Quantification of the water,energy and carbon footprints of wastewater treatment plants in China considering a water–energy nexus perspective

Water and energy are closely connected and both are very important for human development. Wastewater treatment plants (WWTPs) are central to water–energy interactions as they consume energy to remove pollutants and thus reduce the human gray water footprint on the natural water environment. In this work, we quantified energy consumption in 9 different WWTPs in south China, with different treatment processes, objects, and capacities. The energy intensity in most of these WWTPs is in the range of 0.4–0.5 kWh/m³ in 2014. Footprint methodologies were used in this paper to provide insight into the environmental changes that result from WWTPs. A new indicator “gray water footprint reduction” is proposed based on the notion of gray water footprint to better assess the role of WWTPs in reducing human impacts on water resources. We find that higher capacity and appropriate technology of the WWTPs will result in higher gray water footprint reduction. On average, 6.78 m³ gray water footprint is reduced when 1 m³ domestic sewage is treated in WWTPs in China. 13.38 L freshwater are required to produce the 0.4 kWh electrical input needed for treating 1 m³ domestic wastewater, and 0.23 kg CO₂ is emitted during this process. The wastewater characteristics, treatment technologies as well as management systems have a major impact on the efficiency of energy utilization in reducing gray water footprint via these WWTPs. The additional climate impact associated with wastewater treatment should be considered in China due to the enormous annual wastewater discharge. Policy suggestions are provided based on results in this work and the features of China's energy and water distribution.     

Wastewater treatment performance of a vermifilter enhancement by a converter slag–coal cinder filter

The study aimed at investigating rural domestic wastewater treatment performance through vermifilter enhancement by a converter slag–coal cinder filter. The research was carried out by column experiments in a lab scale. Results showed the average removal rate of TCOD, BOD, ammonia nitrogen (NH₄⁺-N) and phosphorus removal by the system were 78.0%, 98.4%, 90.3%, 62.4%, respectively at a hydraulic loading rate of 4 m³ m^?2 day^?1. Vermifiltration was effective for insoluble organic matter and suspended solid removal, and the converter slag–coal cinder filter played an important role in phosphorus removal. The molecular weight of particles in the influent and effluents from every unit of the system were distributed in a wide range between 0.1 kDa and 10,000 kDa with predominance of values between 10 kDa and 300 kDa; the major portion of soluble material are high MW compounds. In addition, the optimal design parameters for vermifiltration and converter slag and coal cinder filters, respectively, were studied in the experiments.     

Comparison of the treatment performances of blast furnace slag-based and gravel-based vertical flow wetlands operated identically for domestic wastewater treatment in Turkey

In 2001, to foster the practical development of constructed wetlands (CWs) used for domestic wastewater treatment in Turkey, vertical subsurface flow constructed wetlands (30 m² of each) were implemented on the campus of the METU, Ankara, Turkey. The main objective of the research was to quantify the effect of different filter media on the treatment performance of vertical flow wetlands in the prevailing climate of Ankara. Thus, a gravel-filled wetland and a blast furnace granulated iron slag-filled wetland were operated identically with primarily treated domestic wastewater (3 m³ d⁻¹) at a hydraulic loading rate of 0.100 m d⁻¹, intermittently. Both of the wetland cells were planted with Phragmites australis. According to the first year results, average removal efficiencies for the slag and gravel wetland cells were as follows: total suspended solids (TSS) (63% and 59%), chemical oxygen demand (COD) (47% and 44%), NH₄⁺–N (88% and 53%), total nitrogen (TN) (44% and 39%), PO₄³⁻-P (44% and 1%) and total phosphorus (TP) (45% and 4%). The treatment performances of the slag-filled wetland were better than that of the gravel-filled wetland in terms of removal of phosphorus and production of nitrate. Since this study was a pioneer for implementation of subsurface constructed wetlands in Turkey using local sources, it has proved that this eco-technology could also be used effectively for water quality enhancement in Turkey.     

