Aerobic decomposition of pollutants in leachate is based on processes with suspended growth microorganisms-activated sludge and/or attached growth microorganisms-with different types of fixed film biological reactors.
Activated sludge is one of the basic methods applied for municipal landfill leachate treatment. This process is used for biochemical oxidation of biodegradable organic compounds, but mainly for the biological removal of nitrogen [20,42-46].
Efficiency of organic compounds removal by means of activated sludge varies strongly and depends on leachate composition to a major extent, mainly on leachate BOD but also on applied process parameters. Klimiuk and Kulikowska  reached 83 and 71% of COD removal in leachate from young landfill in activated sludge SBR at a hydraulic retention time of 12 and 2 days, respectively, and a sludge age of 58 and 19 days, respectively. Similar results were also achieved by Bae and colleagues  in a three-stage system consisting of denitrifying filter and two steps of activated sludge with total hydraulic retention time of 14 days. In both abovementioned cases, the BOD5/COD ratio in raw leachate was unknown.
In leachate treatment using biofilms, the leachate composition and carrier type are both important in affecting achieved results. For leachate with a BOD5/COD ratio of 0.6 applied in biofilters, special open-cell plastic foam characterized by large surface, porosity, and absorption capacity, as well as easy access of leachate into inside parts of the carrier, gave the following treatment results:
• total nitrogen (all organic and inorganic forms), 50% .
Effects of nitrification at leachate treatment usually reached over 90% of ammonia nitrogen oxidation and allowed for the achievement of ammonia nitrogen concentrations in effluents at the level of 10 mg N/L and even [20,42,45,48]. However, these results were reached in different systems of activated sludge, starting with a one-stage system with several phases up to complex multistage systems, where different hydraulic retention times, sludge ages, organic loading rates, and nitrogen loading rates in activated sludge are applied. Multistage biological systems were among the first plants used for sanitary landfill leachate treatment. For example, Figure 6 shows the first leachate treatment plant in Germany, which consisted of four stages of activated sludge .
The different composition of leachate is the reason why many systems are used for its treatment. For instance, different ammonia nitrogen concentrations exist in leachate from different landfills. They may differ even by several hundreds of milligrams per litre. Differences also exist in quantities of refractory organic compounds and toxic substances. In the case of high concentrations of ammonia nitrogen accompanied by large amounts of refractory and toxic substances, activated sludge is not sufficient and other solutions are required, such as extra application of granular activated carbon in the biological reactor. Horan and colleagues  characterized leachate by average concentrations of COD, 2450 mg/L; ammonia nitrogen, 744 mg/L; and low BOD5/COD ratio, 0.08, and applied granular activated carbon to 125 g/L of
mixed liquor. They obtained 90% of ammonia nitrogen oxidation, total BOD removal and 60% of COD removal. For leachate with higher concentrations of COD and ammonia nitrogen, in amounts of 5000 mg/L and 1800 mg/L, respectively, and low BOD5/COD ratio (0.2), Loukidou and Zouboulis  applied 90 g/L granular activated carbon and received removal level of 90% of BOD, 81% of COD, and 85% of ammonia nitrogen. These results are comparable. In both cases, the function of granular activated carbon not only supported attached growth of bacteria but also the absorption of refractory organic compounds and toxic substances. The carbon allowed the bacteria to oxidize ammonia nitrogen and biodegradable substances. As a disadvantage of this process, Loukidou and Zouboulis  pointed to the large amount of residual suspended solids requiring separation and treatment as well as increased operational costs due to the addition of activated carbon.
In addition to high concentrations and toxicity of leachate compounds, there are problems with weak sedimentation properties of activated sludge, which leads to microorganisms washout from installations, and high sensitivity of microorganisms to low temperatures . A decrease of temperature from 20 to 10°C within the activated sludge process can lower the nitrification rate by as much as 50% . The influence of temperature on microorganisms of activated sludge was also examined by Ilies and Mavinic . They controlled the activated sludge reaction at a temperature decrease from 17 to 10°C in the Bardenpho system, which consisted of two aerobic and two anoxic reactors. Leachate in this treatment process originated from the landfill in the methanogenic phase and was characterized by a low BOD value and ammonia nitrogen concentrations of up to 2200 mg N/L. The experiments showed that the temperature decrease from 17 to 14°C had no influence on the intrification process. The drop of nitrification efficiency between 10 and 30% started only after the temperature decreased to 10°C. The unexpected denitrification process appeared to be much more vulnerable to the temperature decrease. Its inhibition started at 14°C and at 10°C denitrification efficiency was lower than 5% of its potential. Figure 7 illustrates the influence of temperature on the activated sludge process.
