Nghiên cứu cải thiện chất lượng nước hồ đô thị bằng giải pháp kết hợp làm thoáng qua hệ thống định hình dòng chảy và bãi lọc trồng cây ttta

MINISTRY OF EDUCATION AND TRAINING
HANOI UNIVERSITY OF CIVIL ENGINEERING

UNG THI THUY HA

RESEARCH ON IMPROVEMENT OF
URBAN LAKE WATER QUALITY WITH THE
SOLUTION OF COMBINING AERATION BY
FLOWFORM SYSTEM AND PLANTED WETLAND

Specialization: Water and wastewater environmental technology
Code: 9520320-2

SUMMARY OF DISSERTATION

Hanoi, 2023


The dissertation was completed at Hanoi University of Civil
Engineering

Supervisor 1: Assoc. Prof. Dr. Leu Tho Bach
Supervisor 2: Assoc. Prof. Dr. Tran Thi Hien Hoa

Reviewer 1: Assoc. Prof. Dr. Nguyen Ngoc Dung
Reviewer 2: Assoc. Prof. Dr. Nguyen Manh Khai
Reviewer 3: Assoc. Prof. Dr. Vo Anh Tuan

The dissertation will be defended before the University-level PhD
Dissertation Assessment Committee at Hanoi National University of
Civil Engineering

At …… hour …… date …… month …… year ……
The dissertation can be found at the National Library and the Library
of Hanoi University of Civil Engineering.


1
INTRODUCTION
1. The necessity of the dissertation topic
Natural or artificial urban lakes often perform many functions such as:
landscape, improving urban environment; micro-regional air conditioning;
flood control; preserving and developing cultural and historical values; serving
the needs of recreation, sports and tourism; economic development.
Wastewater is the main cause of urban lake water pollution. Most of the
characteristic parameters for evaluation of surface water quality such as BOD5,
COD, NH4+-N, PO43--P, dissolved oxygen (DO), etc. do not meet the limits
according to the requirements of QCVN 08-MT:2015/BTNMT column B1.
With the advantages of low energy consumption, nonnecessity of
chemicals, simple operation, solutions for treating urban lake water pollution in
natural conditions such as planted constructed wetland (CW) are very feasible,
especially for the tropical climate in Vietnam. However, one of the biggest
limitations of the above solution is that DO concentration in CWs is often very
low, leading to low nitrogen removal efficiency. One of the passive aeration
methods to enhance DO that has received much attention recently worldwide is
the use of flowforms arranged in a form of waterfall - flowform cascade (FC).
Compared to the traditional passive aeration method by spillway or weirs, the
FC method usually has a higher oxygen replenishment efficiency. On the other
hand, flowform are often made with artistic shapes, so they can also contribute
to landscape decoration for the place of application.
With the desire to provide scientific basis to evaluate the pergofmance and
to propose simple, environmentally friendly technical solutions, suitable for

climatic conditions and the context of urban lakes in Vietnam, author chose the
topic: "Research on improvement of urban lake water quality with the
solution of combining aeration by flowform system and planted wetland" .
2. Aims of the dissertation
(1) Fabricate the flowforms; evaluate the aeration efficiency of the
fabricated flowform;
(2) Evaluate the impact of the FC and the operating conditions of the CW on
the treatment efficiency in tems of the main parameters: BOD5, NH4+-N, TSS,
TN, PO43--P in lake water which is polluted by domestic wastewater (DW);
(3) Choose a suitable kinetic model for modeling treatment processes by CW
combined with the FC in real conditions. Determine removal rate coefficients
for major pollutants: BOD5, NH4 +-N, TSS, TN, PO43 --P in real conditions.
3. Object and scope of research of the dissertation
Research object: urban lake water polluted by wastewater and has low DO


2
content.
Scope of the study: use the experimental models of FC and horizontal
subsurface flow constructed wetland (CW) in the field to study the aeration
capacity of the FC and the ability to improve the water quality of urban lakes
polluted by DW.
4. The scientific basis
- The oxygen replenishment efficiency of each flowform depends on the
flow rate through the sample. At the appropriate level, a flow pattern with the
shape resembling “8” figure appears in the flowform, thus the aeration process
is enhanced, resulting in improved oxygen replenishment efficiency.
- Oxygen replenishment efficiency of the FC increases with the number
of flowforms.
- By combining with FC, DO level in CW would increase, thus the

efficiency of treatment of organic matter and especially nitrogen compounds
would enhanced.
- By adjusting the operating conditions, the aerobic and anoxic zones
in the CW can appear, facilitating the process of nitrification and
denitrification, resulting in improved nitrogen removal efficiency.
5. Research content
(1) Research on the current status of water quality of urban lakes polluted
by DW in Hanoi area;
(2) Research on fabricating flowform with appropriate materials;
(3) Evaluation of the oxygen replenishment performance of the passive
aeration system with flowforms using the experimental setup;
(4) Using the experimental setup in the field, evaluate and compare the
performance of treatment of typical pollutants in urban lake water by CW
in two scenarios: not combined and combined with the FC.
6. Research methods
Methods used for conducting: liturature review; field survey; in-field
experiment; data processing; consulting with experts.
7. New findings of the dissertation
(1) The dissertation has determined the improvement of dissolved oxygen
(DO) content through the FC. DO content could reach 5.6 mg/L for the
studied lake water;
(2) The dissertation has confirmed that the performance of the horizontal
subsurface flow constructed wetland (hereinafter referred to as CW)
significantly enhanced when the influent is aerated by the FC;


3
(3) The dissertation has determined the removal rate coefficients for the
main pollutants i.e: BOD5, TSS, NH4+-N, TN, PO43--P of the model of the
CW combined with the FC.

