AUBURN — City officials are considering updates to the septic design standard in the Lake Auburn watershed, after a recent study showed a new standard could help keep harmful nutrients out of the lake.

The proposal, however, is likely to rekindle debate over development in the watershed.

City staff members said this week that if the proposed standard were used, it would likely make it easier for new systems to be installed and new homes to be built, but said the better-performing systems could achieve a “net improvement” to the lake even with additional systems installed.

The lake is the source of drinking water for Auburn and Lewiston.

The study, which was given to officials late last year, said the septic standard used in the Lake Auburn Watershed Overlay District limits development on a significant part of the watershed by “effectively prohibiting the use of innovative and alternative septic system and leach field designs.”

“These innovative and alternative designs are otherwise allowed by the state and can achieve comparable or better nutrient removal than a traditional system and leach field,” according to the report.

Eric Cousens, director of planning and permitting, said the current standard requires a 36-inch layer of soil between the surface and what’s called the “limiting factor,” which is either groundwater or bedrock. Most standards require 12 inches, he said.

Cousens said that because Auburn’s standard does not allow for alternative soils to be used, some properties simply cannot meet the standard in order to install a system. But, he said, the standard also has not been yielding the best results, and that alternative soils can be used that drain more slowly, filtering out nutrients like nitrogen and phosphorus that are harmful to the lake.

Among the study’s recommendations was that Auburn maintain the 36-inch minimum depth requirement while allowing the use of alternative septic designs. The proposal was included in the updated comprehensive plan, approved by the council last year at the end of its term, and was forwarded to the Planning Board for consideration last week.

The Auburn Water District board of trustees is also slated to discuss the proposal during its next meeting.

Cousens said he anticipated some resistance to the change, but said the current standard is not “using the best science for wastewater,” it is simply making lots unbuildable.

He said because of the restrictive standard, some people who cannot put a system on their land end up buying adjacent properties that might meet the standard, but it is “placing systems in areas that aren’t being treated as well as they could be.”

The Lake Auburn study, completed by FB Environmental, the Horsley Witten Group and the University of Maine, was commissioned by the city to analyze the rules governing the watershed. Officials said they hoped it would provide data to inform a yearslong debate over watershed protections and additional development.

A summary of the report said additional development in the watershed would provide “minimal net economic benefit across all affected stakeholders,” but also recommended a new septic design that could lead to more homes there.

A “build-out analysis” in the study determined that more than 100 additional new homes could be built in the watershed if the septic system requirement was revised, but the study also addresses the “contradiction.”

The consultant group questioned whether the city’s existing standard is effective for water quality protection, or if the water quality benefit is “its de facto restriction of buildable areas in the watershed?”

Cousens said the goal is to build better systems during new construction, but also use the standard as people replace old septic systems. He said the standard should result in systems that provide better treatment and have longer lifespans.

“With our recommended revision, we aim to have the septic design standard achieve its stated purpose of effectively regulating both new septic system construction and replacement of existing septic systems as they age out, so that systems with alternative technologies and innovative phosphorus controls can be phased in,” the study reads. “Restrictions on developable land are better left to base and resource protection zoning than to septic design standards.”

Last week, when the City Council forwarded a number of zone change proposals to the Planning Board, Councilor Rick Whiting motioned that the septic requirement discussion be considered a secondary priority. He argued that the Planning Board be given a “broader directive” to review the entire lake study.

During the discussion, Cousens said he was working with the board chairman to plan extra meetings. He said some public hearings on proposed changes would require public notices for some 2,000 residents.

Mayor Jason Levesque said last week that “by adopting the state standard and implementing new low-impact development standards, we can make the lake better.”

Levesque also said residents should keep in mind that the watershed includes a lot of area that is not on the lake, including outlying towns that he believes should also adopt new standards.