Answer:
2. sand and gravel filtration.
Explanation:
Sand and/or gravel filters are made at home usually comprising of a pipe or tub filled with sand and gravel, while the advanced commercial systems usually filters contain other filtration media and processes such as charcoal filtration phytoremediation reverse osmosis
Wet oxidation
Activated sludge systems
Upflow anaerobic sludge
Vacuum evaporation
blanket digestion
Anaerobic filter
Urine-diverting dry toilet
Vermifilter
Activated sludge model