Effect of plant species on water quality at the outlet of a sludge treatment wetland

A review on the treatment of septage and faecal sludge management: A special emphasis on constructed wetlands

The global concern of the pollution of freshwater resources is associated with faecal sludge (FS) disposal, which is an inevitable component of onsite wastewater management mostly in developing countries. The difficulties with its treatment facilities lies in its higher organic content and low dewaterability of various available treatment systems. Moreover, the higher variability in characteristics and quantity of FS generated at different locations creates hindrances in designing the treatment system. Among the several treatment options, the constructed wetlands (CW) are an organic/green approach towards sanitation of FS with low cost and higher efficiency. The present study is an in-depth literature review on the quality and quantity of FS and septage (stabilized FS) in different regions attributed to the wide variability of its characteristics. This paper highlights the treatment of FS in different systems with a special emphasis on CW systems. Different mechanisms and factors affecting the FS treatment efficacy in CW, such as DO/aeration, macrophytes, substrate, CW configuration, and other environmental parameters, have been studied meticulously. The cost analysis revealed CW to be an economic system, and it can enable hybridization with other technologies to develop a complete treatment system with pronounced efficiencies. Several process modifications, such as augmentation with aeration, recirculation, micro-organisms, and earthworms, can enhance the treatment efficacies of CWs. The present review exhibited that the widely used plant species is Phragmites, and the optimum solid loading rate (SLR) range is 50-250 kg TS/m²/yr. The various factors to construct an optimized CW system for FS treatment were attempted, which may bolster the necessary guidelines for field-scale applications.

Explore the sludge stabilization process in sludge drying bed by modeling study from mesocosm experiments

Sludge drying bed (SDB) treatment is a valuable alternative to conventional sludge treatment methods. However, changes in sludge hydrotexture during dewatering present a barrier for direct modeling of the SDB process. This study proposes a modeling strategy to simulate the sludge stabilization process in SDB treatment by separating sludge dewatering and sludge solids stabilization into independent processes. Two cell decay theories widely used by activated sludge models (ASM), death-regeneration concept and endogenous respiration theory, are compared to describe the biokinetic processes of sludge digestion. Both cell decay theories are found to adequately describe effluent total COD, NH₄-N, NO₃-N, and sludge layer composition, but have pronounced differences in describing effluent COD compositions. Results show that natural aeration does not maintain adequate aerobic/anoxic sludge digestion within the sludge layer to fully nitrify NH₄-N released by cell decay. Results also indicate that the kinetics of sludge digestion are adaptable over time, indicating the need to adopt lumped values for biokinetic simulations. While lowered sludge dewatering rates (outflow) can increase biodegradable COD for cell metabolism, increased sludge loading rates (inflow) lead to higher effluent COD and NH₄-N concentrations. Contrary to conventional judgement, this study demonstrates the merit of sludge layer formation to reduce leaching loss of biodegradable COD. Overall, the proposed modeling strategy is proven capable of simulating deposited sludge digestion processes in an SDB.

Performance of sludge drying reed beds for the leachate purification: Effects of sludge loading frequencies and plant species

Sludge drying reed beds (SDRBs), as a natural biological technology, have positive effects on surplus sludge treatment. However, few studies focus on the sludge leachate purification in the SDRBs regarding the wetland plant species and sludge loading rates. In this study, four SDRBs planted with two wetland plant species (Phragmites australis, Typha angustifolia) were investigated for leachate purification under six sludge loading volumes and feeding frequencies (9L/3d, 6L/3d, 4L/d, 3L/d, 2.5L/d, and 2L/d). Results showed that the lowest Escherichia coli content of 630 number/mL was determined in the P. australis SDRBs, with 86.2-92.8% lower than those in the T. angustifolia controls. However, similar removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN), ammonium (NH₄⁺), total phosphorus (TP), and heavy metals were obtained in the SDRBs with both plant species. Moreover, the optimum sludge loading volume and feeding frequency of 3L/d was determined in the P. australis system, with pollutants (COD, TN, NH₄⁺, and TP) mass removal efficiencies in the leachate being over 94.9%. In addition, the principal component analysis indicated that water loss and oxidation-reduction potential had positive influences on pollutant removal in the planted SDRBs. Overall, the best leachate removal efficiency was obtained in the P. australis SDRBs under the sludge loading volume and feeding frequency of 3L/d.

