Chemical Sludge

Chemicals are widely used in wastewater treatment to precipitate and remove phosphorus and in some cases to improve the efficiency of suspended solids removal. Chemicals can be added to raw wastewater, to a secondary biological process, or to secondary effluent, in which case tertiary filters or tertiary clarifiers are used to remove the chemical precipitates.

Although theoretical rates of chemical sludge production can be estimated from the anticipated chemical reactions, competing reactions can make the estimation difficult. For example, ferric chloride will form ferric hydroxide, which in turn will react with phosphate to form ferric phosphate. Classical jar tests are favored as a means of estimating chemical sludge quantities. Quantities of precipitates in chemical sludge are influenced by such conditions as pH, mixing, reaction time, and opportunity for flocculation. Following are some of the types of precipitates that must be considered in measuring the total sludge production (U.S. EPA, 1979):

• Phosphate precipitates. Examples are AlPO4 or Al(H2PO4)(OH)2 with aluminum salts, FePO4 with iron salts, and Ca3(PO4)2 with lime.

• Carbon precipitates. This is significant with lime, which forms calcium carbonate, CaCO3. If two-stage recarbonation is used, a recarbonation sludge of nearly pure CaCO3 is formed.

• Hydroxide precipitates. With iron and aluminum salts, excess salt forms a hydroxide, Fe(OH)3 or Al(OH)3. With lime, magnesium hydroxide, Mg(OH)2, may form; the magnesium comes from the influent wastewater, from lime, or from magnesium salts.

• Inert solids from chemicals. This item is most significant with lime. If a quicklime is 92% CaO, the remaining 8% may be mostly inert solids that appear in the sludge. Many chemicals supplied in dry form may contain significant amounts of inert solids.

• Polymer solids. Polymers may be used as primary coagulants and to improve the performance of other coagulants. The polymers themselves contribute little to the total mass, but they can greatly improve clarifier efficiency, with a concomitant increase in sludge production.

• Suspended solids from the wastewater. Addition of any chemical to wastewater treatment process affects process efficiency. Therefore, the change in sludge production must be considered.

2.3 SLUDGE CHARACTERISTICS

Knowledge of the characteristics of sludge, along with the quantities produced, is important for the design of a sludge processing system in a waste-water treatment plant. Several factors influence the characteristics of sludge. These include the amount and type of industrial waste contribution, ground garbage, storm flow, sidestreams from sludge processing units, and the treatment process used for the wastewater.

2.3.1 Primary Sludge

Fresh primary sludge is a gray or light brown suspension with solids of different sizes and composition. Because of the high organic content of primary sludge, it decays quickly and becomes septic, which can be identified by its change to a dark gray or black color and an objectionable sour odor. Sludge characteristics vary widely from one treatment plant to another. Table 2.2 lists compositions of sludge typically seen in European and Russian wastewater treatment plants. These can be compared to the constituents of sludge in North America that are shown in Table 2.3. A range of values and in most cases typical values are given.

Concentration is affected by the type of solids in the raw wastewater and the frequency of sludge withdrawn from the primary settling tank. Some plants withdraw the sludge less frequently and allow the sludge to thicken further in the primary settling tanks, thereby increasing the solids concentration of sludge. However, such sludge, because of its long detention time in the tank, can generate unpleasant odors. The quantity of raw primary sludge can be approximately 0.4 to 0.5% by volume of the plant influent flow, or approximately 1.1 m3 (39 ft3) per 1000 people.

