Removal of added nitrate in the single, binary, and ternary systems of cotton burr compost, zerovalent iron, and sediment: Implications for groundwater nitrate remediation using permeable reactive barriers

Recent research has shown that carbonaceous solid materials and zerovalent iron (Fe(0)) may potentially be used as media in permeable reactive barriers (PRBs) to degrade groundwater nitrate via heterotrophic denitrification in the solid carbon system, and via abiotic reduction and autotrophic denitrification in the Fe(0) system. Questions arise as whether the more expensive Fe(0) is more effective than the less expensive carbonaceous solid materials for groundwater nitrate remediation, and whether there is any synergistic effect of mixing the two different types of materials. We carried out batch tests to study the nature and rates of removal of added nitrate in the suspensions of single, binary, and ternary systems of cotton burr compost, Peerless Fe(0), and a sediment low in organic carbon. Cotton burr compost acted as both organic carbon source and supporting material for the growth of indigenous denitrifiers. Batch tests showed that cotton burr compost alone removed added nitrate at a greater rate than did Peerless Fe(0) alone on an equal mass basis with a pseudo-first-order rate constant k=0.0830+/-0.0031 h(-1) for cotton burr compost and a k=0.00223+/-0.00022 h(-1) for Peerless Fe(0); cotton burr compost also removed added nitrate at a faster rate than did cotton burr compost mixed with Peerless Fe(0) and/or the sediment. Furthermore, there was no substantial accumulation of ammonium ions in the cotton burr compost system, in contrast to the systems containing Peerless Fe(0) in which ammonium ions persisted as major products of nitrate reduction. It is concluded that cotton burr compost alone may be used as an excellent denitrification medium in a PRB for groundwater nitrate removal. Further study is needed to evaluate performance of its field applications.     

Effects of macrophytes and external carbon sources on nitrate removal from groundwater in constructed wetlands

Several microcosm wetlands unplanted and planted with five macrophytes (Phragmites australis, Commelina communis, Penniserum purpureum, Ipomoea aquatica, and Pistia stratiotes) were employed to remove nitrate from groundwater at a concentration of 21-47 mg NO3-N/l. In the absence of external carbon, nitrate removal rates ranged from 0.63 to 1.26 g NO3-N/m2/day for planted wetlands. Planted wetlands exhibited significantly greater nitrate removal than unplanted wetlands (P<0.01), indicating that macrophytes are essential to efficient nitrate removal. Additionally, a wetland planted with Penniserum showed consistently higher nitrate removal than those planted with the other four macrophytes, suggesting that macrophytes present species-specific nitrate removal efficiency possibly depending on their ability to produce carbon for denitrification. Although adding external carbon to the influent improved nitrate removal, a significant fraction of the added carbon was lost via microbial oxidation in the wetlands. Planting a wetland with macrophytes with high productivity may be an economic way for removing nitrate from groundwater. According to the harvest result, 4-11% of nitrogen removed by the planted wetland was due to vegetation uptake, and 89-96% was due to denitrification.     

Pressurized CO2/zero valent iron system for nitrate removal

A fluidized zero valent iron (ZVI) reactor pressurized by CO(2) gas for controlling pH was employed for nitrate reduction. The proposed CO(2) pressurized system potentially has advantages of using less CO(2) gas and reaching equilibrium pH faster than CO(2)-bubbled system. However, due to weak acid nature of carbonic acid, system pH gradually increased with increasing oxidation of ZVI and reduction of nitrate. As pH increased with progress of reaction, nitrate removal rate decreased continuously. The results indicate that nitrate removal efficiency increases with increasing initial ZVI dosage but reaches plateau at ZVI doses of higher than 8.25gl(-1), and initial nitrate concentration up to 100mg l(-1) as N has minimal impact on the removal efficiency. Unlike the fluidized system with pH control by strong acid reported in our pervious study, near 100% of nitrogen recovery was observed in the current process, indicating that nitrate reduction by ZVI with different pH controlled mechanisms will have different reaction routes.     

