Hey Owen, great to hear from you. In a perfect world—where the developer actually builds all the homes they promised, plus all of the necessary infrastructure—these large, built-to-a-finished-state developments rarely (if ever) generate enough tax revenue to cover their long-term maintenance obligations. You can learn more here: https://www.strongtowns.org/journal/2017/1/9/the-real-reason-your-city-has-no-money39
Throw in a waste water treatment plant on the balance sheet and I'd imagine that, since the developer bailed, it's nowhere close to servicing enough taxable developments to recoup its cost or cover its long-term maintenance obligations.
As you and your colleagues discuss how to cover the debt from the plant, keep in mind that new projects will have their own long-term maintenance obligations. Sure, they'll generate immediate tax revenue, which you can use to pay off some debt from the plant, but flash-forward a few decades and consider what revenue the town will use to maintain those developments.
At Strong Towns, we call this the growth ponzi scheme: exchanging immediate cash flow for long-term maintenance obligations of new infrastructure. You can learn more about the growth ponzi scheme here: https://www.strongtowns.org/journal/2011/6/15/the-growth-ponzi-scheme-part-3.html34
As you have time, I recommend you check out this video that explains how Fate, Texas, does the math on proposed developments to see if they'll generate enough tax revenue to cover their long-term maintenance obligations: https://community.strongtowns.org/post/5d45cb36e8d4676044b77bb837. Since you're stuck with the plant, it's essential that all new developments—now more than ever—cover their own infrastructure and maintenance costs.