The use of water spinach (Ipomoea aquatica) in domestic wastewater treatment

Nong Lam University, Ho Chi Minh City 49 The use of water spinach (Ipomoea aquatica) in domestic wastewater treatment Thinh V. D. Nguyen∗, Huong N. T. Huynh, Mai N. H. Nguyen, & Thao V. Ngo Department of Environmental Sciences, Nong Lam University, Ho Chi Minh City, Vietnam ARTICLE INFO Research paper Received: March 23, 2018 Revised: April 27, 2018 Accepted: May 05, 2018 Keywords Domestic wastewater Household Hydroponics Wastewater treatment Water spinach ∗Corresponding author Nguyen Vu Duc Thinh Email: ducthinh.env@gmail.com ABSTRACT The main objective of this study was to examine the efficacy and capacity of using hydroponic systems in municipal pollutant removal at household scale. Three pilot scaled hydroponic systems [dimension for each system: 4.5 m (L) x Φ 114 mm] were installed to investigate the optimal age of vegetable, planting density and retention time for household wastewater treatment, respectively. Water spinach (Ipomoea aquatica) planted in 27 plastic cups throughout 4.5-m-length and 114- mm-diameter uPVC pipes filled with wastewater was employed as the treating agent of pollutants. The averaged influent contained proxi- mately 32.5 mg/L suspended solids (SS), 76.0 mg/L biological oxy- gen demand (BOD5), 220.5 mg/L chemical oxygen demand (COD), 26 mg/L NH+4 , 5.0 mg/L NO − 3 , and 8.5 mg/L PO 3− 4 at pH 7.3. Results showed that a designed system consisting of 10 plants of 15-day-old water spinach pre-planted in baked clay in each cup was capable of treating 30 L of domestic wastewater meeting the current municipal wastewater discharge standards in Vietnam (column A standards of QCVN 14:2008/BTNMT) after 4 days of wastewater retention time. If operated under conditions of the above parameters, the pilot-plant hy- droponic system can achieve the removal of 65% SS, 82% BOD5, 74% COD, 90% NH+4 , 30% NO − 3 and 86% PO 3− 4 . The result of this study has provided an applicable domestic wastewater treatment system eco- friendly and suitable for small and medium household areas. Cited as: Nguyen, T. V. D., Huynh, H. N. T., Nguyen, M. N. H., & Ngo, T. V. (2018). The use of water spinach (Ipomoea aquatica) in domestic wastewater treatment. The Journal of Agriculture and Development 17(3), 49-54. 1. Introduction The proportion of domestic wastewater treated is at low levels, and raw wastewater is usually dis- charged directly to environment in urban areas of Vietnam (MONRE, 2016). Currently, 37 col- lective wastewater treatment plants have been in operation in urban centers of grade III or higher cities (MONRE, 2016). Wastewater drainage sys- tems, however, have not been completed, causing difficulties in collecting and leading wastewater to treatment plants (MONRE, 2016). Hence, a domestic wastewater treatment plant at house- hold scale is necessary to reduce pollutant loads to environment. Domestic wastewater can be treated in differ- ent ways: mechanically, chemically or biologically (Luong, 2011; Hoang & Tran, 2014). Among bi- ological treatments, the hydroponic system is a potential way for wastewater treatment at house- hold scale because it is easy to establish and re- quires small space and harvested vegetable can be used as food (VEA, 2010). Hydroponic crops can be almost any type of plants such as vegetables, fruits, flowers, garden trees, herbs, ivy, and peren- nial that crops are harvested after a short plant- ing period (Lem et al., 1990). It is easy to con- trol various environment parameters as nutrients, pH, temperature, oxygen, etc. (Lem et al., 1990). Wastewater would be used instead of chemical fertilizers for growing