Coagulation And Flocculation Method Biology Essay


Physicochemical methods are used along with the biological methods mainly to improve treatment efficiency or make them possible when the biological oxidation process is hampered by the presence of bio-refractory materials. The advantages of chemical treatment methods in general include immediate start-up, easy automation, insensitivity to temperature changes, can produce high quality effluents, adapt to wide variations in flow and chemical composition and have the ability to remove toxic substances from leachate and simplicity of plant and material requirements. However, the advantages are outweighed by the disadvantages of large quantities of sludge generated due to the addition of flocculants and chemicals with high running costs some of the treatment processes are difficult to operate and require highly skilled labor, They require high capital and operational costs and some processes require extensive pretreatment (Al-Harbawi 2008). Thus, chemical and physical treatment is merely used as pre or post treatment of leachate to complement biological processes.

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A brief description and review most of the physical-chemical treatment processes applied to leachate treatment is provided below.

Coagulation and flocculation Method

Coagulation/flocculation is an essential process in water and in industrial wastewater treatment. The coagulation process destabilizes colloidal particles by the addition of a coagulant. To increase the particle size, coagulation is usually followed by flocculation of the unstable particles into bulky floccules so that they can settle more easily (Cheng et al. 1994). The general approach for this technique includes pH adjustment and the addition of ferric/alum salts as the coagulant to overcome the repulsive forces between the particles (Ayoub et al. 2001). This treatment is effective on leachate with high molecular weight organic material such as fulvic and humic acid, since these components are generally difficult to degrade biologically. However, this method may result in only moderate removals of COD (or TOC) content, apart from presenting a number of drawbacks: excessive sludge may be produced, and in certain cases, when the conventional chemical coagulants are used, increased aluminium or iron concentrations may be encountered in the resulting effluent (Trebouet et al. 2001).

Coagulation-flocculation has been employed for the removal of non-biodegradable organic compounds and heavy metals from landfill leachate. It has thus been proposed mainly as a pretreatment method for fresh leachate, or as a post-treatment technique for partially stabilized leachate (Tatsi et al. 2003).

Coagulation the first step destabilizes the particle's charges. Coagulants have an opposite charge to those of suspended solids. The coagulants are used in the leachate in order to defuse the negative charges on dispersed solids which are not settled like color,producing organic substances and clay. When the charge is neutralized, the small particles which are suspended stick together in order to increase the particles size. Sometimes not all suspended particles are neutralized because the coagulant is not enough and needs more coagulant to be added (Ayoub et al. 2001).The next step after coagulation is flocculation which occurs in the moving particles that are not fixed into large flocs so that it can settle very fast. The effectiveness of the process is influenced by the coagulating agent, the coagulant dosage, the solution pH and ionic strength as well as the concentration and the nature of the organic compounds (Randtke 1988).

Inorganic coagulants such as lime (CaO), alum (Al2(SO4)3), aluminum chloride, ferric and ferrous sulfate (Fe2(SO4)3 and FeSO4), have been extensively used individually or in combination to treat the wastewater. Organic polymers have been used as flocculants. The effective pH range for aluminum sulfate is pH 5-6, for ferric sulfate pH 4.5-6 and for ferrous sulfate pH 8.5- 10. Iron salts have been proven to be a more efficient coagulant than aluminium ones (Diamadopoulos 1994).

During coagulation and flocculation processes, the aggregation of colloidal particles and dissolved natural organic matter (NOM) can be achieved by four primary mechanisms (Randtke 1988; Edzwald and Van Benschoten 1990): i) Adsorption and Charge neutralization/Destabilization (colloids only), ii) Enmeshment in precipitated floc particles (colloids only), iii) Complication / Precipitation (NOM only), and iv) Adsorption onto precipitated floc particles (NOM only). Here it should be noted that a combination of these mechanisms may be occur during coagulation and flocculation processes (Al-Harbawi 2008).

Coagulation/flocculation may be used successfully in treating stabilized and old landfill leachates (Silva et al. 2004). It is widely used as a pre-treatment (Amokrane et al. 1997; Ramirez Zamora et al. 2000), prior to biological or reverse osmosis step or as a final polishing treatment step in order to remove non-biodegradable organic matter. The application of bioflocculant, in comparison with traditional inorganics coagulants has been recently investigated by Zouboulis et al. (Zouboulis et al. 2004), for the lowering of humic acids. It revealed as a viable alternative since 20 mg/L bioflocculant dosage was sufficient in providing more than 85% humic acid removal.

