Although biological processes are an effective way of
treating wastewater and ensuring minimum residual impact
on the aquatic environment, they have the serious drawback
of producing huge amounts of excess sludge: the more
effective the process is in removing contaminants from
the wastewater, the more sludge is produced. Historically,
it was common to see schematics that showed the water
treatment scheme in detail with all of the appropriate unit
processes and an arrow at the end that simply said “sludge
to disposal”.