Green walls to treat kitchen greywater in urban areas: Performance from a pilot-scale experiment

An increase in water use in urban areas is forcing scientists and policy makers to find alternative solutions for freshwater management, aimed at attaining integrated water resources management. Here, we tested in a 2-year experiment (June 2017-April 2019) the treatment performance of an innovative wall cascade constructed wetland (WCCW) system. The aim was to combine the multifunctional benefits of green walls (e.g. aesthetic, surface area requirements) with those of constructed wetland systems (e.g. high pollutants removal efficiencies, water recycling) to treat kitchen greywaters. The WCCW was a terraced system of six phytoremediation lines, each of which was composed of three plastic tanks (3 × 0.04 m³), filled with lightweight porous media, and vegetated with different ornamental species, namely Mentha aquatica L., Oenanthe javanica (Blume) DC., and Lysimachia nummularia L. Physicochemical (temperature, pH, electrical conductivity, dissolved oxygen, turbidity) and chemical parameters (chemical oxygen demand, biochemical oxygen demand, anionic surfactants, Kjeldahl, ammonium and nitric nitrogen, total orthophosphate) were monitored at a frequency of at least 15 days, depending on the season and WCCW management. Results showed that the WCCW significantly reduced the main water pollutants (e.g. organic compounds, nutrients), suggesting its potential application in urban environments for water recycling in the context of green infrastructures and ecological sanitation. A culture-independent taxonomic assessment of suspended bacterial communities before and after the treatment showed clear treatment-related shifts, being the functional ecology attributes changed according to changes in greywater chemical parameters. Future research should attempt to optimize the WCCW system management by regulating the nutrients balance to avoid macronutrients deficiency, and setting the most suitable water flow dynamics (hydraulic retention time, saturation-desaturation cycles) to improve the greywater treatment.

Effects of salinity on the treatment of synthetic petroleum-industry wastewater in pilot vertical flow constructed wetlands under simulated hot arid climatic conditions

Petroleum-industry wastewater (PI-WW) is a potential source of water that can be reused in areas suffering from water stress. This water contains various fractions that need to be removed before reuse, such as light hydrocarbons, heavy metals and conditioning chemicals. Constructed wetlands (CWs) can remove these fractions, but the range of PI-WW salinities that can be treated in CWs and the influence of an increasing salinity on the CW removal efficiency for abovementioned fractions is unknown. Therefore, the impact of an increasing salinity on the removal of conditioning chemicals benzotriazole, aromatic hydrocarbon benzoic acid, and heavy metal zinc in lab-scale unplanted and Phragmites australis and Typha latifolia planted vertical-flow CWs was tested in the present study. P. australis was less sensitive than T. latifolia to increasing salinities and survived with a NaCl concentration of 12 g/L. The decay of T. latifolia was accompanied by a decrease in the removal efficiency for benzotriazole and benzoic acid, indicating that living vegetation enhanced the removal of these chemicals. Increased salinities resulted in the leaching of zinc from the planted CWs, probably as a result of active plant defence mechanisms against salt shocks that solubilized zinc. Plant growth also resulted in substantial evapotranspiration, leading to an increased salinity of the CW treated effluent. A too high salinity limits the reuse of the CW treated water. Therefore, CW treatment should be followed by desalination technologies to obtain salinities suitable for reuse. In this technology train, CWs enhance the efficiency of physicochemical desalination technologies by removing organics that induce membrane fouling. Hence, P. australis planted CWs are a suitable option for the treatment of water with a salinity below 12 g/L before further treatment or direct reuse in water scarce areas worldwide, where CWs may also boost the local biodiversity. Graphical abstract.

