The integrated method of landfill leachate pretreatment

In Poland every year about 14 mlns ton of municipal waste are generated. More then 90% is deposited on landfills. Leachate is generated as consequence of rainwater percolation through wastes, chemical-biological process in waste and the inherent water content of wastes themselves

[Afshin Maleki, 2009; Rivas et al. 2004]. The characteristics of the landfill leachate can usually be represented by the basic parameters COD, BOD, the ratio BOD/COD, pH, suspened solids(SS), ammonium nitrogen, (N-NH3), total Kjeldal nitrogen (TKN) and heavy metal [Renou, 2008].  The leachate composition  from different landfills, show a wide variations. There are many factors affecting the quality of leachates: i.e. age, precipitation, seasonal weather variation, waste type and composition. Depending on rainfall conditions, the colour of leachate varies from black to brown. A brownish colour in the leachate is generated by dissolved organic materials such as humic substances [Kurniawan, 2006]. The discharge of landfill leachate can lead to serious environmental problems, since the leachate contains a large amount of organic matter (both biodegradable and non-biodegradable carbon), ammonia –nitrogen, heavy metals, chlorinated organic and inorganic salts [Tatsi, 2003; Uygur, 2004]. Although some of these pollutants can be degraded by microorganisms, only limited removal of bio-refractory organic pollutants can be achieved. Therefore, alternative technologies based on physical-chemical stages are required. Thus, the treatment processes used for landfill leachates often involve a combination of apprioprate techniques. They are designed as modular , multistage units, according to leachate characteristics. Well known processes drawn from wastewater technology have been applied for the treatment of landfill leachates, such as anaerobic and /or aerobic biological degradation, chemical oxidation, coagulation –precipitation, activated carbon adsorption, photo-oxidation and membrane processes [Tatsi, 2003; Trebouet, 2001; Wang, 2002;Calli, 2005].

Several authors [Maleki, 2009; Silva, 2004; Amokrane, 1997] suggest, that coagulation and flocculation is a relatively simple technique that may be employed successfully in treating old landfill leachates. Aluminum sulfate , ferrous sulfate, ferric chloride, were commonly used as coagulants. Iron salts were proved to be more sufficient than aluminum ones.

As a new processes applied to landfill leachates treatment, the use of membrane techniques are suggested, like microfiltration [Piatkiewicz, 2001], ultrafiltration [Syzdek, 1984; Van Dijk, 1997; Bohdziewicz, 2001],  nanofiltration [Peters,1998; Trebouet, 2001], and reverse osmosis [Linde, 1995; Bilstad, 1992].

Several hybrid processes such as activated sludge –ultrafiltration-chemical oxidation and acvtivated sludge—ultrafiltration-reverse osmosis have been tested [Renou, 2008]. Combined methods like stripping, Fenton, SBR and coagulation are found as  highly effective [Guo 2010].

Suitable treatment strategy depends on major criteria: the initial leachate quality and the final requirements given by local discharge water standards.

Chemical oxidation is a widely studied method for the treatment of effluents containing refractory compounds such as landfill leachate. Growing interest has been recently focused on advanced oxidation process (AOP). Sometimes, when conventional methods fail they are the only effective methods of removing toxic or even carcinogenic organic substances resistant to biodegradation. AOP methods consist in the generation of highly reactive hydroxyl radical (• OH). For this purpose various combinations  of hydrogen peroxide, ozone, UV radiation, ultrasound and catalysts, for example TiO2, Fe2+, Fe3+ are applied. The resulting hydroxyl radical has the highest oxidation potential of 2.75 V and acts selectively on the majority of organic compounds. One of the methods of advanced oxidation is the Fenton reaction in which a mixture of iron (II) and hydrogen peroxide is used to generate hydroxyl radicals. Hydroxyl radicals are formed in the Fenton process as it is  shown in the following equation:

Fe2+ + H2O2Fe3+ + OH+ OH–

During the Fenton reaction the iron (III) precipitating in the form of colloidal ferric hydroxide (III) is formed  in addition to hydroxyl radical. The Fenton reaction enables to lead the processes of oxidation and wastewater coagulation parallely. The effect of Fenton's reagent oxidation is highly dependent on pH solution and the ratio of H2O2 : Fe2+. The optimal pH values ??are between pH 3 and 4, while the weight ratio of iron (II) to hydrogen peroxide is 1 : 5 [Swiderska, 2007]. In order to improve the efficiency of coagulation of contaminants the ashes obtained from municipal heat- power plant were used.