8. Scientific and practical contributions of the dissertation
Scientific contribution
- The dissertation has reviewed the sources of pollution, urban lake water
quality and methods for enhancing urban lake water quality;
- Evaluate the aeration efficiency and determine the optimal number of
steps of the FC for urban lake water contaminated by DW;
- Evaluate the performance of treatment of typical pollutants in urban
lake water by CW in two scenarios: CW alone and combined with the FC;
- Assess the impact of FC and operating conditions on the DO distribution
along the CW and on the treatment efficiency of typical pollutants.
- Determine the removal/conversion rate coefficients of typical
pollutants: BOD5, NH4+-N, TSS, TN, PO43--P by CW when combined with
and when not combined with the FC in field conditions.
Practical contribution
- The aeration system using the flowforms has high performance and
can be applied to the water treatment system in practice;
- The research results of the dissertation helps improving the feasibility
of applying CW in treatment of polluted lake water. CW is a solution with
high efficiency, the quality of the efluent can meet the requirement by
QCVN 14:2008/BTNMT (column B). Referring to QCVN 14:2008/
BTNMT, except for NH4+-N, other parameters can meet the requirements
in column B1. Combining CW with the FF system can yields not only a
low-cost but also an environmentally friendly solution which contributes to
the surrounding landscape as well.
- The research results of the dissertation can serve as a reference for
researchers as well as provide more options for managers in pollution
control of urban lakes.
9. Dissertation structure
The structure of the dissertation includes:
Introduction

Chapter 1. Overview of the current status of urban lake water pollution
in Vietnam and solutions to improve lake water quality
Chapter 2. Theoretical basis for treatment of polluted lake water by
combining flowform system and constructed wetlands
Chapter 3. Experimental research


4
Chapter 4. Research results and discussion
Conclusions, Recommendations
The structure of the dissertation is presented in the figure below:

Figure 1.7. Structure of the dissertation
CHAPTER 1. OVERVIEW OF THE CURRENT STATUS OF
URBAN LAKE POLLUTION IN VIETNAM AND SOLUTIONS
TO IMPROVE LAKE WATER QUALITY
1.1. Function and current use of urban lakes
1.1.1. The role of urban lakes
Natural or artificial urban lakes often perform many functions such as
creating landscapes, improve the living environment for urban areas;
regulating microclimate; flood control; preserving and developing cultural
and historical values; serving the needs of recreation, sports and tourism,
economic development.
1.1.2. Current status of use of urban lakes in Vietnam
There are 636 lakes located in urban areas of 46 provinces and cities
across the country [35]. Lakes in urban areas are often used for the
following purposes: regulating storm runoff and receiving wastewater
(353); creating ecological landscapes (47); serving as sources for water
supply, irrigation; preserving historical and spiritual values (16); fish
farming (27); receiving wastewater (12).

5
1.2. Pollution status in urban lakes in Vietnam
1.2.1. Sources of urban lake pollution
Untreated wastewater and urban waste discharged into lakes are among
the main causes of urban lake pollution. Organisms in the lake are
decomposed upon death, the sediment is disturbed by rain, by runoff
causing secondary pollution to the lake.
1.2.2. Pollution status of urban lakes in some cities
Table 1.2. Urban lake water quality in some cities in Vietnam
DO
COD
(mg/L)
(mg/L)
QCVN 08-MT:2015 ( B1)
30
³4
1 Da Nang
1.8 ÷ 5.8 22.75 ÷ 83.53
2 Hue
4.17 ÷ 4.77 21.43 ÷ 53.07
3 Ba Ria-Vung Tau
4.2 ÷ 5.2
18.3 ÷ 60.7
4 Hai Phong
3.93 ÷ 5.43 34.8 ÷ 51.6
5 Viet Tri
3.8 ÷ 4.7 21.17 ÷ 76.05
TT

6

City

Can Tho

3.1 ÷ 4.6

NH4+-N
(mg/L)
0.9
0.51 ÷ 1.2
0.27 ÷ 1.75
0.06 ÷ 1.67
0.14 ÷ 5.36
0.03 ÷ 0.46

PO43--P
No of
(mg/L)
surveyed
lakes
0.3
0.11 ÷ 1.68 9 lakes
0.02 ÷ 2.15 9 lakes
0.12 ÷ 0.17 9 lakes
0.02 ÷ 1.79 9 lakes
0.02 ÷ 0.28 9 lakes