vegetables. However, hy- droponics has disadvantages such as higher ini- tial costs than planting in soil and diseases could spread to the other plants root easily and are dif- www.jad.hcmuaf.edu.vn The Journal of Agriculture and Development 17(3) 50 Nong Lam University, Ho Chi Minh City ficult to control in the case of planting with re- circulation systems (Lem et al., 1990). Ipomoea aquatica, or water spinach, is a herba- ceous perennial trailing vine (Patnaik, 1976). It has hollow stems that grow floating or prostrate (Patnaik, 1976). The roots from the nodes pen- etrate the soil or mud, and the leaves are sim- ple and alternate (Patnaik, 1976). This plant species grows well as a crop in regions where the mean temperature is above 250C (Patnaik, 1976). Hence, hydroponics in Vietnam is a conducive en- vironment for water spinach to flourish. Previous studies have demonstrated that plant- ing Ipomoea aquatica in fishponds can efficiently remove nutrients and improve water quality (Li & Li, 2009; Dai et al., 2012). Accordingly, the cur- rent study expected that water spinach could use the nutrients in domestic wastewater for grow- ing and reducing water pollutant loads. Pilot hy- droponic systems with water spinach were es- tablished to examine the removal percentages of municipal pollutants in wastewater from an apartment. Moreover, the optimal age of water spinach, planting density and retention time were also determined for household guidelines. 2. Materials and Methods 2.1. Domestic wastewater characteristics Domestic wastewater was collected from col- lecting tank of Sunview Apartment, Cay Keo Street, Thu Duc District, HCMC, Vietnam in the morning from January to June 2017 accord- ing to TCVN 6663-1:2011 and ISO 5667-1:2006. The wastewater parameters included: water tem- perature 290C, pH 7.3, SS 32.5 ± 1.5 mg/L, BOD5 76.0 ± 8.0 mg/L, COD 220.5 ± 25.5 mg/L, NH+4 −N 26.0 ± 4.0 mg/L, NO−3 −N 5.0 ± 1.0 mg/L, and PO3−4 8.5 ± 1.5 mg/L and did not vary much throughout the experiments. Wastew- ater was pre-filtered through a kitchen sieve to remove large particles, contained in 30-L plastic buckets and transferred to Environmental Tech- nology Laboratory of Faculty of Environment and Natural Resources, Nong Lam University. The wastewater was then analyzed and employed for the experiments immediately. 2.2. Conditions of water spinach Prior to the experimetns, water spinach was grown hydroponically in baked clay at Institute of Biotechnology and Environment (IBE), Nong Lam University. Water spinach seeds were pro- vided by Phu Nong Seeds Company. 2.3. Experiments 2.3.1. Hydroponic systems Three pilot scaled experiments consisting hy- droponic systems [dimension for each system: 4.5 m (L) x Φ 114 mm] were installed with water spinach to investigate the optimal age of veg- etable, planting density and hydraulic retention time (HRT) for household wastewater treatment, respectively (Figure 1). Water spinach (Ipomoea aquatica) planted in 27 plastic cups throughout 4.5-m-length and 114-mm-diameter uPVC pipes filled with wastewater was employed as the treat- ing agent of pollutants. A similar designed pipe without water spinach was used to make the con- trol. Figure 1. Hydroponic pilot (sizes in cm). The pre-experiments were executed to choose ranges of vegetables’ optimal age (10, 15 and 20 days old), optimal planting density (5, 10 and 15 plants per cup) and optimal retention time (2, 4 and 6 days). 