Measurement Of Ammonia Emissions Research Articles

Method for Measuring Ammonia Emissions from Poultry Houses

We present a measurement method for NH3 emissions from mechanically ventilated poultry buildings, which has been successfully used in a multistate, multidisciplinary research project to establish baseline values for the United States. To accurately determine building emission rate (ER, the product of pollutant concentration and exhaust airflow rate), accurate measurements must be made over representative periods and production phases. We present an innovative, low-cost, and accurate methodology that allows multiple buildings to be sampled in sequential order, provided that appropriate biosecurity measures are taken. Direct ventilation measurement is used in broiler houses, and in some layer operations, the number of fans is few enough that each fan can be individually calibrated in situ with a fan assessment numeration system device. For larger layer buildings with more than 15 fans, building ventilation is obtained from a tracer gas balance, utilizing CO2 generated by birds and feces. Other methods to determine building ventilation rate, which do not account for mechanical condition and degree of maintenance, both of which significantly affect actual fan capacity, introduce large errors. Twenty-eight portable monitoring units were fabricated and used for field acquisition of exhaust NH3 and CO2 concentrations and building static pressure. Ammonia is measured with redundant electrochemical sensors that are cyclically purged to eliminate errors caused by saturation from continuous exposure to air laden with NH3. The redundant NH3 unit minimizes missing data due to sensor failure.

Measurements of ammonia emissions from oak and pine forests and development of a non-industrial ammonia emissions inventory in texas

Estimates of non-industrial source ammonia emissions in Texas were developed through the use of published emission factors and activity data for those sources. A total of 64 non-industrial source emission sub-categories were addressed, each falling into one of seven major source categories: animal husbandry, fertilizer applications, on-road vehicles, non-road sources, municipal wastewater disposal, domestic sources, and natural soil and vegetation. Annual statewide ammonia emissions were initially estimated to be 921,000 metric tons, with greater than 50% originating from natural soil and vegetation. However, estimates for pine and oak forests were characterized as having a great deal of uncertainty. A series of field sampling events were conducted to determine ammonia fluxes from pine and oak forest floors in east Texas. Both dynamic and static chamber methods were used. The ammonia flux averaged 0.09 kg km −2 month −1 for pine forests and 0.13 kg km −2 month −1 for oak forests. These values are significantly lower than those previously measured and cited in the published literature. However, the ammonia fluxes measured in east Texas forests are reasonably consistent with those predicted using mechanistic models for evergreen pine and deciduous broadleaf forests in Alabama, California, Colorado, and Tennessee. Statewide annual ammonia emissions estimates, revised using the newly developed ammonia fluxes for oak and pine forests in Texas, dropped from 921,000 to 467,000 metric tons. The relative contribution of ammonia emissions from pine and oak forests dropped from 49% to less than 1%. Animal husbandry was predicted to be the dominant non-industrial source, accounting for approximately 77% of non-industrial source ammonia emissions.

Measurement of Ammonia Emission Following Surface Application of Urea Fertilizer from Paddy Fields

Measurement of Ammonia Emission Following Surface Application of Urea Fertilizer from Paddy Fields

Overview and assessment of techniques to measure ammonia emissions from animal houses: the case of the Netherlands

In order to comply with the ammonia (NH 3) emission reduction assigned to the Netherlands development of new measures are needed, which should be supported by fast and accurate measurements to arrive at new estimates of the NH 3 emission from each agricultural source. This paper gives an overview of the current methods used in the Netherlands to measure NH 3 emissions from animal houses, and provides alternative methods for some particular situations. For mechanically ventilated animal houses, passive flux samplers placed in the ventilation shafts of the animal house are presented as alternative to measure a larger number of animal houses (replicates) with the same housing system at a low price. For naturally ventilated animal houses, when mixing in the animal house is not good enough to allow measurements within the animal house (internal tracer gas ratio method), two measurement methods are discussed: the Gaussian plume dispersion model, which is usually not suitable for agricultural situations, and the flux frame method, which is not always applicable because of distortion of the flow around the building. Finally, for animal houses with outside yards for the animals, there are at this moment no methods available to measure the NH 3 emissions from these complex situations, although quick box methods (for the outside yards) and a combination of a backward Lagrangian stochastic model with open-path concentration measurements with a tunable diode laser (TDL), look promising.

