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

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

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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

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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;

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(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

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

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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)

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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

TT City (mg/L) DO (mg/L) COD NH (mg/L) 4 -N PO (mg/L) 4 3- -P surveyed No of

lakes QCVN 08-MT:2015 ( B1) ³4 30 0.9 0.3

1 Da Nang 1.8 ÷ 5.8 22.75 ÷ 83.53 0.51 ÷ 1.2 0.11 ÷ 1.68 9 lakes

3 Ba Ria-Vung Tau 4.2 ÷ 5.2 18.3 ÷ 60.7 0.06 ÷ 1.67 0.12 ÷ 0.17 9 lakes

4 Hai Phong 3.93 ÷ 5.43 34.8 ÷ 51.6 0.14 ÷ 5.36 0.02 ÷ 1.79 9 lakes

5 Viet Tri 3.8 ÷ 4.7 21.17 ÷ 76.05 0.03 ÷ 0.46 0.02 ÷ 0.28 9 lakes

6 Can Tho 3.1 ÷ 4.6 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

TT Lake Temp ( 0 C) DO (mg/L) (mg/L) COD (mg/L) TN (mg/L) TP QCVN08-MT:2015 (B1) - ³4 30 - -

2 Seven Gian Lake (1) 24.9±5.3 3.8±0.4 53.8±7.3 4.0±0.9 1.7±0.5

5 Small Lake Kim Lien (3) 25.0±5.4 0.1±0.1 188.3±32.4 25.9±5.0 3.4±0.6

6 Hai Ba Trung Lake (3) 25.1±5.4 2.7±0.4 83.1±14.1 8.9±1.8 2.1±0.6

7 Uncle Ho's Fish Lake (1) 25.1±5.6 4.5±0.4 46.1±6.8 3.3±0.3 1.6±0.5

8 Lake at Van Phu Urban Area (2) 24.8±6.3 3.8±0.4 60.3±5.8 4.8±0.4 2.3±0.4

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

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 first-

order kinetic models, Kadlec and Knight [84] developed the k-C* model - equation (2.16) to describe the processing in horizontal underground flow

𝐶,-$ = 𝐶./𝑒&' ( $ (2.2 3)

In the above formulas: C, C in, C out, 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; k v - coefficient of pollutant treatment rate by volume, day-1; k a -

coefficient of pollutant treatment rate by area, m3/m2.day; k T -reaction rate constant at temperature T, day-1;HLR - hydraulic loading rate m3/m2.day

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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

water, area of air-water phase contact Figure 2.9 Aeration mechanism of overflow weir

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

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shapes) arranged in the form of a ladder to create a waterfall or cascade

2.5.2 Flow morphology in the flowforms

The flowforms

arran-ged in the form of a

cascade will create the

movement of water with

a harmonious rhythm

The aeration effect of the

water flow would

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

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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

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