Optimization of Petroleum Refinery Wastewater Treatment by Vertical Flow Constructed Wetlands Under Tropical Conditions: Plant Species Selection and Polishing by a Horizontal Flow Constructed Wetland

Typha latifolia-planted vertical subsurface flow constructed wetlands (VSSF CWs) can be used to treat petroleum refinery wastewater. This study evaluated if the removal efficiency of VSSF CWs can be improved by changing the plant species or coupling horizontal subsurface flow constructed wetlands (HSSF CWs) to the VSSF CW systems. The VSSF CWs had a removal efficiency of 76% for biological oxygen demand (BOD₅), 73% for chemical oxygen demand (COD), 70% for ammonium-N (NH₄⁺-N), 68% for nitrate-N (NO₃^?-N), 49% for phosphate (PO₄^3?-P), 68% for total suspended solids (TSS), and 89% for turbidity. The HSSF CWs planted with T. latifolia further reduced the contaminant load of the VSSF CW-treated effluent, giving an additional removal efficiency of 74, 65, 43, 65, 58, 50, and 75% for, respectively, BOD₅, COD, NH₄⁺-N, NO₃^?-N, PO₄^3?-P, TSS, and turbidity. The combined hybrid CW showed, therefore, an improved effluent quality with overall removal efficiencies of, respectively, 94% for BOD₅, 88% for COD, 84% for NH₄⁺-N, 89% for NO₃^?-N, 78% for PO₄^3?-P, 85% for TSS, and 97% for turbidity. T. latifolia strived well in the VSSF and HSSF CWs, which may have contributed to the high NH₄ ⁺-N, NO₃^?-N, and PO₄^3?-P removal efficiencies. T. latifolia-planted VSSF CWs showed a higher contaminant removal efficiency compared to the unplanted VSSF CW. T. latifolia is thus a suitable plant species for treatment of secondary refinery wastewater. Also a T. latifolia-planted hybrid CW is a viable alternative for the treatment of secondary refinery wastewater under the prevailing climatic conditions in Nigeria.     

Evaluation of Vertical-Flow Constructed Wetlands for Swine Wastewater Treatment

In this study, the role of Cyperus sp. was evaluated for removal of pollutants from swine wastewater. Vertical-flow pilot scale constructed wetlands (CWs) operating with a hydraulic retention time (HRT) of 72 h were monitored in a greenhouse, located in Viçosa, Brazil. Significant differences were observed for the following parameters: Kjeldahl nitrogen, total phosphorus, alkalinity and electric conductivity, with averages removals of 37.5 and 28.5%, 55.9 and 44.4%, 30.2 and 25.6 and 26.1% and 22.9% (for planted and unplanted CWs, respectively). The rate of dry matter yield from Cyperus sp. was 7.5 g?m^?2 day^?1, and the nutrient uptake capacities were 21.8, 2.1, 14.0 and 0.9 g?m^?2 of N, P, K and Na, respectively. Evapotranspiration (2.7 mm day^?1) was statistically higher in the planted CWs. Plants in the CWs are important for achieving high nutrient removal.     

Improving Performance of Treatment Wetlands: Evaluation of Supplemental Aeration,Varying Flow Direction,and Phosphorus Removing Filters

Pollutant removal was compared among subsurface flow constructed wetland (CW) mesocosms used for dairy farm wastewater treatment. Supplemental aeration, flow direction, and the use of phosphorus-reducing filters (PRFs) were varied among the CWs. The following were compared: (1) vertical flow CWs with and without supplemental aeration, (2) aerated CWs with horizontal and vertical flow directions, (3) single-cell and two-cell treatment systems, and (4) wetland-wetland systems (two CWs in series) and wetland-PRF systems (a CW followed by a PRF). The results from this investigation showed that, first, nearly all treatment strategies, either singly or in pairs, substantially reduced almost all the contaminants we tested. Second, supplemental aeration resulted in higher ammonium-nitrogen (NH₄-N) removal efficiencies in aerated vertical flow CWs, compared to unaerated CWs. However, it caused no further improvement in dissolved reactive phosphorus (DRP), total suspended solids (TSS), E. coli, or BOD₅ removal. Third, there was no difference between aerated horizontal and aerated vertical flow CWs in removal of any of the tested contaminants. Fourth, adding a second stage of treatment significantly improved DRP, TSS, E. coli, and NH₄-N removal, but not BOD₅. Finally, treatment systems with PRFs showed superior performance in DRP and E. coli removal.     

