Values for mass balance computations should be chosen based on the data from treatment plants with similar wastewater concentrations and treatment systems.

Recycle streams are important in the preparation of solids mass balance. The typical approach to the computation is first to assume a fixed percentage of the influent BOD and TSS in the total recycle flow, based on typical plant data. Then an iterative computational procedure is used until the incremental change is less than 5%. However, if a spreadsheet program is used for computation, incremental changes can be made, as little as 1% or less. A variety of simulation software is available that provides mass balance calculations.

TABLE 2.8 Typical Solids Concentration and Capture Efficiencies for Various

Processes

Solids Solids Capture

Concentration (%) Efficiency (%)

Unit Operation Range Typical Range Typical

Gravity thickening

Solids Solids Capture

Concentration (%) Efficiency (%)

Unit Operation Range Typical Range Typical

Gravity thickening

Primary sludge

4

12

6

85

92

90

Waste activated sludge (WAS)

2-

4

3

75-

90

85

Combined primary and WAS

2-

6

4

80

90

85

Flotation thickening of WAS

With chemicals

4

6

5

90

97

95

Without chemicals

2-

5

4

80

95

90

Gravity belt thickening of WAS

With chemicals

4

6

5

90

98

95

Centrifuge thickening of WAS

With chemicals

4

8

5

90

98

95

Without chemicals

3

6

4

80

90

85

Belt filter press dewatering

With chemicals, raw sludge

18-

30

23

90

98

95

With chemicals, digested sludge

12

25

18

90

98

95

Centrifuge dewatering

With chemicals

15

35

24

85

98

92

Filter press dewatering

With chemicals

20

-45

38

90

98

95

Source: Adapted in part from Metcalf & Eddy, 2003.

Source: Adapted in part from Metcalf & Eddy, 2003.

TABLE 2.9 BOD and TSS Concentrations in Recycle Flows

Unit Operation Range Typical Range Typical

Gravity thickening supernatant

Primary sludge

100-

-400

250

100

400

200

Waste activated sludge (WAS)

100-

500

300

100

400

300

Combined primary and WAS

80-

400

300

100

400

250

Flotation thickening subnatant

100-

1000

250

100

2000

300

Gravity belt thickening filtrate

100-

2500

800

100

2000

1000

Centrifuge thickening centrate

200-

3000

1000

500

3000

1000

Belt filter press dewatering filtrate

50-

-600

300

100

2000

1000

Centrifuge dewatering centrate

50-

-300

1000

200

-9000

5000

Filter press dewatering filtrate

50-

-300

200

50

1000

600

Aerobic digestion supernatnat

100

1800

500

100

9000

3500

Anaerobic digestion supernatant

500

5000

1000

800

9000

4500

Sludge lagoon supernatant

100

250

200

10

200

100

Source: Adapted in part from Metcalf & Eddy, 2003.

Source: Adapted in part from Metcalf & Eddy, 2003.

Primary Aeration Final

Clarifier Tank Clarifier

Primary Aeration Final

Clarifier Tank Clarifier

Example 2.1: Computation of Solids Mass Balance This example illustrates the preparation of solids mass balance for a hypothetical wastewater treatment plant. Figure 2.7 is a flow schematic of the plant. Some of the units for sludge processing shown in the figure are typical equipment and processes, and similar equipment and processes may be substituted depending on the selection and design of the sludge handling system. For example, a gravity thickener, floatation thickener, or centrifuge thickener may be substituted for the gravity belt thickener. However, the appropriate characteristics of the processed sludge and solids capture efficiencies, shown in Tables 2.8 and 2.9, should be used in the mass balance computations.

Following is the information required for the mass balance calculations:

average daily flow = 15,000 m3/d (4 mgd)

Influent characteristics:

Note: g/m3 and mg/L are numerically the same.

