or, what not to flush down your toilet
On Saturday D. and I took a tour of the San Jose/Santa Clara Water Pollution Control Plant in Alviso. Yes, it's where sewage is processed. No, we didn't get up close and personal with any sewage. There was a presentation followed by a one-hour tour of the complex by coach. We both took some pics out the coach windows.
The complex is huge, occupying 4.5 square miles. Operations comprise 180 acres, biosolid treatment, 770 acres; the plant also owns Pond A18, an 860-acre former salt evaporation pond, which is idle at the present time, as well as 720 acres of former farm land used as a buffer around the perimiter (which is currently being mowed by grazing sheep). The plant employs 250 people, but it's hard to tell because 2/3 of the plant is underground (there are 4 miles of tunnels). It processes about 100 million gallons per day and has been working non-stop, 24/7/365, for over 50 years.
We arrive at the complex.
Urban Planning Engineer Matt adjusts the projector for a presentation prior to our tour.
The computer control center at the plant.
A rendering of the original plant in the operations center hallway.
Here's the other coach of tour-goers, taken from my seat in the second coach, before we set out.
Matt is the tour guide on our coach.
Wastewater from sinks, toilets and drains in the homes, businesses and schools of 10 local communities can take up to 10 hours to travel through an underground pipe system, called the sanitary sewer, before arriving at the treatment plant. Eighteen hours later, 99% of impurities are removed through a treatment process that simulates and accelerates the way nature purifies water. (By the way, fats, oils and grease washed down the drain are the primary cause of most sanitary sewer blockages, so please keep them out of your drains!)
The first step is Pretreatment, where wastewater passes through vertical bar screens that remove large objects such as sticks, rocks, paper (including baby wipes) and rags, that would otherwise clog plant machinery. Baby wipes are the Number 1 item that gets caught in their screens. Don't flush baby wipes! They clog the machinery! Wastewater then flows to grit chambers that remove heavier particles such as sand and gravel. These materials are taken to a landfill.
Vertical bar screens.
The next step is Primary Treatment, where the wastewater is pumped into settling tanks to sit for about two hours in order to remove 50% of the remaining solids from the water by gravity. Fiberglass bars, called flights, move across the surface to skim off fats, oils, grease and other floating debris. Flights gradually rotate from the top to the bottom and the settled solid particles are moved into the digesters.
Some of the settling tanks.
A sign at the settling tanks.
Crows and other birds perch on the railings to scrounge for seeds floating on the scum.
Some empty settling tanks.
A life ring. In case someone falls into the settling tank. Full of raw sewage.
Looking across the settling tanks to the digesters.
A secondary level of treatment, Biological Treatment follows, which uses naturally-occurring bacteria to decompose organic material and ammonia, producing a reusable biosolid. A total of 90% of wastewater impurities is removed. After 25 to 30 days in an anaerobic digester, biosolids spend a couple of years being solar-dried in large drying beds and then reused as landfill cover. The 16 digesters, each holding about 2 million gallons, have floating roofs that move up and down inside the tank. The digesting process produces methane as a by-product, which is used for energy production at the plant (35% of their energy needs are met by the methane). They are looking at ways of reclaiming even more energy from the treatment process and would like to be able to go off the grid entirely at some point, maybe even selling surplus electricity back to the power company.
Some valve machinery at the side of a digester tank.
Those black hoses coming out of the top of these digesters collect the methane generated by the digesting process.
The next step is Aeration. Aeration is a biological process that produces 95% clean water by pumping air into the flow. The oxygen-rich environment nurtures growth of naturally-occuring aerobic bacteria. 50% of the plant's energy usage goes to aeration. Three 4,000-horsepower electric motors power blowers that pump air (about 80 thousand cubic feet per minute each) to the aeration basins. Fifteen large engines power generators that produce between seven and eight megawatts of energy each month. Two turbo-diesel powered emergency generators (16-cylinder and 12-cylinder) are each able to generate 2.8 megawatts of electricity (enough for about 1,400 homes) in case the power goes out, because the treatment process cannot stop for any reason.
Inside the blower building.
Part of the complex's electrical plant.
Aeration tanks, full of bubbly bubbles.
Then comes Clarification, where wastewater flows into clarifier tanks. Over one to 3 hours, more solids, including the aerobic bacteria, settle out and are removed. The huge tanks are slowly stirred by long mechanical arms to assist the process.
A clarifier tank.
Looking past a clarifier tank to the landfill across the road from the plant.
Looking across a clarifier to the operations building. Yes, the complex's streets are named and numbered.
Filtration is the next step, where wastewater is run through a multi-layer filter bed of stone, sand and anthracite coal. Each of these filters is as big as an Olympic-sized swimming pool and removes small suspended solids from the water. Few communities undertake this tertiary level of water treatment, so it is considered an advanced step. It is necessary in this location because the treated water will be discharged into shallow waters with little tidal action.
Next is Disinfection, where wastewater travels through serpentine disinfection tanks for about 45 minutes. The wastewater is disinfected with gaseous chlorine to kill any remaining bacteria. About 10% of the treated water is further treated and recycled through South Bay Water Recycling pipelines for landscaping, agricultural irrigation and industrial needs. For the rest of the water, residual chlorine is neutralized using sulfur dioxide and it is then discharged into San Francisco Bay via Artesian Slough, adjacent to the Don Edwards San Francisco Bay National Wildlife Refuge. With 99% of the impurities removed, the water now meets or exceeds many drinking water standards and has the appearance of fresh water. Because tidal action is not very strong down at the south end of the Bay, it can take weeks or months before the treated water reaches the mouth of the Bay at the Golden Gate.
Artesian Slough, where the treated water is discharged, is full of wildlife, a testament to the water quality.
The plant is facing several challenges, one of which is the rising sea level. The plant is located at a low point at the edge of San Francisco Bay, and a one-meter rise in sea level means the plant will be underwater. Another challenge is the increasing population density. Yet another is balancing the needs of the plant's functions with the needs of the endangered species that live at or immediately next to the plant (burrowing owl, salt marsh harvest mouse and clapper rail). Yet another is meeting existing and future environmental regulations. And still another is the plant's own aging infrastructure. Some parts of it have been operating 24/7 for 55 years, and have never gone off-line, not even for maintenance. It could cost $1 billion for repair and rehabilitation to bring everything up-to-date.
Everything put down an indoor drain ends up at the plant. Several things are difficult to remove or neutralize, such as harsh cleaning chemicals, pesticides and fungicides, paint, solvents, and pharmceutical drugs. Some or all of them can end up in the Bay. The plant relies on consumers not to flush these substances down the toilet or put them down the drain, because they will only wind up being discharged mainly untreated into the Bay. So do not flush your unused medicines! Or baby wipes! And please conserve water.