Taoiseach Enda Kenny has defended as "common sense" the Government's controversial septic tank registration plans.

But he refused to be drawn on calls for a grant-aid programme for financially struggling countryside householders who cannot afford upgrades or replacements.

Sinn Fein leader Gerry Adams said, despite reduced fees announced on Monday, hundreds of thousands of rural dwellers still face the prospect of punitive bills to bring their septic tanks up to standard.

"Billions have been spent on upgrading and providing a modern sewerage system in urban areas," he said.

"Rural households helped to pay for this through taxes yet they are being forced by your government to bear the burden of the cost of improvements, upgrades and replacements. "

Mr Adams said rural households would be criminalised under the proposals and called for a full grant aid scheme.

But Mr Kenny said Ireland needed to meet water quality standards after a ruling against the country by the European Court of Justice in 2009. The Taoiseach described as "hysterical" and "startling" comments and headlines about the issue in recent weeks.

"This is part of a common sense approach towards having clean water, to have safe water, having septic tanks that do what they are supposed to do," he told the Dail.

Environment minister Phil Hogan announced on Monday that householders may register their septic tanks from March until the end of June for a reduced fee of 5 euro, rather than the full fee of 50 euro.

Mr Hogan said there will be financial assistance for people to fix septic tanks if needed, but could not give details on this until inspections begin next year.