Measuring ammonia emissions from land applied manure: an intercomparison of commonly used samplers and techniques

A number of techniques have been developed to quantify ammonia (NH 3) emissions following land application of manure or fertiliser. In this study, coefficients of variation were determined for three commonly used field techniques (mass balance integrated horizontal flux, wind tunnels and the equilibrium concentration technique) for measuring emissions from a range of manure types. Coefficients of variation (CV) for absorption flasks, passive flux samplers and passive diffusion samplers were 21, 10 and 14%, respectively. In comparative measurements, concentrations measured using passive flux samplers and absorption flasks did not differ significantly, but those measured using passive diffusion samplers were on average 1.8 times greater. The mass balance technique and wind tunnels gave broadly similar results in two out of four field tests. Overexposure of passive diffusion samplers for some sampling periods meant that estimation of cumulative NH 3 emission using the equilibrium concentration technique in the field tests could not be made. For cumulative NH 3 emissions, CVs were in the range of 23–52, 46–74 and 21–39% for the mass balance, wind tunnel and equilibrium concentration techniques, respectively. Lower CVs were associated with measurements following slurry compared with solid manure applications. Our conclusions from this study are that for the measurement of absolute emissions the mass balance technique is to be preferred, and for small-plot comparative measurements the wind tunnel system is preferred to the equilibrium concentration technique.

Simple use of the backwards Lagrangian stochastic dispersion technique for measuring ammonia emission from small field-plots

This study evaluates some simple procedures for measuring ammonia (NH 3) emission from wind speed, NH 3 concentration and an estimate of atmospheric stability using the backward Lagrangian stochastic (bLS) dispersion technique. The bLS technique, which requires measurements from only one height may prove useful when carrying out experiments simultaneously in a small field with several small plots. Commercial software exists that facilitates easy use of the bLS technique. Our study shows the bLS to be reliable using average wind speed at one height and average NH 3 concentration derived from horizontal flux measurements of NH 3 using passive flux samplers. The bLS technique underestimated the emission by 16–24% when applied at a measurement height of 0.5–1.55 m, but the inclination of bLS estimates versus integrated horizontal flux estimates was not different from 1 ( P < 0.01). The emission of NH 3 from urea applied to dry prairie soil was 29% of the nitrogen applied, emphasising the need of developing techniques for reducing NH 3 emission from zero tillage arable soils.

Effect of lowering dietary crude protein on nitrogen excretion, manure composition and ammonia emission from fattening pigs

The aim of this study was to evaluate the effects of three strategies of protein feeding (12%, 16% and 20%), supplying adequate levels of all essential amino acids, on finishing pig performance, nitrogen excretion and ammonia emission in the laboratory and in the field. The animals (15 castrated males) were housed individually in metabolism cages and were fed one of the three diets. The experiment lasted 21 days and urine and faeces were collected separately. Samples of each type of manure (urine alone, fresh slurry and 18-day slurry) were placed for 6 days in a laboratory pilot scale system designed to measure ammonia emission. In the field, measurements of ammonia volatilization were made during storage and surface application of slurry. Lowering dietary crude protein, while maintaining similar levels of essential amino acids, reduced total amount of manure, because of a lower water consumption, without any negative effect on animal performance. Total ammoniacal nitrogen content in the slurry was reduced from 4.32 g N kg −1 to 3.13 and 1.92 g N kg −1 when dietary CP decreased from 20% to 16% and 12%, respectively (the corresponding figures for total nitrogen content were 5.48, 4.30 and 3.05 g N kg −1, respectively). The pH of slurry decreased by 1.3 units when dietary crude protein decreased from 20% to 12%. These changes in slurry composition and pH resulted in a reduction of ammonia emissions at each stage of manure management. Over the whole process of the slurry, ammonia emission was decreased by 63% when dietary protein decreased from 20% to 12%.

In situ measurement of ammonia and greenhouse gas emissions from broiler houses in France

An experimental technique was developed for measuring gaseous emissions including ammonia (NH 3), nitrous oxide (N 2O) and methane (CH 4) from broiler houses. This technique included the monitoring of the air flow rate and the gaseous concentrations. NH 3 was determined using acid trap while N 2O and CH 4 were determined continuously by infrared gas analyser and sequentially by gas chromatography. Moreover, N 2O and CH 4 emissions were monitored above the litter using closed flux chambers at the end of the experiment. No emissions of N 2O and CH 4 were observed neither during the growth of the broiler nor above the litter at the end of the experiment. Ammonia concentration varied between 0.8 and 32 ppm in the building. Total ammonia emissions were estimated to 5.74 gN animal −1 during this experiment. According to this result, ammonia emissions from broiler houses could be estimated to 5.3 kt of N per year in France.

