Table 17 Rejection of Substances from Landfill Leachate by Means of Nanofiltration

Parameter

Raw leachate Permeate Retention coefficient (%

Conductivity (pS/cm)

61 43

17,000 700

3,350 1,420

31,200 2,345

12,800 17,700

2,670 187

1,030 72.7

10,900 5,010

29.5

COD (mgO2/L) Ammonia (mg/L) Sulfate (mg/L) Chloride (mg/L) Calcium (mg/L)

95.8

Magnesium (mg/L) Sodium (mg/L)

Figure 18 Hybrid membrane system for landfill leachate treatment with high permeate recovery (from Ref. 81).

at the removal of substances undergoing biodegradation, especially in the case of effluent heavily loaded with organic substances. As a result of reverse osmosis, refractive substances and inorganic salts are removed. The flux leaving the biological installation is subjected to further treatment with the use of reverse osmosis. With minimum requirements involving the quality of permeate, a single-stage reverse osmosis may be sufficient.

There are two pretreatment possibilities of landfill leachate before reverse osmosis with the biological method [84]. The first involves activated sludge with high sludge loading above 0.25 kg/kg -day. With this solution, it is possible to diminish only TOC and AOX. The advantages of such a solution are as follows:

• removal of scaling and fouling causing compounds; and

• enhancement of permeate flux and concentration factor in the reverse osmosis step.

The second pretreatment involves advanced biological pretreatment including nitrification and denitrification. Figure 19 presents the diagram of a pilot installation utilizing that concept for the treatment of outflows [84].

The task of fabric-drum filters is to enhance the nitrification process and to protect reverse osmosis membranes. The advantages of such a system include a fixed film of nitrifying bacteria and enhanced N-loading rate due to optimum BOD/N ratios of less than 0.1.

The application of an affiliate system of biological treatment and reverse osmosis is more economical than a system where only reverse osmosis is applied, since the permeate flux and/or obtainable concentration factor are considerably increased. Furthermore, the nitrification process lowers the pH of the pretreated stream, and the quantity of added acid is considerably lowered. The procedure of membrane cleaning is carried out with detergent solution and citric acid, with much lower frequency as compared to the previous method. Modules for reverse osmosis are always equipped with tubular membranes from cellulose acetate (retention coefficient of NaCl, 95%) and composite membranes of the retention coefficient equalling 99%. The utilization of retentate most frequently consists in the application of a single-stage evaporator with forced circulation of the liquid and drying the concentrate from the evaporator in the drying machine [114]. As a result, solid dry waste, suitable for storing, is obtained.

Figure 19 Diagram of the installation for multistage treatment of landfill leachate. (From Refs 78, 84.)

The introduction of a biological pretreatment to the system for the treatment of landfill leachate with the application of reverse osmosis results not only in the removal of substances undergoing biodegradation, but it has many other important advantages [84].

• only refractive substances remain in the landfill leachate;

• higher permeate fluxes and higher volumetric capacities of the permeate can be obtained in the process of reverse osmosis, because the biofouling of osmotic membranes is eliminated;

• the removal of calcium and iron compounds effected by aeration of the outflow is taking place, which prevents the formation of membrane scale;

• nitrogen compounds that cause problems in the course of concentrate evaporation after reverse osmosis are removed;

• treatment costs are lowered.

To sum up, biological pretreatment has a strong influence on the reverse osmosis feed. It diminishes not only organic loading factors but also conductivity of each leachate. This offers a possibility for evaluating the effect of primary purification on the RO process. Figure 20 shows the permeate flux vs. the volumetric concentration factor [84]. The more biological pretreatment diminishes the conductivity, the higher is the possible concentration factor. This phenomenon can be explained by the fact that the osmotic pressure of inorganic salts is higher than that of organic salts or ammonia.

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