Kinetics of pollutant removal from domestic wastewater in a tropical horizontal subsurface flow constructed wetland system effects of hydraulic loading rate

Ecological Engineering 36 (2010) 527–535 Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng Kinetics of pollutant removal from domestic wastewater in a tropical horizontal subsurface flow constructed wetland system: Effects of hydraulic loading rate Ngo Thuy Diem Trang a,∗ , Dennis Konnerup b , Hans-Henrik Schierup b , Nguyen Huu Chiem a , Le Anh Tuan c , Hans Brix b Department of Environmental Science, College of Environment and Natural Resources, Can Tho University, Campus I, 3/2 Street, Ninh Kieu District, Can Tho City, Vietnam Department of Biological Sciences, Aarhus University, Ole Worms Allé 1, DK-8000 Århus C., Denmark Department of Environmental and Natural Resources Management, College of Environment and Natural Resources, Can Tho University, Campus I, 3/2 Street, Ninh Kieu District, Can Tho City, Vietnam a b c a r t i c l e i n f o Article history: Received 25 July 2009 Received in revised form November 2009 Accepted 23 November 2009 Keywords: Ammonium Background concentration BOD COD First-order kinetics Nitrogen Phosphorus Phragmites vallatoria a b s t r a c t The treatment capacity of constructed wetlands is expected to be high in tropical areas because of the warm temperatures and the associated higher rates of microbial activity A pilot scale horizontal subsurface flow constructed wetland system filled with river sand and planted with Phragmites vallatoria (L.) Veldkamp was set up in the southern part of Vietnam to assess the treatment capacity and the removal rate kinetics under tropical conditions The system received municipal wastewater at four hydraulic loading rates (HLRs) of 31, 62, 104 and 146 mm day−1 Removals of TSS, BOD5 and COD were efficient at all HLRs with mean removal rates of 86–95%, 65–83% and 57–84%, respectively Removals of N and P decreased with HLRs and were: NH4 -N 0–91%; TN 16–84% and TP 72–99% First-order area-based removal rate constants (k, m year−1 ) estimated from sampling along the length of the wetland from inlet to outlet at the four HLRs were in the range of 25–95 (BOD5 ), 22–30 (COD), 31–115 (TSS), 5–24 (TN and TKN) and 41–84 (TP) at background concentrations (C*) of 5, 10, 0, 1.5 and mg L−1 , respectively The estimated k-values should not be used for design purposes, as site-specific differences and stochastic variability can be high However, the study shows that domestic wastewater can be treated in horizontal subsurface flow constructed wetland systems to meet even the most stringent Vietnamese standards for discharge into surface waters © 2009 Elsevier B.V All rights reserved Introduction The treatment of domestic wastewater is far from satisfactory in many subtropical and tropical countries, including Vietnam As opposed to Europe and North America, where nearly all wastewater is collected in sewer systems and treated in large centralized wastewater treatment facilities, in most other parts of the world, centralized sewer systems and wastewater treatment plants only exist in larger cities, whereas small and medium-sized towns generally discharge wastewater untreated into the environment Hence, for hygienic and environmental reasons, there is an urgent need to implement appropriate wastewater management systems in these areas, as for example constructed wetland (CW) systems, that are low-cost, easy-to-operate, efficient and robust (Denny, 1997; Haberl, 1999; Kivaisi, 2001) The CWs are currently ∗ Corresponding author Tel.: +84 7103 830 635; fax: +84 7103 730 392 E-mail address: ntdtrang@ctu.edu.vn (N.T.D Trang) 0925-8574/$ – see front matter © 2009 Elsevier B.V All rights reserved doi:10.1016/j.ecoleng.2009.11.022 being studied as a technology that can be used for the treatment of many types of wastewater in tropical countries, including highstrength wastewater from agricultural activities and municipal wastewater from small communities (Nahlik and Mitsch, 2006; Bojcevska and Tonderski, 2007; Brix et al., 2007a; Konnerup et al., 2009; Kantawanichkul et al., 2009) Treatment of wastewater in CW systems is based on natural processes, and CWs can tolerate a high variability in loading rate and wastewater quality Furthermore, in addition to treating the wastewater, CW systems planted with ornamental plants can provide a nice-looking environment (Brix et al., 2007a; Zurita et al., 2009) Horizontal subsurface flow CWs are commonly used for secondary and post-treatment of domestic wastewater in temperate regions, and are efficient in solids and organic matter removal but less efficient in nutrient removal (Masi and Martinuzzi, 2007; El Hamouri et al., 2007; Brix et al., 2007b; Vymazal, 2009) Most performance data available for CW systems are from temperate climates, but the treatment performance is expected to be significantly higher in tropical areas because of the high temperatures and the associated higher microbial activity The area-specific removal 528 N.T.D Trang et al / Ecological Engineering 36 (2010) 527–535 capacity of pollutants in CWs is often evaluated using a firstorder reaction kinetics model (Kadlec and Knight, 1996; Kadlec, 2009) Removal rate constants (k) are often estimated based on inlet–outlet performance data from operational CW systems (e.g Kadlec, 2009) and may therefore be affected by many factors, such as wetland configuration, hydraulic