Foul Drianage Smells

[David Cunningham]

I have an external manhole which collects the foul waste and waste water which then distributes this to a septis tank.
There is a unpleaseant smell from the manhole especially in the hot weather, havung lifted the manhole cover it appears some waste has splashed out of the drain pipe route onto the sides of the manhole which then remains thus causing the smell.
The foul drain pipe in question tees into the manhole.
Is there a type of splash guard or cover that is available to avoid this problem.
The house is 4 years old and we have had this smell on and off since we moved in, the septic tank has also been recently emptied.

Any ideas appreciated.
Thanks.