Quantitative study of degradation coefficient of pollutant against the flow velocity＊

Ben-sheng Huang (黃本勝), Chang-hong Hong (洪昌紅), Huan-huan Du (杜歡歡), Jing Qiu (邱靜), Xin Liang (梁馨), Chao Tan (譚超), Da Liu (劉達(dá))

1.Guangdong Research Institute of Water Resources and Hydropower, Guangzhou 510610, China, E-mail: bensheng@21cn.com

2.Estuarine Water Technology National Local United Engineering Laboratory, Guangzhou 510610, China

3.Zhongjiao Guangzhou Chanel Bureau Ltd., Guangzhou 510221, China

(Received April 23, 2015, Revised December 20, 2015)

Quantitative study of degradation coefficient of pollutant against the flow velocity＊

Ben-sheng Huang (黃本勝)1,2, Chang-hong Hong (洪昌紅)1,2, Huan-huan Du (杜歡歡)1,2, Jing Qiu (邱靜)1,2, Xin Liang (梁馨)3, Chao Tan (譚超)1,2, Da Liu (劉達(dá))1,2

1.Guangdong Research Institute of Water Resources and Hydropower, Guangzhou 510610, China, E-mail: bensheng@21cn.com

2.Estuarine Water Technology National Local United Engineering Laboratory, Guangzhou 510610, China

3.Zhongjiao Guangzhou Chanel Bureau Ltd., Guangzhou 510221, China

(Received April 23, 2015, Revised December 20, 2015)

The pollutant degradation coefficient is one of the key parameters to describe the water quality change, for establishing a reasonable water quality model and to determine the water carrying capacity and the environmental capacity. In this research, the environmental channel experiment is conducted to simulate the degradation evolution of the COD and NH3-N under different flow velocity conditions in typical pollution water. It is shown that the processes of the COD and the NH3-N’s concentration over time are quite consistent with the first-order kinetic equation and the degradation coefficients increase with the increase of the flow velocity. When the flow velocity varies from 0 m?s?1to 0.87 m?s?1, the degradation coefficients of the COD and NH3-N increase from 0.011 d?1to 0.071 d?1and 0.038 d?1to 0.258 d?1, respectively. Moreover, the COD and NH3-N’s degradation coefficients both have excellent correlation with the reaction time. There is a good linear relationship between the COD degradation coefficient and the flow velocity as well as a good power exponential function between the NH3-N degradation coefficient and the flow velocity. The comparative analysis of the Youth canal prototype monitoring and the calculation results shows that the quantitative formula obtained from the indoor water channel experiments gives results very close to the prototype observation results, which could reflect the degradation of pollutants in river water with varying flow velocity.

Degradation coefficient, carrying capacity, prototype observation, flow velocity

Introduction

The water pollution becomes a serious issue with the long-term industrialization and the rapid urbanization in China in recent two decades, imposing a real challenge to the river system[1,2]. At present, the water environment management in China is under a change from the concentration control to the total amount control, with a water function area system being set up, tostrictly control the total amount of the sewage flows to rivers and lakes[3].

The calculation and the evaluation of the water carrying capacity and the environmental capacity become the key factor to control the total amount[4,5]. The contaminant degradation coefficient reflects the capacity of the specific water body to degrade the pollutants at a certain time and space. It is not only one of the key parameters for a water quality calculation model but also the important parameter to calculate the water environment capacity and the sewage carrying capacity[6]. Furthermore, it plays an important role in the total-amount-control of pollutions within the regional planning, the scientific allocation of the total load index, and the management of controlling process[7]. Methods commonly used to determine the contaminant degradation coefficient include the empirical formula estimation method, the data analysis analogy method, and the indoor experiment and prototypeobservation method[8,9]. However, the empirical estimation method is more subjective than the others and is hard to reflect the internal pattern of the pollutants degradation, which has a great impact on the reliability of the calculated result. The indoor experiment is usually conducted in a volume container, which is hard to reflect the hydraulic characteristics and the behaviors of the river course, and as a result, the measurement result is often smaller than the actual value. While the results of the prototype observation method are often applied in a particular time period for a particular river, its wide range of the pollutant degradation coefficient brings about many difficulties in practical applications. In addition, the pollutant degradation coefficient could be influenced by many factors, such as the water temperature, the pollutant characteristics, the microbial magnitude and species, the aquatic plant absorption and sediment adsorption[10-15], and especially, the hydraulic condition, which directly affects the spreading and the degradation of pollutants in the water body[16]. Quantitative research reports about the pollutant degradation coefficient and the flow rate of the hydraulic parameters are few.

