JOURNAL OF SCIENCE, Hue University, Vol. 70, No 1 (2012) pp. 275-288 275 ASSESSMENT OF ASSIMILATIVE CAPACITY OF DONG BA AND BACH YEN RIVER BRANCHES, HUE CITY Hoang Ngoc Tuong Van, Tran Quang Loc Institute of Resources, Environment and Biotechnology, Hue University Abstract. The authors have applied mathematical basic and modified Streeter - Phelps equation to calculate the maximum allowable BOD load discharged into Dong Ba and Bach Yen branches, from which to assess their assimilative capacity (AC) according to three different scenarios. The results show that Bach Yen branch (BY) has the lowest AC followed by DB1 and DB2 of Dong Ba branch. Assimilative capacity of these branches in rainy season is much higher than that in the dry one in three scenarios. In addition, outputs of Streeter-Phelps model (represented by travel time and distance to critical sag, deficit at critical sag) indicate that it is necessary to consider nitrification and SOD when applying Streeter-Phelps equation combined with Fair model to assess AC. Moreover, the authors have conducted the calculation of actual BOD load from various sources of wastewater discharged into these branches and the actual BOD load is much lower than the maximum allowable. It indicates that Dong Ba and Bach Yen branches are still likeky to receive organic pollutants without violating their best-designated conditions. Keywords: modified Streeter - Phelps, assimilative capacity (AC), BOD load, Dong Ba river, Bach Yen river. 1. Introduction Hue, a famous city in Vietnam with its world cultural heritage of palaces, monuments, landscape, etc., has been being severely threatened. The existing of boat communities with very low-living conditions and education levels together with the limitation of infrastructure system is causing pollution to the Huong River. Dong Ba and Bach Yen river branches are the tributaries of Huong River flowing through Hue city. Previously, they were in the hydro system of Hue citadel and played an important role in the circulation of water between the Citadel and the outside. Yet, as urban population has increased rapidly, the two branches of this river have become places to receive waste from Hue residents’ daily activities, sewage from Dong Ba and Phu Binh market, and especially from people living on boats. There are about 941 boat households with 6529 dwellers living on “slum-boats”
Transcript
JOURNAL OF SCIENCE, Hue University, Vol. 70, No 1 (2012) pp. 275-288
275
ASSESSMENT OF ASSIMILATIVE CAPACITY OF DONG BA AND BACH YEN RIVER BRANCHES, HUE CITY
Hoang Ngoc Tuong Van, Tran Quang Loc
Institute of Resources, Environment and Biotechnology, Hue University
Abstract. The authors have applied mathematical basic and modified Streeter -
Phelps equation to calculate the maximum allowable BOD load discharged into
Dong Ba and Bach Yen branches, from which to assess their assimilative capacity
(AC) according to three different scenarios. The results show that Bach Yen branch
(BY) has the lowest AC followed by DB1 and DB2 of Dong Ba branch.
Assimilative capacity of these branches in rainy season is much higher than that in
the dry one in three scenarios. In addition, outputs of Streeter-Phelps model
(represented by travel time and distance to critical sag, deficit at critical sag)
indicate that it is necessary to consider nitrification and SOD when applying
Streeter-Phelps equation combined with Fair model to assess AC. Moreover, the
authors have conducted the calculation of actual BOD load from various sources of
wastewater discharged into these branches and the actual BOD load is much lower
than the maximum allowable. It indicates that Dong Ba and Bach Yen branches are
still likeky to receive organic pollutants without violating their best-designated
conditions.
Keywords: modified Streeter - Phelps, assimilative capacity (AC), BOD load,
Dong Ba river, Bach Yen river.
1. Introduction
Hue, a famous city in Vietnam with its world cultural heritage of palaces, monuments, landscape, etc., has been being severely threatened. The existing of boat communities with very low-living conditions and education levels together with the limitation of infrastructure system is causing pollution to the Huong River.
Dong Ba and Bach Yen river branches are the tributaries of Huong River flowing through Hue city. Previously, they were in the hydro system of Hue citadel and played an important role in the circulation of water between the Citadel and the outside. Yet, as urban population has increased rapidly, the two branches of this river have become places to receive waste from Hue residents’ daily activities, sewage from Dong Ba and Phu Binh market, and especially from people living on boats.
