Individual septic tanks are the most common means of on-site sanitation in Malaysia, but they result in a significant volume of septage. A two-staged vertical flow constructed wetlands (VFCWs) system for the treatment of septage was constructed and studied in Sarawak, Malaysia. Raw septage was treated in the first stage wetlands, and the resulting percolate was fed onto the second stage wetlands for further treatment. Here, the effects of a batch loading regime on the contaminant removal efficiency at the second stage wetlands, which included palm kernel shell within their filter substrate, are presented. The batch loading regime with pond:rest (P:R) period of 1:1, 2:2 and 3:3 (day:day) was studied. The improvement of the effluent redox condition was evident with P:R = 3:3, resulting in excellent organic matters (chemical oxygen demand and biochemical oxygen demand) and nitrogen reduction. The bed operated with P:R = 1:1 experienced constant clogging, with a water layer observed on the bed surface. For the P:R = 3:3 regime, the dissolved oxygen profile was not found to decay drastically after 24 hours of ponding, suggesting that the biodegradation mainly occurred during the first day. The study results indicate that a suitable application regime with an adequate rest period is important in VFCWs to ensure efficient operation.

INTRODUCTION

In Malaysia, individual septic tanks are the predominant type of on-site sanitation used for wastewater management. Septage is the combination of sludge, scum and liquid periodically pumped from the septic tanks (Tchobanoglous et al. 1991) and is typically characterized by a higher solids and organic content compared to domestic sewage (Teal & Peterson 1991; Koottatep et al. 2005). Affordable and sustainable wastewater and septage management has become a pressing concern, due to rapid population growth not just in Malaysia, but the surrounding regions as well. For developing nations, conventional wastewater and septage management is a costly investment. This is due to the requirement of expensive treatment plants and infrastructure, and high energy, manpower, and technology requirements. Other management alternatives should be explored, with emphasis on long-term sustainability, and manageable capital, operation and maintenance costs.

Constructed wetlands are an eco-technology that offers a treatment format with reduced technical complexity, and can be operated with low to no energy requirements (Koottatep et al. 2001). Constructed wetlands are cost-effective systems in terms of construction, operation and maintenance. They could also be implemented to support existing systems. While the conventional treatment plants focus on wastewater treatment in larger urban regions, constructed wetland systems could be utilised as an affordable and appropriate treatment method to be implemented in rural and low-density areas in Malaysia.

While constructed wetlands have been widely utilised to treat wastewaters such as greywater and stormwater (Kadlec & Knight 1996), the treatment of septage with constructed wetlands is more complicated than the treatment of other domestic wastewaters, as the strength of the septage contaminants are at least 10–100-fold stronger (Cofie et al. 2006) than those typically handled by the prevalent wetland technology for domestic wastewater treatment. Thus, constructed wetlands meant for this purpose need to be designed with care. Currently, there are no comprehensive design guidelines for designing constructed wetlands for septage treatment, especially in the tropical region.

Vertical flow constructed wetlands (VFCWs) have been successfully utilised in domestic wastewater treatment (Vymazal 2002; Babatunde et al. 2008; Molle et al. 2008). In France, VFCWs have been successfully used since the 1980s, and typically consist of two stages of vertical subsurface flow filters fed with raw wastewaters (Lienard et al. 1990). These beds are operated with alternating phases of feed and rest to maintain aerobic conditions within the bed. The beds retain organic deposits on the surface, due to accumulation of suspended solids from the raw sewage. These organic deposits are removed via mineralization (Molle et al. 2008). For treatment of septage, VFCWs are more attractive as a design option due to their greater oxygen transfer capacity for improved nitrification, and their high efficiency in organic matter and pathogen removal. However, these systems may be susceptible to substrate clogging, which will lead to failure of the system. Therefore, design and operation of the VFCWs should be studied to minimise problems such as overloading and substrate clogging.

As part of a study on determining the efficiency of constructed wetlands in treating septage, a pilot-scale, two-staged VFCW system was designed and constructed within Curtin University Sarawak Campus, Miri, Sarawak, Malaysia, to treat raw septage collected from domestic septic tanks in Miri city (Jong & Tang 2014a, b). The main purpose of the pilot-scale system was to determine the influence of various system-related and operation-related factors on the wetlands' treatment efficiency. Figure 1 shows the side view of the treatment system featuring two stages of the VFCW system.
Figure 1

Side view of the system featuring two stages of VFCWs for septage treatment.

