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Operation and Maintenance
Anaerobic sludge digestion Magnesium hyroxide slug
We have a high rate mesophillic anaerobic digester (.35 MG) treating primary sludge. We recieved a slug load of Magnesium hydroxide (60%)(appox. 2500 gallons), The magnesium settled out in the primary clarifier, and was pumped to our digester. We pump about 10,000 gallons/day at 5% total solids
Before the discharge the volatile acids/alkalinity were at 120mg/l / 2500mg/l, pH was 7.0, in 4 days the volatile acids and alkalinity increased to 1250mg/l / 4500mg/l, pH is now 8.3
The CO2 has decreased from 33% to 13%. (The CO2 decreasing I find very strange), we measure our gas production daily and it appears to have decreased by more than 2/3 rds (pay attention cause there is lots more to the gas production part) We use a waste gas burner and boiler at our WWTP both of these system turn off and on based off digester gas pressure (WGB on at 8.9" off at 8.2"), about 12 hours after the discharge our gas pressures dropped from about (the pressure is in constant flux due to the WGB & boiler) 10"-7" to 8.8"-8.0" Does the high methane content digester gas burn faster, and since the OFF/ON cycles are based off of pressure does that even matter? I think the decrease in gas production (or appearance of a decrease in production) also might be cause by the boiler and WGB burning for a much shorter duration (watching the computer that trends this info, it took about 10 minutes from the time I could watch with my own eyes, the WGB start before it registered on the computer??? It almost looks like the trending on the computer is not registering the first 10-15 minutes off operation of the WGB. Normal gas production is about 30,000 cubic feet, now it's at 10,000 cubic feet. Today I changed the WGB on/off set points from on at 8.9" to 9.0", off 8.2" to 7.5", to allow the gas to burn longer to see if the computer would register, in five hours I had already had 5000 cubic feet. Right now I'm not sure if the magnesium caused a decrease in gas production, (that would tell me a lot about what is really going on in the digester), or if the meter or computer is for whatever reason is not recording all of the flow. the boiler only uses about 10% of the total digester gas, so that why I did not put any info about it.
At present I don't plan on making any process changes, other than monitering volatile acids, alkalinity, CO2, and pH. I guess my questions are these.
1. Did the high pH cause either the acid formers or methane formers a biological advantage?, are the methane formers not working? (methane % is increasing!!!) Are they both working and for right now the acid formers are just out pacing the methane formers?
2. Why did the CO2 decrease???
3. Is the gas production really decreasing or is something else going?
4. Ever hear of an anaerobic digester getting hit with a large dose of Magnesium hydroxide???
5. The magnisium added double the alkalinity to the digester, so why get worried at the current level of volatile acids?
6. Should I let the volatile acids to continue to increase to drop the pH back to normal? (that seems kinda dangerous)
If anyone has any idea's, I would like to hear them,
I am having trouble logging in right now so I do not know if this will post.
There are a couple of papers you can find that will help explain #1 and 2.
#4: The City of Fort Collins WWTP uses MgOH and on occasion has had to drop a tank for maintenance. They would see an increase in gas production relatively quickly
#5-6: As I am replying some 5 months later -- I am sure it is a moot point to make a statement, though I would like to know if/when things returned to normal.
Feel free to contact me if you wish
Just a word of caution. Magnesium is the limiting nutrient in the formation of struvite.