There are several types of septic tank available.  Only the very large ones (for blocks of flats etc.) need powering.
The most common two here seem to be the multi-chamber Remosa ones (http://www.remosa.net/remosa/productos/prod_105.html) and the bell shaped ones.

Some people illegally just dig a hole in the ground then coveer it and feed a pipe in.

You'll need a hole for it (obviously!) and a trench for the input pipe and then a trench for the exit tube (which needs filling with stone chippings to not block the pipe which has perforations in it). The price for the hole depends on the soil/rock - one I was recently involved with cost 1200 euros for the hole as it took 19 hours to chip through the very hard rock in Aldover!

The Remosa tanks cost around 750 euros + IVA for a 1400 litre (4 person) tank and 1250 + IVA for a 2300 litre (7 person) tank.

I'm not sure about permissions but as you're not actually constructing anything, rather just burying something, then you may find you don't need permission (anyone know better??).

Nigel