Use of fixed micro-algae as a direct, simple and quickly measurable indicator of biofilm density in biofilters in order to improve washing operations

Slow sand filtration is a biological means of surface water clarification that involves both biological and physical mechanisms. During this process, as time progresses, microbiological growth occurs, mainly consisting of micro-algae and bacteria which form a fixed microbial ecosystem (biofilm) on the sand of biofilters. The establishment and maintenance of this biofilm are necessary for an effective treatment because some extracellular polymeric molecules, synthesized by fixed bacteria and fixed micro-algae, act as a coagulant. Very often, the presence of macro-algae in too great quantity inevitably leads to a clogging of biofilters and thus to important pressure losses. To prevent these phenomena, washings of variable intensity are made periodically in order to eliminate the macro-algae responsible for clogging but with preserving the biofilm (fixed bacteria and fixed micro-algae). In spite of these precautions, after an in-depth washing, there must frequently be a filter ripening period before a new filter run. At present, the restarting of a biofilter is determinated by analyses carried out on the effluents because there is no direct, simple and quickly measurable parameter which allows the appreciation of the biofilm maturity. This study, carried out during the summer, constitutes a very first approach. Using the number of fixed micro-algae as an indicator of the biofilm density, its goal was twofold: to evaluate the effect of washing operations on the quantity of fixed biomass and to follow the quantitative evolution of this one between two washings of moderate intensity. The results show that washings can involve the elimination of nearly 90% of the fixed biomass. After washing, this latter increases again but finds an equilibrium state only after 20 days. More generally, the simple method suggested here could complement the indirect measurements classically taken before the restarting of the filters. It could also contribute to enhance the ripening period with helping the operators to adapt washings according to the state of the fixed biomass.

Fig. 5. Kinetics of the biofilm’s phytoplanktonic fraction deposit between two washings. day, the number of algae in the water of the fifth rinsing was always higher than 4×103/mL, it has also always represented less than 20% of the number of algae/mL in the biofilm samples. Nevertheless, if the kinetics had to continue, perhaps the rinsing stage would have appeared insufficient. Lastly, Fig. 5 represents the kinetics of the biofilm’s phytoplanktonic fraction deposit during the period of study. It takes the form of a sigmoid in which three successive phases can be observed: slow increase (0–13 days), fast increase (13– 20 days), and finally a pseudo-equilibrium state (6.8×104 to 1.2×105 detached algae/mL in biofilm extracts) after 20 days.