In Nepal, both horizontal bed and vertical bed subsurface flow constructed wetlands have been used for wastewater treatment. However, these units were designed based on the empirical findings from other countries. The rational design criteria developed so far has some limitations as the performance of the units is sensitive to the behaviour of the microorganisms, climatic conditions and other attributes pertaining to the local contexts. Secondly, only limited numbers of studies have been carried out to assess the performance of these systems leading to the development of rationale design criteria. Considering these facts, the major objectives of the study were set to: (1) evaluate the performance of the subsurface flow reed bed system in terms of organic matter and ammonia removal; (2) estimate the reaction rate constant and effective specific surface area; (3) assess the relationship between performance and age of the system; and (4) investigate the dynamic behaviour of the reaction rate constant. The study was carried on three full-scale domestic wastewater treatment units and one pilot-scale ranging the age of horizontal beds from 1 to 5 years.

INTRODUCTION

In developing countries, urbanization and rapid growth of economic activities are leading to an increase in the volume of wastewater generated which is a big threat to protect the surface and subsurface water sources. In the past, significant efforts have been made to develop the conventional wastewater treatment facilities. However, their application is limited due to high capital cost, sophisticated operation and cumbersome maintenance works. Among natural wastewater treatment systems, the constructed wetland (CW) is regarded as a simple cost-effective ecological technology for wastewater treatment. It is designed to optimize the process variables and overcome the disadvantages of natural wetlands by better hydraulic control and management of the vegetation and other components of the system. CWs for wastewater treatment by using aquatic plants have been widely used in various countries such as Germany, America, Australia, England, China, and India (Kurniadie 2011). The most important effects of these plants in relation to wastewater treatment processes are the physical effects including filtration and creation of surface area for biofilm and metabolism of the macrophytes including plant uptake and oxygen release.

In Nepal, both horizontal bed and vertical bed subsurface flow treatment units utilizing local reeds (Phragmites Karka spp.) have been used for wastewater treatment by institutions and small- to medium-sized communities since 1997. Currently, more than a dozen such units with capacity ranging from 0.5 to 115 m3/d are operating in Nepal. These units were designed based on the empirical findings from other countries. Only limited numbers of studies have been carried out to assess the performance of these systems to develop rational design criteria (Bista 2008).

In this context, this study was envisioned to contribute in developing kinetics of organic matter and ammonia removal for reed beds treating municipal wastewater. The major objectives of the study were to: evaluate the performance of organic matter and ammonia removal considering organic loading rates (OLR) and age of horizontal flow beds (HFBs); assess the performance at various sections along the length of the wetland bed; and determine the reaction rate constants and specific surface areas of the media utilized by the biofilm bacteria.

A rational method based on first-order kinetics and plug-flow model as shown in Equation (1) is generally used for engineering design of the constructed wetlands in general. 
formula
1
where Ce = effluent chemical oxygen demand (COD) concentration, mg L−1;Co = influent COD concentration, mg L−1; k = reaction rate constant, day−1 and t = hydraulic retention time (HRT), days.
Khatiwada & Polprasert (1999) provided a rational basis to estimate the reaction rate constant as given in Equation (2). 
formula
2
where k = overall reaction rate coefficient (day−1); ks = reaction rate coefficient of suspended bacteria (day−1); as = effective specific surface area (m2/m3); ηs = effectiveness factor of the biofilm; Lf = biofilm thickness (m); and kb = reaction rate constant in the biofilm (day−1).

The effective specific surface area (as) of an operating constructed wetland is essentially the biofilm area which contributes in decomposing or transforming the substrate be it the organic matter or nutrients in the form of ammonia or phosphorus (Reed et al. 1995; Kadlec & Knight 1996; Khatiwada & Polprasert 1999).

Khatiwada & Polprasert (1999) introduced an area factor (δ), a dimensionless entity, in order to compare the effective specific surface areas of different natural treatment systems having varying available biofilm area. 
formula
3
where h = depth of the system (m).

