UDR Material Flow Clarification
The low-density, undigested substrate in the upper part of the U-Tank is the feedstock of the R- Tank. This material flows directly into the R-Tank via overflow pipe. After further anaerobic digestion inside the R-Tank, all material from there is returned / recirculated into the U-Tank where it is mixed with fresh feedstock in the bottom area where the agitator is located.
Low density, yet undigested organic material (“molecules”) have the tendency to float upwards or simply remain in the upper section of the D-Tank. That’s why all that material is transported / forwarded into the R-Tank, making (at least) one “loop” through there before being returned to the U-Tank again. Only relatively heavy / high density sludge with little or no remaining degradability (and therefore little or no gas bubbles attached to it) sink down to the very bottom of the Downflow tank where they are pumped out as digestate sludge.
The small agitator on top of the D-Tank is rather a “nice to have” homogenisation device and by far not as important as the agitator at the bottom of the U-Tank. In fact, there are UDR systems that don’t contain this mixer.
Usually, the U- and D-Tanks are laid out in such a way that the incoming material (i.e. the blend of fresh feedstock and returned material from the R-Tank) has an average retention time of about 1-2 days in each tank, i.e. 2-4 days in both tanks. For example, if we feed 50 tons of feedstock blend (e.g. comprised of 25 tons of fresh feedstock and 25 tons of material from the Reflow-Tank) into the bottom of the U-Tank, the average hydraulic retention time is (65 x 2) / 50 days = 2.6 days (without considering biologic degradation, that makes the retention time higher, in fact, as some of the blend is converted into biogas). However, this average retention time is really just a layout value and does not reflect the fact that yet undigested material will “loop” through the U- and D-Tanks into the R-Tank and – if still undigested after the first loop – loop through the UDR again until it has reached the heavy sludge characteristics described previously so that it is ready to settle in the lower part of the D-Tank to be taken out as digestate.
Reflow Tank Agitation:
As in every digester, homogenisation is crucial for ensuring high efficiency of anaerobic digestion, that’s why agitators are necessary in the R-Tank as in every other ideally mixed digester vessel. The organic material which is not yet fully digested in the R-Tank (and the subsequent U-Tank) after about 30 days of retention time will “loop” again into the R-Tank so that it has the chance to be fully decomposed into biogas and inactive sludge that will later settle down in the D-Tank and be taken out from there in the form of digestate.