Anaerobic thermophilic digestion of cutting oil wastewater: Effect of co-substrate

This paper describes the thermophilic (55 °C) anaerobic biodegradation of a mixed feed composed of vinasses and cutting oil wastewater (COW) in a laboratory upflow anaerobic fixed-film reactor (UAFF) with a porous support medium. The experimental protocol was defined to examine the effect of increasing the percentage of cutting oil wastewater in the feed.The UAFF reactor was initially started-up with vinasses as the only carbon source at an organic loading rate of 22.3 kg COD/m³ day and HRT of 0.8 days using porous particles as the support (SIRAN). The percentage of organic matter composed of vinasses was subsequently reduced while increasing the amount of cutting oil until 100% of cutting oil wastewater was added in the feed. Four stages were considered in the study (0, 42.4, 66.6 and 100% COW). HRT was adjusted in order to maintain an approximately constant organic loading rate applied to the system. Under theses conditions, the UAFF reactor was subjected to a programme of steady-state operation with hydraulic retention times (HRT) in the range 0.8–0.15 days and organic loading rates (OLR) between 22.3 and 14.9 kg COD/m³ day in order to evaluate the treatment capacity of the system.The COD removal efficiency was found to be 87% COD and 94.6% TOC in the reactor when treating vinasses at 22.3 kg COD/m³ day. The volumetric methane level produced in the digester reached 0.45 m³/m³ day. After an operating period of 120 days, the reactor was fed with cutting oil wastewater (COW) as the only source of carbon. An OLR of 16.7 kg COD/m³ day was achieved with 85.8% COD removal efficiency (58.1%TOC) in the experimental UAFF reactor. Under these conditions the volumetric methane produced in the digester was negligible.Hence, COW can be removed, if not degraded, by anaerobic treatment in the presence of a biodegradable co-substrate. Wine vinasses degradation creates conditions for non-biological removal of COW constituents. More studies are necessary in order to test the mechanisms of organic removal when biodegradation apparently had ceased. Also, toxicity assays of COW are necessary to evaluate the toxicity to the methanogenic community.     

A sustainability analysis of a municipal sewage treatment ecosystem based on emergy

“Discharge of treated wastewater and sewage sludge landfilling” are the most common practice, which poses threats to the local environment. In this work we first constructed a general sewage treatment ecosystem (STE) which integrated the three systems. Emergy synthesis, with several improved emergy-based indicators which considered the waste input contribution and impacts of emissions, was applied to evaluate two alternative scenarios (scenario 1: sewage treatment system + treated water discharge + sludge landfilling; scenario 2: sewage treatment subsystem + reclaimed water reuse subsystem + aerobic compost production subsystem) for Mingjingtan sewage treatment plant in Wanzhou City of Chongqing in China. Results point out the environmental pressure of scenario 2 is much smaller than scenario 1 although its economic performance is somewhat poorer than scenario 1, and finally the sustainability of scenario 2 is still much better than scenario 1. Therefore, the treated water and sewage sludge reuse can further improve the environmental benefit of the sewage treatment process; however, the STE should be optimized in order to enhance its economic benefit. The proposed methods can help policy-makers make decisions and guide designers and operators to improve the comprehensive performance of sewage treatment processes. In addition, this paper also briefly discusses wastewater integrated management strategy.     

Effect of different sewage sludge applications on growth and yield of Vigna radiata L. field crop: Metal uptake by plant

The aim of this study was to assess the suitability of sewage sludge use for mung bean {Vigna radiata L. cv. Malviya janpriya (HUM 6)} plants by evaluating the growth, and yield responses, nutritional quality and heavy metal accumulation at different sewage sludge amendment (SSA) rates. Sewage sludge amendment modified the physico-chemical properties of soil by decreasing pH and increasing organic carbon, total iron and heavy metals. Plants showed increments in shoot length, leaf area and total biomass at all SSA rates, but root length increased only up to 9 kg m^?2 SSA rates. Plants grown at different SSA rates showed higher nutrients and heavy metals in seeds, but protein content declined. Sewage sludge application caused about 39, 76 and 60% more yield at 6, 9 and 12 kg m^?2 treatments, respectively. Concentrations of Pb and Ni in grains were higher than the Indian permissible limits at and above 9 kg m^?2 and of Cd at 12 kg m^?2 SSA rates.The study suggests that SSA at a rate lower than 9 kg m^?2 may be recommended due to better fertilizing value for soil and promoting mung bean yield. Higher rate of sewage sludge application leads to elevated accumulation of heavy metals in seeds, which limits the suitability for human consumption.     

