**3.5 Fluidised bed reactor**

In a fluidised bed reactor, the biomass grows as a biolayer around particles made up of plastic, polymer or sand which are suspended and remain fluidized because of upward movement of water. Some of the advantages are higher treatment capacity, no clogging as in the case of anaerobic filters but the limitation is that sometimes particles aggregate too much with biomass and settles after becoming dense [38]. Tisa et al. [56] found that fluidised bed reactor could remove 80% COD from landfill leachate. The role of the fluidised bed reactor in removing metal ions was explored by Sahinkaya et al. [37] who found that it was able to remove 80 to 99.9% of metals. According to Eldyasti et al. [57], their fluidised bed reactor was capable of achieving COD, nitrogen, and phosphorus removal efficiencies of 85%, 80%, and 70%, respectively at a low carbon-to-nitrogen ratio of 3: 1 and nutrients loading rates of 2.15 kg COD/m3 /d, 0.70 kg N/m3 /d, and 0.014 kg P/m3 /d).

In another study by Sahinkaya et al. [37], treatment of young leachate using a fluidised bed reactor resulted in 80% of COD removal and 60% of sulphate removal.

## **3.6 Leach bed reactor**

This reactor works on an opposite principle to a UASB reactor in that the flow of wastewater is in the opposite direction: downflow direction. However, it shares some similarities with the UASB in terms of the sludge blanket. The difference is that effluent will leach out of the sludge bed and will be re-circulated as influent back into the reactor until maximum treatment is achieved [38]. According to Xu et al. [58] a leach bed reactor is capable of removing up to more than 80% COD. In a recent study by Degueurce et al. [59], a leach bed reactor was able to remove 27% COD from a young leachate; whereas, according to Ko et al. [60], treatment of young leachate via leach bed reactor was able to remove 80% COD.
