I have a small package plant that only does 11,000 gpd. My pH in the 2 aeration zones is down to 5.4. I'd like to raise it & heard that soda ash is good for this.  I have 2 500 gal. mixing tanks with mixers along with metering pumps.  Is there a formula for figuring how much soda ash to mix with water & how much to set the metering pumps for dosing?

thanks......... b

Dogs:

I have a surface water plant. We used to use soda ash before switching over to a caustic soda. Although we used a Volumetric Dry Feeder, Orenda's solution procedure works just the same. You will notice a coating forming in the tanks all around the edges and even on your mixers. I wouldn't let that get to thick before cleaning it off. It won't affect your treatment, but it's kinda troublesome. It will cling to your mixers shaft in the water especially, and weigh it down considerably. That will put strain on that motor. Always keep those mixers going. We even ran our mixers with the plant off. Good luck! Since you have a small storage tank, and a metering pump you could even try caustic soda. It's a little dangerous. A lot of saftey involved but you don't have to deal with bags or mixing. We get it just a little cheaper than soda ash here. So expense wise probably wouldn't be that different for you.

Yes, yes, and yes.    If you pH affects the activated sludge, you have the same microbes in the anoxic zone as in the aeration zone. pH and alkalinity control is important in anerobic treatment. Caustic soda is commercially available as a liquid in concentrations up to 50%, but 25% solutions have the minimum freezing point. Flake caustic soda (sodium hydroxide) is also available, but the solutions are usually easier to use.

With the low flow rates, manual control, and lack of instrumentation, soda ash and Orenda's advice will be less likely to cause you control problems.

grrun  

I have a package plant which averages 100,000 GPD it consists of ( 2 ) separate trains. Each train has ( 3 ) anoxia zones and ( 3 ) aeration tanks. I add ( 1 ) 50 pound bag of
soda ash to the 2nd aeration tank and to the 2nd anox tank by just pouring it directly into each tank slowly which takes me approximately 3-5 minutes per bag. My pH raises 3 to 5 points by the next day, You may want to try this, also remember the plants pH is usually the last indicator of other more serious problems so you may have other issues to address also.

Dogs,
Oxidizing an influent NH4 of 66 to 77 is going to chew up about 500 mg/L of alkalinity.  The amount of denitrification you indicate will recover about 180mg/L.  You need to target an effluent alk of 50+mg/L.  The influent pH of 6.8 indicates a lowish alk.  If you know your influent alk then you can work out the dose rate if you know the equivalent strength (do a google for this) of soda ash to bicarbonate alkalinity.

Geoff

Dogs,
An influent alkalinity of 120mg/L is quite low.  Most sewage I deal with is about 260.  Catchments with water drawn from the ground that has lots of limestone gets to around 330.  Thus pH correction is often not necessary.

I refered to bicarbonate alkalinity as the standard test measures the buffering capacity in terms of bicarbonate; you can use a whole range of pH correction chemicals. Such as:.

Limestone (CaCO3)                           1.00*
Magnesium oxide (MgO)                           0.40*
Slaked lime (Ca(OH)2)                            0.74*
Caustic soda (NaOH)                            0.80*
Quick lime (Calcium oxide) (CaO)             0.56*
Sodium bicarbonate (NaHCO3)                 1.68*
Ammonia gas (NH3)                          0.34*

*Chemical equivalent (mg) to 1mg of CaCO3

Cost effectiveness, handling and safety issues should be the guide.

Geoff


PS
Once nitrification/denitrificaion stablizes you can trim the dose rates to get the alk down closer to 50.  It doesn't hurt being above 50, it's just to make sure you don't waste $.  But keep it above 50!!

G