Thirteen of us sat in a conference room at the Oceanside Wastewater Treatment Plant, all oddly excited to spend the next two hours talking about poop. The walls of the conference room were stacked with hundreds of municipal binders, and a collection of San Francisco Public Utilities Commission staff fumbled with an overhead projector.

“We have a really great sewer system,” communications director Jean Walsh explained to the crowd. “But it’s hella old.”

Once or twice a month, San Francisco’s sewers offer free tours on Saturday mornings, so last weekend I made my way to Oceanside by 10 a.m. Our tour began with 30 minutes in that conference room, learning about everything from the history of the plant to the behind-the-scenes details on a sewer-themed rap video produced by local high school students. It wasn’t until we put on our safety helmets and latex gloves that things got marvelously down and dirty.

Backed up against Lake Merced, alongside the Pacific Ocean and underneath the San Francisco Zoo, Oceanside handles about 20 percent of San Francisco’s raw sewage. The rest gets treated at Southside in the Bayview district, but according to the sewer folks, Oceanside is much fancier. (To hear sewer employees talk about the Oceanside Wastewater Treatment Plant, you’d think they worked at the Ritz-Carlton Half Moon Bay.)

A materials testing aide named Jonathan Smith served as our well-versed tour guide. He very subtly arranged for the one guest in shorts to quickly pop into a safety jumpsuit. I guess bare legs are forbidden at the sewer plant, and the same is presumably true for open-toed shoes.

Our first stop was in what appeared to be Dr. Frankenstein’s laboratory. Yellow-liquid-filled jars contained dead sea creatures, and glass-walled offices housed rows of ongoing tests. San Francisco’s sewer scientists pride themselves on going above and beyond official standards when it comes to waste treatment, and the tour included several lengthy and detailed explanations of exactly how this is done. Honestly, I just wanted to see raw sewage.

Smith guided us downstairs and outside the office building. The ocean air blowing off the Pacific worked as a sort of natural Febreze, masking the smell of all that went on underneath us. As we headed into the inner workings of the plant, Smith warned us that precisely two spots on the tour smelled so foul, he’d actually had guests call it quits. I braced myself for the worst.

Once we ventured into one of the plant’s industrial buildings, we encountered raw sewage right away. It smelled different than I expected, with more complex scent notes than just poop and pee. That sounds disgusting, and it was, but checking out the initial processing of 20 percent of San Francisco’s raw sewage was worth the stink.

Items that have ended up in raw sewage, said Smith and Walsh, include cell phones, toys, a lot of keys, drugs, guns and one human body. Not all of these get flushed down the toilet, however. Wastewater Treatment Plants handle anything that goes down a drain, whether it’s in a dishwasher or a street gutter.

San Francisco’s raw sewage is so well treated that much of it is repurposed. Some is extra-treated and serves as the water used during the city’s street sweeping, for instance, while other solid and treated sewage can be used as landfill cover. The rest from Oceanside is pumped 4½ miles out in the Pacific Ocean. While none of it is potable, the Oceanside Wastewater Treatment Plan can turn raw sewage into a pretty clear glass of water.

And yet perhaps the most interesting part of our sewer tour wasn’t the actual sewage. Apparently the plant is haunted by someone who died in its construction. Lore has it that someone even managed to snap a photo of him.

In addition to loving San Francisco’s sewers, the team in charge has taken a very modern — and very funny — attitude when it comes to marketing. Smith’s hard hat featured two official stickers. One said, “No one takes more crap than me,” and the other read, “Your #2 is our #1.” Much to everyone’s delight, free stickers were included in the tour.

This cheeky marketing is used to help educate San Franciscans on the various facts of life that the sewer people want us to know. For example, they’re very into the flush-ability of “The Three P’s: poop, pee, and (toilet) paper.”

“It’s a toilet, ladies and gentlemen,” announced Smith. “Not a trash can.”

Beth Spotswood’s column appears Thursdays in Datebook. Email: datebook@sfchronicle.com