Thermophilic treatment of bulk drug pharmaceutical industrial wastewaters by using hybrid up flow anaerobic sludge blanket reactor

The hybrid up flow anaerobic sludge blanket reactor was evaluated for efficacy in reduction of chemical oxygen demand (COD) and biochemical oxygen demand (BOD) of bulk drug pharmaceutical wastewater under different operational conditions. The start-up of the reactor feed came entirely with glucose, applied at an organic loading rate (OLR) 1 kg COD/m³ d. Then the reactor was studied at different OLRs ranging from 2 to 11 kg COD/m³ d with pharmaceutical wastewater. The optimum OLR was found to be 9 kg COD/m³ d, where we found 65–75% COD and 80–94% of BOD reduction with biogas production containing 60–70% of methane and specific methanogenic activity was 320 ml CH₄/g-VSS d. By the characterization studies of effluent using GC–MS, the hazardous compounds like phenol, l,2-methoxy phenol, 2,4,6-trichloro phenol, dibutyl phthalate, 1-bromo naphthalene, carbamazepine and antipyrine were present. After the treatment, these compounds degraded almost completely except carbamazepine. Thermophilic methanothrix and methanosaetae like bacteria are present in the granular sludge.     

Carbon sequestration potential of green roofs using mixed-sewage-sludge substrate in Chengdu World Modern Garden City

Green roofs which use sewage sludge to sequestrate urban carbon dioxide may represent a potential opportunity to evaluate carbon sequestration benefits for the urban development under increasing global climate change. In this study, green roofs composed of 6 small green segments with two different substrates, mixed-sewage-sludge substrate (MSSS, volume ratio of sewage sludge and local-natural soil 1:1), and local-natural soil (LNS), three different substrate depths (20 cm, 25 cm and 30 cm), and three types of native plants (Ligustrum vicaryi, Neottia auriculata, and Liriope spicata) in Chengdu City were established to determine carbon sequestration from July 2012 to July 2013 through assessment of the carbon storage and sequestration. Results show that the average carbon storage of MSSS and LNS on green roofs was respectively 13.15 kg C m⁻² and 8.58 kg C m⁻², and the average carbon sequestration followed the order of LNS (3.89 kg C m⁻² yr⁻¹) > MSSS (3.81 kg C m⁻² yr⁻¹). Thus MSSS could be considered as a potential material for carbon sequestration. The carbon storage and carbon sequestration by native plants on the green roofs followed the order of L. vicaryi > L. spicata > N. auriculata. The whole green roof had a mean carbon storage of 18.28 kg C m⁻² and average carbon sequestration of 6.47 kg C m⁻² yr⁻¹ in the combined biomass and substrate organic matter. The best green roof configuration was L. vicaryi together with MSSS substrate, with a middle-high level of carbon sequestration. It will be feasible and worthwhile to scale-up the adaptable green roof configurations in Chengdu World Modern Garden City.     

Microfiltration membrane performance in two-chamber microbial fuel cells

Proton exchange membranes (PEMs) are typically used in two-chamber microbial fuel cells (MFCs) to separate the anode and cathode chambers while allowing protons to pass between the chambers. However, PEMs such as Nafion are not cost-effective. To reduce the cost of MFCs, we examined the performances of cellulose acetate microfiltration membranes in a two-chamber microbial fuel cell using acetate. The internal resistance, the maximum power density and the coulombic efficiency (CE) of the microfiltration membrane MFC (MMMFC) were 263 Ω, 0.831 ± 0.016 W/m² and 38.5 ± 3.5%, respectively, in a fed-batch mode, while the corresponding values of the MFC using a PEM were 267 Ω, 0.872 ± 0.021 W/m² and 74.7 ± 4.6%, respectively. We further used the MMMFC for poultry wastewater treatment. The maximum power density of 0.746 ± 0.024 W/m² and CE of 35.3 ± 3.2% were achieved when the poultry wastewater containing 566 mg/L COD was used, removing 81.6 ± 6.6% of the COD. These results demonstrate microfiltration membranes, compared with PEMs, have a similar internal resistance and reduce pH gradient across the membrane. They parallel PEMs in maximum power density, while CE is much lower due to the oxygen and substrate diffusion. The MMMFC was effective for poultry wastewater treatment with high COD removal.     