There are about 941 boat households with 6529 dwellers living on “slum-boats”
276 Assessment of assimilative capacity of…
and scattering mainly in these branches. A large number of boat dwellers use their boats for both working and living. These boat-dwellers have been livng here for many years on hundreds of old boats parking on the riversides, which affects the aesthetic of Hue city as well as the flow circulation of the river system and water transportation. This makes these branches polluted locally and their water quality decrease significantly.
So far, there have been several articles giving warnings of river water quality, especially the quality of water from the part of the river flowing through Hue city is gradually reduced. Yet, few topis and projects study the self-purification of these tributaries to give warnings and forecasting of pollution levels in order to propose maximum allowable discharge. Thus, the authors have applied Thomas’ transformation of the initial Streeter-Phelps model, modified Streeter - Phelps equation developed by ADEM [6] and the monograph for its solution proposed by Fair et al (1968) to study the assimilative capacity of Dong Ba and Bach Yen branches, Hue. The results from this study will provide input for modeling water quality or water pollution, allow assessment of the impact of waste sources on river water quality. These will provide information necessary for sustainable development planning.
Fig.1. Positions of water samples and sewage wastewater samples
HOANG NGOC TUONG VAN, TRAN QUANG LOC 277
2. Research method
To evaluate the self-purification ability, the authors used the basic Streeter - Phelps equation combined with the monograph of Fair et al (1968) to estimate the maximum BODu load input into the rivers while its DO level is maintained above a specified minimum level and calculate the assimilative capacity (AC). In addition, the authors also calculated critical time and critical point (tc and xc) of the basic and modified Streeter - Phelps equation, then compared these parameters to determine whether it is necessity to consider nitrification and sediment oxygen demand or not.
2.1. Basic Streeter - Phelps equation
( ) tKa
tKtK eDeeKK
LKD 221
12
01 −−− +−−
= (1)
* Dissolved oxygen deficit and critical time:
ctKac eL
K
KD 1
2
1 −= (2);
−−
−=
aac LK
KKD
K
K
KKt
1
12
1
2
12
1ln1
( 3)
The procedure for calculating the critical BOD load is outlined as below:
D (mg/l): the dissolved oxygen deficit at the transition when the substrate is all utilized
Lp (mg/l): the concentration of BODu just prior to the discharge point. If monitoring data do not exist, some measurements are advisable. Lp consists of BODu of the river (L0) and BODu of the waste source (Lp =L0 + LBOD of waste source), if discharge does not exist, Lp = L0
La (mg/l): allowable concentration of BODu just after the discharge point
D0 (mg/l): initial dissolved oxygen deficit.
Ca: the initial dissolved oxygen concentration (prior or just after the discharge point). If monitoring data do not exist, some measurements are advisable. Ca may be assumed close to Cs in the case of very clean rivers.
Cc: the critical (minimum acceptable) dissolved oxygen concentration. This depends on the kind of species to be protected. Cc is normally set by the standards applicable in the study area. In this study, Cc is set by level A2, QCVN 08:2008/BTNMT.
K1 (day-1): BODu removal rate constant
K2 (day-1): re-aeration rate constant
t: travel time, calculating from the equation x/v, in which x: distance and v: velocity .
278 Assessment of assimilative capacity of…
To simplify the Streeter - Phelps model in calculating self-purification of a river (AC) and maximum allowable BOD load discharged into rivers, Fair et al developed a monograph to allow quick estimation of La and then AC and LBOD.
Fig. 2. The monograph of allowable BODu loading of the receiving waters et al (Gayer, 1968)
Definition of the Model Inputs
Qr (m3/s): the river water flow rate, defined from hydrological data and reflecting
minimum flow during the dry (usually summer) season. In this study, Qr is determined as belows:
- Dong Ba river: Qr is determined by the minimum monthly mean monitoring data of Huong river during 7 years (2003 - 2010) and Qr = 15 m3/s, occuring in June, 2003.
