  The "search for extraterrestrial intelligence" (SETI) commensal surveys aim
to scan the sky to find possible technosignatures from the extraterrestrial
intelligence (ETI). The mitigation of radio frequency interference (RFI) is an
important step, especially for the most sensitive Five-hundred-meter Aperture
Spherical radio Telescope (FAST), which can detect more weak RFI. In this
paper, we propose several new techniques for RFI mitigation, and use our
procedure to search for ETI signals from the archival data of FAST's first SETI
commensal survey. We detect the persistent narrowband RFI by setting a
threshold of the signals' sky separation, and detect the drifting RFI (and
potentially other types of RFI) using the Hough transform. We also use the
clustering algorithms to remove more RFI and select candidates. The results of
our procedure are compared to the earlier work on the same FAST data. We find
that our methods, though relatively simpler in computation, remove more RFI
(99.9912% compared to 99.9063% in the earlier work), but preserve the simulated
ETI signals except those (5.1%) severely affected by the RFI. We also report
more interesting candidate signals, about a dozen of which are new candidates
that are not previously reported. In addition, we find that the proposed Hough
transform method, with suitable parameters, also has the potential to remove
the broadband RFI. We conclude that our methods can effectively remove the vast
majority of the RFI while preserving and finding the candidate signals that we
are interested in.
