• Story alerts
  • Letter to Editor
  • Pin it

— The treatment process is labyrinthine; it ends in a simulation of animal digestion. "As a result of the treatment process, you remove solids that are discharged, whether human waste or any other materials. The water is a conveyance system to remove the waste from your household, and this water used for transport is essentially 99.9 percent water and only .1 of 1 percent solids. So the amount of solids that people in the community have in their water is really minuscule. Next to nothing. In order to meet the treatment needs at the reclamation plants, there is nothing left in the water, and if it ends up at Point Loma, we remove anywhere from 83 to 95 percent of the particulates. The suspended particulates form a mass called 'sludge' or 'raw sludge.' The facilities, Point Loma in particular, take the raw sludge and they pump it through anaerobic digesters. Anaerobic digesters digest the organic materials in the raw sludge that we call 'volatile matter.' Raw sludge is typically 75 to 80 percent volatiles. There's bacteria or 'anaerobes' in those digesters that utilize the solids as a food source and also for respiration. There's a large amount of sulfates and sulfites carrying the sulfates, carrying particulates; they break those down and remove the oxygen and release the sulfite.

"The first stage is called acid formation. All digesters carry two types of bacteria: acid-forming bacteria and methane-forming bacteria. The first order of business, when you feed it with a slug of raw sludge, is that those bacteria that are acid-formers will break that material down and essentially form acids that are similar to vinegar. The methane-formers will utilize those acids and break that material down to release CH4 [methane gas]. The end result is reduced solids. You have water with fixed solids left, with a little bit of organic solids left, but mostly broken-down material or 'bio-solids' and methane in the water."

After going through the treatment plant, any water containing solids is pumped back to Miramar's Metro Biosolids Center, or MBC, near the Miramar Landfill. "Those solids are dewatered, then hauled to landfill. Very soon we'll be going to land application as well, and the land application will serve as a supplement for nitrogen, phosphates -- that type of thing, but mostly nitrates. The product is fairly rich in nitrates."

Little of the product is wasted; even the treatment process is used to generate energy. It might be more appropriate to label our waste as "fuel."

"Those digesters -- and they're fairly large units -- produce a mixture of methane and CO2 [carbon dioxide] that's a natural part of the digestion process. We operate the digesters at 98.6 degrees to about 99 degrees Fahrenheit. Essentially, what we're trying to do is extend the digestion process that begins in the human body. We want to reduce these solids to next to nothing. One of the side-products, digester gas -- a combination of 65 to 67 percent methane with CO2 and some trace gasses -- is being used at Point Loma to produce electricity. We use the digestive gas to operate two large engine generators that produce electricity on the order of 2.258 megawatts per unit.

"It's essentially potential energy that we're talking about. Point Loma puts out about 4.56 megawatts of electricity and only uses about 2 megawatts of that power at the facility. The remaining two and a half megawatts are sold back to the utility. So the community is being provided additional service by the production of electricity. Besides the 2.258 megawatts provided by the methane, there's a small hydroelectric facility because of the location of the discharge. It has to go through a drop-structure and spins a turbine. That produces another megawatt to 1.35 megawatts. When you total it all up, a lot of the time we're pushing as much as 3.85 additional megawatts of electricity from Point Loma -- enough to operate approximately 3800 homes. There's also a cogeneration facility at MBC, but they're using a combination of digester gas and landfill gas. They use a small portion and the rest is marketed to the public utility. But that generation facility is owned and operated by a private firm. North City also has a cogeneration facility next to the plant. So we use the waste in a digestion process, which produces electricity. It's not just a waste stream, as many would think; it's also a source of energy."

  • Story alerts
  • Letter to Editor
  • Pin it


Sign in to comment

Win a pair of VIP tickets to
Reader Tacotopia!

Join our newsletter list

Each newsletter subscription
means another chance to win!