Effect of diffusional mass transfer on the performance of horizontal subsurface flow constructed wetlands in tropical climate conditions

2011 ◽  
Vol 63 (12) ◽  
pp. 3039-3045 ◽  
Author(s):  
K. N. Njau ◽  
L. Gastory ◽  
B. Eshton ◽  
J. H. Y. Katima ◽  
R. J. A. Minja ◽  
...  
Keyword(s):  
Mass Transfer ◽  
Flow Velocity ◽  
Rate Constants ◽  
Removal Rate ◽  
Subsurface Flow ◽  
Plug Flow ◽  
Pollutant Removal ◽  
Velocity Correlation ◽  
Climate Conditions ◽  
Horizontal Subsurface Flow

The effect of mass transfer on the removal rate constants of BOD5, NH3, NO3 and TKN has been investigated in a Horizontal Subsurface Flow Constructed Wetland (HSSFCW) planted with Phragmites mauritianus. The plug flow model was assumed and the inlet and outlet concentrations were used to determine the observed removal rate constants. Mass transfer effects were studied by assessing the influence of interstitial velocity on pollutant removal rates in CW cells of different widths. The flow velocities varied between 3–46 m/d. Results indicate that the observed removal rate constants are highly influenced by the flow velocity. Correlation of dimensionless groups namely Reynolds Number (Re), Sherwood Number (Sh) and Schmidt Number (Sc) were applied and log–log plots of rate constants against velocity yielded straight lines with values β = 0.87 for BOD5, 1.88 for NH3, 1.20 for NO3 and 0.94 for TKN. The correlation matched the expected for packed beds although the constant β was higher than expected for low Reynolds numbers. These results indicate that the design values of rate constants used to size wetlands are influenced by flow velocity. This paper suggests the incorporation of mass transfer into CW design procedures in order to improve the performance of CW systems and reduce land requirements.

Mass transfer approach and the designing of horizontal subsurface flow constructed wetland systems treating waste stabilisation pond effluent

2018 ◽  
Vol 78 (12) ◽  
pp. 2639-2646 ◽  
Author(s):  
Anita M. Rugaika ◽  
Damian Kajunguri ◽  
Rob Van Deun ◽  
Bart Van der Bruggen ◽  
Karoli N. Njau
Keyword(s):  
Mass Transfer ◽  
Constructed Wetlands ◽  
Rate Constants ◽  
Removal Rate ◽  
Subsurface Flow ◽  
Oxygen Demand ◽  
Domestic Wastewater ◽  
Organic Load ◽  
Mass Transfer Effect ◽  
Horizontal Subsurface Flow

Abstract Pilot-scale constructed wetlands (CWs) that allowed wastewater to flow with high interstitial velocities in a controlled environment were used to evaluate the possibility of using mass transfer approach to design horizontal subsurface flow constructed wetlands (HSSF-CWs) treating waste stabilisation ponds (WSPs) effluent. Since CW design considers temperature which is irrelevant in tropics, mass transfer approach could improve the design. HSSF-CWs were operated in batch recycle mode as continuous stirred tank reactors (CSTR) at different interstitial velocities. The overall removal rate constants of chemical oxygen demand (COD) at various interstitial velocities were evaluated in mesocosms that received pretreated domestic wastewater. The mean overall removal rate constants were 0.43, 0.69, 0.74 and 0.73 d−1 corresponding to interstitial velocities of 15.43, 36, 56.57 and 72 md−1, respectively. Results showed that the interstitial velocities up to 36 md−1 represented a range where mass transfer effect was significant and, above it, insignificant to the COD removal process. Since WSPs effluent has high flow rates and low organic load, it is possible to induce high interstitial velocities in a HSSF-CW treating this effluent, without clogging and overflow. The performance of these HSSF for tertiary treatment in tropical areas could be improved by considering flow velocity when designing.