Loss‐on‐Ignition Method to Assess Soil Organic Carbon in Calcareous Everglades Wetlands

Measurement of soil carbon (C) is important for determining the effects of Everglades restoration projects on C cycling and transformations. Accurate measurement of soil organic C by automated carbon–nitrogen–sulfur (CNS) analysis may be confounded by the presence of calcium carbonate (CaCO₃) in Everglades wetlands. The objectives of this study were to compare a loss‐on‐ignition (LOI) method with CNS analysis for assessment of soil C across a diverse group of calcareous Everglades wetlands. More than 3168 samples were taken from three soil depths (floc, 0–10, 10–30 cm) in 14 wetlands and analyzed for LOI, total C, and total calcium (Ca). The LOI method compared favorably to CNS analysis for LOI contents ranging from 0 to 1000 g kg^?1 and for soil total Ca levels from 0 to 500 g Ca kg^?1. For all wetlands and soil depths, LOI was significantly related to total C (r² = 0.957). However, LOI was a better predictor of total C when LOI exceeded 400 g kg^?1 because of less interference by CaCO₃. Total C measurement by CNS analysis was problematic in soils with high total Ca and low LOI, as the presence of CaCO₃ confounded C analysis for LOI less than 400 g kg^?1. Inclusion of total Ca in regression models with LOI significantly improved the prediction of total C. Estimates of total organic C by CNS analysis were obtained by accounting for C associated with CaCO₃ by calculation, with results being similar to total organic C values obtained from LOI analysis. The proportion of C in organic matter measured by the LOI method (51%) was accurate and applicable across wetlands, soil depths, and total Ca levels; thus LOI was a suitable indicator of total organic C in Everglades wetlands.     

Evaluation of the Fate of Ciprofloxacin and Amoxicillin in Domestic Wastewater

The main objective of this study was to evaluate the contribution of sorption to the removal of two commonly used antibiotics (amoxicillin and ciprofloxacin) from wastewater. These antibiotics are excreted in large quantities with more than 75% of them being unmetabolized and are therefore likely to end up in domestic wastewater in significant quantities. The specific objectives were to determine the sorption behavior in synthetic wastewater (SWW), the effect of pH and contribution of microbial surfaces, to the sorption of these antibiotics. The SWW, adjusted to various pH levels, was used and sorption kinetics conducted at 100 and 250 ??g L^?1 concentrations. Adsorption isotherms were determined at different pH levels. The SWW (pH 6.6) was inoculated with Rhodococcus sp. B30 strain to determine the contribution of microbial surfaces to sorption. Generally, both antibiotics revealed a decrease in sorption with pH increase, suggesting that lowering the solution pH of the wastewater may reduce their amounts in wastewater solution. Comparatively, ciprofloxacin exhibited higher sorption than amoxicillin. The sorption distribution coefficient (K _d) values for ciprofloxacin ranged from 0.4356 to 0.8902 L?g^?1, with pH?=?5.5 exhibiting the highest K _d, while that for amoxicillin ranged from 0.1582 to 0.3858 L?g^?1 with the highest K _d at pH?=?3.5. There was a significant difference (p?<?0.05) in K _d values between various pH levels for both antibiotics except between the pH of 5.5 and 6.6. Both antibiotics were not degraded within 48 h by Rhodococcus sp. B30 strain. These results indicate that degradation may not be the major process of removal of compounds from wastewater treatment plants and hence the importance of sorption as an intervention technique.     