This paper focuses on the COD and NH3-N, simulates the pollutant degradation process at different flow rates through the indoor environment channel model experiment, to obtain the quantitative relationship between the pollutant degradation coefficient and the water flow velocity. The result would provide a scientific basis for the accurate calculation of the water body’s carrying capacity and the water environmental capacity.

1. Materials and methods

1.1Study area and materials

The experimental raw wastewater is sampled from Huangdong village of the Beijiang River Basin, and it includes the rural domestic sewage and the agricultural effluent with typical characteristics of the nonpoint source pollution in the Guangdong province. DR2800 spectrophotometer is used to analyze the sample concentration, and all chemical reagents come from HACH Company. The flow velocity is monitored through the rotary paddle flow meter. The experiment is conducted for 23 d in total. The water samples are taken from fixed places of the channels at the same time everyday. The water temperature, the flow velocity, the NH3-N and COD concentrations are monitored at a same time in a daily basis in the experimental channels.

1.2Environmental channel experiments

The environmental test channels are self-made on the basis of the open channel hydraulics principle, and the photographs of the test channels are shown in Fig.1. There are five different slope sinks made of Perspex sheet, and theflow velocityis controlled by the adjustment of the slope. The channel water is circulated with the use of the QZ-144 type mixed submersible pumps. The slope is adjusted to make the water flowing freely and make sure that the flow of the pump is equal to that of the channel in order to avoid the hydraulic jump and the aeration which might interfere with the study. Design parameters of the environmental channel are shown in Table 1 in a descending order of the velocities. The dimension of each of the five channels is 14 m long, 0.6 m high and 0.1 m wide, while the water depth is 0.4 m. The water velocities of the five channels are 0 m?s?1, 0.17 m?s?1, 0.36 m?s?1, 0.60m?s?1and 0.87 m?s?1, respectively. The static water (5# channel) is to imitate the hydrostatic lakes and reservoirs, while the rest could represent different flows of plain rivers. The temperature of the sewage in the channels is about 20oC during the experiments.

Fig.1 The environmental test channels for measuring the pollution decaying coefficients with water flowing

Table 1 Parameters of experimental channels and hydraulic characteristic

2. Results and discussions

The pollutant concentration degradation with time can be described by a first-order kinetics model[2,17-19], in the form as followswheretrepresents the reaction time,krepresents the degradation coefficient of pollutants,Lrepresents the concentration of pollutants at timet, andL0is the initial concentration of pollutants.

2.1The variation of COD degradation coefficient

For different flow velocities in different channels, the change rate of the COD concentration varies accordingly, as shown in Fig.1. The fitting results for the first-order kinetics model are shown in Table 2. It can be seen from Fig.2 that the initial concentration of COD is 90 mg?L?1, then the concentration decreases steadily with the reaction time for 16 d, before the concentration of COD in each channel reaches a stable value. The degradation trends of the COD under different velocity conditions show that the degradation rate increases with the acceleration of the water velocity. The concentration of the COD of channel 1# with the maximum velocity is dropped to 30 mg?L?1, while that of channel 5# under static conditions is down to 75 mg?L?1. The first-order kinetics fitting results in Table 2 show a fine fitting effect: the correlation coefficientsRin the first-order kinetics equation are all above 0.99, and the initial concentration is very close to the measured value. This indicates that the COD degradation processes can be described by the first-order kinetics model, which reflects very well the degradation pattern of the COD at different water velocities.

