I'm now studying anaerobic sludge digestion and have started the two-phase reactors, but the sludges in the fermentation reactors still have pH around 6, while it should be around 5.
Here are some details of the reactors:
1. They are 1-L bottles with working volume of 500ml;
2. There are two ports, one for feed & waste and the other one connected to a gas sampling bag;
3. The bottle reactors and the connections are all air-tight (tested and checked for many times);
4. They are semi-batch reactors with SRT=4 days, mixed by shaking, and one is operated at room temperature, the other one at 15℃;
5. After operated for a month, now pH (Reactor 1, 20℃)=6.10, pH(R2, 15℃)=6.35;
6. Feed & waste is done everyday with a syringe through the feeding tube, which is introduced to the bottom of the reactor.
I once tried to adjust the pH to 5 when feeding, but it didn't lower the pH as much. When first started, the original pH was around 7 and it did drop at the beginning. Now I'm very confused and don't know what is wrong with my operation. I'll be so grateful if someone can help.
How new is the microbial community you inoculated with? Sometimes these communities take a while to adapt.
In the low temperatures, bacterial activities drop. So you need to give more time for culture adaption. Moreover, if you don't get biogas as much as you expect (based on the sCOD measurements), it may indicate that the first stage of the AD process (Hydrolysis) is not working actively, which can be related to the substrate.
Check the C/N ratio of the substrate so you may need to increase/decrease the load.
You can increase the SRT for a short period to see if the pH drops to 5 or not. If the pH drops to 5, it means the substrate needs more time to digest in the selected temperature.
Thanks a lot for answering! Matthew B Paddock Seyedbehnam Hashemi
The bacteria in fermenters, which do hydrolysis, acidogenesis and acetogenesis, typically need 3-4 days SRT at 35 ℃. And I did find COD accumulating in the fermenters but not in the secondary reactor. I think this is resulted from low temperature. Therefore, now I've increased the SRT of fermenters to 6 days and I hope their performance would be better.
ps. The C/N ratio was measured and the organic load was also adjusted for the reactors I am using now. And the gas production of the methanogenesis reactors are OK now. I'll keep them monitored.
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