Process

Frequently, conditions of leachate treatment within the landfill change considerably in comparison with the specifications for the planning of the plant. Thus, the task is to utilize the potential of the plant optimally under these new conditions. The most important element of the enlargement conception is the membrane change in the reverse osmosis plant. One replacement of reverse osmosis by nanofiltration membranes. The plant could then be used with these membranes without the need to effect comprehensive conversion works. Similar to reverse osmosis, nanofiltration

Figure 20 Permate flux in RO process as a function of concentration factor (1, leachate after biological pretreatment; 3, raw leachate) (From Refs 78-84).
Unix Family Tree
Figure 21 Schematic of the leachate treatment plant for the landfill at Mechernich (from Ref. 85).

membranes are characterized by the fact that they have a high retention of organic substances from molecular weight ca. 200 g/mol or from a size of approximately 1 nm. Furthermore, nanofiltration membranes are ion selective, which means that the salts with monovalent anions are able to pass through the membrane to a high degree, whereas the salts with polyvalent anions are retained [91,96,101]. Table 19 presents the retention coefficients of large-scale, single-stage reverse osmosis plants with composite membranes for biologically pretreated leachate in comparison with nanofiltration membranes as determined in medium-scale examinations.

By using nanofiltration membranes instead of the first-stage reverse osmosis plant, the organic components, heavy metals, and the bivalent organic salts would be retained to a great

Table 18 Effectiveness of the Leachate Treatment Plant at Mechernich (1994)

Influent leachate Biological treatment Reverse osmosis permeate

Table 18 Effectiveness of the Leachate Treatment Plant at Mechernich (1994)

Influent leachate Biological treatment Reverse osmosis permeate

Parameter

Average

Max

Average

Max

Average

Max

COD (mg/L)

3176

6440

1301

1755

16.8

48

BOD5 (mg/L)

1062

4540

23.7

64

2.11

5.5

Organic N (mg/L)

251

1449

82.1

125

5.71

9.8

NH4-N (mg/L)

884

1153

1.9

92

0.48

4.2

NO3-N (mg/L)

32

110

129

440

10.0

22.5

NO2-N (mg/L)

0.1

0.6

2.4

75.2

0.12

0.79

Ninorg (mg/L)

916

1173

131.2

430

10.6

20.6

Pb (mg/L)

0.593

0.9

0.142

0.19

< 0.001

<0.001

AOX (mg/L)

1261

2750

775

1360

<0.01

0.015

Cl-(mg/L)

2172

5430

2010

2300

29.0

73

Source: Ref. 85.

Table 19 Reference Data for the Retention of Reverse Osmosis and Nanofiltration Membranes for Biologically Pretreated Leachate

Parameter Retention coefficient of RO Retention coefficient of NF

Table 19 Reference Data for the Retention of Reverse Osmosis and Nanofiltration Membranes for Biologically Pretreated Leachate membranes (%)__membranes (%)

COD

92-97

85-95

AOX

90-96

80-91

N03-N

83-93

5-20

Cl-

85-95

5-20

SO42-

95-99

90-96

Heavy

88-97

85-96

metals

extent. The monovalent ions as chloride and all the nitrogen ions would pass through the membrane. In such a situation, biological pretreatment would have to be designed in a way to fulfill the requirements on the discharge of the leachate concerning the inorganic nitrogen compounds. In the nanofiltration plant, a volumetric concentration factor amounting to 10 would be possible, so little volume of retentate should be sent to the evaporation plant, and the evaporation and drying plant would therefore be sufficiently dimensioned. Apart from this, recirculation of the retentate from nanofiltration to biological pretreatment in order to increase COD elimination [102] would only be possible to a limited degree, since without the extraction of sulfates and heavy metals, they would accumulate in the biological stage and soon result in considerable problems with calcium sulfate scaling. Figure 22 presents the concept of nanofiltration application for leachate treatment with biological pretreatment.

Figure 22 Process conception for capacity enlargement of the leachate treatment plant (From Refs 78, 82).

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