Table 2 Fitting results ofkCODand different flow velocities

Fig.2 Relation curves of the concentration of COD and hydraulic retention time

The degradation coefficient increases with the rise of the flow velocity. Under static water conditions, the degradation coefficient of the COD is 0.011 d?1, when the velocity reaches 0.87 m?s?1, the degradation coefficient increases to 0.071 d?1. On one hand, the increase of the velocity enhances the dilution and diffusion capacity of the pollutants as well as the uniformity of microorganisms in water, on the other hand, it consolidates the reoxygenation capacity of the water body, which could increase the dissolved oxygen in water and the reacting probability between the organic pollutants and the dissolved oxygen, thus speed up the degradation process of the pollutants. The result of linear fitting between the degradation coefficients of the COD under different flow velocities and the velocities is shown in Fig.3, in which the correlation coefficientRreaches 0.9946, showing an excellent linear correlation. The quantitative relationship between the degradation coefficients of the COD and the flow velocities is

Thus, a quantitative relationship betweenkCODand the flow velocityvis established, and the defects in existing methodology are removed, and it is shown that the COD degradation coefficient takes basically a value under a particular hydrological condition or a value of a wide range, thus providing a more accurate calculation method for the environmental capacity in a specific drainage basin.

Fig.3 Relation curve ofkCODand flow velocity

Fig.4 Relationship between the concentration of NH3-N and hydraulic retention time

2.2The variation of NH3-N degradation coefficient

The degradation processes of NH3-N at different velocities are shown in Fig.4, in which it could be seen that the initial concentration of NH3-N in each channel is 42.50 mg?L?1, after 14 d, apart from the channel 5#, in which the concentration of NH3-N is 25.75 mg?L?1, those of the other channels are all below 0.20 mg?L?1, indicating that the flow of the water body plays an extremely important role in the degradation of NH3-N. The relation curves between the concentration of NH3-N and the hydraulic retention time are presented in Fig.3, and the parameters of the first-order kinetics model are listed in Table 3.

Table 3 Fitting result ofkNH3-Nand different flow veloci-ties

Fig.5 Relation betweenkNH3-Nand flow velocity

High correlation coefficients show that with the first-order kinetics model, a perfect fitting can be obtained for the degradation of NH3-N in the water body. In a still water, the degradation coefficient of NH3-N is only 0.038 d?1, but it goes up to 0.169 d?1immediately in channel 4# when the velocity increases to 0.17 m?s?1, and the value is 4.45 times higher. The relationship between the degradation coefficient of NH3-N and the velocity is shown in Fig.5. The relation betweenkNH3-Nandvsees a tendency of a power function and the correlation coefficientsRare all above 0.98. Thus, the quantitative relationship betweenkNH3-Nandvis as follows

It is shown that the degradation process is very slow when the velocity is less than 0.17 m?s?1, however, the degradation coefficient of NH3-N increases significantly when the velocity goes up from 0.17 m?s?1to 0.87 m?s?1. For instance, when the flow velocity is 0.87 m?s?1, the degradation coefficient increases to 0.258 d?1, which might be related to the augmentation of the dissolved oxygen in water. The dissolved oxygen concentration maintains at about 2 mg/L in the static water, which increases with the flow velocity in channels 2 through 5# and can reach about 7 mg/L in a motion water finally. From the above results, it can be seen that the pollutant degradation in lakes is significantly slower due to the weak hydraulic exchange, while the degradations in rivers and other water bodies are faster because of the good oxygenation capacity, which is beneficial to the degradation of pollutants.

2.3Prototype monitoring and validation

To further validate the accuracy and the reliability of the indoor water channel experiment results, the Youth Canal in Zhanjiang City of Guangdong Province is selected as a representative river, and the water quantity-quality prototype observation experiments were carried out in the upstream and downstream sections in September 2014. Originating from Hedi Reservoir in Lianjiang City of Guangdong Province, the Youth Canal is 74 km long, 17 m-20 m wide, with its riverbed up to 4 m-5 m, which serves mainly as the living water supply, the industry water, and the water supply for agricultural irrigation and tourism. The prototype observation stations are set at Hedi Reservoir and Jianshe road, with a total length of about 50 km between upstream and downstream sections. The study area mainly covers the upper stream in the rural area of Suixi County, a piece of undeveloped land where the main use of the river is for agricultural irrigation. As the water usage for agricultural irrigation in September is relatively low, the inspected pollutant discharge is also reduced accordingly. Main hydrological and water quality monitoring data are shown in Table 4.

