Most of the landfills in the world do not have an appropriate leachate treatment system. Although some treatment options are available, treatment alternatives for leachate are very limited because they are not usually designed by considering the leachate characteristics (Frascari et al. 2004; Salem et al. 2008). Hence, it is necessary to develop leachate treatment systems with reduced footprint and effective efficiency.
Leachate treatment is depending on the estimated leachate generation rates, Physical-chemical features of the leachate and variations in leachate characteristics/flow over time, landfill age, environmental impact, technological constraints, regulatory requirements, compatibility with other elements of the landfill design and operation, estimated capital, and operation and maintenance costs of treatment and disposal method with respect to reliability and flexibility (Kiely 2007). However, High ammonia and phosphorus deficiency in young leachate constrain the biological treatment applications such as nitrification-denitrification processes following phosphorus addition (Renou et al. 2008; Xing et al. 2008). Some researchers received nitrification efficiency higher than 95% for leachates containing high ammonia by using some expensive biological methods (Klimiuk and Kulikowska 2006; He and Shen 2006). Therefore, a general consensus among researchers is high nitrogen levels which are still hazardous to receiving waters and needs to be removed prior to discharge (Wichitsathian August 2004). The characteristics of leachate differ from landfill to landfill and over the life span of the same landfill (it becomes less biodegradable with time). It has been realized that biodegradation mechanism depends upon the age and origin of the landfill, the type and operation of the treatment system (Kang and Wang 2006; Bilgili et al. 2006). As a result, a combination of biological and physico-chemical treatment processes is required to achieve complete and efficient leachate treatment over the life span of a landfill (Qasim and Chiang 1994) .
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Several wastewater treatment processes have been generally used to treat landfill leachate. The majority of them is adaptations of wastewater treatment techniques and can be divided in two main categories: biological treatment and physical / chemical treatments. The major biological treatment processes comprises of activated sludge (AS), sequencing batch reactor (SBR), rotating biological contactor (RBC), etc and physical and chemical treatment processes comprises of oxidation, coagulation-flocculation, chemical precipitation, activated carbon absorption and membrane processes. Methods whereby leachates are treated separately in stages can be classified in to biological, physicochemical as well as combine biological and physicochemical methods. A brief description and review of most of the biological and physicochemical treatment processes applied to leachate treatment is provided below.
2.3.1 Biological treatment method:
The most common practice for leachate treatment worldwide is a biological treatment by employing microorganisms to degrade organic and nitrogenous matter from wastewater. It involves adapting an environment for growth of a microorganism that can remove the substances. Biological removals of organic substances are done through anaerobic and aerobic decomposition processes. Both can be realized by using different plant concepts; A combination of aerobic, anaerobic and anoxic processes is the main processes used for biological treatment (Im et al. 2001). Biological treatment of landfill leachate usually results in low treatment efficiencies because of high chemical oxygen demand (COD), high ammonium-N content and also presence of toxic compounds such as heavy metals (Kargi and Pamukoglu 2003). The main advantages of this systems have the advantage of microbial transformation of complex organics and possible adsorption of heavy metals by suitable microbes, anaerobic treatment is efficient in treating young landfill leachate due to its high content of COD and BOD, cost effectiveness, low nutrient requirements, low sludge production and production of a useful methane gas (Al-Harbawi 2008) . While the main disadvantages of this process are the Sludge formed especially during aerobic treatment may become a serious disposal problem, especially when it contains specific organic compounds and toxic heavy metals. Also, biological treatment requires highly skilled labor. The high capital cost, long start up periods, the possible requirement for an additional treatment, and its high sensitivity to variable loads and organic shocks, as well as toxic compounds (particularly heavy metals) (Al-Harbawi 2008).
Biological processes have been shown to be very effective in removing organic and nitrogenous matter from immature (young) leachates (< 2yrs) when the BOD5/COD ratio has a high value (>0.5). With time, the major presence of refractory compounds (mainly humic and fulvic acids) tends to limit process's effectiveness (Renou et al. 2008; Lema et al. 1988) .
