Toxic Materials

Although many materials are toxic to the bacteria in an anaerobic digester, heavy metals, light metal cations, ammonia, sulfides, and some inorganic materials are of concern. Toxic conditions normally occur from overfeeding and excessive addition of chemicals. Toxic conditions can also occur from industrial wastewater contributions with excessive toxic materials to the plant influent. Tables 5.1 and 5.2 present toxic and inhibitory materials of concern.

Heavy metal toxicity has frequently been cited as the cause of anaerobic digestion failures, although trace amounts of most heavy metals are necessary for cell synthesis. Domestic wastewater sludge normally has low concentrations of light metal cations (sodium, potassium, calcium, and magnesium). However, significant contributions can come from industrial discharges and

ENVIRONMENTAL FACTORS

181

TABLE 5.1 Selected Toxic and Inhibitory Inorganic Materials in

Anaerobic Digestion

Moderately Inhibitory

Strongly Inhibitory

Substance

Concentration (mg/L)

Concentration (mg/L)

Na+

3,500-5,500

8,000

K+

2,500-4,500

12,000

Ca2+

2,500-4,500

8,000

Mg2+

1,000-1,500

3,000

Ammonia nitrogen, NH4+

1,500-3,500

3,000

Sulfide, S2-

200

200

Copper, Cu2+

0.5 (soluble)

50-70 (total)

Chromium

Cr6+

3.0 (soluble)

200-250 (total)

Cr3 +

2.0 (soluble)

180-420 (total)

Nickel, Ni2+

30 (total)

Zinc, Zn2+

1.0 (soluble)

Source: WEF, 1998.

TABLE 5.2 Selected Toxic and Inhibitory Organic Materials in

Anaerobic Digestion

Concentration Resulting in 50% Reduction

Compound

in

Activity (mM)

1-Chloropropene

0.1

Nitrobenzene

0.1

Acrolein

0.2

1-Chloropane

1.9

Formaldehyde

2.4

Lauric acid

2.6

Ethyl benzene

3.2

Acrylonitrile

4

3-Chlorol-1,2-propanediol

6

Crotonaldehyde

6.5

2-Chloropropionic acid

8

Vinyl acetate

8

Acetaldehyde

10

Ethyl acetate

11

Acrylic acid

12

Catechol

24

Phenol

26

Aniline

26

Resorcinol

29

Propanol

Source: WEF, 1998.

from the addition of alkaline material for pH control. Ammonia, produced during the anaerobic digestion of proteins and urea, may reach toxic levels in highly concentrated sludge. Ammonia nitrogen concentrations of more than 1000 mg/L can be highly toxic.

When wastewater sludge contains high concentrations of sulfide, it can cause a problem in anaerobic digestion because the sulfate-reducing bacteria reduce sulfate to sulfide, which is toxic to methanogens at concentrations over 200 mg/L. This can be controlled by precipitating the sulfide as iron sulfide by adding iron salts to the digesters at controlled amounts.

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