Key reasons to consider using anaerobic processes

Anaerobic processes are regarded as the most efficient of biological technologies (Speece, 1996). In contrast to aerobic biological treatment, anaerobic fermentation processes do not require air input and generate considerably smaller amounts of sludge. Anaerobic treatment normally produces 10 times less refractory biomass than aerobic treatment. Under anaerobic conditions, the majority of food processing waste COD is converted to biogas (methane and carbon dioxide) as an end product. This equivalent energy is not available for biomass synthesis, and thereby considerably lessens wastewater biomass disposal requirements and the financial burden associated with disposal. Biogas produced from anaerobic treatment has been promoted as a part of the solution to energy problems. Bio-methane has a calorific value of 9000 kcal/m3 and can be burned on-site for heat production, to generate electricity or to do both. Little energy (3-5%) is lost as heat in the biological process (Saham, 1984; Speece, 1996; Droste, 1997).

Lettinga and Hulshoff Pol (1991a) have listed criteria to select proper treatment for any given wastewater as follows:

1 They should lead to prevention of the production of additional wastewater(s), or at least a sharp reduction.

2 They should not require any dilution of the pollutants with clean water.

3 They should provide a high efficiency with respect to environmental pollution control.

4 They should lead to maximum recovery and reuse of polluting substances (e.g. to integrated systems) particularly from food processing wastewaters.

5 They should be low cost with respect to construction, required infrastructure (including energy requirement), operation and maintenance.

6 They should be applicable on a small as well as large scale.

7 They should lead to a high self-sufficiency in all respects.

8 They should be acceptable for the local population.

9 The method should provide sufficient treatment efficiency for removal of various categories of pollutants, i.e. biodegradable organic matter, suspended solids, ammonia, organic-N compounds, phosphates and pathogens.

10 The system should be stable for interruptions in power supply, peak loads, feed interruptions and/or for avoiding toxic pollutants.

11 The process should be flexible with respect to future extensions and possibilities to improve the efficiency.

12 The system should be simple to operate, maintain and control, so that good performance does not depend on the continuous presence of highly skilled operators and engineers.

13 Land requirements should be low, especially when little land is available and/or the price of the land is high.

14 The number of process steps should be as low as possible.

15 The lifetime of the system should be long.

16 The system should not suffer from serious sludge disposal problems.

17 The application of the system should not be accompanied by malodor or other nuisance problems.

18 The system should offer good possibilities for recovery of useful byproducts, such as for irrigation or fertilization.

19 There should be sufficient experience with the system to manage it easily.

Based on these criteria, anaerobic treatment technology is regarded as a highly effective method for the treatment of most wastewaters.

The development of new bioreactors is one reason that anaerobic treatment technologies will probably gain in popularity. Organic loading rates over 20 kg COD/m3/day (g COD/L day) are possible for some types of food processing wastes in a modern high-rate anaerobic digester (Table 23.1). High loading rates result in relatively low hydraulic retention times, which reduces capital costs because the vessels holding the waste are smaller.

Table 23.1 Comparison of various anaerobic treatment processes for cheese waste

Reactor

(°C)

(d)

Influent strength (g COD/L)

OLR (g COD/ L d)

Treatment efficiency

(%)

Reference

UFFLR

Sour

35

5

79

14

95

Wildenauer and Winter

whey

(1985)

DSFFR

Whey

35

5

13

2.6

88

De Haast et al. (1985)

FBR

Whey

35

0.4

7

7.7

90

Boening and Larsen

(1982)

FBR

Whey

35

0.1-0.4

0.8-1.0

6-40

63-87

Denac and Dunn (1988)

AAFEB

Powder

28-31

0.4-1.1

10

8.9-27

77-93

Switzenbaum and

Danskin (1982)

Whey

35

0.6-0.7

5-15

8.2-22

61-92

Switzenbaum and

Danskin (1982)

AnRBC

Whey

35

5

64

10.2

76

Lo and Liao (1986)

Whey

35

6-11

61-70

6.3-10

76

Lo and Liao (1986)

SDFA

Whey

4.3

69.8

16.1

99

Barford et al. (1986)

UASB

Deproteinized

35

1.5

11

7.1

94

Schroder and De

Whey

Haast (1989)

UASB

Whey

33

5

5-28.7

0.9-6

97-99

Yan et al. (1989)

DUHR

Whey

35

7

68

10

97

Malaspina et al. (1995)

UASB

Whey

35

2.3-11.6

5-77

1-28.5

95-99

Kalyuzhnyi et al. (1997)

UASB

Whey

22-30

5.4-6.8

47-55

7-9.5

90-94

Kalyuzhnyi et al. (1997)

UASB

Whey

20-29

3.3-12.8

16-50

1-6.7

90-95

Kalyuzhnyi et al. (1997)

UASB

Whey

34

5

25-30

90-98

Yan et al. (1993)

CSTR and

Whey

10

98

Malaspina et al. (1996)

DUHR

CSTR and AT

Whey

33-36

2

2-6

5

90-95

Ince (1998)

MPAR

Whey

34

2.3-2.4

20-37

9-14.7

92-98

Guiot et al. (1995)

Abbreviations are: HRT, hydraulic retention time; OLR, organic loading rate; COD, chemical oxygen demand; UFFLR, upflow fixed-film loop reactor; DSFFR, downflow stationary fixed-bed reactor; FBR, fluidized-bed reactor; AAFEB, anaerobic attached-film expanded-bed reactor; AnRBC, anaerobic rotating biological contact reactor; SDFA, semicontinuous digester with flocculent addition; UASB, upflow anaerobic sludge-blanket; DUHR, downflow-upflow hybrid reactor. CSTR, continuously stirred tank reactor; AT, anaerobic filter; MPAR, multiplate anaerobic reactor.

Abbreviations are: HRT, hydraulic retention time; OLR, organic loading rate; COD, chemical oxygen demand; UFFLR, upflow fixed-film loop reactor; DSFFR, downflow stationary fixed-bed reactor; FBR, fluidized-bed reactor; AAFEB, anaerobic attached-film expanded-bed reactor; AnRBC, anaerobic rotating biological contact reactor; SDFA, semicontinuous digester with flocculent addition; UASB, upflow anaerobic sludge-blanket; DUHR, downflow-upflow hybrid reactor. CSTR, continuously stirred tank reactor; AT, anaerobic filter; MPAR, multiplate anaerobic reactor.

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