Efficiency of Mesocosm-Scale Constructed Wetland Systems for Treatment of Sanitary Wastewater Under Tropical Conditions

Subsurface-flow constructed wetlands technology (SSFW) has been used successfully for treating sanitary wastewater throughout North America and Europe. However, treatment wetland technologies have not been used extensively in the tropics. To advance tropical studies, a pilot-scale SSFW was constructed on the campus of the University of the Atlantic in Barranquilla, Colombia. The systems performance was monitored from January to July of 2009. The treatment system consisted of a 760-L septic tank followed by three mesocsom-scale subsurface-flow constructed wetlands in parallel arrangement. Clarified wastewater was batch loaded to each unit at a rate of 53 L/m²/day to affect a hydraulic retention time of approximately 3 days. One of the treatment units served as a non-planted control (gravel only), while the other two treatment units were planted with either Eriochloa aristata or Eleocharis mutata. The objective of this study was to evaluate the comparative efficacy of the treatment units (planted vs. unplanted), with respect to their abilities to augment treatment of septic tank effluent (sanitary wastewater). Monitored parameters included plant biomass, oxidation?Creduction potential, chemical oxygen demand (COD), temperature, dissolved oxygen, pH, ammonia?Cnitrogen (NH ₄ ⁺ ?CN) nitrate?C and nitrite?Cnitrogen (NO₃?CN, NO₂?CN), phosphates (PO ₄ ^? ), and coliform bacteria. Total biomass (dry matter) was 2.84 and 0.87 Kg/m² for E. aristata and E. mutata, respectively. Redox potential in the plant rizospheres averaged ?172 mV (±164.1) in E. aristata, 29 mV (±251.1) in E. mutata, and 32 mV (±210.5) in the unplanted control. COD removal was superior in planted vs. non-planted systems (>75% vs. 47%). Ammonia and total phosphorus removal averaged 69% and 85%, respectively, in planted systems versus 31% and 59% in the unplanted system. Removal of total and fecal coliforms averaged 96%. Results of this pilot study revealed that SSFW technology in the tropics can provide significant removal of organic matter, nutrients, and bacteria from clarified sanitary wastewater.     

Nutrient Removal in Pilot-Scale Constructed Wetlands Treating Eutrophic River Water: Assessment of Plants, Intermittent Artificial Aeration and Polyhedron Hollow Polypropylene Balls

Seven experimental pilot-scale subsurface vertical-flow constructed wetlands were designed to assess the effect of plants [Typha latifolia L. (cattail)], intermittent artificial aeration and the use of polyhedron hollow polypropylene balls (PHPB) as part of the wetland substrate on nutrient removal from eutrophic Jinhe River water in Tianjin, China. During the entire running period, observations indicated that plants played a negligible role in chemical oxygen demand (COD) removal but significantly enhanced ammonia–nitrogen (NH₄–N), nitrate–nitrogen (NO₃–N) total nitrogen (TN), soluble reactive phosphorus (SRP) and total phosphorus (TP) removal. The introduction of intermittent artificial aeration and the presence of PHPB could both improve COD, NH₄–N, TN, SRP and TP removal. Furthermore, aerated wetlands containing PHPB performed best; the following improvements were noted: 10.38 g COD/m² day, 1.34 g NH₄–N/m² day, 1.04 g TN/m² day, 0.07 g SRP/m² day and 0.07 g TP/m² day removal, if compared to non-aerated wetlands without PHPB being presented.     

Ammonia Nitrogen Removal in Step-Feed Rotating Biological Contactors

The effect of step-feed on biological ammonia nitrogen (NH₃-N) removal in a rotating biological contactor (RBC) system consisting of two three-stage units (one control and one step-feed) treating synthetic wastewater was examined. The performance of the step-feedRBC was evaluated in comparison to a regularly fed RBC in terms of NH₃-N removals and stage-dissolved oxygen (DO) conditions over a range of hydraulic and organic loading rates (HLR = 0.032 to 0.125 m³ m^-2. d^-1 and OLR = 11.03 to 111.6 g COD m^-2. d^-1). The results indicate that the step-feed unit showed better removal efficiency (%) at high HLR and ORL than the regularly fed control unit. Increasing the HLR and the OLR resulted in a decrease in DO in all the stages of the two units. However, DO values in the step-feed system were higher than those recorded for the control system. In addition, O₂ limiting conditions (DO ≤ 2 mg l^-1) and heavy growth of Beggiatoa were detected in stage 1 of the control unit at high loading rates.     

