Metals are a special problem for treatment plants as lead, cadmium in Milwaukee. Materials not readily metabolized such as PCB's, Kepone can also enter municipal sewage and cause problems. Sewer plants are sensitive to anything that will kill their organisms. The sea is also used for waste. Rats - Landfills need to be sensitive to rat problems.
Garbage is covered each night and in some cases covered with plastic sheeting.
Some items may be restricted from a landfill. PCB's should be incinerated and certainly not in a land fill. Radioactive material needs long-term storage as encased in glass. Siting is a real problem as no one wants a toxic waste dump near their area. Another type is to make mounds separated by barriers.