Eight horizontal subsurface flow pilot scale artificial wetlands were constructed to evaluate the effectiveness of broken brick to remove nutrients from hospital wastewater. The average total suspended solids (TSS), 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), NH4-N, NO3-N, and phosphate percent removal efficiency of constructed wetlands were, respectively, 93.2%, 90.4%, 83.7%, 64%, 64.3%, 52.1% and 56.1% in the dry season and 89.7%, 85.8%, 82.9%, 66%, 62.7%, 56.1% and 59.5% in the rainy season. Broken brick bed wetlands provide better removal efficiency of TKN, ammonia, nitrate, and phosphate with an average removal rate of 73%, 71.3%, 79.6% and 77.1% in the dry season and 74.7%, 70.7%, 70.9% and 73.6% in the rainy season, respectively, and it provides better adsorption sites for ammonium, nitrate, and phosphate. Typha with the broken brick bed significantly improved (P < 0.05) the treatment performance of the constructed wetland systems for the removal of ammonia, nitrate, and phosphate. The seasonal variation could not significantly influence the removal of all the pollutants, but better performance of nitrate and phosphate was achieved in a dry season. Use of locally available broken brick as a substrate media can increase the nutrient removal efficiency of wetlands at a cheaper cost when applied in full scale constructed wetlands.

The treatment of wastewater has been a great concern in developing countries, since waterborne diseases such as cholera and other diarrheal illnesses have been persistent problems for the continuous death of children and poor families (Naik & Stenstrom 2012). The cost of construction for conventional wastewater treatment plants and the policy for mitigating environmental pollution have been the major barriers for the implementation of conventional technologies in many third world countries (UNWWDR 2017). Moreover, the supreme challenge in the water and sanitation sector in these countries would be the implementation of low-cost wastewater treatment plants that would be applied in a small community with the requirement for less skillful personnel and technical expertise (Almuktar et al. 2018). It is, therefore, essential that a treatment plant that is an economical, efficient and sustainable technology be inaugurated in developing regions (Wang et al. 2014; UNWWDR 2017).

Among the treatment systems, constructed wetlands (CWs) as a reasonable option have recently received considerable attention to treat a wide variety of wastewater throughout the world (Skrzypiecbcef & Gajewskaad 2017). They are applied to clean up not only municipal wastewaters but also agricultural effluents, landfill leachates, storm water, polluted river water, urban runoff, food wastes, abattoir effluent, acid mine drainage, industrial effluents, and petrochemicals (USEPA 2000; GIZ 2011; Vymazal 2011; Qasaimeh et al. 2015; Skrzypiecbcef & Gajewskaad 2017). They are complex, well-established, self–contained, integrated and environmentally friendly alternative treatment systems that use natural processes (USEPA 2000). CWs involve water, wetland vegetation, gravel/soils, the environment and their associated microbial assemblages for sewage treatment, pollution control and environmental improvements (Qasaimeh et al. 2015). The main advantages of CWs over the other solutions in developing countries are their high-quality effluent production for multi-purpose reuses as well as their sustainability in self-remediation and self-adaptation to the surrounding conditions and environment (Almuktar et al. 2018).

Several researchers have intensively reviewed the removal mechanisms for nitrogen and phosphorus in subsurface flow (SSF) wetlands (Vymazal 2007; Martin et al. 2012; Albalawneh et al. 2016). There is limited nitrogen removal in SSF wetlands as a result of the lack of oxygen in the infiltration beds (Gupta et al. 2016). This totally restricts the nitrification process of wetlands, and its removal mostly depends on denitrification, assimilation, volatilization, and plant uptake through microbial activity (Shi et al. 2018). Based on the filtration materials used, adsorption and burial also play a great role in retaining nitrogen in the beds of CWs (Shi et al. 2018). Harvesting of plant biomass is another removal mechanism since nitrogen is an essential plant nutrient and is stored as organic content in the wetland vegetation (Wu et al. 2013). The removal of phosphorus in CWs occurs through various processes such as adsorption on the media surface, precipitation, retention in sediments and plant uptake (Avsar et al. 2007; Vymazal 2007). Adsorption has been considered to be the most important mechanism for phosphorous removal based on a substrate media type (Bama et al. 2013).