31.43 ÷ 48.17 0.07 ÷ 0.81 0.12 ÷ 0.14

9 lakes

Sources: [27], [32], [35].
1.2.3. Pollution status of lakes in Hanoi city
Urban lakes in Hanoi are classified into the following groups [6]:
- Group 1: not receiving wastewater, only receiving storm runoff;
- Group 2: receiving a mixture of storm runoff and wastewater;
- Group 3: receiving wastewater.
Table 1.3. Lake water quality in Hanoi city area
Temp
(0C)
QCVN08-MT:2015 (B1)
1 Can lake (1)
26.0±5.2
2 Seven Gian Lake (1)
24.9±5.3
3 Tan Mai Lake (2)
25.6±5.2
4 Ho Mot Lake (2)
25.4±5.8
5 Small Lake Kim Lien (3)
25.0±5.4
6 Hai Ba Trung Lake (3)
25.1±5.4
7 Uncle Ho's Fish Lake (1)
25.1±5.6
8 Lake at Van Phu Urban Area (2) 24.8±6.3
9 Ngoc Thuy Lake (3)

24.8±5.5

TT

Lake

DO
COD
TN
TP
(mg/L)
(mg/L)
(mg/L) (mg/L)
30
³4
3.8±0.4 57.5±8.8
3.5± 0.8 2.5±1.0
3.8±0.4 53.8±7.3
4.0±0.9 1.7±0.5
2.6±0.5 75.6±5.4
6.7±0.7 1.9±0.3
2.8±0.7 68.0±7.0
6.1±0.8 2.6±0.7
0.1±0.1 188.3±32.4 25.9±5.0 3.4±0.6
2.7±0.4 83.1±14.1 8.9±1.8 2.1±0.6
4.5±0.4 46.1±6.8
3.3±0.3 1.6±0.5
3.8±0.4 60.3±5.8
4.8±0.4 2.3±0.4
3.6±0.3 74.6±7.6

5.9±0.9 2.6±0.6

Notes: (1), (2), (3): The lakes belong to group 1, group 2, group 3. Sampling
period from November 2019 to April 2020, the number of samples is 6.


6
1.3. Overview of lake water quality pratice
1.3.1. World practice
The world practice includes physical methods: dilution and bottom
discharge, deep aeration, aeration with the FF system, sediment removal
by dredging; biological methods: treatment by micoorganisms, biofilm,
microbiological additives, CW, aquatic plants.
1.3.2. Practice in Vietnam
Solutions applied in Vietnam: sediment removal by dredging, aquatic
plants, chemical additives Redoxy-3C, chemical additives in combination
with microbial additives and aquatic plants, aeration.
1.4. Research content proposition
Most of the urban lakes are polluted with domestic wastewater.
Particularly in Hanoi, the DO concentration in many lakes is very low,
resulting in limitation of their self-cleaning ability.
The solution of combining the CW and the FC would promote the
advantages of pollution treatment in natural conditions: less energy
cónumption, non necessity of chemicals, simple operation, suitability to
Vietnam climate conditions.
CHAPTER 2. THEORETICAL BASIS FOR TREATMENT OF
POLLUTED LAKE WATER BY COMBINING FLOWFORM
SYSTEM AND CONSTRUCTED WETLANDS
2.1. Scientific basis of treatment processes in constructed wetlands
2.1.1. The concept and classification of CW

Constructed wetlands are submerged ecosystems with shallow water
level where plants are grown in moist soil conditions.

Figure 2.1. Types of constructed wetlands [87]
2.1.2. Mechanism of treatment processes in CW
Mechanism of pollutant treatment in CW includes the processes: deposition,
filtration by filter media or plant roots; absorbtion, adsorbtion; evaporation,


7
diffusion, aeration; metabolism by microorganisms, plant uptake, ...
2.1.3. The role of microorganisms and plants in CW
Microbial system participates in nitrogen and phosphorus metabolism,
heavy metal processing and affects the absorption capacity of plants [129].
In CW, plants participate in many mechanisms: roots, stems help
evenly distribute the flow, increase the capacity of sediment retention,
reduce the growth of algae [124]; plant roots absorb nutrients and transport
oxygen into the water [58].
2.2. Factors affecting the treatment efficiency of the CW
- Hydraulic retention time (HRT): determines the contact time between
the substrate in the water and the microorganisms, so it directly affects the
treatment efficiency.
- Temperature: in natural conditions, the rate of metabolism of
substances in CW is usually covariate with temperature.
- pH: The pH of the water affect the processing efficiency of CW.
- Dissolved oxygen (DO): DO plays an important role in the
nitrification and biodegradation of organic compounds occurring in CW.
- Hydraulic conductivity and filter media: filter media are the substrates
where microorganisms can attach on and grow. High hydraulic conductivity
helps to reduce clogging in CW.

2.3. Kinetics of the treatment of pollutants in the CW
Pollutants treatment processes in CW can be characterized by first order
kinetic model, which is represented by equation (2.15). On the basis of firstorder kinetic models, Kadlec and Knight [84] developed the k-C* model equation (2.16) to describe the processing in horizontal underground flow
CW.
!"
"!"# &" ∗
=
−𝑘
.
𝐶
(2.15)
;
= 𝑒 &'' /)*+ (2.16)
%
#$
" &" ∗
%&

Reed et al. [112] also proposed a kinetic model for treatment of
pollutants BOD5, NH4+-N, TN in CW as equation (2.23) :
𝐶,-$ = 𝐶./ 𝑒 &'( .$
(2.2 3)
In the above formulas: C, Cin, Cout, C* - are respectively concentration of
pollutant at the observation point, in the influent, in the effluent and
background concentration of pollutants in CW, mg/L; t - hydraulic retention
time, day; kv - coefficient of pollutant treatment rate by volume, day-1; ka coefficient of pollutant treatment rate by area, m3/m2.day; kT - reaction rate
constant at temperature T, day-1; HLR - hydraulic loading rate m3/m2.day.