2.3.2. Determination of the optimal age of veg- etables After 10, 15, and 20 days pre-planted in baked clay at IBE, water spinach was transferred to three hydroponic systems, respectively in 27 plas- tic cups. Each cup contained 10 plants. The con- trol system was made without vegetables. Thirty liters of domestic wastewater were added to each hydroponic systems with HRT = 4 days. Treated wastewater was collected after HRT to analyze The Journal of Agriculture and Development 17(3) www.jad.hcmuaf.edu.vn Nong Lam University, Ho Chi Minh City 51 SS, BOD5, COD, NH − 4 , NO − 3 , and PO 3− 4 concen- trations remaining. 2.3.3. Determination of the optimal planting density Fifteen-day-old water spinach was planted in 27 plastic cups with 3 different densities of 5, 10 and 15 plants per cup throughout the pipes, re- spectively. The control system was made without vegetables. Thirty liters of domestic wastewater was added to each hydroponic systems with HRT = 4 days. Treated wastewater was collected after HRT to determine SS, BOD5, COD, NH − 4 , NO − 3 , and PO3−4 concentration residues. 2.3.4. Investigate the optimal retention time Thirty liters of domestic wastewater was added to each hydroponic systems. Fifteen-day-old wa- ter spinach was removed from baked clay and put in 27 lastic cups with the density of 10 plants/cup. There were 3 hydroponic systems with 3 different HRTs of 2, 4, and 6 days, re- spectively. A control system was made without vegetables. Treated wastewater was collected af- ter HRT to analyze SS, BOD5, COD, NH − 4 , NO − 3 , and PO3−4 concentrations remaining. 2.4. Water analysis The concentrations of SS, BOD5, COD, NH − 4 , NO−3 , and PO 3− 4 and pH of the wastewater out of the hydroponic systems were checked after hy- draulic retention time. The water sample was col- lected stochastically from three locations of each hydroponic system from 8 AM to 9 AM with 100 mL per model. Chemical oxygen demand was analyzed accord- ing to SMEWW 5220 D (2012). BOD5 was ana- lyzed according to TCVN 6001-1:2008 and ISO 5815-1:2003. NH−4 (LoD = 0.2 mg/L, LoQ = 0.5 mg/L), NO−3 (LoD = 4 mg/L, LoQ = 10 mg/L) and PO3−4 (LoD = 0.04 mg/L, LoQ = 0.1 mg/L) concentrations were determined by Sera Test Kits (Germany). In addition, the samples have con- centrations of NO−3 less than 20 mg/L were de- termined by Tropic Marin Test Kits (Germany) with LoD = 0.5 mg/L and LoQ = 1.5 mg/L. pH was measured by LAQUAtwin portable pH meter (HORIBA Scientific, Japan). Temperature was measured by mercury thermometer. Each mea- surement was made 3 times. 3. Results 3.1. Optimal age of water spinach After 4 days, SS, BOD5, COD, NH − 4 , NO − 3 , and PO3−4 concentrations of wastewater in the hydroponic systems containing 10, 15, and 20- day-old water spinach were 13.0 ± 1.5, 15.0 ± 2.0, 61.0 ± 5.0, 4.0 ± 1.0, 3.0 ± 0.5 and 2.0 ± 0.5 mg/L; 11.8 ± 1.3, 13.5 ± 2.5, 57.5 ± 5.5, 2.5 ± 0.5, 3.5 ± 0.5 and 1.2 ± 0.2 mg/L; and 16.0 ± 1.0, 15.5 ± 2.0, 67.5 ± 6.5, 3.5 ± 0.5, 4.0 ± 1.0 and 2.5 ± 0.5 mg/L, respectively (Figure 2). The pH values ranged from 7.9 to 8.1 in the three systems. As a result, the efficiency of the system with 15- day-old water spinach was greater than that of the other systems. Therefore, 15-day-old water spinach was employed for the next experiments. Figure 2. Treated wastewater parameters in hydro- ponics with different initial ages of water spinach. 3.2. Optimal planting density After 4 days, treated SS, BOD5, COD, NH − 4 , NO−3 , and PO 3− 4 values of hydroponic sys- tems with 5 plants/cup, 10 plants/cup, and 15 plants/cup were 15.0 ± 1.5, 16.0 ± 2.0, 68.0 ± 7.0, 3.0 ± 0.5, 4.0 ± 0.5 and 1.5 ± 0.5 mg/L; 11.0 ± 1.0, 14.0 ± 2.0, 55.0 ± 5.0, 2.5 ± 0.5, 3.0 ± 1.0 and 1.2 ± 0.2 mg/L; 10.0 ± 1.0, 14.0 ± 2.0, 57.5 ± 5.5, 2.5 ± 0.5, 3.5 ± 1.0 and 1.4 ± 0.2 mg/L, respectively (Figure 3). The pH values ranged from 7.5 to 8.0. Consequently, the optimal density was 10 plants each cup and used in the last experiment. www.jad.hcmuaf.edu.vn The Journal of Agriculture and Development 17(3) 52 Nong Lam University, Ho Chi Minh City Figure 3. Treated wastewater parameters in hydro- ponics with different planting densities. 