Phosphorus inactivation

Phosphorus inactivation aims at P removal in the water body by addition ofP binding substances with subsequent sedimentation to the sediment.

Scientific background. Depending on the chemical sub stance used (iron, aluminum, or calcium compounds), soluble P is precipitated as a salt with very low solu bility or it is sorbed by colloidal aggregates, whereas the particulate P (e.g., P incorporated in biomass) can coagulate. High doses of chemicals remove not only the P from the water but also increase the P binding capa city in the sediment so that P release from sediments is decreased for a longer period. The use of iron and aluminum salts results in the formation of hydroxides with a simultaneous release of H+ ions. This can lead to a complete loss of the buffering capacity and, ulti mately, to an ecologically unacceptable low pH value. Dissolved Al + compounds or Al(OH)2+ formed at pH below 6.0 are toxic in varying degrees. The deficiency of alkalinity can be ameliorated by adding lime. In contrast to Al salts, the efficiency of Fe salts depends on the redox conditions in the water and in the sedi ments. Under strongly reductive conditions, a portion of Fe(III) hydroxides can be reduced to Fe(II) and the sorbed P is released again. Al salts are stable under the reductive conditions in deeper sediment layers. Different calcium compounds naturally or artificially induce the process of calcite precipitation that leads to sorption of P at the calcite surface, to co crystallization of soluble and particulate P, and to flocculation and coagulation processes. Algae and other P containing particles act as condensation nuclei and are a precondition for crystallization. Phosphorus removal by calcium may also occur due to the forma tion of hydroxyapatite or other calcium phosphates in the water body. However, this process only takes place at high pH values, with high concentrations of Ca and P. The lowest solubility of hydroxyapatite is reached above pH 9.5. With increasing CO2 concentration and lower pH values the solubility of calcite and hydro xyapatite increases strongly. Therefore, calcium carbonate can be dissolved in the hypolimnetic water and in the sediment, losing its P binding capacity.

Techniques. Depending on the lake size and chemicals the distribution is realized by piping, by distribution on the ice cover, by airplane or boats, or by aeration devices (Figure 5). Aluminum is generally used as aluminum sulfate (Al2(SO4)3), sodium aluminate (Na2Al2O4), or as

Figure 5 Equipment for the addition of chemicals to induce P inactivation combined with hypolimnetic aeration.
Figure 6 Phosphorus inactivation by alum treatment.

aluminum chloride (AlCl3), at a dosage between 3 and 30 gm Al (Figure 6). Iron is applied as iron sulfate (FeSO4), iron(II) chloride (FeCl2), iron(III) chloride (FeCl3), or iron chloride sulfate (FeClSO4), with a dosage between 1 and 150 gm 3 Fe. Calcium carbonate or calcium hydroxides are added once or repeatedly with a dosage between 10 and 250gm Ca. Ca(OH)2 addition may cause an immediate eradication of submerged macrophytes due to a short term rise in pH. The addition of iron is often combined with oxidation measures such as destratification, hypolimnetic aeration, and nitrate addition. In shallow lakes with high pH values during summer, the resuspen sion of Fe and Al precipitates can lead to P release by exchange with OH ions. The efficiency of addition of P binding chemicals is high when (1) the water residence time is long and (2) a delayed response to measures decreasing the external P loading is expected. The in lake P inactivation was successfully performed in many stratified and nonstratified lakes in North America and Europe, but only short term effects were observed in cases of continued external loading that quickly substituted the eliminated P.

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