If the HOA is ONLY to run the sewer plant, why have any rules at all? If you have rules beyond the sewer plant, then you must have something more in mind than just the sewer. You can easily structure it so that its only purpose is to operate the sewer plant.
Like others, I would never buy in an HOA. I've lived in areas, Houston, specifically, where HOAs were all-powerful since there was no planning and zoning commission. I've seen them run amuck. Their rule seems to be "The HOA has absolute and total control of your property. We can tell you what you can do to your property. We can tell you who can live there. We can bill you at any time for any amount and if you don't pay we will place a lien on your property."
Even if that's not what you intend, think about the situation where an entirely new group of people takes over the HOA (that will happen, eventually) and DOES have that in their mind. Can your rules be interpreted to give them that degree of control
Associations always change over time. Once in a while nobody really cares, and association just fades away. More often, there is gradual changeover from reasonable HOA management to little dictators who have too much time and want too much control over their neighbors. These folks like HOAs because they can exercise power over their neighbors.
I grew up in a tiny town in rural MO. We had a saying "small town politics are so bitter because the stakes are so small." That's how I see HOAs.
IMHO, the fundamental problem with HOAs is that you don't fully own your property. The HOA is always a partial owner and always has some ability to control your property. While I am tempted by the cheap condo prices in Florida right now, this single issue keeps me from seriously considering buying one.