Abstract
Nitrogen and phosphorus are essential for the growth of micro-organisms used in wastewater treatment. When a treatment system is designed to remove a quantity of nutrients، larger than the ordinary metabolism requirements of bio-cell، the operation is called biological nutrient removal BNR. The BNR system consists of two processes: the biological nitrogen removal and the enhanced biological phosphorus removal (EBPR). There are several arrangements for biological nutrient removal processing. The selection of the suitable treatment scheme depends on the characteristics of the raw wastewater، quality of treated effluent required and the economical consideration. Several studies showed that the A2O system is the most suitable among different configurations because of its simplicity in operation، low oxygen requirements (i.e. low operational cost)، high efficiency of nutrient removal and a good settling characteristics of the sludge produced. A continuously flow laboratory scale of A2O was used in the current study. A raw municipal wastewater from the environmental engineering department is flowing at a rate of 0.6ml/sec. The mean cell residence time was 12 days. The experiments were conducted under three different internal cycle ratios (IR) and three returned activated sludge ratios، The effective sizes of the anaerobic، anoxic and aerobic compartments were 6.4، 8.65 and 17.21 L respectively، The results showed that the percentages of internal cycle ratio have a significant effect on the process of biological removal of nitrates while the percentage of returned activated sludge (RAS) has a significant impact on the biological removal of phosphorus. The A2O system removal efficiency of COD، orthophosphate and ammonia were: 85.7%، 93.5%، and 92.4%، respectively. The effluent quality is within the Iraqi standards with respect to organic and nutrients contents.
