According to the installation instructions, the tank is to be vented to the sewage vent system:
http://www.pumpbiz.com/omi_files/Flotec_FPOS1800LTS.pdf
Doing that will relieve the pressure in the tank while it is being filled as well as letting air in while it is being drained ... and yes, you should have a P-trap under the sink. Even gray water can smell badly while there in that tank, and the absence of a P-trap would leave your sink drain open to sewer odor after the tank has been properly vented.





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