Simultaneous Pollutant Removal and Electricity Generation in a Combined ABR-MFC-MEC System Treating Fecal Wastewater

Simultaneous power generation and fecal wastewater treatment were investigated using a combined ABR-MFC-MEC system (anaerobic baffled reactor-microbial fuel cell-microbial electrolysis cell). The installation of multi-stage baffles can benefit retaining the suspended solids in the system and help separate the hydrolysis-acidification and the methanogen processes. The efficiencies of the nitrification-denitrification process were improved because of the weak current generation by coupling the microbial electrochemical device (MFC-MEC) with the ABR unit. Maximum removal rates for chemical oxygen demand (COD) and ammonia nitrogen (NH₄ ⁺-N) were 1.35 ± 0.05 kg COD/m³/day and 85.0 ± 0.4 g NH₄ ⁺-N/m³/day, respectively, while 45% of methane (CH₄), 9% of carbon dioxide (CO₂), and 45% of nitrogen gas (N₂) contents in volume ratio were found in the collected gas phase. An average surplus output voltage of 452.5 ± 10.5 mV could be achieved from the combined system, when the initial COD concentration was 1500.0 ± 20.0 mg/L and the initial NH₄ ⁺-N concentration was 110.0 ± 5.0 mg/L, while the effluent COD could reach 50.0 mg/L with an HRT of 48 h. The combined process has the potential to treat fecal wastewater efficiently with nearly zero energy input and a fair bio-fuel production.     

Nitrogen And Phosphorus Removal In Substrate-Free Pilot Constructed Wetlands With Horizontal Surface Flow In Uganda

In constructed wetlands (CWs) with horizontal sub-surface flow, nutrient removal, especially phosphorus, is limited because the root biomass fills the pore spaces of the substrate (usually gravel), directing wastewater flow to deeper wetland media; plants are not regularly harvested; the litter formed by decomposing vegetation remains on the surface of the substrate and thus does not interact with the wastewater; and the substrate media often used provide only limited adsorption. Effective nutrient removal including rootzone oxidation, adsorption and plant uptake therefore requires sufficient interaction of wastewater with the treatment media. We assessed the feasibility of biological nutrient removal from wastewater using substrate-free CWs with horizontal flow, planted with two tropical macrophytes namely, Cyperus papyrus and Miscanthidium violaceum. The objectives were to evaluate the system treatment efficiency under semi-natural conditions, and to assess microbial and plant biomass contributions to nutrient removal in the CWs. Results showed high removal efficiencies for biochemical oxygen demand, ammonium-nitrogen (NH₄–N) and phosphorus (P) fractions in papyrus-based CWs (68.6–86.5%) compared to Miscanthidium (46.7–61.1%) and unplanted controls (31.6–54.3%). Ammonium oxidizing bacteria in CW root–mats (10⁸–10⁹ cells/gram dry weight) and residual nitrite and nitrate concentrations in the water phase indicated active system nitrification. Papyrus showed higher biomass production and nutrient uptake, contributing 28.5% and 11.2%, respectively, of the total N and P removed by the system compared to 15% N and 9.3% P removed by Miscanthidium plants. Compared to literature values, nitrification, plant uptake and the overall system treatment efficiency were high, indicating a high potential of this system for biological nutrient removal from wastewaters in the tropics.     

Chemical Oxygen Demand and Ammonia Nitrogen Removal in a Non-saturated Layer of a Strengthened Constructed Rapid Infiltration System

A strengthened constructed rapid infiltration (SCRI) system is a sewage treatment system derived from a constructed rapid infiltration (CRI) system. The SCRI tank structure primarily includes saturated and non-saturated layers. The degradation of the chemical oxygen demand (COD) and the conversion of ammonia nitrogen (NH₄ ⁺-N) are primarily performed in a non-saturated layer. To study the COD and NH₄ ⁺-N removal process in a non-saturated layer, two organic glass columns with a radius of 2.5 cm and a height of 70 cm were loaded with layers of soil from the Shunyi district of Beijing. The primary goal of this research is to quantify the removal effect factors and the relationship of the COD and NH₄ ⁺-N in the non-saturated layer. The SCRI system functioned successfully under a wetting-drying ratio of 1:5 with hydraulic loading at 1.0 m³/ (m²·d) for over 2 months. Our results show that the removal rate of NH₄ ⁺-N is approximately 69.11%, and the removal efficiency of COD is approximately 90.46%. The removal of COD is only slightly affected by pH, while the removal of NH₄ ⁺-N is greatly influenced by pH.     

