Characteristic of the sludge from the wastewater treatment plants near Varna city and possibilities for use in agriculture

During the last few years numerous stations were built for purification of wastewaters in the country. Such a station was opened in Varna town on the Black sea cost. At purification of wastewaters a huge amount of sludge is received. The sludge is accumulated in the WWTP premises, thus hampering its functioning and polluting the environment. The aim of this investigation was to determine the chemical characteristics of the sludge from this station, considering the different modes of treatment: the sludge compactor and the drying beds after some stay. With the sludge from Varna Purification station a pot experiment was set to determine at what levels of loading with sludge the soils differ from each other by the biomass yield and heavy metals content. During the experiment two crops were grown: rape-fodder and rye-grass harvested in two cuts to follow the effects of the sludge use and the post effect in controlled conditions. The experiment was carried out on four soil units: Calcareous Chernozem, Leached Smolnitsa, Leached Cinnamonic Forest soil and Grey Forest soil (Calcic Chernozem, Haplic Vertisol, and two Chromic Luvisols from South and North Bulgaria, respectively).