Gas production in aquatic sediments in the presence and absence of nitrate

In the overlying water of a water-sediment system the pH was controlled at 7.0, the nitrate-nitrogen concentration at 25.0 mg 1⁻¹ and the dissolved oxygen concentration above 6.1 mg t⁻¹. The temperature of the whole system was kept at 15°C. The average rate of nitrate removal from the system as a result of denitrification amounted to 160 mg NO₃⁻ -N m⁻² day⁻¹. By means of Eh measurements at various depths in the sediment, it was attempted to figure out the course of the penetration fronts of nitrate and oxygen in the sediment during 241 days of incubation. From these results the layer in which denitrification occurred was derived. The course of the denitrification zone was followed during the incubation period. As a result of the depletion of the available hydrogen donors in the sediment, oxygen reached the bottom of the sediment after 235 days of incubation.     

Nitrate reduction by denitrifying bacteria in single and two stage continuous flow reactors

Denitrification by a mixed bacterial population of medium containing 1000 mg NO₃⁻-N1⁻¹ and acetate as carbon source was studied in batch, a single stage continuous flow stirred reactor (CFSTR) and a two stage CFSTR at 30°C. The optimum pH for denitrification, nitratase, nitrite reductase activities and growth was found to be 7.5 in batch culture. A single stage CFSTR growth limited by nitrate had an optimum denitrification rate of 0.13 mg NO₃⁻-N mg⁻¹ cells h at a residence time of 8 h. The experimentally observed carbon to nitrate ratio (mg CH₃ COO⁻-C mg⁻¹ NO₃⁻-N)was 1.7 for the dilution rates of 0.02–0.18 h⁻¹. For the second stage CFSTR, bacteria growing at the maximum rate of 0.25 h⁻¹ and not limited by nitrate had a denitrification rate of 0.24 mg NO₃⁻-N mg⁻¹ h. Dissolved oxygen (up to 9.5 mg 1⁻¹) did not effect denitrification rates in the second stage CFSTR. As the second stage CFSTR runs progressed extensive wall growth occurred and concurrently the output gas contained increasing quantities of nitrous oxide. A development from this study would be a two stage CFSTR with wall growth in the second stage which would make an efficient nitrate removal process.     

Denitrification with a bacterial disc unit

An enclosed rotating disc unit was operated anaerobically as a denitrifying system, with methanol as the hydrogen donor. As the bacterial population became established, denitrification rate increased by 1·5 mg NO₃⁻—N reduced m⁻² h⁻², to a maximum rate of 260 mg NO₃⁻—N reduced m⁻²h⁻¹. The C:N ratio necessary for complete denitrification was found to be 2·6:1. Optimum pH for denitrification lay in the range between pH 7·0 and 8·5. Q₁₀ values were 1·38 between 10 and 30°C, −2·66 above 30°C and 13·06 below 10°C.     

Rising sludge in secondary settlers due to denitrification

High suspended solids concentrations in settler effluents can be caused by rising sludge, which is the effect of flotation of solids by nitrogen gas resulting from biological denitrification. Many factors influence the nitrogen gas bubble evolution. The most important factor is the rate of biological denitrification. Factors like nitrogen gas solubility and oxygen concentration in settler influent only play a minor role. The hydraulic retention time in the bottom part of the settler is, for all practical purposes, so high that sufficient nitrogen gas will be generated at temperatures above 20°C, if the nitrate content in the influent to the settler is above the critical one. For temperatures around 20°C the critical nitrate-nitrogen concentration is 6–8 g NO₃⁻-N/m³. The best measure in order to avoid rising sludge is to denitrify the wastewater in the treatment process ahead of the settler.     

The relation between redox potential and denitrification in a water-sediment system

In the overlying water of a water-sediment system the pH was controlled at 7.0, the nitrate-nitrogen concentration at 25.0 mg 1^?1 and the dissolved oxygen concentration above 6.1 mg t^?1. The temperature of the whole system was kept at 15°C. The average rate of nitrate removal from the system as a result of denitrification amounted to 160 mg NO₃^? -N m^?2 day^?1. By means of Eh measurements at various depths in the sediment, it was attempted to figure out the course of the penetration fronts of nitrate and oxygen in the sediment during 241 days of incubation. From these results the layer in which denitrification occurred was derived. The course of the denitrification zone was followed during the incubation period. As a result of the depletion of the available hydrogen donors in the sediment, oxygen reached the bottom of the sediment after 235 days of incubation.     