- Bach Yen river: Qr = 5.5 m3/s, the minimum value among the 4 monitoring periods in 2010, occurs in June, 2010.
Qw (m3/s): the waste water flow rate (required only in cases where Qw is significant in relation to Qr, otherwise Qw can be assumed to be zero).
Dc (mg/l): the critical DO deficit, from Equation Dc = Cs - Cc
Da (mg/l): the initial DO deficit, from Equation Da = Cs - Ca
HOANG NGOC TUONG VAN, TRAN QUANG LOC 279
Calculation Procedure
Cs (mg/l): the saturation concentration of the dissolved oxygen in the water.
Da/Dc ratio, (Note: the limiting values are 0 for very clean waters, in which case the permitted BODu loads are maximum, and 1 for critically polluted waters, in which case zero BODu loads are allowed).
La/Dc ratio, obtained from the monograph of Fair et al (1968) as function of f and Da/Dc.
La: the allowable concentration of BODu just after the discharge point, from Equation La = Dc(La/Dc) ;
f: Self-purification constant, from Equation f = K2/K1. Alternatively, a typical value of f can be selected, depending on the kind of receiving water, from the ranges of applicable values of f given in the upper part of the monograph of Figure 2. It should be noted that although K2 and K1 depend on temperature, their ratio f, is much less temperature dependent.
Bu, the maximum allowable BODu load discharge at the river, from Equation:
AC, assimilative capacity, from Equation: AC = 24×3600×La × Qr/1000 (kg/day) (5)
2.2. Modified Streeter - Phelps equation
One water quality model sometimes used by the Alabama Department of Environmental Management (ADEM) to develop waste load allocations (WLAs) and total maximum daily loads (TMDLs) for oxygen demanding wastes is the Spreadsheet Water Quality Model (SWQM). The version of SWQM sometimes used by ADEM in the development of TMDLs for dissolved oxygen is a steady-state model relating dissolved oxygen concentration in a flowing stream to carbonaceous biochemical oxygen demand (CBOD), nitrogenous biochemical oxygen demand (NBOD), sediment oxygen demand (SOD) and reaeration. The model allows the loading of CBOD, NBOD and SOD to the stream to be partitioned among different land uses (nonpoint sources) and wastewater treatment facilities (point sources).
The SWQM is based on the Streeter-Phelps dissolved oxygen deficit equation with modifications to account for the oxygen demand resulting from nitrification of ammonia (nitrogenous oxygen demand) and the organic demand found in the waterbody sediment. Equation (6) shows the Streeter-Phelps relationship with the additional components to account for nitrification and SOD.
280 Assessment of assimilative capacity of…
( ) ( ) tKtKtKtKtKtK eDeHK
SODee
KK
NKee
KK
LKD 222321
0232
03
12
01 )1( −−−−−− +−+−−
+−−
= (6)
where: D = dissolved oxygen deficit at time t, mg/l ;
L0 = initial CBOD, mg/l
N0 = initial NBOD, mg/l (NBOD = NH3-N x 4.57)
D0 = initial dissolved oxygen deficit, mg/l
K1 = CBOD decay rate, 1/day;
K2 = reaeration rate, 1/day
K3 = nitrification rate, 1/day;
SOD=sediment oxygen demand, g O2/ft2/day
H = average stream depth, ft;
t = time, days
CBOD concentration, expressed as L0 in Equation (6), is the ultimate carbonaceous biochemical oxygen demand. Oxygen demand by benthic sediments and organisms can represent a significant portion of oxygen consumption in surface water systems. Benthic deposits at a given location in an aquatic system are the result of the transportation and deposition of organic material. The material may be from a source outside the system, such as leaf litter or wastewater particulate CBOD, or it may be generated inside the system as occurs with plant growth. In addition to oxygen demand caused by decay of organic matter, the indigenous invertebrate population can generate significant oxygen demand through respiration. The sum of oxygen demand due to organic matter decay plus demand from invertebrate respiration is equal to the sediment oxygen demand (SOD).
The process by which oxygen enters a stream is known as reaeration. Equation (6) shows the net effect on dissolved oxygen concentration of the simultaneous processes of deoxygenation through the decay of carbonaceous organic matter, nitrification of ammonia, SOD and reaeration.
Commonly used method for estimating a stream’s reaeration rate is the O’Conner-Dobbins formulation shown in Equation (4). This formulation generally works best for streams with a depth of greater than 5 feet and a slope of less than 2 feet/mile.
2/3
2/1
2
93.3
H
UK = (7)
where: K2 = reaeration rate at 20°C, 1/day
HOANG NGOC TUONG VAN, TRAN QUANG LOC 281
U = stream velocity, m/s
H = stream depth, m
Temperature affects the rate at which reactions proceed. Reaction rates are generally expressed with units of per day at 20°C. If the reactions are occurring at a temperature other than 20°C, then the reaction rates must be corrected for the new temperature. The most commonly used expression to adjust reaction rates for temperature is the modified Arrhenius relationship shown in Equation (4).
( ) ( )2020
2
2
−° Θ= TCT KK (8)
where: KT2 = reaction rate at the new temperature, 1/day
K20°C = reaction rate at 20°C, 1/day
The θ values for each of the reaction rates shown in Equation (6) vary slightly from reference to reference but in the SWQM the θ values for COD decay, reaeration, nitrification and SOD are respectively listed: 1.048, 1.024, 1.080 and 1.060.
The dissolved oxygen saturation concentration at a pressure of 1 atmosphere and a given temperature is computed using Equation (4) taken from Standard Methods for the Examination of Water and Wastewater, 16th Edition.
* To solve Equation 6, it is assumed that:
K1 often ranges from 0.1 to 0.5 for most of the streams, selecting K1 = 0.23 day-1 [9].
K3: often ranges from 0.1 to 0.5, selecting K3,20 = 0.25 day-1.
SOD = 0.5 g/ft2.day at 200C for natural waters with low flow rate, sandy bottom.
3. Results and discussion
3.1. Streeter - Phelps model
To study the application of Streeter – Phelps model to evaluate the assimilative capacity of these two branches. Dong Ba river is divided the rivers into these following sections :
+ DB1: the part of river, from Dong Ba market to Thanh Long Bridge, most of the main town’s drainage is gathered in this section.
+ DB2: section from Thanh Long Bridge to Bao Vinh confluent, this section receives wastewater mainly from people living along river banks and there is a little drainage from the main town.
In this paper, the authors have carried out calculation of maximum allowable BOD and assimilative capacity according to two different scenarios:
282 Assessment of assimilative capacity of…
Regarding Dong Ba river branch:
- Maximum scenario: wastewater generated from main town drain, market, industry and 70% runoff;
- Medium scenario: wastewater generated from main town drain, market, industry and 30% runoff.
Regarding Bach Yen river branch (BY):
- Maximum scenario: 100% runoff discharged into the river.
- Medium scenario: 30% runoff discharged into the river.
Replacing input data into equation 2.5 and 2.6, results are shown in the following table:
Table 1. Maximum allowable BOD and Assimilative Capacity of Dong Ba-Bach Yen rivers
Input and output
data Symbol Unit
DB1 DB2 BY
Dry
season
Rainy
season
Dry
season
Rainy
season
Dry
season
Rainy
season
Temperature of
river water T (oC) 32 28 31 28 31 28
Initial DO (prior
to discharge
point)
Ca mg/l 6.0 6.1 6.2 6.3 5.3 5.4
Critical DO Cc mg/l 5.0 5.0 5.0 5.0 5.0 5.0
Saturation DO at
ToC Cs mg/l 7.30 7.82 7.43 7.82 7.43 7.82
Dc = Cs – Cc Dc mg/l 2.30 2.82 2.43 2.82 3.43 3.82
Da = Cs – Ca Da mg/l 1.43 1.72 1.23 1.52 2.83 3.02
Capacity (AC) AC kg/day 7154 8405 7874 8772 2546 2657
Annotation: (a): medium scenario, (b): maximum scenario.
