Engineered septic systems are more approved in Wisconsin than a few years ago when the choice was "Mound" or "standard." Holding tank is not really a septic system. It's just a tank that holds the effluent until the honey wagon comes to suck it out. That expense is what makes them not very attractive... a last chance option to build. I don't know the exact numbers, but if it's going to cost you $.25 for every flush, you'll understand the saying "If it's yellow, let it mellow, when it's brown, flush it down." There is a possibility that with the new rules, the Holding tank could be replaced with an engineered system. Again, a big expense.