The owner of a North Bay septic system cleaning business was sentenced Wednesday to a year in Sonoma County Jail and ordered to pay up to $400,000 in fines after being convicted of repeatedly dumping untreated human waste into the sewer network near his home on Rohnert Park’s eastern outskirts, causing costly backups to the city’s system.

Carlos Velarde Chavez, 64, owner of Carlos’ Petaluma Septic Services and a subsidiary septic hauling firm in Novato, pleaded no contest last month to one felony count of theft of utility services and one misdemeanor count of advertising construction work without a proper license, according to Sonoma County District Attorney’s Office.

An additional felony and misdemeanor count were dismissed in the plea agreement but were taken into consideration by Judge Robert LaForge during the sentencing Wednesday, prosecutors said. A hearing on the fines facing Chavez is set for Oct. 30.

“It’s a fairly significant case in that it’s large fines and we did proceed with a felony for the reason that it caused significant harm to the sewer,” said Scott Jamar, chief deputy district attorney. “Cities have to pay for that real cost, and it’s thievery, and it has environmental connotations if it’s not disposed of appropriately given that volume.”

In court filings, investigators said Chavez illegally drained the contents of his 2,800-gallon commercial tanker into Rohnert Park pipes six days a week, on average, before his Dec. 13 arrest. The illegal dumping amounted to more than 300 separate incidents, a search warrant indicated. He disposed of the waste through a secret pipe installed in his backyard of his home on East Cotati Avenue, authorities said.

The scheme was uncovered last year, when investigators with the Santa Rosa Police Department began an undercover probe that included 24-hour video surveillance and remote underground sensors in the area near Chavez’s home, which sits just outside Rohnert Park city limits.

The probe was launched based on reports of repeated blockages in city drains, with “grit” material normally found in septic systems causing overflows, hampering drains that lead to Santa Rosa’s sewage plant, prosecutors said.

Authorities confirmed underground flows increased almost immediately after Chavez would return home from a job emptying residential septic tanks, according to court documents.

His arrest and arraignment came in December after a search of the Rohnert Park home, where authorities found the secret pipe, and a second home in Petaluma.

Roy E. Miller, Chavez’s attorney, declined to offer any comment Wednesday. Chavez pleaded not guilty to two dozen charges in March, when he declined an interview request.

“We’re working on it,” he said at the time when reached by phone at his Petaluma business. He declined to answer any additional questions.

District Attorney Jill Ravitch said Chavez operated a business “with an unfair advantage” over competitors “as he repeatedly dumped septic waste in an unpermitted location. His conduct was unfair and harmful to the sewer system and to the cities that rely upon fees associated with lawful disposal.”

The sentence handed down by LaForge includes three years of probation.

Chavez has two prior misdemeanor convictions in Sonoma County for unlawful dumping of sewage, in 1999 and 2000. He has a history of similar conduct dating back more than 30 years, the District Attorney’s Office said without specifying any violations.

In the two prior convictions, he pleaded no contest in exchange for a year of probation. He also paid fines totaling $823 in the second case.

In the sentencing, LaForge ordered Chavez to pay a $30,700 penalty and report to county jail early next month. The Oct. 30 restitution hearing will determine how much he will be required to pay in damages to Santa Rosa and Rohnert Park. Santa Rosa’s claims amount to $353,977 and Rohnert Park’s total $15,576, according to prosecutors.

Chavez has continued to operate his business during the criminal proceedings and has been in compliance with Sonoma County health regulations, according to the District Attorney’s Office.

Should he return to the business upon his release from jail, the terms of his probation require he abide by the law and not engage in criminal behavior.

You can reach Staff Writer Kevin Fixler at 707-521-5336 or at kevin.fixler@pressdemocrat.com. On Twitter @kfixler.