Effect of High Organic Loading Rates of Particulate and Dissolved Organic Matter on the Efficiency of Shallow Experimental Horizontal Subsurface-flow Constructed Wetlands

Two identical experimental subsurface-flow constructed wetlands were operated at relatively high organic loading rates (23 g COD m^?2 day^?1) for 4 months to evaluate their relative ability to remove either dissolved organic carbon (glucose, considered to be a readily biodegradable substrate) or particulate organic carbon (starch, considered to be a slowly biodegradable substrate). The systems were built using plastic containers (0.93 m long, 0.59 m wide and 0.52 m high) that were filled with an 0.35 m layer of wetted gravel (D₆₀?=?3.5 mm, uniformity coefficient C_u?=?D₆₀/D₁₀?=?1.7) and the water level was maintained at 0.05 m under the gravel surface to give a water depth of 0.30 m. The results indicated that there was no significant difference in COD removal between the two systems. Both systems generally had COD removal rates of over 90%, which is quite high if the heavy load applied is taken into account. The removal of ammonium was greater in the glucose-fed system (57%) in comparison with the starch-fed system (43%). Based on mass balance calculations and stoichiometric relationships, it was estimated that denitrification and sulphate reduction were minor pathways for the removal of organic matter. Indirect observations allowed to assume that methanogenesis made a highly significant contribution to the removal of organic matter.     

Application of Pilot-Scale-Constructed Wetland as Tertiary Treatment System of Wastewater for Phosphorus and Nitrogen Removal

This paper presents the results of using a pilot-scale-constructed wetland as a tertiary system to simulate the treatment conditions of wastewater effluents from the metal-mechanical industry, aiming to achieve the Brazilian legal standards of phosphorus and nitrogen emission. The macrophytes were placed in 1 m³ polyethylene tanks, daily estimating the treatment of 2 m³ of effluents. The effluents were circulated in a horizontal subsurface flow through a porous matrix of thick sand and gravel, in which the roots of the macrophytes of the species Reed (Scirpus sp.) and Cattail (Typha sp.) were fixed. Monitoring of the pilot plant was performed through a battery of physical?Cchemical and biological analyses. Despite the load variations and operational problems, the system presented a positive degree of pollutant efficiency removal, especially for phosphorus (73% medium), TKN (61% medium), and NH₄?CN (56% medium). Peak results were achieved during the last 3 months of monitoring. The chemical analysis of the support layer, plus the root system and aerial portion of the plants, revealed that these wastes could be used as fertilizer.     

Performance of the Giant Reed (<Emphasis Type="Italic">Arundo donax</Emphasis>) in Experimental Wetlands Receiving Variable Loads of Industrial Stormwater

Two emergent macrophytes, Arundo donax and Phragmites australis, were established in experimental subsurface flow, gravel-based constructed wetlands (CWs) and challenged by untreated stormwater collected from the hard-pan and other surfaces of a dairy processing factory in south-west Victoria, Australia. The hydraulic loading rate was tested at two levels, sequentially, 3.75 and 7.5 cm day⁻¹. Some of the monitored variables were removed more efficiently by the planted beds in comparison to unplanted CWs (biochemical oxygen demand (BOD), total nitrogen (TN) and total phosphorus (TP); p < 0.007) but there was no significant difference between the A. donax and P. australis CWs in removal of BOD, suspended solids (SS) and TN (p > 0.007) at 3.75 cm day⁻¹ or SS and TN at 7.5 cm day⁻¹. At 3.75 cm day⁻¹, BOD, SS, TN and TP removal in the A. donax and P. australis CWs was 71%, 61%, 78% and 75% and 65%, 60%, 73% and 41%, respectively. Nutrient removal at 7.5 cm day⁻¹ in the A. donax and P. australis beds was 87%, 91%, 84% and 71% and 96%, 94%, 87% and 55%, respectively. As expected, the A. donax CWs produced considerably more biomass (10 ± 1.2 kg wet weight) than the P. australis CWs (2.7 ± 1.2 kg wet weight). This equates to approximately 107 and 36 tonnes ha⁻¹ year⁻¹ biomass (dry weight) for A. donax and P. australis, respectively (assuming 250 days of growing season and single-cut harvest). The performance similarity of the A. donax- and P. australis-planted CWs indicates that either may be used in HSSF wetlands treating dairy factory stormwater, although the planting of A. donax provides additional opportunities for secondary income streams through utilisation of the biomass produced.     