Primary clarifier:

Sludge concentration: 6%

Effluent:

1. Daily influent mass values:

2. Recycle: Solids mass balance should be calculated using an iterative approach. For the first iteration, assume the following for the total recycle b. TSS

3750kg/d

3000kg/d flow:

Flow: 1% of influent flow BOD: 2% of influent BOD

TSS:

4% of influent TSS

a. Flow = (15,000 m3/d)(0.01) = 150 m3/d b. BOD = (3000kg/d)(0.02) = 60kg/d c. TSS = (3750kg/d)(0.04) = 150kg/d

3. Primary clarifier:

a. Influent flow = (15,000 + 150) m3/d = 15,150 m3/d b. Influent BOD = (3000 + 60) kg/d = 3060 kg/d c. Influent TSS = (3750 + 150) kg/d = 3900 kg/d d. BOD removed = (3060 kg/d)(0.30) = 918 kg/d e. TSS removed = (3900 kg/d)(0.60) = 2340 kg/d f. Effluent BOD = (3060 - 918) kg/d = 2142 kg/d g. Effluent TSS = (3900 - 2340) kg/d = 1560 kg/d h. VSS removed = (2340 kg/d)(0.65) = 1521 kg/d i. Effluent VSS = (2340 - 1521) kg/d = 819 kg/d j. At 6% concentration, sludge flow =_2340 kg/d_

= 39m3/d k. Effluent flow = (15,150 - 39) m3/d = 15,111 m3/d

4. Plant effluent: In the first iteration, assume the plant effluent flow to be the same as the plant influent flow, although this may vary depending on the recycle flows and primary sludge and WAS flows discharged to the sludge processing system.

5. Secondary process:

a. Operating parameters:

RAS and WAS concentration = 0.8% (8000 g/m3)

MLSS = 80%of MLSS

(15,000m3/d )(10g/m3 ) _ 103 g/kg (15,000m3/d)(10g/m3) _ 103 g/kg _

150 kg/d

c. Influent BOD concentration = (2142kg/d)(103g/kg)

15,111m3/d

d. Biomass produced that must be wasted: Using the biomass portion of equation (2.5), we have

103g/kg

Note: In reality, the substrate in the effluent (influent soluble BOD escaping treatment) should be determined based on the BOD of the biodegradable effluent TSS. However, the error in using the effluent BOD as the effluent substrate is negligible. In sizing the aeration basin, some designers ignore the effluent BOD altogether, as it is usually a small amount.

e. Determine the solids to be wasted based on the fact that the MLVSS is 80% of MLSS:

()- 150kg/d = 1246 kg/d f. Determine the WAS flow at a solids concentration of 0.8%:

g. Return activated sludge: Assuming an MLSS of 3500 g/m3, compute the RAS ratio:

Qr/Q = RAS = 0.78 (78% return rate) RAS flow = (15,111 m3/d)(0.78) = 11,787 m3/d h. Total mixed liquor (ML) flow =(15,111 +11,787) m3/d

= 26,898 m3/d i TSS in RAS _(11,787mVd)(0.80)(10,000g/m3) . 103g/kg _ 94,296 kg/d j. TSS in ML to the aeration tank _ (1560 + 94,296)kg/d (3.g + 5.i)

_ 95,856 kg/d k. TSS in ML from the aeration tank _ (95,856 +1117) kg/d

l. TSS concentration in the mixed liquor = -

26,898m3/d

Note: If solids are wasted from mixed liquor, the volume to be wasted based on an MLSS concentration of 3368 g/m3 is

6. Gravity belt thickening:

a. Operating parameters:

WAS flow = 156 m3/d (see 5.f) WAS solids _ 1246 kg/d (see 5.e) WAS concentration _ 0.8% Solids capture efficiency _ 95% Thickened sludge concentration _ 5%

Belt wash water flow and the weight of polymer are not considered in the mass balance calcwulations.

b. Solids in thickened sludge _ (1246 kg/d)(0.95) _ 1184 kg/d c. Thickened sludge flow _-1184 kg/d— _ 24 m3/d

d. VSS in thickened sludge _ (1184)(0.80) _ 947 kg/d e. Filtrate flow _ (156 - 24) m3/d _ 132 m3/d f. TSS in filtrate _ (1246 - 1184) kg/d _ 62 kg/d g. Assuming that the BOD of the WAS solids is 50%,

