Disinfection of advanced wastewater treatment effluent by chlorine, chlorine dioxide and ozone: Experiments using seeded poliovirus

Chlorine, chlorine dioxide and ozone were tested as chemical disinfectants against seeded poliovirus and naturally-occurring fecal coliform organisms in wastewater effluent that had received secondary treatment followed by bench scale advanced wastewater treatment (AWT). The AWT sequence consisted of chemical treatment with lime or alum followed by mixed media filtration. The resulting effluent had low suspended solids concentrations but chemical oxygen demand and nitrogen concentrations only slightly lower than those of secondary effluent. Lime treatment produced greater reductions than alum treatment in virus numbers, but not in fecal coliform organisms.

With both chlorine and chlorine dioxide, in order to reduce seeded poliovirus to less than detectable levels, it was necessary to use doses comparable to those required to disinfect secondary effluent. The required contact times of 30–60 min were also comparable. Utilized ozone doses of 2–4 mg l⁻¹ were required to reduce seeded poliovirus to less than detectable levels in AWT effluent. Naturally-occurring fecal coliform organisms were unaffected at these ozone doses, but were inactivated at higher doses. Because they were more resistant than seeded poliovirus to ozone, fecal coliform organisms show promise as indicators for ozone disinfection.     

Degradation of sulphur containing s-triazines during water chlorination

The reactions of four sulphur containing s-triazines (prometryne, terbutryne, ametryne and desmetryne) with hypochlorous acid (HClO) and chlorine dioxide (ClO₂) have been investigated using an 11 ppm/3 ppm oxidant/herbicide ratio. The main objective of the study was the identification of by-products. Additionally, to study the effect of oxidant concentration on the reaction rate, two more oxidant/herbicide ratios (3 ppm/3 ppb and 11 ppb/3 ppb) have been investigated only for prometryne. Oxidation reactions were monitored by high performance liquid chromatography (HPLC), while, the identification of by-products was initially carried out by low resolution HPLC-mass spectrometry (HPLC-MS) and confirmed by accurate mass measurement. Under the experimental conditions (T = 20°C, pH = 8, reaction TIME = 48 h), the results indicate that all the investigated triazines react in the same way with each oxidant. The reactions with HClO occur much faster than those with ClO₂ and give rise to three identified oxidation by-products: the sulfoxide, the sulfone and the sulfone's hydrolysis product. The reactions with ClO₂, instead, give rise to a sole oxidation by-product: the sulfoxide. With both oxidants, as expected, the lower the oxidant concentration the slower the oxidation rate.

Based on the obtained results, a general pathway for the oxidation of sulphur containing s-triazines is proposed.     

Cinetiques et mecanismes de la degradation de la creatinine sous l'action de l'hypochloriteKinetics and mechanisms of hypochlorite oxidation of creatinine

An area of substantial interest in current research on chlorination is the formation, stability and nature of chloramines formed by the interaction of chlorine with nitrogen organic compounds of biological origin in natural water or swimming pool water. It is desirable to be able to predict the lifetime of these harmful compounds under various conditions. The research described here constitutes an effort to gather important baseline data regarding the rate of formation of creatinine chloramines, the stabilities of these products and their identities.

Some researchers have studied the effect of the presence of chlorinated creatinine compounds in swimming pool water. Lomas (1967), showed that the presence of urine in water allowed the formation of compounds which reacted with DPD like dichloramine. He reported that the presence of this apparent dichloramine could be due to a chlorine derivative of creatine and creatinine derived from urine. Hamence (1980) confirmed this work and found that urine and particularly creatinine were responsible for the apparent nitrogen trichloride. As a result of this work it was concluded that the DPD-fast titrimetric method of analysis did not determine nitrogen trichloride but other chlorine compounds, particularly those of chlorinated creatinine and creatine. We found it interesting to examine in this study, for a range of hypochlorite creatinine ratios and pHs, the kinetics and mechanisms of formation and decomposition of N-chlorocreatinines.

