Low flows are a good measure of the waste water recipient self-purification capability. The regional statistical analysis is an objective way of assessment of the required low flows. This paper presents the principles and key phases of the regional statistical analysis. The advantages of this method are a reduction of the outliers influence and the assessment of low flows on ungauged streams and stream profiles. The method is illustrated with the results obtained by the required low flows regional statistical analysis in Serbia (Yugoslavia) with 59 hydrological gauge stations in the scope, which data has the same length of 39 years, from the year 1956 to 1994.

Over the last several years, the investigations of the natural zeolites application in the sorption processes have been intensified. Purification of waste waters in order to remove lead, cadmium, copper and other heavy metals, is one significant example of such application. In this paper, the investigations results on characteristics of the natural and chemically activated zeolites from the region of Vranje (Serbia, Yugoslavia), are presented. The experiments with zeolites were performed after determination of their physico-chemical characteristics. Adsorptive characteristics were investigated under laboratory conditions, in a liquid medium, depending on granulation and concentration of the heavy metals. As the obtained results show, these natural materials can be used to remove heavy metals from the waste waters.

Neither soil nor water has very distinct function when seen in isolation. Therefore, analyses of water and soil can get real value only by their linkages. Taking into consideration irrigated land, water and soil interaction degree can cause different changes of soil, plants and aquifer A. All negative changes which are occur in the soil (in case of using unsuitable water quality for irrigation or applying of not rational fertilizers amount) have influence to the changes aquifer A quality. Interaction between soil and aquifer A are require investigation of groundwater quality changes relating to plant production, especially in irrigation. Intensity and type of aquifer A properties change, in case of different soil use in agricultural production, have been studying since 1991 at the Lysimetric Station Rimski Sancevi (Serbia, Yugoslavia). The results are point out satisfactory quality of aquifer A after irrigation with wastewater and well water, and applying of different mineral fertilizers amount. The soil capability to receive water with unsuitable quality, to influence their change and change of aquifer A quality as the final recipient, are showed by received results.

The work describes optimization on the process of separation of a synthetic dye, methyl orange, from water by sorption on iron(II) hydroxide, using response surface methodology. It was established that the optimum sorption conditions for dye removal on iron(II) hydroxide are: pH 9.1, iron(II) concentration 128 mg/l, dye concentration 10 mg/l, whereby the highest efficiency of dye removal is close to 90%.