Biological Phosphorus Removal Potential Test

Introduction

Phosphorus removal from domestic and industrial wastewater is a key factor in preventing eutrophication of surface waters. Biological phosphorus removal (BPR) in activated sludge systems is one of the most economical and efficient methods for phosphorus removal. BPR is achieved by growing microorganisms that are capable of storing phosphorus intracellularly as polyphosphate. The growth of phosphorus-accumulating organisms (PAOs) is favored by subjecting the activated sludge to a cycle of anaerobic and aerobic conditions. Phosphorus is then removed by wasting excess sludge. The treatment efficiency of a BPR process depends not only on the size of anaerobic and aerobic basins but also on intrinsic wastewater characteristics. All wastewater may not be suitable for BPR. If the characteristics of the wastewater are not well defined, the BPR process may be improperly designed. Thus, it would be useful to develop a screening method to assess the feasibility of BPR for a wastewater of interest.

Parameters that have been used to evaluate the feasibility of BPR include ratios of biochemical oxygen demand (BOD₅) or chemical oxygen demand (COD) to phosphorus (P) and total Kjeldahl nitrogen (TKN) to COD. The BOD/P and COD/P ratios indicate that the strength of organic substrate is important for BPR. Having a large quantity of fermentation products, such as acetate and propionate, available in the anaerobic zone will increase the phosphorus removal efficiency.

Effluent phosphorus has been related to influent BOD/P and COD/P ratios. It has been known that total BOD/P ratio in the range of 20-30 would provide effluent, soluble phosphorus concentration < 1 mg/L for systems with relatively low sludge age (Sedlak 1991) and that an influent total COD/P ratio > 35 would result in an effluent total phosphorus concentration < 1 mg/L (Randall et al. 1992).

The TKN/COD ratio indicates the inhibitory effect of nitrate (NO₃^-) and nitrite (NO₂^-) in BPR. Nitrate is the by-product of nitrification. It can be introduced into the anaerobic zone by the returned sludge from final clarifiers. Nitrate depletes the limited amount of the readily biodegradable substrate required for the growth of PAOs. Therefore, it is important to control nitrate in BPR systems.

For normal domestic wastewater, complete denitrification can only be achieved for TKN/COD ratios < 0.08 without the addition of an external energy source (Ekama et al. 1984). If TKN/COD > 0.14, it is unlikely that BPR will be achieved with any configuration because of the inability to achieve sufficient denitrification.

A major problem with using ratios of BOD or COD to phosphorus and TKN to COD for evaluating BPR is that only total BOD or COD is evaluated. The total BOD or COD ratios may exceed the criteria and yet not ensure that sufficient soluble biodegradable substrate will be available in the anaerobic zone. This may be the case where a treatment plant receives significant industrial wastewater discharges or where little fermentation occurs during transport in the sewer.

A second problem with using these ratios is that they do not reflect the effect of internal recycles on the BPR process. The degree of nitrification in a plant can result in fluctuating nitrate levels in the anaerobic zone. Nitrate is known to significantly inhibit BPR processes.

Thus, these ratios are not specific enough to show the feasibility of BPR under varying wastewater compositions. Other methods for evaluating BPR feasibility include long-term pilot testing or complete wastewater characterization along with computer modeling (Park et al. 1997).

The purpose of this study is to develop a simple procedure for determining whether BPR can be adapted for a wastewater of interest. A screening test developed by Kang et al. (1991) was evaluated, and an improved test, the BPR potential test, was developed and evaluated using wastewater samples from five wastewater treatment plants in Wisconsin.

The BPR potential test offers a rapid, low-cost alternative for assessing BPR feasibility and predicting the effluent soluble phosphorus concentration.

BPR Potential Test Report

Introduction | Screening Test | BPR Potential Test | Evaluation of the BPR Potential Test | Conclusions | Literature Cited | Credits

More information on this topic: Gerry Novotny