8

2.4. The principle of aeration
2.4.1. The principle of passive aeration
Mass transfer of oxygen into the
stream water through the dam, overflow
weir occurs due to the diffusion of
oxygen through the air-water interface.
The effectiveness of that prosess
depends on the drop height,
temperature, oxygen deficiency in Figure 2.9. Aeration mechanism of
overflow weir
water, area of air-water phase contact.
2.4.2. Factors affecting the aeration performance
a. Oxygen exchange between air and water: the rate of dissolution of
oxygen from the air into the water is directly proportional to the area of
the water-air Phase contact and the oxygen deficiency in the water .
b. Temperature and salinity: as the temperature and salinity of the water
increases, the solubility of oxygen in the water will decrease .
c. Mixing intensity, water depth: the rate of dissolution of oxygen from the air
into the water is proportional to the intensity of the air-water interface
disturbance. The DO concentration is unevenly distributed according to the
depth of the water body, the deeper the water layer, the lower the DO [99].
d. Photosynthesis: DO concentration during the day increases due to
photosynthetic activity and decreases at night due to plant respiration.
e. Other factors: The process of decomposing organic matter, nitrification
because microorganisms in water consume oxygen and reduce DO; The
process of transpiration in the rhizosphere helps oxygen transport through
the stem and roots into the water to increase DO.
2.5. The principle and applicability of flowforms for treatment of lake
water contaminated by domestic wastewater
2.5.1. The concept and working principle of flowforms

Figure 2.11. Flowforms cascade [121]
Flowform: is an aeration device, which can facilitate an "8" shaped flow
pattern, which allows the water to be replenished with oxygen from the air .
Flowforms cascade: a system of flowforms (of the same or different


9
shapes) arranged in the form of a ladder to create a waterfall or cascade.
2.5.2. Flow morphology in the flowforms
The flowforms arranged in the form of a
cascade will create the
movement of water with
a harmonious rhythm.
The aeration effect of the
water flow would signiFigure 2.1.3. Flow
Figure 2.1.4. Flow
ficantly enhanced in a
compact space.
morphologies [121]
pattern of figure “8”
2.5.3. Factors affecting the aeration capacity of the flowforms
- Flow morphology: facilitates the turbulence and mixing intensity.
- Flow rate: at suitable level, the figure “8” flow pattern can occur.
- Drop height of water: the aeration performance increases with the
increase of the drop height between the flowforms.
- Shape of the flowform: more complex shapes would have higher
aeration capacity over the simpler shapes.
- The open surface area of the flowform: the amount of oxygen
transported into the water is proportional to this area.

- Wind speed: wind causes waves to increase the contact area between
the air and the water surface, helping to create convection currents and
increase the mixing intensity.
- Number of aeration steps: dissolved oxygen concentration in water
increases with the number of aeration steps until reaching saturation value.
- Temperature, water quality: the increase of temperature or the presence of
salts and ions in water reduces the efficiency of oxygen replenishment as well
as the saturation DO value.
2.6. Aplicability of combining CW and flowforms cascade for
treatment of lake water contaminated by domestic wastewater
2.6.1. Processing organic matter
Dissolved oxygen (DO) is important factor in the process of organic
matter treatment, affecting the activities of microorganisms to pollutant
treatment. In wastewater treatment, aerobic decomposition is commonly
used, high DO concentration helps the decomposition of organic matter
take place faster [103], [105].
2.6.2. Processing nutrients
In CW, phosphorus is mainly treated by deposition and adsorption


10
processes; Nitrogen is removed mainly by nitrification under aerobic
conditions followed by denitrification under anoxic conditions. The
arrangement of artificial aeration before the CW helps to maintain the
appropriate DO levels for nitrification and denitrification occurring in the
CW, thereby greatly improving the nitrogen removal.
2.6.3. Effect of DO on the performance of the constructed wetland
DO is an important factor affecting the efficiency of pollutant treatment
by CW. [DO] >1.5 mg/L is good condition for nitrification process [149];
DO below 0.5 mg/L is favorable for denitrification process to occur [88].

2.6.4. Applicability of constructed wetland for improving the quality
of polluted lake water
Horizontal subsurface flow CW has many advantages: low construction
cost, easy operation, environmental friendliness, high treatment efficiency for
organic matter, suspended solids, color of water [39]. On the other hand, the
small height allows easy placement along the banks of urban lakes. However,
this work has the disadvantage that the nitrogen treatment efficiency is not
high due to the limited DO concentration, which directly affects the
nitrification process [55] which is a prerequisite for nitrogen treatment. This
problem can be solved by applying aeration measures to increase DO
concentration in water. One of the simple solutions to increase the amount of
dissolved oxygen is to incorporate an FF system placed in front of the CW.
Urban lakes are basically embanked and embellished (about 85% in Hanoi),
so the space around the lakeshore can be used to arrange the CW.
2.7. Chapter 2 comments
Thanks to the improved oxygen level when combined with FC, CW is
a low cost solution that can effectively remove common pollutants in
urban lake water such as organic matter, nutrients, suspended solids, and
contribute to improvement of urban lake water quality.
With the aesthetic design of the FC, appropriately selected plants, this solution
has the potential to contribute to enahncing landscape and value of urban lakes.
CHAPTER 3. US EXPERIMENTAL RESEARCH
3.1. Set up empirical studies
The experimental studies are presented in Figure 3.1.


11

Figure 3.1. Experimental studies
3.2. Research and fabrication of flowform

3.2.1. Research purposes
Selection of design, materials, fabrication of flowforms.
3.2.2. Experiment setup
The FC consists of 7 flowforms (2 made of enameled ceramic, 2 made of
white cement and 3 made of Ultra High Performance Concrete - UHPC),
placed outdoors and operated with wastewater for a period of two years.
3.2.3. Selection of flowforms model
The selected flowforms model is capable of generating a stable "8"
digital flow, operating in a wide flow range (100-500L/h), and is simple
to manufacture.

Figure 3.2. Selected flowforms
Figure 3.3. The flowforms cascade
3.2.4. Selection of materials for fabricating flowforms
After 2 year of operation, the surface of the flowforms made of UHPC
was smooth, free of cracks, and demonstrated good tolerance to outdoor
climates. Therefore, the UHPC is selected for flowforms fabrication.


12
3.2.5. Fabrication of flowforms using UHPC
The mixing ratio for flowforms fabrication: UHPC ready mix: Water:
Pigment was 81.5: 16.3: 2.2%.
3.2.6. Fabricated flowforms
The fabricated flowforms are
shown on Fig. 3.6 and meets the
technical requirements, the surface
of the product is glossy and
smooth, favorable for creating a
steady flow with the shape of the

Figure 3.6. Fabricated flowforms
number "8" repeated many times .
3.2.7. Comments on the flowform
Flowform model of UHPC motor produces a flow pattern number "8",
with high durability, maintain a smooth surface under outdoor operating
conditions. The shape of the flowform model is aesthetically pleasing .
3.3. Study on the ability to add oxygen from the air to the water of the
flowform system
3.3.1. Research purposes
Determine the optimal number of steps and flow rate of FF system.
3.3.2. Experimental model, operating mode
a. Experimental model
The FC consists of 11 flowforms with size L×B×H = 400×400×140
mm, the operating flow ranged from 100 to 400 L/h.

Figure 3.7. Process flowchart of the actual system
b. Operation flow rates
Operation flow rates: Q1=100 L/h; Q2=150 L/h; Q3=200L/h; Q4=250L/h;
Q5=300L/h; Q6=350L/h; Q7=4 00 L/h .
3.3.3. Dissolved oxygen concentration at different flow rates
a. Experiment time
Experiment period: from March 28 to May 2, 2019.
b. Measuring frequency and method
3 times/day at 9AM at each flow rate;
DO measurement method: place the probe at a position where no


13
bubbles appear and make sure the electrode membrane and temperature
sensor are fully submerged in water.

3.4. Evaluation of the impact of the FC and operating flow rates on the
distribution of DO along the CW and the efficiency of pollutant treatment
3.4.1. Research purposes
Evaluate the level of improvement DO in the CW through the FF
system; Compare the treatment performances of Phase 1 and Phase 2;
Determine the removal rate coefficent for pollutants.
3.4.2. Selection of subjects and locations for field experiment
The object of the study is an urban lake polluted by DW. The Kim Lien
small lake, which regularly receives DW from 30 households nearby, was
selected for field experiments. The lake has the area of about 3000m2, water
depth 1.0÷1,2m, water volume 3000÷3600 m3.
3.4. 3. Field experiment design
Table 3.5. Design parameters of the experiment setup
Design parameters
Symbol
1. Horizontal subsurface flow CW
Surface area of CW
As
Length, width of CW
L×B
Construction height of CW
Hxd
Depth of water layer
H
Filter media height
hfm
Media size
Dfm
Filter media porosity
n

Hydraulic conductivity
kf
Hydraulic slope
dH/ds
2. Flowform cascade
FC dimensions:
LFC×HFC
Length×height
Number of steps
n
Flowform dimensions:
L×B×H
Flow rate
QFC

Unit

Value

Note

m2
m
m
m
m
mm
%
m/day
%

5.76
4.8×1.2
0.8
0.60
0.65
10×20
42
2000
2

Concrete, brick

mm

2890×1630

step
mm
L/h

8
400×400×140
200

Crushed stone
Measured
As per [129]
As per [129]

3.4.4. Construction and installation of experimental models
Experimental model at the field including the FC, CW and equipment was
built and installed in the period from July 15 to September 30, 2019. CW is
built on the ground with concrete bottom, brick walls, waterproofed with
cement and planted with Cyperus Involucratus.
3.4. 5. Experimental plan
a. Operation modes of the experimental model


14

Figure 3.18. Model operating modes
b. Operation diagram of the experimental model
* Phase 1: Operating the CW alone, without the FC

Figure 3.19. Operation diagram of Phase 1 - without the FC
* Phase 2: Operating the CW in combination with the FC