Aerobic Biological Treatment
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Microorganisms consume organic materials as their energy sources in the presence of oxygen. The presence of oxygen in large quantities is essential. In addition to degrading organic materials, aerobic biological processes convert ammonium into biomass or oxidize to nitrate. An aerobic treatment should allow a partial abatement of biodegradable organic pollutants and should also achieve the ammonia nitrogen nitrification. Aerobic biological processes based on suspended-growth biomass, such as aerated lagoons, conventional activated sludge processes.
Generally efficient aerobic biological treatment is possible and is very similar to domestic wastewater treatment except for some unique issues. These issues include high ammonium concentration, low BOD/N-ratio, precipitation of inorganics and foaming which could cause clogging of aerators and other operational problems (Ehrig 1984). Also the organic biodegradation could be predicted by BOD5/COD ratio. The lower this ratio, the lower the COD reduction achieved. For further reduction, a physico-chemical treatment technique must be used (Ehrig 1984). Due to excess nitrogen, the biological process may not run reliably producing unacceptable concentrations of ammonia (Fletcher and Ashbee 1994). The pH of old leachate could be in the range of 8 to 8.3. When aerated, the pH could rise to 9 or higher. Under these circumstances the ammonia equilibrium shifts from ammonium to free ammonia affecting the growth of denitrification bacteria through inhibition. During the same time, conversion of ammonium to nitrate reduces pH. Therefore pH control is essential in getting low effluent ammonium levels (Sumanaweera 2004).
The most common aerobic biological treatment methods are (aerated lagoons , activated sludge plants and sequencing batch reactors (SBR), rotating biological contactors (RBC), trickling filter, sequential batch plant, co-treatment with sewage parts of the landfill body used as a reactor, circulating fluidized bed bioreactor (CFBBR) (Li and Zhao 2001; Lin and Chang 2000; E. Neczaj et al. 2005; Uygur and Kargi 2004; Renou et al. 2008; Cortez et al. 2009; Eldyasti et al. 2011).
Aerobic biological processes based on suspended-growth biomass and attached-growth systems. The suspended-growth biomass such as aerated lagoons, conventional activated sludge processes and SBR, have been widely studied and adopted (Li and Zhao 2001; Bae et al. 1999). While attached-growth systems have recently attracted major interest: the rotating biological contractors, the moving bed biofilm reactor (MBBR), and biofilters. The combination of membrane separation technology and aerobic bioreactors, most commonly called membrane bioreactor, has also led to a new focus on leachate treatment (Abbas 2010). Next sections present a brief review of some biological treatment:
i) Aerated Lagoon
A lagoon is normally an artificial pond with aeration (Fig. 2.3a). Aerated lagoons have generally been viewed as an effective and low-cost method for removing pathogens, organic and inorganic matters. Their low operation and maintenance costs have made them a popular choice for wastewater treatment, particularly in developing countries since there is a little need for specialised skills to run the system (Zaloum and Abbott 1997). A full scale large aerated lagoon that had more than ten days of retention time tested by Robinson and Grantham (Robinson and Grantham 1988). Better than 97% removal of COD was obtained, together with excellent removal of ammonia, iron, manganese and zinc. Maehlum (Maehlum 1995) used on-site anaerobic-aerobic lagoons and constructed wetlands for biological treatment of landfill leachate. Overall N, P and Fe removals obtained in this system were above 70% for diluted leachate. Abatement of 55-64% of COD and 80-88% of phenol was achieved. However, as stricter requirements are imposed, lagooning may not be a completely satisfactory treatment option for leachate in spite of its lower costs (Zaloum and Abbott 1997). In particular, authors claimed that the temperature dependence of lagooning is a significant limitation because it mainly affects microbial activity.