The hypochlorite oxidation of creatinine in aqueous solution has been investigated in the dark. The following of creatinine and chloramines concentrations by the DPD-fast titrimetric method and by their u.v. spectra confirmed Lomas' and Hamence's works. However we observed dichloramine formation (Fig. 4) when the molar ratio of hypochlorite and creatinine was sufficient to decompose all chlorinated creatinine forms. The creatinine determination (HPLC method) suggested that N-chlorocreatinines were formed rapidly at an initial stage. Then they were decomposed by an apparent first order reaction at pH 8. With equimolar (1:1 mmol) amounts of hypochlorite and creatinine at pH 8, it appeared that N-chlorocreatinines were decomposed by hydrolysis to regenerate creatinine. We observed then the formation of creatine, 1-methylhydantoin, chlorocreatinines and NH₂Cl (Fig. 3). When the molar ratio was greater, the N-chlorocreatinines decomposed completely to form carbon dioxide, chlorite ion and mineral chloramines (see Table 1).

The reaction in the initial stage should be considered as an electrophile substitution followed slowly by hydrolysis when pH remained around 8 (Scheme 2). If the addition of hypochlorite affects the amine group of the molecule, 1-methylhydantoin is produced (Scheme 3) with NH₂Cl. Reaction yield was about 10% of initial creatinine.

In acid aqueous solution, with a molar ratio of 3, we also obtained trichlorocreatinine. This reaction is due to the various form of creatinine after addition of proton on amino of N-H groups of the molecule. In these conditions N-chlororcreatinines remained stable in aqueous solution for many days. However in the presence of free chlorine, we observed the production of carbon dioxide and mineral chloramines. After 4 days the residual concentration of N-chlorocreatinines was half the initial value.

It appears that N-chlorocreatinines formed during the chlorination of natural or swimming pool water were relatively stable, leading to the increase of the combined chlorine level. This stability was a function of the molar ratio of hypochlorite and creatinine, and pH. However, since most of the difference types of water had a pH in the range of 6–9, there would be little effect of pH at ambient temperature.     

Chlorine dioxide disinfection of drinking water—An evaluation of a treatment plant

A treatment plant for lake Kinneret water, comprising treatment by two filtration steps, flocculation and disinfection with chlorine dioxide, was studied with a view to evaluating the effect of ClO₂ disinfection on drinking water quality and determining the optimal mode of operation for the treatment plant. Four modes of operation were studied and the optimal mode was defined as that in which the flocculant (aluminium sulphate) was introduced before the first, and ClO₂ after the second, filtration.

The finished water contained a residue of approx. 0.2 mgl⁻¹ ClO₂, approx. 0.35 mgl⁻¹ ClO₂⁻ and low concentrations of suspended matter (1.5 mgl⁻¹) and of chlorophyll (0.1 μgl⁻¹). Trihalomethane concentrations were negligible, and the bacteriological quality of the water was within the health authorities' requirements. It was shown that disinfection of treated water (after flocculation and filtration) was much more effective than that of raw water. Furthermore, disinfection in the optimal mode prevents accumulation of high chlorite concentrations leaving a residue of ClO₂.     

Reactions of chlorine with inorganic and organic compounds during water treatment-Kinetics and mechanisms: a critical review

Numerous inorganic and organic micropollutants can undergo reactions with chlorine. However, for certain compounds, the expected chlorine reactivity is low and only small modifications in the parent compound's structure are expected under typical water treatment conditions. To better understand/predict chlorine reactions with micropollutants, the kinetic and mechanistic information on chlorine reactivity available in literature was critically reviewed. For most micropollutants, HOCl is the major reactive chlorine species during chlorination processes. In the case of inorganic compounds, a fast reaction of ammonia, halides (Br(-) and I(-)), SO(3)(2-), CN(-), NO(2)(-), As(III) and Fe(II) with HOCl is reported (10(3)-10(9)M(-1)s(-1)) whereas low chlorine reaction rates with Mn(II) were shown in homogeneous systems. Chlorine reactivity usually results from an initial electrophilic attack of HOCl on inorganic compounds. In the case of organic compounds, second-order rate constants for chlorination vary over 10 orders of magnitude (i.e. <0.1-10(9)M(-1)s(-1)). Oxidation, addition and electrophilic substitution reactions with organic compounds are possible pathways. However, from a kinetic point of view, usually only electrophilic attack is significant. Chlorine reactivity limited to particular sites (mainly amines, reduced sulfur moieties or activated aromatic systems) is commonly observed during chlorination processes and small modifications in the parent compound's structure are expected for the primary attack. Linear structure-activity relationships can be used to make predictions/estimates of the reactivity of functional groups based on structural analogy. Furthermore, comparison of chlorine to ozone reactivity towards aromatic compounds (electrophilic attack) shows a good correlation, with chlorine rate constants being about four orders of magnitude smaller than those for ozone.     

