Vista looking for new sewage spill safeguards

VISTA - In the aftermath of two large sewage spills caused by exterior corrosion on relatively young pipes, Vista officials are faced with a difficult problem: How will they find similar weaknesses, which are seemingly undetectable, before it's too late?

According to corrosion engineers, they could start by taking a closer look at the soil surrounding existing lines.

Both of last month's sewer breaks occurred on cement-lined, cast-iron sewer pipes, which were designed to last at least two more decades. Analysts later determined that "severely corrosive soil" ate through the pipes, according to city reports submitted to water-quality regulators.

Schiff Associates, a consulting firm hired by the city in the wake of the leaks, has suggested using radio frequencies to determine where soil is salty, and thus corrosive to metal. While not commonly used by public utilities, this type of electromagnetic survey is nonintrusive, and has been used to find groundwater in the Middle East, among other things, said Robert Pannell, a senior corrosion technologist with the firm.

"Soils in San Diego County, generally, are pretty salty," Pannell said.

While corrosive soil would not necessarily point to weaknesses in the sewer lines, it could help the city better assess the risk of a leak and plan replacements accordingly.

Of the two breaches discovered last month, the first - on a sewer main jointly owned by Vista and Carlsbad -- caused more than 7.3 million gallons of raw sewage to pour into Buena Vista Lagoon.

Days later, more than 400,000 gallons of waste escaped from a fissure on a Vista-owned line near South Melrose Drive in Carlsbad, though much of that spill was captured in an emergency containment basin.

Short of digging up the pipes, and running the risk of more damage, there would have been no way for the city to see the external corrosion before the ruptures occurred, said Larry Pierce, the city's director of engineering and public works.

The pipes were installed in 1982 and 1988, respectively, and had life expectancies of 50 years, according to city reports. In both cases, sections just a few feet away from the fissures were in good condition, reports stated.

The city maintains about 310 miles of sewer lines, but only about a half-mile is the type that failed last month - a cast-iron "force main" that is actively used, Pierce said last week.

A force main is a pressurized sewer line that carries wastewater to a point where gravity can take over. Breaks in these pipes typically create bigger spills, since the sewage is being forced through, Pierce said.

As for spotting corrosion on existing force mains, the city is researching methods used by other industries, such as oil and gas transport, but most have limitations, Pierce said.

It's not yet clear, for example, if inspection techniques used on steel pipes would be as effective on iron, Pierce said.

"We may run into a bunch of dead ends … (but) if there's one that plays out, then we'll continue tracking that."

In the meantime, the city is talking with the Encina Waste Water Authority about bolstering the monitoring and alarm systems to detect when leaks occur. The flow monitors already in place were installed for billing purposes and do not give real-time updates.

Because the pressure in a force main varies substantially over a 24-hour period, it will be a challenge to calibrate the monitoring equipment, but "we'll get them in as soon as we can get it figured out," Pierce said.
