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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 INTRODUCTION 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 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" 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 Object and scope of research of the dissertation Research object: urban lake water polluted by wastewater and has low DO 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 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 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 Research methods Methods used for conducting: liturature review; field survey; in-field experiment; data processing; consulting with experts 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) 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 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 Dissertation structure The structure of the dissertation includes: Introduction Chapter Overview of the current status of urban lake water pollution in Vietnam and solutions to improve lake water quality Chapter Theoretical basis for treatment of polluted lake water by combining flowform system and constructed wetlands Chapter Experimental research Chapter 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 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) 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 Da Nang 1.8 ÷ 5.8 22.75 ÷ 83.53 Hue 4.17 ÷ 4.77 21.43 ÷ 53.07 Ba Ria-Vung Tau 4.2 ÷ 5.2 18.3 ÷ 60.7 Hai Phong 3.93 ÷ 5.43 34.8 ÷ 51.6 Viet Tri 3.8 ÷ 4.7 21.17 ÷ 76.05 TT 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 lakes 0.02 ÷ 2.15 lakes 0.12 ÷ 0.17 lakes 0.02 ÷ 1.79 lakes 0.02 ÷ 0.28 lakes 31.43 ÷ 48.17 0.07 ÷ 0.81 0.12 ÷ 0.14 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) Can lake (1) 26.0±5.2 Seven Gian Lake (1) 24.9±5.3 Tan Mai Lake (2) 25.6±5.2 Ho Mot Lake (2) 25.4±5.8 Small Lake Kim Lien (3) 25.0±5.4 Hai Ba Trung Lake (3) 25.1±5.4 Uncle Ho's Fish Lake (1) 25.1±5.6 Lake at Van Phu Urban Area (2) 24.8±6.3 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 Sampling period from November 2019 to April 2020, the number of samples is 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 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, 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 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 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 flowforms (2 made of enameled ceramic, made of white cement and 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 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 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 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 Field experiment design Table 3.5 Design parameters of the experiment setup Design parameters Symbol 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 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 Concrete, brick mm 2890×1630 step mm L/h 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 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 - without the FC * Phase 2: Operating the CW in combination with the FC Figure 3.20 Operation diagram of Phase – 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; 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 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 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 - CW alone Figure 4.3 Change of water quality parameters along the FC in Phase pH of wastewater according to CW length little change at all 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 levels of HLR; DO was almost unchanged along the the CW in all 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 (