Effects of water characteristics on nitrate reduction by the Fe0/CO2 process

In this study, CO2 was bubbled into Fe0-contained solution to create an acidic environment favorable to reduction of aqueous nitrate under various water qualities. Results showed that nitrate of 30 mg l(-1) could be removed from solutions almost completely within 30 min under the conditions of 2 g Fe0 l(-1) and CO2 bubbling flow rate of 200 ml min(-1). It was observed from the Fe0/CO2 system that one mole of nitrate reduced by Fe0 led to the formation of 6.6 mol of ferrous ions. The removal of nitrate increases with increasing Fe0 dosage, however, the removal makes no difference as the Fe0 is applied at a relatively higher dosage. In the system with various water qualities, nitrate removal was inhibited significantly in the presence of humic acid. Calcium ions strongly retard nitrate removal, whereas chloride ions promote the reduction of nitrate in a significant way. Sodium ions impose only slight inhibitive effect on nitrate removal. Water molecule in the studied system can be of significance due to its competitive capability of electrons released from Fe0.     

Gaseous ammonia uptake in compost biofilters as related to compost water content

Sewage sludge and yard waste compost were used as biofilter materials and tested with respect to their capacity for removing ammonia from air at different water contents. Ammonia removal was measured in biofilters containing compost wetted to different moisture contents ranging from air dry to field capacity (maximum water holding capacity). Filters were operated for 15 days and subsequently analyzed for NH3/NH4+, NO2-, and NO3-. The measured nitrogen species concentration profiles inside the filters were used to calculate ammonia removal rates. The results showed that ammonia removal is strongly dependent on the water content in the filter material. At gravimetric water contents below 0.25 g H2O g solids(-1) for the yard waste compost and 0.5 g H2O g solids(-1) ammonia removal rates were very low but increased rapidly above these values. The sewage sludge compost filters yielded more than twice the ammonia removal rate observed for yard waste compost likely because of a high initial concentration of nitrifying bacteria originating from the wastewater treatment process and a high airwater interphase surface area that facilitates effective ammonia dissolution and transport to the biofilm.     

Electrochemical removal of nitrate using ZVI packed bed bipolar electrolytic cell

The present study investigates the performance of the zero valent iron (ZVI, Fe⁰) packed bed bipolar electrolytic cell for nitrate removal. The packing mixture consists of ZVI as electronically conducting material and silica sand as non-conducting material between main cathode and anode electrodes. In the continuous column experiments for the simulated groundwater (initial nitrate and electrical conductivity of about 30 mg L⁻¹ as N and 300 μS cm⁻¹, respectively), above 99% of nitrate was removed at the applied potential of 600 V with the main anode placed on the bottom of reactor. The influx nitrate was converted to ammonia (20% to maximum 60%) and nitrite (always less than 0.5 mg L⁻¹ as N in the effluent). The optimum packing ratio (v/v) of silica sand to ZVI was found to be 1:1-2:1. Magnetite was observed on the surface of the used ZVI as corrosion product. The reduction at the lower part of the reactor in acidic condition and adsorption at the upper part of the reactor in alkaline condition are the major mechanism of nitrate removal.     

Hydraulic constraints on the performance of a groundwater denitrification wall for nitrate removal from shallow groundwater

Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.     

Nitrous oxide flux from landfill leachate-sawdust nitrogenous compost

Composted nitrogenous waste has the potential to produce excessive amounts of nitrous oxide (N2O), a potent greenhouse gas that also contributes to stratospheric ozone depletion. In this laboratory study, sawdust was irrigated with varying amounts of landfill leachate with high NH4+-N content (3950 mg l(-1)). Physicochemical properties, including the amount of N2O produced, were monitored during the composting process over 28 days. A rapid decline in NH4+-N in the first 4 days and increasing NO3--N for 11 days was followed by lower but stabilized levels of available-N, even with repeated leachate irrigation. Less than 0.03% of the leachate-applied N was lost as N2O. Higher leachate applications as much as tripled N2O production, but this represented a lesser proportion overall of the total nitrogen. Addition of glucose to the composting process had no significant effect on N2O production. The derived sawdust-leachate compost supported healthy growth of Sesbania rostrata. It is concluded that compost can be produced from sawdust irrigated with landfill leachate without substantial emission of N2O, although excessive flux of N2O remains about high application rates over longer time periods.     