For a large shallow basin with only floor area as effective surface area available for the biofilm bacteria, the δ value would be unity. If the emergent vegetation is considered as an additional biofilm area, a dense stand of plants can yield δ ≈ 5 and inclusion of the litter can further increase the value to δ ≈ 10 (Kadlec & Knight 1996).

The theoretical and practical values of as, δ and porosity of young and matured reed beds along with the assumptions made are also discussed in this paper. The practical values of these parameters were obtained from the actual reaction rate constants and using Equations (2) and (3).

MATERIAL AND METHODS

Description of the reed bed units

The study was carried on three full-scale domestic wastewater treatment units comprised of CWs and owned by the Kathmandu University (KU), Shrikhandpur Community (SC), and Dhulikhel Hospital (DH) and one pilot-scale research unit in Pulchowk Institute of Engineering (PE). These treatment units of KU, SC, DH and PE were in operation for 0.8 years, 2 years, 4 years and 5 years, respectively. The description of each unit and the operating conditions are given in Table 1.

Table 1

Description of reed bed units and operating conditions

Location Type Size (L × B × H), (m) Age of bed (year) HRT (days) OLR (kgCOD/ (ha.day)) OLR (kg NH3N/(ha.day)) 
Kathmandu University (KU) Institutional 27 × 8 × 0.6 0.8 1.03 121 31 
Pulchowk Engineering Campus (PE) Pilot scale 41 × 7 × 0.45 4.84 75 13 
3.54 103 17 
2.58 140 25 
Shreekhandapur Community (SC) Community 25 × 7 × 0.6 0.64 139 40 
Dhulikhel Hospital (DH) Institutional 20 × 7 × 0.6 0.44 340 83 
Location Type Size (L × B × H), (m) Age of bed (year) HRT (days) OLR (kgCOD/ (ha.day)) OLR (kg NH3N/(ha.day)) 
Kathmandu University (KU) Institutional 27 × 8 × 0.6 0.8 1.03 121 31 
Pulchowk Engineering Campus (PE) Pilot scale 41 × 7 × 0.45 4.84 75 13 
3.54 103 17 
2.58 140 25 
Shreekhandapur Community (SC) Community 25 × 7 × 0.6 0.64 139 40 
Dhulikhel Hospital (DH) Institutional 20 × 7 × 0.6 0.44 340 83 

L, B and H indicate length, width and depth of the wetland bed. HRT, hydraulic retention time; OLR, organic loading rate.

Figure 1 provides a cross-sectional view of horizontal flow reed bed planted with Phargmities Karka.
Figure 1

Typical cross section of the reed bed.

Figure 1

Typical cross section of the reed bed.

Sampling and analysis

All units consist of a settling tank and HFBs. However, the KU unit and DH unit also consisted of vertical flow beds (VFBs). The treated effluent from a HFB is passed to the VFB. The units were constructed to maintain the continuous flow using the force of gravity. This study was focused only on the HFBs in order to make a distinct comparison. Samples were collected at every four hour interval in KU and DH units. The sampling was conducted for three consecutive days in one cycle and four cycles were carried out for each unit. In the PE unit, samples were collected and analysed every day for a 1 week period. Same protocol was applied for three different flows, i.e. 8, 11 and 15 m3/d. However, samples were collected for three consecutive days in the SC unit.

The altitude of KU, DH and SC units was 1550 m and that of the PE unit was 1324 m. Average annual precipitation in these two locations were 1711 mm and 1430 mm, respectively. More than 90% of the precipitation occurs during June–August period in these areas. Similarly, the average annual temperatures were 17 °C and 18 °C. The evapo-transpiration losses were not considered as the study was conducted in the winter season. The samples were analysed according to Standard Methods for the Examination of Water and Wastewater (APHA, AWWA and WEF 1993).

RESULTS AND DISCUSSION

Characteristic of the wastewater

Table 2 provides a summary of the characteristics of the wastewater fed to the reed beds. The data shown indicated that the influent wastewater of KU, PE and DH units fell in the category of medium strength wastewater whereas that of the SC unit fell in the category of high strength wastewater (Metcalf & Eddy 1995).

