Study on the combination model between constructed wetlands and stabilisation ponds to treat wastewater from suburban residential areas in cau river basin

high efficiency of nitrogen treatment, consuming a large area to construct, high cost for filtration materials, arising odor and harmful organisms. The kinetics of the removing pollutants in the CWs are described according to the model of first-order reaction with plug flow, the reaction rate constant (k) should be determined by local experiments to ensure fit of design. tK C C T e i .ln       RW TT RRT KK  . 9 (3). WSPs and CWs are technologies to treat wastewater in natural conditions. The process of removing pollutants depends heavily on natural conditions and other operating factors such as hydraulic regimes, retention times and pollutant loading rate. The mechanism of removing typical pollutants of domestic wastewater such as organic compounds, suspended substances, N, P and pathogenic microorangnisms of WSPs and CWs has many similar characteristics, but the conditions and effectiveness of treatment in these technologies are not the same. Therefore, combining WSPs and CWs in a system to treat wastewater met to environmental standards is perfectly appropriate. CHAPRTER 3: EXPERIMENTAL RESEARCH 3.1. Selecting study location The study site is a typical residential area for peri-urban residential areas in Cau river basin, with centralized drainage. The study site has stable wastewater, available land for building pilot models, without affecting on the surrounding residents. After the survey in the Song Cong city in Thai Nguyen province, residential area in Bach Quang ward was selected as the location for the thesis. This is a typical residential area like other peri-urban areas in Cau river basin. Bach Quang ward is established since 2011, the distance from the center of Song Cong city to it about 1 km and has convenient terrain and roads ... The total land area of 8,525 km 2 , population of 5,142 people (2014) [31]. Song Cong city has relatively flat terrain, the average height of the inner city is 15-17m. The climate is in the tropical region, with two distinct seasons: the hot season from April to October, often with southeast wind blowing, heavy rain and the cold season is from November to March next year, usually the northeast monsoon comes down, the lower temperature drops and it is cold. The annual average temperature is about 22 0 C, the highest average temperature is 38 0 C (in July and August) and the lowest is about 15 0 C - 16 0 C (in January). Number of sunshine hours in a year reaches 1,628 hours; and radiation energy is 115 kcal/cm 2 . According to the plan, domestic wastewater from residential areas, offices and schools of Bach Quang ward is collected and flowed to the centralized treatment system of the ward, which has been operating since 12/2013. It has a capacity of 750 m 3 /day. It is built on an area of 5,000 m 2 by the Center for Environmental Technology and Consulting under the General Department of Environment. The technology diagram of this system is shown in Figure 3.1. After nearly one year, the Center has sampled to analyze the inlet and outlet of the system. The analytical results of the indicators reach the permissible limit Column B of QCVN 14: 2008/BTNMT. Until now, the system has operated. However, only the planned residential areas belonging to Binh Minh and La Dinh residential groups have wastewater collection systems that are brought to the system. The flow rate is estimated at 31.1 m 3 /day (calculated according to the water supply norm of 120 liters/person) equal to 4.15% of the designed capacity. Each week, the sewage is pumped from the pumping pit to the distribution tank from one to two times, so the system operates intermittently, not at full capacity. Ecological pond after HF has a function of stabling wastewater and disinfection. However, the pond is stocked with dense fish, adding fish food daily