ii) Activated sludge processes
Activated sludge facilities have considerably shorter retention times. The reason is that the sludge content (amount of bacteria) can be controlled which is several times higher than in aerated lagoons. This is achieved by installing a settling tank behind the aeration tank and recirculating the sludge back into the activated sludge tank (Fig. 2.3b). A certain amount of sludge has to be removed as excess sludge out of the system. The processes are extensively applied for the treatment of domestic wastewater or for the co-treatment of leachate and sewage. However, this method has been shown in the more recent decades to be inadequate for handling landfill leachate treatment (Lin et al. 2000). Even if processes were proved to be effective for the removal of organic carbon, nutrients and ammonia content, too much disadvantages were listed below tend to focus on others technologies:
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- Inadequate sludge settleability and the need for longer aeration times (Loukidou and Zouboulis 2001),
- High energy demand and excess sludge production (Hoilijoki et al. 2000),
- Microbial inhibition due to high ammonium-nitrogen strength (Lema et al. 1988).
Consequently, only few works are recently available concerning landfill leachate treatment by activated sludge methods. Hoilijoki et al. (Hoilijoki et al. 2000) investigated nitrification of anaerobically pre-treated municipal landfill leachate in lab-scale activated sludge reactor, at different temperatures (5-10°C) and with the addition of plastic carrier material. Aerobic post-treatment produced effluent with 150-500 mg/L of COD, less than 7 mg/L of BOD5 and on an average, less than 13 mg/L of NH3-N. Addition of PAC to activated sludge reactors enhanced nitrification efficiency on biological treatment of landfill leachate.
Besides BOD reduction, the nitrification of ammonium is an important aspect of the treatment in many activated sludge facilities. Nitrogen elimination becomes more and more important with the aging of a landfill, due to the increase in nitrogen levels. (Kheradmand et al. 2010) treated landfill a high-strength leachate using a combined anaerobic and activated sludge system. A two-stage laboratory-scale anaerobic digester under mesophilic conditions and an activated sludge unit were used. He was found that the system could reduce the COD of the leachate by 94% at a loading rate of 2.25 g COD/L/d and 93% at loading rate of 3.37 g COD/L/d, alkalinity and ammonia removal efficiency were 49-60% and 48.6-64.7%, respectively.
C:\Users\riyadh\Desktop\phd\thesis ch\chapters\figs\Aerated lagoon.tif
Fig. 2.3 A generalized diagram of Lagoon and the activated sludge process.
iii) Sequencing batch reactor (SBR)
The SBR technology is a sequential suspended growth (activated sludge) process designed to operate under non-steady state flow conditions and there is a degree of flexibility associated with working in a time rather than in a space sequence (Laitinen et al. 2006). The cycles comprise periods for leachate filling, aeration (react), settling and decanting (draw) (Fig. 2.4b). An SBR are commonly used as a biological treatment for leachate treatment. It is ideally suited to nitrification-denitrification processes since it provides an operation regime compatible with concurrent organic carbon oxidation and nitrification (Diamadopoulos et al. 1997). When biological nutrient removal (BNR) is desired, the steps in the cycle are adjusted to provide anoxic or anaerobic periods within the standard cycles (Mahvi 2008). Some advantages and disadvantages of SBRs are listed below (EPA 1999):
Advantages of SBRs
• Equalization, primary clarification, biological treatment, and secondary clarification can be achieved in a single reactor vessel.
• Operating flexibility and control.
• Minimal footprint.
• Potential capital cost savings by eliminating clarifiers and other equipment.
Disadvantages of SBRs
• A higher level of sophistication is required especially for larger systems, of timing units and controls.
• Higher level of maintenance associated with more sophisticated controls, automated switches, and automated valves.
• Potential of discharging floating or settled sludge during the draw or decant phase with some SBR configurations.
• Potential plugging of aeration devices during selected operating cycles, depending on the aeration system used by the manufacturer.
• Potential requirement for equalization after the SBR, depending on the downstream processes.
