Steep Soil Pipe Fall to Septic Tank

[Pinoy]

I have a septic tank 15m from the manhole cover on the terrace.
I need to lay a soil pipe from the manhole cover to the septic tank.
Horizontal distance from manhole to septic tank is 15m.
Vertical drop is 3m.
In the three pictures attached the manhole cover is by the wheelbarrow on the terrace.
The two other pictures are taken halfway between septic tank (violet flowers) and manhole cover.
I appreciate such a steep slope will cause problems since water and soil will not flow at the same rate....but what do I do ?
Please bear in mind this is France so regulations are meaningless here !
I just want to avoid blockages...

 

[lostinthelight]

You need some back drop chambers.
http://www.draindomain.com/drainage backdrops.html

 

[Pinoy]

Thanks...the total drop is about 3m....so two backdrops enough ?
Are there UK regs on this for best practice ?