Figure 3.20. Operation diagram of Phase 2 – combining with the FC
c. Sampling plan
Model operated for 21 days. Water samples were taken at 9-10AM on
15th, 17th, 19th and 21st days since the start model operation.
3.5. Methods of sampling and analysis
3.5.1. Sampling methods
Sampling method: lake water, wastewater samples were collected in
accordance with the TCVN 5994-1995 and TCVN 5999:1995; water
samples were preserved in accordance with the TCVN 6663-3:2008.
3.5.2. Analytical methods

15
Analytical methods: pH, DO, temperature were measured in the field;
BOD5 as per the TCVN 6001-1:2008; TSS as per the TCVN 6625:2000;
TN as per the TCVN 6638:2000; NH4+-N as per the TCVN 6179-1:1996;
NO3--N as per the SMEWW 4500 NO3-, E:2012; Alkalinity as per the
TCVN 6636-1:2000; PO43--P as per the TCVN 6202:2008.
3.6. Method for determining the removal rate coefficient of pollutants
3.6.1. Kadlec and Knight model
Equation (2.16) is written as:𝐶1 − 𝐶 ∗ = (𝐶./ − 𝐶 ∗ )𝑒 &'' .3/)*+
Where: Cx is the concentration of pollutants measured at the sampling
points. Microsoft software Excel was used to build the fitting curves, then
the above equation has the form: 𝑦 = 𝑌. 𝑒 &41
Where: y = 𝐶1 − 𝐶 ∗ , mg/L; Y = 𝐶./ − 𝐶 ∗ , mg/L.
For BOD5, C* was determined according to the instructions in TCVN
7957:2008: C* = 3.5+0.053.Cin ; for the parameters NH4+-N, TN, PO43 --P
and TSS, C* were chosen to be 1.5, respectively; 1.5; 0; 0 mg/L [84].
3.6.2. Model by Reed SC et al.
The treatment processes of BOD5, NH4+-N, TN were represented by
equation (2.23). Using Microsoft Excel software to build the fitting curves,
then equation (2.23) will have the form: 𝑦 = 𝑌. 𝑒 &'1
In there: y - contaminant concentration at the observation site, mg/L;
Y-pollutant concentration at CW inlet, mg/L; k: Contaminant removal rate
coefficient, day-1 ; x: Water retention time, day.
CHAPTER 3. RESEARCH RESULTS AND DISCUSSION
4.1. Evaluation of the aeration performance of the FC
4.1.1. Change of DO over the FC steps

Figure 4.1. Change of DO over FC Figure 4.2. DO values at the 8th
steps
step at various flow rates

4.1.2. Findings of the experiment
Formation of the flow pattern of figure “8” depends on the flow rate.


16
Aeration performance was stable at flow rate ranged from 150 to 350 L/h and
reach the maximum value at flow 200 L/h. The optimal number of flowform
steps were 8÷9. At Q = 200 L/h, the FC with 8 steps helped increasing the DO
from 0.1 to 5.6 mg/L.
4.2. Operation of the CW and the FC
4.2.1. Performance of the model in Phase 1 - CW alone

Figure 4.3. Change of water quality parameters along the FC in Phase 1
pH of wastewater according to CW length little change at all 3 levels
HLR1, HLR2, HLR3 respectively 0.031; 0.063; 0.125 m3/m2.day, pH
value respectively reached 6.8÷8.1. Alkalinity decreased slightly from
197.2 to 171.3 mg CaCO3 /L at all 3 levels of HLR; DO was almost
unchanged along the the CW in all 3 experiments, the value reached 0.1±
0.1 mg/L; BOD5 values of the influent were from 74.4 to 77.0 mg/L, the
removal efficiency reached 76.9; 62.8; 62.8% respectively to HLR1,
HLR2, HLR3. The NH4+-N content in influent were from 26.4 to 27.4
mg/L; the processing efficiency of NH4+-N reached 62.7; 54.2; 28.0%
respectively for HLR1, HLR2, HLR3; NO3--N at all three levels of HLR
was always low (<0.4 mg/L), with little change along the CW. The inlet
TN content was from 27.8 to 32.9 mg/L, the removal efficiency reached
49,4; 25,5; 19,6% for respectively for HLR1, HLR2 and HLR3 levels; The
input PO43--P content was from 2.34 ÷ 3.55 mg/L, the removal efficiency
reached 88.8; 67.3% HLR1 and HLR3 levels, respectively.
4.2.2. Performance of the model in Phase 2 - CW combined with FC
pH of wastewater according to the length of CW is little changed at all 3

levels of HLR, with values 7.1÷7.8; DO of influent was significantly improved
thanks to the FC, reaching values of 5.4÷5.6 mg/L. The increase in NO3--N


17
value together with the decrease in alkalinity indicated that nitrification occured
in about first ¼ of the CW length. Increasing HLR to the level of HLR2 = 0.063
m3/m2.day helped to expand the aerobic zone to about ½ length of the CW. NO3-N value and alkalinity also showed similar developments as in HLR1 and
confirmed the presence of nitrification in the first half of CW. The value of
influent BOD5 reached 78.8÷87.8 mg/L. Removal performance reached 86.6;
82.8; 77.5% respectively for HLR1, HLR2, HLR3. NH4+-N content in the
influent was from 26.3 to 32.1 mg/L. Processing efficiency reached 79.6; 65.6;
51.0% respectively for HLR1, HLR2, HLR3. The TN content in the influent
ranged from 30.2 to 34.7mg/L. The processing efficiency reached 71.2; 58.0;
41.4% respectively for HLR1, HLR2, HLR3.