reducing of its role. Therefore, it is difficult to assess the removing efficiency of the system. To determine characteristic wastewater, wastewater samples were collected behind the screen and at the pump pit of the system on three, four and five of May 2014 and analyzed at the Water Environment Laboratory of the Division of Water Supply and Drainage - National University of Civil Engineering. The analytical results of the samples are shown in Table 3.1. Figure 3.1. Diagram of the centralized wastewater treatment system in Bach Quang, Song Cong, Thai Nguyen. Manhole to collect water screen Sand settling tanks Anaerobic filtration tank Pump pit Clarifier tank Distribution tank Overflow gutters ladder type Manhole distribution CWs (HF) Ecological ponds Receiving water 10 Table 3.1. The characteristic domestic wastewater from residential areas of Bach Quang ward, Song Cong city, Thai Nguyen province Ordinal number Indicator Unit M1 M2 QCVN 14:2008 /BTNMT Column A 1 pH - 6,9÷7,4 7,2÷7,5 5÷9 2 TSS mg/L 69,67±13,80 67,00±13,00 50 3 COD mg/L 184,41±24,31 154,55±26,07 - 4 BOD5 mg/L 102,53±14,30 85,00±11,14 30 5 NH4 + -N mg/L 29,79±0,44 23,44±1,23 5 6 NO3 - -N mg/L 4,47±1,00 1,73±0,47 30 7 PO4 3- -P mg/L 0,79±0,06 0,61±0,03 6 8 Coliform MPN/100 mL 131.670±8.020 91.000±10.540 3000 Note: M1, M2: Wastewater sample after screen and at pump pit in the wastewater treatment system of Bach Quang ward, Song Cong city, Thai Nguyen province and number of sampling n is three. The influent characteristics have a low pollution due to the wastewater flow passed through septic tanks of households. The values of parameters exceeding the limit in QCVN 14:2008/BTNMT are not much. The values of pH, NO3 - -N, PO4 3- -P parameters are within the limits column A of QCVN 14:2008/BTNMT. The value of TSS, BOD5, NH4+-N parameters and Coliform exceeds the limit column A of QCVN 14:2008/BTNMT, respectively, is 1.39; 3.42; 5.96 and 43.89 times. Comparison of values of typical wastewater parameters of M1 and M2 samples showed that the values of parameters in M2 samples were not significantly reduced compared to M1 samples. This proves that the anaerobic filtration tank of the system with short HRT, has negligible efficiency in removing organic matter, suspended matter, nitrogen, phosphate and Coliform. Anaerobic filtration tank acts as a wastewater tank to pump up to HF. The influent into the pilot models was taken at the pump pit of the system because the wastewater at this location was not significantly different from residential areas and stability. The point of collecting wastewater is presented in Figure 3.3. 3.2. The pilot models 3.2.1. Designing the pilot research models * Research goals The research assumption is that the domestic wastewater flow from septic tanks of suburban residential areas will be collected and treated by the system of combining CWs and WSPs and treated wastewater will meet limit Column A of QCVN 14:2008/BTNMT. On that basis, there are four research goals. The first is researching the domestic wastewater removal from the suburban residential areas in Cau river basin by using the combinated CWs and WSPs models. The second is assessing the adaptation and growth level of selected plant species grown in the CWs in the pilot models. The third is determining organic removal rate coefficient in WSPs with natural conditions of Cau river basin. The fourth is determining the removal rate coefficients to pollutants in CWs with natural conditions of Cau river basin. * Choosing the pilot models Screen Sand settling tanks The anaerobic filtration tank Pump pit The wastewater treatment system in Bach Quang ward Pilot models The domestic wastewater flow from the resident areas of Bach Quang ward Figure 3.3. Diagram of the taking wastewater location into the experimental model from the wastewater treatment system of Bach Quang ward Figure 3.5. Diagram of domestic wastewater treatment system from Bach Quang ward by the combined