3.3. Optimal retention time After HRT = 2 days, SS, BOD5, COD, NH − 4 , NO−3 , and PO 3− 4 concentrations of wastewater in the experimental hydroponic system were 19.5 ± 1.5, 53.0 ± 6.0, 97.0 ± 15.0, 3.0 ± 0.5, 4.0 ± 1.0 & 2.0 ± 0.5 mg/L, respectively (Figure 4a) and pH was 7.5 ± 0.1 while those of the control system were 24.0 ± 1.0, 68.0 ± 8.0, 160.0 ± 20.0, 24.0 ± 4.0, 5.0 ± 1.0 and 7.5 ± 0.5 mg/L, respectively (Figure 4b) and pH was 7.1 ± 0.2. After HRT = 4 days, SS, BOD5, COD, NH − 4 , NO − 3 , and PO 3− 4 concentrations of wastewater in the experimental hydroponic system were 11.5 ± 1.5, 13.5 ± 5.5, 57.0 ± 8.0, 2.5 ± 0.5, 3.5 ± 0.5 and 1.2 ± 0.3 mg/L respectively (Figure 4a) and pH was 7.8 ± 0.1 while those of the control system were 18.0 ± 1.5, 60.0 ± 6.0, 146.5 ± 18.0, 24.0 ± 4.0, 20.0 ± 2.0 and 7.0 ± 0.5 mg/L respectively (Figure 4b) and pH was 6.8 ± 0.1. These parameters met the current municipal wastewater discharge stan- dards in Vietnam (column A standards of QCVN 14:2008/BTNMT). After HRT = 6 days, SS, BOD5, COD, NH − 4 , NO−3 , and PO 3− 4 concentrations of wastewater in the experimental hydroponic system were 3.5 ± 0.5, 6.0 ± 1.0, 36.0 ± 7.0, 2.5 ± 0.5, 3.0 ± 0.5 and 1.2 ± 0.5 mg/L respectively (Figure 4a) and pH was 8.1 ± 0.1 while those of the control system were 7.0 ± 1.0, 52.0 ± 6.0, 112.0 ± 15.0, 22.0 ± 4.0, 25.0 ± 3.0 and 7.0 ± 1.0 mg/L respectively (Figure 4b) and pH was 6.5 ± 0.1. Figure 4. Treated wastewater parameters in (a) hy- droponics with different HRTs and (b) the control system. 4. Discussion 4.1. Hydroponics with water spinach In general, a hydroponic system consisting of 10 plants of 15-day-old water spinach pre-planted in baked clay in each cup could process 30 L of do- mestic wastewater to meet the current municipal wastewater discharge standards in Vietnam (col- umn A standards of QCVN 14:2008/BTNMT) at a HRT of 4 days. 4.1.1. pH pH of the wastewater out of the hydroponic systems increased slightly from 7.3 to over 7.5 in all experiments. That was because the wa- ter spinach in the hydroponic systems absorbed The Journal of Agriculture and Development 17(3) www.jad.hcmuaf.edu.vn Nong Lam University, Ho Chi Minh City 53 CO2 for photosynthesis, so the pH of water was increased. CO2 in the water reacts with water to produce H+ and bicarbonate to decrease pH of water according to the mechanism: : CO2 + H2O  H2CO3  H+ + HCO−3 (Kanabkaew & Puetpaiboon, 2004). Because CO2 for photosyn- thesis of aquatic plants is absorbed faster than the amount of CO2 generated from the respira- tory process of the quatic plants, plants must take CO2 from the metabolism of HCO − 3 (2HCO − 3 → CO2 + CO 2− 3 + H2O) (Kanabkaew & Puetpai- boon, 2004). Therefore, the pH of water increases. 4.1.2. SS removal The SS concentration decreased from 32.5 ± 1.5 mg/L to 11.8 ± 1.3 mg/L (Figure 4a), which means 65% of SS was removed from the wastew- ater. The removal of SS may be due to sedimen- tation or/and breakdown of microorganisms and plants. 4.1.3. COD and BOD5 removal Previous research has show that COD and BOD5 can be assimilated by plants (Vymazal & Kropfelova, 2009). The microbes around the roots can also contribute to the purification. The flourishing roots can provide a comfortable envi- ronment for microbes. Thus, the organic matter can be removed effectively. The concentrations of COD and BOD5 decreased from 220.5 ± 25.5 mg/L to 57.5 ± 5.5 mg/L and from 76.0 ± 8.0 mg/L to 13.5 ± 2.5 mg/L, respectively (Figure 4a). 