Effects of Potassium and Magnesium in the Enhanced Biological Phosphorus Removal Process Using a Membrane Bioreactor 2 Hee-Jeong Choi', Sung-Whan Yul, Seung-Mok Lee'*, Seung-Young Yu
ABSTRACT: sium
2
(Mg ID,
This study assessed the role of potassium (K+), magneand calcium (Ca2l) ions in the enhanced biological
phosphoros removal (EBPR) from wastewaters using a membrane bioreactor (MBR). A linear relationship in the anaerobic and aerobic/ anoxic phases for P,p,,,k, versus P,,],.se was obtained using the known equation APRyInk, = a X AP,,.e.,,+ b, where the constants "a" and "b" were found to be 0.44 and 8.40, respectively. Both potassium and magnesium were soluble with phosphate in the anaerobic phase, but they
accumulated again during the successive aerobic/anoxic phase. The linear 2 correlation coefficients (R2 ) of KW/7O 4 -P and Mg 'IPO4-P were calculated as 0.6682 and 0.8884, respectively. The molar ratio of CK,p during anaerobic phosphorus release was observed to be 0.20 mol/hiol, whereas CMW1p was 0.21 mol/mol. Furthermore, unlike potassium and magnesium, calcium was not co-transported with phosphorus during the release and
uptake processes. Water Environ. Res., 83, 613 (2011). KEYWORDS: enhanced biological phosphorus removal, membrane biureactor, metal cations, phosphate, wastewater treatment. doi: 10.2175/106143010X 12851009156808
Introduction The enhanced biological phosphorus removal (EBPR) process is a preferable alternative to the chemical precipitation of phosphorus in wastewater treatment (Yang et al., 2010). Many microorganisms (e.g., Acinetobacler and Pseudononas Aerobac-
ter) assimilate phosphorus via the EBPR process, which becomes a constituent of several macromolecules in the cell (LopezVazquez et al., 2009; Seviour et al., 2003). The same microorganisms have the ability to store phosphorus as polyphosphatCs in volutin granules. Microorganisms also require various cations. Cations such as potassium, magnesium, calcium, and iron must be present above critical concentrations in culture media (Adov et al., 2008). The potassium and magnesium concentrations in the influent of the EBPR process are responsible for fulfilling specific requirements and activating the microorganisms involved in this process. Potassium and magnesium play the role of principal inorganic cations in cells. As the cofactors for the some enzymes, potassium and magnesium stimulate enzyme reactions 1 Department of Environmental Engineering, Kwandong University, Gang=mung 210-701. Korea. 2 Department of Ophthalmology, KyungHee University Medical Center. Scottl, Korea. *Department of Environmental Engineering, Kwandong University, Gangneung 210-70t, Korea; e-mail:
[email protected]. July 2011
associated with the synthesis of cell materials (Choi et al., 2008). Potassium defines cell membrane permeability and plays a major role in the phosphate transport between the surrounding environment and the cell. Moreover, this cation is an essential counter-ion for polyphosphate in the cell, and it is a generally important factor in cellular energy generation (Mino et al., 1994; Rickard and McClintock, 1992). An enzyme catalyzes polyphosphate biosynthesis in the presence of magnesium ions by transferring the terminal phosphoryl group from ATP to a polyphosphate chain. Polyphosphate degradation is driven by several enzymes that depend on inorganic cations. Magnesium also acts as an important counterion of polyphosphates; it is taken up and released simultaneously with phosphate. Consequently, these cations (potassium and magnesium) are necessary for polyphosphate accumulation in biological phosphorus removal, although the binding mechanisms are not known in detail (Pattarkine and Randall, 1999). The principal questions in this area of research are whether the potassium (K+), magnesium (Mg2+), and calcium (Ca 2+) concentrations in the wastewater influence the phosphorus-removal process and whether there is any relationship among the elemental compositions of polyphosphate granules in the EBPR process. With these questions in mind, this study systernatically investigated the influence of K+, Mg 2 +, and Ca 2+ concentrations on the removal of biological phosphorus (bio-P) using the EBPR process via a membrane bioreactor (MBR) for treatment of domestic wastewater samples. Results obtained with this study should be helpful in elucidating the practical implications of using the MBR process for the removal of bio-P from wastewater contaminated with phosphorous with associated metal cation impurities, such as K', Mg2 +, and Ca2 +. Materials and Methods Membrane Bioreactor Laboratory Plant and Batch Reactor. Figure Ia shows a -schematic diagram of the MBR plant. The MBR plant consists of one anaerobic (AN), three aerobic (AE), and four anoxic (AX) chambers, with an external filter chamber. The volume of the plant (VR) could be varied between 210 and 300 L. The wastewater is partly re-circulated from the last anoxic chamber into the anaerobic chamber (RI) and from the filter chamber into the first aerobic chamber (R2). Excess sludge is removed from the filter chamber. The water moves through the filter chamber, which allows the filtrate water to be collected. Table 1 shows the operating parameters of the MBR plant. The plant is adjusted with a total hydraulic retention time (HRT) of 613
Choi et al.
