Issues on building around septic tank

I have a sloping garden that i want to tier. The septic tank is very close to where I want to build the first retainer wall. I know have to keep the ground level arount the tanksso that the tank doesn't get covered. My question is How close can I build the wall to the tank?

It is a concrete cover tank.

Thanks for any advice.

[broken link removed]
see table 2

I'm reluctant to comment further as things have changed since the above document was published with the introduction of the Water_Services_Bill_2011, so you REALLY SHOULD get a professional (that is registered on your local councils waste treatment assessment panel) to view your proposal. soon enough we'll be paying for inspectors to assess our septic tanks as were getting fined 25,000€ per week(not sure about that figure) by the EU for environmental misbehaviour.