Current World Environment

Introduction

China is experiencing rapid urban growth. Mounting population, industrialization, urbanization, and changing life style are resulting in the random generation and disposal of wastewater which in turns pollute the water environment. Nowadays, domestic wastewater has been widely studied by many researchers using different treatment processes, either by application of high-rate aerobic systems,^{1, 2, 3} or by application of low-rate systems³ and vertical flow constructed wetlands.⁴ Constructed wetlands are considered as an appropriate technology for treating domestic sewage in the rural areas of China with climatic, population, and socioeconomic considerations.⁵ To meet the standard effluent quality discharge of China, integrated A/O system and constructed wetlands is considered to be the best option.

Material and Methods

Experimental set-up

The experimental system is made up of an influent tank, pumps (model BT100), the integrated A/O system and artifical wetlands, an air compressor for aeration, and automatic aeration mixers to provide aeration in the oxic zones. The anoxic reactor has a total effective volume of 60 L and the oxic reactor has three segments with a total capacity of 27L. The reactor was connected in series with an internal diameter of 0.2m and a height of 2m for the anoxic columns. The oxic column has width size of 30cm and effective depth of 10cm, 20 and 30cm for first, second and third oxic segment. The height difference between each adjacent unit of Oxic is 0.5m. The domestic sewage drops to some porous baffles after it was pumped to a certain height. Both of the reactor’s columns (A/O) were filled with a non-woven fabric filter materials. The artificial wetland has a plot area of 1.2m² and bottom slope of 0.003%. Each chamber was composed of three different layers of matrix particles of different sizes: the 200mm layer of coarse gravel (20 to 40mm in diameter) and the 250 mm layer of grit gravel in the bottom, in the upper is a 100mm layer of fine sand (0.5-1.2 mm in diameter). The artificial wetland was planted with Chinese celery (Oenanthe javanica). Temperature was maintained in the range of 20 to 26.

Wastewater for the experiment and Analytical Procedures

Raw wastewater from a campus main manhole was pumped into a storing tank for sedimentation, and then fed into the reactor. The A/O reactor was inoculated with sludge obtained from Wuxi municipal sewage treatment plant. The raw domestic wastewater composition is shown in Table 1. The influent and effluent samples were collected in separate bottles after every two days and stored in refrigerator at 4^oC before experimental tests. The internal recycle ratio, R, can be defined as the ratio of returned flow rate (Qr) to that of the main influent flow rate (Q0). All the analyses were carried out in accordance to⁶ The influent and the effluent COD, NH4+-N, NO3--N and TP were measured according to.⁷
 

Table 1: Composition of the studied area domestic raw wastewater Click here to View table

Result and Discussion

Organic Material Removal (COD)

The average concentrations of COD in the influent and effluent during the experimental operations were 91.1mg/L and 17.4mg/L respectively.  It can be noted that, COD removal efficiency increases with increase in HRT (fig. 1 (a). The overall removal efficiency of COD was 70.4%. This result obtained is quite similar to the study conducted by.^{8, 9} In this study, the average removal of COD was 42, 68, 74, 83, and 85% with effluent concentration of 34, 17.5, 11.7, 7.7 and 15.9mg/l under HRT of 1.5, 4, 2, 3 and 5hrs, recycle ratios of 3, 2 and 1 respectively.

NH₄⁺-N and NO₃^--N Removal

The overall removal of NH₄ ⁺-N by the reactor was 85% almost similar to that obtained by.⁸ The average concentrations of NH₄ ⁺-N in the influent, anoxic oxic and artificial wetland during the experimental operations were 11, 2.4, 1.1 and 1.5mg/L respectively. The average removal of NH₄ ⁺-N was 78, 85, 85, 88 and 89 % with effluent concentration of 1.9, 1.2, 1.3, 2.1 and 1.2mg/l operated with HRT of 5, 3, 2, 4 and 1hr respectively fig. 1(b). According to literatures and a study by,¹⁰ the optimum pH condition for nitrifying bacteria is 7.5 to 8.6. For this study, the value of pH was maintained between the ranges of 7.6 to 8.1 indicating satisfactory condition of the reactor. In order for simultaneous nitrification and denitrification to take place in the anoxic reactor, the reactor was intermittently aerated. The average DO concentration in the reactor was in range of 2.2 to 3.1 mg O₂/l, respectively. During the operation, there is high concentration NO₃^--N in the effluent of the nitrification reactor (oxic). The overall removal of NO₃^--N by the system was 94%. The average removal of NO₃^--N was 92, 94, 94 and 97% with effluent concentration of 0.3, 0.2and 0.5mg/l operated with HRT of 2, 3, 5and 4hrs and recycle ratios of 2, 3, and 1 respectively fig. 1 (c). Increase in recycle ratio (R) enhances the denitrification efficiency of A/O but according to,¹¹ high increase of recycle ratio also inhibits denitrification.

Total Phosphorus Removal

The removal of TP was quite stable despite the concentration of TP in the influent was ranging between 2.9mg/l to 4.5mg/l. The effluent TP was mostly below 0.5 mg/. Most of the TP removal was accomplished in artificial wetland. The overall removal efficiency was 83%. The average removal of TP was 78, 85, 88 and 89% with influent and effluent concentration of 4.3, 3.5, 4.5, 2.9, 3.8mg/l and 0.5, 0.6, 0.7, 0.5 and 0.8mg/l operated with HRT of 5, 3, 2, and 1hr with recycle ratios of 3, 2 and 1 respectively fig. 1(d). Although temperature fluctuates and reed in the artificial wetland withered steadily during the winter season, TP removal wasn’t affected.

Figure 1: Effect of HRT on removal efficiency of  (a) average COD,  (b) average NH4+-N,  (c) average NO3--N and (d) average TP concentration in the influent, anoxic, oxic reactors and artificial wetland  Click here to View figure

Conclusion

This study used A/O and constructed wetland for treatment of campus domestic wastewater at different operating condition. According to the result obtained during the whole experimental operation, the A/O reactor and constructed wetland emerge to be well suited to the treatment of this kind of low strength campus domestic wastewater. Since heterotroph uses organic substrates as a source of carbon, heterotrophic denitrification is responsible for COD degradation. In the constructed wetland, most of the TP removal was accomplished by medium interception, microbial transformation and plant adsorption. The result obtained revealed that, the A/O reactor and constructed wetland is suitable and efficient in organic matter and nutrient removal

Acknowledgment 

This research has been supported by the National Natural Science Foundation of China (51078074) and the Key Project of Chinese Ministry of Education (308010).

References
 

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