Figure 1

Side view of the system featuring two stages of VFCWs for septage treatment.

MATERIALS AND METHODS

Description of the experimental studies

This paper reports on the effects of varying ponding and resting periods on the performance of batch-fed wetlands at the second stage of the VFCW system, as shown in Figure 1. The VFCW treatment system is located in a tropical environment that is generally hot and humid all year round. The system consists of two stages of constructed wetlands planted with Phragmites karka. The first stage of the VFCW system is focussed on septage dewatering, where most of the solids are separated from the liquids. The filtrate collected from the first stage wetlands was treated in these second stage beds under a batch loading regime, where the beds were fed in batches, with the wetlands filled rapidly to capacity, left idle for an extended period of time before being drained completely and left idle, and repeated.

The second stage wetlands were constructed from polyethylene drums with a surface diameter of 550.0 mm and a total substrate height of 800.0 mm. The base of the drums was filled with 50.0 mm of 37.5 mm diameter crushed limestones, overlaid by 200.0 mm of 8.0–10.0 mm diameter crushed limestones, 200.0 mm of 3.0 mm pea gravel, 250.0 mm of palm kernel shell (PKS) and finally topped with 100.0 mm of river sand. The wetlands at the second stage were operated by intermittent flooding onto the bed surface, with the hydraulic load applied in one go (single feeding per batch at full volume). The effluent loaded onto the second stage wetlands consists of the filtrate from the first stage wetlands, with the septage originating from domestic sources. The effluent was left ponded in the wetland for a period of time before it was drained from the wetlands. Subsequent bed resting followed, with the wetlands being left idle for an extended time period. The batch loading with sequencing fill–pond–drain–rest mode (Figure 2) was examined with different periods of wetland ponding (P) and resting (R) at P:R (day:day) of 1:1, 2:2 and 3:3; where P:R = 1:1 was subjected to 1 day ponding and 1 day resting, for instance. The ponding to resting time ratio remained at 1 to 1, with the beds ponded at the same number of days as resting. The wetlands were thus fed once every 2, 4 and 6 days for each cycle. The volume of influent applied per batch was 21.0 litres with a constant hydraulic load of 0.1 m3/m2·batch.
Figure 2

Batch-operated wetlands at the second stage of the VFCW system.

Figure 2

Batch-operated wetlands at the second stage of the VFCW system.

Sampling and water quality analysis

To evaluate the performance of the wetlands, influent and effluent samples were collected once a week and analysed for their organic matter, nitrogen and particulate solids contents. Other parameters such as pH, dissolved oxygen (DO), electrical conductivity (EC), oxidation–reduction potential (ORP) and temperature changes were monitored also. The influent was sampled before the filtrate was pumped into the wetlands and the effluent samples were collected from the outlet of each unit at the end of each loading cycle. Samples from all wetlands were collected for 10 consecutive weeks following 3 weeks of wetland acclimatization period. No replicates were taken for analysis unless stated otherwise in the standard methods for testing.

The water quality parameters of chemical oxygen demand (COD), NH3-N, NO3-N and total nitrogen (TN) were analyzed using a spectrophotometer (Hach-DR2800, USA) according to the procedure manual (Hach 2007). Biochemical oxygen demand (BOD) and total suspended solids (TSS) testings were carried out in accordance with Standard Methods for the Examination of Water and Wastewater (APHA 1998). Temperature, pH, DO and EC of the samples were measured immediately upon collection using a multi-parameter digital meter (Hach-HQ40d, USA), and the ORP was tested with an Ezdo ORP 7011 handheld tester. A volumetric method (using beaker and stopwatch) was employed to measure the wetlands' inflow and outflow rates on a weekly basis.

Mass removal rates

The performance of the batch-loaded wetlands is evaluated by determining the mass removal rates using the equations below: 
formula
 
formula
where Ci, influent concentration (mg/L); Ce, effluent concentration (mg/L); Vi, influent volume per batch (L); Ve, effluent volume per batch (L); A, bed surface area (m2); I, interval between water drainage and refilling (d).

Statistical analyses

Statistical analyses on the study results were performed using SPSS Statistics 19.0 for Windows. The effects of variation in P:R under the batch loading mode were investigated by comparing all experimental results (P:R of 1:1, 2:2 and 3:3) using the post hoc test after analysis of variance (ANOVA). The statistical significance of the mean differences between the wetlands performance were determined at a confidence level of 95%. Thus, the differences were regarded as significant at p ≤ 0.05.

