As Mr. Rizzo says, the first ...

As Mr. Rizzo says, the first thing is to carry out a process of separation of coarse solids. The variety of equipment for this is enormous, but I recommend that special attention be paid to the ease of cleaning and operational continuity. Next goes the biological process. We have used a system developed in Chile by Dr. Tohá of the University of Chile, which currently does not require patent payment. We have implemented it successfully in industrial pig raising farms (more than 10,000) and the point is the availability of space, although I suppose that due to the nature of the industry, it should not be a limitation.
The Tohá system allows the removal of BOD, SST, NKT and P, with minimum energy requirements and without the problem of the production and disposal of biological sludge, obtaining as a by-product the so-called earthworm humus, which can be used directly as fertilizer, and you get totally stabilized.

Of course, the selection of the system will also depend of the final use of the water, the quality required, if you have electric energy and the cost, how difficult is the solid disposal, etc.