The important adsorbents that have been tested for phosphorous removal as a CW substrate include broken brick (Wang et al. 2012; Mateus et al. 2016), basic oxygen furnace slag (BOFS) (Hussain et al. 2015), biochar (Gupta et al. 2016), dolomite (Zibiene et al. 2015), laterite (Mansing & Rout 2013), zeolite, limestone, calcite and other substrates rich in iron, aluminum and calcium (Yun et al. 2015). Broken brick is well known to remove phosphorus as it has a greater surface area to provide better adsorption (Mateus et al. 2016). Additionally, Wang et al. (2012) stated that it is a good medium for the enrichment of microorganisms and growth of plants in CWs. The contents and chemical forms of broken brick could also be the principal factors for phosphorus removal by the precipitation process (Wang et al. 2012). There are few reports on the phosphorus absorption potential of broken brick as a substrate of vertical flow wetlands for municipal and industrial wastewaters. However, such absorbents have not been thoroughly investigated in SSF wetlands for the treatment of complex wastewaters generated from health care institutions. Therefore, the aim of the current study is to evaluate the efficiency of a pilot-scale SSF CW with broken brick as the filter media in removing nitrogen and phosphorus from the wastewater.

The experiments were carried out from December 2016 to December 2017 in the compound of Hawassa University Referral Hospital, Hawassa City, Southern Ethiopia, which is located at 7.06° latitude and 38.48° longitude with an elevation of 1,697 meters above sea level. Its climate is tropical with an average annual rainfall of 945 mm. During the data collection period, the hospital had around 350 beds in six wards and 200 to 350 patients were visiting per day. Eight parallel pilot-scale SSF wetlands were constructed with cement blocks of 4 m × 1.2 m and with a depth of 0.6 m (Figure 1). The efficiency of two growing media, namely gravel and broken brick, were evaluated. Burnt bricks were ground into a uniform particle size of 20–25 mm. Based on the guidelines of GIZ (2011) and USEPA (2000), coarse gravel of 40 to 50 mm in diameter was arranged at the inlet and outlet zones of the wetlands in order to prevent clogging and facilitate wastewater distribution. The main treatment zone of five CWs was filled with gravel and the other three with broken brick substrate having a particle size of 20–25 mm at a depth of 45 cm (Figure 1). The upper top layer (15 cm) of all wetlands was filled with gravel of 5–10 mm to provide better rooting of plants.

Figure 1

Cross-sectional representation of the constructed wetland and its dimensions.

Figure 1

Cross-sectional representation of the constructed wetland and its dimensions.

Close modal

Four of the gravel bed wetlands were planted with emergent macrophytes including cattails (Typha domingensis), Cyperus papyrus, dark green bulrush (Scirpus atrovirens) and sugarcane (Saccharum officinarum), which are known to be suitable for use in CWs (Vymazal 2011; Mateus et al. 2016). Two broken brick wetlands were also planted with Typha domingensis and Cyperus papyrus. The remaining two from both substrates were left unplanted to act as the control (Table 1 and Figure 2). Part of the primarily treated wastewater was pumped into a temporary collection tank prior to entering the experimental wetland. The wastewater was then allowed to flow constantly into each bed at a loading rate of 237.6 liters/day for one consecutive year. The bed capacity measured from the porosities of the gravel and the broken brick filter was 950.4 liters, with 4 days' hydraulic retention time (HRT).