RESULTS AND DISCUSSION

Table 1 summarises the physico-chemical characteristics of the wetlands' influent and effluent. During the cyclic ponding and resting period, the pH values fluctuated marginally and remained below 7 for all the effluent samples collected from the wetlands. The average pH value of the influent was basic with values ranging between 7.6 and 8.3. The EC readings of the effluent were found to have increased significantly, with the effluent from wetland B-PR3 (P:R = 3:3) having the highest EC value at 2.5 ± 0.2 mS/cm. Throughout the experimental period, it was observed that the DO concentration increased with the improved redox status in the effluent after treatment, and the improvement was more evident at wetland B-PR3 with P:R = 3:3. Wetland B-PR1 with P:R = 1:1 was observed to experience clogging issues with a notable thin layer of influent waterlogging the bed surface after 1 day of resting, before the subsequent feeding cycle.

Table 1

Physico-chemical parameter statistics for influent and effluent of the wetlands fed with batch mode at P:R (days) of 1:1 (wetland PR1), 2:2 (wetland PR2) and 3:3 (wetland PR3)

Parameter Sampling point Statistics
 
Range Mean Std dev. 
Temperature (°C) Influent 11 27.0–29.0 27.7 0.8 
B-PR1 11 26.9–30.7 28.5 1.3 
B-PR2 11 26.5–32.9 28.3 1.7 
B-PR3 11 26.9–30.9 28.6 1.2 
pH Influent 11 7.6–8.3 7.8 0.3 
B-PR1 11 6.6–6.8 6.7 0.1 
B-PR2 11 6.5–6.8 6.6 0.1 
B-PR3 11 6.4–6.8 6.6 0.1 
DO (mg/L) Influent 11 0.5–0.9 0.6 0.1 
B-PR1 11 0.5–1.4 0.9 0.3 
B-PR2 11 0.7–1.6 1.1 0.3 
B-PR3 11 1.1–1.9 1.5 0.2 
ORP (mV) Influent −113.0–(−84.0)   
B-PR1 −82.0–2.0   
B-PR2 −56.0–17.0   
B-PR3 111.0–178.0   
EC (mS/cm) Influent 11 1.4–1.8 1.6 0.2 
B-PR1 11 1.7–2.1 1.9 0.1 
B-PR2 11 1.8–2.2 2.1 0.1 
B-PR3 11 2.3–2.9 2.5 0.2 
Parameter Sampling point Statistics
 
Range Mean Std dev. 
Temperature (°C) Influent 11 27.0–29.0 27.7 0.8 
B-PR1 11 26.9–30.7 28.5 1.3 
B-PR2 11 26.5–32.9 28.3 1.7 
B-PR3 11 26.9–30.9 28.6 1.2 
pH Influent 11 7.6–8.3 7.8 0.3 
B-PR1 11 6.6–6.8 6.7 0.1 
B-PR2 11 6.5–6.8 6.6 0.1 
B-PR3 11 6.4–6.8 6.6 0.1 
DO (mg/L) Influent 11 0.5–0.9 0.6 0.1 
B-PR1 11 0.5–1.4 0.9 0.3 
B-PR2 11 0.7–1.6 1.1 0.3 
B-PR3 11 1.1–1.9 1.5 0.2 
ORP (mV) Influent −113.0–(−84.0)   
B-PR1 −82.0–2.0   
B-PR2 −56.0–17.0   
B-PR3 111.0–178.0   
EC (mS/cm) Influent 11 1.4–1.8 1.6 0.2 
B-PR1 11 1.7–2.1 1.9 0.1 
B-PR2 11 1.8–2.2 2.1 0.1 
B-PR3 11 2.3–2.9 2.5 0.2 

During the experimental run, the top sand and PKS layers of wetland B-PR2 with P:R = 2:2 was occasionally disturbed and burrowed by rats. It was unclear if the incident had affected the performance of the wetland and so the results from wetland B-PR2 shall be used with care, bearing in mind that the disturbed substrate may have impacted the hydraulics (short circuit flow and possible increase in water loss) of the wetland.