BOD in filtrate _ (62 kg/d)(0.50) _ 31 kg/d h. BOD in thickened sludge = (1184kg/d)(0.50) = 592kg/d 7. Aerobic sludge digestion:

a. Operating parameters:

VSS destruction in digestion = 38% BOD in supernatant = 500 g/m3 TSS in suppernatant = 3000 g/ m Digested sludge draw-off concentration = 5%

b. TSS to digester = (2340 +1184)kg/d (see 3.e and 6.b)

=3524kg/d c. VSS to digester = (2340 +1184)kg/d (see 3.h and 6.d)

= 2468 kg/d d. Flow to digester = (39 + 24)m3/d (see 3.j and 6.c)

= 63 m3/d e. Non-VSS to digester = (3524 - 2468)kg/d

=1056kg/d f. VSS remaining after digestion = (2468 kg/d)(1 - 0.38)

=1530kg/d g. TSS remaining after digestion = (1056 +1350) kg/d

=2586kg/d h. To determine the flow distribution between supernatant at 3000 g/m3 (0.3%) concentration and digested sludge draw-off at 5% concentration, let Qs be supernatant flow and Qd be the digested sludge draw-off. Then

[(Qs m3/d)(0.003) + (Qd m3/d)(0.05)](103 kg/m3 ) = 2586 kg/d 3Qs + 50Qd = 2586 (i) Qs + Qd = 63 (ii)

Multiplying (ii) by 3 gives

• uon- t t (12mVd )(500g/m3) i. BOD in supernatant = -- ' .——-- = 6 kg/d

103 g/kg j. TSS in supernatant = (12mVd )(3«X>g/m3) = 36kg/d

103 ^kg k. TSS in digested sludge draw-off = (2586 - 36) kg/d = 2550kg/d

Note: If there is no supernatant recycle, flow to and from the digester is the same (63 m3/d), and then

TSS conc. in digested sludge = 2586 kg^d

8. Belt filter press (BFP) dewatering a. Operating parameters:

Solids capture efficiency = 95% Dewatered cake solids = 20% Specific gravity of cake = 95%

Belt wash water flow and the weight of polymer are not considered in the mass balance calculations.

b. Sludge cake solids = (2550kg/d)(0.95) = 2423kg/d c. Cake volume =-2423k^^ — = 12mVd

d. Filtrate flow = (52 - 12) m3/d = 39 m3/d e. Filtrate TSS = (2550 - 2423) kg/d = 127 kg/d f. Assuming that the BOD of the filtrate is 50% of the filtrate TSS,

9. Total recycle quantity versus quantity assumed:

Flow = (132 + 12 + 39) m3/d = 186 m3/d vs. 211 m3/d BOD = (32 + 6 + 65) kg/d = 103 kg/d vs. 101 kg/d TSS = (34 + 36 +130) kg/d = 229 kg/d vs. 225 kg/d

10. These new recycle quantities should be used for the second iteration of mass balance, and if needed, additional iterations should be performed until the change in quantities of the recycle is less than 5%. If mass balance calculations are performed using a spreadsheet program, additional iterations are easier and can be repeated until the change in quantities is 1% or less.

11. The following quantities are total recycle quantities after a second iteration and are shown versus quantities in the first iteration:

Flow = (132 + 12 + 40) m3/d = 186 m3/d vs. 211 m3/d BOD = (32 + 6 + 65) kg/d = 103 kg/d vs. 101 kg/d TSS = (63 + 36 +130) kg/d = 229 kg/d vs. 225 kg/d

Mass Balance Calculation Wastewater
Figure 2.8 Mass balance of WWTP in Example 2.1. (Units: Flow in m3/d, and BOD and TSS in kg/d.)

12. The quantities of flow (m3/d), BOD (kg/d), and TSS (kg/d), after the second iteration, for all the unit processes of treatment are shown in Figure 2.8.

Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment