Long-term analysis on determination of deoxygenation rate of urban river water

Water quality modeling is one approach in developing river management strategies. The deoxygenation rate is an important coefficient in the water quality of rivers modeling. This study aims to obtain a coefficient of deoxygenation rate using long-term method that can represent the condition of urban rivers with high levels of pollution. The study location was the Cikapundung River, which runs through Bandung, Indonesia. The method used to determine the rate of deoxygenation is long term, with an analysis period of 30 days in the laboratory. The Slope Method and empirical equations from Hydroscience were used to process the data. The results showed that the deoxygenation rate ranged from 0.230 to 0.291 per day. The value of the deoxygenation rate ranges from 0.40 to 0.81 per day using the Hydroscience empirical equation. The overall biochemical oxygen demand Ultimate (La) ranged from 62.03 to 77.18 mg/L. The deoxygenation rate value obtained using the long-term method shows a relatively higher result than the value obtained using the short-term time range. The long-term method is better than the short-term method because the results obtained from the long-term method are closer to the results of the empirical equation.


Introduction
Rivers located in big cities in Indonesia experience relatively heavy pollution [1,2,3].Therefore, various efforts must be made to restore the quality of the river to its original state as designated.One method to recover the quality of the urban river is the model results usage in developing a rehabilitation strategy.
Modeling of river water quality was evolved since the 1920s.A deoxygenation rate coefficient that describes the speed of oxygen consumption in the self-purification activity is included in the formula used to simulate river quality.The deoxygenation rate used in water quality modeling must be consistent with the conditions of the river being modeled.
The Cikapundung River is strategic in West Java with the highest population density in Indonesia.Along with problems in urban development, the critical problem is water resources as a raw source for the main supply of drinking water.The Cikapundung River is the water source for the Water Supply Company of Bandung City [4,5].However, human activities make rivers a place for sewage, domestic waste disposal, and municipal waste.
Waste discharged from domestic activities can pollute the Cikapundung River.Some measures can be taken to recover the environmental quality of the Cikapundung River.River modeling on the water quality may provide basic information to execute a priority rehabilitation program for the river.
The results of river water quality modeling can support the development of management strategies for rehabilitating contaminated urban rivers.Dissolved oxygen (DO) and biochemical oxygen demand (BOD) are important parameters to be considered during water quality modeling.DO is often used as a river quality indicator [6].The variation in the DO concentration can be used to describe the river's ability to perform self-purification [7,8].Decreasing DO in the self-purification process is occurred due to the decomposing organic matter by microorganisms.Deoxygenation is defined as a decrease in DO concentration, while an increment in DO concentration due to DO fixation is known as reaeration, which may be due to the turbulence in river flow [9].The main processes involved in the rise and fall of DO and BOD concentrations are decomposition and aeration, respectively.A contaminated river may remove pollutants, particularly organic pollutants from domestic activities during self-purification.Naturally, pollution in river water can be recovered when microorganisms and oxygen are available.Organic contaminants can be degraded naturally.
The deoxygenation rate in the Cikapundung River was determined in a previous study [10].However, the method used was a 10-day short-term treatment in an incubator at 20C.The long-term period in determining the deoxygenation rate is a better duration for determining the coefficient [11].The use of the right coefficients can present simulation results that can depict the actual conditions [12,13].
This study obtained the coefficient of the deoxygenation rate of the Cikapundung River using the long-term laboratory analysis method and empirical formula.The result is compared with the deoxygenation rate value of previous short-term method research results.

Methods
In this study, sampling was performed at 3 (three) locations, representing the stream areas in Bandung City, i.e.Point 1 is on the Cikapundung River, passing through Dago Bangkok Street, representing the upstream segment.Point 2 is located at the Viaduct area, representing the middle segment, and Point 3 is at Soekarno Hatta Street, representing the downstream segment.Figure 1 shows the sampling sites.

Figure 1. Map of sampling locations
Direct measurements are performed to identify the onsite conditions of the sampling location, then direct inspections at the sampling location i.e., discharge, temperature, pH, and DO were performed.
Sampling was performed in the early rainy season.At the time of sampling, the weather was sunny and slightly cloudy.The points determined are areas representing urban rivers, with activities in the Bandung City.Water samples are stored in jerry cans.During the journey, the water samples were kept in a cool box at 4C of temperature.After arriving at the laboratory, the water samples were transferred into BOD bottles and incubated at 20C.
After collecting the water sample, the water quality was handled at the Environmental Engineering Water Laboratory, at Universitas Pasundan.The DO parameter was used to examine the rate of deoxygenation.The incubation process was performed using a Winkler bottle for 30 days in an incubator at 20°C.The DO was measured for days, i.e., 0, 0.2, 0.8, 1.2, 2.2, 2.8, 3.2, 4.0, 5.0, 7.0, 12.0, 20.0, and 30.0.The concentration of dissolved oxygen was analysed based on Winkler's modified technique [14].
Empirical formula (equation 1 and 2) to measure the deoxygenation rate of rivers considering the water depth has been developed [15,16].The formula can be used to calculate the oxygenation rate for normal flow [17].
(2) where K1 is the deoxygenation rate (/day), and H is the water depth (ft).

