The effect of chemically pretreated landfill leachate on the properties of activated sludge has been investigated. The leachate was pretreated by a two-stage method consisting of coagulation and oxidation. The mixture of pretreated leachate (from 2.5 to 15 wt. %) and municipal wastewater was fed into a bioreactor. Results of the studies showed that the pretreated leachate contributed to the dispersion of the activated sludge flocs. As a result, a very low value of volume index (30 cm 3 /gTS) and a significant turbidity of supernatant liquid were observed at the same time. The dispersion phenomenon was also confirmed by floc size distribution. An increase of the amount of small and very small flocs was observed.

Treatment of leachate from an exploited since 2004 landfi ll by using two methods of advanced oxidation processes was performed. Fenton's reagent with two different doses of hydrogen peroxide and iron and UV/H 2 O 2 process was applied. The removal effi ciency of biochemically oxidizable organic compounds (BOD 5 ), chemically oxidizable compounds using potassium dichromate (COD Cr ) and nutrient (nitrogen and phosphorus) was examined. Studies have shown that the greatest degree of organic compounds removal expressed as a BOD 5 index and COD Cr index were obtained when Fenton's reagent with greater dose of hydrogen peroxide was used -effi ciency was respectively 72.0% and 69.8%. Moreover, in this case there was observed an increase in the value of ratio of BOD 5 /COD Cr in treated leachate in comparison with raw leachate. Application of Fenton's reagent for leachate treatment also allowed for more effective removal of nutrients in comparison with the UV/H 2 O 2 process.

A modified Fenton process using sodium percarbonate (SP), as an alternative source of H 2 O 2 , and UV radiation was investigated to discoloration of dye wastewater containing Acid Green 16 (concentration 100 mg/dm 3). This acid dye is characterized by the harmful effect on aquatic organisms and it may cause adverse effects in the aquatic environment. The experiments were carried out in two systems, in which two lamps were used as a UV radiation source: low pressure (system I) and medium pressure (system II). The effect of SP dosage (100-400 mg/dm 3-system I; and 100-250 mg/dm 3-system II), Fe 2+ /SP ratio (from 0.2 to 0.4), reaction pH (3 and 4) and reaction time (from 10 to 30 min) on colour removal efficiency was examined. The modified Fenton process was found to be very efficient for discoloration of simulated wastewater. For a system with a low pressure UV lamp the optimal doses of SP and Fe 2+ /SP ratio were 400 mg/dm 3 and 0.2, respectively at pH 3 and 20 minutes reaction time. For a system with a medium pressure UV lamp the optimal doses of SP and Fe 2+ /SP ratio were 200 mg/dm 3 and 0.33, respectively at pH 3 and 20 minutes reaction time. In both cases, at described conditions total visual discoloration was achieved. Better results of colour removal (concentration of Acid Green 16 was in the range of 0.64-0.96 mg/dm 3) were achieved when the initial pH value equalled 3.0 than at initial pH value of 4.0 (concentration of Acid Green 16 was in the range of 0.80-6.87 mg/dm 3).

The effect of chemically pretreated landfill leachate on the properties of activated sludge has been investigated. The leachate was pretreated by a two-stage method consisting of coagulation and oxidation. The mixture of pretreated leachate (from 2.5 to 15 wt. %) and municipal wastewater was fed into a bioreactor. Results of the studies showed that the pretreated leachate contributed to the dispersion of the activated sludge flocs. As a result, a very low value of volume index (30 cm 3 /gTS) and a significant turbidity of supernatant liquid were observed at the same time. The dispersion phenomenon was also confirmed by floc size distribution. An increase of the amount of small and very small flocs was observed.

Organic dyes are widely used in many industries (textiles, food, cosmetics, medicine and biology). These plants produce wastewater containing dyes. Even small amounts of dyes can cause a strong colour of wastewater. Therefore, it is very important to effectively remove residues of these pollutants from the wastewater, before discharging them into the environment. The sorption process is one of the methods used to remove dyes. However this method is often unprofitable economically in comparison with other dye removal processes, due to the high cost of commercial sorbents. Therefore, research is currently conducted in order to find waste materials that can be used as sorbents. The static sorption process of two dyes were carried out (Acid Red 18 and Acid Green 16) with the use of dried excess sludge. The activated sludge (excess) came from a municipal sewage treatment plant that purifies wastewater from carbon, nitrogen and phosphorus compounds. During the study the most favourable pH of the process and the contact time of the sorbent with the dyes were determined. It was observed that for both dyes the highest effectiveness of dye removal was obtained at pH = 2. The most favourable contact time was equal to 60 and 180 minutes for Acid Red 18 and Acid Green 16 respectively. In addition, in order to establish process parameters, a different models of sorption isotherm was examined. The studies showed that the sorption capacity (calculated based on Langmuir model) was much higher in the case of Acid Green 16 (qm = 434.8 mg/g) than for Acid Red 18 (qm = 109.9 mg/g). The experiments to evaluate the effect of pH, contact time on the process effectiveness and to determine the sorption isotherm were conducted at 293.15 K.