In October, engineers at the Coastside’s sewer authority saw a curious spike in one of their meters. The Sewer Authority Mid-coastside monitors a host of contaminants several times a week, and there appeared to be an inordinate amount of organic waste coursing through SAM’s system of sewer lines.
When the contaminants reached the treatment plant, they were sucking up more oxygen than the treatment was designed to handle. This led to ocean-bound treated water that had unhealthy, low levels of oxygen.
SAM uses a metric called the biochemical oxygen demand to keep track of how much oxygen is used in the treatment process. For municipal sewer agencies like SAM, BOD levels measured when leaving a treatment plant should be less than 50 milligrams per liter. This ensures that the water flowing into the ocean is safe for marine life, some of which depends on higher levels of oxygen in the water they ingest to survive.
In late September and the first day of October, BOD readings hovered around 17 milligrams per liter. Steadily that number rose to 41, then 59, and by Oct. 28, levels were in the hundreds. At its highest reported level, BOD was measured at 398 milligrams per liter on Nov. 2. That’s nearly eight times higher than what is permitted by law.
The source of the contamination remains unknown, but there are a host of possibilities: breweries and distilleries, frozen fisheries, farms and more. These sites can dump products like bad batches of beer or fertilizer that lead to high BOD levels.
In the days immediately after the first abnormal reading, SAM staff conducted its own investigation. They installed sampler devices, which collect a small amount of sewer water every hour, at four different locations outside of the treatment plant: Princeton, El Granada, Moss Beach and Half Moon Bay. The samples were then sent out for testing.
A normal reading at those sites (before they reach the plant) should fall within 200 and 600 milligrams per liter. But when the results came back, the highest results came from Princeton at 1,041, with Half Moon Bay at 720 and Moss Beach at 610.
“That’s when we raised a red flag,” said Kishen Prathivadi, general manager at SAM. “That was really high and therefore we began to wonder what had to be done.”
In late November, SAM brought on a specialist who has been trying to track down the source of the contamination. To begin, the specialist interviewed businesses in the coverage area, especially those where high BOD levels were observed, to determine if there were any changes to the way they were dumping their products.
“We don’t want to scare the public or get anyone in trouble. We just want to work with everyone to solve the problem,” Prathivadi said.
High BOD, or low oxygen levels in the water, has no direct impacts on human health.
However, the sewer authority invested in more equipment to stave off any further stress on the sewer system. Staff introduced additional treatment processes to remove more particulate matter and powered up two more aeration tanks. They also increased sampling and testing from twice per week to daily.
These enhancements have raised operation costs and energy use, but they are working, according to a presentation to the SAM board of directors on Dec. 14. Oxygen levels are back to normal, but the modifications should be temporary, Prathivadi said.
A return to normal operations will depend on ensuring the original source is under control. If results trace the problem to establishments within the jurisdiction of any of SAM’s member agencies — the city of Half Moon Bay, Granada Community Services District and the Montara Water and Sanitary District — these jurisdictions may have to absorb the responsibility of additional treatment.
The investigation is expected to conclude sometime in January.