It is a fact, i'm afraid, septic tank effluent SMELLS!  The smell is not too bad on a gravity system, but your pumped system is stirring it up and the smell will come from a point prior to the absorbtion bed.  The only way to stop the smell is to replace the septic tank with a sewage treatment unit whose effluent is odourless.  Pump switches last longer with sewage treatment unit effluent as there are much lower levels of suspended solids in the effluent.  It is these solids which @gum-up' the switches and cause them to fail.  If you want any information about anything to do with sewage systems, please ring me.  It's FREE  Tel 01757 288022