Effect of passive ventilation on the performance of unplanted sludge treatment wetlands: heavy metal removal and microbial community variation

Sludge treatment wetlands (STWs) have been applied worldwide to treat excess sludge; however, the performance of STWs is generally limited by weather partly due to the plants vegetated on the STWs. In this study, ventilation is suggested to assist unvegetated STWs. Solid samples from different depths were analysed. Additionally, the variation of microbial community in STW unit was analysed and the fate of heavy metals in the sludge was determined. Results indicate that the STW unit with suitable parameters has better performance in stabilising and maturing the sludge than planted STW, which may contribute to the variation of the microbial community; additionally, ventilation exerts a positive influence on these bacteria during the variation of microbial community and on heavy metal removal through the substrate and positively impacts the Cd and Pb in reduction state. Furthermore, ventilation decreases the bioavailability of Cr. With ventilation in STWs, Bacillus and Streptomyces play a necessary role in enhancing the possibility of sludge to be used as microbial inoculants.

Phytoremediation potential and control of Phragmites australis as a green phytomass: an overview

Phragmites australis (common reed) is one of the most extensively distributed emergent plant species in the world. This plant has been used for phytoremediation of different types of wastewater, soil, and sediments since the 1970s. Published research confirms that P. australis is a great accumulator for different types of nutrients and heavy metals than other aquatic plants. Therefore, a comprehensive review is needed to have a better understanding of the suitability of this plant for removal of different types of nutrients and heavy metals. This review investigates the existing literature on the removal of nutrients and heavy metals from wastewater, soil, and sediment using P. australis. In addition, after phytoremediation, P. australis has the potential to be used for additional benefits such as the production of bioenergy and animal feedstock due to its specific characteristics. Determination of adaptive strategies is vital to reduce the invasive growth of P. australis in the environment and its economic effects. Future research is suggested to better understand the plant's physiology and biochemistry for increasing its pollutant removal efficiency.

Designing and construction of simulated constructed wetland for treatment of sewage containing metals

A simulated horizontal flow constructed wetland (CW) has been designed with gravel medium and aquatic plants Typha latifolia and Polygonum hydropiper to assess its performance efficiency for sewage treatment. Monitoring of fully developed CW revealed a high removal of nutrients and metals from sewage after treatment at varying retention times. The percent (%) removal of biological oxygen demand, total dissolved solids, total suspended solids, PO₄-P and total nitrogen in CW planted with T. latifolia were 88.20, 61.9, 72.12, 74.23 and 66.78%; however, with P. hydropiper, reductions were 79.47, 53.47, 55.46, 60.40 and 52.87%, respectively, at 8 d retention time. In addition, T. latifolia and P. hydropiper accumulated substantial amount of metals in their tissues particularly in roots. T. latifolia root accumulated maximum amount of Zn (40.44 µg/g dw) followed by Cu (39.24 µg/g dw), Pb (37.78 µg/g dw) and Cr (19.95 µg/g dw) as compared to P. hydropiper, which was 17.85, 33.43, 36.19 and 9.67 µg/g dw, respectively. Further, plant-specific high translocation factor (>1) of metals were observed at different retention times. Results suggest that simulated CW may be applied as an ecofriendly and low-cost tool to treat sewage before discharge into a fresh water body.

Designing living walls for greywater treatment

Greywater is being increasingly used as an alternative water source to reduce potable water demand and to alleviate pressure on sewerage systems. This paper presents the development of a low energy and low maintenance greywater treatment technology: a living wall system, employing ornamental plants (including vines) grown in a sand filter on a side of a building to treat shower, bath, and washing basin wastewaters. The system can, at the same time, provide critical amenity and micro-climate benefits to our cities. A large scale column study was conducted in Melbourne, Australia, to investigate the following design and operational factors of the proposed system: plant species, saturated zone design, rest period, hydraulic loading rate and pollutant inflow concentration. The results indicate that the use of ornamental species (e.g. Canna lilies, Lonicera japonica, ornamental grape vine) can contribute to pollutant removal. Vegetation selection was found to be particularly important for nutrient removal. While a wider range of tested plant species was effective for nitrogen removal (>80%), phosphorus removal was more variable (-13% to 99%) over the study period, with only a few tested plants being effective - Carex appressa and Canna lilies were the best performers. It was also found that phosphorus removal can be compromised over the longer term as a result of leaching. Excellent suspended solids and organics removal efficiencies can be generally achieved in these systems (>80% for TSS and >90% for BOD) with plants having a relatively small impact. Columns had an acceptable infiltration capacity after one year of operation. When planted with effective species (e.g. Carex appressa and Canna lilies), it is expected that performance will not be significantly affected by longer rest periods and higher pollutant concentrations in the early years of system operation. The results of this study, thus, demonstrate that innovative and aesthetically pleasing living walls can be designed for treatment of greywater at the household scale.