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Several studies have been reported on the examination of coagulation-flocculation for the treatment of landfill leachates, aiming at process optimization, i.e. selection of the most appropriate coagulant, identification of optimum experimental conditions and assessment of pH effect (Amokrane et al. 1997). Synthesis of recent works clearly reveal that iron salts are more efficient than aluminum ones, resulting in sufficient chemical oxygen demand (COD) reductions (up to 50%), whereas the corresponding values in case of aluminium or lime addition were moderate (between 10 and 40%) (Loizidou et al. 1992). Nevertheless, combination of coagulants (Loizidou et al. 1992) or addition of flocculants together with coagulants may enhance the floc-settling rate (Amokrane et al. 1997)] and so the process performance (COD abatement up to 50%). COD removal with this process for young leachates is between 25-38% and for stabilised leachate with low BOD5/COD ratio it is about 75% (Wiszniowski et al. 2006).

However, this treatment presents some disadvantages: consistent sludge volume is produced and an increase on the concentration of aluminum or iron, in the liquid phase, may be observed (Silva et al. 2004).

The removal of heavy metals from stabilized leachate containing high concentrations of organic and inorganic matter was investigated using coagulation with FeCl3 (Urase et al. 1997). The metal removal performances were reported to be higher at pH 9.0 than at pH 4.0. The results demonstrated the effectiveness of precipitation at basic pH for the removal of heavy metals (Urase et al. 1997).

Another application of coagulation-flocculation for the removal of non-biodegradable organic compounds from hazardous landfill leachate was studied by Amokrane et al. (Amokrane et al. 1997).

Although the doses required were identical (0.035 mol/L of Fe or Al), with an initial COD concentration of 4100 mg/L, ferric chloride was found to give higher removal of organic compound (55%) than alum (42%). These results were in agreement with the previous study undertaken by Diamadopoulos (Diamadopoulos 1994) in the Thessaloniki landfill (Greece). At an initial concentration of 5690 mg/L and at pH 4.8, the maximum COD removal of 56% was achieved with 0.8 g/L of FeCl3, as compared to 39% with 0.4 g/L of Al2(SO4)3. The results of both studies suggest that FeCl3 is more effective than alum as a coagulant.

In a similar study, the application of coagulation-flocculation for the treatment of stabilized leachate from the Thessaloniki landfill (Greece) was reported by Tatsi et al. (Tatsi et al. 2003).Without pH adjustment, the addition of 1.5 g/L of FeCl3 was able to increase the COD removal rate to 80%, while 1.5 g/L of Al3+ ions resulted in up to 38% reduction of COD. These results were in agreement with another study carried out by Kargı and Pamukoglu in Turkey (Kargi and Pamukoglu 2003). After 30 h of fed-batch operation, coagulation-flocculation treatment using 2 g/L lime achieved 86% COD removal with an initial COD concentration of 7000 mg/L (Kargi and Pamukoglu 2003).

Overall, it is found that coagulation-flocculation technique using FeCl3 is effective for the removal of organic compounds and heavy metals. To improve the removal of COD from leachate, lime can be employed as a coagulant. The other drawbacks of this technique include the high operational cost due to high chemical consumption, the sensitivity of the process to pH and the generation of sludge. It is important to note that the velocity gradient, settling time and pH play major roles in increasing the probability of the settling of colloidal particles.

The application of coagulation-flocculation as a pretreatment process for young landfill leachate in order to prevent fouling in the ultra filtration membranes employed for the separation of biomass in the biological plant by Maranon et al. (Maranon et al. 2008). They tested ferric chloride, aluminium sulphate and aluminium polychloride (PAX) as the coagulants, along with different types of flocculants (anionic and cationic polyelectrolytes). Optimum pH values were around 4.0 and 6.0 for ferric chloride and aluminium sulphate, respectively. It was not necessary to alter the pH of the leachate when using PAX, as the optimum value was found to be similar to that of the leachate (around 8.3). Optimum dosages were 0.4 g Fe3+L-1, 0.8 g Al3+ L-1 and 4 g PAX L-1, although there was not much difference in the results for lower dosage of PAX. The best results were found with this coagulant, obtaining 98% turbidity removal, 91% colour removal and 26% COD removal. The volume of the sludge generated represents around 4.5-5.0% when using ferric chloride or aluminum sulphate, and 15% when using aluminum polychloride.