As shown in Table 2, wetland B-PR1 with P:R = 1:1 was found to have the poorest treatment performance amongst the three wetlands. The organic matter concentrations in the effluent of wetland B-PR1 were constantly higher than those observed in the effluent of wetland B-PR3 which was left ponded and rested for a longer period. In terms of mass loading rate, a mean of 451.4 g COD/m2 and 24.0 g BOD/m2 was fed onto the batch-loaded wetlands per cycle (Table 2). Comparing the treatment performances of the three batch-loaded wetlands, statistical analysis on the data showed significant differences in the COD removal efficiencies between wetlands B-PR1 and B-PR3, and wetlands B-PR2 and B-PR3. Wetland B-PR3 was found to outperform wetland B-PR1 in terms of the COD mass treatment efficiency by 6.1%. A significant difference between wetland B-PR1 and B-PR3 was also found for the BOD5 reduction efficiencies. The removal of BOD5 at wetland B-PR3 was 96.0%, which was significantly greater than the treatment at wetland B-PR1 with 92.2% of BOD5 mass removed.

Table 2

Performance of batch-fed wetlands at P:R (days) of 1:1 (wetland PR1), 2:2 (wetland PR2) and 3:3 (wetland PR3) in terms of mass removal rate (g/m2.batch) and mass removal efficiency (RE, %) with n = 11

 Mass (g/m2.batch)
 
Removal
 
IN OUT g/m².batch RE (%) 
COD 
 B-PR1 451.4 ( ± 251.8) 28.9 ( ± 19.6) 422.5 91.7 
 B-PR2 451.4 ( ± 251.8) 20.8 ( ± 15.0) 430.6 93.7 
 B-PR3 451.4 ( ± 251.8) 7.7 ( ± 4.0) 443.8 97.3 
BOD 
 B-PR1 24.0 ( ± 6.7) 1.7 ( ± 0.5) 22.3 92.2 
 B-PR2 24.0 ( ± 6.7) 1.3 ( ± 0.4) 22.8 94.1 
 B-PR3 24.0 ( ± 6.7) 0.9 ( ± 0.3) 23.1 96.0 
NH3-N 
 B-PR1 7.5 ( ± 4.0) 1.6 ( ± 0.7) 5.9 75.5 
 B-PR2 7.5 ( ± 4.0) 1.2 ( ± 0.4) 6.3 81.5 
 B-PR3 7.5 ( ± 4.0) 0.7 ( ± 0.5) 6.8 91.1 
NO3-N 
 B-PR1 0.8 ( ± 0.6) 0.1 ( ± 0.1)   
 B-PR2 0.8 ( ± 0.6) 0.1 ( ± 0.1)   
 B-PR3 0.8 ( ± 0.6) 0.2 ( ± 0.1)   
TN 
 B-PR1 20.5 ( ± 7.1) 3.3 ( ± 1.2) 17.2 82.1 
 B-PR2 20.5 ( ± 7.1) 2.5 ( ± 0.9) 18.0 86.8 
 B-PR3 20.5 ( ± 7.1) 1.3 ( ± 0.6) 19.1 92.9 
Total solids 
 B-PR1 561.3 ( ± 91.3) 118.5 ( ± 28.8) 442.8 78.0 
 B-PR2 561.3 ( ± 91.3) 121.0 ( ± 36.5) 440.3 77.8 
 B-PR3 561.3 ( ± 91.3) 92.9 ( ± 34.7) 468.4 83.0 
TSS 
 B-PR1 205.2 ( ± 127.0) 8.7 ( ± 7.6) 196.5 95.6 
 B-PR2 205.2 ( ± 127.0) 9.4 ( ± 6.6) 195.8 95.1 
 B-PR3 205.2 ( ± 127.0) 4.7 ( ± 3.2) 200.6 97.3 
 Mass (g/m2.batch)
 
