Sewage spills declined 17.3 percent in 2009, the city Department of Environmental Services reported.

It's the third consecutive year that the number of wastewater spills has gone down, the department said in a statement.. There were 101 spills last year, down from 122 in 2008.

Sewage spills, also called "sanitary sewer overflows," can occur with the malfunction of gravity mains, lower laterals, pump stations, pre-treatment facilities and force mains break down.

The department said that this year, the leading causes were roots, pipe failure, debris, grease and third-party contractor error.

The agency has put in than $1.1 billion to refurbish the city's wastewater collection system and infrastructure since January 2005 and expects to appropriate $1.6 billion more over the next five years.