Table 1

Dimension and features of CWs

ParameterCW1CW2CW3 (control)CW4CW 5CW6CW7CW8 (control)
Type of bed Broken brick Broken brick Broken brick Gravel Gravel Gravel Gravel Gravel 
Type of reed Papyrus Typha No plant Sugar cane Typha Papyrus Bulrush No plant 
ParameterCW1CW2CW3 (control)CW4CW 5CW6CW7CW8 (control)
Type of bed Broken brick Broken brick Broken brick Gravel Gravel Gravel Gravel Gravel 
Type of reed Papyrus Typha No plant Sugar cane Typha Papyrus Bulrush No plant 
Figure 2

Picture of the pilot-scale SSF wetlands to treat hospital wastewater.

Figure 2

Picture of the pilot-scale SSF wetlands to treat hospital wastewater.

Close modal

Wastewater treatment performance was monitored over six sampling periods, i.e. three times in the dry season (November to December 2017) and three times in the rainy season (June to August 2017) on a monthly base. A total of six composite samples from inflow wastewater and 48 composite samples from outflow wastewater were collected simultaneously. The wastewater samplings were undertaken three times a day with a three hour interval in the morning and afternoon (9:00 AM, 12:00 AM and 3:00 PM) in 250 ml cleaned and sterile screw-capped containers and transported with a cold box (≈4 °C) and stored in a refrigerator at 4 °C. Then all the samples pooled into 500 ml sized cleaned and sterilized containers to be analyzed within 24 hours. Wastewater parameters were analyzed according to APHA (1998) standard methods.

The Fisher's Least Significant Difference test was used to determine any significant differences in the mean influent and effluent values of parameters, and pollutant removal efficiencies of the wetlands with different vegetation combinations were also compared. A p-value of less than 0.05 was considered significant. A comparison between wastewater analysis results for planted and unplanted cells in broken brick and gravel bed during the dry and rainy seasons was performed to evaluate the effect of seasons and the plant/media combination on the pollutants' removal.

The average removal percentage of total suspended solids (TSS) is shown in Figure 3. Influent suspended solids concentration was ranged from 335 ± 22 mg/l in the dry season to 306 ± 11.7 mg/l in the rainy season. Its level in influent was significantly higher than in the effluent (P < 0.05) throughout the study period. Gravel-based beds containing different plants managed to reach the removal values of 90.7–97.3% in a dry season and 84.6–95.3% in a rainy season (Figure 3). Similarly, the removal percent of wetlands containing broken brick as substrate reached 90.3% and 96.3% in the dry season and 88.7% and 93.1% in the rainy season. These high suspended solids removal rates were consistent with others' reports (Andreo-Martínez et al. 2016; Gupta et al. 2016). The best performance was attained by the sugar cane planted gravel bed wetland with an almost constant removal rate above 97% and an average effluent concentration of less than 10 mg/l.

Figure 3

Organic matter removal efficiency of the constructed wetlands.

Figure 3

Organic matter removal efficiency of the constructed wetlands.

Close modal

There was no statistical difference between media types in the removal of TSS among the eight wetlands in dry and rainy seasons, which agrees well with the findings of Prost-Boucle et al. (2015) and Wang et al. (2017). Moreover, there was no significant difference observed between the performance of the planted and unplanted wetlands in both substrates. Similar recent studies done in Cameroon and Korea also confirmed that plants usually have no effect on the removal of suspended solids (Martin et al. 2012; Gupta et al. 2016). The similarity in TSS treatment between planted and unplanted systems in this study persisted in all seasons of the year for both the broken brick and gravel bed wetlands. This result agrees well with the results of Elfanssi et al. (2018). This result confirmed that TSS removal is greatly attributed to physical processes like mechanical filtration and microbial breakdown of the organic portion of suspended solids (Gupta et al. 2016). Additionally, flocculation and settling of colloids by sedimentation, straining, physical capture, and adsorption onto the substrate play a great role in their reduction (USEPA 2000).