Several studies have been conducted to find out the applicability of SBR in leachate treatment. (Doyle et al. 2001) conducted a study of high-rate nitrification in SBR on a mature leachate obtained from a domestic landfill. The leachate possessed high ammonia content with an average concentration 880 mg/L, while the average BOD5 and COD concentrations were 60 and 1,100 mg/L, respectively. The ammonium oxidation rates up to 246 mg N/L.h and specific ammonium oxidation rates of 36 mg N/mg VSS.h were achieved in this study. A complete ammonia oxidation of the leachate could be achieved with a HRT of 5 h.
Kargi and Panukoglu (Kargi and Pamukoglu 2004) have reported that the COD removals up to 75%. Also, 99% NH3-N removal has been observed by Lo (Lo 1996) during the aerobic treatment of domestic leachates in a SBR with a 20-40 days residence time. The greater process flexibility of SBR is particularly important when considering landfill leachate treatment, which have a high degree of variability in quality and quantity (Kennedy and Lentz 2000). Zhou et al. (Zhou et al. 2006) studied the capability of SBR in treating landfill leachate containing high concentration of NH3-N. The study resulted in up to 94, 98, 85 and 99% in COD, BOD, TN, and NH3-N, respectively. This study showed high nitrification and denitrification achievement.
In recent years, some modifications of SBR have been used by researchers to provide secondary, advanced secondary treatment, nitrification, denitrification and biological nutrient removal. An intermittent cycle extended aeration system (ICEAS) is modified version of the SBR. In the ICEAS system, influent wastewater flows into the reactor on a continuous basis. As such, this is not a true batch reactor, as is the conventional SBR. A baffle wall may be used in the ICEAS to buffer this continuous inflow. The design configurations of the ICEAS and the SBR are otherwise very similar (EPA 1999). Another modification was carried out such as continuous flow SBR (Mahvi et al. 2004), sequencing batch biofilm reactor (SBBR) (Tengrui et al. 2007), anaerobic sequencing batch reactor (ASBR) (Timur and Ozturk 1999) and anaerobic-aerobic sequencing batch reactor (Bernet et al. 2000). An anaerobic sequencing batch reactor (ASBR) is similar to aerobic SBR, except that ASBR is not aerated during reaction phase and has a cover to exclude air (Fu et al. 2001).
iv) Membrane bioreactors (MBRs)
Membrane bioreactor systems are an example of an emerging advanced leachate treatment technology. MBRs are essentially composed of two primary parts, the biological unit or a bioreactor responsible for the biodegradation of the waste compounds and the membrane module for the separation of the treated water from bio solids or microorganisms (Cicek 2003). MBRs are categorized into two configurations depending on the location of the membrane unit: submerged (or immersed) MBRs and side stream (or external) MBRs (Fig. 2.4a) (Ahmed and Lan 2012).
A wide range of COD removal efficiencies were reported for landfill leachate treatment using MBRs, from as low as 23% (Korajlija Jakopovic et al. 2008) to as high as above 90% (Chen and Liu 2006). Otherwise a high BOD removals between 90 and 99% were achieved regardless of the leachate age or the operating conditions used (Ahmed and Lan 2012). PuszczaÅ‚o et al. (Puszczao et al. 2010) varied the landfill leachate percentage in the feed over a range of 3-40 vol.% and determined 10 vol.% of lLandfill leachate to be the most favourable composition of the synthetic sewage+ Landfill leachate mixture (corresponding to the highest COD removal of 89% in the MBR).