ii) Ammonia stripping method

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Ammonium stripping is the most widely employed treatment for the removal of NH3-N from landfill leachate (Marttinen et al. 2002; Collivignarelli et al. 1998). Ammonia stripping is based on the change of conditions in the effluent enabling the transition of ammonium ion to ammonia gas which must be efficiently removed from the liquid phase with air (Arogo et al. 1999). NH3-N is transferred from the waste stream into the air and is then absorbed from the air into a strong acid such as sulphuric acid or directly flux into the ambient air (Bonmati and Flotats 2003).

Ammonia stripping is a first-order reaction, however, which means that the mass transfer rate from liquid to gas depends on the initial concentration of ammonia. Thus, the ammonia stripping rate is expected to be somewhat lower with low strength leachates than with concentrated leachates (Srinath and Loehr 1974).

Air stripping of ammonia involves passage of large quantities of air over the exposed surface of the leachate, thus causing the partial pressure of the ammonia gas within the water to drive the ammonia from the liquid to the gas phase.

Ammonia is normally present in water as soluble ammonium ion (NH4+). This has to be converted to gaseous ammonia (NH3) molecule for stripping to take place. This conversion is accomplished by raising the pH of wastewater to between 10.8 ̶ 11.5 with NaOH or Ca(OH)2 and subsequently aerating the wastewater, ammonia gas can be stripped from the water to the air, thus removing the ammonia from the wastewater . The following equation (Eq. (1)) defines the relationship between ammonium and ammonia in aqueous solution (Forgie 1988):

NH4+ + OH- ↔ H2O + NH3 (1 )

The equilibrium between ammonium ion (NH4+) concentration and dissolved ammonia gas (NH3) in water depends on pH and temperature as shown in the following Eq. 2.3 calculated at 25 °C.


The quantity of alkali required depends not only on ammonia concentration but also on the carbon dioxide, alkalinity, calcium, magnesium, iron, manganese and organic acid levels (Abbas 2010). This pH correction adds to the cost of treatment. But, it also helps to remove substantial portion of heavy metals and some of the organic load. Therefore, it is beneficial in terms of overall treatment process.

High levels of ammonia nitrogen are usually found in landfill leachates, and stripping can be successful for eliminating this pollutant, which can increase wastewater toxicity. Performances of this process can be evaluated in term of ammonia-nitrogen removal efficiency. The treatment of stabilized leachate from the Thessaloniki landfill (Greece) using ammonium stripping for 24 h was investigated (Diamadopoulos 1994). With an initial NH3-N concentration of 2215 mg/L, about 95% NH3-N was removed at pH 11.5. The NH3-N removal was found to improve with an increasing flow rate, as a result of a greater interaction between the liquid and the air phases.

Another study on the treatment of leachate from the Junk Bay landfill (Hong Kong) using ammonium stripping was carried out in laboratory scale (Cheung et al. 1997). About 10 g/L lime was used to adjust the pH of leachate to 11. After pH adjustment, approximate 90% NH3-N with an initial concentration of 500 mg/L was removed after ammonium stripping for 12 h . This can be due to the fact that at pH 11, most NH3-N was in the form of NH3 gas, thus resulting in a higher removal of NH3-N. Under the same conditions, 47% COD removal was achieved. The results suggest that ammonium stripping was more effective for the removal of NH3-N than for organic compounds removal.

The treatment of young leachate from the Mustankorkea landfill (Finland) was explored by separately employing ammonium stripping and nanofiltration (NF) (Marttinen et al. 2002). At pH 11, ammonium stripping with 24 h of retention time was able to remove 89% NH3-N and 21% COD with initial concentrations of 220 and 920 mg/L, respectively. However, only 50% NH3-N and 66% COD removal were achieved by nanofiltration alone at the same concentrations. The results of ammonium stripping treatment were in agreement with those obtained in another study undertaken by Ozturk et al. (Ozturk et al. 2003) in the Oyaderi landfill (Turkey) using anaerobically pre-treated leachate, where 85% NH3-N with an initial concentration of 1025 mg/L was removed by the stripping process alone.