Observations and impacts of bleach washing on indoor chlorine chemistry

Ambient levels of chlorinated gases and aerosol components were measured by online chemical ionization and aerosol mass spectrometers after an indoor floor were repeatedly washed with a commercial bleach solution. Gaseous chlorine (Cl₂, 10's of ppbv) and hypochlorous acid (HOCl, 100's of ppbv) arise after floor washing, along with nitryl chloride (ClNO₂), dichlorine monoxide (Cl₂O), and chloramines (NHCl₂, NCl₃). Much higher mixing ratios would prevail in a room with lower and more commonly encountered air exchange rates than that observed in the study (12.7 h^?1). Coincident with the formation of gas‐phase species, particulate chlorine levels also rise. Cl₂, ClNO₂, NHCl₂, and NCl₃ exist in the headspace of the bleach solution, whereas HOCl was only observed after floor washing. HOCl decays away 1.4 times faster than the air exchange rate, indicative of uptake onto room surfaces, and consistent with the well‐known chlorinating ability of HOCl. Photochemical box modeling captures the temporal profiles of Cl₂ and HOCl very well and indicates that the OH, Cl, and ClO gas‐phase radical concentrations in the indoor environment could be greatly enhanced (>10⁶ and 10⁵ cm^?3 for OH and Cl, respectively) in such washing conditions, dependent on the amount of indoor illumination.     

二氧化氯应用于水处理消毒的探讨

介绍了二氧化氯水处理原理和特点,并就消毒的效果、使用范围、建设费用、用地等方面与液氯、臭氧、紫外线作了比较,说明二氧化氯是一种新型高效多功能的水处理消毒剂。     

从DOC、AOC的变化分析二氧化氯消毒工艺

考察了二氧化氯（CIO2）及其组合消毒工艺中溶解性有机物（DOC）浓度和可同化有机碳（AOC）浓度的变化规律。结果表明,以DOC表征的有机物浓度在消毒反应前后变化不大;CIO2的强氧化能力体现在将高分子有机物氧化成中、小分子有机物方面,氧化产物以草酸类物质为主,这导致了AOC浓度的升高,降低了出水的生物稳定性;增加CIO2的投量（〉4mg／L）或与氯胺组合消毒能有效降低出水的AOC浓度,消毒反应24h后出水的AOC浓度均比进水的（59μg/L）低。     

生长环境对消毒剂灭活粪肠球菌影响效果的试验研究

通过改变培养基稀释度和培养温度以及添加无机生长元素（氯化钾、硫酸镁、三氯化铁）研究了自由氯、氯胺和二氧化氯对不同生长环境下所培养的粪肠球菌的灭活情况。结果表明，当粪肠球菌在100倍稀释的肉汤培养基上生长时，达到3log去除率氯和二氧化氯所需的0值分别增加了2．1和5．5倍，而氯胺所需的D值变化很小；将粪肠球菌的生长温度从22℃调整至44．5℃时，达到3log去除率氯胺所需的C≠值下降了2倍，自由氯和二氧化氯所需的C￡值则变化不大；在营养肉汤培养基中投加K＋（4mmol／L）、Mg2＋（0．4mmol／L）和Fe3＋（0．08mmol／L）后，粪肠球菌对自由氯和二氧化氯的抗性提高了，对氯胺的抗性则变化不大，达到3log去除率自由氯和二氧化氯所需的C￡值最大分别增加了约2．3和19倍。     

Rate constants of reactions of bromine with phenols in aqueous solution

The kinetics of bromination of six ortho- and para-substituted phenols was investigated between pH 5 and pH 12 in aqueous solution. Kinetics was followed with a continuous-flow reactor previously validated by studying the fast reaction between chlorine and ammonia. The overall reaction rate between bromine and phenols is controlled by the reaction of HOBr with the phenoxide ion between pH 6 and pH 10. The reaction of HOBr with the undissociated phenols and the reaction of BrO(-) with the phenoxide ions become only significant for pH<6 and pH>10, respectively. The second-order rate constants for the reaction of HOBr with phenoxide ions vary between 1.4(+/-0.1)x10(3) and 2.1(+/-0.5)x10(8)M(-1)s(-1) for 2,4,6-trichlorophenol and 4-methylphenol, respectively. Hammett-type correlation was obtained for the reaction of HOBr with the phenoxide ions (log(k)=8.0-3.33 x Sigmasigma) and was compared with Hammett-type correlations of HOCl and HOI. The reaction rate of bromine with phenol-like organic compounds was estimated to be about 10(3)-fold higher than with chlorine and 10(3)-fold lower than with ozone in drinking water treatment conditions.     

