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An anaerobic digester sees a doubling of alkalinity and volatile acids cut in half. This coincides with the use of salt on city streets in winter time, and also foaming in the anerobic digester. Has there been a study showing the relationship of salt and foaming in an anaerobic digester, and if so, what can be done?
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Answer #1
I know of no study or relationship between road salt and digester performance. We have anaerobic digestion and have never seen this.
I would suggest there is some other culprit, not road salt, like inconsistent sludge feed, highly variable Ph in the feed sludge, low alkalinity in the sludge. The operator can test the chloride level in his wastewater with and without the road salt events. He can also test the sludge for chloride levels in each case and compare the difference. Typically salt is dissolved in water and dissolved substances usually don’t accumulate in sludge.
MM - Mansfield
Answer #2
Have the facility test both the influent and effluent for TDS specifically looking for the level of chlorides. Have the test taken during dry weather to establish a baseline and then after a “washing” rain event to see if the TDS (chlorides) are escalated. Also, is the facility a combined sewer community or a separate sewer community?
TA - Warren
Answer #3
In making my response, I assume that this is an actual problem with an existing facility as opposed to a theoretical supposition.
Adding to TA's response, I have found that a good resource document is "The Microbiology of Anaerobic Digesters" by Michael H. Gerardi, 2003, SBN 0-471-20693-8. Table 17.2 indicates that sodium may have a toxic threshold of 3,500 mg/l. If they are using magnesium chloride for road de-icing, the toxic threshold for magnesium is 1,000 mg/l. TA is correct in looking at these parameters since there is suspicion between problems being faced and road de-icing. This may be a case where the primary cause of the problems being realized is due to toxicity.
Judging from the question, it appears that the affected plant is also seeing a toxic impact as a result of a change in the VA/Alkalinity ratio. Foaming is a clear manifestation of inhibition in the process microbiology, most likely in the acid forming process. Several follow up questions to the author include:
(1) has this change impacted gas production in a negative way?
(2) if the answer to #1 is yes, has there been a change in the persentage of methane?
(3) has the Volatile Solids Reduction (VSR) been reduced?
(4) are there mixing, heating, or sludge pre-thickening difficulties in the existing system, which would pre-dispose the digestion process to this problem? Poorly mixed and heated systems have dead spots, which may render them susceptive to problems.
(5) I am assuming that the sewer system is combined - not stated in the question.
Often digester upsets, which generate foaming, are attributable to increases in solids loading well above about 0.15 PPD VS/CF, which creates organic acid overload, which in turn consumes alkalinity, and hence pH suppression. However, this would increase the VA/Alkalinity ratio as opposed to decrease it assuming that there are no changes to the loading rate. DK, Columbus
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