Influence of flow velocity on the removal of faecal coliforms in horizontal subsurface flow constructed wetland

2012 ◽  
Vol 66 (12) ◽  
pp. 2808-2813 ◽  
Author(s):  
W. S. Lohay ◽  
T. J. Lyimo ◽  
K. N. Njau
Keyword(s):  
Flow Velocity ◽  
Rate Constants ◽  
Membrane Filtration ◽  
Removal Rate ◽  
Plug Flow ◽  
Flow Equation ◽  
Faecal Coliforms ◽  
Pathogen Removal ◽  
Removal Rates ◽  
Filtration Technique

In order to determine the influence of flow velocity on the removal of faecal coliforms (FC) in constructed wetlands (CWs), removal rate constants of FC (kFC) were studied at various flow velocities (u). Membrane filtration technique was used during analysis. Values of kFC were determined using Reed's equation of pathogen removal; the results were compared with the plug flow equation. According to Reed's equation, kFC values ranged from 1.6 day−1 at a velocity of 4 m/day to 34.5 day−1 at a velocity of 42.9 m/day. The removal rates correlated positively with flow velocity (r = 0.84, p < 0.05). On assuming a plug flow equation, removal rates constants ranged from 0.77 to 11.69 day−1; a more positive correlation (r = 0.93, p < 0.05) was observed. Optimum removal rate constants were observed for the velocity ranging 36 to 43 m/day. Generally, the increase of flow velocity improved FC removal rate constants: implying that pathogen removals are influenced by diffusion of the microorganisms into the biofilms on CW media. The velocity dependent approach together with the plug flow equation is therefore proposed for incorporation in the design of CW in a tropical climate where temperature variations are minor.

Hydraulic tracer studies in a pilot scale subsurface flow constructed wetland

1997 ◽  
Vol 35 (5) ◽  
pp. 189-196 ◽  
Author(s):  
Andrew C. King ◽  
Cynthia A. Mitchell ◽  
Tony Howes
Keyword(s):  
Dispersion Model ◽  
Plant Root ◽  
Subsurface Flow ◽  
Plug Flow ◽  
Pilot Scale ◽  
Pollutant Removal ◽  
Tracer Studies ◽  
Operational Strategies ◽  
Design Procedures ◽  
Removal Mechanisms

Current design procedures for Subsurface Flow (SSF) Wetlands are based on the simplifying assumptions of plug flow and first order decay of pollutants. These design procedures do yield functional wetlands but result in over-design and inadequate descriptions of the pollutant removal mechanisms which occur within them. Even though these deficiencies are often noted, few authors have attempted to improve modelling of either flow or pollutant removal in such systems. Consequently the Oxley Creek Wetland, a pilot scale SSF wetland designed to enable rigorous monitoring, has recently been constructed in Brisbane, Australia. Tracer studies have been carried out in order to determine the hydraulics of this wetland prior to commissioning it with settled sewage. The tracer studies will continue during the wetland's commissioning and operational phases. These studies will improve our understanding of the hydraulics of newly built SSF wetlands and the changes brought on by operational factors such as biological films and wetland plant root structures. Results to date indicate that the flow through the gravel beds is not uniform and cannot be adequately modelled by a single parameter, plug flow with dispersion, model. We have developed a multiparameter model, incorporating four plug flow reactors, which provides a better approximation of our experimental data. With further development this model will allow improvements to current SSF wetland design procedures and operational strategies, and will underpin investigations into the pollutant removal mechanisms at the Oxley Creek Wetland.

Nutrient Removal from Polluted River Water by Combining Vertical and Horizontal Subsurface Flow Constructed Wetlands

2013 ◽  
Vol 663 ◽  
pp. 1029-1032 ◽  
Author(s):  
Cheng Xin Qin ◽  
Gang He ◽  
Yu Huan Duan ◽  
Xiao Ping Pang ◽  
Zong Lian She
Keyword(s):  
River Water ◽  
Removal Rate ◽  
Subsurface Flow ◽  
Ammonia Removal ◽  
Polluted River ◽  
Main Layer ◽  
Second Stage ◽  
N Removal ◽  
Different Depths ◽  
Horizontal Subsurface Flow

A lab-scale hybrid constructed wetland system was constructed to purify polluted river water. The system was composed of a first stage of the vertical subsurface flow filter, followed by a second stage of horizontal subsurface flow bed. Both beds used furnace slag with a size of 4-60 mm for the main layer. The system was continuously fed. Different depths of unsaturated layer (0 cm, 15 cm and 30 cm) in vertical filter were tested. The unsaturated layer of 30 cm in vertical filter presented the most effective ammonia removal of 89.1%, while lowest NO3--N removal rate of 74.1% for the system. High TN removal efficiencies (77.3%-81.0%) could be observed during operation of three depths. The removals of COD and TP were in the range of 97.1%-98.4% and 76.4%-88.9%, respectively.