The impact of ferrous ion reduction of chlorite ion on drinking water process performance

The use of chlorine dioxide (ClO₂) as a primary disinfectant and pre-oxidant in drinking water treatment is being explored as an alternative to chlorine for reducing disinfection by-product formation and to assure compliance with United States Environmental Protection Agency's Stage 1 Disinfection/Disinfection By-Products Rule. However, the ClO₂ by-product chlorite ion (ClO₂⁻) is also regulated by the same regulation. Ferrous iron (Fe(II)) has been shown to effectively reduce chlorite ion to chloride ion (Cl⁻) and this study was conducted to evaluate the impact on overall treatment process performance due to the ferric hydroxide solids that form from the reaction. Ferrous iron application was explored at three different points in a pilot-scale water treatment system: pre-rapid mix, pre-settling and pre-filter. Chlorite ion concentrations were effectively reduced from 2 mg/L to less than 0.3 mg/L using an Fe(II) dose of approximately 6 mg/L for all trials. Fe(II) addition at the rapid mix caused no adverse effects and, in fact, allowed for reduction of the alum dose due to the newly formed ferric hydroxide acting as a supplemental coagulant. An increase of 241 and 247% of total suspended solids influent to the filter process was observed when Fe(II) was applied at the pre-settling and pre-filter locations. Pilot-scale filter runs during these trials were less than 2 h and never obtained true steady state conditions. Jar testing was performed to better understand the nature of the ferric hydroxide solids that are formed when Fe(II) was oxidized to Fe(III) and to explore the effectiveness of Fe(II) addition at intermediate stages in the flocculation process.     

Photodecomposition du bioxyde de chlore et des ions chlorite par irradiation u.v. en milieu aqueux-partie II. Etude cinetiquePhotodecomposition of chlorine dioxide and chlorite by U.V.-Irradiation—Part II. Kinetic study

The decomposition of ClO₂ and ClO₂⁻ by u.v. radiation leads to the production of chlorate, chloride and oxygen as end-products via complex reactions which are initiated by the products generated by the primary reactions of photolysis (Buxton and Subhani, 1972a; Mialocq et al., 1973; Karpel Vel Leitner et al., 1992). As far as the rate of decomposition is concerned, Bowen and Cheung (1932) and Zika et al. (1985) have shown that the quantum yield of photodecomposition of chlorine dioxide (overall reaction) increases when the wavelength decreases [Zika et al. (1985): φ = 0.46 at 366 nm and 1.4 at 296.7 nm]. However, the values of the quantum yield of photodecomposition of ClO₂ and ClO₂⁻ at 253.7 nm as well as the quantum yields for the primary reactions of photolysis of ClO₂ and ClO₂⁻ at different wavelengths are not given in the literature.

The aim of this work was to study the kinetics of photodecomposition of chlorine dioxide and of chlorite by u.v. irradiation.     

Sequential inactivation of Cryptosporidium parvum using ozone and chlorine.

Inactivation of bovine-derived C. parvum oocysts was studied at bench-scale in oxidant demand free 0.05 M phosphate buffer using free chlorine alone or ozone followed by free chlorine at temperatures of 1°C, 10°C and 22°C at pH 6. Animal infectivity using neonatal CD-1 mice was used for evaluation of oocyst viability after treatment. Kinetic models based on the linear Chick–Watson model were developed for free chlorine inactivation and ozone/free chlorine sequential inactivation for 0.4 or 1.6 log-units of ozone primary kill. At 22°C, ozone pre-treatment increased the efficacy of free chlorine for about 4–6 times depending on the level of ozone primary kills. Gross kills of the ozone/free chlorine sequential inactivation were a function of ozone primary kills and increased linearly with the free chlorine C_avgt (arithmetic average of the initial and final residual×contact time) product. Temperature was critical for both single and sequential inactivation, and the efficacy of free chlorine after 1.6 log-units of ozone primary inactivation decreased by a factor of 1.8 for every 10°C temperature decrease. Given an ozone primary kill of 1.6 log-units, the free chlorine C_avgt products required for a gross kill of 3.0 log-units were 1000, 2000 and 3300 mg min/L for 22°C, 10°C and 1°C, respectively.     