Removal of nitrate from effluent following discharge on surface water

The loss of nitrate nitrogen over a 800-m long reach of canal was studied in a field experiment during a 20-days period. The nitrate originated mainly from sewage effluent. Fifty-six percent of the nitrate had disappeared during its flow through the 800-m long reach, where the average retention time was 1.7 days. The average rate of nitrate disappearance during the 20-day period was 913 mg NO₃⁻-N m⁻² day⁻¹. Laboratory experiments with undisturbed water-sediment profiles from the canal showed that the disappearance of nitrate was caused mainly by denitrification in the sediment. Increased knowledge of this phenomenon may lead to an effective and cheap means in inducing denitrification.     

High nitrate denitrification in continuous flow-stirred reactors

Continuous flow stirred reactors were used to evaluate the maximum denitrification specific removal rates for influent solutions made from NH₄NO₃, CaNO₃, KNO₃ and UO₂ fuel fabrication waste water. Nitrate substrate concentrations ranged from 0.01 to 20 kg NO₃/m³. Values for U_max (maximum specific substrate removal rate per unit mass of microorganisms per unit time, days⁻¹) were determined using graphical solutions to the Lineweaver-Burk equations. For NH₄NO₃ solutions at nitrate substrate concentrations <6 kg NO₃/m³ the value for U_max was found to be 1.73 days⁻¹. At nitrate substrate concentrations >6 kg NO₃/m³ a nonlinear relationship was observed in the Lineweaver-Burk plots indicating nitrate substrate inhibition. Specific removal rates at nitrate concentrations >6 kg NO₃/m³ averaged <1.0 days⁻¹. Ammonia toxicity may also have occurred as the pH of the mixed liquor was near 8. Methanol concentrations as high as 11.6 kg CH₃OH/m³ did not inhibit denitrification rates. The highest specific removal rates recorded (3.13 ± 0.56 days⁻¹) were with influents made from UO₂ fuel fabrication waste water.     

Low temperature removal of nitrate by bacterial denitrification

The efficiency of two denitrifying sludges enriched at 5 and 20°C were compared using methanol as an electron donor. Both sludges were exposed to the same hydraulic and chemical conditions using an influent containing methanol and mineral salts. The low temperature sludge seemed to have several advantages over the sludge selected for at the higher temperature. In the range 0–17°C, the specific denitrification rate was 1.5–4 times the rate for the high temperature sludge, temperatures below 8°C being the most favourable. At 2°C, under nitrate limiting conditions, 98% nitrate reduction was obtained at a hydraulic residence time of 3.5 h, with an effluent concentration of 0.8 mg NO₃---Nl⁻¹. Sedimentation characteristics were always better for the low temperature sludge, and the utilization of methanol equally good as the high temperature sludge. The low temperature sludge appeared to be biochemically and microbiologically stable to temperature changes within the range 0–17°C, the latter temperature being close to the limit for maintaining the psychrophilic characteristics of the sludge. Studies on pure culture isolates of the denitrifying bacteria showed >90% dominance of one bacterial strain in both sludges. Studies of the isolates also showed that the low-temperature sludge consisted predominantly of psychrotrophs/psychrophiles, and not well-adapted mesophiles, which were only present in low concentrations. The dominant strain in both sludges was unable to grow on methanol in pure culture without access to nutrient growth factors. Only a few minor strains were obligate methylotrophs.Low temperature sludges were tested in a 3-stage biological process receiving domestic sewage. Each stage; carbon oxidation, nitrification and denitrification had separate sludge recycle, and methanol was added to the denitrification stage. These sludges were grown and selected for at temperatures 5°C. At 5°C the laboratory scale process gave 90% removal of total nitrogen at hydraulic residence times of 1.5, 9 and 4 h for the two aeration stages and the anaerobic stage respectively. Overall nitrification/denitrification was 95%, while denitrification separately was 98%. The effluent contained 0.4 mg NO₃---Nl⁻¹. The critical step in the process was unquestionably nitrification. Oxidation of ammonium was satisfactory at low temperature, but the reaction was somewhat vulnerable to changes in external conditions. The low temperature denitrifying sludge was originally enriched on synthetic waste but did not appear to change its microbial composition or characteristics by exposure to municipal wastewater.     

