## Materials and Methods  
### Experimental Set-up  
Laboratory experiments were conducted to analyse the correlation between four grain size fractions of a selected sandy soil and the biodegradation rates of organic substances. The selected fractions are: 125 um to 200 um, 200 um to 630 um, 630 um to 1000 um and 1000 um to 2000 um, which represent different soil textures that are characteristic to filter layers in SAT facilities. The selection considered the proportion of different pore sizes, mean pore diameter as well as WFPS. After sieving, the fractions were sterilized and inoculated with _Sphingobium yanoikuae_ (DSMZ-German Collection of Microorganisms and Cell Cultures, registration code 7235), a bacterium occurring naturally in soils. To achieve a homogeneous distribution of the bacteria in the soil, the inoculated solution was mixed with the soil for 24 h using an overhead shaker at 5 rot/min. After mixing, the fractions were dried for three days at 40 \({}^{\circ}\)C, the low temperature being selected to prevent bacteria dying. All working steps were carried out under sterile conditions.  
After drying, the different fractions were packed in sterilized glass columns (9.5 cm long, 10.5 cm in diameter) (Figure 1). The upper parts of the columns were open to the atmosphere, and a synthetic solution containing organic substances and nutrients was infiltrated from the top. The solution was pumped from a storage bottle and distributed uniformly over the soil surface by a perforated plate (1 hole of 0.3 cm diameter per cm\({}^{2}\)) installed 1 cm above the soil surface. The perforated plate was covered with aluminium foil to prevent its contamination with other bacteria from the air. The experiment was run at constant temperature (13 \({}^{\circ}\)C) for 2 h (infiltration period) followed by a dry period of 22 h. This particular setup was selected to reproduce the infiltration scenarios of treated wastewater in SAT facilities, where infiltration basins are flooded for a specific period followed by a drying phase.  
The hydraulic loading rate was set at 15 m per year, which corresponds to a flow rate of 3.0 mL/min during the wet period. This ensures an average residence time in the soil of about one day. This rather high infiltration rate is common in SAT facilities, which are operated using infiltration rates from 10 to 150 m per year, depending on the soil type [42]. The wet/dry ratio of 1:11, the ratio between the length of infiltration and following dry phase, was chosen to strengthen the effect of different pore size distribution on moisture content, oxygen and nutrient supply in the different fractions. A perforated PVC plate (2 holes of 0.3 cm diameter per cm\({}^{2}\)), covered with a nylon membrane (30 \(\upmu\)m mesh size), was placed at the bottom of the column to ensure the free drainage of the water.  
Two columns were used for each fraction: an active column and a sterile control column to correct the results from the effects of sorption and chemical oxidation. Microbiological activity in the sterile control system was prevented by adding NaN\({}_{3}\) (1 g/L) to the inflow solution and outflow collectors. Dissolved organic carbon (DOC) and oxygen were measured at the inflow and outflow of the columns. The soil moisture content was measured continuously by weighting the columns during the entire duration of the experiment. The water evaporation from the columns, determined by a water mass balance of the system (evaporation = \(\Delta\)inflow-\(\Delta\)outflow-\(\Delta\)storage in the soil), amounted to 2 mL per 24 h and was neglected due to its insignificance in the calculations. The field capacity was reached after different times depending on the water holding capacity of the different fractions. The experiment was run for 30 days for the fraction 125 \(\upmu\)m to 200 \(\upmu\)m, 35 days for the fraction 1000 \(\upmu\)m to 2000 \(\upmu\)m, and 48 days for the fractions 200 \(\upmu\)m to 630 \(\upmu\)m and 630 \(\upmu\)m to 1000 \(\upmu\)m.