In connection with studies aimed at developing low-energy wastewater treatment processes considerable interest has been shown in the possibility of removal of phosphorus biologically rather than chemically.

The results over one year from bench and pilot scale tests conducted with synthetic and settled domestic wastewaters respectively are reported.

The pilot plant was installed in one of the Rome urban wastewater treatment works and consisted of a two stage (anaerobic-aerobic) system. The wastewater was withdrawn downstream from the primary settling tank and conveyed to the anaerobic reactor of the pilot plant.

A similar experiment was conducted in a three reactor (two anaerobic and one aerobic) bench scale plant, where a synthetic solution (peptone, sodium acetate and potassium phosphate) was fed under rigorously controlled conditions of temperature, pH and dissolved oxygen. Both bench and pilot scale tests were designed to verify and quantify the biological removal of phosphorus; the performances of both phosphorus removal test scale systems were compared with those from a reference aerobic plant operating under the same conditions.

The impact of several parameters on the biological removal of phosphorus were examined. These parameters were: nature of carbonaceous substrate, carbonaceous/phosphorus ratio, sludge age; etc.

Considerable care and attention was given to checking phosphorus balances, once the steady state conditions had been obtained.

The results can be summarized as follows:

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    Phosphorus removal was found to be greater in plants with an anaerobic stage than in the reference plants. At pilot scale level this is clearly shown by the plot of the daily phosphate concentration in the effluents. At bench scale it is shown by comparison between P contents in sludges (9.9% vs. 5.0%) and by the fact that, at the end of the experiment, large deposits of inorganic phosphate (mainly of Ca and Mg) were found on the walls of both anaerobic reactors. The unstable operating conditions of the reference plant due to the poor sedimentation characteristics of the sludges leading to uncontrolled losses of biomass meant it was impossible to obtain a reliable mean phosphorus abatement value, which in any case was always well below that found for the P-removal plant.

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    As reported above, the deficit in the phosphorus balance was due to inorganic phosphate precipitation. In this connection an excellent quantitative agreement was found between the deficit in the balance and the quantity of phosphorus collecting in the anaerobic reactors.

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    Phosphorus release from the biomass under anaerobic conditions has already been completed in the first anaerobic reactor (where carbonaceous substrate uptake also occurs) and the use of the second anaerobic reactor seems to allow a more complete phosphate precipitation.

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    Part of the carbonaceous substrate which disappears during the first anaerobic stage is metabolized. In fact, in this reactor, the complete disappearance of organic nitrogen is accompanied by the release of large quantities of ammoniacal nitrogen.

Lastly, despite the fact that the values of the main operative parameters, such as hydraulic retention time, sludge age, temperature, pH and dissolved oxygen, were kept rigorously constant, a certain instability was found in the biomass as regards the maintaining of such high phosphorus removal efficiencies over long periods of time. This may be due to the highly variable biological population dynamics.

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