**3. Anaerobic technologies treating landfill leachate**

Anaerobic technologies are widely utilized for the treatment of wastewater, more precisely for the treatment of landfill leachate as they have following merits over aerobic technologies; Remarkably less sludge production, energy production in the form of methane, and efficient removal of pollutants [32]. Some of the treatment technologies/reactors are mentioned in **Table 2**.


**Table 2.**

*Some of the anaerobic treatments of leachate.*

#### **3.1 Anaerobic contact reactor**

Anaerobic contact reactors are widely used for anaerobic treatment process. ACR consists of a main reactor and a sedimentation tank from where the settled sludge is brought back into the parent reactor. The ACR reaches steady state due to proper mixing and can even work for short HRTs getting higher removal efficiencies. The drawback usually encountered is the gas formation in the settling tank which causes reactor upset [38]. The drawback of this reactor is the development of gas in the settling tank, which upsets the solid settlement process. Şentürk et al. [39] studied an anaerobic contact reactor treating potato-chips wastewaters (COD = 5500 mg/l, OLR = 0.6 to 8 kg COD/m3 /d). The performance of ACR was evaluated based on COD removal, VFA production and the composition of biogas. The removal of COD was 86–97% and the methane content of the biogas production was about 68–89% accounting an yield of 0.42 m3 CH4/kg COD removed. El-Gohary and Kamel [33] recently found that an anaerobic contact reactor was able to remove 37.5 and 40.5% COD and BOD, respectively, from young leachate.

#### **3.2 Anaerobic membrane bioreactor**

The membrane bioreactor works on the application external membrane filter before/after the anaerobic reactor. This helps to capture the solids preventing the solids washout and getting them returned to the reactor sludge. Membrane bioreactor (MBR) technology became viable and popular as compared to activated sludge systems because of the following additional merits; MLSS concentration is high, low cost of treatment, less sludge production and quality of effluent is high [12]. The limitation of the system is the high probability of organic fouling in the membrane. Bohdziewicz and Kwarciak [40] found that using an anaerobic membrane bioreactor as much as 90% COD removal was possible for landfill leachate treatment. In another study by Zayen et al. [41], 90% COD removal was obtained using this type of reactor. In a separate study, an anaerobic membrane bioreactor achieved 26% COD removal at a low HRT of 0.4 days during the anaerobic treatment of leachate [34]. However, Trzcinski and Stuckey [42] demonstrated that the same reactor achieved 60% COD removal during the treatment of young leachate. Nuansawan et al. [43] found that treatment of young leachate using an anaerobic membrane bioreactor attained 81 and 92.1% removal of COD and BOD, respectively.

#### **3.3 Up-flow anaerobic sludge blanket (UASB)**

In an UASB reactor the sludge blanket provided at the bottom of the reactor serves the purpose of a filter and medium helping the anaerobic microbes to grow and utilize the organic matter. Influent wastewater is introduced by an inlet at the bottom and goes in an up-flow manner with the help of a pump. When the wastewater passes the anaerobic sludge blanket it is being treated by the microorganisms. This is the principle which governs the mechanism of UASB globally. Singh and Mittal [44] found that treatment of old leachate by UASB was only able to remove 35% of COD. Abood et al. [45] studied leachate treatment by UASB and found that the treatment could achieve COD, NH3-N, and BOD5 removal percentages of 69.27%, 92.18% and 23.81%, respectively. In a separate study by Tauseef et al. [46] found that leachate treatment by UASB was able to remove 80% COD and produce 70% methane. Montalvo et al. [47] found that treatment of leachate via UASB was capable of removing 92.4% nitrate, whereas a study conducted by Liu et al. [48] reported that leachate treatment by UASB could achieve removal of NH3-N, TN and COD as high as 99.3%, 85.4% and 90.3%, respectively. In support of this, Moharram et al. [49] also found out that UASB could achieve 50 to 75% of COD removal. Lu et al. [50] stated that UASB could achieve COD removal rates between 77% and 91%. Alvarino et al. [51] stated that they could achieve 96.7% COD removal via UASB. Intanoo et al. [52] discovered that by using UASB, up to 60% COD removal could be attained, while according to Wu et al. [16] leachate treatment via UASB could achieve COD removal of 95%. Lu et al. [53] found that leachate treatment by UASB could attain COD removal rates of 93%.

#### **3.4 Anaerobic filters**

An anaerobic filter consists of a filter media usually made up of packed material (non-degradable polymer) having high surface area to volume ratio. These filters facilitate microorganisms to get developed as a biofilm and forming an anaerobic channel mat. The problem in such type of reactors arises when the wastewater is rich in solids causing clogging. Wang et al. [54] revealed that by applying an anaerobic filter in leachate treatment more than 90% COD removal could be accomplished. A recent study by Zayen et al. [17] reported 40% COD removal from young leachate.

Nanayakkara et al. [55] studied the treatment of 10% diluted landfill leachate using downflow anaerobic filters. One of the columns was filled with a mixture of Washed Sea Sand (WSS), Dewatered Alum Sludge (DAS) and Firewood Charcoal (FWC) while in the other the same materials were used but in layers. The parameters studied and their removal efficiencies using both columns are given below.

