State of the art in anaerobic treatment of landfill leachate: a review on integrated system, additive substances, and machine learning application by Zamrisham, Nur Ain Fitriah, Abdul Wahab, Abdul Malek, Zainal, Afifi, Karadag, Dogan, Bhutada, Dinesh, Suhartini, Sri, Musa, Mohamed Ali, Idrus, Syazwani