CWs with WSPs teachnologies Note: The pilot model 1; (b). The pilot model 2 (a) (b) Domestic wastewater flow pretreated FWS Facultative pond Receiving water Domestic wastewater flow pretreated HF Maturation ponds Receiving water 11 * Designing the pilot models: Results of calculating the CWs and WSPs dimensions of the pilot models follow: - The pilot model 1: Dimensions of facultative pond: L x B x H = (1.22 x 1.22 x 1.45) m; Dimensions of FWS: Lx B x H = (1.20 x 0.80 x 1.45) m. - The pilot model 2: Dimensions of HF: L x B x H = (1.20 x 0.80 x 0.75) m; Maturation pond: Lx B x H = (2.40 x 1.00 x 1.00) m. 3.2.2. Arranging the layout and determining the elevation of the pilot models - Determining layout of the pilot models: HF bottom elevation is 0.00 m. Maturation pond bottom elevation is - 0.25 m. Facultative pond bottom elevation is – 0.55 m. FWS bottom elevation is – 0.55 m. - Arranging the layout of the pilot models: To save space and operate conveniently, the pilot models is built of blocks and arranged in layout as shown in Figure 3.6 3.3. Materials and research methods 3.3.1. Selecting research materials - Filter materials used in the CWs include filter stones (d = 2 x 3 cm), support gravel (d = 4 x 6 cm) and planting sand (d = 1 x 2 mm). The selected plants grown in the CWs include Canna generalis and Cyperus alternifolius. 3.3.2. Process of operating the pilot models The pilot models operated in 2 phases and divided into 5 unit phases, from 12/2014 to 5/2016. + Phase 1: From 7/12/2014 to 29/8/2015 (includes period 1). The goal of this phase is to evaluate the ability to remove pollutants in domestic wastewater flow from Bach Quang residential area by two pilot models. Operating parameters showed in Table 3.4. + Phase 2: From 26/9/2015 to 29/5/2016 (includes period 2, 3, 4 and 5). The goals of these phases are to assess the load capacity of two pilot models when changing the flow rate into the models and comparing the removal capacity of Canna generalis and Cyperus alternifolius planted on the HFs. The operating parameters of the models in phase 2 show in Table 3.5. Table 3.4. Operating parameters of two pilot models in phase 1 Operating parameters Unit Period Unit works of the pilots Model 1 Model 2 Facultative pond FWS HF 1 HF2 Maturation pond Q L/h Period 1: 7/12/2014- 29/8/2015 4 4 2 2 4 HRT Day 20,16 6,36 5,72 4,57 20 HLR m 3 /m 2 /day - 0,1 0,05 0,05 - Tree species - Canna generalis - Canna generalis - Table 3.5. Operating parameters of two pilot models in phase 2 Operating parameters Unit Period Unit works of the pilots Model 1 Model 2 Facultative pond FWS HF 1 HF2 Maturation pond Q L/h Period 2: 5 5 2 3 5 3 2 4 Figure 3.6. Diagram of the pilot models layout Note: Number 1 is storge wastewater tank into the system. Number 2 is HF for tree planting. Number 3 is HF not plant trees. Number 4 is maturation pond. Number 5 is FWS. Number 6 is facultative pond. Number 7 is ditch distributing wastewater into HF. Number 8 is suspended shield. Number 9 is ditch collecting effluent from maturation pond. Number 10 is the system of collecting effluent from FWS. The path of waste water in the pilot models. 7 8 9 6 5 10 1 Influent Effluent Effluent 12 HRT Day 26/9/2015- 13/12/2015 16,12 5,09 4,572 3,05 16 HLR m 3 /m 2 /day - 0,125 0,05 0,075 - Tree species Canna generalis Cyperus alternifolius Canna generalis Q L/h Period 3: 13/12/2015- 21/2/2016 6 6 3 3 6 HRT Day 13,44 4,24 3,05 3,05 13,33 HLR m 3 /m 2 /day - 0,15 0,075 0,075 - Tree species Canna generalis Cyperus alternifolius Canna generalis Q L/h Period 4: 21/2/2016- 3/4/2016 7 7 3,5 3,5 7 HRT Day 11,52 3,64 2,61 2,61 11,43 HLR m 3 /m 2 /day 0,175 0,0875 0,0875 Tree species Canna generalis Cyperus alternifolius Canna generalis Q L/h Period 5: 3/4/2017- 29/5/2016 8 8 4 4 8 HRT Day 10,08 3,18 2,29 2,29 10 HLR m 3 /m 2 /day 0,2 0,1 0,1 Tree species Canna generalis Cyperus alternifolius Canna generalis * Operating the pilot models + Phase 1 (period 