Vertical flow-constructed wetlands for domestic wastewater treatment under tropical conditions: effect of different design and operational parameters

This study assessed the treatment of domestic wastewater to find the optimum vertical flow-constructed wetland (VFCW) configuration under tropical conditions. Eight pilot-scale configurations units were studied to compare between fine sand and medium gravel used as substrate, two feeding frequencies (20 pulses d^-1 and 10 pulses d^-1), and the presence or absence of tropical plants (Heliconia psittacorum). The results showed that the sand beds were significantly more efficient in the removal of organic matter, ammonia nitrogen, and total suspended solids than gravel beds, presenting average removal rates of 48 and 24 g m^-2 d^-1 of COD; 35 and 16 g m^-2 d^-1 of BOD₅; 7 and 4 g m^-2 d^-1 of [Formula: see text]; 9 and 0 g m^-2 d^-1 for sand and gravel, respectively. The oxygen consumption rates were calculated and a value of 65 g m^-2 d^-1 was obtained for sand beds while for the gravel beds the consumption rate was 30 g m^-2d^-1. The assessment of different kinds of nitrogen showed interesting dynamics in the nitrification processes. The presence of H. psittacorum showed positive effects in the total nitrogen (TN) removal. The different loading frequencies applied did not show significant statistical differences in the removal of the tested contaminants. Preliminary results were found in the pathogen removal, where the sand is favorable as the substrate. This work represents the first step in the research of optimum VFWC design and operation parameters for Colombia as well as the use of plants of the genus Heliconia.

Preliminary investigation on the effect of earthworm and vegetation for sludge treatment in sludge treatment reed beds system

Sewage sludge treatment is becoming one of the most significant challenges for domestic wastewater management. Optimization of sludge management for reducing sludge handling cost in wastewater treatment plant is highly demanded. Sludge treatment reed bed system (STRB) is an eco-environmentally friendly technology which has a low investment input and reduced costs for operation and maintenance. The objective of this study is to evaluate the effect of earthworm assistant STRB in terms of sludge dewatering and stabilization of surplus sludge. The results show that draining and evapotranspiration (ET) take the main role for sludge dewatering; with maximum of 77 and 43 % water was removed through draining and ET, respectively. Plants improved ET rate up to 13.1 % in the planted STRB compare with the unplanted STRB. The combination of plants and earthworms increased ET rate of 20.9 % more than the control STRB (unplanted without earthworms). The planted STRB with earthworm reached the lowest water content in accumulated sludge of 46 %. There was a systematic increase of total solids (TS) concentration from 0.5 % in the influent to 25-54 % in the accumulated sludge. Earthworms enhanced the sludge stabilization dramatic with the ratio of volatile solids (VS)/TS decreased from 49 % in the influent to 18 % in the accumulated sludge in the earthworm assistant STRB. The results demonstrated a good efficiency for sludge dewatering and stabilization with the assistant of earthworms in STRBs, which can be an alternative technology for sludge treatment in wastewater treatment plants.

Performance of a pilot demonstration-scale hybrid constructed wetland system for on-site treatment of polluted urban river water in Northwestern China

