Conventional Systems Anaerobic Systems

Anaerobic treatment of leachate is similar to the anaerobic processes occurring inside municipal waste landfills. Organic substances are first hydrolyzed to simpler forms. Amino acids are produced from proteins, monosaccharides from polysaccharides, fatty acids from lipids, purines and pyrimidines from nucleic acids. Then hydrolyzed products are subjected to a fermentation process, forming higher organic acids-nonmethanogenic substrates (~75% of organic matter) such as propionate and butyrate. Nonmethanogenic substrates as well as small quantities of input organic matter are transformed by acetogenesis and dehydrogenation and, as a result, methanogenic, substrates are generated. Among the methanogenic substrates are H2, CO2, formate, methanol, methylamines, and acetate, which is the main substrate. This entire phase is called acidogenesis because its main products are fatty acids, whose presence is responsible for the pH decrease. Next, the methanogenic phase occurs, and methane in combination with CO2 is produced (Fig. 5). In each phase, another group of microorganisms is active. One group of bacteria is responsible for hydrolysis of organic polymers to their monomers. The second group-decomposes monomers into organic acids. There are facultative and obligate anaerobic bacteria called acidogenic bacteria. Methanogenic bacteria, which are anaerobes responsible for methane and CO2 production from acetate as well as from hydrogen and carbon dioxide, are the last group of bacteria involved in anaerobic processing of organic matter decomposition [32].

Anaerobic treatment is applied mainly to young landfill leachate, which is characterized by very high values of BOD5 from several to several tens of thousands of milligrams per liter. Moreover, it contains large quantities of easily biodegradable organic pollutants susceptible to fermentation such as volatile fatty acids, alcohols, and aldehydes [11,34,35]. Low BOD5 values applied in anaerobic treatment vary from 1000 to 1500 mg/L [33]. Kettunen and Rintala [35] write, however, that that favorable effects can be achieved at leachate COD values higher than 800 mg/L and BOD/COD ratios higher than 0.3.

Anaerobic processes for landfill leachate treatment have been examined in typical systems applied in wastewater treatment and systems:

• UASB (upflow anaerobic sludge blanket) reactor [35];

• ASBR (anaerobic sequencing batch reactor) [36];

• AHBF (anaerobic hybrid bed filter—merged upflow sludge blanket at the bottom and anaerobic filter on the top of the filter) [36].

Figure 5 Diagram of organic matter decomposition within anaerobic process. (From Refs. 32, 33.)

Recent literature indicates that anaerobic processes allow for the complete removal of organic carbon from leachate of 70-80% and BODs removal beyond 90% [35,36]. Table 5 shows details concerning the results reached in different reactor types presented in literature during the 1980s and late 1990s. All results are comparable and differences are provoked probably more by leachate composition and organic loading rate than by type of reactor.

Some difficulties in anaerobic leachate treatment must also be mentioned. Disadvantages of anaerobic process that should be taken into account during leachate treatment are:

• Decrease of anaerobic treatment efficiency along with the development of methanogenic phase of waste decomposition in the landfill. Consequently, when the methanogenic phase is predominant, the change of the anaerobic process into an aerobic process is necessary.

• Possible lack of ammonia nitrogen removal. This component is one of the major pollutants in leachate.

• Considerable vulnerability to changes of environmental factors such as temperature, pH, as well as to toxic substances [11,41].

The influence of temperature on anaerobic process efficiency is illustrated in Table 5. In experiments carried out by Kettunen and Rintala [35], the decrease of temperature from 21-23°C to 13°C lowered the anaerobic process efficiency by 8-22%. Methanogenic phase inhibition caused the drop in COD removal, as evidenced by the increase of volatile acids (acetic

Table 5 Different Anaerobic Reactors Performance

Type of

Volume

Hydraulic Temperature

COD

Biogas

Methane

Source

reactor

of reactor

retention

(°C)

removal

production

contents

Ref.

(L)

time (days)

COD destroyed)

in biogas (%)

Anaerobic

24

5

25-34

70

400-500

80

[38, cit.

filter

after 37]

Packed bed

3

8

Ambient, 21-

95

580

74

[18]

anaerobic

25

reactors

UASB

12.7

1.9

29

83

499

75.5

[39, cit.

reactor

after 37]

Full-scale

135,000

NM

35

95

500

72

[40, cit.

AHBFs

after

(anaerobic

37]

filter/

UASB)

AHBFs

2.75

2.4

35

37.5-76

NM

NM

[36]

(anaerobic

(available

filter/

volume)

UASB)

ASBR

2

1.5-10

35

57-74

NM

NM

[36]

UASB

40

0.96

23

57-63

NM

NM

[35]

reactor

0.54

21

71-77

1.3

13

49-55

and propionic acids) concentrations in the effluent. The authors also described the significant quantities of inorganic precipitates accumulated in the reactor sludge. At longer operation times of the reactor, the problem of accumulated precipitates can arise, which authors suggested remedying by periodical replacement of the sludge [35].

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