Denitrification of nitrified and non-nitrified swine lagoon wastewater in the suspended sludge layer of treatment wetlands

One method for managing livestock-wastewater N is the use of treatment wetlands. The objectives of this study were to (1) assess the magnitude of denitrification enzyme activity (DEA) in the suspended sludge layers of bulrush and cattail treatment wetlands, and (2) evaluate the impact of nitrogen pretreatment on DEA in the suspended sludge layer. The study used four wetland cells (3.6 m × 33.5 m) with two cells connected in series. Each wetland series received either untreated or partially nitrified swine wastewater from a single-cell anaerobic lagoon. The DEA of the suspended sludge layers of the constructed wetlands was measured by the acetylene inhibition method. The control DEA treatment for the sludge layer had a mean rate of 18 μg N₂O-N g^?1 sludge h^?1. Moreover, the potential DEA (nitrate-N and glucose-C added) mean was very large, 121 μg N₂O-N g^?1 sludge h^?1. These DEA rates are consistent with the previously reported high levels of nitrogen removal by denitrification from these wetlands, especially when the wastewater was partially nitrified. Stepwise regression using distance within the wetland, wastewater nitrate, and wastewater ammonia explained much of the variation in DEA rates. In both bulrush and cattail wetlands, there were zones of very high potential DEA.     

Comparative kinetic study between moving bed biofilm reactor-membrane bioreactor and membrane bioreactor systems and their influence on organic matter and nutrients removal

New technologies regarding wastewater treatment have been developed. Among these technologies, the moving bed biofilm reactor combined with membrane bioreactor (MBBR-MBR) is a recent solution alternative to conventional processes. This paper presents the results obtained from three wastewater treatment plants working in parallel. The first wastewater treatment plant consisted of a membrane bioreactor (MBR), the second one was a MBBR-MBR system containing carriers both in anoxic and aerobic zones, and the last one consisted of a MBBR-MBR system which contained carriers only in the aerobic zone. The reactors operated with a hydraulic retention time of 26.47 h. During the study, the difference between the experimental plants was not statistically significant concerning organic matter and nutrients removal. However, different tendencies regarding nutrients removal are shown by the three wastewater treatment plants. In this sense, the performances in terms of nitrogen and phosphorus removal of the MBBR-MBR system which contained carriers only in the aerobic zone (67.34 ± 11.22% and 50.65 ± 11.13%, respectively) were slightly better than those obtained from another experimental plants. As a whole, the pilot plant which consisted of a MBR showed better performance from the point of view of the kinetics of the heterotrophic and autotrophic biomass with values of μ_m,H = 0.00858 h⁻¹, μ_m,A = 0.07646 h⁻¹, K_M = 2.37 mg O₂ L⁻¹ and K_NH = 1.31 mg N L⁻¹.     

The investigation of dairy industry wastewater treatment in a biological high performance membrane system

The dairy industry is generally considered to be the largest source of food processing wastewater in many countries. The highly variable nature of dairy wastewaters in terms of volumes and flowrates and in terms of high organic materials contents such as COD 921–9004 mg L⁻¹, BOD 483–6080 mg L⁻¹, TN of 8–230 mg L⁻¹ and SS of 134–804 mg L⁻¹ makes the choice of an effective wastewater treatment regime difficult. A high performance bioreactor, an aerobic jet loop reactor, combined with a ceramic membrane filtration unit, was used to investigate its suitability for the treatment of the dairy processing wastewater. The oxygen transfer rates of the bioreactor were found to be very high (100–285 h⁻¹) on the operating conditions. A loading rate of 53 kg COD m⁻³ d⁻¹ resulted in 97–98% COD removal efficiencies under 3 h hydraulic retention time. The high MLSS concentrations could be retained in the system (up to 38,000 mg L⁻¹) with the contribution of UF (ultrafiltration) unit. During the filtration of activated sludge, the fluxes decreased with increasing MLSS. Cake formation fouling was determined as dominant fouling mechanisms. The results demonstrate that jet loop membrane bioreactor system was a suitable and effective treatment choice for treating dairy industry wastewater.     