1): From 7/12/2014 to 29/8/2015, model 1 and model 2 operated through the following steps: step 1: preparing materials; step 2: plant trees; Step 3: Load wastewater into the model; step 4: operating the model, monitoring plants growth and sampling. + Phase 2 (periods 2, 3, 4 and 5): From 26/9/2015 to 29/5/2016 - Model 1 is operated through the following steps: Step 1: Adjusting the influent flow rate into the model according to the plan shown in Table 3.5 respectively for periods 2, 3, 4, 5. Step 2: Maintaining the models and monitoring the plants growth and taking samples in the respective periods. - Model 2 operated through the following steps:  Phase 2: From 26/12/2015 to 13/12/2016: Step 1 is restarting model. The model restarted on the September 26, 2015. First taking all the filter gravel out of the non-tree filtration area (HF1) to wash, dry and rearrange filter material layers in the HF1. These layers include one filter gravel layer of 0.6 m thick below and one sand planting layer 0.15 m thick on the top. Pump all the wastewater in the maturation pond and clean it. Step 2 is planting. Cyperus alternifolius planted in HF1 from 4/10/2015. Cyperus alternifolius cut all the leaves. Then they split into clusters of 3-5 trees and planted on HF with a distance of 20 cm between the clusters. Step 3 is supplying wastewater to the pilot model. Supplying wastewater to HF plants to Cyperus alternifolius (HF1') with the flow rate of 2 liters per hour (q = 2 L/h. Then adjusting the influent flow rate to the HF to grown Canna generalis (HF2) with flow rate of 3 liters per hour (q = 3 L/h). Step 4 are maintaining the model, monitoring the plants growth and taking samples.  Period 3, 4 and 5: From 13/12/2016 to 29/5/2016: Step 1: Adjust the influent flow rate into the model according to the plan shown in Table 3.5, respectively for periods 3, 4 and 5; Step 2: Maintain the model, monitor the growth of plants and take samples of the respective periods. * Sampling and analysis plan Table 3.6. Sampling plan during the experiment periods Experiment period Content Sampling interval Frequency of sampling (one time per week) Number of sampling cycle Number of samples per cycle Total samples Period 1 8/3/2015 -29/8/2015 2 12 7 84 Period 2 8/11/2015 - 13/12/2015 1 6 7 42 13 Period 3 27/12/2015 -31/1/2016 1 6 7 42 Period 4 28/2/2016 - 3/4/2016 1 6 7 42 Period 5 17/4/2016 - 29/5/2016 1 6 7 42 + Sampling locations: Sampling locations follow as: (1).The influent sample- M1; (2). Effluent sample of facultative pond - M2; (3). Effluent sample of FWS - M3; (4). Effluent sample of HF without plant - HF1 (period 1) or HF with plants - HF1' (periods 2, 3, 4 and 5) n- M4; (5). Effluent sample of HF with Canna generalis (HF2) - M5; (6).Influent sample of maturation pond - M6; (7). Effluent sample of maturation pond - M7 * Analytical parameters: pH, TSS, COD, BOD5, TN, N-NH4 + , N-NO3 - , PO4 3- , Coliform, biomass and plant height. 3.3.3. Research Methods Research Methods include sampling methods, analytical methods in the laboratory, methods of measuring and controlling the wastewater flow rate, Data processing methods and methods of determining kinetic coefficients. CHAPTER 4: RESULTS AND DISCUSSIONS 4.1. Removal of domestic wastewater from Bach Quang ward by the conbined facultative pond and FWS (The model 1) 4.1.1. Wastewater characteristics into the model 1 Table 4.1. The results of analyzing the average concentration of the characteristic parameters of wastewater into model 1 Parameters Unit Periods Average concentration QCVN 14:2008 /BTNMT Column A Period 1 Period 2 Period 3 Period 4 Period 5 pH - 7,1 ÷ 8,5 7,1 ÷7,5 7,6 ÷8,9 7,1 ÷7,3 6,9 ÷7,4 6,9 ÷ 8,5 5÷9 BOD5 mg/L 83,45 ±22,63 84,83 ±6,44 83,38 ±8,05 83,27 ±5,60 81,95 ±8,14 83,39 ±13,62 30 TSS mg/L 70,21 ±19,29 39,83 ±4,40 48,33 ±10,17 47,33 ±4,93 43,33 ±7,71 53,21 ±16,49 50 TN (*) mg/L 26,00 ±2,85 34,00 ±4,36 35,33 ±3,51 47,33 ±9,81 36,33 ±7,37 35,19 ± 8,84 - NH4 + -N mg/L 23,53 ±8,87 33,18 ±5,01 34,58 ±3,06 40,55 ±7,65 43,67 ±14,35 33,17 ±10,71 5 NO3 - -N mg/L 2,51 ±0,88 1,30 ±0,27 1,43 ±0,16 2,25 ±0,33 2,43 ±0,30 2,07 ±0,70 30 PO4 3- -P mg/L 0,35 ±0,31 1,73 ±0,31 1,33 ±0,05 2,42 ±0,62 2,84 ±0,61 1,51 ±0,98 6 Coliform (**) (MPN/ 100mL) 92.125 ±8.107 - - - - 92.125 ±8.107 3000 The influent is low pollution; there is not much variation between periods, with a neutral pH ranging from 6.90 to 8.50. The parameters NO3 - -N, PO4 3- -P are all lower than the limit Column A of QCVN 14:2008/BTNMT. Wastewater is mainly polluted by TSS, BOD5, NH4 + -N and Coliforms. The average value of these parameters exceeds limit Column A of QCVN14: 2008/BTNMT, respectively 1.06; 2.78; 6.63 and 30.71 times. 4.1.2. Pollutants removal in facultative pond The pollutants load into the facultative pond and pollutants removal during the experiment periods are summarized in Table 4.2 and Figure 4.1. 14 Table 4.2. The average pollutants load into the facultative pond during the experiment periods (kg/ha/day). Ordinal number Parameters Periods Period 1 Period 2 Period 3 Period 4 Period 5 1 BOD5 53,83±14,60 68,38±5,19 80,67±7,79 93,99±6,33 105,72±10,50 2 TN 16,77±1,67 27,41±3,51 34,18±3,40 53,43±11,08 46,87±9,51 3 NH4 + -N 15,17±5,72 26,75±4,04 33,45±2,96 45,77±8,64 56,33±18,51 4 NO3 - -N 1,62±0,57 1,05±0,22 1,39±0,16 2,54±0,38 3,44±0,39 5 PO4 3- -P 0,23±0,20 1,40±30,77 1,29±0,04 2,73±0,70 3,67±0,78 - pH change in the effluent: The pH in the effluent is in the neutral and slightly alkaline range, ranging from 7.6 to 10.6. - Biodegradable organics removal: The removal of biodegradable organics was average, quite stable between experiment periods and ranged from 43.80% to 68.44%. When increased the influent flow rates in the range from 4 to 8 L/h (also reduced HRT from 20.16 to 10.08 day) has negligible impact on biodegradable organics removal of this pond. - TSS removal: The TSS concentration in the influent has high volatility and decreases gradually through periods. The mean average concentration of TSS is the highest in period 1 and lowest in period 4 as 22.67 mg/L and 11.50 mg/L respectively. The TSS average removal ranges from 45.54% to 75.70%, with the lowest in period 2 and the highest in period 4 When increased the influent flow rates from 4 to 8 L/h and reduced HRT from 20.16 to 10.08 days has negligible impact on the algae growth. - Nitrogen removal: The average TN loads in the influent vary from 16.77 to 53.43 kg/ha/day. The average TN concentration in the effluent tends to increase gradually in the experiments, ranging from 8.00 to 27.00 mg/L, reaching the highest treatment efficiency in period 1 (69.23%) and the lowest in period 5 (32.11%). The average NH4 + -N concentration of effluent from period 1 to period 5, ranges from 5.24 to 20.75 mg/L. The average NO3 - -N concentration of effluent was quite variable, ranging from 2.75 to 5.3 mg/L. The average NH4 + -N removal ranges from 49.24% to 77.72%, reaching the highest value in period 1 and the lowest value in period 4. This range is lower than the NH4 + -N removal in facultative pond can reach up to 90%. When increasing the influent flow rates from 4 to 8 L/h and reduced HRT from 20.16 to 10.08 days, the NH4 + -N removal reduced not much. This demonstrates the stability of facultative pond when changing the influent flow rates. - PO4 3- -P removal: The average PO4 3- -P loads in the influent vary from 0.23 to 3.67 kg/ha/day. The average PO4 3- -P concentration in the effluent is increasing gradually from period 1 to period 5, ranging from 0.18 to 1.89 mg/L. The average PO4 3- -P removal has a fluctuation in the experiments, increases slightly in period 2 and gradually decreases in period 3, 4 and increases in period 5. The highest PO4 3- -P removal is 52.30% in period 1 and the lowest PO4 3- -P removal is 21.68% in period 4. When increasing the influent flow rates in period 2, 3, 4 and 5 and reduced HRT from 20.16 days to 10.08 