Mathematical analysis of oxygen and nitrate consumption in deep microbial films

The simultaneous diffusion and consumption of oxygen and nitrate into microbial films with aerobic and anaerobic layers was modeled by a system of differential equations and solved by numerical methods. Fickian diffusion was assumed. Models used Monod kinetics and zero-order kinetics, for growth limitation by oxygen and nitrate. Denitrification was assumed to be repressed by oxygen concentrations greater than a finite critical value. Both models indicate that oxygen concentration in the overlying water would have little effect on the nitrate uptake of thick films.     

Biocenosis diversity and denitrification efficiency

During denitrification carried out in the presence of methanol, glycerol, acetic and lactic acids as a C-source, the microbial growth parameters such as: maximum attainable biomass production (P_{x max}), “optimal” biomass residence time (Θ_x), biomass yield coefficients (Y_N and Y_C) and specific biomass loss rate constants (b_N and b_C) were estimated. Also, the following process parameters: maximum attainable biomass N and C removal rates (ΔB_{x, Nmax} and ΔB_{x, Cmax}) and corresponding rate constants (K_{Bx, N} and K_{Bx, C}) were calculated. The values of these parameters were interpreted in relation to the chemical structure of the C-sources for denitrification and degree of their assimilability by the bacteria and fungi which dominated in biocenoses.     

Combined aerobic heterotrophic oxidation, nitrification and denitrification in a permeable-support biofilm

A biofilm reactor, termed the permeable-support biofilm (PSB), was developed in which oxygen was supplied to the interior of the biofilm through a permeable membrane. The reactor was tested on filtered sewage supplemented with nutrient broth; the bulk solution was anoxic and the interior of the biofilm was supplied with pure oxygen. All tests were performed on a non-steady state biofilm with a depth of 1 mm. Mass balances on total organic carbon, ammonia, organic nitrogen and nitrate showed that combined heterotrophic oxidation of organics, denitrification and nitrification occurred simultaneously within the biofilm. The advantages of such a reactor are discussed.     

中空纤维膜生物反应器去除地下水中硝酸盐的效能

采用聚氯乙烯中空纤维膜生物反应器处理受硝酸盐污染的地下水,在生物膜驯化成熟后硝酸盐还原得较彻底.在进水硝酸盐浓度为10 mgN/L和HRT为37.5 min的条件下,平均反硝化速率达383.01 gN/(m<'3>·d),平均表面去除负荷达1.39 gN/(m<'2>·d).反应器运行稳定阶段的出水水质好,出水中氢气散逸所致空气中的氢气浓度低于其爆炸极限.这表明,将聚氯乙烯中空纤维膜生物反应器用于净化受硝酸盐污染的地下水具有经济和技术上的可行性.     

短程硝化反硝化工艺简析

指出短程硝化反硝化工艺是目前国内外生物脱氮技术研究应用的热点,通过介绍短程硝化反硝化工艺原理,分析了不同工艺稳定亚硝态氮积累实现短程硝化的工艺控制措施,对短程硝化反硝化工艺今后的研究和应用进行了展望。     

Competition in denitrification systems affecting reduction rate and accumulation of nitrite

During the process of denitrification of wastewater nitrite has often been observed to accumulate, most probably because of the nitrite reduction rate falling behind the rate of nitrate reduction. The hypothesis to be investigated in this study was that microbial communities could be enriched for facultative anaerobes capable of reducing nitrate, but only to nitrite. A mathematical model was developed, and experiments were conducted to study results of enhanced proliferation of facultative anaerobes, on the expense of true denitrifiers, in activated sludge biocommunities. A lab-scale sequencing batch reactor system was employed for the studies. As predicted, the rate of nitrite reduction progressively decreased whereas the nitrate reduction rate remained almost unaffected, when fermentation conditions were introduced into the process schematic. Implications in design and operation of wastewater treatment plants are discussed.     

Oxygen uptake rates for determining microbial activity and application

The method presented involves using the oxygen uptake rates of activated sludges for determining the microbial activity and viability. The microbial activity correlates well with the substrate reduction first order rate constant. K₁, determined in batch operations, and with the specific substrate utilization rate, q, of steady state operations. The microbial viability determined based on respiratory activity would result in slightly higher values than other means such as plate counting, since some of the respiratory activity is contributed by nonviable yet active cells. The extent of respiratory activity attributed to the nonviable cells increases as the biological solids retention time increases (or net growth rate decreases). Since the loss of ability of multiplication is not necessarily associated with loss of biochemical activity, the viability determined by the respiratory activity would represent more realistically the activity of microbes and thus is suggested as the preferred technique.     

