Hydrogen Sulfide (H2S) in wastewater odor control case study

El Centro - Southern California, United-States

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Magnesium hydroxide ( Mg(OH)2 ) reduces the odor to manageable levels at El Centro, Southern California, United-States.

Case study summary


El Centro, a city 100 miles east of San Diego, was experiencing significant issues with odour control in its sewer system, despite many different attempts from different chemical dosing suppliers to resolve the issue.

Rotten egg gas (H2S) measurements at some man-holes in the system were exceeding 4000ppm, which is both highly dangerous as well as highly corrosive to the system.

  • Control pH and reduce odor along the Imperial Valley Mall sewer line.
  • Reduce potential hazard to El Centro residents from excessive H2S emissions.

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Magnesium hydroxide ( Mg(OH)2 ) reduces the odor to manageable levels at El Centro, Southern California, United-States.

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Benefits of Magnesium hydroxide ( Mg(OH)2 )

Broke-up a fatberg and cleaned the line

Significantly reduce hydrogen sulphide levels from 3000 ppm to 70 ppm


El Centro is city 100 miles east of San Diego, which sits on the Colorado canal system.
The city has a population of around 45,000 and runs its own wastewater treatment plant. Chemical dosing for odor control is carried out at a few lift stations.

Sewer line 3, designed to transport waste from Imperial Valley Mall, was particularly known for extreme odor issues. The city staff had unsuccessfully tried a variety of odor mitigation systems.


Sewer line 3 is a 12 inch diameter line that starts from the Imperial Valley Mall and increases to 27 inches by the time it reaches the lift station. It was designed to serve the projected population growth around the mall, however, this growth never eventuated, making the line too big for the relatively small volume sewer flow.

The first and only lift station, E Villa Road is positioned roughly 2 miles from the start of the line. The retention time from the lift station to the treatment plant is roughly three days with a flow of 0.13 million gallons per day (MGD). While the retention time in the gravity flow up to the lift station is estimated to be around four days. A reverse syphon, upstream of the lift station adds to the challenge.


Due to these factors very high H2S readings, around 3,000 ppm, were measured in this sewer line.

The city tried dosing various chemicals to mitigate the odors, including enzymes and bacterial treatment products. However, these methods were unsuccessful and only marginally reduced the H2S.


Calix took on the challenge and committed to reduce the odor to manageable levels. The solution was to methodically and consistently dose Calix’s  magnesium hydroxide liquid at strategic locations. Due to the unusually long retention time, the benefits of dosing magnesium hydroxide for odor reduction were not be apparent for some weeks.

The magnesium hydroxide dosing started at the E Villa Road pump station at the end of January, 2019. The odor level before dosing magnesium hydroxide was very high (Fig 3). A few weeks of shock dosing at a higher dosing rate of 80 to 100 gln/MG was carried out. This was followed by dosing in the upstream manhole along with the dosing at the lift station.


After commencing magnesium hydroxide dosing in early February 2019, there was a sharp reduction in Peak and Average H2S levels over the first 4 weeks, but the levels were still too high. However, after the fourth week, soap bubbles were observed downstream of the receiving manhole, evidence that the efficacy of the magnesium hydroxide was being used to saponify the accumulated fats. The peak odor level had reduced to 1/3rd by end of 8th week. Around the 9th week a yellow precipitate started to flow out, which on aboratory testing, was found to be fats like glycerine.

The H2S levels continued to drop over the 8 weeks, the odor graph from end May, shows odor to have reduced below below 100ppm. The magnesium hydroxide had effectively cleaned the accumulated fats in the sewer pipe as well as significantly reduced the odor.


The odors have dropped significantly. H2S levels have continued to remain at manageable levels as long as magnesium hydroxide is dosed consistently at strategic points.

The City of El Centro is of the opinion that,

“Magnesium hydroxide Technology has been applied successfully in sewer line 3. The dangerous levels of H2S have been greatly reduced and we now have a safer sewer system. The success of this project lies of course in the product, but also equally in the high level of customer service, sincerity and professionalism practiced by all Calix personnel.”

Magnesium hydroxide ( Mg(OH)2 ) for Odour Control

Magnesium hydroxide has a higher neutralising value per kilo when compared with caustic while being significantly safer to handle than other traditional alkalis, making it a very safe and cost competitive option for hydrogen sulphide gas (H2S) control in sewers.

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