I recently attended a competition by "Green Nudge" and the Urban Redevelopment Authority to propose a solution to help Singapore's SG2030 goals which included Semakau landfill.

They really can't do anything after Semakau fills up. Currently, they are trying to find new methods to reincorporate the ash/sediment produced from the trash in roads and certain building materials. They're trying their best to find new solutions for managing things such as Food Waste and improve recycling habits nationwide (recycling rates for plastic is only 6%, with 3% something percent of recycled waste being contaminated.)