Subterranean gravity-flow graywater filtration apparatus and system

A gravity-flow graywater filtration system, comprising:

• a tank assembly having a lower portion and an upper portion with a top structure;
• a filter cartridge located in the upper portion of the tank assembly and comprising:
• a filter-cartridge housing having an open top facing the top structure and a bottom region located and configured to empty water into the lower portion of the tank assembly, and
• a plurality of filter layers extending between the open top of the filter-cartridge housing and the bottom region of the filter-cartridge housing, the plurality of filters comprising a thermo-plastic mesh layer adjacent the open top, a gravel layer adjacent the bottom region, and a sand layer between the thermo-plastic mesh layer and the gravel layer;
• an inflow pipe located and configured to empty graywater through the open top of the filter-cartridge housing and into the thermo-plastic mesh layer;
• an outflow pipe located and configured to transport non-potable water from the lower portion of the tank assembly;
• and
• a sump pump in the lower portion and coupled to the outflow pipe and configured to pump non-potable water from the lower portion into the outflow pipe.