It is now known that since cyanobacteria (blue-green algae) occur in both swimming and drinking water supplies, and lakes and rivers, they represent an increasing hazard to animal life and human population. Moreover, high algal contents pose also a number of operation problems for water purification plants. The objective of the work is to study the elimination of a Microcystis strain of cyanobacateria by the use of an ozoflotation process which associates the oxidizing properties of ozone and the physical aspects of flotation. The functioning and the efficiency of a pilot unit is presented according to such parameters as: ozone dose, flow rate, coagulants and raw water quality. The use of ozone in pretreatment leads to an inactivation of the algal cells. Experiments let us calculate the specific ozone utilisation rate of Microcystis and the [C.t] (ozone concentration, contact time) curve is determined versus algal removal. Under real conditions, a previous coagulation stage is necessary; best results are obtained with ferric chloride. Preozonation is also of influence on the enhancement of the coagulation efficiency. Association of the ozoflotation process and bilayer filtration can solve the algae problems of waters presenting low turbidity and low organic content, and improve water quality.

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The simultaneous removal of carbonaceous and nitrogenous pollution by the activated sludge process is becoming common in industrial and municipal wastewater treatment plants. An oxygenation monitoring process has been developed, which is based on the dynamic analysis of ORP and DO signals and allows the detection of specific characteristic points at the end of the biological nitrification and denitrification. The aim of this study is to validate this process in a food-processing industry WWTP (slaughterhouse) having large variations of carbonaceous and nitrogenous loads. In order to treat during the peak period, pure oxygen is used. The first part of the study provides a precise diagnosis of the WWTP operation by the analysis of the ORP and DO signals. It is particularly easy to estimate the level of nitrogen treatment actually achieved and the oxygen requirements, and to detect the over- or under-oxygenated phases. Thanks to the monitoring process, the aerobic period of each cycle is reduced to the optimal duration, providing a reduction of 30% on the energy consumption compared to a traditional schedule. We have demonstrated that the use of pure oxygen associated with the existing air system is particularly relevant for the peak period. The revamping of an existing plant to simultaneously treat the carbon and the ammonia in the same basin is now technically feasible.

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