The build-up of sulfides, which are characterized by the blackwater, is common in sand filters. Septic-Scrub can help restore flow to the system. You would add the Septic-Scrub directly to the sand filter. If this is not possible, then it can be added through the pump chamber. The biggest concern regarding sand filters is whether the sand has compacted so much that flow through the filter is hindered by this "mechanical" problem and not a build-up of sulfides. I do not see where the latex paint or the washing detergents would have caused a failure unless you have used an excessive amount of either. Adding the residues of latex paint to the septic system is obviously not recommended. An normal amount of washing powder should not cause harm, but if there is a concern, you might go to a liquid or a more bio-degradable detergent. If you have a choice, I would suggest an attempt at a restoration rather that an anaerobic system installation. If you have any questions, please do not hesitate to call.