A law requiring low-nitrogen septic systems for all new residences, for existing buildings — including commercial ones — that expand their floor area by 25 percent, and for nonresidential properties that require site plan review was adopted by the Sag Harbor Village Board on March 12. The law, which will take effect April 1, also gives the village’s harbor committee, and the Suffolk County Department of Health Services the authority to require the installation of upgraded systems. Nitrogen pollution from conventional septic systems is excessive, widespread, and threatens the health of drinking water, bays, estuaries, and coastal waters, the law states. The legislation, passed in a unanimous vote, and without discussion among board members, follows similar laws enacted by Southampton and East Hampton Towns, which include sections of the village. At a public hearing on the measure on Jan. 8, there was no opposition. To monitor compliance, the law empowers the village building inspector to require reports on the performance and maintenance of systems. The penalty for violations would be a fine of not less than $1,000, in addition to making the premises compliant with county standards.