To study the biodegradability of organic substances (hydrocarbons, phenols, polyoses, alcohols, polyesters, amino acids, and so on) three types of water have been used. The first type is water samples taken from natural water bodies (lakes, rivers, ponds, and sea). The second type is a “model” of natural water containing bacterial cultures, and the third type is a “model” of sewage water. The “model ” of natural water was prepared on the basis of activated sludge. In order to choose a more informative descriptor a treatment of experimental BOD values for organic chemicals has been done. From 15 examined descriptors the standard heat combustion (H⁰_HC) has been chosen. The standard heat combustion can be estimated from the relation: H⁰_HC = - (204.2n + 44.4 m + S x) kJ / mol. Here n is the number of oxygen atoms, which is necessary for the full combustion of substance “i”; m - number of moles of arises water; x - thermal characteristics; x = constant within the homologous series. By means of regression analysis the following QSBR modeling equation was obtained (for sewage water): lgBOD₂₀ = 1.06 lgH⁰_HC - 1.35. The statistical parameters were n = 55; r = 0.99; s = 0.05; F_c = 2326.4; F_t = 4.05, where n - number of the chemicals used in the model; r - correlation coefficient; s - standard error; F_c - Fisher’s variance ratio (calculated); F_t - Fisher’s variance ratio for the level of significance 95% (taken from statistical tables). The data show that the model is adequate (F_c > F_t ) and can be used for prediction (F_c / F_t = 574.4 >> 4). Specific information on each experiment with water samples taken from natural water bodies located in Russia is shown in Table I.

Table I. Relationships between BOD_t and H⁰_HC (natural water bodies)

As may be seen from the Table I all QSBR models are adequate (F_c > F_t ) and can be used for prediction (F_c / F_t > 4 ).