1. Literature Review
1.5 Treatment Methods for Landfill Leachate
1.5.1 Biological treatment
Biological treatment processes are simple and economical when compared to the majority of other leachate treatment options. The economic viability of the method is further improved by the inherent ability of an acclimated microbial population to utilize organic carbon and other essential nutrients present in the leachate (Venkataramani and Ahlert, 1984).
The BOD:COD ratio provides a solid indication of the degree of biodegradability of the organic content present in the leachate. A ratio of approximately 0.5 (typical of young leachate) is indicative of proficient biodegradation of the organic content in the leachate whereas the opposite holds true for an older leachate with a ratio <0.5, and in the case of the latter biological treatment of the leachate is not recommended (Cossu et al., 1989). Therefore the nature of the leachate being treated plays a central role in determining the mode of biological leachate treatment i.e. aerobic or anaerobic (Kennedy and Lentz, 2000).
Aerobic treatment
Aerobic bio-stabilization of sanitary landfill leachate has been extensively investigated.
The principle modes that have been assessed include: activated sludge, aeration lagoons, extended-aeration, and oxidation ditch processes (Venkataramani et al., 1984). Other systems of note include: biological filters, rotating biological contactors (RBC) (Knox, 1985; Cossu et al., 1989), semi-continuous fed batch systems (Cecen and Aktas, 20010, continuous-flow systems (Harper et al., 1996; Cecen and Aktas, 2001), and sequencing batch biofilm reactors (White and Schnabel, 1998; Kennedy and Lentz, 2000).
Bull et al. (1983) revealed that aerated processes were capable of treating organic wastes to stringent levels of quality. They concluded that organic and heavy metal constituents of leachate were rapidly removed by aerobic oxidation.
The treatment of high-ammonia landfill leachate has received considerable attention (Knox, 1985; Carley and Mavinic, 1991; Robinson and Luo, 1991; Harper et al., 1996; Cecen and Aktas, 2001). Harper et al. (1996) established that a single-sludge, nitrification- denitrification process was capable of removing significant concentrations of ammonia and total nitrogen from landfill leachate under aerobic solid retention times (SRT) ranging from six to ten days. Carley and Mavinic (1991) arrived at a comparable conclusion and further stated that the addition of an external carbon source to carbon deficient methanogenic leachate was imperative for the occurrence of optimum nitrification-denitrification of high-ammonia landfill leachate. Martiensen and Schops (1997) also tackled the problem of high-ammonia landfill leachate by using a novel aerobic/anoxic fixed film reactor and a activated sludge bioreactor.
Studies have reported biological system failures as a consequence of bio-available phosphorus deficiencies (Palit and Qasim, 1977; Robinson, Barber and Maris, 1982; Scott, 1982) and heavy metal toxicities (Harper et al., 1996). Conversely, Cameron and Koch (1980) reported impressive heavy metal removal efficiencies in aerobic treatment systems.
Activated sludge systems do not function efficiently when treating high-strength leachates (young leachates). Aerobic treatment technologies generally have quicker treatment rates (Bull et al., 1983) but have the added disadvantage of heavy sludge production (Venkataramani et al., 1984; Frigon et al., 1997) coupled to the necessity of sludge disposal and greater operational costs (Bull et al., 1983; Frigon et al. 1997).
Anaerobic treatment
Anaerobic treatment methods offer an impressive alternative to the aerobic options, by virtue of the technology’s immense potential for the production of treated effluents of comparable quality, in addition to associated advantages of low costs, energy production through methane generation (Frigon et al., 1997; Kennedy and Lentz, 2000), production of a solids residue that can be used as a cover material in landfills (Kennedy and Lentz, 2000), and the production of smaller quantities of sludge (Venkataramani et al., 1984; Frigon et al., 1997). However, the inability of the anaerobic system to treat ammonia present in the leachate ranks as a major disadvantage for the system (World Bank Technical Paper, 1989).
Numerous systems have been proposed and evaluated for the anaerobic treatment of landfill leachate. These include: upflow anaerobic sludge blanket (UASB) reactors (Kennedy, Hamoda and Guiot, 1988; Britz et al., 1990; Kennedy and Lentz, 2000), bench-scale anaerobic digesters (Cameron and Koch, 1980; Lin, 1991; Myburg and Britz, 1992; Myburg and Britz, 1993); anaerobic filters (Chian and DeWalle, 1976); and anaerobic lagoons (Cossu et al., 1989). Treatment success rates vary between researchers, with the quality of the treatment achieved often dependant on the character of the leachate being treated (Bull et al., 1983; Lin, 1991; Frigon et al., 1997; Kennedy and Lentz, 2000).
Boyle and Ham (1974) showed a 90 % removal of BOD when the hydraulic retention time in an anaerobic system was greater than ten days with temperatures ranging between 23 °C and 30 °C. Bull et al. (1983) further demonstrated a 95 % BOD removal and 100 % soluble iron removal as a sulphide precipitate. Myburg and Britz (1993) went even further to demonstrate 80 – 95 % COD removal efficiency in a hybrid digester operated at mesophilic
temperatures at a hydraulic retention time of one day. Kennedy and Lentz (2000) published COD removal rates ranging between 71 – 92 % for UASB and sequencing batch reactors with hydraulic retention times ranging between 12 – 24 hours. Anaerobic systems are often sensitive to shock loads and toxic substances (Venkataramani et al., 1984). However, Myburg and Britz (1993) demonstrated the ability of their anaerobic hybrid digester to withstand shock loads within specific limits.
Final discharge of anaerobically treated leachate requires further physico-chemical treatment of the organic- and nitrogenous content (Bull et al., 1983); and sulphide- and chloride content (Kennedy and Lentz, 2000). A combined approach, incorporating biological- and physico-chemical treatment technologies will enable the complete treatment of landfill leachate (Venkataramani and Ahlert, 1984).