After optimizing the amount of Magnesium hydroxide dosed to 75 gal/MGD, H2S levels were significantly reduced, averaging 9 ppm and peaking at 169 ppm.
Very safe and cost competitive option for hydrogen sulphide gas (H2S) control in wastewater treatment.
South Coast Water District (SCWD), located on the Pacific Coast just south of Orange County, provides water and wastewater services to around 35,000 residents, 1000 businesses and 2 million annual visitors. Regarding wastewater services, SCWD is responsible for managing and operating the sanitary sewer collection system comprising of 13 lift stations (LS) and around 140 miles of sewer lines. The sewer system removes around 3.2 million gallons per day (MGD) of wastewater and transports it to the wasterwater treatment plant (WWTP), which is operated by the South Orange County Wastewater Authority (SOCWA).
SCWD had been working with an external consultant to evaluate odor mitigation technologies suitable for its existing sewer system. Magnesium hydroxide liquid dosing was earmarked as a suitable technology and a decision was reached to trial it. SCWD asked us to supply the dosing product and equipment for the trial.
The main odor hotspot was downstream of lift station (LS) Y, around the long beachside tunnel. Dosing for this hotspot would be a large undertaking.
There was, however, also an odor issue downstream from LS X, with a much smaller flow.
It was therefore decided that LS X was more suitable for a trial project.
The trial project location of LS X and the monitoring manhole (end of forced main) are shown in figure 1.
The aim of the project was to dose magnesium hydroxide liquid to reduce the average hydrogen sulfide (H2S) levels below 10 ppm and confirm the product dosage rate in gal/MGD to reach this goal.
<== Figure 1
Location of dosing site and monitoring manhole
We took on the challenge to reduce the odor to the required level by dosing its magnesium hydroxide at LS X (figure 2).
Figure 2: Lift station X
Figure 4: Dosing pump and entry into wet well
The sewer flow rate at LS X was confirmed to be 0.1 MGD. As this was quite a small flow rate, the decision was taken to use a tote for site product storage in the trial project (figure 3).
Figure 3: Product storage tote
Dosing was carried out using a peristaltic pump. The dosing control was time based. The product was dosed directly into the LS wet well. Refer figures 3,4 and 5.
We had informed SCWD that the dose rate would be set high for the first few weeks and odor reduction would not be evident at the start. Sewer lines that have not been dosed in the past, have a propensity of FOGs (fat, oil and grease) accumulation. The magnesium hydroxide efficacy thus gets used for saponification of these fats before any odor reduction can be
We were also responsible for monitoring the H2S levels using an odor logger. Odor and sewage pH data was collected every 2-3 weeks (figure 6). This data was used to set and optimize the product dosage rate.
Figure 6: Odalog in the monitoring manhole
As can be seen in figures 7 and 8, the H2S levels before magnesium hydroxide dosing averaged 42 ppm and peaked at 1083 ppm.
Dosing was started at a high rate of 150 gal/MGD which reduced the H2S or odor level to an average of 15 ppm, albeit over a 2-3 week period. Thereafter magnesium hydroxide dosing was halved to 75 gal/MGD. The odor levels, however kept reducing, averaging 9 ppm and peaking at 169 ppm. The trial was concluded at this stage.
Figure 7 ==>
Dose rate and average H2S
The odors were succesfully reduced as per the project objective. There is a possibility that magnesium hydroxide dosing can be further reduced and optimized by synchronizing the dose pump with the LS pump. Setting up a permanent dosing unit would further assist in optimizing the process.
<== Figure 8
Dose rate and peak H2S
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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