Table 2

Influent characteristics of the wastewater

  Average values
 
Parameters Units KU PE PE PE SC DH 
Flows m3 d−1 16 11 15 52 34 
BOD5 mg L−1 163 281 270 273 467 156 
COD mg L−1 258 407 391 396 700 244 
Temperature o19 13 18 17 
NH3-N mg L−1 42 47 46 48 81 33 
HLR cmd−1 7.4 2.7 3.7 5.1 4.9 24.3 
  Average values
 
Parameters Units KU PE PE PE SC DH 
Flows m3 d−1 16 11 15 52 34 
BOD5 mg L−1 163 281 270 273 467 156 
COD mg L−1 258 407 391 396 700 244 
Temperature o19 13 18 17 
NH3-N mg L−1 42 47 46 48 81 33 
HLR cmd−1 7.4 2.7 3.7 5.1 4.9 24.3 

BOD5, biochemical oxygen demand; COD, chemical oxygen demand; HLR, hydraulic loading rate.

Overall performance of units

The reed bed units in KU had an average removal performance of 65.6% and 52.0% for the organic matter and ammonia-nitrogen, respectively. These figures were 86.6 and 54.7% in PE units; 80.0 and 4.8% in SC; and 65 and 5.2% in DH units. The experiment shows that the removal performance depended on process variables such as HRT and age of reed beds. Overall performance of the units is given in Table 3.

Table 3

Removal performance of pollutant in each reed beds

  Organic matter (COD), mg/L
 
Ammonia, mg/L
 
Details Influent mg L−1 Effluent mg L−1 Removal % Influent mg L−1 Effluent mg L−1 Removal 
KU 258 89 66 42 21 52 
SC 700 140 80 81 78 
PE (HRT = 4.8 d) 407 30 93 47 80 
PE (HRT = 3.5 d) 391 36 91 46 21 54 
PE (HRT = 2.6 d) 396 79 80 48 28 43 
DH 244 89 63 33 31 
  Organic matter (COD), mg/L
 
Ammonia, mg/L
 
Details Influent mg L−1 Effluent mg L−1 Removal % Influent mg L−1 Effluent mg L−1 Removal 
KU 258 89 66 42 21 52 
SC 700 140 80 81 78 
PE (HRT = 4.8 d) 407 30 93 47 80 
PE (HRT = 3.5 d) 391 36 91 46 21 54 
PE (HRT = 2.6 d) 396 79 80 48 28 43 
DH 244 89 63 33 31 

Performance along the bed length

The removal performance of organic matter and ammonia-nitrogen along the length of constructed wetland in PE unit is shown in Figure 2. Significant improvement on the performance was seen along the length of the beds for both parameters. However, the performance was found diminished with increasing loading rate in both cases. Two clearly distinct decomposition trends are also seen on the plots. The first half being more linear while the second indicating a saturation stage of the removal. Conversely, Figure 2 also explains that decomposition of organic matter is accomplished more rapidly at the first half section of the reed bed whereas the ammonia nitrification takes place at the later half section of the bed.
Figure 2

Performance of organic matter removal vs. length of HFB (OLR for organic matter is given in kg COD.ha−1.day−1 while OLR for ammonia is given in kg NH3-N.ha−1. day−1).

Figure 2

Performance of organic matter removal vs. length of HFB (OLR for organic matter is given in kg COD.ha−1.day−1 while OLR for ammonia is given in kg NH3-N.ha−1. day−1).