The results show that: assimilative capacity and maximum allowable BOD of these rivers in rainy season is much higher than those in dry season. This is consistent with the reality. The ability of scrambling and decomposing organic matter in the rainy season is higher than that in the dry season because wastewater is diluted by rainwater. The assimilative capacity in rainy season is therefore increased.
Among the study sections, Bach Yen has the lowest assimilative capacity due to its low velocity (0.10 m/s - 0.15 m/s), small and narrow basin (1.7m - 3.2 m) and its current is often closed for the irrigation on the upper area farm land. Meanwhile, the assimilative capacity of DB2 is the highest. In addition, the difference in the maximum allowable BOD discharged into the river according to the calculations of different scenarios is not significant.
Table 2. Comparing the actual BOD with maximum allowable BOD discharged into the river
Input data Symbol Unit
DB1 DB2 BY
Dry season
Rainy season
Dry season
Rainy season
Dry season
Rainy season
Maximum allowable
BODu Bu kg/day
3840(a) 5306 4368 5652 1193 1370
3862(b) 5326 4382 5666 1198 1378
Assimilative Capacity (AC)
AC kg/day 7154 8405 7874 8772 2546 2657
284 Assessment of assimilative capacity of…
The maximum actual BOD
kg/day 204(a) 298 73 108 20 38
462(b) 663 243 285 67 85
The fact is that the DO level is more than 5 mg/l, meeting the National Technical Guidance QCVN 08:2008/BTNMT (level A2), BODu allowed to discharge into these rivers is less than 3862 kg/day, 4382 kg/day và 1198 kg/day (dry season) and 5326 kg/day, 5666 kg/day và 1378 kg/day (rainy season), respectively to DB1, DB2 and Bach Yen. Yet, the real discharge into these rivers is less than the maximum allowable. This means that they are able to receive organic pollutants within their assimilative capacity.
The authors also use equation 2 and 3 to calculate the critical time and distance tc and xc to evaluate the time and distance from the discharge to critical point. The results are shown as table 3.
Table 3. Values of tc, xc and Dx, Cx at the end of the study sections
Input data Symbol Unit
DB1 DB2 BY
Dry season
Rainy season
Dry season
Rainy season
Dry season
Rainy season
Critical time tc day 2.9 2.8 3.2 2.9 2.4 2.3
Velocity v m/day 8640 16416 9504 24192 8640 11232
Critical distance xc km 27.1 46.7 29.9 56.7 22.4 22.2
Length of the river
x km 1.5 1.5 1.3 1.3 1.0 1.0
DO deficit at x, km
Dx mg/l 1.5 1.8 1.3 1.5 2.2 2.5
DO at x, km Cx mg/l 5.8 6.1 6.2 6.3 5.2 5.4
Thus, to maintain dissolved oxygen in these sections at 5 mg/l (level A2 in QCVN 08:2008/BTNMT: used for domestic activities but (subject?) have to apply appropriate treatment measures, conservation of aquatic flora and fauna), the distance which the dissolved oxygen reaches the minimum, 5 mg/l out of the length of the river . It means that organic pollutants is not promptly degraded when going out of the river and indicates that these sections are still capable of receiving organic pollutants within their assimilative capacity. Values of dissolved oxygen at the end of study sections are around 6 mg/l for DB1 and DB2 = 6.1 - 6.2 mg/l, and 5 mg/l for BY and greater than the DO deficit (5 mg/l).
3.2. Modified Streeter-Phelps model
- From the above designed data, input data was entered in the equation 6 for these sections as follows:
HOANG NGOC TUONG VAN, TRAN QUANG LOC 285
Table 4. Input data of the model by season
Parameters Unit
DB1 DB2 BY
Dry season
Rainy season
Dry season
Rainy season
Dry season
Rainy season
Temperature 0C 32 28 31 28 31 28
Depth m 2.85 3.20 3.3 3.75 2.45 2.95
Velocity m/s 0.10 0.19 0.11 0.28 0.10 0.13
K1,t0C day-1 0.23 0.23 0.23 0.23 0.23 0.23
K2, t0C day-1 0.33 0.35 0.28 0.34 0.41 0.32
K3, t0C day-1 0.61 0.61 0.59 0.59 0.57 0.57
SOD, t0C g/m2.day 0.98 0.77 0.96 0.79 0.94 0.77
BODu mg/l 2.56 2.41 2.71 2.41 2.71 2.85
NBOD, N0 mg/l 0.75 0.74 0.93 0.97 1.05 0.88
CBOD, L0 mg/l 1.90 1.75 1.88 1.52 1.76 2.09
D0 mg/l 1.43 1.72 1.23 1.52 2.13 2.42
It is assumed that the critical (minimum acceptable) dissolved oxygen concentration of these sections is 5 mg/l (level A2 of QCVN 08:2008).
