Supernatant

Supernatant from an anaerobic system generally contains high concentrations of dissolved and suspended solids, organic materials, nitrogen, phosphorus, and other materials. When returned to the treatment plant influent, these materials may impose an extra load on the liquid treatment processes. Table 5.6 presents reported characteristics of supernatant from two-stage digestion systems at different wastewater treatment plants. These characteristics represent a summary of the wide range of data observed at facilities in which the feed sludge has been (1) primary sludge alone, (2) primary and trickling filter sludge, or (3) primary and activated sludge.

As can be seen from the table, supernatant quality varies widely. Suspended solids, BOD, ammonia, and phosphorus can all cause problems in a treatment plant. The finely divided suspended solids in supernatant settle poorly and can build up in the plant, causing process overloading. High BOD imposes a high oxygen demand, requiring increased aeration requirement in activated sludge treatment systems. In plants that must achieve nitrogen limits in their effluents, the high ammonia loadings increase the cost of providing the oxygen required for treatment. In biological phosphorus removal, phosphorus is taken up by the growing cell mass and is removed from the waste-water stream in the waste biological sludge. This cell-bound phosphorus may be released during anaerobic digestion. Return of this phosphorus to the

Concentration" (mg/L)

Primary Primary and Primary and

Parameter Sludge Trickling Filter Sludge Activated Sludge

Primary Primary and Primary and

Parameter Sludge Trickling Filter Sludge Activated Sludge

Total solids

9,400

4,545

1,475

2,160

Total volatile solids

4,900

2,930

814

983

Suspended solids

Average

4,277'

2,205

1,518

7,772c

383

143

740

1,075

4,408'

Maximum

17,300

12,400

14,650

Minimum

660

100

100

Volatile suspended solids

Average

2,645

1,660

4,403

299

118

750

3,176

Maximum

10,850

17,750

10,650

Minimum

420

60

75

BOD

Average

713

1,238

515

667

Maximum

1,880

6,000

2,700

Minimum

200

135

100

COD

4,565

2,230

1,384

1,310

1,230

TOC

1,242

443

320

Total (p04)-p

143

85

63

87

100

nh3n

853

253

559

480

Organic nitrogen

291

678

53

91

360

560

pH

8.0

7.3

7.2

7.0

7.8

7.0

7.3

Volatile acids

264

322

250

Alakalinity (as CaC03)

2,555

3,780

1,349

1,434

Phenols

Average

0.23

0.23

0.35

Maximum

0.80

0.50

1.00

Minimum

0.06

0.06

0.08

11 Unless noted, all values are average for the sampling period studied. b Values in this column are a composite from seven treatment plants. " Values in this column are a composite from six treatment plants.

11 Unless noted, all values are average for the sampling period studied. b Values in this column are a composite from seven treatment plants. " Values in this column are a composite from six treatment plants.

TABLE 5.7 Alternatives for the Treatment of Anaerobic Digestion Supenatant

Constituent

Treatment Alternatives

Suspended solids Coagulation with iron salts; filtration

BOD Removal with suspended solids; stripping of volatile acids;

aerobic biological treatment; activated carbon adsorption Carbon dioxide Precipitation with lime; stripping; ion exchange Nitrogen Removal of organic nitrogen with suspended solids; chemical precipitation; ion exchange; ammonia stripping may occur at pH > 8.3

Phosphorus Removal with suspended solids; chemical precipitation; ion exchange

Source: Lue-Hing et al., 1998.

liquid stream can substantially reduce the net phosphorus removal efficiency of the plant and may necessitate removal by chemical treatment.

Problems associated with the recycling of supernatant from an anaerobic digestion system to the headworks of the plant include odor problems, possible sludge bulking, increased chlorine demand, and higher concentrations of nutrients in the effluent (Lue-Hing et al., 1998). High concentrations of some of the constituents may require removal by physical, biological, or chemical treatment of the supernatant before it is recycled to the liquid stream. Process alternatives for the treatment of supernatant are summarized in Table 5.7.

It is preferable to eliminate, rather than treat, a highly polluted digester supernatant. Modern digesters are designed as single-stage digesters without supernatant draw-off. In addition to eliminating supernatant discharge, preth-ickening the feed sludge reduces digester volume and heating requirements.

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Responses

  • myrtle roper
    Where should Supernatant from sludge digesters be sent?
    7 months ago
  • Annabella Sabbatini
    What to do with supernatant of sluge anaerobic digestion?
    7 months ago
  • sarah hartmann
    What affects supernate wastewater?
    3 months ago
  • lemlem nasih
    What is supernatant waste water?
    11 days ago

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