## 4 Conclusions  
The present study contributed to the identification of a relationship between the biodegradation of organic matter and the texture of a sandy soil typically used in SAT applications, characterized by the pore size distribution; mean pore diameter, and water-filled pore space (WFPS) of the soil. These parameters were selected as evaluation criteria for degradation since they describe both limiting factors in the microbiological development: available soil moisture (responsible for cell desiccation stress when too low) and aeration (supply of oxygen).  
The highest biodegradation rate of organic substances percolating under specific experimental conditions (infiltration cycle, water quality, temperature) through the columns filled with the selected soil was obtained from the grain size fraction 630 \(\upmu\)m 1000 \(\upmu\)m. This is characteristic of the mean pore diameter of 230 \(\upmu\)m and a rather wide range of pore sizes (pore size distribution index n = 4.91) and a WFPS of 58% estimated in the dry phases of the infiltration cycle. The high availability of oxygen and nutrients, and the sufficient soil moisture favored the microbial activity and the corresponding metabolization of DOC. By contrast, these processes were partially limited in the smaller fractions (due to insufficient supply of oxygen and nutrients) and the largest fraction (low moisture content).  
The values obtained for the selected sandy soil and under the present experimental setup confirm the results reported by previous studies where conditions for microbiological activity were optimal at WFPS of 60%. Contrary to previous investigations, results of the present experiments demonstrate that optimal conditions can be achieved in the soil fraction 630 \(\upmu\)m to 1000 \(\upmu\)m with an average pore diameter of 230 \(\upmu\)m. In the case of embedding sand with very low organic matter in the shallowest soil layer of SAT in filtration basins, it is therefore recommended to use the fraction with grain sizes around the range of 630 \(\upmu\)m to 1000 \(\upmu\)m. When water is infiltrated into natural sandy soils, the recommended pore diameters are between 90 \(\upmu\)m and 320 \(\upmu\)m and the mean pore diameter is about 230 \(\upmu\)m.  
The outcomes of this study can be very useful for planning the infiltration of water with high organic load. In this case, biodegradation processes can be enhanced by providing adequate aeration, nutrient supply and soil moisture content in the shallowest soil layer. The results help in the selection of the granulometric characteristics of the soil filter in order to ensure optimal conditions for purification processes.  
Conceptualization, T.F., C.S.; Investigation, T.F., N.G.; Methodology, T.F., N.G.; Validation, T.F., C.S.; Writing original draft, T.F.; Writing, review & editing, T.F., C.S.  
Figure 4: Correlation between experimental biodegradation rates and WFPS in the used grain size fractions.  
**Funding:** This work was supported by the German Federal Ministry of Education and Research (BMBF), grant no. 01 LN1311A (Junior Research Group \"INOWAS\").  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.