TABLE 2.2 Wastewater Sludge Characteristics in Europe and Russia

Item

Raw Primary Sludge

Digested Primary Sludge

Unthickened Activated Sludge

Digested Mixture of Primary and Thickened Activated Sludge

Total dry solids (TS) (%)

4.5

6.0

0.5

3.0

Volatile solids (% of TS)

70

50

75

60

Grease and fats

18

11

6

5

Protein (% of TS)

25

18

37

22

Ammonia nitrogen (% of TS)

3.0

2.0

5.0

3.5

Phosphoric acid (% of TS)

1.4

2.0

4.0

3.0

Potash (% of TS)

0.5

0.4

0.4

0.4

pH

6

7

7

7

TABLE 2.3 Wastewater Sludge Characteristics in North America

Primary Sludge Activated Sludge

Item Range Typical Range Typical

TABLE 2.3 Wastewater Sludge Characteristics in North America

Item Range Typical Range Typical

Total dry solids (TS) (%)

2-7

5

0.4-1.5

1

Volatile solids (% of TS)

60-80

65

60-80

75

Specific gravity

1.02

1.01

Grease and fats

Ether soluble (% of TS)

6-30

Ether extract (% of Ts)

7-35

5-12

Protein (% of TS)

20-30

25

32-41

Nitrogen (N, % of TS)

1.5-4.0

2.5

2.4-5.0

Phosphorus (P2O5, % of TS)

0.8-2.8

1.6

2.8-11.0

Potash (K2O, % of TS)

0-1

0.4

0.5-0.7

Cellulose (% of TS)

9-13

10

7

Iron (not as sulfide, % of TS)

2-4

2.5

Silica (SiO2, % of TS)

15-20

8

pH

5-8

6

6.5-8.0

7

Alkalinity (mg/L as CaCO3)

500-1,500

600

580-1,100

Organic acids (mg/L as HAc)

200-2,000

500

1,100-1,700

Energy content:

kJ/kg

23,300

18,600

23,300

Btu/lb

10,000

8,000

10,000

Source: Adapted in part from U.S. EPA, 1979.

Source: Adapted in part from U.S. EPA, 1979.

TABLE 2.4 Major Mineral Constituents of Sludge"

Content

Raw Primary Sludge

Raw Activated Sludge

Digested Mixture of Primary and Activated Sludges

SiÜ2

21.5-55.9

17.6-33.8

27.3-35.7

AI2O3

0.3-18.9

7.3-26.9

8.7-9.3

Fe3O4

4.9-13.9

7.2-18.7

11.4-13.6

CaO

11.8-35.9

8.9-16.7

12.5-15.6

MgO

2.1-4.3

1.4-11.4

1.5-3.6

k2o

0.7-3.4

0.8-3.9

1.8-2.8

Na2O

0.8-4.2

1.9-8.3

2.6-4.7

SO3

20-7.5

1.5-6.8

3.0-7.2

ZnO

0.1-0.2

0.2-0.3

0.1-0.3

CuO

0.1-0.8

0.1-0.2

0.2-0.3

NiO

0.2-2.9

0.2-3.4

0.2-1.0

Cr2Os

0.8-3.1

0.0-2.4

1.3-1.9

" Values shown are a percentage of total mineral constituents.

" Values shown are a percentage of total mineral constituents.

The concentration of volatile solids in sludge will approximately match the concentration of volatile solids in the influent wastewater unless large amounts of sludge-processing sidestreams are returned to the primary tank influent flow. A volatile solids content below 70% of total solids is usually influenced by the presence of groundwater infiltration, stormwater inflow, sludge-processing sidestreams, a large amount of grit, industrial waste with a low volatile solids content, or wastewater solids that have a long detention time in the sewers. Volatile solids will also be at the lower end of the range if chemicals are used to precipitate phosphorus in the primary settling tank.

Major chemical constituents of raw sludge include grease and fats, protein, nitrogen, phosphorus, potash, cellulose, iron, and silica. The major mineral constituents of sludge are shown in Table 2.4. Wastewater sludge may also contain heavy metals, such as cadmium, chromium, cobalt, copper, lead, mercury, nickel, and zinc.

Alkalinity and pH are the most important of the easily measured chemical parameters affecting sludge conditioning. Raw primary sludge has a pH range of 5.0 to 8.0 and an alkalinity of 500 to 1500 mg/L as CaCO3. The organic acid content is 200 to 2000 mg/L as HAc.

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