Numerical simulation of a fine-grained denitrification layer for removing septic system nitrate from shallow groundwater

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. It has been proposed that adding labile carbon sources to septic distribution fields could enhance heterotrophic denitrification and thus reduce nitrate concentrations in shallow groundwater. In this study, a numerical model which solves for variably saturated flow and reactive transport of multiple species is employed to investigate the performance of a drain field design that incorporates a fine-grained denitrification layer. The hydrogeological scenario simulated is an unconfined sand aquifer. The model results suggest that the denitrification layer, supplemented with labile organic carbon, may be an effective means to eliminate nitrogen loading to shallow groundwater. It is also shown that in noncalcareous aquifers, the denitrification reaction may provide sufficient buffering capacity to maintain near neutral pH conditions beneath and down gradient of the drain field. Leaching of excess dissolved organic carbon (DOC) from the denitrification layer is problematic, and causes an anaerobic plume to develop in simulations where the water table is less than 5-6 m below ground surface; this anaerobic plume may lead to other down gradient changes in groundwater quality. A drain field and denitrification layer of smaller dimensions is shown to be just as effective for reducing nitrate, but has the benefit of reducing the excess DOC leached from the layer. This configuration will minimize the impact of wastewater disposal in areas where the water table is as shallow as 3.5 m.     

Biodegradation of wastewater nitrogen compounds in fractures: laboratory tests and field observations

Throughout several coastal regions in the Mediterranean where rainfalls rarely exceed 650 mm per year municipal treated wastewater can be conveniently reused for soil irrigation. Where the coastal aquifer supplies large populations with freshwater in such area, an assessment of ground water quality around spreading sites is needed. In this study, the efficacy of natural filtration on nitrogen degradation in wastewater spreads on the soil covering the Salento (Southern Italy) fractured limestone was quantified by using laboratory tests and field measurements. In the laboratory, effluent from municipal wastewater treatment plants was filtered through a package of fractures made by several slabs of limestone. An analysis of wastewater constituent concentrations over time allowed the decay rates and constants for nitrogen transformation during natural filtration to be estimated in both aerated and non-aerated (i.e., saturated) soil fractures. A simulation code, based on biodegradation decay constants defined in the laboratory experiments, was then used to quantify the total inorganic nitrogen removal from wastewater injected in an aquifer in the Salento region (Nardò). Here the water sampled in two monitoring wells at 320 m and 500 m from the wastewater injection site and downgradient with respect to groundwater flow was used to verify the laboratory nitrification and denitrification rates.     

Denitrification in cypress swamp within the Atchafalaya River Basin, Louisiana

Nitrogen has been implicated as a major cause of hypoxia in shallow water along the Louisiana/Texas, USA coasts. Excess nitrogen (mainly nitrate) from Mississippi and Atchafalaya River drainage basins may drive the onset and duration of hypoxia in the northern Gulf of Mexico. Restoring and enhancing denitrification have been proposed to reduce and control coastal hypoxia and improve water quality in the Mississippi River Basin. Sediments were collected from six baldcypress restoration sites within the Atchafalaya River Basin, Louisiana, USA. The acetylene blockage technique was used to measure background and potential sediment denitrification rates. Denitrification fluxes were measured before nitrate addition (background rates) and after nitrate addition of 100mgNl(-1) (potential denitrification) at three seasonal temperatures. Background denitrification was low across all cypress swamp sites ranging from 0.9 to 8.8, 0.6 to 28.5 and 8.8 to 47.5g N evolved ha(-1)d(-1) at water/sediment column temperatures of 8, 22 and 30 degrees C, respectively. After nitrate addition, temperature had a significant effect on sediment denitrification potential. Maximum rates measured at 8, 22 and 30 degrees C were approximately 250-260, 550 and 970gNha(-1)d(-1), respectively. Most of the added nitrate in water columns, incubated at 8 degrees C, was removed after 65d compared to 32d and 17d at 22 and 30 degrees C, respectively. These results indicate cypress swamps have the potential to assimilate and process elevated levels of floodwater nitrate with denitrification being a major removal mechanism.     