The effects of active chlorine on photooxidation of 2-methyl-2-butene

Active chlorine comprising hypochlorite (OCl⁻), hypochlorous acid (HOCl) and chlorine (Cl₂) is the active constituent in bleach formulations for a variety of industrial and consumer applications. However, the strong oxidative reactivity of active chlorine can cause adverse effects on both human health and the environment. In this study, aerosolized Oxone® [2KHSO₅, KHSO₄, K₂SO₄] with saline solution has been utilized to produce active chlorine (HOCl and Cl₂). To investigate the impact of active chlorine on volatile organic compound (VOC) oxidation, 2-methyl-2-butene (MB) was photoirradiated in the presence of active chlorine using a 2-m³ Teflon film indoor chamber. The resulting carbonyl products produced from photooxidation of MB were derivatized with O-(2,3,4,5,6-pentafluorobenzyl) hydroxyamine hydrochloride (PFBHA) and analyzed using gas chromatograph-ion trap mass spectrometer (GC/ITMS). The photooxidation of MB in the presence of active chlorine was simulated with an explicit kinetic model using a chemical solver (Morpho) which included both Master Chemical Mechanism (MCM) and Cl radical reactions. The reaction rate constants of a Cl radical with MB and its oxidized products were estimated using a Structure-Reactivity Relationship method. Under dark conditions no effect of active chlorine on MB oxidation was apparent, whereas under simulated daylight conditions (UV irradiation) rapid MB oxidation was observed due to photo-dissociation of active chlorine. The model simulation agrees with chamber data showing rapid production of oxygenated products that are characterized using GC/ITMS. Ozone formation was enhanced when MB was oxidized in the presence of irradiated active chlorine and NO_x.     

Sequential inactivation of Cryptosporidium parvum oocysts with chlorine dioxide followed by free chlorine or monochloramine.

The main objective of this study was to assess the effect of temperature (4-30 degrees C) on the inactivation kinetics of Cryptosporidium parvum oocysts with sequential disinfection schemes involving the use of chlorine dioxide as the primary disinfectant and free or combined chlorine as the secondary disinfectant in synthetic water. The synergy previously reported for sequential inactivation of C. parvum oocysts with ozone/free chlorine or ozone/combined chlorine did not occur when chlorine dioxide was used. instead of ozone, as the primary disinfectant within the temperature range (4-30 degrees C) and the pre-treatment levels investigated. Sequential ozone/chlorine dioxide and chlorine dioxide ozone experiments revealed that the lower level or absence of synergy for chlorine dioxide/free chlorine and chlorine dioxide, monochloramine was likely the result of chlorine dioxide reacting with oocyst chemical groups that are mostly different from those reacting with ozone, free chlorine, or monochloramine. The CT concept was found to be valid for the primary inactivation kinetics of C. parvum oocysts with chlorine dioxide, thus allowing the use of the simpler CT approach for the development of C. partum inactivation requirements with chlorine dioxide. General consistency was found between the secondary inactivation kinetics of C. parvum oocysts with free chlorine and monochloramine after chlorine dioxide pretreatment obtained in this study with oocyst viability determined by a modified in vitro excystation method and those reported in the literature for the same sequential disinfection schemes based on an animal infectivity assay.     

Photodecomposition du bioxyde de chlore et des ions chlorite par irradiation U.V. en milieu aqueux—partie I. Sous-produits de reactionPhotodecomposition of chlorine dioxide and chlorite by U.V.-Irradiation—Part I. Photo-products

Literature data about the photochemistry of oxychlorine compounds in aqueous solutions (flash photolysis, pulse radiolysis, solar radiation) indicate that the products (ClO, Cl, O, O⁻,…) generated from the primary photochemical reactions of decomposition of chlorine dioxide and hypochlorite can then initiate complex reactions which lead to the formation of many secondary products (Cl₂O₂, Cl₂O₁, ClO⁻,…) and of chlorate, chloride and molecular oxygen as end-products (Table 1). The aim of this work was to study the photodecomposition of aqueous solutions of chlorine dioxide and chlorite in u.v. reactors equipped with low pressure mercury vapour lamps in order to show the effects of pH and of the initial concentrations on the nature of photoproducts and on the rate of photodecomposition of ClO₂ and ClO₂⁻. This paper presents the data obtained for the identification of photoproducts. The kinetics of photodecomposition will be presented in another paper (Part II).     

