Code does not allow the use of a garbage disposal on a septic
installation. The food particles can clog up your leech field. Get rid
of the garbage disposal and install the type of food grinder that is
designed for use with a septic system. It's a hell of a lot cheaper
than paying a plumber to install a new leech field.
As for the grease, hot water will help grease flow through your pipes.
However, inevitably that grease gets into your septic tank. The grease
in your septic tank will impede the ability of the bacteria in your
tank to continue with the aerobic digestion of the particulate. Grease
in your septic tank is a bad idea. Don't run hot water to flush the
grease through your drains. Instead, dump your grease into an old soup
can and toss it in the garbage can.
Sounds like you don't know how your septic tank works. Those septic
systems are very pricey to repair or replace. Maybe you should nose
around the internet and learn a little more about how your septic tank
works before you fuck it up by continuing your poor practice of
dumping crap like food particles, grease and nitrates into your
It might be cheaper to modify how you treat that septic system before
it costs you more than a new car to fix.