Effect of oxygen concentration on the rates of denitratification and denitrification in the sediments of an eutrophic lake

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 and denitrification in water and soil environments

An overview of nitrification and denitrification in the natural environment is presented and the basic mechanisms of nitrification and denitrification in water/soil environments are described in this paper. Nitrification/denitrification is one of the most important techniques for removing inorganic nitrogenous compounds in water and soil environments. Nitrification and denitrification in aquatic and soil environments can be described by using First‐order kinetic and the methods for parameter estimation are discussed. The study indicated that nitrification and denitrification could take place simultaneously in aerobic and anaerobic zones in soil and water environment. The factors effecting nitrification and denitrification are discussed in this study. External carbon sources are the most important factors for completing denitrification and dissolved oxygen (DO) is very significant for nitrification.     

喷射环流反应器同步硝化反硝化机理的研究

喷射环流反应器在好氧条件下具有良好的脱氮性能,其对氨氮和总氮的去除率分别达到80%和70%以上,且两者的去除率成正比.试验测定了反应器出水中NOx^--N的含量,结果表明出水中的氮主要以氨氮和亚硝酸盐氮的形式存在,证明该反应器在硝化过程中实现了对亚硝酸盐的积累.反应器的脱氮效果随进水C/N值的增加而提高,证明了异养硝化细菌的存在.对废水处理过程中产生的废气进行气相色谱分析,结果表明废气中氮气的含量比空气的增加了0.24%,证明反应器中发生了反硝化反应.综合试验结果表明,喷射环流反应器中的脱氮机理为亚硝酸盐型同步硝化反硝化.     

Achieving biofilm control in a membrane biofilm reactor removing total nitrogen

Membrane biofilm reactors (MBfR) utilize membrane fibers for bubble-less transfer of gas by diffusion and provide a surface for biofilm development. Nitrification and subsequent autotrophic denitrification were carried out in MBfR with pure oxygen and hydrogen supply, respectively, in order to remove nitrogen without the use of heterotrophic bacteria. Excessive biomass accumulation is typically the major cause of system failure of MBfR. No biomass accumulation was detected in the nitrification reactor as low-level discharge of solids from the system balanced out biomass generation. The average specific nitrification rate during 250 days of operation was 1.88 g N/m² d. The subsequent denitrification reactor, however, experienced decline of performance due to excessive biofilm growth, which prompted the implementation of periodic nitrogen sparging for biofilm control. The average specific denitrification rate increased from 1.50 g N/m² d to 1.92 g N/m² d with nitrogen sparging, over 190 days thus demonstrating the feasibility of stable long-term operation. Effluent suspended solids increased immediately following sparging: from an average of 2.5 mg/L to 12.7 mg/L. This periodic solids loss was found unavoidable, considering the theoretical biomass generation rates at the loadings used. A solids mass balance between the accumulating and scoured biomass was established based on the analysis of the effluent volatile solids data. Biofilm thickness was maintained at an average of 270 μm by the gas sparging biofilm control. It was concluded that biomass accumulation and scouring can be balanced in autotrophic denitrification and that long-term stable operation can be maintained.     

Bedform mobility and benthic oxygen uptake

A non-consumptive probe was used to measure DO concentrations in mobile sand dunes of the Tarawera River, New Zealand, which receives pulp-mill effluent. Net oxygen depletion rates were low in the top 5–10 cm (occasionally 10–20 cm) of the sediments which indicates that turbulent dispersion transports oxygen to these depths. Below 10–20 cm the net depletion rate was independent of depth which implies that the oxygen resupply rate was negligible. Minimum DO concentrations were invariably found at the upstream (scouring) face of the dunes indicating that advection through the sediments was negligible. A simple analytical model showed that dune turnover explained 30% of the observed river deoxygenation rate. Benthic uptake arising from dune turnover was uniform down the river because an increase in dune height along the river was offset by a decrease in river DO. This helps explain why the river deoxygenation rate is uniform. A fragile layer of fluidized sediment covered the crests of each dune. It was inferred that oxygen uptake in this layer explained 60% of the river deoxygenation rate but an improved understanding of oxygen dynamics within the fluidized layer is required. In other mobile-bed rivers it is likely that dune turnover will be important in determining the benthic mass transfer rate.     

