Updated: July 2002

1. Outline

The biological phosphorus removal process is widely utilized in Japan because it is economical and easy to apply to existing activated sludge processes. However, attention must be paid to the sludge treatment in order not to return phosphate to the main stream. Otherwise, the phosphate accumulated in the sludge is released, then recycled to the main stream, resulting phosphate removal deterioration.
This report is the result of full-scale application of MAP (magnesium ammonium phosphate) crystallizer after anaerobic digestion of the excess sludge from the biological phosphate removal process to recover phosphate for fertilizer use.

2. Process Description

Schematic diagram of the plant is as shown in the figure. The plant receives about 54,000m³-sewage/day. The hydraulic retention time of the anaerobic zone and aerobic zone is 1.6 and 4.8 hours, respectively.
There are two MAP crystallizers of 0.6m diameter and 4.5m height of a reaction part. The phosphorus concentration of the supernatant of anaerobically digested sludge is between 80 to 120mg/l. The flow rate to the each MAP crystallizer is 5.4m³/hr. and the pH is controlled at 8.3 automatically. Magnesium salt is supplemented to make the molar ratio of Mg to P at 1.5.

3. Result

As for BOD, the screened sewage is around 150mg/l and the effluent of anaerobic aerobic treatment is less than 10mg/l.
The total phosphorus concentration in the screened sewage is between 3 to 6mg/l. Before construction of the MAP crystallizers, the phosphorus concentration of effluent had been 2 to 3mg/l in spite of the operation of the activated sludge process as an anaerobic aerobic mode. After operation of the MAP crystallizers, phosphorus concentration of the effluent decreased to less than 1.0mg/l.
The performance of the MAP crystallizers is satisfactory such that orthophosphate removal is more than 80% and that the effluent orthophosphate concentration is less than 20mg-P/l. Recovered phosphate as MAP granules is usually around 50% of that fed into MAP crystallizers. The rest is thought to flow out from the MAP crystallizers as small MAP particles because of the high concentration of acid labile phosphate in the effluent.
The recovered MAP granules have a diameter of 2 to 5mm. They have been dried and recycled as a fertilizer.