Supplementation of ammonium bicarbonates for the treatment of fruit cordial wastewater by anaerobic digestion process

Lack of nitrogenous substrate and buffering capacity have been identified as causing failure in previous work on the treatment of fruit cordial wastewater using anaerobic continuous stirred tank reactors. In this study, ammonium bicarbonate was proposed to be used as the substrate for nitrogenous and buffering resources. In order to determine the toxicity effect of the ammonium salts on the anaerobic system, a series of concentration from 0 to 40 mg L^?1 was tested. Biogas production was used as the indicator for NH₄⁺ toxicity. The results showed no indication that methanogen was affected by the additional ammonium salt within the dosing regime. Application of the specific mathematical function (G = G_m^k/t) to describe the kinetic of biogas production, suggested that the optimal concentration of ammonium bicarbonate that can be used is 10 mg L^?1. This study also shows that the dosage regime up to 40 mg L^?1 can be used to supplement the lack of nitrogenous and buffering capacity for the anaerobic digestion process of the fruit cordial wastewater using CSTR.     

Treatment of high-strength wastewater in tropical constructed wetlands planted with Sesbania sesban: Horizontal subsurface flow versus vertical downflow

Treatment of various types of wastewaters is an urgent problem in densely populated areas of many tropical countries. We studied the potential of using Sesbania sesban, an N₂-fixing shrub, in constructed wetland systems for the treatment of high-strength wastewater. A replicated horizontal subsurface flow system and a saturated vertical downflow system was established with planted and unplanted beds to assess the effects of system design and presence of plants on treatment performance. The systems were loaded with a mixture of domestic and pig farm wastewater at three hydraulic loading rates of 80, 160 and 320 mm d^?1. The S. sesban plants grew very well in the constructed wetland systems and produced 17.2–20.2 kg dry matter m^?2 year^?1 with a high nitrogen content. Mass removal rates and removal rate constants increased with loading rate, but at 320 mm d^?1 the effluent quality was unacceptable and hydraulic problems appeared. Mass removal rates and removal rate constants were much higher than reported in other studies probably because of the high-strength wastewater, the high loading rates and the tropical conditions. Planted systems removed pollutants much more efficiently than the unplanted controls. Direct plant uptake constituted only up to 8% of the total-N removal and 2% of the P removal at the lowest loading rate, and was quantitatively of low importance compared to other removal processes. The significant effects of plants were therefore related more to their indirect effects on the removal processes. This study for the first time documents that S. sesban can be used in constructed wetland systems for the treatment of polluted water while at the same time producing a valuable N rich biomass that can be used for animal fodder or soil amendment.     

Kinetics of organic carbon removal by a mixed culture in a membrane bioreactor

The aim of this work was to model the biological activity and anticipate the kinetic behaviour of microorganisms and the overall performance of the process according to a specific model and running parameters. The bacterial inoculum used in these experiments was a mixture of cultures taken from the wastewater treatment plant in Montpellier. The fermentor, used in association with an ultrafiltration separation stage (with a filtration area of 0.2 m²) had a working volume of 15.8 l. For various working conditions (different solid retention times, different hydraulic retention times and substrate concentrations), the biomass concentration and the residual substrate concentration, expressed in terms of dry weight and chemical oxygen demand, respectively, were measured. The basic idea of modelling was related to the concept of maintenance. The coefficient of maintenance, E, and the theoretical conversion yield, y, were therefore calculated. The values of E and y, measured for total cell recycling experiments and for experiments with various solid retention times, remained similar and were found to equal 0.040 mgCOD mgVSS h⁻¹ and 0.36 mgVSS mgCOD⁻¹, respectively. Determining these two constants and modelling the treatment process made it possible to anticipate the optimal biomass concentration for a defined removal efficiency under different steady-state operating conditions.     

