**5. Landfill sustainability**

Over time, preventing, recycling, or re-using wastes have not been found to take away the need for a landfill in any waste management system as not all wastes can be recycled or recovered under all circumstances [32]. Oftentimes, the quantity of wastes to be disposed far exceeds the capacity for recovery, recycling, or incineration making the need for landfills even more inevitable, as they (landfills) have been used to dispose large quantity of wastes [32]. All of these indicate that in a typical recycling and recovery waste management practice, landfilling is a "go-to" method of waste management [32]. In landfill designs, impermeable liners are the standard for preventing environmental pollution [32]. Liners may stay intact for a long period, up to 50 or even 100 years but eventually, they will fail; this necessitates the need for landfill aftercare, which may not be sustainable [32]. Landfills should, therefore, be managed and operated so that future generations do not have to worry about their adverse effect on the environment; they should be managed in a sustainable way.

A sustainable landfill is one which attains stable conditions within a short period of time and reaches a state where the undisturbed contents no longer pose environmental risk to human health and the environment [32]. At this stage, the landfill is said to be in a state of completion and aftercare can be discontinued [32]. In sustainable landfills, waste materials are safely absorbed into the surrounding environment, whether or not they have been treated and landfill gases (LFGs) are controlled so as to minimize environmental impact [32]. In order to achieve landfill completion, waste pretreatment or bioreactor operations can be carried out in the landfill facility. Common chemicals used for pretreatment include aluminum sulfate, ferrous sulfate, ferric chloride, and ferric chlorosulfate [3]. Bioreactor operations are employed for waste treatment in a landfill [33]. A landfill bioreactor facilitates the degradation of waste by microbial activity, which may be achieved by controlling parameters such as moisture, oxygen, nutrients, pH, and temperature [33]. Since water limits microbial activity in a landfill, leachate recirculation can be used to create landfill bioreactor [33]. Recirculation increases the moisture content of the waste in the landfill thereby promoting waste degradation. Landfill sustainability can be measured (**Figure 13**) in terms of environmental, economic, and social impacts of the landfill on host community [33].
