THE PROBLEM
Buffering is need to prevent souring. If a digester does not contain enough buffering capacity, the pH of the sludge will drop. This harms the methane formers because they can only reproduce in a pH range of 6.6 to 7.6. If the pH falls too far, methane production will taper off and the digester is said to be “sour” or “stuck”.
Souring
Key Indication: VA/Alk > 0.1:1, pH 7.0 – 7.2
Possible Cause: Excessive feeding of raw sludge, sludge withdrawal rate too high, shock-load.
Stuck
Key Indication: VA/Alk >0.5:1, pH <7.0 and dropping
Possible Cause: Not correcting a “souring” digester.
Balance
The Volatile Acid/ Alkalinity relationship (VA/Alk ratio) is out of balance. As long as volatile acids remain low and the alkalinity (buffering capacity) remains high, the digestion process will remain stable. Each treatment plant has its own acceptable ratio of volatile acids to alkalinity. However, this ratio is usually less than 0.1 part volatile acids to each 1 part alkalinity (10 times as much alkalinity as volatile acids). A CHANGE IN THE VA/ALK RATIO PROVIDES THE FIRST INDICATION THAT SOMETHING IS WRONG WITH THE DIGESTER. For this reason it is imperative that the VA/Alk ratio remain consistent.
THE SOLUTION
The Microbial Energy Device (MED) was designed to buffer Va's so the VA/ALK Ratio remains consistent. The MED buffers the VA's because the MED uses an Electrogenic Bacteria (EB) with microbial electrochemistry to consume and control VA’s. The MED produces hydrogen which in turn is converted to methane (RNG) in an anaerobic digester. RNG production is then increased up to 5-fold by using electrochemistry. The threshold rate is no longer a factor because the VA's are simply converted to methane. With the MED, we can increase the loading rate to increase VAs which increases RNG production. The MED removes the acidic VA's from the digester acting to buffer the reactor while it converts the acid to more methane thus producing more RNG. The MED enhances the economics of RNG production enabling smaller, less expensive digesters to produce more RNG, lowering both capital expenditures and increasing revenue. The MED is designed to retrofit existing anaerobic digesters and is applied as an exterior solution. It can also be incorporated inside new digesters.