Biological Phosphorus Removal in Connection with Oxygen Aerated Activated Sludge Plants

Abstract

Since the beginning of 1984 the phostrip process has been investigated in combination with an oxygen aerated activated sludge plant. This combination of phostrip and oxygen aeration can be expected to yield the following advantages:-Savings in stripper volume and lime dosage. If detention time of return sludge in the stripper is kept constant, the amount of phosphorus released is directly proportional to the dry weight of organic matter being treated. Since oxygen aeration renders an activated sludge with excellent settleability, the return sludge has a high concentration of suspended solids and thus a given amount of phosphorus can be eliminated in a smaller stripper volume. Furthermore, the concentration of phosporus in the stripper supernatant will be higher. As lime precipitation is not stoichiometric on calcium hydroxide but pH-dependent, the smaller amount and more highly concentrated stripper supernatant requires less calcium hydroxide for phosphorus precipitation.-High levels of dissolved oxygen in the aeration basin, as maintained with oxygen aeration, prevent undesired phosphorus release in the final clarifier.-The potential problem of pH-decline in an oxygen aerated basin due to carbon dioxide accumulation especially hazardous to nitrification can be overcome by side stream precipitation with lime. After precipitation, the side stream has an elevated pH and a high buffer capacity and will therefore antagonize the pH decline after being recharged into the aeration basin. Test trials in semi-technical scale were performed using a stripper of only 30% of the volume of the oxygen aerated basin. The stripper was a 2,6 m high column equipped with a rotating elutriant distribution system near the bottom of the column. Elutriation was employed to maintain an upward current of the liquid phase throughout the stripper in order to transfer most of the released phosphorus into the stripper supernatant. The stripper therefore does not accomplish any sludge thickening. Design and operation criteria of the stripper are comprehensively listed in Table 1. Results of two first test series are listed in Table 2. In both series the stripper was equally operated. The F/M ratio of the aeration basin was lowered in phase 2. It is obvious, that with respect to the total P input with primary treated sewage the P-elimination was not sufficient. In phase 1 only 57% of influent phosphorus was eliminated (34% by the stripper supernatant, 23% in the surplus sludge). It was anticipated that a higher P-elimination could be obtained in phase 2 by lowering the flow rate of the influent to the aeration basin and maintaining the same conditions in the stripper as in phase 1. Surprisingly the stripper efficiency was considerably lower. Consequently phosphorus uptake and release are determined by the F/M ratio in the aeration basin. other potential reasons for decreasing stripper efficiency could be excluded:-Nitrification had barely started and nitrate was not detectable in the return activated sludge.-Elutriation conditions were the same in both phases and elutriation efficiency of the stripper was about 40%.-P-release in the final clarifier was not observed, for concentrations of phosphate found in the effluent of the aeration basin were equal to those found in the secondary effluent. The low wastewater temperature could be a possible reason for insufficient P removal and might also be responsible for insufficient nitrification. It can be anticipated that for satisfactory phosphorus removal the stripper should have at least twice the volume of the stripper investigated, which is then 60% of aeration basin volume. The subject of further research is the optimization of the stripper operation in order to minimize necessary stripper volume.