loading characteristics, hydraulic efficiency, and input pollutant concentration Another approach to estimate removal rate constants that is less affected by some of these factors is to explore the concentration profiles of the pollutants from inlet to outlet in the wetland and to use these profiles to estimate removal rate constants (Kadlec, 2003) The aim of the present study was to estimate the removal capacity of horizontal subsurface flow CW systems under tropical conditions We used an existing experimental CW system at Can Tho University, Vietnam, that had been loaded with municipal wastewater at a hydraulic loading rate of 31 mm day−1 since 2003 with consistent efficient removals of suspended solids, BOD5 , COD, as well as nitrogen and phosphorus The experimental system was equipped with facilities to sample water internal in the system at different positions along the flow path from inlet to outlet The system was run at four hydraulic loading rates, and the first-order area-based removal rate constants (k) were estimated based on the pollutant concentration profiles within the system It is to our knowledge the first time that this procedure has been used in tropical climates to estimate removal kinetics in horizontal subsurface flow CWs Materials and methods 2.1 Experimental setup An experimental constructed wetland (CW) system with horizontal subsurface flow (HSF) was built at Can Tho University, Vietnam (10◦ 00′ N, 105◦ 46′ E), in 2003 (Figs and 2) The CW received a mixture of domestic sanitary wastewater, grey water from dormitories and storm water collected in the sewer system of the campus The CW system contained a 6.4 m3 storage tank into which the wastewater from the sewer system was pumped, a 0.96 m3 pretreatment section, a 21.1 m3 wetland filter planted with Phragmites vallatoria (L.) Veldkamp, and finally a m3 effluent section The first part of the pretreatment section was filled with charcoal placed under a layer of ∅ 40–60 mm gravel for deodorization and followed by a coconut fiber filter The wetland filter (12 m × 1.6 m × 1.1 m; length × width × depth) was filled with 0.25–0.43 mm (D10 –D60 ) river sand with a saturated hydraulic conductivity of 9.8 m d−1 Five sampling pipes were placed at the bottom of the bed at different distances from the inlet for water sampling (Fig 2) Effluent from the wetland filter passed a gravelfilled (∅ 10–20 mm) effluent section where the outlet water level was controlled by a tiltable vertical pipe Prior to the present experiment, the operation of the system was stopped for months and the system renovated The sand at the front m of the wetland filter was washed, and P vallatoria was subsequently replanted at a density of 25 stems m−2 using 60 cm cut culms from a nearby natural reed bed Then the bed was saturated with wastewater and the plants allowed establishing for a 4-week period prior to continuous wastewater loading 2.2 Wastewater loadings and samplings From November 2007 to April 2008 four hydraulic loading rates (HLRs) of 31, 62, 104 and 146 mm day−1 were applied to the CW system, each for a period of about 40 days starting with the lowest HLR The loading rates were controlled by filling the inlet storage tank twice or three times daily with the desired amount of wastewater from the sewer system using a manually controlled pump From the storage tank the wastewater was drained slowly into the system through two valves to achieve even loading over time The outlet level was set just below the bed surface to secure water saturated conditions in the wetland filter At each HLR, water samples were collected during the last days of the loading period when performance were assumed to be representative for the particular HLR Grab samples were taken in the morning every second day at the inlet (WL1), after the pretreatment unit (WL2), at the outlet (WL8), and at different intermediate positions in the wetland filter (WL3 to WL7) corresponding to distances from the inlet of 1.9, 3.8, 5.9, 7.9 and 9.9 m using a perforated plastic pipe to withdraw water from the sand bed (Fig 2) All samples were collected and stored at ◦ C in L plastic bottles until analysis in the laboratory 2.3 Water quality analyses The pH, dissolved oxygen (DO), temperature and electric conductivity (EC) of the water samples were analyzed on-site using portable pH, DO and EC meters (HI 8424 and HI 9146, Hanna Instruments, Romania; Orion 011510, Thermo Electron Corporation, USA) Total suspended solids (TSS) were analyzed by the filtration method using Whatman GF/F filters Ammonium nitrogen (NH4 N) and orthophosphate (PO4 -P) were analyzed in filtered samples using the salicylate method (modified from Lachat, Quikchem No 10-107-06-3-A or B), and standard colorimetric methods (APHA, 1998), respectively Total phosphorus (TP) was analyzed by the ascorbic acid method after acid hydrolysis in an autoclave for 30 (APHA, 1998) Total Kjeldahl nitrogen (TKN) was analyzed at HLRs of 104 and 146 mm day−1 using a Kjeldahl block digestion unit (Kjeldatherm KB 20S, Gerhardt, Germany) and a semi-automatic steam distillation unit (Vapodest 20, Gerhardt, Germany), and total nitrogen (TN) was analyzed at HLRs of 31 and 62 mm day−1 using addition of Devarda’s alloy during distillation For technical reasons, we could not analyze nitrate nitrogen (NO3 − ), but later analyses of the wastewater in the sewer system and the outlet of the bed indicate that the concentrations were low (