Biological Phosphorus Removal Potential Test

Screening Test

A screening test proposed by Kang et al. (1991) provided a simple procedure for testing the feasibility of BPR for a wastewater. The basic idea behind this screening test is the comparison of phosphorus release rates between wastewaters with and without acetate addition. By controlling parameters such as sludge composition and operational conditions, only the difference of substrate (with and without acetate addition) will determine the phosphorus release rate. If the phosphorus release rate with acetate addition is greater than that without acetate addition, it may indicate the lack of short-chain fatty acids (SCFAs) for PAOs in the wastewater. By comparing the differences between the phosphorus release rates, the feasibility of BPR for the wastewater of interest can be evaluated. This method can also be used to assess the effect of toxicants existing in wastewater on PAOs responsible for BPR.

In order to conduct the screening test developed by Kang et al., the following steps are required:

1. Obtain PAO-containing activated sludge.
2. Obtain the wastewater to be tested.
3. Conduct the screening batch test.

Phosphorus-Accumulating Organisms (PAOs)

A bench-scale sequencing batch reactor (SBR) was inoculated with activated sludge from the Nine Springs Wastewater Treatment Plant (WWTP) in Madison, Wisconsin. The SBR was used to develop PAOs for the screening test when PAO-containing sludge was not available. The SBR operation procedure used in this study is as follows:

1. Prepare an SBR with a total reactor volume of 6 L.
2. Feed with 4 L of primary effluent from Nine Springs WWTP, Madison, Wisconsin.
3. Operate the SBR with 2 hours/5 hours of the anaerobic/aerobic detention times and 1 hour of settling and decanting. Decant volume is 4 L.
4. One cycle lasts 8 hours. The hydraulic residence time (HRT) is 12 hours.

The schematic of the SBR is shown in Figure 1. The SBR is mixed with a magnetic stirrer equipped with three blades. Dissolved oxygen (DO) and pH are recorded. An air pump, a magnetic stirrer, and influent and effluent pumps are connected to the timer. Dissolved oxygen is typically maintained at < 0.02 mg/L for the anaerobic stage and 2-6 mg/L for the aerobic stage.

After 2 months of operation, 600 ml of activated sludge was wasted every day at the end of the reaction cycle to achieve a sludge age of about 10 days. SBR performance data are summarized in Table 1. When the effluent phosphorus concentration was < 1 mg P/L, the PAOs were considered fully developed in the SBR.

The behavior of phosphorus release/uptake and the readily biodegradable COD uptake by PAOs after 30 days of operation is shown in Figure 2. The biodegradable soluble COD was consumed from 155 to 95 mg/L, and phosphorus was released from 9 to 43 mg/L during the anaerobic stage. The ratio of phosphorus release to readily biodegradable COD uptake was 0.57 mg P/mg COD. Nitrate from recycled sludge, about 5 mg N/L, was totally denitrified in 10 minutes during the anaerobic stage (not shown in Figure 2). Phosphorus was taken up from 43 to 0 mg/L in 3 hours during the aerobic stage. PAOs appeared to be fully established in the SBR.

Figure 2. Change of phosphorus and biodegradable soluble COD in SBR.

Wastewater Tested Using Screening Test

Wastewater samples from the Oakfield, Nine Springs, and Badger Army Ammunition Plant WWTPs were evaluated using the screening test developed by Kang et al. (1991). The Madison Metropolitan Sewerage District’s Nine Springs wastewater treatment plant has an average design capacity of 57 million gallons/day (MGD). Currently, flows average about 40 MGD of residential, commercial, and industrial wastewater. The plant was recently modified for BPR using a variation of the University of Cape Town (UCT) process. Sludge is digested anaerobically. Oakfield treats primarily domestic wastewater in a conventional activated sludge plant with a design flow of about 0.3 MGD. Sludge is aerobically digested. The Badger Army Ammunition Plant WWTP has a nominal design flow of 0.5 MGD and currently treats about 0.046 MGD of domestic wastewater. The plant consists of an Imhoff tank, trickling filter, and final clarifier.

Screening Test Procedure

The screening test procedure developed by Kang et al. is as follows:

1. Prepare two batch reactors, each containing 2 L of the wastewater to be tested.
2. Add 25 mg COD/L of sodium acetate to one reactor only.
3. Add 2 L of PAO-containing activated sludge to each reactor and mix anaerobically.
4. Obtain filtered samples in 1-hour anaerobic and 3-hour aerobic stages.
5. Examine the phosphorus release rate.
6. Evaluate the feasibility of BPR for the wastewater of interest.

Screening Test Results

Figure 3 shows the phosphorus profile during the screening test for the Oakfield WWTP sample. The phosphorus release rates with and without acetate addition were totally different. According to the method of Kang et al., the Oakfield wastewater is unfavorable for BPR. Figure 3 shows good phosphorus uptake but no phosphorus release. The PAO-containing activated sludge developed in the SBR may have contained enough energy (poly-b-hydroxybutyrate) for PAOs to take up phosphorus even if it did not release phosphorus.

Figure 3. Phosphorus profile for Oakfield Wastewater Treatment Plant.

Figure 4 shows the phosphorus profile during the screening test for the Nine Springs WWTP. The phosphorus release rates with and without acetate addition are almost the same. This indicates the wastewater can be treated by a BPR process. The screening test results are validated by the fact that the Nine Springs WWTP is currently removing phosphorus below 1 mg TP/L using a BPR process.

Figure 4. Phosphorus profile for Nine Springs Wastewater Treatment Plant.

Figure 5 shows the phosphorus profile during the screening test for the Badger Army Ammunition Plant WWTP. The phosphorus release with and without acetate addition was the same in the first 15 minutes, but decreased afterwards for raw wastewater. Using the method of Kang et al., no conclusive answer can be given for this situation.

Figure 5. Phosphorus profile for Badger Army Ammunition Wastewater Treatment plant.

A major problem with the screening test is that there are no absolute criteria to compare the difference between phosphorus release rates with and without acetate addition; therefore, no definitive answer is given by the screening test when phosphorus release rates are neither identical nor completely different. A better method must be developed to properly evaluate the feasibility of BPR.

More information on this topic: Gerry Novotny