Effect of hydraulic retention time on the treatment of secondary effluent in a subsurface flow constructed wetland

This study evaluates the potential of subsurface flow (SSF) constructed wetlands (CWs) for tertiary treatment of wastewater at four shorter HRTs (1–4 days). The CWs were planted with Typha angustata, which was observed in our earlier study to be more efficient than Phragmites karka and Scirpus littoralis. The CWs comprised four rectangular treatment cells (2.14 m × 0.76 m × 0.61 m) filled with layers of gravel of two different sizes (approximately 2.5 cm and 1.5 cm diameter) to a depth of 0.61 m. The inflow rates of the secondary effluent in the four cells were accordingly fixed at 300 L d^?1, 150 L d^?1, 100 L d^?1 and 75 L d^?1, respectively, for 1, 2, 3 and 4 days HRT. The hydraulic loads ranged between 59.05 mm d^?1 and 236.22 mm d^?1.The wastewater inflow into the CW system as well as the treated effluent were analyzed, using standard methods, at regular intervals for various forms of nitrogen (NH₄-N, NO₃-N and TKN), orthophosphate-P and organic matter (BOD and COD) concentrations over a period of five weeks after the development of a dense stand.The higher HRT of 4 days not only helped maximum removal of all the pollutants but also maintained the stability of the treatment efficiency throughout the monitoring period. For the nutrients (NH₄-N, NO₃-N and TKN), HRT played a more significant role in their removal than in case of organic matter (BOD₃ and COD). More than 90% of NO₃-N and TKN and 100% of NH₄-N were removed from the wastewater at 4 days HRT.At lower HRTs, the mass loading rate was higher with greater fluctuation. However mass reduction efficiency of the T. angustata CW for all forms of nitrogen was >80% with the HRTs of 2, 3 and 4 days.     

Minimizing land requirement and evaporation in small wastewater treatment systems

Water supplies in the Middle East arid climate are a scarce commodity making treated wastewater an economically attractive source for increasing the limited existing water resources for agricultural purposes. In order to minimize water losses with the corresponding increased salinity and to reduce land demand, an integrated system based mainly on high-rate semi-intensive treatment units is being tested and demonstrated. The units include an upflow anaerobic sludge blanket (UASB) reactor and vertical and horizontal flow wetlands. The units are characterized by simple and low-cost maintenance with minimal energy input. Three years of pilot plant results from the combined system are presented in this paper. The results show a high organic removal rate for the combined system: 140 g COD/m²/day for the scheme, which included a UASB reactor followed by two PAVB units and subsurface horizontal flow CW. Even higher rates of 900 g COD/m²/day were achieved for the same scheme by replacing the final CWL with another PAVB unit. These high rates allow for a small treatment plant footprint equivalent to 0.13–0.9 m² per person, assuming 125 g COD per person per day.     

Treatment of domestic wastewater in tropical,subsurface flow constructed wetlands planted with Canna and Heliconia

Constructed wetlands have a good potential for wastewater treatment in developing countries due to the simple operation and low implementation costs. Ornamental plants like Canna and Heliconia are used in the wetlands to increase their aesthetic value and these two species were compared in this study. Six pilot scale horizontal subsurface flow constructed wetland units were constructed at the Asian Institute of Technology (AIT) campus in Bangkok, Thailand, of which three were planted with Heliconia psittacorum L.f. × H. Spathocircinata (Aristeguieta) and three with Canna × generalis L. Bailey. The beds were loaded with domestic wastewater in four trials with hydraulic loading rates ranging from 55 to 440 mm d^?1 corresponding to nominal detention times between 12 h and 4 days. Both plant species grew well in the systems and especially Canna had high growth rates (3100 ± 470 g DW m^?2 yr^?1) compared to Heliconia (550 ± 90 g DW m^?2 yr^?1). TSS mass removal rates were very high with efficiencies >88% even at hydraulic loading rates of 440 mm d^?1. COD mass removal rates varied between 42 and 83% depending on the loading rates. The removal rate constants for COD as fitted by the first-order k–C^* model were estimated to be 0.283 and 0.271 m d^?1 for Canna and Heliconia beds, respectively (C^* = 28.1 and 26.7 mg l^?1). Removals of nitrogen (N) and phosphorus (P) were low compared to the loading rates, but removal of total-N was higher in the beds planted with Canna than in beds with Heliconia because of the higher growth rate of Canna. It is concluded that ornamental species like Canna and Heliconia can be used to enhance the aesthetic appearance and hence the public acceptance of wastewater treatment systems in tropical climates. Canna is the preferred species from a treatment perspective because of its more vigorous growth, but since Heliconia has an economic potential as cut flowers may be preferred in many cases.     