Hybrid constructed wetland (HCW) systems have been used to treat various wastewaters across the world. However, large-scale applications of HCWs are scarce, particularly for on-site improvement of the water quality of highly polluted urban rivers in semi-arid regions. In this study, a large pilot-scale HCW system was constructed to improve the water quality of the Zaohe River in Xi'an, China. With a total area of about 8000 m(2), the pilot HCW system, composed of different configurations of surface and subsurface flow wetlands, was operated for 2 years at an average inflow volume rate of 362 m(3)/day. Local Phragmites australis and Typha orientalis from the riverbank were planted in the HCW system. Findings indicate a higher treatment efficiency for organics and suspended solids than nutrients. The inflow concentrations of 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), suspended solids (SS), total nitrogen (TN), NH3-N, and total phosphorus (TP) were 125.6, 350.9, 334.2, 38.5, 27.2, and 3.9 mg/L, respectively. Average removal efficiencies of 94.4, 74.5, 92.0, 56.3, 57.5, and 69.2%, respectively, were recorded. However, the pollutant removal rates were highly seasonal especially for nitrogen. Higher removals were recorded for all pollutants in the autumn while significantly lower removals were recorded in the winter. Plant uptake and assimilation accounted for circa 19-29 and 16-23% of the TN and TP removal, respectively. Moreover, P. australis demonstrated a higher nutrient uptake ability and competitive potential. Overall, the high efficiency of the pilot HCW for improving the water quality of such a highly polluted urban river provided practical evidence of the applicability of the HCW technology for protecting urban water environments.

Carbon storage potential by four macrophytes as affected by planting diversity in a created wetland

Wetland creation has become a commonplace method for mitigating the loss of natural wetlands. Often mitigation projects fail to restore ecosystem services of the impacted natural wetlands. One of the key ecosystem services of newly created wetlands is carbon accumulation/sequestration, but little is known about how planting diversity (PD) affects the ability of herbaceous wetland plants to store carbon in newly created wetlands. Most mitigation projects involve a planting regime, but PD, which may be critical in establishing biologically diverse and ecologically functioning wetlands, is seldom required. Using a set of 34 mesocosms (∼1 m(2) each), we investigated the effects of planting diversity on carbon storage potential of four native wetland plant species that are commonly planted in created mitigation wetlands in Virginia - Carex vulpinoidea, Eleocharis obtusa, Juncus effusus, and Mimulus ringens. The plants were grown under the four distinctive PD treatments [i.e., monoculture (PD 1) through four different species mixture (PD 4)]. Plant biomass was harvested after two growing seasons and analyzed for tissue carbon content. Competition values (CV) were calculated to understand how the PD treatment affected the competitive ability of plants relative to their biomass production and thus carbon storage potentials. Aboveground biomass ranged from 988 g/m(2) - 1515 g/m(2), being greatest in monocultures, but only when compared to the most diverse mixture (p = 0.021). However, carbon storage potential estimates per mesocosm ranged between 344 g C/m(2) in the most diverse mesocosms (PD 4) to 610 g C/m(2) in monoculture ones with no significant difference (p = 0.089). CV of E. obtusa and C. vulpinoidea showed a declining trend when grown in the most diverse mixtures but J. effusus and M. ringens displayed no difference across the PD gradient (p = 0.910). In monocultures, both M. ringens, and J. effusus appeared to store carbon as biomass more effectively than the other species, suggesting that the choice of plant species may play an important role in facilitating the development of carbon accumulation/storage in created wetlands. Plant community diversity provides many ecosystem services (e.g., habitat and floristic quality) other than carbon storage function. Thus, a further study is needed that will focus on investigating how other design elements such as microtopography and hydrologic connectivity may interact with PD in terms of enhancing the carbon storage potential of newly created wetlands.

Biomass, Nutrient, and Trace Element Accumulation and Partitioning in Cattail ( L.) during Wetland Phytoremediation of Municipal Biosolids

Biomass and contaminant accumulation and partitioning in plants determine the harvest stage for optimum contaminant uptake during phytoremediation of municipal biosolids. This wetland microcosm bioassay characterized accumulation and partitioning of biomass, nutrients (N and P), and trace elements (Zn, Cu, Cr, and Cd) in cattail ( L.) in a growth room. Four cattail seedlings were transplanted into each 20-L plastic pail containing 3.9 kg (dry wt.) biosolids from an end-of-life municipal lagoon. A 10-cm-deep water column was maintained above the 12-cm-thick biosolids layer. Plants were harvested every 14 d over a period of 126 d for determination of aboveground biomass (AGB) and belowground biomass (BGB) yields, along with contaminant concentrations in these plant tissues. Logistic model fits to biomass yield data indicated no significant difference in asymptotic yield between AGB and BGB. Aboveground biomass accumulated significantly greater amounts of N and P and lower amounts of trace elements than BGB. Maximum N accumulation in AGB occurred 83 d after transplanting (DAT), and peak P uptake occurred at 86 DAT. Harvesting at maximum aboveground accumulation removed (percent of the initial element concentration in the biosolids) 4% N, 3% P, 0.05% Zn, 0.6% Cu, 0.1% Cd, and 0.2% Cr. Therefore, under the conditions of this study, phytoremediation would be most effective if cattail is harvested at 86 DAT. These results contribute toward the identification of the harvest stage that will optimize contaminant uptake and enhance in situ phytoremediation of biosolids using cattail.

