3.3.5. Effect of the Organic Loading Rate

In general, the expected trend for *HRT* and *OLR* was confirmed in all reactors, as the biogas production increased when increasing *HRT* or *OLR*, with some exceptions: R1 during P4, probably due to the ammonia inhibition effects mentioned before. Also increases of *OLR* in R3 during P2 and in R4 during P2 and P4 did not increase the *MY*, probably due to the decreased *RR* in these reactors and periods. An increase of acetic and propionic acid concentrations was also observed with *OLR* increases, such as in R1 and R2 during both P3 and P4 periods.

### 3.3.6. Effect of Total Solid Content

As the *OLR* depends on *inlet-TS* concentration, similar performances were evaluated. At higher *inlet-TS* concentrations, higher *MY* values were obtained. The maximum *MY* obtained was in R3 during P3 (373.7 <sup>±</sup> 49.7 LCH4 kgVS−<sup>1</sup> ) with 6.81 ± 1.35 % TS. H.M. El-Mashad et al. [51] concluded that a higher TS content resulted in higher VFA concentrations. Although there was not a clear correlation in our results between *inlet-TS* contents and VFA (see Table 4), a similar trend was exhibited as the reported study of Page et al. [55]; when evaluating dairy manure effluent at 2.4% TS, a range of 89–439 mg·L −1 of VFA was obtained.

### **4. Conclusions**

A novel modified plug-flow anaerobic reactor for treating animal slurries was developed and tested successfully with appropriate performance. The mixing power inputs and the intermittent mixing pattern allowed for the avoidance of operational problems such as solid stratification, foaming, or scum generation. In general, there was no inhibition due to ammonia concentration, except for reactor R1 during P4. VFA generation allowed stable conditions. The methane yields were optimized during some of the periods, showing how significant the operational parameters evaluated are. However, it seems that complex automation and equipment design involves facing the important problem of the scale factor.

**Author Contributions:** D.G., J.L.R., B.F. and F.H. wrote the paper; J.L.R. and B.F. designed the experiments; D.G., E.M., L.T. and M.R. performed the experiments.

**Funding:** This research was funded by private funds from ProCycla SA, a privately held company. Research was conducted in IRTA—Institut de Recerca i Tecnologia Agroalimentàries (Spain). IRTA thanks the financial support of CERCA program of the Generalitat de Catalunya.

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **References**


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