I'm trying to analyze the operation parameters of an anaerobic bio gas digester related to urban sludge. there is a two-stage digester system including recirculation system and has not any mixer. To analyze TS, VSS, VFA and VS changes, the inflow sludge should be compare with which type of outflow sludge (before/ after post thickening or after dewatering)? the recirculation system should be turn on or off? What are the details on sampling? If any one suggest me a reference, I would appreciate it
not very sure if i understood your process well, but if you want to see the performance of the digester I would do the tests on the sludge that exits the system, prior post-treatment, and compare it with the parameters of the sludge that comes in. The tests you want to carry out should always be related to you want to investigate, but generally speaking pH, temperature, maybe heavy metals, phosphorus and nitrogen may also be worth measuring.
If the goal is to document the VSS degradation of the digester, you should measure the Influent to the digester and effluent before post thickening. You must take several samples (I.e once per day) over a period of at least equal to 3 times the HRT - which means a long sampling period ! and then compare the average in fluent with average effluent.
If you want to analyz the performance of the digester you need to estimate TS,VSS,VFA in the influent and effluent , and you can investigate the effects of these parameter in the influent with the production of bio gas
It is not very clear from what you have posted whether the digester is completely mixed or not. Recirculation within the first stage would induce mixing. If second stage is being used for thickening and the first stage is completely mixed, you can measure VSS, VFA, TSS in the influnet and effluent of the first stage to measure the performance of AD process. Digestion, if any, in the second stage may be neglected, being much smaller in comparison with that in the first stage.
The schematic structure of the two-stage digester system is attached. At the time of sampling, the circulation system related to second digester should be turn on or off?
If the Recirculation is running permanent during normal operation it should be on also during sampling.
Looking to the schematic you have sent, it seems that the second digester is not simply for layer separation and thickening. In such a case I assume that there is no loss of water (supernatant) until the sludge is taken for thickening. Thus, keep circulation on in both the stages (until the contents are completely mixed) and analyse influent, effluent of D1 and effluent of D2 for various parameters to find overall and stage-wise efficiency.
Dear Maryam,
VS/TS/COD (whatever parameter you rely on) of the influx material should be measured before entering the system. The same parameters should be measured in the native outflow sludge (before post-treatment). By comparison of both data sets combined with the outcome of your biogas yield/composition you should be able to characterize the performance of your system.
Two small pieces of advice from my own experience.
1. Measure VFA not too long after sampling. Another option is deep-freeze the sludge sample right after taking the probe. But even after deep-freezing don't take too much time for measuring your samples (after 7-14 days should be still fine) because VFA are volatile and concentrations in the sample change quickly.
2. Make sure that the samples you rely on come from a thoroughly mixed bioreactor content or that the bioreactor content is at least thoroughly mixed before sampling. Always condisder: The probe will be only as good as the sampling conditions were set and followed. Otherwise you will have falsified and not very meaningful measuring results.
Good luck for your further research!
Best, Yann
Dear Maryam
all answers are good and I recommend Yann in his answer.
Best Wishes
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