Identification and reliability of microbial aerobic respiration and denitrification kinetics using a single-well push-pull field test

Methods to derive reaction rates of microbial processes are important since these processes are determining many chemical reactions influencing groundwater quality. Thereby, it is not only important to derive the parameters, but also to have a firm idea about the reliability with which these are determined. Analysis of residuals, sensitivity analyses and analysis of joint confidence intervals provide an interesting tool for this purpose. The method is illustrated in this paper using a push-pull test designed to derive aerobic respiration and denitrification. Therefore, a test solution containing dissolved oxygen and nitrate as reactive tracer and bromide as non-reactive tracer was injected in organic matter rich sediment. Afterwards, this test solution was extracted and water quality was monitored. ReacTrans, a finite-difference, axial-symmetric groundwater flow and solute transport model was developed to simulate the test and derive hydraulic, solute transport and chemical parameters. Aerobic respiration and denitrification were simulated with Michaelis-Menten kinetics. Maximum reaction rates (10.4 and 2.4 mmol/ld for aerobic respiration and denitrification respectively) and Michaelis constants (0.14 and 0.1 mmol/l for aerobic respiration and denitrification respectively) were determined. The reliability with which these parameters are derived is indicated by analysis of residuals, sensitivities and joint confidence intervals. This shows that the Michaelis-Menten parameters can be derived reliable with a push-pull test, whereas the test is insensitive to effective porosity and hydraulic conductivity. Because of the small scale of the test, longitudinal dispersivity was very small and therefore unidentifiable.     

Denitrification with acrylamide by pure culture of bacteria isolated from acrylonitrile-butadiene-styrene resin manufactured wastewater treatment system

Acrylamide is widely used in industrial applications as cement binder and solidification agent. Due to its carcinogenicity and toxicity, discharge of acrylamide to the natural water and soil systems may lead to an adverse environmental impact on water quality and thus endanger public health and welfare. This study attempts to isolate and identify the denitrifying bacteria, which utilize acrylamide as the substrate from the acrylonitrile-butadiene-styrene resin manufactured wastewater treatment system. The performance of the denitrifying bacteria for treating different initial acrylamide concentrations was also investigated under aerobic and anaerobic conditions. The test results indicated that the Pseudomonas stutzeri could remove acrylamide at concentrations below 440 mg/l under aerobic conditions. The acrylic acid and ammonia intermediates were used as carbon and nitrogen sources, respectively. However, P. stutzeri did not show the capability of metabolizing acrylonitrile under aerobic conditions. Furthermore, the P. stutzeri could utilize both acrylamide and acrylic acid in the presence of nitrate (denitrification) and acrylamide could be removed completely from the wastewater.     

New methods of nitrate removal from water

Nitrate contamination in groundwater resources originates mainly from the excessive use of fertilisers and uncontrolled land discharges of treated wastewater. This can cause potential health hazards to infants and pregnant women, thus limiting the direct use of the groundwater resources for the human consumption in several parts of the world, including India. The conventional processes used to eliminate nitrate from water are ion exchange, reverse osmosis and electro-dialysis. The utility of these processes has been limited due to their expensive operation and subsequent disposal problem of the generated nitrate waste brine. This paper presents a comprehensive account of the methods/techniques used for the removal of nitrate ion from water during the last 10 years with special reference to the biological denitrification and fate of the metals in decontamination processes.     

Correlations between in situ denitrification activity and nir-gene abundances in pristine and impacted prairie streams

Denitrification is a process that reduces nitrogen levels in headwaters and other streams. We compared nirS and nirK abundances with the absolute rate of denitrification, the longitudinal coefficient of denitrification (i.e., K_den, which represents optimal denitrification rates at given environmental conditions), and water quality in seven prairie streams to determine if nir-gene abundances explain denitrification activity. Previous work showed that absolute rates of denitrification correlate with nitrate levels; however, no correlation has been found for denitrification efficiency, which we hypothesise might be related to gene abundances. Water-column nitrate and soluble-reactive phosphorus levels significantly correlated with absolute rates of denitrification, but nir-gene abundances did not. However, nirS and nirK abundances significantly correlated with K_den, as well as phosphorus, although no correlation was found between K_den and nitrate. These data confirm that absolute denitrification rates are controlled by nitrate load, but intrinsic denitrification efficiency is linked to nirS and nirK gene abundances.     

Occurrence and behavior of wastewater indicators in the Santa Ana River and the underlying aquifers.