A laboratory-scale study of the treatment of young leachate from the Komurcuoda landfill (Istanbul) by using ammonium stripping for 12 h was carried out (Calli et al. 2005). About 94% NH3-N removal with an initial concentration of 3260 mg/L was achieved by adding 11 g/L of lime. However, under the same conditions, with air stripping, the COD removal was always lower than 15%. This suggests that ammonium stripping treatment alone was not effective for the removal of non-biodegradable compounds from young leachate.

As a whole, ammonium stripping gives a NH3-N treatment performance in the range of 85-95% with concentrations ranging from 220 to 3260 mg/L with duration of (12 ̶ 24) h. The reduction in COD, however, is relatively low of less than 47% with its concentration ranging from 500 to 47,800 mg/L. Prior to treatment, pH of leachate can be easily adjusted to basic conditions (pH 11-12) to improve the removal of NH3-N by stripping process.

Another advantage of this is that it is possible to meet the NH3-N discharge standard using ammonium stripping (Bae et al. 1999). In terms of operational cost, ammonium stripping was found to be more economically appealing than other treatments such as reverse osmosis or nanofiltration.

In spite of its advantages, the major drawbacks of ammonium stripping are the environmental impact due to the release of NH3 gas into the atmosphere. Therefore, there is a need for further treatment of the gas with HCl or with H2SO4, thus increasing the operational cost of waste treatment due to chemicals. The other limitations of this technique are the CaCO3 scaling of the stripping tower when lime is employed for pH adjustment, the need for pH adjustment of the treated effluent prior to discharge and the difficulty in removing ammonia with concentrations of less than 100 mg/L (Li and Zhao 1999; Tanaka and Matsumura 2002; Li et al. 1999).

iii) Chemical precipitation

In the precipitation process suitable chemicals are added to leachate to precipitate soluble contaminants as insoluble (particle) compounds which can be separated by sedimentation or filtration. The unit operations typically required in this technology include neutralization, precipitation, coagulation / flocculation, solids / liquid separation and dewatering. The effectiveness of a chemical precipitation process is dependent on several factors, including the type and concentration of soluble contaminants present in solution, the precipitant used, the reaction conditions (especially the pH of the solution), and the presence of other constituents that may inhibit the precipitation reaction (Abbas 2010). Chemical precipitation is widely used as leachate pretreatment in order to remove high strength of NH3-N.

Li and Zhao (Li and Zhao 2001) precipitated ammonium ions as Magnesium Ammonium Phosphate (MAP) with the addition of MgCl2.6H2O and Na2HPO4.12H2O with a Mg/NH4/PO4 ratio of 1:1:1 at a pH of 8.5-9. Ammonium concentration was reduced from 5600 to 110 mg/L within 15 min by this method. Yangin et al. (Yangin et al. 2002) and Altinbas et al. (Altinbas et al. 2002) studied MAP precipitation after anaerobic pre-treatment of domestic wastewater and landfill leachate mixture. Maximum ammonia lowering was obtained as 66% at a pH of 9.3 at the stochiometric ratio whereas ammonia lowering reached to 86% at the same pH above the stochiometric ratio. In MAP precipitation at the stochiometric ratio and above the stochiometric ratio, ammonia concentration, in the UASB reactor, was reduced to 31 mg/L and 13 mg/L, respectively. struvite precipitation (Mg: NH4: PO4 = 1:1:1) was applied to anaerobically pretreated effluents for ammonia removal. Ammonium nitrogen depletion was observed as 85, 72 and 20% at pH of 9.2, 12 and 10-11, respectively.

Zhang et al. (Zhang et al. 2009) investigated optimum pH, optimum molar ratio, and different kinds of chemicals combinations for magnesium ammonium phosphate precipitation. The results indicated that ammonium in landfill leachate could be removed with the optimum pH of 9.5. The Mg2+:NH4+:PO43− molar ratio was practically controlled at 1.15:1:1 to remove ammonium effectively and avoid higher concentration of PO43− in the effluent. Highest salt concentration was generated by using MgCl2·6H2O plus Na2HPO4·12H2O. Compare to MgCl2·6H2O and Na2HPO4·12H2O, adding MgO and 85% H3PO4 could significantly minimize the salt concentration, although ammonium removal ratio was 9 percents lower. The lowest ammonium removal ratio was generated by adding Ca(H2PO4)2·H2O and MgSO4·7H2O.

iv) Chemical oxidation

Chemical oxidation processes are potential treatment options for the removal of specific organic and inorganic pollutants from landfill leachates, but are unlikely to provide full treatment of the wide range of contaminants present in typical samples ?.