Reaction rate constants

The reaction rate constants obtained from Equation (1) using the performance data are summarized in Table 4. The standard deviations shown reflect the variation on influent characteristics and the performance of the beds. An average reaction rate constant (k) of 1.1 d−1 found in the HFB of the Kathmandu University, is in close agreement with the suggested value of 1.104 day−1 for subsurface flow CW system having media porosity of 0.25 to 0.45 (Reed & Brown 1995). The reaction rate constants were 0.83, 1.4, and 1.3 d−1 for three HRTs in the PE unit, which was in operation for the last 2 years. The reaction rate constants were also found depended on the age of the beds. In the span of 2 years, a two-fold increase on their values was observed (Bista et al. 2004). Past studies have described the k values as a function of both external and internal factors. Kadlec (1997) mentioned that k value is strongly dependent on the HLR and OLR or the HRT of the unit. Similarly, the k values were also attributed as the function of the specific surface area available for the biofilm bacteria (Kadlec & Knight 1996; Poh-Eng & Polprasert 1998; Khatiwada & Polprasert 1999). However, the findings and the conclusions made in these studies, which highlight the significance of k values, point clearly towards an optimally running constructed wetland which implies that wastewater is transported to all potential sites where biofilm bacteria is available. However, it seems to be unrealistic and impractical to make such an assumption. Because of the fact that a constructed wetland would never achieve an ideal situation derived out of optimal values of hydraulic conductivity and effective specific surfaces. Even if such a situation is achieved, it would be a transient phenomena only.

Table 4

Reaction rate constants for organic matter and ammonia removal

  Organic matter
 
Ammonia
 
Description Age of reed bed (year) Reaction rate constant (d−1Standard deviation Reaction rate constant (d−1Standard deviation 
KU unit 0.8 1.1 0.32 0.73 0.27 
PE unit (HRT = 2.6 days)  1.30 0.09 0.25 0.40 
PE unit (HRT = 3.5 days) 1.4 0.15 0.21 0.01 
PE unit (HRT = 4.8 days)  0.83 0.09 0.27 0.03 
SC unit 2.91 0.09 0.09 0.01 
DH unit 2.75 0.09 0.16 0.44 
  Organic matter
 
Ammonia
 
Description Age of reed bed (year) Reaction rate constant (d−1Standard deviation Reaction rate constant (d−1Standard deviation 
KU unit 0.8 1.1 0.32 0.73 0.27 
PE unit (HRT = 2.6 days)  1.30 0.09 0.25 0.40 
PE unit (HRT = 3.5 days) 1.4 0.15 0.21 0.01 
PE unit (HRT = 4.8 days)  0.83 0.09 0.27 0.03 
SC unit 2.91 0.09 0.09 0.01 
DH unit 2.75 0.09 0.16 0.44 

Hydraulic conductivity is dependent on particle diameter, their size distribution and shape, porosity of the bed and arrangement of the particles. Plant roots increase their size and numbers changing the porosity which is also continuously changed by litter and settled particles. These attributes are beyond the control of an designer and or an operator and are also difficult to predict. Similarly, full control of the external factors such as loading rates and HRT is also not possible which makes it rather cumbersome to keep on the control of k values in an operating unit. As shown in Table 4 and discussed above, for the PE unit, the highest k value of 1.4 day−1 was obtained for an OLR of 103 kg/(ha. day) and HRT of 3.5 days which were the medium values. This indicates that a high loading rate or a shorter HRT vis-à-vis a low loading rate or a longer HRT represents conditions of a non-optimal system or under utilization of the potential sites where biofilm bacteria is available.

Effective specific surface area

Using Equations (2) and (3), the data shown in Table 4 and the estimated values of ηs, Lf, and kb suggested by Khatiwada & Polprasert (1999), the values of as and δ for each of the reed bed units were estimated and shown in Table 5. The figures indicated that the wetland bed would provide an effective surface area in the range of 27–71 m2/m3 meaning that the biofilm area available is 15–42 times larger than the floor area of the wetland bed. Kadlec & Knight (1996) estimated the theoretical as and δ values as 360 m2/m3 and 162 for a subsurface flow wetlands assuming wetland bed to be filled up with spherical particles of diameter 1 cm. Similarly, Khatiwada & Polprasert (1999) estimated the theoretical values for subsurface flow wetlands assuming the bed full of roots only. The authors reported as and δ values of 24,000 m2/m3 and 16,800 for roots of diameter 100 μm. The underlying assumption made in the later study, while estimating the two values, was a cylindrical geometry of the biofilm which is attached to the roots. Nonetheless, if the opportunities for the formation of cylindrical biofilm are not available as of clogging, short circuiting, poor hydraulic conductivity, relatively large size of the root or accumulation of the litter, etc., then the bio-film can be considered to follow the geometry of a slab. Such surfaces will have less potential sites for the biofilms leading to lower values of as and δ. Using the data shown in Table 4 and the equations for slab geometry instead of cylindrical geometry as suggested by Khatiwada & Polprasert (1999), the as and δ values for the reed bed units under consideration were found to be in the range 10–26 m2/m3 and 5–15 only. These finding point towards an important corollary that lower k values should be expected for wetland beds whose effective specific surface area is reduced drastically. This argument is also helpful in describing the difference in the k values between young and matured wetland beds.