Removal of organic matter and nitrogen in an horizontal subsurface flow (HSSF) constructed wetland under transient loads

2009 ◽  
Vol 60 (7) ◽  
pp. 1677-1682 ◽  
Author(s):  
A. Albuquerque ◽  
M. Arendacz ◽  
M. Gajewska ◽  
H. Obarska-Pempkowiak ◽  
P. Randerson ◽  
...  
Keyword(s):  
Organic Matter ◽  
Constructed Wetland ◽  
Removal Rate ◽  
Subsurface Flow ◽  
Point Sources ◽  
Operating Conditions ◽  
Mass Removal ◽  
Mass Load ◽  
Horizontal Subsurface Flow ◽  
Transient Loads

A monitoring campaign in a horizontal subsurface flow constructed wetland under the influence of transient loads of flow-rate, organic matter, nitrogen and suspended solids showed an irregular removal of COD and TSS and lower both removal efficiencies and mass removal rates than the ones observed in other studies for similar operating conditions. This circumstance is associated to the presence of large amount of particulate organic matter from non-point sources. The mass removal rate of ammonia increased 39% as both the water and soil temperatures increased from weeks 1–8 to weeks 9–14. A good correlation between mass load and mass removal rate was observed for all measured parameters, which attests a satisfactory response of the bed under to transient loads.

Treatment of Domestic Sewage in Macrohydrophyte Ponds

1992 ◽  
Vol 26 (7-8) ◽  
pp. 1639-1649 ◽  
Author(s):  
H. Xu ◽  
B. Wang ◽  
Q. Yang ◽  
R. Liu
Keyword(s):  
Flow Pattern ◽  
Water Flow ◽  
Water Hyacinth ◽  
Removal Rate ◽  
Plug Flow ◽  
Domestic Sewage ◽  
Pollutant Removal ◽  
K Value ◽  
Tp 75 ◽  
D Values

An experimental study was carried out on the treatment of domestic sewage by macrohydrophyte ponds, including water hyacinth and water peanut (AlterantheraPheloxiroides) ponds. The study indicated that both the water hyacinth and water peanut ponds were more effective and efficient in removing BODs (90-98%), TP (75-85%), PO4(90-99%), SS (90-95%), total bacteria (99.9-99.99%), and E. Coli (99-99.9%) in their growth seasons compared with the algae/bacterial symbiotic ponds with the pollutant removal rate constant ratio (k value in WHP or WPP/k value in ABP ponds) being: COD, 4.0-5.0, BOD5, 1.5-2.0, TN, 1.5-2.0, NH4+−N,2.0-3. 01 TP, 3.5-4.0; and PO43−2.8-5.7. The study found that the intensification effect was mainly ascribed to the microbial community in the root system, which was found to consist of Bacillus, Pseudomonas, Alcaligens, Brevibacterium, Ammonia fier, Nitrosonionas, Nitrobacter, and Denitrifier. A mathematical model to describe the water flow pattern and BOD5 removal rule across the WPP pond based on the diffusion flow pattern model was developed with very low D values, which means that the water flow in the WHP and WPP ponds is near plug flow pattern.

scholarly journals Efficiency of horizontal subsurface flow constructed wetlands cultivated with grasses of different root systems

2020 ◽  
Vol 20 (8) ◽  
pp. 3318-3329
Author(s):  
Fernanda Lamede Ferreira de Jesus ◽  
Antonio Teixeira de Matos ◽  
Mateus Pimentel de Matos
Keyword(s):  
Constructed Wetlands ◽  
Subsurface Flow ◽  
Oxygen Demand ◽  
Root Systems ◽  
Sodium Concentration ◽  
Pollutant Removal ◽  
Organic Loading Rate ◽  
Vetiver Grass ◽  
Horizontal Subsurface Flow ◽  
Subsurface Flow Constructed Wetlands

Abstract The objective of this study was to evaluate the influence of stoloniferous and fasciculated root systems, of Tifton 85 and vetiver grass respectively, on pollutant removal for primary treatment of sewage in horizontal subsurface flow constructed wetlands (HSSF-CWs). For this, three HSSF-CWs measuring 4 m × 1 m × 0.25 m, filling with gneiss gravel # 0 (D60 of 7.0 mm and 48.4% porosity) as substrate, were used. One unit was cultivated with Tifton 85 grass (HSSFT-CW), one with vetiver grass (HSSFV-CW) and one remained uncultivated (HSSFC-CW) as a control. Sewage was applied at a flow rate of 0.53–0.80 m3 d−1, corresponding to an organic loading rate of approximately 350 kg ha−1 d−1 (biochemical oxygen demand – BOD), which resulted in a hydraulic retention time of 0.6–0.9 day. The HSSFV-CW was more efficient than the HSSFC-CW in removing dissolved solids (measured as electrical conductivity) and reducing the total suspended solids (TSS), BOD5, turbidity and sodium concentration, while the HSSFT-CW was not superior in any way. The results indicate that cultivation of vetiver grass provided increased efficiency for removing pollutants from sewage when compared with Tifton 85-grass, in the HSSF-CW.