a
RI
Table 1-Operating parameters of the MBR laboratory plant. Volume (L)
Reactor
b
Ventilation over membrane kose
Figure 1-Schematics of the (a) MBR laboratory plant and (b) batch reactor. 21 hours and sludge age of 15 days. The average removal efficiencies for biochemical oxygen demand (BOD) (218 mg/L --3 mg/L), total phosphorus (16 mg/L --), 0.36 mg/L), and total nitrogen (72 mg/L -- > 11 mg/L) were 99%, 98%, 84%, respectively. Wastewater was collected from the domestic wastewater treatment plants in Berlin-Ruhleben, Germany. The chemical composition of the raw wastewater is represented in Table 2. The content of metal ions was not artificially modified. The variation of metal ions concentration in the influent represents the normal fluctuation of the composition of domestic wastewater. • The cylindrical batch reactor, which is made of Plexiglas, had an inner diameter of approximately 143 mm, a height of approximately 283 mm, and a volume of approximately 4.5 L (Figure 1b). The reactor had a lid with holes for the aeration hose and sampling and a double wall for the cooling water. The mix of content was done via an agitator sheet (width = 70 mm and height = 100 mnm), with approximately 125 rpm. The aeration was carried out via a membrane hose with a grid of stainless steel held on the bottom of the reactors. Sampling and Analysis. Approximately 3.8 L (1.9 L of AN and 1.9 L of AX3) of sample was collected from the MBR laboratory plant for batch test. The first sample of batch test was drawn immediately after the reaction start, and subsequent samplings were done with definite time interval (AN, AE, and AX phase = 0, 10, 20, 30, 60, and .120 minutes). Based on previous observations, fixed parameters in this experiment included a 2-hour anaerobic phase and minimum total 4-hour aerobic/anoxic phases. The effect of the contact time of the anaerobic phase on bio-P was evaluated directly based on the data 614
Contact time (hours)
Anaerobic chamber (AN) 37 1.85 Aerobic chamber (AE) 74 2.05 Filter chamber (FC) (33) (1.65) Anoxic chamber (AX) 99 1.76 Total 210 ---Volume flow (net) = 10 L/h Recirculation aerobic-anoxic R, = 10 L/h (100%) Recirculation anoxic-anaerobic R 2 = 10 L/h (100%) Sludge age = 15 days
HRT (hours) 3.7 7.4 9.9 21
obtained. Previously, Oehmen et al. (2007) confirmed that the phosphorus-removal rate is 80% at tk,.annerob = 0.7 hours and 37% at tk.anaerob = 0.27 hours. Miyake and Morgenroth (2005) observed that the phosphorus-removal rate is higher than 80% at t k,anaerob = 0.93 hours; in other words, they concluded that the phosphorus removal could not be enhanced beyond 2 hours of contact time in the anaerobic phase. Therefore, the required contact time of the anaerobic phase for bio-P removal was between I and 2 hours. Moreover, the temperature was set between 18 and 22*C during the entire experiment using a thermostat. Most of the bacteria in solids and water are mesophilic, and they grow effectively in the temperature range 20 to 42*C (Panswad et al., 2003). Oehmen et al. (2007) studied the phosphorus release and phosphorus uptake at various temperatures and showed that the phosphorus-release rate increased by 1.95 to 2.1 times with a temperature increase of 10'C. Also, the results showed that the optimal temperature for biological phosphorus removal is between 20 and 28°C. The pH is also an important factor for bio-P removal. For example, Oehmen et al. (2005) stated that the optimal pH for bio-P removal is 6.5 to 7.5. Furthermore, the phosphorus-uptake rate decreases significantly at pH levels either above 8 or below 6. In this experiment, the pH was kept constant between 7.0 and 7.2 using either sodium hydroxide (NaOH) or hydrochloric acid (HCI). Moreover, during the 6-hour experiment, continual pH adjustment also prevented precipitation. Samples were collected both before and after filtration and were submitted for Plotal, PO4 -P, COD, K+, Mg.2 , and Ca2 + analysis using ion chromatography (Dionex ISP 2000, Dionex, Sunnyvale, California). In addition, the total solids (TS) and total. volatile
Table 2-Composition of the influent for batch test. Parameter
Concentration (mg/L)
TCOD BOD Total phosphorus Total nitrogen Metal ions (filtered sample) Calcium Magnesium Potassium Iron
470 to 203 to 7 to 12 to
680 280 19 47.9
42 to 57 5 to 15 10 to 35
