## 1 Introduction
As a consequence, optimal bacterial activity and development require a trade-off between oxygen (low soil moisture content) and nutrient availability (high soil moisture content). Extreme situations such as very wet or very dry soil conditions significantly reduce the biodegradation rates [27; 28]. Aerobic microbial activity increases with soil water content until a point is reached where the diffusion and availability of oxygen are limited by the water [29; 30; 31]. There is an optimum for microbial activity at a specific water content coupled with a specific exchange of air in the pore space [32]. Often the moisture content in the soil, given in percentage of water saturation, field capacity (FC) or water holding capacity (WHC), is used as a criterion for the evaluation of conditions for biodegradation of organic substances. The field capacity is a range of soil moisture content that represents the amount of water which can be held against gravity through capillary and adsorption forces [33; 34]. Water holding capacity is defined as the water retained between field capacity and wilting point [32; 34]. Thereby, the soil moisture content is influenced by the bulk density and the pore size distribution [35; 36]. Another parameter that can be used as indicator for the suitability of soils for biodegradation of organic substances is the water-filled pore space (WFPS). Determination of WFPS requires knowledge of the soil volume, gravimetric soil water content and soil bulk density. Some studies [19; 37; 38; 39; 40; 41] indicate that maximum microbial activity can be achieved at WFPS = 60%. In the case of SAT basins, most microbiological activity occurs in the shallowest soil layer where trade-off conditions for water, nutrients and oxygen supply are easier to be met. Therefore, it is important to select soils that provide a WFPS which can ensure optimal conditions for bacterial growth. Selection of soils with unsuitable grain size distribution can have a negative effect on the overall system's efficiency, such as poor supply of nutrients and oxygen or low water retention capacity. In practice, sands or sandy soils in their natural condition are used in the shallowest layer of SAT basins. So the aim of this paper is to identify the relationship between different soil fractions of a sandy soil typically used in SAT systems and the biodegradation of organic compounds. The parameters considered in the analysis are the pore size distribution, the mean pore diameter and the water-filled pore space of four different grain size fractions. Knowledge of the relationship can be helpful during the planning and operation of SAT systems. Thus, it is possible to ensure optimal availability of oxygen and nutrients as well as sufficient soil moisture content needed for purification processes in the shallowest soil layer. For comparison purposes, the four fractions were obtained from the same soil type, so the results are applied only for this particular example. Nevertheless, the values achieved will be compared with those obtained from experiments run under different boundary conditions in order to confirm the applicability of present results for other site-specific conditions (soil type, organic compounds).