The occurrence and behavior of wastewater indicator compounds in the Santa Ana River (SAR) water and the underlying aquifer recharged by the SAR has been studied. The SAR contains a high proportion of tertiary treated wastewater effluents, up to 100% during summer and fall. The following water quality parameters were quantified: four specific wastewater indicator compounds, ethylene diaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), a naphthalene dicarboxylate (NDC) isomer, alkylphenol polyethoxy carboxylates (APECs), and selected haloacetic acids (HAAs), nitrate, dissolved oxygen (DO), DOC, total carbohydrate, and phenolic substances. Statistical analysis indicated that normal distribution was adequate to describe the probability distribution of the constituents in most cases. In the river, the concentrations of wastewater indicator compounds decreased as the fraction of storm runoff increased. EDTA and NDC were detected in a monitoring well near the river and in two production wells 1.8 and 2.7 km down gradient with little apparent attenuation. By contrast, NTA, APECs, bromochloro- and dibromoacetic acids appeared to be attenuated significantly during infiltration of river water and groundwater transport.     

Occurrence of steroid hormones and antibiotics in shallow groundwater impacted by livestock waste control facilities

Wastewater impoundments at concentrated animal feeding operations (CAFOs) represent a potential source of veterinary pharmaceuticals and steroid hormone contamination to shallow groundwater. This study investigates the occurrence of seventeen veterinary pharmaceuticals and thirteen steroid hormones and hormone metabolites in lagoons and adjacent groundwater at operating swine and beef cattle facilities. These sites were chosen because subsurface geology and previous monitoring of nitrate, ammonia and chloride levels in shallow ground water strongly indicated direct infiltration, and as such represent worst cases for ground water contamination by waste water. Pharmaceutical compounds detected in samples obtained from cattle facilities include sulfamerazine; sulfamethazine; erythromycin; monensin; tiamulin; and sulfathiazole. Lincomycin; ractopamine; sulfamethazine; sulfathiazole; erythromycin; tiamulin and sulfadimethoxine were detected in wastewater samples obtained from swine facilities. Steroid hormones were detected less frequently than veterinary pharmaceuticals in this study. Estrone, testosterone, 4-androstenedione, and androsterone were detected in wastewater impoundments at concentrations ranging from 30 to 3600ng/L, while only estrone and testosterone were detected in groundwater samples at concentrations up to 390ng/L. The co-occurrence of veterinary pharmaceutical and steroid hormone contamination in groundwater at these locations and the correlation between pharmaceutical occurrence in lagoon wastewater and hydraulically downgradient groundwater indicates that groundwater underlying some livestock wastewater impoundments is susceptible to contamination by veterinary pharmaceuticals and steroid hormones originating in wastewater lagoons.     

饮用水中硝酸盐的脱除

饮用水中硝酸盐氮的污染问题日趋严重,对人类的健康有多方面的危害.离子交换、反渗透、电渗、生物反硝化、化学和化学催化反硝化都可从水中脱除硝酸盐,但目前投入实用的只有离子交换、生物反硝化、反渗透三种工艺.这些脱硝方法各有优缺点.本文综述了饮用水脱硝的应用与研究的现状,并对其发展的趋势进行了简单的论述.     

太阳能光伏废水处理设施脱氮功能优化实例

某太阳能光伏企业的生产废水约4000 m³/d,废水中的主要污染物是少量的氟离子和大量硝态氮,原废水工艺中缺少去除硝态氮的功能。在原废水处理设施基础上,将生化处理工艺优化为“A/O反硝化”工艺,增加碳源投加设备。工程实践表明,工艺优化后,硝态氮得以有效去除,TN去除率达到95%,出水水质达到《电池工业污染物排放标准》(GB30484—2013)表2的间接标准。     

Direct measurement of denitrification activity in a Gulf coast freshwater marsh receiving diverted Mississippi River water

Wetland loss along the Louisiana Gulf coast and excessive nitrate loading into the Gulf of Mexico are interrelated environmental problems. Nitrate removal by soil denitrification activity was studied in a ponded freshwater marsh receiving diverted Mississippi River water for the purpose of reversing or slowing wetland loss. Labeled ¹⁵N-nitrate was applied at 3.8 g N m⁻² into four replicate study plots after removing above ground vegetation. Nitrogen gas (N₂) and nitrous oxide (N₂O) emissions from the plots were determined by isotope ratio mass spectrometry (IRMS). Nitrous oxide emissions were also compared with the results determined by gas chromatograph (GC). Results showed that it took 2 weeks to remove the added nitrate with N₂O emission occurring over a period of 4 d. The apparent denitrification dynamics were assumed to follow the Michaelis–Menten equation. The maximum denitrification rate and K_m value were determined as 12.6 mg N m ⁻² h⁻¹, and 6.5 mg N l⁻¹, respectively. Therefore the maximum capacity for nitrate removal by the marsh soil would be equivalent to 110 g N m⁻² yr⁻¹, with more than 30% of nitrogen gas evolved as N₂O. For typical nitrate concentrations in Mississippi River water of about 1 mg N l⁻¹, nitrate would be removed at a rate of 14.7 g N m⁻² yr⁻¹ with N₂O emission about 1.5%. A denitrification dynamic model showed that the efficiency of nitrate removal would largely depend on the water discharge rate into the ponded wetland. Higher discharge rate will result in less retention time for the water in the marsh where nitrate is denitrified.     