Water quality dynamics and hydrology in nitrate loaded riparian zones in the Netherlands

Riparian zones are known to function as buffers, reducing non-point source pollution from agricultural land to streams. In the Netherlands, riparian zones are subject to high nitrogen inputs. We combined hydrological, chemical and soil profile data with groundwater modelling to evaluate whether chronically N loaded riparian zones were still mitigating diffuse nitrate fluxes. Hydraulic parameters and water quality were monitored over 2 years in 50 piezometres in a forested and grassland riparian zone. Average nitrate loadings were high in the forested zone with 87 g NO(3)(-)-N m(-2) y(-1) and significantly lower in the grassland zone with 15 g NO(3)(-)-N m(-2) y(-1). Groundwater from a second aquifer diluted the nitrate loaded agricultural runoff. Biological N removal however occurred in both riparian zones, the grassland zone removed about 63% of the incoming nitrate load, whereas in the forested zone clear symptoms of saturation were visible and only 38% of the nitrate load was removed.     

Determination of the origin of groundwater nitrate at an air weapons range using the dual isotope approach

Nitrate is one of the most common contaminants in shallow groundwater, and many sources may contribute to the nitrate load within an aquifer. Groundwater nitrate plumes have been detected at several ammunition production sites. However, the presence of multiple potential sources and the lack of existing isotopic data concerning explosive degradation-induced nitrate constitute a limitation when it comes to linking both types of contaminants. On military training ranges, high nitrate concentrations in groundwater were reported for the first time as part of the hydrogeological characterization of the Cold Lake Air Weapons Range (CLAWR), Alberta, Canada. Explosives degradation is thought to be the main source of nitrate contamination at CLAWR, as no other major source is present. Isotopic analyses of N and O in nitrate were performed on groundwater samples from the unconfined and confined aquifers; the dual isotopic analysis approach was used in order to increase the chances of identifying the source of nitrate. The isotopic ratios for the groundwater samples with low nitrate concentration suggested a natural origin with a strong contribution of anthropogenic atmospheric NOx. For the samples with nitrate concentration above the expected background level the isotopic ratios did not correspond to any source documented in the literature. Dissolved RDX samples were degraded in the laboratory and results showed that all reproduced degradation processes released nitrate with a strong fractionation. Laboratory isotopic values for RDX-derived NO(3)(-) produced a trend of high delta(18)O-low delta(15)N to low delta(18)O-high delta(15)N, and groundwater samples with nitrate concentrations above the expected background level appeared along this trend. Our results thus point toward a characteristic field of isotopic ratios for nitrate being derived from the degradation of RDX.     

Characterization and reactivity assessment of organic substrates for sulphate-reducing bacteria in acid mine drainage treatment

Acid mine drainage (AMD), which contains high concentrations of sulphate and dissolved metals, is a serious environmental problem. It can be treated in situ by sulphate reducing bacteria (SRB), but effectiveness of the treatment process depends on the organic substrate chosen to supply the bacteria's carbon source. Six natural organic materials were characterized in order to investigate how well these promote sulphate reduction and metal precipitation by SRB. Maple wood chips, sphagnum peat moss, leaf compost, conifer compost, poultry manure and conifer sawdust were investigated in terms of their carbon (TOC, TIC, DOC) and nitrogen (TKN) content, as well as their easily available substances content (EAS). Single substrates, ethanol, a mixture of leaf compost (30% w/w), poultry manure (18% w/w), and maple wood chips (2% w/w), and the same mixture spiked with formaldehyde were then tested in a 70-day batch experiment to evaluate their performance in sulphate reduction and metal removal from synthetic AMD. Metal removal efficiency in batch reactors was as high as 100% for Fe, 99% for Mn, 99% for Cd, 99% for Ni, and 94% for Zn depending on reactive mixtures. Early metal removal (0-12d) was attributed to the precipitation of (oxy)hydroxides and carbonate minerals. The lowest metal and sulphate removal efficiency was found in the reactor containing poultry manure as the single carbon source despite its high DOC and EAS content. The mixture of organic materials was most effective in promoting sulphate reduction, followed by ethanol and maple wood chips, and single natural organic substrates generally showed low reactivity. Formaldehyde (0.015% (w/v)) provided only temporary bacterial inhibition. Although characterization of substrates on an individual basis provided insight on their chemical make-up, it did not give a clear indication of their ability to promote sulphate reduction and metal removal.     