Investigating synergism during sequential inactivation of Bacillus subtilis spores with several disinfectants

The sequential application of ozone, chlorine dioxide, or UV followed by free chlorine was performed to investigate the synergistic inactivation of Bacillus subtilis spores. The greatest synergism was observed when chlorine dioxide was used as a primary disinfectant followed by secondary disinfection with free chlorine. A lesser synergistic effect was observed when ozone was used as the primary disinfectant, but no synergism was observed when UV was used as the primary disinfectant. When free chlorine was used as the primary disinfectant (i.e., sequential application in the reverse order), the synergistic effect was shown only when chlorine dioxide was applied as the secondary disinfectant. The synergistic effect observed could be related to damage to the spore coat during primary disinfection, suggested by the loss of proteins from spores during disinfectant treatment. The greatest synergism observed by the chlorine dioxide/free chlorine pair suggested that common reaction sites might exist for these disinfectants. The concept of percent synergistic effect was introduced to quantitatively compare the extent of synergistic effects in the sequential disinfection processes.     

Effect of oxidants on microalgal flocculation

The effects of chlorine, ozone and chlorine dioxide on Scenedesmus sp. cultures were studied. Algal cell viability and chlorophyll concentration decreased, and the concentration of dissolved organic substances increased with increasing applied oxidant concentration. Pretreatment with chlorine dioxide (1, 3 or 5 mg l⁻¹) or ozone (2.6, 4.6 or 8.1 mg l⁻¹) on algal cultures enhanced algal flocculation with alum, while prechlorination with 10 or 20 mg l⁻¹ increased the required dosage of alum by 15%. Scanning electron micrographs of oxidized cells revealed drastically adverse effects upon the cell surface architecture: in addition to the oxidation of noncellular organic materials, the oxidants damaged both cell surface structures and intracellular components. A model explaining the effects of the different oxidants on microalgal flocculation is suggested.     

Reactivity of natural organic matter with aqueous chlorine and bromine

While both aqueous bromine (HOBr/OBr(-)) and chlorine (HOCl/OCl(-)) react with natural organic matter (NOM) during water treatment, limited direct parallel comparison of bromine versus chlorine has been conducted. Experiments with model compounds and natural waters indicated more efficient substitution reactions with bromine than chlorine. Kinetic experiments with NOM isolates with and without pre-ozonation were conducted to obtain second-order rate constants (k) with bromine and chlorine. Two-stage reaction kinetics (rapid initial and slower consumption stages) were observed. Bromine reacted about 10 times faster than chlorine with NOM isolates during both stages. The rapid initial stage reactions were too fast to quantify k values, but qualitative estimates ranged between 500 and 5000 M(-1)s(-1). For the slower second stage k values for bromine were 15 to 167 M(-1)s(-1) over the pH range of 5-11, and lower for chlorine (k = 0.7-5M(-1)s(-1)). Values of k correlated with initial SUVA values of NOM (UVA measured at 254 nm divided by DOC). Based upon UV/VIS and solid-state (13)C-NMR spectroscopy, chlorine addition to a NOM isolate resulted in significant oxidation of aromatic and ketone groups while bromine had significantly less change in spectra. Overall, the improved knowledge that bromine reacts faster and substitutes more efficiently than chlorine will be useful in developing strategies to control disinfection by-product formation during water treatment.     

Ammonia oxidation in nitrosomonas at NH3 concentrations near km: Effects of pH and temperature

Nitrosomonas europaea from continuous pure cultures was incubated with 26.4 μ M NH₃(= 0.37 mg NH₃-N l⁻¹) at various NH₄⁺ concentrations, pH values and temperatures. Measured rates of nitrite formation were significantly influenced by pH. Likewise unexpectedly, the maximum ammonia oxidation rate occurred between pH 6.7 and 7.0. Temperature had an even stronger effect on the rate of ammonia oxidation than the availability of NH₃. It is concluded that the assumption of a strict dependence of the rate of ammonia oxidation on substrate concentration is an unjustified oversimplification. Among the mechanisms which could explain ammonium uptake and oxidation near or below pH 7.0, the formation of NO from HNO₂ is considered.