Influent 5-day biochemical oxygen demand (BOD5) concentration was 221 ± 31.3 mg/l in the dry season and 185 + 11.6 mg/l in the rainy season. The BOD5 levels in the influent were significantly (P < 0.0001) higher than in the effluent in both planted and unplanted CWs. Gravel-based beds containing different plants managed to reach a removal of BOD5 of 87–96.5% in the dry season and 83.6–95.5% in the rainy season (Figure 3). Correspondingly, there was a higher percent removal of BOD5 in broken brick bed wetlands that ranged from 83.3% to 95% in the dry season and 79.2% to 95.5% in the rainy season. A similar report by Gikas et al. (2007) showed that the mean BOD5 removals were 89% and 93.5% for temperatures below and above 15 °C, respectively. The best performance was attained by the sugar cane planted gravel bed wetland, with an almost constant removal rate above 96.5% and an average effluent concentration of less than 10 mg/l.

The average chemical oxygen demand (COD) concentration of influent wastewater was 713 ± 36.5 mg/l in the dry season and 673 ± 31.9 mg/l in the rainy season. The average COD removal efficiency of both planted and unplanted wetlands ranged from 76.2% to 88.1% in the dry season and 75.8% to 88.4% in the rainy season, which is consistent with other researchers' findings (Martin et al. 2012). With respect to BOD5 and COD, the wetlands didn't show a performance difference throughout the different seasons of the year. Comparable findings were also reported by Prost-Boucle et al. (2015) and Wang et al. (2017). The least significant difference (LSD) test analysis for percent removal of BOD5 and COD showed no significant difference in the planted and unplanted cells. Therefore, it can be said that the presence of macrophytes did not lead to an increase in the wetland performance in terms of BOD5 and COD reduction. Likewise, the vegetated cells in both substrates did not differ significantly (P > 0.05), which agrees well with the results of Mairi et al. (2012). Thereby, bacterial degradation may play a key role in the removal of BOD and COD in horizontal subsurface flow (HSSF) CWs (Abdul & Ganapathyvenkatasubramanian 2016). The organic matter in wastewater was dominantly degraded by facultative and anaerobic heterotrophic microorganisms in the wetland reactors due to a minimum oxygen concentration in the bed (USEPA 2000). Furthermore, filtration, adsorption, sedimentation, and oxidation are also responsible for organic matter reduction in wetlands (Lee et al. 2004; Skoczko et al. 2017).

Nitrogen removal efficiency was only modest in SSF CWs. The results of this study showed that broken brick bed wetlands provide better removal of nutrients than gravel beds. The average concentration of total Kjeldahl nitrogen (TKN) in the inflow wastewater was 86.3 ± 11.7 mg/l in the dry season and 98 ± 3 mg/l in the rainy season. The level of TKN in inflow wastewater was significantly higher (P < 0.05) than in the wetlands outflow in both seasons. In the dry season, broken brick bed planted wetlands exhibited a higher removal percentage of TKN (75.6%) than unplanted broken brick bed wetland (67.7%). Similarly, planted gravel bed wetlands achieved higher TKN removal (61.6%) than unplanted gravel bed wetland (46.8%) in the same season. Both planted and unplanted broken brick bed cells had significantly higher (P < 0.05) removal efficiency than unplanted gravel bed wetland in rainy season. Moreover, Typha plants in broken brick bed wetland revealed significantly higher (P < 0.05) removal efficiency than bulrush plants in gravel bed wetland in the rainy season. A maximum of 76.5% to 79% of TKN removal was achieved by Typha species, which is in agreement with other literatures (Sun et al. 2009; Basker et al. 2014). This result indicated that the removal of nitrogen in CWs is mainly due to the plant uptake in the planted wetlands compared to denitrification, which occurs in the unplanted wetlands under anoxic conditions (Vymazal 2007). However, there was no significant difference in the removal percentage of TKN between planted and unplanted gravel bed wetlands, which is comparable to the reports of Sirianuntapiboon & Jitvimolnimit (2007).