Biological nitrification/denitrification of high sodium nitrite (navy shipyard) wastewater

In the hydroblasting of ships' boiler tubes, a wastewater high in nitrite (as high as 1200 mg litre(-1)) is produced by the US Navy. This research has evaluated the use of a suspended-growth biological system to treat this wastewater by denitrification. Two biological treatment configurations were evaluated (direct denitrification versus nitrification/denitrification) with nitrification/denitrification producing better nitrite removal efficiencies (54 to 62% versus 40%, respectively). The introduction of metals (cadmium, chromium, lead, copper and iron) in concentrations typical for this wastewater did not inhibit the nitrite removal efficiencies. The influent metal concentrations ranged from 0.02 mg litre(-1) for cadmium to 22 mg litre(-1) for iron and the metal removal efficiencies ranged from 4.8% for cadmium to 50% for copper. Increasing sludge age resulted in improved nitrite removal efficiencies (52%, 57% and 74% for sludge ages of 4, 6 and 8 days, respectively). The resulting biokinetic constants were similar to those reported by others for lower influent concentrations of nitrite or nitrate (Ygs=0.02 mg/mg; Ygn=0.16 mg/mg; Yb=0.8 mg/mg; and b=0.006 h(-1)).     

低C/N比水产养殖废水生物脱氮实验研究

随着短程硝化-反硝化理论研究的发展,在低C/N比条件下,实现污水的生物脱氮处理已成为可能。为此,设计了水产养殖用水的三级生物膜短程硝化-反硝化处理工艺,并对该工艺在去除模拟水产养殖废水主要污染物的作用进行了初步研究。研究结果表明,在进水pH值7.5~8.5,温度为28~32℃,溶解氧为0.5~1 mg/L,游离氨浓度为5~10 mg/L的条件下,模拟废水的COD、NH4+-N和TN的平均去除率分别达到94.4%、91.6%和70.1%;并且低C/N比对出水氨氮NH4+-N的去除率影响不大,NO2--N的平均浓度控制在5.2 mg/L以下,低于鱼类的耐受浓度。表明该短程硝化-反硝化工艺设计,可用于低C/N比水产养殖废水主要污染物的生物处理,尤其是可消除NO2--N对水产养殖的潜在威胁,基本达到养鱼回用标准。     

Implications of non-equilibrium transport in heterogeneous reactive barrier systems: evidence from laboratory denitrification experiments

Organic substrates in reactive barrier systems are often heterogeneous material mixtures with relatively large contrasts in hydraulic conductivity and porosity over short distances. These short-range variations in material properties imply that preferential flow paths and diffusion between regions of higher and lower hydraulic conductivity may be important for treatment efficiency. This paper presents the results of a laboratory column experiment where denitrification is investigated using a heterogeneous reactive substrate (sawdust mixed with sewage sludge). Displacement experiments with a non-reactive solute at three different flow rates are used to estimate transport parameters using a dual porosity non-equilibrium model. Parameter estimation from breakthrough curves produced relatively consistent values for the fraction of the porosity consisting of mobile water (β) and the mass transfer coefficient (α), with average values of 0.27 and 0.42 d(-1), respectively. The column system removes >95% of the influent nitrate at low and medium flow, but only 50-75% of the influent nitrate at high flow, suggesting that denitrification kinetics and diffusive mass transfer rates are limiting the degree of treatment at lower hydraulic residence times. Reactive barrier systems containing dual porosity media must therefore consider mass transfer times in their design; this is often most easily accommodated by adjusting flowpath length.     

A review of denitrification in on-site wastewater treatment systems

The literature on denitrification and its relationship to on-site wastewater systems was reviewed. Denitrification is an important nitrogen removal mechanism in some alternative on-site wastewater systems. Nitrogen removal rates for conventional septic tank systems may vary from 0% to 35% but should be increased by pressure dosing. Denitrification rates for conventional on-site wastewater systems will depend upon loading rate, soil types and the height of the water table below the soil adsorption bed. Some alternative on-site wastewater systems may remove 70% to 80% of the nitrogen if a supplemental carbon source is used. Methanol and household wastes have been used as carbon sources. Crust formation in a soil adsorption bed may improve denitrification.     

