More stringent effluent criteria with regard to nitrogen calls for improved nutrient removal techniques in wastewater treatment plants (WWTPs). Besides optimisation of the liquid treatment train of the plants, attention has increasingly centred on the problem of return flows from sludge treatment. One of the most recent developments aimed at the reduction of this nitrogen load is deammonification which has been used at one of Ruhrverband's plants since 2002 by applying a moving bed system. To gain additional experience in operating this process, another full scale plant was modified in 2007 by integration of deammonification, using a SBR system with suspended biomass based on the DEMON(®) control scheme. By using seeding sludge from Strass WWTP in Austria, start-up has been achieved within only 1 day. After stable operation for several months, increasing nitrate concentrations were observed in the effluent of the system indicating growing activity of nitrite oxidising bacteria (NOB). Following severe process deterioration, it was decided to re-start the system again but the same behaviour, i.e. increasing levels of nitrate, was observed once again. Several approaches were used to suppress NOB organisms in full-scale without success, e.g. low oxygen levels and high free ammonia concentrations. Finally, the reduction of the aerobic cycle length during intermittent aeration down to 8 min, followed by an anoxic mixing period of only 18 min was successful in inhibiting the activity of NOB organisms, most probably due to their elevated lag-phase compared with ammonium oxidising bacteria. Today, nitrogen elimination that has been stabilised at more than 80% at a daily volumetric loading rate of 0.5 kg N/(m³ d). The total costs amount to €2.3/kg N(eli).
During the start-up phase of an enhanced biological phosphorus removal (EBPR) plant, the amount of eliminated phosphorus during wastewater treatment and the subsequent release during anaerobic sludge digestion was investigated. Different approaches were used to determine the mechanisms of enhanced phosphorus removal. From a comparison of the EBPR plant with a control, a strong correlation between the potassium, the magnesium and the phosphorus content of the sludge and the results gained from phosphorus fractionations we conclude that the major part of the eliminated phosphorus was stored in form of polyphosphate. During digestion of excess and a mixture of excess and primary sludge a complete release of the stored polyphosphate was found. The release of phosphorus was accompanied by a release of potassium and magnesium ions, from which only potassium remains in soluble form. Therefore, the soluble potassium concentration seems to be a good measure for the amount of phosphate released. Only a part of the released phosphate remains in soluble form. When digesting excess and mixed sludge this accounts for approximately 40% of the total phosphorus brought into the digester. The difference between the measured soluble phosphate concentration and the amount of released phosphorus was fixed, mainly due to chemical precipitation. It was found that a fixation in the form of magnesium ammonium phosphate (struvite) was likely to occur under the conditions of anaerobic sludge digestion. The amount of phosphate precipitation as struvite could be estimated using theoretical calculations at approximately 20% of the total phosphorus in the digester. Calcium dosing experiments show that calcium-phosphate precipitation plays only a minor role in phosphate fixation.
Phosphate release and phosphate fixation during sludge treatment of waste activated sludge (WAS) was investigated with a pilot plant for enhanced biological phosphorus removal (EBPR), complemented by laboratory investigations of sludge samples from different large EBPR plants. The major part of the eliminated phosphorus in the pilot plant was due to the storage of polyphosphate (poly-P) in the W AS and was accompanied by an uptake of magnesium and potassium. Thickening and stabilizing WAS from the EBPR pilot plant results in a hydrolysis of poly-P which could be modeled with first-order kinetics and the Arrhenius relationship for the temperature dependence of the reaction constant. As a result of poly-P hydrolysis in stabilizing systems, phosphate, magnesium, and potassium are released, but only potassium remains in solution, whereas magnesium and a part of the released phosphate were precipitated as struvite. Another large fraction of the released phosphate was fixed by aluminium. Water Environ. Res., 68, 965 (1996).
Phosphate release and phosphate fixation during sludge treatment of waste activated sludge was investigated with a pilot plant for enhanced biological phosphorus removal, complemented by laboratory investigations of sludge samples from different large enhanced biological phosphorus removal plants. The major part of the eliminated phosphorus in the pilot plant was due to the storage of polyphosphate in the waste activated sludge and was accompanied by an uptake of magnesium and potassium. Stabilising waste activated sludge from the enhanced biological phosphorus removal pilot plant results in a hydrolysis of polyphosphate. As a result of polyphosphate hydrolysis in stabilising systems, phosphate, magnesium and potassium are released, but only potassium remains in solution whereas magnesium and a part of the released phosphate was precipitated as struvite. Another large fraction of the released phosphate was fixed by participation of aluminium.
More stringent effluent criteria with regard to nitrogen call for improved nutrient removal techniques in wastewater treatment plants (WWTPs). Besides optimisation of the liquid treatment train of the plants, over the last years, attention has increasinly centred on the problem of return flows from sludge treatment. Depending on sludge handling and treatment, some 15 to 25 % of the influent nitrogen load are usually returned from the sludge dewatering facility to the inlet of the WWTP. By minimising this extra nitrogen load, it can be expected to substantially improve the effluent quality. On a full-scale basis, mainly ammonia stripping and different biological processes have been applied, in Europe, for the treatment of process water streams with the overall goal to reduce the return nitrogen load. A recently performed survey on fullscale plants shows that only eight plants use ammonia stripping. Whereas the majority of WWTPs have been upgraded by implementation of biological measures for the treatment of return flows. Most of these biological systems use classical nitrification and denitrification orto reduce the consumption of energy and organic substrate-nitritation and denitritation. One of the most recent developments in this field is deammonification which has so far been applied in three full-scale plants. Based on the experience gained from operation of two of these plants, it can be said that a stable nitrogen elimination of no less than 80 % is possible irrespective of the process configuration used, like the fixed film system at the Hattingen WWTP or the Sequential Batch Reactor (SBR) process at the Strass WWTP. While, in both cases, the operating costs are relatively low, the investment costs vary significantly as these strongly depend on the specific site conditions, i.e. the possibility to use existing reactors and machinery of the WWTP.
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