The concentration of NH4+-N in the effluent after the CWs treatment varied from 16.6 ± 4.1 to 30.9 ± 0.5 mg/l in the dry season and 16.1 ± 4.3 to 30.1 ± 0.9 mg/l in the rainy season. The level of NO3-N in hospital wastewater was very low, with an average concentration of 0.9 ± 0.2 mg/l in the dry season and 1.1 ± 0.5 mg/l in the rainy season. The concentrations of both NH4+-N and NO3-N in the influent were significantly higher (P < 0.05) than in the effluent in both seasons, which was in agreement with others (Cui et al. 2016). This study showed that the type of media has an influence on the removal of both NH4+-N and NO3-N. It was revealed that the wetlands with broken brick were more efficient compared to the wetlands with gravels (Figure 5 and 6). A similar study by Abdul & Ganapathyvenkatasubramanian (2016) also showed that a good removal performance was achieved by using brickbats as a CW media.

Typha in the broken brick bed wetland significantly improved (P < 0.05) the treatment performance of the constructed wetland systems for NH4+-N and NO3-N compared to Typha in a gravel bed wetland (Figure 4). Likewise, C. papyrus in broken brick had higher removal performance than C. papyrus in a gravel bed (Figure 5). This might be due to the higher potential of broken brick for the enrichment of microorganisms and growth of plants as well as its adsorption capability for pollutants (Wang et al. 2012). The higher elimination rate of ammonium can also be explained by the higher cation exchange capacity of broken brick (Yang et al. 2016). Moreover, Typha in broken brick wetland exhibited a higher removal percentage of NH4+-N (77.4%) and NO3-N (88.9%), which were significantly greater (P < 0.05) than those of planted and unplanted gravel bed wetlands in both seasons. In the rainy season, broken brick bed wetlands had significantly (P < 0.05) higher removal efficiency than unplanted gravel bed wetlands. In a similar study, NH4+-N removal was significantly improved in planted systems compared to unplanted systems (P < 0.05) (Caselles-Osorio et al. 2017). Likewise, Martin et al. (2012) showed that the vegetated wetland had higher removal efficiencies for nitrate than the non-vegetated control in both seasons. Villalobos et al. (2013) and Rana & Laura (2014) also explained that plants have a positive influence in the removal of ammonia and nitrate by direct assimilation or uptake and indirectly due to translocation of oxygen from the upper parts of the plants to the roots, which facilitates the nitrification of ammonia. Sugarcane planted wetland also had significantly (P < 0.05) higher removal efficiency of ammonium than unplanted gravel bed wetland. In a similar study, better nutrient removal was reported in sugar cane planted CWs (Mateus et al. 2016). Compared to the other wetlands, the lower removal of ammonia and nitrate was observed in the unplanted gravel bed wetland in both seasons. This could be due to the fact that the nitrification/denitrification processes may have been limited by inadequate microbial activity in an unplanted medium.

Figure 4

Comparison of nutrient removal percentage of broken brick and gravel bed Typha planted wetlands. (a) Dry season. (b) Rainy season.

Figure 4

Comparison of nutrient removal percentage of broken brick and gravel bed Typha planted wetlands. (a) Dry season. (b) Rainy season.

Close modal
Figure 5

Comparison of nutrient removal percentage of broken brick and gravel bed C. papyrus planted wetlands. (a) Dry season. (b) Rainy season.

Figure 5

Comparison of nutrient removal percentage of broken brick and gravel bed C. papyrus planted wetlands. (a) Dry season. (b) Rainy season.

Close modal

The core removal mechanisms of nitrogen in CWs include nitrification/denitrification, volatilization, ammonification, plant uptake, and matrix adsorption (Vymazal 2007). However, the nitrification process in HSSF wetlands is usually considered to be limited due to lower oxygen concentration released by plant roots, and the available small concentration is mostly consumed by competitive microorganisms to degrade organic matter (Gupta et al. 2016). Additionally, the oxygen released from roots in the anaerobic condition was too low to enhance ammonia oxidation (Keffala & Ghrabi 2005). Denitrification seems to be the dominant mechanism of removal in this study due to the fact that the anoxic and/or anaerobic condition, as well as the pH and temperature of wastewater, is conducive for denitrification (Shi et al. 2018). Adsorption of nitrogen ions by brick media may also play a part in its removal. In general, according to Wu et al. (2013) and the present study, plant uptake and sediment storage were the key factors limiting nitrogen removal.

