Other mechanical and biological techniques

In addition to the many techniques used in large-scale water treatment, several small-scale, less (or non)-polluting techniques are also being used to treat polluted water. These techniques include those based on mechanical and biological processes. An overview:

• mechanical systems: sand filtration, lava filter systems and systems based on UV-radiation)
• biological systems:
  • plant systems as constructed wetlands and treatment ponds (sometimes incorrectly called reedbeds and living walls) and
  • compact systems as activated sludge systems, biorotors, aerobic biofilters and anaerobic biofilters, submerged aerated filters, and biorolls ^[9]

In order to purify the water adequately, several of these systems are usually combined to work as a whole. Combination of the systems is done in two to three stages, namely primary and secondary purification. Sometimes tertiary purification is also added.