Many secondary treatment methods exist for decentralized wastewater treatment, each having advantages and disadvantages (Table 2). Considering that sand is the most common and available media for filters, sometimes media filter is equivalent to sand filter. Generally, in areas with deep, permeable soils, septic tank–soil absorption systems can be used. On the other hand, in areas with shallow, very slowly permeable or highly permeable soils more complicated onsite systems will be required.