Chemical oxidation converts molecular structure of hazardous contaminants to non-hazardous or less toxic compounds that are: more stable, less mobile, and/or inert. The oxidizing agents most commonly used are ozone, hydrogen peroxide, hypochlorite, chlorine, chlorine dioxide and UV-radiation. These oxidants have been able to cause the rapid and complete chemical destruction of many toxic organic chemicals; other organics are amenable to partial degradation as an aid to subsequent bioremediation (Yu 2007 ).

Chemical oxidation is required for the treatment of wastewater containing soluble organic non-biodegradable and/or toxic substance (Marco et al. 1997). Commonly used oxidants such as chlorine, ozone, potassium permanganate and calcium hydrochloride for landfill leachate treatment resulted in COD removal of around 20-50%. The most processes based on direct reaction of oxidant (O3-selective) with contaminates or via generated hydroxyl radicals (•OH) (Amokrane et al. 1997).

Compared to biological oxidation, treatment with chemical oxidation/reduction methods is costly, because of the oxidizing/reducing agent's demand of the target organic chemicals and the unproductive agents' consumption of the formation. Therefore, chemical oxidation/reduction is not a realistic method for complete treatment for leachate, but could be an alternative in combination with other treatment methods.

Among the treatment process of chemical oxidation advanced oxidation processes (AOPs) are frequently used to oxidize complex organic constituents found in wastewaters, which are difficult to be degraded biologically into simpler end products (Metcalf and Eddy 2003). AOPs are defined as the oxidation processes in which the hydroxyl radicals (OH-) are derived in sufficient quantity to effect wastewater treatment (Anotai et al. 2010) .

The main purpose of AOP is to enhance chemical oxidation efficiency by increasing generation of hydroxyl radicals. Advanced oxidation processes include both of Non-photochemical methods generating hydroxyl radicals without light energy i.e (Ozonation (O3) at elevated pH (> 8.5), Ozone + hydrogen peroxide (O3/H2O2), Ozone + catalyst (O3/catalyst) and Fenton process (H2O2/Fe2+) and Photochemical methods such as ( O3/UV, H2O2/UV, O3/H2O2/UV , Photo-Fenton and Photocatalysis (UV/TiO2) (Wiszniowski et al. 2006). Specifically, common drawback of AOPs is the high demand of electrical energy for devices such as ozonizers, UV lamps, ultrasounds, which results in rather high treatment costs (Abbas et al. 2009; Lopez et al. 2004).

Advanced oxidation processes have been proposed in the recent years as an effective alternative for mineralization of recalcitrant organics in landfill leachate (Wang et al. 2003). However, these techniques in application for the treatment of large-scale effluents are not economically acceptable. A significant decrease of overall leachate treatment cost could be obtained by the combination of AOPs with a biological process.

Fenton treatment (Fe2‏+/H2O2) and different ozone-based Advanced Oxidation Processes (AOPs) (O3, O3/OH ̶ and O3/H2O2) were evaluated as pre-treatment of a mature landfill leachate by Cortez et al. (Cortez et al. 2011), in order to improve the biodegradability of its recalcitrant organic matter for subsequent biological treatment. With a two-fold diluted leachate, at optimized experimental conditions (initial pH 3, H2O2 to Fe2‏+‏ molar ratio of 3, Fe2‏+‏ dosage of 4 mmol L ̶ 1, and reaction time of 40 min) Fenton treatment removed about 46% of chemical oxygen demand (COD) and increased the ratio of BOD5/COD from 0.01 to 0.15. The highest removal efficiency and biodegradability was achieved by ozone at higher pH values, solely or combined with H2O2. These results confirm the enhanced production of hydroxyl radical under such conditions. After the application for 60 min of ozone at 5.6 g O3 h ̶ 1, initial pH 7, and 400 mg L ̶ 1 of hydrogen peroxide, COD removal efficiency was 72% and BOD5/COD increased from 0.01 to 0.24.

v) Adsorption

The adsorption process refers to a substance adhering to the surface of a solid. Adsorption is one of the physico-chemical processes using either activated carbon or other adsorbents such as zeolite, activated alumina or low cost adsorbents such as limestone, rice husk ash and peat (Halim et al. 2010).