Utilization of created wetlands to upgrade small municipal wastewater treatment systems

Created wetlands offer a low cost, low maintenance, and practical alternative for upgrading secondary municipal wastewater treatment systems. The removal efficiencies, effects of seasonal temperature variations, and effects of increased loading rates on contaminant removal within such a system was studied by Auburn University researchers at a created wetland site in Hurtsboro, Alabama. The 0.16 ha system consisted of a two cell wetlands planted with cattails (Typha latifolia), bulrush (Scirpus validus), arrow duck potatoes (Sagitaria latifolis), burr reeds (Spargaminum eurycarpun), water pennywort (Hydrocotyl ranunculoides), and parrotfeather (Myriophyllum brasiliense). Testing occurred from January through September of 1988 at hydraulic loading rates of 169, 289, and 345 m³ ha^?1 d^?1. The monthly average total suspended solids influent: effluent mg L^?1 concentration ratio during the study period was 135:19 while the monthly average total BOD₅ influent: effluent mg L^?1 concentration ratio was 38:8. Once the system stabilized, the monthly average total BOD₅ effluent concentration remained essentially constant over the range of average BOD₅ loading rates employed in this study. Total Kjeldahl N removal was more effective at loading rates of 2.6 kg ha^?1 d^?1. The monthly average influent: effluent TKN mg L^?1 concentration ratio was 15:4.     

Controlled irrigation and drainage of a rice paddy field reduced global warming potential of its gas emissions

The effect of controlled drainage on methane (CH₄) and nitrous oxide (N₂O) emissions from a paddy field under controlled irrigation (CI) was investigated by controlling the sub-surface drainage percolation rate with a lysimeter. CI technology is one of the major water-saving irrigation methods for rice growing in China. Water percolation rates were adjusted to three values (2, 5, and 8 mm d^?1) in the study. On the one hand, the CH₄ emission flux and total CH₄ emission from paddy fields under CI decreased with the increase of percolation rates. Total CH₄ emissions during the growth stage of rice were 1.83, 1.16, and 1.05 g m^?2 in the 2, 5, and 8 mm d^?1 plots, respectively. On the other hand, the N₂O emission flux and total N₂O emissions from paddy fields under CI increased with the increase of percolation rates. Total N₂O emissions during the growth stage of rice were 0.304, 0.367, and 0.480 g m^?2 in the 2, 5, and 8 mm d^?1 plots, respectively. The seasonal carbon dioxide (CO₂) equivalent of CH₄ and N₂O emissions from paddy fields under CI was lowest in the 2 mm d^?1 plot (1364 kg CO₂ ha^?1). This value was 1.4% and 19.4% lower compared with that in the 5 and 8 mm d^?1 plots, respectively. The joint application of CI and controlled drainage may be an effective mitigation strategy for reducing the carbon dioxide equivalents of CH₄ and N₂O emissions from paddy fields.     

Treatment of Domestic Wastewater by Three Plant Species in Constructed Wetlands

Three common Appalachian plant species (Juncus effusus L., Scirpus validus L., and Typha latifolia L.) were planted into small-scale constructed wetlands receivingprimary treated wastewater. The experimental design includedtwo wetland gravel depths (45 and 60 cm) and five plantingtreatments (each species in monoculture, an equal mixture of the three species, and controls without vegetation), with two replicates per depth × planting combination. Inflow rates (19 L day^-1) and frequency (3 times day^-1) were designed to simulate full-scale constructed wetlands as currently used for domestic wastewater treatmentin West Virginia. Influent wastewater and the effluent from each wetland were sampled monthly for ten physical, chemical and biological parameters, and plant demographic measurements were made. After passing through these trough wetlands, the average of all treatments showed a 70% reduction in total suspended solids (TSS) and biochemical oxygen demand (BOD), 50 to 60% reduction in nitrogen (TKN), ammonia and phosphate, anda reduction of fecal coliforms by three orders of magnitude. Depth of gravel (45 or 60 cm) had little effect on wetland treatment ability, but did influence Typha and Scirpus growth patterns. Gravel alone provided significant wastewater treatment, but vegetation further improved many treatment efficiencies. Typha significantly out-performedJuncus and Scirpus both in growth and in effluent quality improvement. There was also some evidence that the species mixture out-performed species monocultures.Typhawas the superior competitor in mixtures, but a decline in Typha growth with distance from the influent pipe suggested that nutrients became limiting or toxicities may have developed.     