days, the PO4 3- -P removal reduced. - Coliforms removal: The average Coliforms number in the influent and effluent are 9.21 x 10 4 and 8.5 x 10 3 MPN/100 mL (4.91 and 3.70 logs respectively). Thus, the average Coliforms removal is 1.03 logs. This result fit to the synthesis of Ansa E. D. O. et al. (2015) about that in facultative pond to domestic wastewater. That is in the range of from 1 to 5 logs. 4.1.3. Pollutants removal in FWS The influent pollutants loads and pollutants removal in FWS summarize in Table 4.4, Table 4.5 and Figure 4.2. Figure 4.1. The average removal in facultative pond to characteristic parameters of domestic wastewater 15 Talble 4.4. The average pollutants loads in the influent into FWS through experiment periods (kg/ha/day) Ordinal number Parameters Periods Period 1 Period 2 Period 3 Period 4 Period 5 1 BOD5 46,91±24,09 41,11±11,67 39,47±12,68 57,14±2,86 60,80±11,50 2 TN 8,00±2,71 20,42±5,05 24,50±6,24 47,25±3,03 49,33±4,16 3 NH4 + -N 5,24±2,00 18,51±6,23 16,72±5,18 36,02±4,91 41,50±6,05 4 NO3 - -N 5,3±1,21 3,44±0,63 4,80±1,01 7,38±1,91 8,07±1,03 5 PO4 3- -P 0,18±0,16 0,88±0,21 1,33±0,11 3,32±0,45 3,55±0,59 Talble 4.5. The average concentration of characteristic parameters in the effluent from FWS through experiment periods Parameters Periods Parameters Period 1 Period 2 Period 3 Period 4 Period 5 pH - 6,7 ÷ 7,8 6,5 ÷ 6,8 6,6 ÷ 6,9 6,5 ÷ 6,7 6,5 ÷ 6,9 BOD5 mg/L 8,43±4,24 10,95±2,88 10,35±3,05 15,49±0,68 15,07±2,57 TSS mg/L 4,5±1,13 1,5±0,55 3,33±1,03 6,67±2,73 5,33±2,58 TN (*) mg/L 1,5±0,58 4,33±1,53 8,67±1,53 14,67±2,31 15,67±0,58 NH4 + -N mg/L 0,81±0,24 4,10±1,60 5,79±1,54 12,1±1,57 13,08±2,14 NO3 - -N mg/L 1,35±1,09 0,72±0,10 1,13±0,21 1,87±0,28 2,17±0,17 PO4 3- -P mg/L 0,12±0,13 0,56±0,24 0,63±0,06 1,39±0,18 1,36±0,23 Coliform (**) MPN/100 mL 1.750± 1.269 Note: The number of samples in period 1 is 12 (n = 12). The number of samples in periods 2, 3, 4 and 5 are 6 (n = 6); (*): The number of samples in period 1 is 4 (n = 4). The number of samples in periods 2, 3, 4, 5 are 3 (n = 3); (**): The number of Coliform analysis samples in period 1 is 4 (n = 4). The number of Coliform analysis samples in periods 2, 3, 4, 5 are 0 (n = 0). - pH change in the effluent: pH of the effluent has neutral values and be quite stable during experiments, ranging from 6.50 to 7.80. - Biodegradable organics removal: The average BOD5 loads in the influent range from 46.91 to 106.46 kg/ha/day. The average BOD5 concentrations in the influent increase gradually from period 1 to period 5 with the corresponding values of 8.43, 10.95, 10.35, 15.49 and 15.07 mg/L. Average BOD5 removals decrease from period 1 to period 5 with the corresponding levels of 82.04, 66.72, 60.67, 52.55 and 50.43%. Thus, when increased HLR into the FWS and reduced HRT corresponding to the experiments are (0.10, 0.125, 0.15, 0.175, 0.20) m 3 /m 2 /day and (6.36, 5.09, 4.24, 3.64, 3.18) days, the BOD5 removals reduce negligibly. The BOD5 removals in FWS in periods 2, 3, 4 and 5 decreases compared to the period 1 are 15.32, 21.37, 29.48 and 31.60% respectively. - TSS removal: The TSS concentrations in the effluent increase gradually through experiments. The average TSS concentrations range from 1.50 to 6.67 mg/L, the lowest in period 2 and the highest in period 4. Average TSS removals from period 1 to period 5 are 80.13, 92.68, 84.50, 42.03 and 60.98% respectively. When increasing HLR from 0.125 to 0.15 m 3 /m 2 /day, the TSS removals in FWS negligibly impacted on. Figure 4.2. The average removal in FWS for characterisic parameters of domestic wastewater 16 However, when increased HLR from 0.175 to 0.20 m 3 /m 2 /day, the TSS removals in FWS reduce significantly. - Nitrogen removal: The average loads of TN, NH4 + -N and NO3 - -N in the influent vary in the respective intervals as 8.00-49.33, 5.24 - 41.50 and 5.30 - 8.07 kg/ha/day. The average concentrations of TN, NH4 + -N and NO3 - -N in the influent into FWS increase gradually