Granular (GAC) or powdered activated carbon (PAC) is a good adsorbent and is frequently used for wastewater treatment. It can be made from several materials, of which, the most popular are coal, wood, and coconut shells due to the large size of their surfaces and the extent to which they are porous. The bigger the pores, the longer the activated carbon functions at a time (Abbas 2010).

The adsorption process is used as a stage of integrated chemical-physical-biological process for landfill leachate treatment (Geenens et al. 2001), or simultaneously with a biological process (Kargi and Yunus Pamukoglu 2003). Carbon adsorption permits 50-70% removal of both COD and ammonia nitrogen (Amokrane et al. 1997). Consequently, activated carbon adsorption aim is to (i) ensure final polishing level by removing toxic heavy metals or organics i.e. AOXs, PCB, etc. (ii) support microorganisms. The main disadvantage of adsorption is the requirement for repeated renewal of columns or high utilization of activated carbon.

Rodriguez et al. (Rodriguez et al. 2004) studied PAC and different resins efficiency in the reduction of non-biodegradable organic matter from landfill leachate. Activated carbon presented the highest adsorption capacities with 85% COD decrease and a residual COD of 200 mg L−1. In Malaysia, a comparative study for the removal of ammonium nitrogen has been undertaken by Aziz et al. (Aziz et al. 2004a) using granular activated carbons and limestones in the Burung Island landfill. Approximately 40% of ammonium nitrogen with an initial concentration of 1909 mg/L was eliminated with 42 g/L of GAC while 19% removal was achieved using 56 g/L of limestone under the same concentration. Halim et al. (Halim et al. 2010) conducted study to investigate the adsorption properties of NH3-N and COD in semi-aerobic leachate from the Pulau Burung landfill site on zeolite, activated carbon and a new composite media in terms of adsorption isotherm and kinetic. A comparison study indicated that the adsorption capacity of composite adsorbent towards NH3-N was higher than zeolite and activated carbon and comparable to activated carbon for COD.

4.1.1 Filtration ye 2007 sweden

Filtration is a physical process whereby suspended solids are removed from leachate by forcing the fluid through a porous medium. The most common and conventional one is a soil filter. After that, the aritificial soil filtration bed and sand filter are developed and applied in wastewater and leachate treatment. Soil filters are permeable upland areas that soak up and cleanse runoff as it travels through the soil toward groundwater. The soil acts as a filter by removing sediment and other pollutants (Yu 2007 ).

Sand filters have proven effective in removing several pollutants from waste leachate. There are two main sand filter designs currently in common use: the conventional sand filter and continuous up-flow sand filter (Pipeline et al. 1997).

The filter used in the filtration process can be compared to a sieve or microstrainer that traps suspended material between the grains of filter media. However, since most suspended particles can easily pass through the spaces between the grains of the filter media, straining is the least important process in filtration. Filtration primarily depends on a combination of complex physical and chemical mechanisms, the most important being adsorption. Adsorption is the process of particles sticking onto the surface of the individual filter grains or onto the previously deposited materials. The forces that attract and hold the particles to the grains are the same as those that work in coagulation and flocculation. In fact, some coagulation and flocculation may occur in the filter bed, especially if coagulation and flocculation of the water before filtration was not properly controlled. Complexation is the combination of metal ions with non-metallic ligands by covalent bounds. The humic-like substances formed from wastewater decomposition can serve as ligands for metal complexes . Precipitation occurs when a metal species falls out of solution as a solid. (Metcalf and Eddy 2003).

The criteria for selection of a filter material are also related to the purpose of treatment, but usually include the following consedrations: material availability, cost, Physical characteristics; pH, porosity, Chemical composition, and Sorption capacity (Renman 2008).

