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Tijuana Sewage Explained

Steve SmullenWater, plants, birds, coyotes and sewage. All these things still get shared between the U.S. and Mexico regardless of whether you put up a fence or not. AND when it comes to sewage — It’s boring, it’s stinky and it’s technical, but what happens at the sanitation plant in our neighborhood is also vital for our health and safety.

Steve Smullen gave me a tour of the treatment plant whose only purpose is to capture 25 million gallons of Tijuana sewage per day and then treat it for bacteria so that it poses no danger to the public. Once treated with very harsh chemicals, the sewage is sent elsewhere. That’s it. That’s all they do. But that also happens to be a large operation — to the tune of $90 million or the cost of the treatment plant in the 1990s.

The IBWC’s Role In Sewage

The IBWC’s big concern is flood control, not just in San Diego but throughout the 1,952 miles of border. In Texas, for example, the IBWC has many levees. The border area in the Tijuana River Valley used to be all agricultural land, but in the 1990s there was a large public outcry about the raw sewage flow in the river. High bacteria counts and apocalyptic-looking waste along the TRV roads pushed the IBWC to build a treatment plant. The land in the Tijuana River Valley was then bought by government entities and today is mostly county and state parks.

Steve Smullen, area operations manager in the San Diego office of the IBWC, showed me a map of the Tijuana River Valley, which is a flood plain. The IBWC bought the property from the Hofer’s and Nelson & Sloan. Today, some of the area is leased to AMSOD, a company from Escondido that grows sod near the sanitation plant. They are the ones who keep the flood plain clear. The IBWC also has an MOU with Border Patrol to keep this area free of vegetation.

Currently, the IBWC field office has four people, including Steve, who are in charge of administrative duties such as making sure the permits are complied with. They also work with Veolia North America, a French company that contracts to run the sanitation plant.

Aerial Map Of BorderSewage flow in Tijuana goes to the lowest point in the city, which happens to also be at the border. Mexico currently has a pump station at that point. They pump their raw sewage from that point over the hills to their treatment facility, which is six miles south.

Before the sanitation plant was built, any time there was a failure in that station, the water would go into the U.S. river. The U.S. then had an emergency pump connection that was actually a connection between Tijuana and the City of San Diego. We could, prior to the construction of the treatment plant, send waste water to the city of San Diego (Point Loma) to be treated. We did that for many years and it was very expensive. The taxpayers paid for it.

In the 1990s, federal taxpayer money paid for the plant and ever since, the agreement has been that the U.S. takes 25 million gallons of waste from Mexico per day and treats it. That’s about one half the daily amount of sewage produced by Tijuana.

The Brain Center

Veolia has about 23 employees who monitor the sewage via computers. Few employees are at the actual stations where the machinery works. Everything nowdays is automated.

Computers display the sewage in the pumps and the flows. A typical average flow is 25 million gallons. In the afternoon, it can push 40 million gallons, then at night it can drop to 15 to 20 million. It all averages out at 25 million gallons, which is the permit limit given by the state.


Outside the brain center the first thing to notice are the scrubbers that treat the air in the buildings, so that it doesn’t disturb residents within the vicinity.

Four of these odor control facilities exist on the property, which are towers packed with plastic media. What happens is, the air goes up and there’s a solution of chemicals that goes down and strips out the hydrogen sulfide from the air, so the air discharged to the atmosphere is fairly clean.


Mechanical Bar Screens

Mechanical Bar Screens

An underground pipe runs from Mexico into a building called the headworks. The pipe is deep underground. Each morning at 7 a.m. amounts of the sewage are captured in a bottle that then goes to a lab. Samples are taken on a daily basis and tested for many constituents. If the concentrations are very low or high we know we’re getting something really odd. Acid, metal plating, discharges from the food industry, or things that have a high biological content.

The sewage first goes onto mechanical bar screens. The mechanical bar screens at this plant happen to be some of the tallest in the industry. They do primary treatment, aka getting rid of the visible solids in the waste water. There’s a rake that lifts the garbage up into a conveyor. The conveyor brings the solid waste across the building and dumps it into a bin. All that solid waste then gets placed into a truck that gets hauled into Mexico.

Two gates allow the trucks to exit into Mexico. Veolia will open a remote gate, wait for the driver to leave and then close the gate. When the driver comes back, Veolia has to come out and open the gate again.

After the primary solids are taken out, the remaining liquid waste goes through a sanitation process whereby all solids are extracted.

Primary solids are what you flush down your toilet. Secondary solids are aerobically digested. Primary solids are heavy and dense. Secondary solids are fluffy and light. They differ in consistency. The secondary solids are more stable, while the primary have no real treatment whatsoever.

