Strong adsorption enhanced nitrogen removal from landfill leachate by PVA/CMC/WPU pellets immobilized microorganisms

Landfill leachate contains high concentrations of ammonia nitrogen (NH₄⁺-N), which threatens human health and the biosphere. In this study, novel polyvinyl alcohol (PVA)/sodium carboxymethyl cellulose (CMC)/waterborne polyurethane (WPU) composite carriers were prepared to immobilize microorganisms for denitrification from landfill leachate. The raw materials ratios for preparation of the carrier were optimized, and the optimized dosages of PVA, CMC and WPU were 10 wt%, 1.0 wt% and 1.0 wt%, respectively. The adsorption experiments (with a carrier dosage of 1.0 g/L) showed that the adsorption of NH₄⁺-N on PVA/CMC/WPU carriers accords with the Langmuir model with a maximum adsorption capacity of 1013.63 mg/g, and is a thermodynamic spontaneous process. After treatment of landfill leachate with the optimal carrier for 48 h, the NH₄⁺-N and total nitrogen (TN) removal efficiencies reached 97.5 % and 93.3 %, respectively. The strong adsorption of carrier itself and metabolism of the embedded microorganisms synergistically contributed to the superior denitrification effect. After 15 cycles of repeated use, the NH₄⁺-N and TN removal efficiencies maintained over 90 % and 85 %, respectively, demonstrating the good use stability of the carrier. The 16S rRNA sequencing results showed that after a long-term treatment of landfill leachate, a large number of denitrifying bacteria and heterotrophic nitrifying-aerobic denitrifying bacteria were present in the carrier, which are probably responsible for the superior nitrogen removal effect of the carrier. The novel pore structure (dense outside and loose inside) of carrier provides abundant attachment sites for microorganisms, and allows the microorganisms in wastewater entering into the carrier. A localized anoxic zone was generated inside the carrier, consequently facilitating the synchronous nitrification and denitrification of microorganisms inside the carrier. The obtained carrier shows great potential for application on denitrification of industrial wastewater with high NH₄⁺-N concentration.