The effect of the pH level on the dissociative behavior of calcium ion from oyster shell was evaluated in batch experiments. The results indicated that when the initial pH changed from 3.0 to 10.0, equilibrium pH possibly maintained at about 8.0. Nitrification behaviors at various pH and NH4+-N loads were investigated simultaneously in two aerated biofilters packed with oyster shell and plastic balls, respectively. The results showed that when the influent NH4+-N concentrations varied from 15 to 60 mg l− 1, 63.6 to 97.5 percent of NH4+−N could be removed by the oyster shell biofilter, but only 13.3 to 29.0% could be removed by the plastic ball biofilter at the influent pH of 5.3∼ 5.7. Furthermore, due to the sufficient alkalinity supply in oyster shell biofilter, the effluent pH could be maintained above 6.5, indicating that oyster shell shows greater buffer capacity.
Increase of nitrogen concentrations in natural waters may cause severe eutrophication and has led to blooms of micro-algae and red tide in freshwater and seawater bodies. Ammonia is the main N species in domestic sewage; therefore, ammonia removal from wastewater has become vitally important.
Stoichiometric calculation of biological nitrification shows that removal of 1.0 g NH4+-N consumes 7.14 g alkalinity based on the weight of CaCO3 (Xu, 1994). Since the activity of ammonia oxidizing bacteria is strongly inhibited at relatively low pH (Anthonisen et al., 1976; Villaverde et al., 1997), sufficient alkalinity supply is required to ensure the nitrification process. Research has been carried out on the removal of biologically produced ammonia by adding extra NaHCO3 or NaOH as the alkalinity supplier (Bao and Cai, 1995; Ruiza et al., 2003). High efficiency of ammonia removal in most wastewater treatment plants has been achieved by addition of Ca(OH)2 or NaOH. However, large amounts of chemical additions increase operation cost, so it is necessary to develop an alternative technique to improve ammonia removal from wastewater without chemical additions. Aerated biofilters are usually aerated with enriched oxygen and can provide optimal mass transfer conditions for excessive growth of microbial at the surface of biocarriers (Rogalla et al., 1990). These features enable the biofilter to achieve higher removal rates of organics and N (Pujol et al., 1992; Fdz-Polanco et al., 2000).
Oyster shell, an aqua-culture waste product, not only has ample specific surface favoring the growth of nitrifying bacteria, but also contains large amounts of CaCO3 for alkalinity supply (Li, 2001). Compared with conventional carriers, oyster shell demonstrates higher rates of ammonia and phosphorus removal in aerated biofilter systems (Xiong et al., 2002). However, so far there has been little research using oyster shell as the alkalinity supplier in the biological nitrification process.
In this work, the dissociative behavior of calcium ion from oyster shell at different pH levels was evaluated based using a batch equilibration method. Two aerated biofilters packed with either oyster shell or plastic balls were employed. Nitrification behavior was investigated simultaneously under different pH values and ammonia loads in the influent.
Materials and methods
Batch equilibration experiment
To evaluate effect of pH on the disassociation rate of calcium ion from oyster shell, the experiments were carried out at various pH values in eight Erlenmeyer flasks. In these flasks, 2.0 g dry debris oyster shell were added initially and pH values were adjusted to 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0, respectively, by adding hydrochloric acid and sodium hydroxide. These flasks were shaken at 200 rpm and 25°C for 8 h. Samples were taken at regular time intervals. The pH and calcium ion concentrations were measured. No other hydrogen ion was involved during the experiments.
Aerated biofilter experiment
A schematic diagram of the apparatus is shown in Figure 1. The aerated biofilter column was a submerged Plexiglas filter, 1.0 m high and 75 mm in diameter. Two columns with working volumes of 9–l were operated simultaneously. Column A and column B were packed with oyster shells (porosity of 81.0%) and plastic balls (diameter of 5cm), respectively. The height of the packing carrier in both columns was 0.5 m. Air was supplied through a sand stone diffuser at the bottom of the filter. The synthetic wastewater was fed at the top of column by a peristaltic pump and the liquid level was kept constant by overflow. The pH of the influent was adjusted in the range of 5.3 to 6.6. The system was operated continuously with hydraulic residence time of 8h at room temperature.
The composition of synthetic wastewater was as follows: 70.7 to 282.8 mg l− 1 (NH4)2SO4, 94 mg l−1 glucose, 44 mg l−1 KH2PO4, 75mg l−1 MgSO4, 50 mg l−1 NaHCO3 and 50 mg l−1 NaCl. The pH was adjusted by hydrochloric acid.
Incubative sludge was taken from a domestic wastewater plant in Xiamen city. Two columns were operated continuously after adding the sludge. At the start-up period, biofilm was formed gradually at the surface of the oyster shell, and became established in four weeks. At the steady-state treatment period, the nitrification efficiency for column A and B was investigated at different ammonia loads and pH values in the influent. The experimental conditions at the different runs are described in Table 1. Samples were taken from the influent and effluent, and the nitrification efficiency was calculated based on the produced NO2 + 3−-N concentration during the experiments.
