In this paper the results of preliminary investigations of microbiological quality of seawater and sediment are presented. Analyses are performed by membrane filtration. The samples were collected near the sawage outfalls in Bokakotorska bay. It is noticed much higher number of bacteria – indicators of fecal pollution in sediment surface that seawater overlaying the sediment. Concentrations of bacteria diminished in layers of sediment laying deeper that sediment surface. Influence of wastewater is the most evident on 1 and 3 positions.

Microthrix parvicella is dilamentous bacterium common present in the activated sludge on the plants with biological nutrient removal. Microthrix parvicella is described as a bacterium which present in a large numbers, has an extremely negative impact on the sludge settling, and its considered as the primary cause of bulking sludge on the WWTP-s with biological nutrient removal. Also, this bacterium considered as the primary foam-forming microorganisms in activated sludge, because those hydrophobic cells stabilized air bubbles or affected by the oil droplets in foam. Although there are numerous data regarding the growth of certain filamentous bacteria, as well as their impact on the waste waters treatment and deposition of sludge, they are often vague, and not always in clear correlation, especially when comparing data from different plants. The aim of this paper was toexamine the growth and population size of Microthrix parvicella in correlation to water temperature (seasonal dynamics of growth), and a correlation between population size of filamentous bactereia (filamentous sludge index-FI), especially M. parvicella growth and the sludge settlement (Imhoff 30 '), or the ratio of FI and the volume of sludge (sludge volume index – SVI) on WWTP of Subotica during 2011.