Santa Rosa will sell up to $18 million in bonds to finance upgrades to the aging regional wastewater treatment system.

The funds are needed for a wide variety of projects, from infrastructure upgrades to flood control studies to protection of a high-pressure water line.

The bonds will be repaid over 15 years at an average rate of 3.34 percent. They will be added to the system's approximately $310 million in outstanding debt, much of it from the construction of the 40-mile pipeline recharging The Geysers geothermal fields with treated wastewater.

The bonds' terms are structured to keep the city's total wastewater debt between $28 million and $26 million per year over the term, according to city staff.

The largest chuck of money will be used to fund $6.5 million in routine capital upgrades to the treatment system, such as new pipes and pumps.

Additional major costs include $4 million for seismic upgrades to the Llano Road plant, $1.5 million for the stabilization of Pine Flat Road, under which the city's Geysers high-pressure wastewater pipeline runs, and $1.5 million for a slurry processing station that will allow the plant's digesters to turn food waste into energy.

Other uses include $1 million to study how to protect plant operations during a flood, $1 million for a study of the plant's disinfection system, and $1 million to fund projects to help the plant comply with permit rules restricting it from adding to the phosphorus level in the Laguna de Santa Rosa.

The city wants to get away from funding routine capital upgrades with debt and instead use cash in an effort to reduce interest costs and keep rates down. But it just started squirreling money away for that purpose and hasn't yet saved enough to allow it to fund projects with cash, Utilities Director David Guhin said.

Another debt issuance of approximately $30 million will likely be needed in 2017 to fund additional upgrades to the plant, including the flood control and expansion of the disinfection system the two studies will explore, Guhin said.

The City Council signed off on the sale of the additional debt Tuesday evening on a 7-0 vote.

(You can reach Staff Writer Kevin McCallum at 521-5207 or On Twitter @citybeater.)