NH4+-N was analyzed by the Indophenol blue method (HP8453 UV-VIS spectrophotometer). Nitrite- and nitrate-N were analyzed based on American standard methods of 4500-NO2-B and 4500-NO3-E, respectively (APHA, 1995). pH was measured by an acidity electrode (PHB-4) and calcium ion was detected by a cation liquid chromatogram (732 IC Detector).
Results and discussion
Effect of pH value on the disassociation extent of calcium ion from oyster shell
Figure 2 shows the relationship between the initial pH and calcium ion concentration obtained from the batch experiments. When the initial pH was at 3.0, calcium ion from the shell was disassociated significantly with the concentration of 131.7 mg l−1. When the pH was increased to 7.0, the concentration of disassociated calcium ion dropped rapidly to 31.8 mg l−1, and then tended to become constant at 23.2 mg l−1 when pH was above 7.0. It was observed that the disassociation of calcium ion from oyster shell ceased at pH of 8.0, suggesting that disassociation could only occur under acidic conditions. This result was similar to the finding reported by Damir and Ljerka (1995).
The relationship between the initial pH and equilibrium pH in batch experiments is shown in Table 2. It was found that even though the pH changed from 3.0 to 10.0, equilibrium was still possibly maintained at about 8.0, indicating that the oyster shell had a substantial buffering effect against the lower pH of the influent caused by disassociating calcium ion. Furthermore, since oyster shell contains large amounts of CaCO3, there is no doubt that oyster shell is a very suitable alkalinity supplier and can play an important role in the biological nitrification process.
Comparison of nitrification efficiency at different influent pH levels
Figure 3 shows a comparison of nitrification efficiency between column A and column B at pH of 5.3 to 5.7 and 6.4 to 6.6, and NH4+−N concentration of 60 mg l−1. When influent pH was in the range of 5.3 to 5.7, in column B, NH4+−N removal efficiency was below 10%. The activity of nitrifying bacteria was completely inhibited. In contrast, in column A, NH4+-N removal efficiency was still maintained at above 60%, and no significant drop was observed in nitrifying activity at low pH levels. This finding indicated that the oyster shell really played a very important role in protecting nitrifying bacteria from inhibition at low pH. When influent pH was in the range of 6.4 to 6.6, although the restoration of nitrification activity likely took place in both columns, NH4+-N removal efficiency increased rapidly to 83.3% in column A, but to only 31.5% due to the lack of alkalinity supply in column B.
Nitrification behavior at different influent NH4+-N concentrations
Figure 4 shows the transient behavior of NH4+-N and NO2 + 3−-N concentrations in columns A and B at pH 5.3 to 5.7. When influent NH4+-N concentration rose from 15 to 30 mg l−1 by 20 d, NH4+-N concentration became stable at about 0.55 mg l−1 in column A, but at 10.0 to 21.6 mg l−1 in column B. As high as 97.5% of NH4+-N removal was achieved in column A, whereas only 29% of NH4+-N was removed in column B. When the influent NH4+-N concentration rose to 60 by 40 d, NH4+-N removal efficiency dropped to 63.6% and 13.3% in columns A and B, respectively. With the increase in influent NH4+-N concentration, nitrification in column A still operated. This finding indicated that owing to the supply of alkalinity from oyster shell, nitrifying bacteria could tolerate the low pH effect. However, NH4+-N removal efficiency in column B was limited in a low level due to the lack of alkalinity supply, and decreased gradually with the increase in influent NH4+-N concentration.
Comparison of effluent pH at different influent NH4+-N concentration
Figure 5 shows the comparison of the effluent pH between column A and B at influent NH4+ -N concentrations of 15 (0∼ 20 d), 30 (20∼ 40 d), and 60 mg l−1 (40∼ 60 d), respectively, while the influent pH was in the range of 5.3 to 5.7.
In column A, the pH value was maintained at about 7.0 when influent NH4+-N concentrations were at 15 and 30 mg l− 1, and decreased slightly to 6.5 after NH4+-N concentration increased to 60 mg l−1. Although hydrogen ion was generated progressively by increasing the NH4+-N load, the pH was still maintained at about 6.5. This result is attributable to the buffer effect of shell to the acidity from influent and NH4+-N oxidation. In contrast, in column B, the pH appeared to be near 6.0 at NH4+-N concentration of 15 mg l− 1 in the influent, but dropped rapidly to 5.3 when the influent NH4+-N concentration increased to 30 and 60 mg l−1.
Compared with the conventional carrier of plastic balls, oyster shell carrier showed excellent nitrification performance. Consequently, we suggest that the oyster shell packed biofilter should be selected for treating wastewater with relatively high ammonia load and low pH value.
The authors acknowledge the funding for this study from the Natural Science Foundation, Fujian Province, China (D0210004).