Transport and reduction of nitrate in clayey till underneath forest and arable land

Transport and reduction of nitrate in a typically macroporous clayey till were examined at variable flow rate and nitrate flux. The experiments were carried out using saturated, large diameter (0.5 m), undisturbed soil columns (LUC), from a forest and nearby agricultural sites. Transport of nitrate was controlled by flow along the macropores (fractures and biopores) in the columns. Nitrate reduction (denitrification) determined under active flow mainly followed first order reactions with half-lives (t(1/2)) increasing with depth (1.5-3.5 m) from 7 to 35 days at the forest site and 1-7 h at the agricultural site. Nitrate reduction was likely due to microbial degradation of accumulated organic matter coupled with successive consumption of O2 and NO3- in the macropore water followed by reductive dissolution of Fe and Mn from minerals along the macropores. Concentrations of total organic carbon measured in soil samples were near identical at the two study sites and consequently not useful as indicator for the observed differences in nitrate reduction. Instead the high reduction rates at the agricultural site were positively correlated with elevated concentration of water-soluble organic carbon and nitrate-removing bacteria relative to the forest site. After high concentrations of water-soluble organic carbon in the columns from the agricultural site were leached they lost their elevated reduction rates, which, however, was successfully re-established by infiltration of new reactive organics represented by pesticides. Simulations using a calibrated discrete fracture matrix diffusion (DFMD) model could reasonably reproduce the denitrification and resulting flux of nitrate observed during variable flow rate from the columns.     

湖岸缓冲带反硝化作用的研究进展

反硝化作用是湖岸缓冲带去除硝酸盐的重要途径。湖岸缓冲带是联系陆地与湖泊生态系的纽带,不仅为许多动植物提供适宜生境,而且通过反硝化作用能去除地下水中的硝酸盐,提高湖泊水质。概述不同检测反硝化速率的方法,并对比各种方法的优点与缺点。阐述反硝化作用的影响因素：厌氧环境、有机碳、湖岸坡度、缓冲带坡度、pH与温度、硝酸盐浓度。介绍反硝化速率模型的研究开发状况。最后,提出了目前国内外反硝化研究中存在的不足及发展方向。     

Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers. I. Model formulation and performance

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. The objectives of this study are to develop and assess the performance of a mechanistic flow and reactive transport model which couples the most relevant physical, geochemical and biochemical processes involved in wastewater plume evolution in sandy aquifers. The numerical model solves for variably saturated groundwater flow and reactive transport of multiple carbon- and nitrogen-containing species in a three-dimensional porous medium. The reactive transport equations are solved using the Strang splitting method which is shown to be accurate for Monod and first- and second-order kinetic reactions, and two to four times more efficient than sequential iterative splitting. The reaction system is formulated as a fully kinetic chemistry problem, which allows for the use of several special-purpose ordinary differential equation (ODE) solvers. For reaction systems containing both fast and slow kinetic reactions, such as the combined nitrogen-carbon system, it is found that a specialized stiff explicit solver fails to obtain a solution. An implicit solver is more robust and its computational performance is improved by scaling of the fastest reaction rates. The model is used to simulate wastewater migration in a 1-m-long unsaturated column and the results show significant oxidation of dissolved organic carbon (DOC), the generation of nitrate by nitrification, and a slight decrease in pH.     

Inhibition of sulfide generation in a reclaimed wastewater pipe by nitrate dosage and denitrification kinetics.

Sulfide generation should be avoided during wastewater transportation. The efficiency of nitrate dosing for the inhibition of sulfide generation was evaluated during reclaimed wastewater transport with two nitrate doses, 2.5 and 5 mg/L nitrate-nitrogen (NO3-N). A calcium nitrate [Ca(NO3)2] solution was injected at the beginning of the 61-km-long gravity pipe, which is part of the Reclaimed Wastewater Reuse System of South Tenerife (Spain). During transportation, after dissolved oxygen depletion, a denitrification process took place. With the 5 mg/L NO3-N dose, nitrate was not completely removed at the end of the pipe, whereas with 2.5 mg/L NO3-N, a complete denitrification was achieved. Sulfide generation was completely inhibited with the 5 mg/L dose. However, with 2.5 mg/L, sulfide generation was not completely inhibited but delayed and minimized to a great extent. Denitrification was stoichiometrically limited by the availability in biodegradable matter. An empirical equation enables one to predict the nitrate concentration.