Removal
 
IN OUT g/m².batch RE (%) 
COD 
 B-PR1 451.4 ( ± 251.8) 28.9 ( ± 19.6) 422.5 91.7 
 B-PR2 451.4 ( ± 251.8) 20.8 ( ± 15.0) 430.6 93.7 
 B-PR3 451.4 ( ± 251.8) 7.7 ( ± 4.0) 443.8 97.3 
BOD 
 B-PR1 24.0 ( ± 6.7) 1.7 ( ± 0.5) 22.3 92.2 
 B-PR2 24.0 ( ± 6.7) 1.3 ( ± 0.4) 22.8 94.1 
 B-PR3 24.0 ( ± 6.7) 0.9 ( ± 0.3) 23.1 96.0 
NH3-N 
 B-PR1 7.5 ( ± 4.0) 1.6 ( ± 0.7) 5.9 75.5 
 B-PR2 7.5 ( ± 4.0) 1.2 ( ± 0.4) 6.3 81.5 
 B-PR3 7.5 ( ± 4.0) 0.7 ( ± 0.5) 6.8 91.1 
NO3-N 
 B-PR1 0.8 ( ± 0.6) 0.1 ( ± 0.1)   
 B-PR2 0.8 ( ± 0.6) 0.1 ( ± 0.1)   
 B-PR3 0.8 ( ± 0.6) 0.2 ( ± 0.1)   
TN 
 B-PR1 20.5 ( ± 7.1) 3.3 ( ± 1.2) 17.2 82.1 
 B-PR2 20.5 ( ± 7.1) 2.5 ( ± 0.9) 18.0 86.8 
 B-PR3 20.5 ( ± 7.1) 1.3 ( ± 0.6) 19.1 92.9 
Total solids 
 B-PR1 561.3 ( ± 91.3) 118.5 ( ± 28.8) 442.8 78.0 
 B-PR2 561.3 ( ± 91.3) 121.0 ( ± 36.5) 440.3 77.8 
 B-PR3 561.3 ( ± 91.3) 92.9 ( ± 34.7) 468.4 83.0 
TSS 
 B-PR1 205.2 ( ± 127.0) 8.7 ( ± 7.6) 196.5 95.6 
 B-PR2 205.2 ( ± 127.0) 9.4 ( ± 6.6) 195.8 95.1 
 B-PR3 205.2 ( ± 127.0) 4.7 ( ± 3.2) 200.6 97.3 

Water quality loads (g/m2.batch) for influent and effluent are given in terms of bioavailable COD, BOD, nitrogen and solids (SD for means given in parentheses to indicate range).

Values presented as the average REs (%) and mass removal rates (g/m².batch) calculated from 11 sets of samples collected throughout the experimental period.

Comparisons between the batch-loaded wetlands revealed significantly higher removal efficiencies of all nitrogen species examined at wetland B-PR3 with P:R = 3:3 (Table 2). The NH3-N mass reduction performance at wetland B-PR3 was about 20.8% greater compared to that of wetland B-PR1. The poor removal of ammonia in wetland B-PR1 was attributed to the occurrence of minor clogging in the unit, with inadequate bed resting period. Wetland B-PR1 (P:R = 1:1) was constantly observed to experience clogging issues due to the insufficient bed resting period. Shrinkage cracks were observed to develop on the surface of the septage deposit layer before the subsequent feeding cycle at wetland B-PR3 (P:R = 3:3). This suggested effective influent draining and deposit drying after the bed was left idle for 3 days. The extended resting time enhanced organic matter degradation and nitrification, with the dried septage layer encouraging oxygen diffusion into substrate biofilm, where the subsequent batch feeding promotes oxygen replenishment via convection. In the event of waterlogging due to accumulation of particulate solids on the bed surface, the standing water can hinder the oxygenation of the wetlands and create an anoxic state that decreased the treatment performance of most pollutants.

VFCWs remove pollutants with physical retention and by endogenous decay between batches (Mitchell & McNevin 2001). Since clogging is a commonly known problem for VFCWs, a proper application regime with an adequate rest period is important to allow for sufficient endogenous decay to restore the substrate porosity. Wetland B-PR1 was found to experience clogging with P:R of 1:1, and the DO concentration in the effluent was observed to be relatively lower than the other wetlands (Table 1). Standing water observed on the bed as a result of surface waterlogging could explain the less oxygenated environment in the substrate, which subsequently impacted the overall bed performance. An average DO concentration below 1.0 mg/L was found in the effluent with ORP readings ranging between −82.0 mV and 2.0 mV (Table 1). It is suggested that the clogging was the result of insufficient resting period of 1 day, considering the high applied organic loading rates on the beds.

During the experimental period, some effluent from wetland B-PR3 was withdrawn after 30 minutes and after 1 day of ponding to examine the DO content. A mean of 4.5 ± 0.7 and 1.7 ± 0.3 mg/L of DO was recovered in the effluent after ponding for 30 minutes and 1 day, respectively. As shown in Figure 3, the DO profile dropped significantly after ponding for 1 day, but prolonged ponding of the effluent for more than 1 day did not seem to cause further substantial drop in its DO content. This indicates that the prolonged ponding period of 3 days had left the wetland in a less aerobic state, as shown in Figure 3, where no oxygen renewal was allowed. Based on the results, it was suggested that the rapid biodegradation of organic matter and the transformation of NH3-N happened mainly during the first day of ponding, after which the rate of organic decomposition decreased due to less available oxygen in the bed. No drastic changes or drops in DO content were observed in wetland B-PR3 after the extended ponding period of up to 3 days.
Figure 3

Effluent DO concentrations for wetland B-PR3 after 30 minutes, 1 day and 3 days of ponding.