Table 5

Specific surface area and area factors

Description Age of the bed, years Reaction rate constant (k), day−1 Specific surface area (as), m2/m3 Area factor (δ) 
KU unit 0.8 1.1 26.66 16.0 
PE unit 1.4 33.93 15.3 
SC unit 2.91 70.52 42.3 
DH unit 2.75 66.64 40.0 
Description Age of the bed, years Reaction rate constant (k), day−1 Specific surface area (as), m2/m3 Area factor (δ) 
KU unit 0.8 1.1 26.66 16.0 
PE unit 1.4 33.93 15.3 
SC unit 2.91 70.52 42.3 
DH unit 2.75 66.64 40.0 

Dynamic characteristics

Efforts were made to demonstrate a relationship between the effluent substrate concentration and HRT as shown in Figure 3. Using Equation (1) and three categories of the wetlands namely young (<1 year), intermediate (1 to 4 years) and matured (>4 years), the plots were obtained by varying the HRT. The plot revealed that when the wetland becomes matured, same level of performance can be obtained at lower HRT. This can be explained using the information provided in Table 5 and the following two theoretical relationships (Kadlec & Knight 1996): 
formula
4
 
formula
5
where θ is the porosity of the bed and dp is the diameter of the particle.
Figure 3

A plot showing the dynamic kinetic behaviour of CW (Co = 300 mg/L).

Figure 3

A plot showing the dynamic kinetic behaviour of CW (Co = 300 mg/L).

As the reaction rate constant and consequently the effective specific surface area of the wetland bed increases with time, the variation of these two parameters must be linked with the increase in the number and diameter of the roots and reduction of the porosity of the bed. As an incremental change in the root diameter would bring a substantial change in the porosity of the bed, the effective specific surface area increases as per the relationship shown in Equations (4) and (5).

A relationship between the size and shape of the particles and resulting porosity is given by Zou & Yu (1996). Bista (2003) measured the volume of the roots at a span of 1 year and found a 3% reduction on the porosity of the KU unit. Nonetheless, the as value and consequently the reaction rate constant are anticipated to increase only to a threshold value because of clogging, short circuiting, poor hydraulic conductivity, relatively large size of the root or accumulation of the litter, etc. Future research studies are expected to look at this trade-off value.

CONCLUSIONS

The aim of this study was to internalize the design attributes of horizontal flow reed bed systems. Experimental investigations were carried out in three full-scale units and one pilot scale reed bed unit. The organic matter removal performance in HRBs varied in the range of 65 to 87% while ammonia-nitrogen removal rates ranged from 5 to 80%. Organic matter removal rate was found higher than ammonia-nitrogen if absolute removals were considered. This clearly indicated that the decomposition of the organic matter and removal of ammonia nitrogen takes place in sequential order. This fact was supported by the measurement of removal rates along the bed length. The reaction rate constant, the kinetic parameter used for designing the HRBs, varied from 1.1 to 2.91 d−1 in the case of organic matter and 0.09 to 0.73 d−1 in the case of ammonia-nitrogen. The values of specific surface areas ranged from 27 to 71 m2/m3 and the corresponding area factors from 16 to 42. These values were found to be in agreement with the findings of other researchers. The reaction rates and the corresponding effective biofilm areas were found to increase as the units became older. Nonetheless, the results also indicated threshold values after which a decline may occur due to clogging, short circuiting, poor hydraulic conductivity, relatively large size of the root or accumulation of the litter, etc.

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