The synthesis of the dissimilatory nitrate reductase under aerobic conditions in a number of denitrifying bacteria, isolated from activated sludge and drinking water

A number of denitrifying bacteria were isolated from activated sludge and drinking water. These bacteria were tested for the synthesis of the dissimilatory nitrate reductase under aerobic conditions (dissolved oxygen concentration above 4 mg · l⁻¹). The synthesis of this enzyme varied from total repression by oxygen in some bacteria, especially those isolated from drinking water, until a nearly non oxygen-repressed synthesis in other bacteria (strains 15 and N4). The effect of the dissolved oxygen concentration during growth of the bacteria on the synthesis of the dissimilatory nitrate reductase in cells of strain 15 was studied more extensively. A considerable repression of the enzyme synthesis was obtained when the dissolved oxygen concentration was relatively high (approx 15 mg·l⁻¹). Addition of chlorate to the growth medium of strain 15 (using NH⁺₄-N as nitrogen source) also resulted in a serious repression of the nitrate reductase synthesis during aerobic growth (dissolved oxygen above 4 mg·l⁻¹). The dissimilatory nitrate reductase of aerobically grown cells of strains 15 and N4 was found to be mainly localized in the membrane fraction.     

Oxygen uptake of bottom sediments studied in situ and in the laboratory

Oxygen uptake by soft bottom sediments was measured in situ with an oxygen electrode in a bell jar. Values in the range 0·3-3·0 g O₂ m⁻² d⁻¹ were obtained at 19 localities in fresh and brackish water. Comparative measurements were made in the laboratory on sediment cores. These gave consistently lower values than the in situ measurements. Laboratory experiments showed that the oxygen uptake depended on the oxygen concentration and that the temperature coefficient decreased with increasing temperature. There was no simple correlation between oxygen uptake and content organic matter in sediments.     

Denitrification of aerobically stabilised pig waste

Experiments for the study of denitrification of biologically stabilised, nitrified pig waste are described. The rate of denitrification was increased by the addition of an energy source or by an increase in temperature. A supplementary energy source in field-scale stabilisation systems can be replaced by the residual chemical oxygen demand of a partially nitrified waste. A semi-continuous denitrification process was capable of reducing input inorganic N concentrations of several hundred mgl⁻¹ to less than 50 mgl⁻¹. Denitrification also improved final effluent quality by reducing soluble phosphorus, total and dissolved solids and chemical oxygen demand.     

The relationship between dissimilatory nitrate reduction and oxygen uptake by cells of an Alcaligenes strain in flocs and in suspension and by activated sludge flocs

The oxygen uptake and the dissimilatory nitrate reduction by anaerobically grown cells of a denitrifying Alcaligenes strain, occurring in floc form or in suspension, were studied at different oxygen concentrations in the surrounding medium. When the oxygen concentration in the medium fell below 1·5 mg l⁻¹, the nitrate reduction by the cells within flocs increased considerably. The cells in suspension showed an increased nitrate reduction when the oxygen concentration was below 0·1 mg l⁻¹. When anaerobically grown cells had been aerated for 24 h in a nitrogen-free medium, the cells became sensitive to respiration inhibition by nitric oxide, resulting from nitrate reduction. This gave rise to an increased nitrate reduction below 2·5 mg oxygen l⁻¹ when the cells were aggregated in flocs and below 0·1 mg oxygen per litre when the cells were in suspension. The nitrate reduction by the denitrifying, floc-forming pure culture was compared with that of activated sludge flocs.     

Transport of C-labelled PCB compounds from sediment to water and from water to air in laboratory model systems

Tetra-, hexa- and octachlorobiphenyls were added to aquatic model systems composed of undisturbed sediment cores with an overlying water phase. Using impactor plates transport of the compounds from sediment to air was observed. About one per cent of the sediment-bound PCBs recovered in the systems left the water by jet drops from bursting bubbles. The transport of PCBs from the sediment to the air was nearly constant over time, with a transport rate of 0.62 μg · dm⁻² week⁻¹ for tetrachlorobiphenyl. Tetrachlorobiphenyl was mobile in systems with and without macroinvertebrates and in those fixed with HgCl₂. Hexa- and octachlorobiphenyls were transported from sediment to water mainly by bioturbation processes. The two latter substances had a higher adsorption to particles than tetrachlorobiphenyl. Compared to tetrachlorobiphenyl, more hexa- and octachlorobiphenyls accumulated in chironomids and tubificids.     

