This section illustrates that MBT systems can be described as two simple concepts: either to separate the waste and then treat; or to treat the waste and then separate. In some systems only biological treatment is required to treat all the residual MSW before disposal to landfill. Whilst a variety of treatment and mechanical separation options are offered, these need to be optimised in terms of the outputs in order to find outlets for the various materials / fuels derived from the process (see Markets for the Outputs section). .