Measuring ammonia emission rates from livestock buildings and manure stores?part 1: development and validation of external tracer ratio, internal tracer ratio and passive flux sampling methods

Abstract There is a need for robust methods for measuring ammonia emission rates from livestock buildings and manure stores, to guide efforts to abate emissions from livestock farming. This paper reports research to develop and validate three candidate measurement techniques: (a) An external tracer ratio method, where concentrations of ammonia and sulphur hexafluoride are measured downwind of an animal house or manure store. (b) An internal tracer ratio (ITR) method, suited to animal housings, where concentrations of ammonia and sulphur hexafluoride are measured just before air leaves the building. (c) A flux sampler method, which uses sets of passive flux sampling devices positioned so as to intersect all significant flows of air out of an animal house or manure store source. All three of the measurement techniques were validated at a building section simulating a naturally ventilated (space-boarded) cattle house, with the external tracer ratio method also being validated at a simulated slurry store. In the validation tests on the external tracer ratio method the derived ammonia emission rates from the slurry store and cattle house validation studies were 25% below and 43% above the measured release rate, respectively. These biases were shown by t-tests to be statistically highly significant, but no clear explanation could be found for the different signs and magnitudes in the two cases. For the ITR method, recovery rates of 78% and 101% of released NH3 were achieved, with low and high release rates, respectively. Validation tests conducted on the flux samplers gave an average of 66% (standard deviation 2.9%) ammonia recovery. The cause of this non-ideal level of recovery has not yet been identified. However, given the low standard deviation, it was concluded that these samplers could be used to measure ammonia emission rates from real farm buildings, provided that a correction factor for the non-ideal recovery was applied.

Measuring ammonia emission rates from livestock buildings and manure stores?part 2: Comparative demonstrations of three methods on the farm

Abstract Comparative demonstrations of three methods (flux sampling, external tracer ratio, and internal tracer ratio), were mounted in four real farm situations. A flux sampling method was demonstrated at a commercial dairy cow house (slurry-based), at a commercial piggery (straw-based), at a full-scale above-ground cylindrical slurry store (dairy cow slurry) and a full-scale earth-bank lagoon (pig slurry). An external tracer ratio method was demonstrated, in parallel with the flux sampling method, at the dairy cow house and at the above-ground slurry store. An internal tracer ratio method was demonstrated at the dairy cow house only. At the dairy cow house, the corrected emission rates from the flux sampling method and from the external tracer ratio method agreed to within the estimated experimental range, while the emission rate from the internal tracer ratio method was significantly lower. The overall conclusion of the study is that all three methods can have a useful role, the choice of which to deploy depending on the particular measurements needed in each case. The paper includes a table (No. 7) which indicates which is the recommended method when each of various considerations has top priority.

Ammonia emissions after application of human urine to a clay soil for barley growth

Important amounts of plant nutrients excreted by humans are found in human urine. This provides the motivation for separating urine and recycling it, as fertiliser, back to agricultural land for food or fodder production. There are some housing estates in Sweden, both blocks of flats and separate houses, in which urine-separating toilets have been installed. The urine is stored in covered basins and spread on agricultural land. The objective of the present study was to evaluate the influence of application rate, application techniques and time on NH3 emissions after application of source-separated human urine. Human urine was spread at different application rates (10, 20 and 60 Mg ha–1) before sowing (year 1 to 3) and when the barley crop was 20-30 cm high (year 2). Urine was spread with a plot spreader using two band-application techniques (with bands 0.25 m apart): trailing hoses and trailing shoes. In spring, band-spread urine with trailing hoses was incorporated with a harrow four hours after application. The four (year 1 and 3) or six (year 2) treatments were organised into a randomised block design with three replicates. An equilibrium concentration method was used for measuring ammonia emissions directly after application. After spring application with trailing hoses and harrowing after four hours, the nitrogen [N] loss as ammonia [NH3], average over 3 years, was 5% of the applied N, irrespective of the application rate. The largest loss (10% of the applied N) was measured after application of 60 t of urine per hectare in spring. Hardly any NH3 loss occurred after incorporation with a harrow, with the exception of the highest application rate. Loss of NH3 was very low, close to 1% of the applied N, when the urine was incorporated directly into the soil in spring by band application with trailing shoes. Virtually no emissions were detected when the urine was applied to the growing crop, neither by trailing hoses nor by trailing shoes. This study shows that it is possible to apply human urine on bare soil or in growing barley crop with very low losses of N as NH3. Together with careful handling and the use of covered storage, nutrients in human urine could be recycled from households to agricultural land with low NH3 emissions.