Denitrification of nitrate wastewater using packed-bed columns.

Columnar packed-bed (PB) reactors with a specific surface area of 127 m2/m3 were investigated in this study for treating nitrate wastewater. This study demonstrated that a single-stage packed bed was able to achieve total nitrogen (TN) and chemical oxygen demand (COD) removal efficiencies higher than 83 and 75%, respectively. The highest achievable TN and COD removal rates were 47.2 g N/m2 x d and 158.0 g COD/m2 x d, respectively. The substrate removal rate in the PB column was found to follow half-order reaction kinetics, with a reaction coefficient, kappa, of 53.62 (mg/L)1/2/d. A dual-stage PB system was capable of achieving TN and COD removal efficiencies greater than 99 and 98%, respectively. Effluent TN and COD concentrations less than 6.5 mg NO3(-)-N/L and 50.0 mg COD/L, respectively, were obtained when the dual PB system was used.     

Visual determination of nitrite and nitrate in waters by color band formation method

A spot test for aqueous nitrate and nitrite for controlling nitrogen removal performance in small-scale wastewater treatment facilities is proposed. In this method, NO(2)(-) ion in water samples was allowed to react with sulfanilic acid and 1-naphthol to form an anionic azo dye. The resulting colored solution was introduced onto a mini column (similar to a gas detecting tube) packed with PVC particles coated with benzyl cetyl dimethyl ammonium chloride (BCDMA) and biphenyl. The NO(2)(-)-N concentration was determined visually by measuring the color band length (CBL) in the column. The CBL correlates linearly with nitrite concentration in the 4-20 mg-N l(-1) range. The concentration of nitrite+nitrate was determined after reduction to nitrite with zinc. The concentration of NO(3)(-)-N species was calculated by difference. This method was used to visually determine the concentrations of NO(2)(-)-N and (NO(2)(-)+NO(3)(-))-N in domestic wastewater samples with maximum suspended solid (SS) and chemical oxygen demand (COD) concentrations of 114 mg l(-1) and 73.9 mg l(-1), respectively.     

Formation of chloromethoxybenzaldehyde during composting of organic household waste

Standardized household waste was mixed with different litter amendments, straw, leaves, hardwood shavings, softwood shavings, paper, and sphagnum peat, resulting in six compost mixtures. In addition non-amended household waste was composted. Composting was done in small rotatable bins and compost samples were taken on a regular basis until day 590. Extraction and analysis of wet compost samples showed no evidence for the presence of chloroorganic compounds. Drying and re-wetting of compost samples, however, revealed that chloromethoxybenzaldehyde (CMBA) was formed in all composts at concentrations varying between 5.6 and 73.4 microg kg(-1) dry matter. CMBA was not present in the original materials. During composting, there was a clear positive relation between formation of CMBA and microbial activity, as indicated by C losses and temperature. Formation took place during the most intensive phase of composting when C losses were highest. Under anaerobic conditions, however, which prevailed initially in the non-amended compost, no CMBA was formed. Calculation of total amounts of CMBA in composts revealed that there was a small decrease during storage in the hardwood, peat, and softwood composts. However, all composts contained CMBA after 590 days. The mean concentration was 33.4 microg kg(-1) dry matter (s.d. = 21.9). Possible biocidal effects of composts when used in cultivation may be explainable by the presence of natural toxic compounds formed during composting.     

