Hey folks- sorta new, but teachable!  Im in N FLA, county says if washer is attached to kitchen sink its sewage. Permit time, fine- so far I see my best fix is to run washer into septic tank- I know many say NO but old tank for washer lost its bottom!. So tank for house is 100' away and 4' inlet at tank is only 16" below topsoil so gravity? I dont think so unless I buy 4" pipe!. "Liberty pumps" make a tank/pump/ alarm with all the fixins thats 2" and the state says I can DIY with permit- advice? concerns?  thanx

Are you talking about the little 40 gallon pump combos?  Not sure of the cost of that with 2 inch pipe and electrical stuff vs. just running 4" schedule 40.  For 100 feet you need a minimum 1/8 fall per foot so you could gravity flow.  Is the top of the lid 16" or the top of the inlet pipe 16"?  The top of the inlet pipe is probably another 5-6 inches below top of lid.  You probably have an older tank without a baffle.  Pumping to the tank, will keep contents stirred up in the tank.  Obviously, this will depend how often you wash,etc.  May not make a big difference on a weekend house.  You will need to put a turn down on the inlet pump line ( in tank), or make sure you have an inlet tee in the tank if you tie into the existing line.  Turn down will help to not disturb the tank contents as much, rather that just blowing the wm water into the tank. 

Side note....I work for HD in SWGA.   I have saw many of the little pump combos fail over the years.  Most of the time the pumps cannot keep up with the cycle demands. I would guess the pump will run with each load you wash.  I would shoot for gravity flow.  Also, not sure in Fl, but in GA, your pump and alarm have to be on two separate circuits.  In other words, you don't want one circuit powering everything.  If the pump fails, the alarm will still work ( which is what is supposed to do).  Some of the pump tank combo control boxes do not come like this.

I might would ask if since it is only washing machine can you use a 2 or 3 inch line to tank.  There are no solids food scraps, paper, etc. only lint.  The 4 inch line requirement is not for flow, but for air exchange between house, tank and drainfield.  Systems need to breathe.  Running a gravity flow 2 inch line will not hurt the air exchange because all your other plumbing should be 4inches. 

Sorry to be long winded.