74% of the COD and 82% of the BOD5 were removed from the wastewater. The efficiency of removal at different HRTs was quite difference. The efficiency of short HRT (2 days) was lower than that of middle HRT (4 days) (Figure 4). This could be because the plants needed a period of time to adapt to the new environment. When the roots grew flourishing, the plants could purify the water by assimilation of organic matters and nutrients. 4.1.4. Nitrogen removal The concentrations of NH+4 and NO − 3 in wastewater decreased from 26.0 ± 4.0 mg/L to 2.5 ± 0.5 mg/L and from 5.0 ± 1.0 mg/L to 3.5 ± 0.5 mg/L, respectively (Figure 4a). 90% of the NH+4 −N and 30% of the NO−3 −N were removed from the wastewater. The nitrogen in wastewater existed in the form of organic nitrogen, NH+4 −N and NO−3 −N. In the current study, the removal of odd nitrogen in wastewater relied on the assim- ilation of these compounds by water spinach in hydroponic systems. Firstly, NH+4 was converted to NO−3 and a portion of NO − 3 would then be denitrificated to N2 by microorganisms. Another NO−3 portion was absorbed by water spinach via roots for growing. However, which process con- tributed more to the NO−3 removal was not clar- ified. In other words, NO−3 could be assimilated by plants or sent back to the atmosphere by the effect of denitrifying microorganisms (Xu et al., 1999). 4.1.5. Phosphorus removal Phosphorus is the essential nutrient for plant growth. It can be assimilated by plants and be converted into various kinds of organic matter of plants (Gu et al., 2008). Water spinach, therefore, could assimilate PO3−4 in wastewater and make a reduction from 8.5 ± 1.5 mg/L to 1.2 ± 0.2 mg/L. Eighty six percent of PO3−4 were removed from the wastewater. 4.2. Control system On one hand, after HRT we observed moss stricking on the inner surface of pipes in the control system. On the other hand, SS created a visible layer of sediment on the inner surface. Moreover, activities of microorganisms could also break organic matters down in wastewater. Con- sequently, SS, BOD5 and COD decreased (Fig- ure 4b). Level of pH declined from 7.3 to 6.5. That was probably because NH+4 was nitrificated to NO−3 as evidenced by decreasing NH + 4 and in- creasing NO−3 concentrations at the end of the experiment. 4.3. Suggested household hydroponic system A family with 4 people release approximately 400 L of wastewater a day (MONRE, 2016). A tank of 1600 L is needed to store wastewater in 4 days. According to the design in this study, 240 m of Φ14-mm uPVC pipe are enough to treat the total amount of wastewater in 4 days. Pipes can be arranged as in Figure 1 or in tower shapes to save space. Total pipe investment costs VND 18,163,200. www.jad.hcmuaf.edu.vn The Journal of Agriculture and Development 17(3) 54 Nong Lam University, Ho Chi Minh City 5. Conclusions The averaged influent contained proximately 220.5 mg/L chemical oxygen demand (COD), 76.0 mg/L biological oxygen demand (BOD5), 32.5 mg/L suspended solids (SS), 26 mg/L NH4+, 5.0 mg/L NO−3 , and 8.5 PO 3− 4 at pH 7.3. The designed system consisting of 10 plants of 15- day-old water spinach pre-planted in baked clay in each cup was capable of treating 30 L of do- mestic wastewater meeting the current municipal wastewater discharge standards in Vietnam (col- umn A standards of QCVN 14:2008/BTNMT) af- ter 4 days of wastewater retention time. If oper- ated under conditions of the above parameters, the pilot-plant hydroponic system can achieve the removal of 74% COD, 82% BOD5, 64% SS, 90% NH+4 , 30% NO − 3 and 86% PO 3− 4 . 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