Evaluation of heavy metal contents in co-composts of poultry manure with barley wastes or chestnut burr/leaf litter

The presence of heavy metals in composts is a main cause of adverse effects on animal and human health, transmitted through the food chain from the soil, groundwater and plants. In this study, the contents of Zn, Cd, Pb and Cu present in co-composts of poultry manure (liquid or solid) with a co-composting material (barley wastes or chestnut burr/leaf litter) were assessed. A compost of solid manure was used as control because a compost cannot be obtained from the liquid manure. The original solid poultry manure showed a Zn content of 2134+/-75 mg/kg, exceeding the current legal limit in Spain of 1100 mg/kg. In the solid poultry manure co-compost with chestnut burr/leaf litter and barley wastes, Zn content decreased to 813+/-25 mg/kg and 883+/-37 mg/kg, respectively. The contents in heavy metals (Zn, Cd, Pb and Cu) of the co-composts were under the maximum limit permitted under the Spanish legislation, excepting for the Zn level in liquid poultry manure co-composted with chestnut burr/leaf litter.     

Accumulation of nitrite in denitrifying barriers when phosphate is limiting

Permeable in situ denitrifying barriers can remove nitrate from groundwater. Barriers may be constructed by filling an excavated area with a porous mixture of sand, fine gravel, and substrate or by the injection of a nonaqueous phase substrate into an aquifer. The substrate stimulates the development of a denitrifying microbial community by providing an electron donor. The objective of this study was to determine the ability of denitrifying barriers to function under low-phosphate conditions. Sand columns injected with a soybean oil emulsion were used as laboratory models of denitrifying barriers. When a natural groundwater containing 17 mg l(-1) nitrate-N and 0.009 mg l(-1) phosphate-P was pumped through the columns, only a small amount of nitrate was removed from the water and, in some effluent fractions, 52% to 88% of the influent nitrate had converted to nitrite. Nitrite also accumulated when the phosphate concentration of the groundwater was increased to 0.040 or 0.080 mg l(-1) phosphate-P. Only when a 0.160 mg l(-1) phosphate-P supplement was added to the groundwater was there a loss of nitrate without a large accumulation of nitrite. The addition of solid calcium phosphate or rock phosphate to the sand columns was found to provide adequate phosphate for denitrification in short-term studies. These studies point out the need to ensure that adequate phosphate is present in denitrifying barriers especially when such barriers are used beneath phosphate-binding soils.     

Biodegradation of heavy crude oil Maya using spent compost and sugar cane bagasse wastes

Experiments were carried out to evaluate the use of some agroindustrial wastes as supports in solid state cultures for the biodegradation of crude oil Maya in static column reactors over 15-20 days periods. Spent compost and cane bagasse wastes showed superior qualities over peat moss waste as support candidates with the advantage that they contain appreciable densities of autochthonous microorganisms in the order of 10(2) cfu g(-1). Mercuric chloride (2%) was able to completely inhibit growth of these microfloras. Biodegradation was enhanced in the presence of the IMP consortium and highest when microflora from cane bagasse only was the bioaugmentation partner (180.7 mg kg(-1) day(-1)). Combination of these waste materials (3:1 ratio, respectively) was observed to significantly biodegrade the crude oil by approximately 40% in 15 days from an initial concentration of 10,000 mg kg(-1) with a four order of magnitude increase in microbial density during this period. Spent compost and cane bagasse wastes are veritable solid support candidates for use in the biodegradation of crude oil polluted systems.     