The average phosphate concentration of the inflowing wastewater was 8 ± 1.4 mg/l in the dry season and 13.4 ± 4.9 mg/l in the rainy season. The obtained data for phosphate removal from each wetland is summarized in Figure 6 and 7. The planted wetlands with broken brick media had significantly (P < 0.05) higher average removal efficiency (84.4%) of phosphate compared to the wetlands with gravel beds (47.5) in a dry season. Typha plants in broken brick bed wetland significantly (P < 0.05) removed more phosphate (77.6% in a rainy season and 87.7% in a dry season) than unplanted gravel bed wetland. A similar study reported by Sun et al. (2009) showed that phosphorus removal rate reached 88.9% by cattail (Typha). Likewise, a higher performance was also reported by Mateus et al. (2016). Abdul & Ganapathyvenkatasubramanian (2016) and Wang et al. (2012) showed that a vertical flow wetland with broken brick media achieved 80% to 90% phosphorus removal. The result indicated that plants and broken brick media must work together for a better removal of phosphate, suggesting a synergistic mechanism. In the dry season, the unplanted broken brick wetland also had significantly (P < 0.05) higher removal efficiency of phosphate than an unplanted gravel bed wetland.

Figure 6

Nutrient removal efficiency of CWs during the dry season.

Figure 6

Nutrient removal efficiency of CWs during the dry season.

Close modal
Figure 7

The nutrient removal efficiency of wetlands during the rainy season.

Figure 7

The nutrient removal efficiency of wetlands during the rainy season.

Close modal

It is well known that broken brick substrate has better phosphorus removal abilities as they have a greater surface area to provide better adsorption (Mateus et al. 2016). Additionally, Wang et al. (2012) stated that broken brick plays a vital role in the enrichment of microorganisms and growth of plants as a filter medium in CWs. The contents and chemical forms of broken brick could also be the principal factors for phosphorus removal by precipitation process (Wang et al. 2012). Phosphorus precipitation can also occur when the wastewater comes into contact with available clay minerals, aluminum, iron and calcium in the substrate, as explained by Albalawneh et al. (2016). There was no significant difference between planted and unplanted gravel bed wetlands in the removal of phosphate in the dry season. Similarly, in the rainy season, planted and unplanted broken brick and gravel bed wetlands didn't have a significant difference in phosphate removal. This supports the fact that the plants have limited ability for the uptake of phosphate and that adsorption or precipitation by bed media contributes to phosphorus removal (Mateus et al. 2016). On the other hand, phosphorus removal was dependent on water temperature. In this study, better removal of phosphate was recorded in planted broken brick wetlands during the dry season than in the rainy season. Similarly, Villalobos et al. (2013) stated that seasonal variations strongly affected the phosphorous removal in HSSF. This might be related to the increase in plant growth and microbial activity in the warm season (Elfanssi et al. 2018).

This study showed that horizontal subsurface flow CWs with a broken brick bed had an excellent removal potential for organic and nutrient pollutants from hospital wastewater. The removal of BOD5, COD, and TSS in the planted bed wetlands was not significantly different from the removal in the unplanted bed. Broken brick bed wetlands provide better removal of TKN, ammonia, nitrate, and phosphate from wastewater than gravel bed wetlands and they provide better adsorption sites for ammonium, nitrate, and phosphate. Typha with the broken brick bed significantly improved (P < 0.05) the treatment performance of the constructed wetland systems for the removal of ammonia, nitrate, and phosphate. No seasonal differences were observed in pollutant removal by the wetlands except in the case of nitrate and phosphate, which were more efficiently removed in the dry season. Use of locally available broken brick as a wetland substrate would increase the nutrient removal performance as well as decrease the cost of the medium when applied in full scale CWs.

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