pH control in secondary clarifier with Mg(OH)2 2

[Bartosz Adamas]

Hi All,

We are thinking about using Mg(HO)2 powder to elevate alkalinity and pH in our secondary clarifier (refinery's wastewater treamtent plant). Can anyone share their experience with Mg(OH)2 in wastewater treatment.

Some additonal questions:

Where woud you dose it ? - DAF outlet ? nitrification / denitrification chamber ? secondary clarifier ?
Will it generate any addiotnal issues ?


I apprieciate your help,

Kind regards

 

[bimr]

Magnesium Hydroxide [Mg(OH)2]. Also commonly referred to as mag, this chemical is effective in neutralizing acids and has been pushed by some chemical companies. Despite some attempts to advertise it as such, mag is no treatment panacea. As with lime, magnesium hydroxide is more economical to use than caustic (NaOH), however, the difference is not significant unless substantial volumes are in use. As with lime, magnesium hydroxide is also much more difficult to handle than caustic (NaOH).

Magnesium hydroxide is relatively insoluble in water at neutral pH values and higher. Given this, mag has little or no effect on water alkalinity above a pH of 7.0. This means that even in a poorly designed system mag will not raise the pH above a pH of 7.0, rendering this chemical safe to use from a control point of view. Additionally, magnesium hydroxide, also known as milk of magnesia, is quite safe to handle and is not normally toxic.

The characteristic that limits mag’s ability to raise the pH above 7.0 (solubility) also contributes to very long reaction times. Typical reaction times for complete neutralization are in the order of 90 minutes. This means that a single reactor tank must have the capacity of 90 minutes of flow. In a 100 gpm system, for example, the first stage reactor must be approximately 9,000 gallons in volume. The alternative is to discharge with an incomplete reaction. This means that the pH will continue to change as the effluent is discharged (never favorable) and an excessive amount of mag must be used. Also as a result if solubility the use of magnesium hydroxide will significantly increase solids loading in the effluent.

Also as a result of solubility, magnesium hydroxide is difficult to handle. Magnesium hydroxide is a slurry that will rapidly separate from solution. The storage tank must be constantly agitated and chemical delivery lines must be kept in motion. Typically recirculation loops are employed with a metering valve, inline, for chemical delivery. Static lines are not acceptable because the slurry will separate, and lines will plug, over time.

There are a few applications where magnesium hydroxide will produce favorable results, however, due to its solubility and long reaction time, magnesium hydroxide is not normally a good choice.

Digital Analysis

Because of the long reaction time, you would have to inject into your aeration tanks.

 

[pierreick]

Hi,
Like Lime, Mag will make a mess of your wastewater plant. You will have spills all over the place, make it very dirty and difficult to maintain.
Why don't you go for caustic soda solution? Kinetic is much better, same for handling (liquid), better control on flow measurement.
My 2 cents
Pierre

 

[Bartosz Adamas]

Hi,

Bimr and pierreick thanks for your comments and opinions. I will definitely reconsider using Mag to adjust the pH.
Speaking about low pH in the secondary clarifier, in your experience, what is the most probable cause for the drop in alkalinity and then in pH in the secondary clarifier. My setup is as follows:
DAF -> denitrification -> nitrification -> pressure tower -> secondary clarifier
The role of the tower is to degas the sludge so technically it should be a little heavier and should stop it from floating in the clarifier.
The issue that I’m having is the pH drop from approx. 7,4 in DAF out to 5,8 in secondary clarifier out. I am suspecting accelerated nitrification that is causing low alkalinity and in turn low pH. What are you thoughts ?
I appreciate your help

 

[bimr]

During nitrification, 7.14 mg of alkalinity as CaCO3 is destroyed for every milligram of ammonium ions oxidized. Lack of carbonate alkalinity will stop nitrification. In addition, nitrification is pH-sensitive and rates of nitrification will decline significantly at pH values below 6.8. Therefore, it is important to maintain an adequate alkalinity in the aeration tank to provide pH stability and also to provide inorganic carbon for nitrifiers. At pH values near 5.8 to 6.0, the rates may be 10% to 20% of the rate at pH 7.0. A pH of 7.0 to 7.2 is normally used to maintain reasonable nitrification rates, and for locations with low-alkalinity waters, alkalinity is added at the water resource recovery facility to maintain acceptable pH values. The amount of alkalinity added depends on the initial alkalinity concentration and amount of NH4-N to be oxidized. After complete nitrification, a residual alkalinity of 70 to 80 mg/L as CaCO3 in the aeration tank is desirable. If this alkalinity is not present, then alkalinity should be added to the aeration tank.

CWEA