Figure 4.4. Change of water quality parameters along the CW in Phase 2
4.2.3. Comparison and evaluation of performance in treatment of Kim
Lien small lake water by experiment models in two Phases
BOD5 treatment efficiency: Compared with the Phase 1 model, treatment
efficiency BOD5 of the Phase 2 model showed improvement at all HLR levels
(Figure 4.5a), most notably at HLR3. BOD5 treatment efficiency increased by
about 5; 6 and 15 %, respectively, the retention time was 8 days, 4 days and 2
days.
NH4+-N treatment efficiency: Phase 2 model showed a significant increase
in NH4+-N treatment efficiency compared to the Stage 1 model at all HLR
levels (Figure 4.5b). Processing efficiency increased by about 17, 11 and 23
% respectively for HLR1, HLR2 and HLR3.
- TN treatment efficiency: In Phase 2, due to the supply of oxygen, the
nitrification-denitrification occurs, so the treatment efficiency increased

significantly by 22; 18 and 22% respectively for HLR1, HLR2 and HLR3.
- Processing efficiency of PO43--P, TSS: The treatment efficiency in both
models is similar at all HLR levels (Figure 4.5d). Thus, the FC did not affect
the removal efficiency of PO43--P, TSS by CW.


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Figure 4.5. Processing performance of the two models in Phases 1 and 2
4.3. Determination of removal rate coefficient of typical pollutants
under different operating modes
4.3.1. BOD5 removal rate coefficient

Figure 4.6. The 𝑘5678) of Phase 1 and Phase 2
+

4.3.2. NH4 -N processing rate coefficient
In Phase 1, R2 = 0,81; 0,80; 0,58 corresponds to the level of HLR1,2,3 live
temperature from 26,3 ÷ 29,5oC. The value 𝑘59)*+ &9 increased when
increasing HLR into the filter field from 0,031 ÷ 0,063 m3/m2/day,
respectively from 13,5 ± 6,6 ÷ 19,9 ± 4,2 m/year. In level HLR3, the value
𝑘59)*+ &9 tends to decrease to 15,8 ± 3,2. In Phase 2, R2 is high in all three
levels of HLR1, 2, 3, with values 0,73 ÷ 0.86. The coefficient


19
𝑘59)*+ &9 increases with the increase of the corresponding HLR 19,4 ± 1,4 ÷
31,6 ± 2,8 m/year.

Figure 4.7. Kinetics of NH4+-N processing according to modes operate

4.3.3. TN removal rate coefficient

Figure 4.8. The kaTN of Phase 1 and Phase 2
The removal rate coefficient kaTN of CW in Phase 1, there is a high
correlation, R2 = 0.81 at the HLR1 level, the HLR2 and HLR3 levels have
an R2. correlation are 0,67 and 0,55 respectively. The coefficient of kaTN
gradually increased from the level of HLR1 to HLR3 corresponding to the
value increasing from 8.1 ± 1.1 to 16.6 ± 5.6 m/year. In Phase 2, the
correlation was higher than that in Phase 1, R2 values at HLR1 and HLR2
levels were 0.88 and 0.80 respectively, and at HLR3 the coefficient was
0.65. The kaTN increased from 15.4 ± 1.6 m/year at HLR1 to 26.5 ± 7.1 at
HLR3. When combined with FC, the removal rate coefficient kaTN of CW


20
was about 1.5 ÷ 2 times of that of the CW alone.
4.3.4. PO43--P removal rate coefficient

Figure 4.9. The 𝑘5:7*,- &: of Phase 1 and Phase 2
In Phase 1, R2 for the removal rate coefficient 𝑘5:7*,- &: of CW was in
the range of 0.75 ÷ 084. 𝑘5:7*,- &: increased gradually with increasing the
level of HLR1, 2, 3, the value reaches from 25.8±8.2 to 47.5±10.7 m/year.
In Phase 2, the correlation for coefficient of 𝑘5:7*,- &: CW was a high, R2
was 0.74 ÷ 0.81 and increased with HLR, the value 𝑘5:7*,- &: reaches
28.7±7.95 ÷ 46.7±14.8 m/year. Compared to Phase 1, this value did not
change significantly. This showed that the treatment efficiency of PO43--P
was not affected by DO value in CW.
4.3.5. TSS removal rate factor

Figure 4.10. The kaTSS of Phase 1 and Phase 2

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In Phase 1, the coefficient kaTSS has high correlation, R2 ranged from
0.85 to 0.94, the kaTSS value was from 30.2±6.0 to 74.0±20.7 m/year,
respectively HLR1, 2, 3. In Phase 2, R2 value was from 0.89 to 0.95. kaTSS
increased gradually with the level of HLR1, 2, 3, reaching from 29.5±7.7
to 75.6±10.8 m/year for HLR1, 2, 3 levels respectively. Compared with
Phase 1, results in Phase 2 did not changed much, showing that enhancing
DO concentration in CW did not improve TSS treatment efficiency.
4.4. Proposal of application of the research results in the treatment of
lake water contaminated by domestic wastewater
4.4.1. Evaluating the advantages/disadvantages of the two models
Based on the comparison results from the experiment of the two models
in Phase 1 and Phase 2, the scientific basis and application of CW and FC in
wastewater treatment pactice, the advantages/disadvantages of each models
are evaluated as follows:
Table 4.1 2. Comparison of two experimental models
No

Criteria
Evaluate

Comparison
Ph1 Ph2 Equiv.