Nitrate removal and greenhouse gas production in a stream-bed denitrifying bioreactor

Denitrifying bioreactors are currently being tested as an option for treating nitrate (NO₃^?) contamination in groundwater and surface waters. However, a possible side effect of this technology is the production of greenhouse gases (GHG) including nitrous oxide (N₂O) and methane (CH₄). This study examines NO₃^? removal and GHG production in a stream-bed denitrifying bioreactor currently operating in Southern Ontario, Canada. The reactor contains organic carbon material (pine woodchips) intended to promote denitrification. Over a 1 year period, monthly averaged removal of influent (stream water) NO₃^? ranged from 18 to 100% (0.3–2.5 mg N L^?1). Concomitantly, reactor dissolved N₂O and CH₄ production, averaged 6.4 μg N L^?1 (2.4 mg N m^?2 d^?1), and 974 μg C L^?1 (297 mg C m^?2 d^?1) respectively, where production is calculated as the difference between inflow and effluent concentrations. Gas bubbles entrapped in sediments overlying the reactor had a composition ranging from 19 to 64% CH₄, 1 to 6% CO₂, and 0.5 to 2 ppmv N₂O; however, gas bubble emission rates were not quantified in this study. Dissolved N₂O production rates from the bioreactor were similar to emission rates reported for some agricultural croplands (e.g. 0.1–15 mg N m^?2 d^?1) and remained less than the highest rates observed in some N-polluted streams and rivers (e.g. 110 mg N m^?2 d^?1, Grand R., ON). Dissolved N₂O production represented only a small fraction (0.6%) of the observed NO₃^? removal over the monitoring period. Dissolved CH₄ production during summer months (up to 1236 mg C m^?2 d^?1), was higher than reported for some rivers and reservoirs (e.g. 6–66 mg C m^?2 d^?1) but remained lower than rates reported for some wastewater treatment facilities (e.g. sewage treatment plants and constructed wetlands, 19,500–38,000 mg C m^?2 d^?1).     

Effects of plant diversity on biomass production and substrate nitrogen in a subsurface vertical flow constructed wetland

Most biodiversity experiments have been conducted in grassland ecosystems with nitrogen limitation, while little research has been conducted on relationships between plant biomass production, substrate nitrogen retention and plant diversity in wetlands with continuous nitrogen supply. We conducted a plant diversity experiment in a subsurface vertical flow constructed wetland for treating domestic wastewater in southeastern China. Plant aboveground biomass production ranged from 20 to 3121 g m^?2 yr^?1 across all plant communities. In general, plant biomass production was positively correlated with species richness (P = 0.001) and functional group richness (P = 0.001). Substrate nitrate concentration increased significantly with increasing plant species richness (P = 0.046), but not with functional group richness (P = 0.550). Furthermore, legumes did not affect biomass production (P = 0.255), retention of substrate nitrate (P = 0.280) and ammonium (P = 0.269). Compared to the most productive of the corresponding monocultures, transgressive overyielding of mixed plant communities did not occur in most polycultures. Because greater diversity of plant community led to higher biomass production and substrate nitrogen retention, thus we recommend that plant biodiversity should be incorporated in constructed wetlands to improve wastewater treatment efficiency.