Critics claiming Maryland Gov. Larry Hogan’s rollback of modern septic tank requirements will modestly increase Bay pollution are misguided.

It’ll be a lot worse than they think.

Hogan’s administration is opening the gate not only for more-polluting septic systems, but for a lot more of them — for a return to the sprawl development that Maryland has spent most of the last 20 years trying to channel into smarter, cleaner growth.

A little background: Most of the 465,000 septic tanks that serve Maryland homes not hooked to sewage treatment plants are one step up from outhouses and cesspools. They remove bacteria, but not the nitrogen, from wastes flowing to groundwater, streams and rivers, and ultimately to the Bay, where it degrades aquatic life.

The newest septics remove twice as much nitrogen, but not nearly as much as do Maryland’s rapidly upgrading sewage treatment plants. Last month, Hogan’s Department of the Environment said the state will no longer require nitrogen-removing septics, except on lots close to the water. This will make it cheaper for developers to build in rural landscapes.

The ties between septic tanks and the countryside are widely underappreciated. State health laws have long served as a crude substitute for more protective rural zoning, which bar development on significant acreages where soils were too soggy, too sloped, too rocky to pass “percolation” tests required to site septic tanks.

“Without septic, you don’t have sprawl,” says Richard Hall, who was Maryland’s secretary of state planning for eight years under Gov. Martin O’Malley, Hogan’s predecessor.

Historically, in the absence of protective rural zoning, septic perc tests steered development toward prime farm soils and bigger lots — toward the suburban sprawl that’s well-documented to increase air pollution through more driving; raise taxes as counties extend services; and gobble up an average eight times as much land per household than do homes connected to sewers.

So with Maryland looking at a projected increase of 1 million people and 500,000 households by 2040, one of the biggest questions for the environment and for quality of life is this: How many will be on sewers, how many on septic?

Minimizing septic tanks seemed the logical answer to Hall and his boss, O’Malley. In 2012, they crafted a widely accepted law that dramatically limited development on septic tanks wherever the landscape was “predominantly agriculture and forest.” About the same time, O’Malley required all new septics to remove nitrogen, making sprawl development more expensive, but also less polluting.

Some rural counties chafed, most notably Cecil in northern Maryland. They submitted for state planning’s review a zoning map that essentially said “in your face” to restrictions on septic-based development on farms and forestland. A county planning official compared then-Secretary Hall to Adolf Hitler.

The septic “tier mapping law” as it is known, left ultimate land use power with the counties; but it gave Hall’s Department of State Planning, and the MDE broad latitude to pressure counties into compliance, even to hold up development if it was contrary to the law’s anti-sprawl intent.

Hogan has quietly reversed all of this. Letters sent to Cecil County from both his environment and planning departments say, in effect, the county can go its own way.

The signals from the state are clear, not just to Cecil, but to Calvert, Queen Anne’s and other rural counties under growth pressure. They need no longer fear state intervention against sprawl. Smart growth is out; dumb growth is back.

The majority of Maryland counties have largely complied with the new law’s requirement that “Tier Four” lands, those where farms and forests predominate, allow only minor development on septic, which is to say only limited development.

But there’s little now to keep them from backsliding, and you can bet that’s not going to be lost on the development community, a powerful political force at the county level everywhere.

State planners these days “pay more attention to the casual Fridays dress code” than they do to Smart Growth laws, says longtime land use advocate Dru Schmidt-Perkins, head of 1000 Friends of Maryland.

“The message to the counties is, ‘Do what you want’,” Hall says of his old department.

Fifty thousand new septic systems would be prevented from being installed in rural landscapes, the O’Malley administration calculated when its 2012 law went into effect. No one should expect that now. We can’t know yet how many will be built, but we know most won’t have to control nitrogen.

Hogan might have helped developers without harming the environment if he had looked at why less polluting septic tanks cost so much — $10,000 to $15,000 apiece.

I’ve seen them done well for half that, by Rich Piluk, a sanitarian in Anne Arundel county. I had one installed myself. But apparently only a few big companies met the state’s requirements for certification and accountability. It’s been suggested counties could set up their own septic management districts to lower costs.

But it’s easier to tout Maryland as “open for business,” with talk of “getting the state off your backs.” With policies that reach well beyond gutting septic restrictions — such as shifting transportation money from mass transit to more roads — Hogan seems determined to defeat the “war on rural Maryland” that his supporters claim O’Malley waged. “Victory” for them means a return to sprawl development.