Application of image analysis in activated sludge to evaluate correlations between settleability and features of flocs and filamentous species.

Digital image analysis is a useful tool to estimate some morphological parameters of flocs and filamentous species in activated sludge wastewater treatment processes. In this work we found the correlation between some morphological parameters and sludge volume index (SVI). The sludge was taken from a pilot-scale activated sludge plant, owned by ENEA, located side stream to the Trebbo di Reno (Bologna, Italy) municipal WWTP and fed by domestic wastewater. In order to use image analysis, we developed a correct method to acquire digital microbiological observations and to obtain images altogether representative of the sludge properties. We identified and assessed the parameters needed to estimate the settleability of the sludge and evaluated the morphological filamentous features. It is known that several conditions (i.e. low F/M, nutrient deficiency, low dissolved oxygen) select specific filamentous species and their excessive growth decrease floc-forming/filaments ratio, correspond to the worse settleability properties; we found a relationship between the relative abundance of filamentous species and SVI. We also evaluated the fractal dimension parameter (FD) and determined a threshold value useful to distinguish between the "weak" and "firm" floc and we found a correlation between FD and SVI.

Testing the effect of selectors in the control of bulking and foaming in full scale activated-sludge plants.

Selectors were operated at four full-scale activated sludge plants to control bulking and foaming problems due to filamentous microorganisms. Selector effectiveness was not related to reduction of biodegradable organic matter in the contact zone, but was related to soluble COD levels in selector effluent. Significant reductions in the numbers of filamentous m icroorganisms were reported.

Spatial distribution of microorganisms and measurements of oxygen uptake rate and ammonia uptake rate activity in a drinking water biofilter.

The biofilm characteristics (population dynamics and biofilm composition) in a biological filter for the removal of iron, manganese and ammonium were studied in a drinking water treatment plant. The objective was to examine the spatial distribution and biological composition of active biomass that grows in a biological filter and to verify the effect of the backwashing on the quantity of fixed biomass and on the density and activity of the biological population. Heterotrophic microorganisms activity was higher in the upper layer of the filter. Nitrifying microorganisms colonized the biofilter in a stratified manner and their activity was higher in the second layer of the filter. A total of 14 species of ciliated protozoa and 7 species of filamentous microorganisms were found in the biofilters. Ciliates were concentrated in the filterbed layer in which the heterotrophic activity was higher. The grazing activity of ciliates on heterotrophic bacteria reduced the competition pressure on nitrifying microorganisms, supporting their growth and thus raising the ammonium removal efficiency. In general, filamentous microorganisms appeared to be indifferent to operating changes in the plant such as backwashing and filtering cycles. Crenothrix was the prevalent filamentous microorganism in terms of both frequency and abundance; it was found prevalently in the first layer where the oxidisation of iron and manganese occurred.

The acute toxicity of nickel to freshwater ciliates.

The acute toxicity of nickel to 12 species of freshwater ciliates was examined in laboratory tests. After exposing standard cultures of the ciliates to a soluble compound of nickel (NiCl(2).6H(2)O) at several selected concentrations close to the lethal limit for 24 h, the mortality rate was registered and the LC(50) values (with 95% confidence intervals) were calculated. Large differences appeared in sensitivities of the 12 species to the metal. Ciliated protozoa such as Spirostomum teres (0.17 mg Ni l(-1), 24-h LC(50)) and Paramecium bursaria (0.36 mg Ni l(-1), 24-h LC(50)) showed the highest sensitivity to the nickel, while Euplotes patella (7.7 mg Ni l(-1), 24-h LC(50)) was the most tolerant species. A comparison of these results with data obtained for other heavy metals indicates that S. teres is an excellent and convenient bioindicator for evaluating the toxicity of waters polluted by heavy metals.