Ozonation of water containing chlorine or chloramines. Reaction products and kinetics

Ozone reacts with free aqueous chlorine when present as hypochlorite ion (OCl⁻) with a second order rate constant of 120 ± 15 M⁻¹ s⁻¹ at 20°C. About 77% of the chlorine reacts to produce Cl⁻ and 23% is oxidized to ClO⁻₃. No ClO⁻₄ is formed. Conversion of chlorine to monochloramine reduces the ozone reaction rate to 26 ± 4 M⁻¹ s⁻¹, independent of pH, NH₂Cl is transformed quantitatively to NO⁻₃ and Cl⁻ by O₃. Rate data for other chloramines are also presented. The direct reaction of ozone with chlorine accounts for a significant amount of the chlorine and ozone demand found when the two oxidants are used in combination under water works conditions.     

Sorption and accumulation of cadmium in the sediment of the Tama River

Cadmium contents in the water and the sediment samples collected from the Tama River and several branches were measured. Cadmium (above 0.005mgl⁻¹) was detected in only four of the water samples, while the sediment samples showed cadmium content of 1.0–9.8 μg g⁻¹ dry sediment. Cadmium concentration in the sediments of the main stream was correlated against ignition loss of the samples and it was found that 1 g of ignition loss (organic matter) corresponded to 35 μg of cadmium.The batch adsorption experiment in the laboratory using an aqueous solution of cadmium for 14 sediment samples with a higher concentration of cadmium indicated that the amount adsorbed by the sediment is highly dependent on the ignition loss. The amount adsorbed on unit mass of ignition loss q_IL could be correlated by a Freundlich-type equilibrium relation as where C is the equilibrium concentration in the aqueous phase ranging between 7 × 10⁻³ and 10 mg l⁻¹, while k_IL and n are equilibrium constants.The adsorption rate measurement showed that the intraparticle diffusion coefficient of cadmium in the sediment was about 1.1 × 10⁻⁶ cm²s⁻¹, which is of a reasonable order of magnitude assuming the pore diffusion mechanism inside the particle.The results suggest that suspended solid particles of high organic content in flowing water contribute significantly to the transport of cadmium along the river.     

The inactivation kinetics of H-1 parvovirus by chlorine

A newly isolated enteric virus has recently been found to be associated with large outbreaks of waterborne gastroenteritis. Most commonly referred to as the Norwalk agent, this virus appears to be morphologically and biophysically similar to the parvoviruses. In this study the parvovirus H-1, a putative human virus containing single-stranded DNA, was used as a model virus for chlorine inactivation experiments. The objective of this research was to investigate the kinetics of inactivation of this virus by low levels of free chlorine (0.05–0.20 mgl⁻¹) at pH 7 and at 5, 10, 20 and 30°C.Inactivation occurred in the usual dose-response relationship, that is, increasing the chlorine dose caused an increase in the rate of inactivation. The energy required for the inactivation reaction using 0.05 mgl⁻¹ free chlorine from 5 to 30°C was graphically determined to be 2.4 kcal mol⁻¹. The change in entropy was calculated to be -52.34 entropy units. For disinfection purposes, the time required for 99% inactivation of H-1 parvovirus at pH 7.20°C and a chlorine dose of 0.2 mgl⁻¹ free chlorine was 3.2 min. The parvovirus H-1 appeared to be less resistant to free chlorine than poliovirus type 1 (LSc).     