Emissions of CO2 and CH4 from sludge treatment reed beds depend on system management and sludge loading

Sludge treatment reed beds (STRB) are considered as eco-friendly and sustainable alternatives to conventional sludge treatment methods, although little is known about greenhouse gas emissions from such systems. We measured CO2 and CH4 emissions and substrate characteristics in a STRB, an occasionally loaded sludge depot (SD) and a natural reed wetland (NW). The aim was to compare (i) emissions among the sites in relation to substrate characteristics and (ii) emissions before and after sludge loading in the STRB. The STRB emitted twice as much CO2 (1200 mg m(-2) h(-1)) as the SD, whereas the SD emitted four times more CH4 (2 mg m(-2) h(-1)) than the STRB. The NW had the lowest emissions of both gases. The differences in gas emissions among the sites were primarily explained by differences in the availability of oxygen in the substrate. As a consequence of overloading and poor management, the SD had no vegetation and a poor dewatering capacity, which resulted in anaerobic conditions favoring CH4 emission. In contrast, the well-managed STRB had more aerobic conditions in the sludge residue resulting in low CH4 emission rates. We conclude that well-designed and well-managed STRBs have a low climate impact relative to conventional treatment alternatives, but that overloading and poor sludge management enhances the emissions of CH4.

Nitrate removal from eutrophic wetlands polluted by metal-mine wastes: effects of liming and plant growth

Wetlands are highly effective systems in removing large amounts of N from waters, preventing eutrophication processes. However, when wetlands are polluted by metal-mine wastes their capacity to act as green filters may be diminished. The objective of this study was to evaluate the effect of liming and plants (Sarcocornia fruticosa and Phragmites australis) on the removal of NO3(-) from eutrophic water in slightly acidic, wetland soils polluted by metal-mine wastes. Simulated soil profiles were constructed and six treatments were assayed: (1) no liming + no plant, (2) no liming + S. fruticosa, (3) no liming + P. australis, (4) liming + no plant, (5) liming + S. fruticosa and (6) liming + P. australis. Three horizons were differentiated: A (never under water), C1 (alternating flooding-drying conditions) and C2 (always under water). The eutrophic water used to flood the soil profiles was enriched in N and organic carbon (pH ~ 7.5, electrical conductivity ~ 11 dS m(-1), NO3(-) ~ 234 mg L(-1) and dissolved organic carbon ~ 106 mg L(-1)). The pH, Eh and concentrations of dissolved organic carbon (DOC), N-NO3(-) and N-NH4(+) were measured regularly for 18 weeks. Liming stimulated the growth of plants, especially for S. fruticosa (20-fold more plant biomass than without liming), increased the soil pH and favoured the decline of the Eh values, enhancing the removal of NO3(-) via denitrification. Of all the treatments assayed, liming + S. fruticosa was the only treatment that removed almost completely the high concentration of NO3(-) from the eutrophic flooding water, reaching ~1 mg L(-1) N-NO3(-) at the end of the experiment, at all depths. The higher content of DOC in the pore water of this treatment could explain this behaviour, since more labile carbon was available to the soil microorganisms in the rhizosphere, favouring NO3(-) removal through denitrification processes. However, the treatment liming + P. australis (2-fold more plant biomass that without liming) did not remove completely the high concentrations of NO3(-) from the eutrophic water, except in the C2 horizon - which was permanently under water. Hence, our results show that the effectiveness of liming, regarding the removal of NO3(-) from eutrophic flooding water in wetland soils polluted by metal-mine wastes, depends on the presence of plants, their growth and the production of organic compounds in the rhizospheric environment.