Increased efficiency of leachate treatment by means of biological processes is achieved by biofilms application. An attached biomass on different moving and fixed carriers is often used
for leachate treatment [3,20,44,47,50,53-56]. Biofilm reactors are widely used because microorganisms change their properties when attached to biofllm. They are more resistant to changes of environmental parameters and toxic substances than microorganisms of activated sludge and are not washed out from reactors to as high a degree as activated sludge. The carrier application for microorganisms attachment is widely used for high concentration removal of ammonia nitrogen by means of biological nitrification. The bacteria responsible for this process (nitrifiers) are recognized as vulnerable to temperature and pH changes as well as the presence of toxic substances; moreover, they grow very slowly . Benefits from attached-growth nitrifying bacteria application are well illustrated by their reaction to a decrease in temperature. A temperature decrease from 20 to 5°C resulted in a nitrification rate drop from 6.2 g NH4+N/m3hour to 4.8 gNH4+-N/m3hour, or about 22.6%. In the case of activated sludge nitrification, a smaller temperature decrease (from 20 to 10°C) resulted in a 50% nitrification efficiency decrease .
System with attached-growth microorganisms have also some disadvantages. The cost of the carrier, which can comprise a significant part of total installation costs, is one. Another is the need to maintain high concentration of dissolved oxygen in reactors with a moving bed in order to assure maximal nitrification rate .
Almost all types of filters are applied for landfill leachate treatment-moving-bed filters [3,44,50,54], rotating biological contactors [20,48,53,55], and trickling filters . Extremely favorable results of organic compounds and nitrogen removal are achieved on rotating biological contactors. The thick biofilm formed on contactor discs facilitates quick oxidation processes (Fig. 8).
Figures 9 and 10 present an example of results achieved for a rotating biological contactor treating leachate characterized by a BOD5/COD ratio equal 0.6, and COD and ammonia nitrogen average concentrations amounting to 1638 mg/L and 404 mg/L, respectively. The contactor treated leachate at organic and nitrogen loading rates of up to 27 g COD/m2 day and
Figure 9 Chemical oxygen demand (COD) removal from leachate for a rotating biological contactor (from Ref. 20).
5.64 g NH4+-N/m2day. Organic compounds and ammonia nitrogen were oxidized very quickly in the first compartment of the contactor. In the next two compartments, only nitrites oxidation to nitrates was observed .
A long retention time of biomass in rotating biological contactors and significant thickness of biofilm facilitate nitrogen removal in addition to classic nitrification (oxidation of ammonia nitrogen first to nitrites and then to nitrates) in a deaminification process. This phenomenon is observed in treatment of leachate without biodegradable organic matter. The efficiency of this process has reached up to 70% of total nitrogen removal [48,55].
In general, removal of nitrogen from landfill leachate requires an external source of organic carbon. This occurs in a given biological treatment system because the ratio of biodegradable organic carbon to nitrogen in leachate is too low. The lack of an external source of organic carbon is responsible for the presence of oxidized forms of nitrogen up to several hundreds of milligrams per liter in effluents from leachate treatment plants. The problem with denitrification because of insufficient amounts of organic carbon is observed even then, when nitrification concludes with nitrites as the main products and by requirement for organic carbon is limited [42,45]. Therefore, research on low-cost denitrification systems is carried out without the costs for external sources of organic carbon. Jokela and colleagues  carried out investigations on denitrification processes with landfill solid wastes as a source of organic carbon. They hypothesized that for small landfill technology, leachate would be introduced again after nitrification into the landfill body where oxidized nitrogen would be subjected to denitrification with organic carbon originating from solid wastes. Contrary to expectations, the experiments carried out on a laboratory scale with rates up to 3.8 g NOx-N/t TSwaste-day showed a lack of denitrification influence on methanogenesis in a landfill. The authors attributed this to a much lower oxidized nitrogen loading rate used in their experiments in comparison with loading rates applied in experiments where a negative influence on methanogenesis was observed. They pointed to the need for further research to verify the denitrification sufficiency of a landfill body for long-term operation .
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