Active schemes involve one or more of chemical dosing, filtering, mechanical aeration, etc. Passive schemes typically involve running the effluent through reed beds or other media.
Both active and passive treatment schemes may involve pumping the effluent if the available treatment site lies at a higher altitude than the point of discharge.
Passive schemes (preferably without pumping) are preferred on the grounds of running costs. Passive schemes require a lot of land for lagoons and reed beds, and in many valleys suitable land simply isn't available.
Frongoch was always going to be a relatively cheap and relatively easy hit. Even so, it has been a long time in being implemented - I was involved in low-key preliminary studies and proposals 30 years ago that came to naught as no money was then available for even the modest stream diversion which is the single most important element within the current scheme.
The other major mid Wales polluting sites are mostly much more problematic, both due to types of discharge (e.g. the high iron content of Cwmrheidol) or due to a shortage of flat land (e.g. Cwmystwyth). There seems to be a general feeling in NRW to avoid schemes that will involve tank farms and miniature chemical works as the running costs are to high and the visual impacts are out of place in rural areas. Even pumping is avoided whenever possible. It'll be interesting to see which sites are progressed (Abbey Consols looks likely to be next) and which sites are more or less left alone (maybe Cwmystwyth).
The 1990s 'worst 50' list, mentioned up-thread seems to have been quietly forgotten by NRW as its errors of inclusion and errors of omission have become recognised.
Some sites to watch for potential proposals: Parys Mountain, a number of the Gwydyr mines, Abbey Consols, Cwmrheidol, Cwmystwyth, Nantymwyn. I'm less well-informed on Clwyd and Montgomeryshire sites but would guess at Van.