Values of A, B, C constant are assigned by these following equations in the modified Streeter - Phelps model:
Values of these constants are calculated as follows:
Table 5. Input values of the modified Streeter- Phelps model
Constant
Value by season
Dry season
Rainy season
Dry season
Rainy season
Dry season
Rainy season
A 4.36 3.35 8.66 3.17 2.25 5.34
B -1.64 -1.74 -1.76 -2.29 -3.74 -2.01
C 1.04 0.69 1.04 0.62 0.93 0.82
Replace these constants into equation 6 and solve equations using mathematical software Algebrator 4.0.1, we obtain xc and tc
as following table:
12
01
KK
LKA
−=
32
03
KK
NKB
−=
HK
SODC
2=
286 Assessment of assimilative capacity of…
Table 6. Output data of the modified Streeter - Phelps model
Output data Symbol Unit
DB1 DB2 BY
Dry season
Rainy season
Dry season
Rainy season
Dry season
Rainy season
Time criticial tc day 0.35 0.71 0.27 0.85 1.17 1.33
Distance critical xc km 3.04 11.73 2.55 20.54 10.14 14.91
Length of the river
x km 1.5 1.5 1.3 1.3 1.0 1.0
According to the calculations, it can be seen that time and distance critical in the rainy season is higher than those in the dry season. Yet, these values calculated by different scenarios do not have a significant difference.
The results show that the distance criticals are more than the actual length of the river. This means that organic pollutants are not decomposed completely when coming out of the river. It is indicated that Dong Ba River is still able to receive additional organic compounds within its self-purification to maintain the minimum acceptable dissolved oxygen concentration.
However, compared with the results calculated by basic Streeter-Phelps model, it can be seen that time critical and distance critical of modified model is much smaller than the basic (4-11 times). This means that the modified model reflects the actual condition of the river better and therefore it is necessary to consider nitrification (NBOD) and sediment oxygen demand (SOD) when applying Streeter - Phelps model to evaluate assimilative capacity of a river.
4. Conclusion and recommendation
4.1. Conclusion
This study is aimed at determining self-purification and assimilative capacity of biodegradable organic compounds in three sections of Dong Ba and Bach Yen rivers. The results show that Bach Yen river has the lowest assimilative capacity followed by DB1 and DB2.
- Maximum allowable BOD discharged into the river in order to maintain the minimum acceptable dissolved oxygen concentration in the rainy season is higher than that in the dry season. Its values calculated by different scenarios do not make a great difference.
- The actual BOD load discharged into the river is much lower than the maximum allowable discharged into rivers. Waste load of section DB1 is the largest, followed by BY and DB2. This means that these sections are capable of receiving
HOANG NGOC TUONG VAN, TRAN QUANG LOC 287
organic polluants discharged within their assimilative capacity.
- Calculation results of critical time and distance by basic and modifed Streeter - Phelps models show that the modified model describes the condition of the river better. Its values of critical time and distance are many times less than those of the basic Streeter-Phelps model. So, it was thought that the modified Streeter-Phelps model should be applied when considering the assimilative capacity of a river.
4.2. Recommendation
It is necessary to consider sediment oxygen demand (SOD) and nitrification when studying Streeter - Phelps to assess self-purification of rivers. These factors affect the oxidation of organic matters in water and thus affect time and distance critical. In addition, other because they affect the curve of dissolved oxygen as well as assimilative capacity of a river.
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