The pollutant comprehensive degradation coefficient of the Youth Canal can be obtained from the measured data from the upstream and the downstream, and the calculating formula is as follows[19]

In formula (4),C1is the pollutant concentration in the upstream section of the river,C2is the pollutant concentration in the downstream section of the river, and x is the distance of the calculation zone.Since thereis a temperature difference between the actual river and the experimental water, the temperature compensation correction is made for the pollutant degradation coefficient according to the Arrhenius empirical formula[7]

Table 4 The hydrologic conditions, COD and NH3-N concentrations of Longhua river

Table 5 The results ofkCODandkNH3-Ncalculated by experiment formula and prototype observation

wherek20is the COD degradation coefficient at 20oC, andθis the temperature correction factor, which is a dimensionless empirical coefficient with a value of 1.047.

During the inspection period, the DO concentration in the stream prior to the dam of Hedi Reservoir is 9.88 mg/L, while that of the Jianshe Road section of the lower stream is 8.70 mg/l. A relatively high DO concentration of above 8 mg/L occurs through the whole inspection period, which is similar to the experimental channels. Therefore, it can be seen that the oxygen-consuming speed of the sediments is lower than the water re-oxygenation speed and the aerobic reaction of the sediments has little impact on the pollutant degradation.

The results obtained from the experimental formula and the prototype observations after the temperature compensation are shown in Table 5. It can be seen from the observation results that the COD and NH3-N concentrations at the Hedi Reservoir are 5.4 mg?L?1and 0.21 mg?L?1, respectively, then after the degradation for 50 km, the COD and NH3-N concentrations are reduced to 5.0 mg?L?1and 0.14 mg?L?1, respectively. The comprehensive degradation coefficients of COD and NH3-N in this prototype observation of the Youth Canal are 0.0554 d?1and 0.2019 d?1, respectively.

During this prototype observation, the flow velocity of the Youth Canal is about 0.5 m?s?1. The calculation by the formula obtained from the water channel experiment shows that the comprehensive degradation coefficients of COD and NH3-N at 20oC are 0.0441 d?1and 0.2003 d?1, respectively. Taking into consideration of the temperature compensation coefficient 1.3 at 31oC, the calculated comprehensive degradation coefficients of COD and NH3-N are finally 0.0571 d?1and 0.260 d?1, respectively. The above calculated results are very close to the inversed prototype observation results, where the COD degradation coefficient differs only by about 3%, while NH3-N degradation coefficient is slightly higher by about 11%. This may own to the sediment adsorption and purification by aquatic plants along the river, which can increase the removal of NH3-N. The prototype observation results show that the formula obtained by the indoor water channel experiment can well describe the degradation process of the river water pollutants against the flow velocity variation.

3. Conclusion

The degradation coefficient of pollutants is an important parameter that describes the variation of the water pollution extent. It is also an important parameter to establish the water quality model and calculate the water carrying capacity for pollutants and the environmental capacity. In this study, the environmental experiment channel is used to simulate the hydraulic conditions of different flow velocities. The degradation processes of COD and NH3-N under different flow velocities are studied, the fitting relations of COD and NH3-N with the first-order kinetics model are successfully obtained. The quantitative relationship between the degradation coefficients of COD and NH3-N and the flow velocities are also established, in an excellent linear correlation and a power function relationship, respectively. The comparative analysis of the Youth canal prototype monitoring and the calculation results shows that the results calculated by the quantitative formula obtained from the indoor water channel experiment are very close to the prototype observation results, which could describe the degradation of pollutants in river water against the flow velocity. The resultwould provide a more scientific key parameter for the calculation of the water quality model.

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* Project supported by the Guangdong Provincial Hydraulic Technology Innovation Project (Grant Nos. 2009-24, 2011-08, 2012-03) the Guangdong Provincial Hydraulic Technology Key Innovation Project (Grant No. 2014-06).

Biography:Ben-sheng Huang (1965-), Male, Ph. D., Professor

Chang-hong Hong, E-mail:changjianghong@126.com