Last November, environmental regulators agreed to take steps to prevent increases in nitrogen discharges.

Upgrading city plants to cut nitrogen discharges could cost several billion dollars and present formidable technical and construction obstacles.

The state bond act allocated $200 million to assist sewer plants to cut nitrogen discharges.

Aerobic conditions promote nitrification and subsequent denitrification thus contributing to nitrogen discharge of the system.

The plan seeks to reduce nitrogen discharges, linked to reduced levels of dissolved oxygen in the Sound.

New York State and Connecticut officials must upgrade sewage plants to stop any increases in nitrogen discharges.

Connecticut has committed $15 million to modify 15 sewage plants to meet the ceiling on nitrogen discharges.

But despite the cost, the city cannot back away from eventually reducing nitrogen discharges, he said.

In particular, sewage-treatment plants can be modified now to limit nitrogen discharge, he said.

Environmental officials said efforts to limit nitrogen discharges and in some cases reduce them had had beneficial effects.