Figure 3

Effluent DO concentrations for wetland B-PR3 after 30 minutes, 1 day and 3 days of ponding.

Jia et al. (2010) reported almost immediate change in the DO concentrations after synthetic wastewater was pumped into the wetlands, and fast depletion of DO concentration was observed during the first 5 hours. The authors also found minimal and insignificant changes in the DO concentrations after 5 hours of ponding. Unlike the batch feeding regime, the intermittent feeding strategy does not involve prolonged ponding and allows free drainage at the wetlands, where the beds are re-oxygenated via passive aeration by drawing in atmospheric air into the substrate with every dose of influent applied. This has helped to maintain the system DO level with constant renewal of fresh air into the wetlands and subsequently leads to better organic matter and NH3-N removal in the beds operated under this regime.

An increased ponding period can extend the hydraulic retention time (HRT) of the influent, which lengthens the contact time between the influent and the biofilm in the substrate for improved pollutant treatment performance. During the ponding period, the hydrated wetlands provide food resources for microorganisms under aerobic (early period of ponding after sufficient rest period) and anaerobic (extended period of ponding) conditions; and when drained, the wetlands are re-aerated with oxygen replenishment by convection, but no food resources are supplied. The batch feeding frequency is important to ensure the chronological processes of adding oxygen, adding food, a reduced condition and then re-oxygenation in the wetlands to sustain the microbial populations.

Effective removal of TN was also found in wetland B-PR3 as shown in Table 2, most likely related to the high nitrification prior to denitrification. As stated by Lowrance et al. (1998) and Sartoris et al. (2000), denitrification rates are positively influenced by increased NO3-N concentrations. Improved nitrification is advantageous to TN removal if provided with an anaerobic environment and a sufficient carbon source to boost denitrification at a later stage. This explains the importance of both ponding and resting period for effective TN removal, which is to promote an oxygen-deficient microenvironment in the wetland and ensure a sufficient contact period between the influent and the PKS via prolonged ponding to encourage denitrification, and to support bed re-oxygenation for nitrification by sufficient bed resting after the ponding period.

In our study with batch-loaded wetlands filled with PKS substrate and operated at P:R = 3:3, the elimination efficiency of TN concentration was as high as 86.3% on average, which is greater than those reported in the literature using hybrid wetland systems. A study conducted in Italy with a hybrid system featuring one vertical flow and one horizontal flow subsurface wetland to treat sewage achieved an overall TN removal efficiency of 78.0% with a hydraulic load of 123.0 L/m2.d, and organic loads of 87.0 g COD/m2.d and 10.0 g TKN/m2.d (Foladori et al. 2012). Meanwhile, Oovel et al. (2007) reported 63.0% of TN reduction in a hybrid system, consisting of a two-chamber vertical subsurface flow filter bed followed by a horizontal subsurface flow filter bed for treatment of sewage in Estonia.

In terms of TSS removal, wetland B-PR3 yielded a mean reduction efficiency of 97.3% which was found to be statistically higher than wetland B-PR1 and B-PR2. As described previously, 1 day of rest period for wetland B-PR1 applied with influent volume of 21.0 L/batch was found to be inadequate for the septage deposit layer to dry up sufficiently. Standing water at the surface hindered oxygenation, creating an anoxic state that decreased removal performance of most of the parameters (such as BOD5, NH3-N and TN). With inadequate septage drying and mineralisation time, the porosity and consequently the hydraulic conductivity of the substrate are expected to be negatively affected. Therefore, controlling the ponding and resting periods is of great importance to ensure the durability and reliability of the system.

CONCLUSIONS

With the batch feeding strategy, the bed operated at the P:R period of 3:3 (day:day) was found to significantly outperform the wetland with P:R of 1:1 in terms of organic matter, ammonia nitrogen, TN and particulate solids removal. A sufficient period of resting was found to be imperative in restoring aerobic conditions within the bed and to ensure sufficient treatment of the wastewater. Besides, batch feeding can increase the HRT of the influent to increase the treatment efficiency and at the same time create less aerobic conditions in the wetland during the extended ponding period to promote denitrification. This is important to enhance the coupled process of nitrification–denitrification for improved TN removal from the wetland influent.

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