Sadri et al. (Sadri et al. 2005) reported COD removal efficiency of 54-78% for organic loading rates (OLR) of 0.90-2.74 g COD L−1 day−1. Given that this study used leachate with BOD/COD ratios as low as 0.1 (indicating a biodegradable fraction of only 10%) coupled with HRTs of 1 to 2 days only, the COD removals achieved are remarkable. Also observed full nitrification in approximately 40 days after the start-up of their MBR treating old Landfill leachate from Brady Landfill, Manitoba. Despite large variations in influent NH3-N concentrations (662±176 mg L−1), the performance of the MBR remained constant with no residual NH3-N detected in the effluent. Aloui et al. (Aloui et al. 2009) used MBR to treat old Landfill leachate (BOD/COD 0.2) with high ammonia concentrations (1000-2800 mg L−1). Effluent NH3-N concentrations as low as 100 mg L−1 were observed with HRTs of 2 to 3 days. Around 76% mean COD removal for old landfill leachate at HRT of 2-3 days and OLR of 1.9-2.7 g COD L−1 day−1.
SBR Process Stages
External, b) submerged (Ng and Kim 2007)
Fig. 2.4 Schematic of SBR process and MBR configurations.
v) Rotating Biological Contactor (RBC)
The rotating biological contactor (RBC) is an attached-growth, aerobic biological treatment process consume low amount of energy to supply air. The biological contactor oxidation process is adopted to treat the organic pollutant in the leachate. Even with a low concentration or remarkable load fluctuation of organic pollutant, the stable and effective treatment efficiency could be achieved.
Knox (Knox 1985) showed that trickling filters could be useful in treating old leachate that had low organic materials but relatively high ammonium nitrogen concentration.
Kulikowska (Kulikowska et al. 2010) used a two -stage rotating biological contactor (RBC) to treat landfill leachate. They concluded that the ammonium load was an important parameter effecting the nitrification efficiency and its main products. At an ammonium load of 1.92 g NH4-N/m2.d, a single RBC was sufficient in obtaining complete nitrification, however, at a higher load 3.6 g NH4-N/m2.d, a two -stage system was needed. The increase of ammonium load above 4.79 g NH4-N/m2.d caused a decrease in nitrification efficiency to 70 %. Cortez et al. (Cortez et al. 2011) studied the removal of nitrate from a mature landfill leachate with high nitrate load in a lab-scale anoxic rotating biological contactor (RBC). Under a phosphorus-phosphate concentration of 10 mg P-PO43− L−1 and nitrogen-nitrate concentrations above 530 mg N-NO3− L−1 the reactor achieved nitrogen nitrate removal efficiencies close to 100%, without nitrite or nitrous oxide accumulation. They concluded that the anoxic rotating biological contactor is very effective having a great potential in the denitrification of a mature landfill leachate with high nitrate load, using acetate as additional carbon source. The supplementary addition of phosphorus played a determinant role on nitrate removal. Fig.2.5a depicts a rotating biological contactor.
A trickling filter (TF) is an attached-growth; aerobic biological treatment process in which leachate is continuously distributed over a bed of rocks or plastic medium that supports the growth of micro-organisms. It consists of a fixed bed of highly permeable media on whose surface a mixed population of microorganisms is developed as a slime layer (Fig. 2.5b). This method has been investigated for the biological nitrogen lowering from municipal landfill leachate. Biofilters remain an interesting and attractive option for nitrification due to low-cost filter media (Jokela et al. 2002) .
Trickling filters operate under short hydraulic retention times that do not allow for complete biodegradation of organics; as a result, effluent recirculation is required to increase the net contact time of the leachate with the biomass and achieve high organic removal efficiency (Yu 2007 ).
This type of system is common to a number of technologies such as rotating biological contractors and packed bed reactors (bio-towers). Trickling filters are very efficient at removing BOD5 and ammonia from wastewater, and they use a minimal amount of power (Renou et al. 2008).
Above 90% nitrification of leachate was achieved in laboratory and on-site pilot aerobic crushed brick filters with loading rates between 100 and 130 mg/L/day of NH3-N at 25°C and 50 mg L-1.day NH3-N even at temperatures as low as 5-10°C, respectively (Jokela et al. 2002) .
a) rotating biological contactor
(b) Trickling filter
Fig. 2.5 Schematic drawing for rotating biological contactor and trickling filter .