Treatment of Reverse Osmosis Retentate

The purification of landfill leachate with membrane techniques helps prevent contamination of natural water sources. Besides this ecological aspect, commercial feasibility must also be considered. In this regard, membrane filtration has proved to be a justifiable and economic solution in most cases, even when the overall costs for the purification are compared with other approaches. This evaluation includes the handling of the retentate (concentrate) produced in the reverse osmosis plant, because of high concentrations of organic and inorganic compounds that must be further treated.

In the past, treatment of reverse osmosis retentate was considered a very expensive treatment step. In many old plants in operation, this step consisted of evaporation and drying followed by deposition of the dry residues in a special landfill. A good example is the treatment system used for leachate of a landfill in Mechernich, Germany [85], schematically represented in

Table 15 Landfill Leachate Treated by the Reverse Osmosis Plant with DT-Module in Two Stages in Schwabach

Parameter Feed water (mg/L) Permeate, second stage (mg/L) Retention coefficient

(%)

Sulfate

22,100

4.80

99.9

Chloride

6,360

14.0

99.8

Ammonium

1,955

42.0

97.9

Nitrate

455

18.0

96.0

COD

912

15.0

98.4

TOC

289

4.00

98.6

Hydrocarbons

13.4

0.30

97.8

Nickel

2.78

0.10

96.4

Chromium

2.18

0.10

95.4

Copper

0.97

0.10

89.6

Vanadium

290

2.20

99.2

Source: Refs 81, 82, 99.

Table 16 Typical Plant Performance in Leachate Purification

Parameter_Raw leachate Permeate I Permeate II Retention coefficient (%)

Table 16 Typical Plant Performance in Leachate Purification

Parameter_Raw leachate Permeate I Permeate II Retention coefficient (%)

PH

7.7

6.8

6.6

-

Conductivity (pS/cm)

17,250

382

20

99.9

COD (mgO2/L)

1,800

15

<15

>99.2

Ammonium (mg/L)

366

9.8

0.66

99.9

Chloride (mg/L)

2,830

48.4

1.9

99.9

Sodium (mg/L)

4,180

55.9

2.5

99.9

Heavy metals (mg/L)

0.25

<0.005

<0.005

>98

Fig. 17 [80]. The total capacity is 150 m3/day; the first RO unit uses tubular membranes in order to avoid fouling; the second reverse osmosis unit uses spiral-wound modules. The concentrate from the second RO is brought back to the inlet of the first RO unit. The concentrate from the first reverse osmosis unit is evaporated in two stages, upon which the residue is dried in a fluidized bed. The dried material is landfilled; the distillate from the evaporation is brought back to the second RO unit.

This scheme can be simplified for relatively new landfills where the leachate is relatively biodegradable, and concentrate can then be directly reinjected into the landfill. During the recirculation, the quality of the concentrate will improve and equilibrium between leaching and biodegradation will be reached for the organic fraction [81,82]. However, this method is not applicable for older landfills and for leachate containing a large inorganic fraction. Evaporation and drying is then necessary and may be followed by a solidification, so that a material is obtained with low water permeability and with low leaching of heavy metals, which can be landfilled without any additional environmental risk.

Today, other possibilities that meet the best ecological and economical requirements are considered. These are [81,82]:

• Transport of the concentrate to an incineration plant equipped for the burning of liquid hazardous waste. This option allows energy to be extracted from the waste material. The incineration must be carried out in a rotating kiln furnace, which provides better incineration than a grate furnace and is thus more suitable for hazardous materials.

• The solidification of the concentrate with different materials, as fly ash [82] or sludge from wastewater treatment plants [82], and disposal of this kind of dry residue on the

Figure 17 Schematic of a landfill leachate treatment in Mechernich, Germany (From Refs 80, 85).

landfill itself If these solids are produced under controlled conditions, no further pollution is to be expected. • The abovementioned controlled reinjection of the concentrate into changing areas of the landfill in order to improve the biochemical degradation process in the waste itself and accelerate the immobilization of the organic matter.

Was this article helpful?

0 0
Waste Management And Control

Waste Management And Control

Get All The Support And Guidance You Need To Be A Success At Understanding Waste Management. This Book Is One Of The Most Valuable Resources In The World When It Comes To The Truth about Environment, Waste and Landfills.

Get My Free Ebook


Post a comment