1 BOD5 removal
performance
2 NH4+-N treatment efficiency
3 TN removal

performance
4 PO43 –P removal efficiency
5 TSS removal
performance
6 Processing rate

x
x
x
x
x
x

7 Vector issues

x

8 Odor problem
x
9 Footprint
x
10 Energy

x

Description

The removal performance of the Phase 2 is
higher than that of the Phase 1
The treatment efficiency of the Phase 2 is higher

than that of the Phase 1
The removal performance of the Phase 2 is
higher than that of the Phase 1
The removal performances of the 2 phases are
equivalent
The removal performances of the 2 phases are
equivalent
The removal rate of Phase 2 is faster than Phase
1.
Both models are less likely to have vetor issues
since the CW is of subsurface flow.
The model setup in the Phase 1 is likely to emit
some odor because the processing takes place
mainly under anaerobic conditions. In Phase 2,
DO is improved by the FF system, greatly
reducing the risk of odor generation.
The processing rate of Phase 2 is faster than
Phase 1, so with the same flow, the same quality
of influent and same requirements on effluent,
the footprint of the model used in Phase 2 would
be smaller than that of Phase 1.
Because the water is brought to a certain height in


22
No

Criteria
Evaluate

Comparison
Ph1 Ph2 Equiv.

consumption
11 Contribution to
landscaping

x

Description

oder to pass through the FF system, the model in
Phase 2 consumes more energy than the model in
Phase 1.
Since the flowforms are designed with the idea
of simulating natural flow patterns, they would
blend into surounding landscape and contribute
to creating landscape highlights.

Note: “x”: Better ability of control and handling
4.4.2. Proposition of application of the results from the research
a. Lakes of Group 1
The water quality in lakes of this group is generaly good, the paramaters DO,
NH4+-N, PO43--P could generally meet the requirements of B1 column, QCVN
08-MT:2015/BTNMT, while organic paramaters (COD, BOD5) might not meet.
The proposed process flowchart for those lakes is shown in Figure 4.11.

Figure 4.11. The process flowchart for urban lakes of Group 1
b. Lakes of Groups 2 and 3
Although there is a difference in terms of pollution parameters between

the two groups, the lakes these two groups of still receive generally
untreated DW, so the lake water needs to be treated according to both
organic (COD, BOD5) and nitrogen parameters. The following process
flowchart (Figure 4.12a ) would help improving water quality to meet
requirements of B1 column, QCVN 08-MT:2015/BTNMT.
For the purpose of improving the lake water quality, it is possible to
arrange an FC after the CW as shown in Figure 4.12b for additionnal
oxygen enrichment of the treated water before returning to the lake.

Figure 4.12a. The process flowchart of general application to urban lakes


23

Figure 4.12b. The process flowchart for the cases when difference in
elevation between the outlet of CW and water level in the lake is suitable
CONCLUSIONS
The main conclusions obtained from the study are as follows :
1) Experimental FC with flowform of size H×L×B : 140×400×400 mm,
150 mm water drop height at each step, has high and stable aeration
efficiency at flow in the range of 150-350L/h; the optimal flow range is
150-200 L/h; the optimal number of aeration steps is 8-9; for lake water
polluted by DW, a FC of 8 steps was capable of increasing DO value from
0.1 mg/L to 5.6 mg/L.
2) The arrangement of the flowforms cascade prior to the CW did not
considarably affect the treatment efficiency of TSS and PO43--P, but
significantly improved the efficiency of organic matter and especially
nitrogen treatment, however. At HLR of 125 mm/day, BOD5 removal
efficiency increased from 62.8% to 77.5%, TN removal increased from 19.6%
to 41.4%. At HLR of 31.25 mm/day, BOD5 removal efficiency increased from

80.9% to 86.1%, while TN removal increased from 49.4% to 71.2%.
3) HLR affects the distribution of DO concentration along the CW. At the
level of 125 mm/day, the environment was mostly aerobic, TN treatment
efficiency was at 41.4%. At the HLR of 31.25 mm/day, the environment
at approximately first third length of the CW was aerobic, followed by an
anoxic zone, which contributed to a significant increase in the TN
treatment efficiency to the lowest level 71.2%.
4) Water retention time directly affects the treatment efficiency. For CW
combined with aeration, increasing HRT from 2 days to 4 days significantly
increased treatment efficiency. By doubling HRT to 8 days the performance
for NH4+-N and TN improved clearly, and would meet column A of
QCVN14:2008/BTNMT.
5 ) Kadlec & Knight's k-C* model shows a better degree of similarity than
the Reed SC model On the other hand, this model Kadlec & Knight uses
only one form of kinematic equation for all parameters, should be more
convenient when applied in practice.