2. Materials and methods

The process of pretreatment was conducted in six solutions of landfill leachate of CODKMnO4=2400 gO2/m3. The leachate samples were  taken. on 5 July 2010 from a lechate  tank located in the municipal landfill in Promnik. Samples were collected in plastic bottles and transported to the laboratory and stored in 4oC. Leachates had strong black color, the specific smell of petroleum substances and pH = 7.81.


Fig.1  The test stand for the oxidation and coagulation of leachate from the landfill in Promnik

The test was conducted at pH = 3 (concentration correction H2SO4) for different doses of iron (50 and 100 g Fe/m3) and for two ratios of Fe2+ : H2O2 (1:5 i 1:10)  at 20 ° C ± 1 ° C. After 4 hours of oxidation on Conbest JLT6  device (fig.1) Samples were neutralized to pH 7.0  by Ca(OH)2 . To carry out the flocculation the samples of  FA/m3 0-2000 g were mixed for 30 minutes with the ashes collected from Kielce Power Plant and then subjected to a 30-minute sedimentation.

After removal of the suspension by filtration through a filter of = 0.45 µm the colour of leachate was tested by optical method using SPECOL and COD was determined by permanganate method.

Table 1. Results of AOP oxidation of  leachates from a landfill in Promnik


Lp

Designation

Unit

Raw leachate

Coagulant dose

1.

-

gFe/m3

0

50

50

100

100

100

100

 

Hydrogen peroxide dose

2.

-

gH2O2/m3

0

500

500

500

1000

1000

1000

 

Ash dose

3.

-

gFA/m3

0

0

200

200

400

800

2000

4.

pH

-

7.81

3.0

3.0

3.0

3.0

3.0

3.0

5.

Colour

gPt/m3

1500

320

230

230

280

300

500

6.

% of colour reduction

%

0

79

85

85

81

80

67

7.

Permanganate index

gO2/m3

2400

125

120

120

160

400

450

8.

% of CODKMno4 removal

%

0

94

95

95

93

83

81

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        


Fig. 2. The dependence of  COD of  treated  leachates versus Fe(II) and FA dose

Results

The results show that oxidation by the Fenton's reagent is an effective pretreatment method for landfill leachate (Table 1). In all cases, the colour was reduced by approximately 80%. The exception was the dose of 100 Fe2+/m3 with a ratio of iron to hydrogen peroxide  1:10 and ash content gFA/m3 . The resulting percentage of colour reduction was 67%.

In addition, in hydrogen peroxide dose of 500 gH2O2/m3 approximately 94% of COD removal for different doses of iron and /or ash  (Fig. 2) was observed. Tests have shown that satisfactory results were obtained even for a dose of iron of  50 gFe/m3 (94.8%), and therefore, there is no need to increase the dose. However, at the ratio Fe2+ : H2O2 = 1:5 the dose of 100 g Fe 2+/ m3 allows 85% reduction in colour. At a ratio of Fe2+: H2O2 = 1:10 the greatest efficiency was obtained for a 200 gFA/m3 dose of ash .

Summary
Advanced oxidation processes enable mineralization of refractory organic compounds, which are not readily biodegradable, harmful to humans and the environment, to carbon dioxide and water. In the case of incomplete decomposition of organic compounds, the resulting forms have a simpler structure and lower molecular weight, and thus are easily assimilated by microorganisms in the biological processes of water and wastewater treatment or better absorbed on activated carbon. The addition of ashes  (0.2%o)  during flocculation is beneficial for the efficiency of the removal of  color and COD from landfill leachate. This refers to a weight of 0.2 kg of  ashes for  1 m3 of landfill leachate.

REFERENCES
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УДК 628.54
Zygadlo M. The integrated method of landfill leachate pretreatment [Електронний ресурс]  / [Zygadlo Maria, Latosinska Jolanta, Gawdzik Jaroslaw ] // Збірник наукових статей “ІІІ-го Всеукраїнського з’їзду екологів з міжнародною участю”. – Вінниця, 2011. – Том.1. – С.65–68. Режим доступу: http://eco.com.ua/

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