An improved hydrazine reduction method for the automated determination of low nitrate levels in freshwater

An automated nitrate determination is described in which nitrate is reduced to nitrite with hydrazine sulphate under alkaline conditions in the presence of Cu²⁺ and Zn²⁺. Interferances encountered in natural water samples were eliminated by the addition of Zn²⁺ to the Cu²⁺ catalyst solution.The method is suitable for the determination of low NO₃−N concentrations and compares favourably with the manual copperised cadmium technique for freshwater samples containing 10–800 mg m⁻³ NO₃−N. The method is also linear at nitrate concentrations below 10 mg N m⁻³. The standard deviations (S.D.) of blanks and of samples containing 2 mg NO₃−N m⁻³ were 0.013 and 0.06 mg N m⁻³ respectively at an analysis rate of 30 samples h⁻¹.     

Inhibitory effect of oxygen on denitratification and denitritification in sludge from an oxidation ditch

The effect of oxygen concentration on the rates of denitratification and denitritification was investigated by the acetylene inhibition method during initial 1-h incubation period after preculture under complete anaerobic conditions for wide ranges of nitrate and nitrite concentrations using sediment sample collected from a highly eutrophic lake. A maximum denitratification rate of 4 μmol (g-dry mud)⁻¹ h⁻¹ was obtained under anaerobic conditions. The denitratification rate was found to be a decreasing function of the oxygen concentration below 60 μM. Approximately the same rate was observed for denitritification in the range below 30 μM O₂. Beyond 30 μM O₂, this rate dropped to the half of the maximum, and remained almost constant until a critical oxygen concentration was attained. The critical concentration, above which denitritification was suppressed thoroughly, depended on nitrite concentration.     

Nitrification kinetics in activated sludge at various temperatures and dissolved oxygen concentrations

Activated sludge from a domestic sewage works was enriched with nitrifying bacteria by running a laboratory fermenter on ammonia-supplemented sewage. This enriched culture was used to determine respirometrically the kinetics of microbial nitrification. It was demonstrated that the reaction fits the Michaelis-Menten model for temperatures from 10 to 35°C, having a temperature optimum at 15°C (K₃ 0.72 mg 1⁻¹ NH₃). Nitrification is unaffected by high dissolved oxygen concentration 38 mg 1⁻¹ O₂ at 30°C) after acclimatisation. Nitrite concentrations > 20 mg 1⁻¹ are inhibitory to the reaction.     

Effect of organic enrichment and thermal regime on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in hypolimnetic sediments of two lowland lakes

We analyzed benthic fluxes of inorganic nitrogen, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates in hypolimnetic sediments of lowland lakes. Two neighbouring mesotrophic (Ca' Stanga; CS) and hypertrophic (Lago Verde; LV) lakes, which originated from sand and gravel mining, were considered. Lakes are affected by high nitrate loads (0.2-0.7 mM) and different organic loads. Oxygen consumption, dissolved inorganic carbon, methane and nitrogen fluxes, denitrification and DNRA were measured under summer thermal stratification and late winter overturn.Hypolimnetic sediments of CS were a net sink of dissolved inorganic nitrogen (−3.5 to −4.7 mmol m⁻² d⁻¹) in both seasons due to high nitrate consumption. On the contrary, LV sediments turned from being a net sink during winter overturn (−3.5 mmol m⁻² d⁻¹) to a net source of dissolved inorganic nitrogen under summer conditions (8.1 mmol m⁻² d⁻¹), when significant ammonium regeneration was measured at the water-sediment interface. Benthic denitrification (0.7-4.1 mmol m⁻² d⁻¹) accounted for up to 84-97% of total NO₃⁻ reduction and from 2 to 30% of carbon mineralization. It was mainly fuelled by water column nitrate. In CS, denitrification rates were similar in winter and in summer, while in LV summer rates were 4 times lower. DNRA rates were generally low in both lakes (0.07-0.12 mmol m⁻² d⁻¹). An appreciable contribution of DNRA was only detected in the more reducing sediments of LV in summer (15% of total NO₃⁻ reduction), while during the same period only 3% of reduced NO₃⁻ was recycled into ammonium in CS.Under summer stratification benthic denitrification was mainly nitrate-limited due to nitrate depletion in hypolimnetic waters and parallel oxygen depletion, hampering nitrification. Organic enrichment and reducing conditions in the hypolimnetic sediment shifted nitrate reduction towards more pronounced DNRA, which resulted in the inorganic nitrogen recycling and retention within the bottom waters. The prevalence of DNRA could favour the accumulation of mineral nitrogen with detrimental effects on ecosystem processes and water quality.