Measurement of ammonia emissions using various techniques in a comparative tunnel study

A field-based intercomparison study for ammonia measurements was conducted using seven analytical methods. It included sulphuric acid impinger, citric acid denuder, differential optical absorption spectroscopy (DOAS), Fourier transform infrared spectroscopy (FTIR), photoacoustic spectroscopy (PAS), and continuous aqueous extraction followed by measurement of conductivity (Airrmonia). Measurements were done at the entrance and the exit of the Gubrist highway tunnel near Zurich, Switzerland. For DOAS, FTIR, PAS and Airrmonia, 24 hour means were calculated based on a time resolution of 10 minutes. At the tunnel exit, all 24 hour averages were within 13%, and the continuous data of the time-resolved methods agreed well. At the tunnel entrance, a slightly reduced method comparison included four methods, and daily mean values agreed within 23%. Ammonia emission factors, based on 4 weeks of continuous measurements with the Airrmonia, were 31 ± 4 mg km-1 for light-duty vehicles and 14 ± 7 mg km-1 for heavy-duty vehicles.

Improved Passive Flux Samplers for Measuring Ammonia Emissions from Animal Houses, Part 1: Basic Principles

At present, precise, expensive and laborious methods with a high resolution in time are needed, to determine ammonia emission rates from animal houses. The high costs for equipment, maintenance and labour limit the number of sites that can be measured. This study examines a new, simpler concept for carrying out these measurements. The basis of the concept is a passive ammonia flux sampler, which improves on Ferm's original design by having a central orifice (thus halving the number of tubes per sampler and hence halving the number of analyses needed) and also by having a greatly increased ammonia-capturing capacity. Sets of the samplers can be placed in ventilation ducts, or in openings of a naturally ventilated animal house, to give a direct and simple measure of ammonia flux and hence of overall ammonia emission rate. Calibration of any sampler design based on this new concept is quick and easy: this is important because of the huge variation in designs of systems for ventilation in animal houses. Moreover, no power source is needed to perform cheap and accurate time-integrated ammonia emission measurements. The high ammonia-capturing capacity, allowing a long-term exposure, of at least 1 week, is achieved by using a glass-fibre insert to hold a large charge of a mineral acid. First results from validation tests with the simplest possible sampler design on the farm were promising. The flux samplers proved easy to handle and deploy, and gave results which agreed closely with those from a reference method based on fan-wheel anemometers to give ventilation rate and on the continuous measurement of ammonia concentration by chemiluminescence. In a companion paper, the performance of different forms of the flux sampler as a function of angle of incidence of airflow is reported.

Improved Passive Flux Samplers for measuring Ammonia Emissions from Animal Houses, Part 2: Performance of Different Types of Sampler as a Function of Angle of Incidence of Airflow