Short-term impact of a wildfire on net and gross N transformation rates

Wildfires often modify soil properties, including the N status and net N mineralization rates, but their impacts on gross N fluxes have been scarcely evaluated. We aimed to ascertain the immediate effects of a medium–high severity wildfire on soil N transformations. Net and gross N rates were analytically and numerically (FLUAZ) quantified in burned (BS) and unburned (US) topsoils from the temperate–humid region (NW Spain). Analytical and numerical solutions were significantly correlated for both gross N mineralization (m) (r ²?=?0.815; p?<?0.001) and gross nitrification (n) (r ²?=?0.950; p?<?0.001). In BS, all NH₄ ⁺-N fluxes (net m, gross m and gross NH₄ ⁺-N immobilization, ‘ia’) increased, while those of NO₃ ^?-N decreased (gross n and gross NO₃ ^?-N immobilization, ‘in’) or did not vary (net n). In US and BS, gross m (0.26–3.60 and 4.70–15.42 mg N kg^?1 day^?1, respectively) predominated over gross n (0.026–2.45 and 0.001–0.002 mg N kg^?1 day^?1, respectively), and the same was true for the net fluxes. Compared with the few available data on recently burned soils (m?=?8–55 mg N kg^?1 day^?1; n?=?0.50–1.83 mg N kg^?1 day^?1), our gross m and n rates were similar and very low, respectively; gross n showed that nitrifiers were active in US and also in BS, despite the 98 % reduction observed immediately after the fire. For gross fluxes, m increased more than ia suggesting an NH₄ ⁺-N accumulation, but there is no risk of NO₃ ^?-N leaching because n decreased more than in.     

Investigation of Nitrification and Denitrification in the Sediment of Wastewater Stabilization Ponds

Kinetic studies of nitrification and denitrification were carried out on reconstituted cores of sediments taken from wastewater stabilization ponds. This study aims to quantify the nitrification and denitrification in the sediment and to offer kinetic models to describe the processes. Sediment cores were collected, and laboratory studies were performed. The result showed that nitrification and denitrification processes are absent in the water column of stabilization ponds of the Bertrix (Belgium) wastewater treatment plant. On the contrary, nitrification and denitrification rates measured on 18 cores of sediment are, respectively, in the range of 0.12?C1.56 g N-NH ₄ ⁺ /m² day and 0.1?C1.2 g N/m² day. In order to describe nitrification and denitrification processes, two kinetic models were developed using the Monod standard equation.     

The effects of catena positions on greenhouse gas emissions along a seasonal wetland (dambo) transect in tropical Zimbabwe

Wetlands are major natural sources of greenhouse gases (GHGs). In central and southern Africa, one of the most extensive wetlands are dambos (seasonal wetlands) which occupy 20–25% of land area. However, there are very little data on GHG methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions from dambos, and this study presents the first estimates from dambos in Zimbabwe. The objective was to evaluate the effects of catena positions; upland, dambo mid-slope and dambo bottom, on GHG emissions along an undisturbed dambo transect. Methane emissions were ?0.3, 29.5 and ?1.3 mg m^?2 hr^?1, N₂O emission were 40.1, 3.9 and 5.5 µg m² hr^?1, while CO₂ emissions were 2648.9, 896.2 and 590.1 mg m^?2 hr^?1 for upland, mid-slope and bottom catena, respectively. Our results showed that uplands were important sources of N₂O and CO₂, and a sink for CH₄, while the dambo mid-slope position was a major source of CH₄, but a weak source of CO₂ and N₂O. Dambo bottom catena was weak source GHGs. Overall, dambos were major sources of CH₄ and weak sources of N₂O and CO₂.We concluded that, depending on catenal position, dambos can be major or minor sources of GHGs.