Induction of PAH-catabolism in mushroom compost and its use in the biodegradation of soil-associated phenanthrene

This paper describes the induction of phenanthrene-catabolism within Phase II mushroom compost resulting from its incubation with (1) phenanthrene, and (2) PAH-contaminated soil. Respirometers measuring mineralization of freshly added 14C-9-phenanthere were used to evaluate induction of phenanthrene-catabolism. Where pure phenanthrene (spiked at a concentration of 400 mg kg(-1) wet wt.) was used to induce phenanthrene-catabolism in compost, induction was measurable, with maximal mineralization observed after 7 weeks phenanthrene-compost contact time. Where PAH-contaminated soil was used to induce phenanthrene-catabolism in un-induced compost, induction was observed after 5 weeks soil-compost contact time. Microcosm-scale amelioration of soil contaminated with 14C-phenanthrene (aged in soil for 516 days prior to incubation with compost) indicated that both induced (using pure phenanthrene) and uninduced Phase II mushroom composts were equally able to promote degradation of this soil-associated contaminant. After 111 days incubation time, 42.7 +/- 6.3% loss of soil-associated phenanthrene was observed in the induced-compost soil mixture, while 36.7 +/- 2.9% loss of soil-associated phenanthrene was observed in the uninduced-compost soil mixture. These results are notable as they indicate that while pre-induction of phenanthrene-catabolism within compost is possible, it does not significantly increase the extent of degradation when the compost is used to ameliorate phenanthrene-contaminated soil. Thus, compost could be used directly in the amelioration of contaminated land i.e. without pre-induction of catabolism.     

An in situ study of the effect of nitrate on the reduction of trichloroethylene by granular iron

The effect of nitrate on the reduction of TCE by commercial granular iron was investigated in column experiments designed to allow for the in situ monitoring of the iron surface film with Raman spectroscopy. Three column experiments were conducted; one with an influent solution of 100 mg/l nitrate+1.5 mg/l TCE, and two control columns, one saturated directly with 100 mg/l nitrate solution, the other pre-treated with Millipore water prior to the introduction of a 100 mg/l nitrate solution. In the presence of nitrate, TCE adsorbed onto the iron, but there was little TCE reduction to end-products ethene and ethane. The iron used (Connelly, GPM, Chicago) is a product typical of those used in permeable granular iron walls. The material is covered by an air-formed high-temperature oxidation film, consisting of an inner layer of Fe(3)O(4), and an outer, passive layer of Fe(2)O(3). In the control column pre-treated with Millipore water, the passive Fe(2)O(3) layer was removed upon contact with the water in a manner consistent with an autoreduction reaction. In the TCE+nitrate column and the direct nitrate saturation column, nitrate interfered with the removal of the passive layer and maintained conditions such that high valency protective corrosion species, including Fe(2)O(3) and FeOOH, were stable at the iron surface. The lack of TCE reduction is explained by the presence of these species, as they inhibit both mechanisms proposed for TCE reduction by iron, including catalytic hydrogenation, and direct electron transfer.     

Comparison of biotic and abiotic treatment approaches for co-mingled perchlorate, nitrate, and nitramine explosives in groundwater

Biological and abiotic approaches for treating co-mingled perchlorate, nitrate, and nitramine explosives in groundwater were compared in microcosm and column studies. In microcosms, microscale zero-valent iron (mZVI), nanoscale zero-valent iron (nZVI), and nickel catalyzed the reduction of RDX and HMX from initial concentrations of 9 and 1 mg/L, respectively, to below detection (0.02 mg/L), within 2 h. The mZVI and nZVI also degraded nitrate (3 mg/L) to below 0.4 mg/L, but none of the metal catalysts were observed to appreciably reduce perchlorate ( approximately 5 mg/L) in microcosms. Perchlorate losses were observed after approximately 2 months in columns of aquifer solids treated with mZVI, but this decline appears to be the result of biodegradation rather than abiotic reduction. An emulsified vegetable oil substrate was observed to effectively promote the biological reduction of nitrate, RDX and perchlorate in microcosms, and all four target contaminants in the flow-through columns. Nitrate and perchlorate were biodegraded most rapidly, followed by RDX and then HMX, although the rates of biological reduction for the nitramine explosives were appreciably slower than observed for mZVI or nickel. A model was developed to compare contaminant degradation mechanisms and rates between the biotic and abiotic treatments.