The basic principle of the improved passive flux sampler has been reported in a companion paper. In the present paper, the performance of different types of the sampler as a function of angle of incidence of airflow was studied in a wind tunnel. The following types of sampler were examined: (a) a basic straight sampler (220 mm long); (b) straight samplers (220 mm long) with openings that had been modified in one of several different ways, all aimed at reducing turbulence around the openings and thus achieving nearer-to-perfect performance as angle of incidence changed; (c) a cranked sampler, designed to reduce the distance between the planes of the two openings to 80 mm (type useful in confined spaces); and (d) two types of sampler (dual curved and angular) for both of which the two openings are in the same plane, albeit offset transversely (useful for space-boarded buildings). The dependency on the angle of incidence of the different sampler types was compared by measuring the pressure drop across the sampler, and hence calculating a normalised air velocity through each sampler's precision central orifice for each angle of incidence. Ideal performance would yield a cosine relationship between air velocity through the sampler and angle of incidence. Any deviations from such a cosine relationship imply a potential under- or over-estimation of ammonia flux rate, the magnitude of which depends on the relative persistence of the relevant angle of incidence during the exposure period of the sampler. The closest approach to ideal performance as angle of incidence changes was achieved by the straight sampler modified by the addition of solid tear-drop shapes around both its openings. However, the overall length of the straight sampler means that it is not suitable for all sampling scenarios. For confined spaces e.g. complex geometries in the roof ridges of livestock buildings, the cranked sampler is more suitable, accepting that it does not so closely approach ideal performance as the angle of incidence changes. Likewise, the dual curved and angular samplers are more suitable for ammonia fluxes through space-boarded walls or similar: in both the above scenarios, non-ideal performance is mitigated by the fact that, with defined air pathways in the roof ridge, and with a space-boarded wall of finite thickness, respectively, the range of angles of incidence likely to be seen by the cranked, dual curved and angular samplers is generally limited to angles near to 0 or 180°.

Establishing the link between ammonia emission control and measurements of reduced nitrogen concentrations and deposition.

In the context of international efforts to reduce the impacts of atmospheric NH3 and NH4+ (collectively, NHx). it is important to establish the link between NH3 emissions and monitoring of NHx concentrations and deposition. This is equally relevant to situations where NH3 emissions changes are certain (e.g. due to changed source sector activity), as to cases where NH3 abatement technologies have been implemented. Correct interpretation of adequate atmospheric measurements is essential, since monitoring data provide the only means to evaluate trends in regional NH3 emissions. These issues have been reviewed using available measurements and modelling from nine countries. In addition to historic datasets, the analysis here considers countries where NH3 source sector activity changed (both increases and decreases) and countries where NH3 abatement policies have been implemented. In The Netherlands an 'ammonia gap' was identified between the expected reduction and results of monitoring, and was attributed initially to ineffectiveness of the abatement measures. The analysis here for a range of countries shows that atmospheric interactions complicate the expected changes, particularly since SO2 emissions have decreased at the same time, while at many sites the few years of available data show substantial inter-annual variation. It is concluded that networks need to be established that speciate between NH3 and aerosol NH4+, in addition to providing wet deposition, and sample at sufficient sites for robust regional estimates to be established. Such measurements will be essential to monitor compliance of the international agreements on NH3 emission abatement.

Measurement of ammonia volatilization from a field, in upland Japan, spread with cattle slurry

Ammonia volatilization from livestock manure is one of the most important pathways of nitrogen loss from agricultural cultivated fields. In this paper, we report the measurement of ammonia emission from cattle slurry manure applied to upland in Miyazaki, Japan. It has been determined that after the cattle slurry was sprayed on the upland surface, the emission flux of the first day was 110 μg N ha −1 s −1. The loss of NH 4 +–N in the applied slurry was 60% after 5 days following the spraying of cattle slurry.

SOME POTENTIAL PROBLEMS FOR MEASURING AMMONIA EMISSIONS FROM FARM STRUCTURES

The ability to accurately measure ammonia emissions from farm buildings is an important issue both forestablishing emissions regulations and for effective evaluation of mitigation techniques. Most techniques for measuringemissions rely on subsampling within a space. This study examines the influence of subsampling under a variety ofconditions on estimated ammonia recoveries. Tests were made using a large environmental chamber with controlled releasesof ammonia from a gas cylinder from one of two positions within the chamber. Ammonia concentrations were measured incontinuous air samples from either a single exhaust duct or a sampling port in the exhaust plenum where exhaust gases werewell mixed. The chamber temperature was maintained at 22.2. C with an airflow of 10.5 air exchanges per hour. Formeasurements made in the exhaust duct, ammonia recoveries were 217% when the release position and the sampling ductwere aligned in terms of chamber airflow, and were 52% when the positions were not aligned. Placing a wooden barrierbetween aligned release and measurement positions only reduced ammonia recoveries to 173%. In contrast, using anoscillating fan to disperse the ammonia release reduced ammonia recoveries to 78%. When measurements were made in theplenum, recoveries were essentially 100%, and placement of a continuously wetted barrier between the release position andthe exhaust ducts did not influence recoveries. It is suggested that measurement of ammonia emissions be restricted to farmstructures with welldefined airflows and a limited number of exhaust openings, and that the most accurate method forestimating ammonia emission rates would be to collect and mix all of the exhaust streams from a structure prior tocontinuously measuring ammonia concentration and airflow.

