Elsevier

Water Research

Volume 35, Issue 13, September 2001, Pages 3190-3196
Water Research

Stability of enhanced biological phosphorus removal and composition of polyphosphate granules

https://doi.org/10.1016/S0043-1354(01)00025-2Get rights and content

Abstract

The influence of varying Ca- and Mg-concentration of the influent wastewater on the enhanced biological phosphorus removal was investigated in an anaerobic-aerobic bench-scale plant. The artificial enhancement of the Mg-concentration in the influent from 15 to 24 mg l−1 and 31 mg l−1, respectively, caused a raise of the mean P-removal efficiency from 85 to 97%. The P-elimination was very stable in time. A chemical precipitation of magnesium ammonium phosphate could be excluded. The elemental composition of polyphosphate granules was investigated by electron microscopy and energy dispersive X-ray spectroscopy. The elements Ca, Mg and K were the principal metal components of polyphosphate granules. Concerning the metal composition, different types of granules could be distinguished. The quantitative ratios of Ca, Mg and K varied in dependence on the influent concentration of these metals. A relation between the Mg/Ca-ratio of the granules and the efficiency of enhanced biological phosphorus removal can be supposed.

Introduction

The enhanced biological phosphorus removal process is a preferable alternative to the chemical precipitation of phosphorus in wastewater treatment. It has been used in numerous full-scale plants for many years. But sudden breakdown of EBPR under full-scale conditions has been observed in many cases, and several theories are available to give an explanation.

It seems that the cation concentration and composition of the influent wastewater plays an important role in maintaining the stability of the enhanced biological P-elimination process and in the binding mechanisms of phosphorus in the activated sludge.

Many authors found, as a result of X-ray analysis, that metal ions are incorporated in the structure of volutin granules (Buchan, 1981; Röske et al., 1989; Bonting et al., 1993). Besides phosphorus, the metals calcium, magnesium and potassium were the most frequently detected components of polyphosphate granules.

It can be supposed that these metals are the counter-ions of phosphate-anions in the polyphosphate-chains, although the binding mechanisms are not known in detail. In the present research work it was tried to find out, by detailed quantitative X-ray analyses, whether varying Ca2+- and Mg2+-concentrations in domestic sewage influence the cationic composition of volutin granules in activated sludge bacteria. The principal question was whether the Ca2+- and Mg2+-concentration in the influent may have an influence on the stability of the enhanced biological phosphorus elimination and whether there is any relationship between the elemental composition of polyphosphate granules and the long-term stability of the EBPR process.

Section snippets

Pilot plant

The object of investigation was a small anaerobic–aerobic bench-scale plant, which was supplied with mechanically treated domestic sewage from a small town (Table 1, Table 2). It was operated without nitrification and denitrification. A partial nitrification occurred during the summer but the nitrate concentration in the return sludge was negligible.

The content of calcium and magnesium in the influent was modified during different operational periods by addition of CaCl2 · 6H2O and MgCl2 · 6H2O,

Stability of the EBPR process at varying cationic composition of the influent wastewater

Figure 1 shows the cumulative frequency plots of P-removal efficiency in the bench-scale plant during different experimental periods:

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    without any additional dosage of metal ions into the influent wastewater,

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    with addition of calcium,

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    with addition of magnesium

During the experimental period (Table 3) without any additional dosage of metals, the P-elimination process was quite unstable. The arithmetic mean of P-removal efficiency was 84.9%. The standard deviation amounted to 19.7%. The effluent

Cationic composition of influent wastewater and stability of the EBPR process

During the additional dosage of magnesium, the mean P-elimination efficiency was 12% higher than without addition of metals and 7% higher than during the addition of calcium. The standard deviation of daily measured P-removal efficiency amounted to only 4% during the addition of magnesium, compared with 19.7% without additional dosage of metals and 15.1% with addition of calcium, respectively. The standard deviation can be regarded as a characteristic of the stability of EBPR in time. It can be

Conclusions

The additional dosage of calcium and magnesium, respectively, into the influent wastewater of an EBPR pilot plant influenced the P-elimination in the following way:

During the addition of 30, 60 or 90 mg l−1 Ca (resulting influent concentration: 80–158 mg l−1 Ca), respectively, the mean P-elimination efficiency was only slightly enhanced as compared to the operation without any addition of metallic cations.

During the addition of 10 or 20 mg l−1 Mg (resulting influent concentration: 24–31 mg l−1 Mg),

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