The sludge will accumulate at the bottom of the tank. ... The outgoing water will be filtered by an apropriate filter...
What you need to do is design the tank so that sludge will settle. This will require that it be large enough relative to peak flow rate through it, that settled sludge won't be disturbed. Settling rate is also related to overflow surface area. A "filter", per se (as conventionally understood), would probably require more maintenance (regular cleaning, unclogging or replacement) than you'd like to have to do.
Normally don?t add drinking water. It should be added only if the level of the rain or used water is very low, because we always need water for the toilet.
I would expect your used water to always exceed the amount of water you would need for toilet flushing, eliminating the added expense of connecting fresh water to the tank. (And if you ever
did need more flushing water, just run water in one of the sinks or shower.)
I don?t understand well, why to seperate solid form the water.
The reason to separate, or at least to be able to control the concentration of solids in the wastewater is so that you can regulate hydraulic retention time vs. organic loading rate for the digester for optimum operation.
The septik is in fact a cleaner, should have some treatment as a 3 level mini cleaner with some active elements. The output from the digester is good fertilizer and plus 3 level septik(or mini cleaner) I beleived it would was enough.
Whether or not it is safe enough to re-use for irrigation all depends on exactly what those 'active elements' consist of. E.g. do they include secondary treatment? Some method of reliable (fail-safe) sterilization? And, of course, it will also depend on what crops you irrigate. Irrigating trees, ornamentals and berry bushes might be safe, where irrigating lettuce, carrots & other root crops might not.