Effect of Adding Alum or Zeolite to Dairy Slurry on Ammonia Volatilization and Chemical Composition

Development of cost-effective amendments for treating dairy slurry has become a critical problem as the number of cows on farms continues to grow and the acreage available for manure spreading continues to shrink. To determine effectiveness and optimal rates of addition of either alum or zeolite to dairy slurry, we measured ammonia emissions and resulting chemical changes in the slurry in response to the addition of amendments at 0.4, 1.0, 2.5, and 6.25% by weight. Ammonia volatilization over 96h was measured with six small wind tunnels with gas scrubbing bottles at the inlets and outlets. Manure samples from the start and end of trials were analyzed for total nitrogen and phosphorus, and were extracted with 0.01 M CaCl2, 1.0 M KCl, and water with the extracts analyzed for ammonium nitrogen, phosphorous, aluminum, and pH. The addition of 6.25% zeolite or 2.5% alum to dairy slurry reduced ammonia emissions by nearly 50 and 60%, respectively. Alum treatment retained ammonia by reducing the slurry pH to 5 or less. In contrast, zeolite, being a cation exchange medium, adsorbed ammonium and reduced dissolved ammonia gas. In addition, alum essentially eliminated soluble phosphorous. Zeolite also reduced soluble phosphorous by over half, but the mechanism for this reduction is unclear. Alum must be carefully added to slurry to avoid effervescence and excess additions, which can increase soluble aluminum in the slurry. The use of alum or zeolites as on-farm amendment to dairy slurry offers the potential for reducing ammonia emissions and soluble phosphorus in dairy slurry.

Field evaluation of the equilibrium concentration technique (JTI method) for measuring ammonia emission from land spread manure or fertiliser

Three experiments were conducted in which intercomparisons were made between the equilibrium concentration technique, developed at JTI, Sweden, and the integrated horizontal flux technique for measuring ammonia emissions following applications of urea fertiliser, cattle slurry and solid pig manure to land. Mean square prediction error analysis was used to compare the emission rates measured by the two techniques. There were no significant differences between the measurement techniques, although there was some evidence that emission rates were overestimated by the equilibrium concentration method relative to the integrated horizontal flux technique at higher emission rates (>400 g.N ha −1 h −1). The equilibrium concentration method provides a practical and relatively inexpensive technique for measuring emissions under ambient conditions from small plots but good sampler preparation, adequate replication of emission measurements and appropriate choice of duration of sampling periods are necessities for obtaining reliable results.

Measurement and analysis of atmospheric ammonia emissions from anaerobic lagoons

Ammonia-nitrogen flux (NH 3-N=(14/17)NH 3) was determined from six anaerobic swine waste storage and treatment lagoons (primary, secondary, and tertiary) using the dynamic chamber system. Measurements occurred during the fall of 1998 through the early spring of 1999, and each lagoon was examined for approximately one week. Analysis of flux variation was made with respect to lagoon surface water temperature (∼15 cm below the surface), lagoon water pH, total aqueous phase NH x (=NH 3+NH 4 +) concentration, and total Kjeldahl nitrogen (TKN). Average lagoon temperatures (across all six lagoons) ranged from approximately 10.3 to 23.3 °C. The pH ranged in value from 6.8 to 8.1. Aqueous NH x concentration ranged from 37 to 909 mg N l −1, and TKN varied from 87 to 950 mg N l −1. Fluxes were the largest at the primary lagoon in Kenansville, NC (March 1999) with an average value of 120.3 μg N m −2 min −1, and smallest at the tertiary lagoon in Rocky Mount, NC (November 1998) at 40.7 μg N m −2 min −1. Emission rates were found to be correlated with both surface lagoon water temperature and aqueous NH x concentration. The NH 3-N flux may be modeled as ln(NH 3-N flux)=1.0788+0.0406 T L+0.0015([NH x ]) ( R 2=0.74), where NH 3-N flux is the ammonia flux from the lagoon surface in μg N m −2 min −1, T L is the lagoon surface water temperature in °C, and [NH x ] is the total ammonia-nitrogen concentration in mg N l −1.