Grit Chamber

The grit chamber is totally covered and has raw sewage inside. It’s very large and there’s some blowers located at the basement of the building. They blow air into the grit chamber, which makes the waste water flow in a circular motion. The grit then drops out into hoppers and pumps take the grit out of those hoppers and delivers it back to the headworks.

Primary Sedimentation Tanks

Primary Sedimentation TankMeanwhile, the remaining liquid gets further treatment in the primary sedimentation tanks where Veolia injects ferric chloride, which is a coagulant. A piece of metal moves along two chains. The metal moves very slowly and collects the floatables at the far end of the tank. The metal rotates around like a circle, going to the bottom of the tank and, on the return, scraping the bottom of the tank. The solids begin to settle out on the bottom. Those solids are then pumped over to the two unstabilized sludge storage tanks.

Skimming Wet Well

While the solids settle to the bottom, the liquid gets pushed over into a trough and gets sent to the skimming wet well.

This is advanced primary effluent. Until 2010, this was going into the ocean. Today, however, the liquid goes into a channel and flows into a secondary plant.

Secondary Treatment Plant

Secondary Sedimentation TanksHere, you’ll see a vast stretch of aeration basins that look like thermal spas. It’s actually mixed liquor suspended solids. This aeration process takes the bacteria out of the liquid.

On the bottom of each of these tanks there’s an array of ceramic diffusers. The bottom of the whole tank is covered with them. There’s a blower in that building, blowing air through the pipes and the air is bubbling up through the waste water. What that does is provide an environment for aerobic bacteria to grow, which is capable of breaking down the waste water. So the aerobic bacteria is what’s doing the treatment and the air enables them to grow. This recirculating liquid foam is like detergent going through the liquid.

Interestingly, Mexican sewage is a lot stronger than U.S. sewage because their per capita use of water is extremely low in comparison, so their wastewater is twice as strong as ours. That means, there’s more cost associated with treating the sewage. The equivalent volume of sewage would produce less solids in the U.S.


Covered channels are everywhere at the plant. They transfer the sewage from one destination at the treatment plant to another. The last channel sends the sanitized water to the “outfall”, meaning a pipe that goes to the ocean.

Meanwhile, large tanks pull return activated sludge or the treated solids into either a thickening facility or back to the aeration basins to help the bacteria grow and further clean the sewage.

Dissolved Air Flotation Thickeners

Aeration TanksThe solid waste from the secondary treatment plant is now fluffy, so it must go to a thickening facility. You have to thicken up the secondary solids so they mix well with the primary solids before being shipped off.

This fluffy waste gets pumped into a tank called a dissolved air flotation thickener. Here, water and bubbles are injected, making the solids float. A skimmer moves in a circle and collects all the floatable solids and pushes them into a trough. That’s what becomes the thickened solids.

These get pumped into the unstabilized sludge storage tank. Those then pull the sludge from the tanks and inject it into a pipe which goes to the solids dewatering facility, the smelliest building they have. The building is also filled with the scent of ammonia.

Here, several belt filter presses squeeze the water out of the solids. The solids coming off the press are called cake.

From Lime Silos To Dump Trucks

Trucks To MexicoThe solids are lastly mixed with lime from lime silos. The solid waste then goes into the trucks along with the primary solid wastes.

Why is there still a sewage problem?

Because 1.6 million people live in Tijuana. The city continues to grow, but officials can’t keep up with the infrastructure needs of the population. There’s also clandestine dumping. Additionally, putting up fences impedes the IBWC’s work.

The on-going solution? AWARENESS

The U.S. and Mexico have engineering solutions and they have treaty obligations that the IBWC tries to meet. The most important part of sewage problems, though, is cooperation.

Thank you, Steve, for the tour!

(See Part I of this story.)

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4 Responses to Tijuana Sewage Explained

  1. Gil April 25, 2015 at 12:28 AM #

    Sure is a lot more to a sewerage treatment plant than I ever envisioned. Thank you for posting.

  2. bzzaragoza April 25, 2015 at 4:58 PM #

    Sewer is probably the least reported, yet most necessary part of a city to understand, wouldn’t you say? Thanks, Gil!

  3. Gil April 25, 2015 at 5:36 PM #

    Totally agree with you reply. People have no idea.

  4. Sandra M Solano February 15, 2017 at 7:12 AM #

    I am a new homeowner at the Coral Gate Community which is nearby and the smell is horrible especially at night. I now understand why, I just wish I would’ve known this before. This has been very informative. Thank you.

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