3.
Results and Discussions Using the measurement data of cross-sectional profile and velocity, the river water discharge was calculated.Table 1 shows the results of the calculation of the Cikapundung River discharge.The discharge obtained from the calculation is relatively low because it is performed in the absence of rain.Slow velocity causes slow reaeration in the water column [18].Temperature, DO, and pH are important parameters that influence the river water quality and the deoxygenation rate.A higher degree of water temperature may accelerate the metabolism, and chemical reactions, as well as reduce the solubility of oxygen in water [19].The pH is a limiting factor for organisms that live in water.The pH of natural water ranges from 6 to 9. DO concentration is used to describe the ability to perform self-purification.Changes in the DO concentration in water are influenced by the process of reducing dissolved oxygen due to bacterial activity in decomposing organic matter in water [19,20].The results of direct measurements in the field are shown in Table 2.The water quality standard for DO parameters in the Cikapundung River is at least 4 mg/L.The river in the upstream area has a very low DO concentration (Table 2), whereas it is relatively close to the minimum standard, which is 4 mg/L in the middle and downstream areas.The Slope or Thomas Method uses the results of the accumulated DO loss calculation, which are then plotted into the graph.Based on the data from the upstream, middlestream, and downstream segment points, each point has 3 samples that were analyzed individually from day 0 to 30.The accumulated DO losses at each sampling point are shown in Figures 2-4.The average of these 3 samples in each sampling point is shown in Fig. 5.  Overall, the deoxygenation rate for the upstream segment ranges from 0.27 to 0.28 per day, with an average value of 0.273 per day.In the middle segment, it ranges from 0.28 to 0.303 per day with an average value of 0.291 per day.While for the downstream segment, it ranges from 0.22 to 0.24 per day with an average value of 0.230 per day.
Each sample's Ultimate BOD values were obtained through laboratory analysis.BOD Ultimate is the dissolved oxygen required by microorganisms to completely decompose organic compounds.The highest deoxygenation rate value is found the middle stream, indicating the high pollution that occurs because of the high domestic activities that were being degraded by the microorganisms supported by high oxygen availability.At point 1 (upstream), the BOD value is high compared to other points.
The biological decomposition of organic matter in river bodies depends on the dynamic conditions of the environment around the river, the types of microorganisms present in the river body, and the number of microorganisms [9].The deoxygenation rate at point 3 (downstream) was lower than that at points 1 and 2. Low deoxygenation rate values indicate low oxygen consumption by microorganisms or lack of healthy microorganisms in the organic matter degradation processes [7,21].
Meanwhile, the Ultimate BOD range on the Cikapundung River for upstream points ranges from 76.37 to 78.14 mg/L with an average value of 77.18 mg/L.The Ultimate BOD (La) value of the middle stream point ranges from 60.6 to 63.75 mg/L with an average value of 62.03 mg/L, whereas the Ultimate BOD (La) value for the downstream point ranges from 60.91 to 67.17 mg/L with an average value of 64.78 mg/L.The overall value of the Ultimate BOD (La) range is 62.03-77.18mg/L.
The results of previous studies on the Cikapundung River using 10-day laboratory observations showed that the deoxygenation rate ranges from 0.03 to 0.24 per day [10], 0.01 to 0.17 per day [22], whereas the deoxygenation rate range of the Cikapundung River in this study ranges from 0.230 to 0.291 per day.Comparing those deoxygenation rate coefficient values, using a long-term period of laboratory analysis yields a higher rate.A higher deoxygenation rate indicates a more rapid self-purification in rivers.The study with the long-term method also proves that the longer the observation period, the better it is to represent the actual condition of the river.
The empirical formula of Hydroscience for calculating the deoxygenation rate considers water depth with the assumption that oxygen is abundant in shallow water.The number of microorganisms increases in rich oxygen and organic matter.When water contains high concentration of oxygen, organic matter and decomposing microorganisms, the rate of decomposition will be faster, indicating the high value of the deoxygenation rate.
The depth of the Cikapundung River is 1.5 m at point 1, 0.3 m at point 2, and 0.4 m at point 3. Point 1 has the deepest water column and the lowest deoxygenation rate based on empirical formula calculation.Table 4 shows the value of the deoxygenation rate calculated for each sample using the empirical formula.4 shows that the deoxygenation rate of the Cikapundung River water using the empirical formula ranges from 0.40 to 0.81 per day.A previous study in the same river showed a deoxygenation rate of 0.5-0.6 range per day [10].This value depends only on the water depth.

Conclusion
Using a 30-day long-term laboratory analysis, it was found that the value of the deoxygenation rate coefficient analysis ranges from 0.230 to 0.291 per day.Whereas using the Hydroscience empirical formula, it was obtained that the deoxygenation rate ranges from 0.40 to 0.81 per day.The BOD Ultimate value ranged from 62.03 to 77.18 mg/L.The deoxygenation rate value obtained using the long-term method shows a relatively higher result than the value obtained using the short-term time range.The long-term method is better than the short-term method because the results obtained from the long-term method are closer to the results of the empirical equation.

Figure 2 .Figure 3 .
Figure 2. Accumulation of DO loss in the upstream points

Figure 4 . 5 Figure 5 .
Figure 4. Accumulation of DO loss in the downstream point

Table 1 .
River velocity and discharge

Table 2 .
Temperature, DO, and pH

Table 3 .
Deoxygenation rate and ultimate BOD using the Thomas or Slope Method

Table 4 .
Value of deoxygenation rate with empirical formula