From the headworks the water flows to primary clarifiers--tanks through which the wastewater flows extremely slowly, not unlike the settlement basins at the water plant, and with the same goal: allowing solids to settle to the bottom, creating a "primary sludge blanket," though here grease and oil also float to the top. Scrapers make a circuit along the surface, scooping grease into a small flume, then cycling to the bottom where they shepherd the sludge to its own pumps. Water cleanliness is measured by biochemical oxygen demand (BOD)--the amount of oxygen the bacteria in the water use to remove its organic impurities. The lower the BOD, the smaller the next basins can be and the lower the plant's operating costs. This is a crucial point, given how much energy the next step takes. That step is what Lynch calls "the absolute heart and soul" of his plant: the activated sludge process.
So after a few hours in the clarification tank, the water flows into aeration basins, six concrete pools of several million gallons apiece, the bottoms of which are crisscrossed by air nozzles. These enormous tanks of what looks like boiling brown sewage are just what you imagine when you think "sewage treatment." But it's not boiling: What's happening is aeration, which provides oxygen, in the presence of which bacteria love to eat poop. "We are bug farmers," Lynch says. Bacteria in the basins multiply rapidly, like the starter for sourdough bread. The wastewater provides the food, the nozzles provide the oxygen, and the bacteria feel like they're on a cruise: nothing but breathing, eating, and reproducing, with free food all day long.
This process removes all the harmful chemicals from the water--except nitrates, which feed algae in rivers. These algae propagate wildly and then die. "And the decomposition sucks up all the oxygen in the river, killing the fish. So now we're going into the same tanks and we put walls in there and we have different zones where we stimulate the bacteria to do different things" that remove nitrogen. After about a day of that, the water spends 2 days flowing very slowly through secondary clarification basins: a dozen large circular tanks, each the size of an aboveground swimming pool. "It's very quiet," Lynch says. "The bacteria settle down and create what we call the sludge blanket," which sinks to the bottom and is pumped out. The water trickles out through V-notch weirs around the top of the tank, by which point it has a barely yellowish tinge and no odor whatsoever.
We'll get to the next stage of water filtration, but what's more interesting at this point is what happens to the sludge. Most of it becomes RAS--return activated sludge--and rejoins new wastewater in the aeration basins, providing the starter for a whole new treatment reaction. Bacteria can go around the cycle half a dozen times, Lynch says, but eventually they become WAS--waste activated sludge. In the WAS stream, the bacteria go to four big covered tanks called aerobic sludge digesters, in which instead of wastewater nutrients they eat each other. We climbed up to the top of one of the basins and looked through a porthole inside: You could almost hear the screaming. Okay, not really; it's just a tank of sludge. "If it's good sludge, it's got that nice brown color--it's oxygenated, and it don't smell bad up here," Lynch says. If it's black, something's out of whack--it probably needs more oxygen.
Aerobic digestion, Lynch says, is actually inefficient--it costs money, since the plant has to pump in oxygen; anaerobic digestion, to which the plant hopes to switch, is the opposite: It generates methane, which the plant can use to generate electricity to operate its pumps and blowers; the waste heat from the generators can even help provide the heat the anaerobic digesters need to keep the bugs at optimum eating-each-other temperature. "There are plants now that generate their entire power needs from that methane," he says. "They pull no energy from the grid."