Development of a horizontal subsurface flow modular constructed wetland for urban runoff treatment

2012 ◽  
Vol 66 (9) ◽  
pp. 1950-1957 ◽  
Author(s):  
J. Y. Choi ◽  
M. C. Maniquiz ◽  
F. K. Geronimo ◽  
S. Y. Lee ◽  
B. S. Lee ◽  
...  
Keyword(s):  
Removal Efficiency ◽  
Energy Requirement ◽  
Subsurface Flow ◽  
Bottom Ash ◽  
Pollutant Removal ◽  
Maintenance Cost ◽  
Gravel Layer ◽  
Flow Length ◽  
Horizontal Subsurface Flow ◽  
Media Layer

Constructed wetlands (CWs) are well recognized as having low construction and maintenance cost and low energy requirement. However, CW design has been mainly based on rule-of-thumb approaches. In this study, the efficiency of a modular horizontal subsurface flow (HSSF) CW using four different design schemes was investigated. Based on the results, the four systems have attained more than 90% removal of total suspended solids and more than 50% removal efficiency for total phosphorus, PO4-P and Zn. The planted system achieved higher pollutant removal rates than the unplanted system. In terms of media, bottom ash was more effective than woodchip in reducing the pollutants. Considering the flow length, optimum removal efficiency was achieved after passing the sedimentation tank and vertical media layer; with respect to depth, more pollutants were removed in the upper sand layer than in the lower gravel layer. This study recommended a surface area of 0.25 to 0.8% of catchment area for planted CW and 0.26 to 0.9% for unplanted CW using the 7.5 to 10 mm design rainfall.

scholarly journals Multistage Horizontal Subsurface Flow vs. Hybrid Constructed Wetlands for the Treatment of Raw Urban Wastewater

2020 ◽  
Vol 12 (12) ◽  
pp. 5102
Author(s):  
José Alberto Herrera-Melián ◽  
Mónica Mendoza-Aguiar ◽  
Rayco Guedes-Alonso ◽  
Pilar García-Jiménez ◽  
Marina Carrasco-Acosta ◽  
...  
Keyword(s):  
Constructed Wetlands ◽  
Subsurface Flow ◽  
Oxygen Demand ◽  
Pollutant Removal ◽  
Urban Wastewater ◽  
Total Coliforms ◽  
E Coli ◽  
Effect Of Water ◽  
Horizontal Subsurface Flow ◽  
Water Height

In this study, pilot-scale hybrid constructed wetlands (CWs) and multistage horizontal subsurface flow CWs (HF CWs) have been studied and compared for the treatment of raw urban wastewater. In the hybrid CWs, the first stage was a mulch-based horizontal subsurface flow CW and the second stage was a vertical subsurface flow CW (VF CW). The VF CWs were used to determine if sand could improve the performance of the hybrid CW with respect to the mulch. In the multistage HFs, mulch, gravel and sand were used as substrates. The effect of water height (HF10: 10 cm vs. HF40: 40 cm) and surface loading rate (SLR: 12 vs. 24 g Chemical Oxygen Demand (COD)/m2d) has been studied. The results show that the use of sand in the vertical flow stage of the hybrid CW did not improve the average performance. Additionally, the sand became clogged, while the mulch did not. The effect of water height on average pollutant removal was not determined but HF10 performed better regarding compliance with legal regulations. With a SLR of 12 g COD/m2d, removals of HF10 were: 79% for COD, 75% for NH4+-N, 53% for dissolved molybdate-reactive phosphate-P (DRP), 99% for turbidity and 99.998% for E. coli and total coliforms. When SLR was doubled, removals decreased for NH4+-N: 49%, DRP: −20%, E coli and total coliforms: 99.5–99.9%, but not for COD (85%) and turbidity (99%). Considering the obtained results and the simplicity of the construction and operation of HFs, HF10 would be the most suitable choice for the treatment of raw urban wastewater without clogging problems.

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