Filtration is the process of passing a liquid through a porous medium, for example sand, either in natural formation or filter constructions with the expectation that the effluent will have a better quality than the influent. High concentrations of dissolved organic matter can result in increased sorption but aqueous complexation with metal ions can also result in a decreased sorption (Jonsson et al. 2006). Organic matter is responsible for different kinds of clogging that can occur in filter constructions.

Many recent investigations have shown that the removal efficiency of particular contaminants can be enhanced if a filter medium of high sorption capacity is used in treatment systems such as constructed wetlands for leachate treatment (Maehlum 1998). Besides the filter construction, the most important part is the selection of material or sorbent (Brix et al. 2001). The sorbent is 'reactive' for one or several contaminants that have to be removed from the wastewater. The term sorbent refers not only to adsorption, but also to processes such as precipitation, ion exchange, complexation and mechanical filtration (McKay 1995). Sorption depends heavily on conditions such as pH, concentration of pollutants, ligand concentration, competing ions and particle size.

Filtration is useful as a pretreatment step for adsorption processes, membrane separation processes and ion exchange processes, which are rapidly plugged or fouled by high loadings of suspended solids. Filtration may also be used as a polishing step after precipitation/flocculation or biological processes for removal of residual suspended solids in the clarifier effluent. In these applications, filtration should be preceded by gravity sedimentation of suspended solids to minimize premature plugging and backwashing requirements. Filtration are well developed processes currently being used in a wide varity of application and is judged to be a good candidate for leachate treatment.

In 1989, this research work was initiated to investigate the use of local peat for the treatment of leachate from a small rural landfill site. In 1997, following the award of grant-aid under the EU LIFE Programme, a full-scale leachate treatment plant was constructed, using local un-drained peat as the treatment medium. When the LIFE Project ended in February 2001, leachate treatment research continued at the site using a pre-treated peat as the treatment medium. The treatment levels achieved using both types of peat are discussed in this paper. It is concluded that landfill leachate may be successfully treated using a low-cost peat bed to achieve almost 100% removal of both BOD and ammonia.

(Heavey 2003) investigated the use of local peat for the treatment of leachate from a small rural landfill site. A full-scale leachate treatment plant was constructed, using local un-drained peat as the treatment medium. His conclusion was that landfill leachate may be successfully treated using a low-cost peat bed to achieve almost 100% removal of both BOD and ammonia. Kietlinska and Renman conducted a laboratory bench-scale column to evaluate permeable reactive filter materials as a new method for removal of heavy metals and inorganic nitrogen from landfill leachate. Mixtures of sand and peat, blast furnace slag (BFS) and peat, and Polonite and peat were tested by loading columns with leachate collected from a pond at Tvetaverket Landfill, Sweden. Sand, peat and Polonite represent natural materials. BFS is a by-product from steel-works. The metal treatment efficiencies of the media were assessed and Polonite was found to perform best, where Mn, Fe, Zn and Cu concentrations were removed by 99%, 93%, 86% and 67%, respectively. This material was also able to reduce inorganic N by 18%. The BFS showed good removal efficiency for Cu (66%), Zn (62%), Ni (19%) and Mo (16%). The sand-peat mixture did not demonstrate a promising removal capacity for any of the elements studied with the exception of Cu (25%) (Kietlinska and Renman 2005). Lind and Nordh investigated the possibility of decreasing the ammonium concentration in leachate by filtration through ash and at the same time wash out salts from the ash with acceptable process water as a result. The idea was to filtrate the leachate through the ash and by the reaction between ammonium ions in the leachate and hydroxide ions in the ash release ammonium as ammonia gas. He concluded that when leachate is filtrated through ash, ammonium is found to be decreased only in the beginning. The ammonium concentration also seems to decrease more when the leachate is mixed with the ash rather than filtrated through the ash. This may be an effect of the longer contact time between the ash and the leachate and can also be the result of that the leachate comes in contact with larger area of the ash (Lind et al. 2004).

Aziz et al. investigated the suitability of limestone to attenuate total iron (Fe) from semi aerobic leachate at Pulau Burung Landfill Site in Penang, Malaysia through a batch process or by filtration technique. The limestone media used in the experiment contain more than 90% CaCO3 with particle sizes ranging from 2 to 4 mm. Initial results indicated that 90% of Fe can be removed from the leachate based on retention time of 57.8 min and surface loading of 12.2 m3/m2 day (Aziz et al. 2004b).