What would the best word be for the sewage that comes out of a septic tank, like the simple septic tank (Βόθρος) one would find in a country house be please? Is it αμολήματα or απόβλητα or something else?
I think you may have meant υπολείμματα or απόβλητα. Occasionally, both these terms are used to refer to sewage but strictly speaking they're more general terms referring to all kinds of waste products. The most common and direct translation for sewage is, as daeman suggests, λύματα (or more specifically, in the case of septic tank sewage - βοθρολύματα).