The relationship between denitrifying bacteria and methanogenic bacteria in a mixed culture system of acclimated sludges

The interaction between denitrification and methanogenesis, with methanol functioning as an electron donor, has been examined through usage of a mixed culture system of denitrifying sludge and methanogenic sludge in an anaerobic bioreactor. Competition for methanol between these two kinds of biocommunity could not be observed, whereas methanogenesis was suppressed as long as nitrate and nitrite were made available in the mixed system. The inhibition of methanogenesis in the methanogenic sludge caused by nitrogen oxides was studied. The redox potential (E_h) of the culture was monitored and/or controlled for the sake of characterizing the behavior of the biocommunities. An addition of nitrite elevated the E_h of the culture less than nitrate did. Nitrite addition, however, exerted a stronger inhibitory effect on methanogenesis as compared to nitrate at the same concentration. The influence of redox potential on the methanation of methanol was examined by using a methanogenic sludge in the E_h-stat batch culture. The hypothesis that the inhibitory effect being expressed by the nitrogen oxides is not simply attributed to an elevation of the redox potential of the culture is supported by the experimental results. The toxic effect of the nitrogen oxides themselves could also have possibly contributed significantly.     

Sedimentary microbial oxygen demand for laminar flow over a sediment bed of finite length

Dead organic material accumulated on the bed of a lake, reservoir or wetland often provides the substrate for substantial microbial activity as well as chemical processes that withdraw dissolved oxygen (DO) from the water column. A model to estimate the actual DO profile and the "sedimentary oxygen demand (SOD)" must specify the rate of microbial or chemical activity in the sediment as well as the diffusive supply of DO from the water column through the diffusive boundary layer into the sediment. Most previous experimental and field studies have considered this problem with the assumptions that the diffusive boundary layer is (a) turbulent and (b) fully developed. These assumptions require that (a) the flow velocity above the sediment bed is fast enough to produce turbulent mixing in the boundary layer, and (b) the sediment bed is long. In this paper a model for laminar flow and SOD over a sediment bed of finite length is presented and the results are compared with those for turbulent flow. Laminar flow near a sediment bed is encountered in quiescent water bodies such as lakes, reservoirs, river backwaters, wetlands and ponds under calm wind conditions. The diffusive oxygen transfer through the laminar diffusive boundary layer above the sediment surface can restrict the microbial or chemical oxygen uptake inside the sediment significantly. The developing laminar diffusive boundary layer above the sediment/water interface is modeled based on the analogy with heat transfer, and DO uptake inside the sediment is modeled by Michaelis-Menten microbial growth kinetics. The model predicts that the rate of SOD at the beginning of the reactive sediment bed is solely dependent on microbial density in the sediment regardless of flow velocity and type. The rate of SOD, and the DO penetration depth into the sediment decrease in stream-wise direction over the length of the sediment bed, as the diffusive boundary layer above the sediment/water interface thickens. With increasing length of the sediment bed both SOD rate and DO penetration depth into the sediment tend towards zero if the flow is laminar, but tend towards a finite value if the flow is turbulent. That value can be determined as a function of both flow velocity and microbial density. The effect of the developing laminar boundary layer on SOD is strongest at the very lowest flow velocity and/or highest microbial density inside the sediment. Under quiescent conditions, the effective SOD exerted by a reactive sediment bed of a lake or wetland approaches zero, i.e. no or very little oxygen demand is exerted on the overlying water column, except at the leading edge.     

Hydraulic performance of percolating biological filters and consideration of oxygen transfer

A distinct difference exists between the experimentally measured and theoretically calculated values of residence time in percolating biological filters. From existing information it was shown that the measured values are usually at least three times higher than those calculated theoretically. Based on several premises, it was concluded that two liquid film layers should be distinguished. One “free” liquid film flowing on top of the biofilm, the second “captured” flowing through the biofilm.From experimentally measured residence time, it becomes evident that the total liquid film depth will be in the region of 150–550 μm for a hydraulic load range of 0.02–5 m³m⁻²m⁻¹. It can be proved that oxygen cannot penetrate up to such a depth, especially when the captured liquid film thickness is in the region of 110–380 μm. The largely anoxic conditions in the captured liquid film are in drastic contrast with the prevailing conditions close to oxygen saturation in the free flowing liquid layer. Due to the large developed surface the outflowing liquid can be saturated with oxygen. The measured oxygen concentration in filter effluent does not reflect the aerobic conditions neither in the captured liquid film nor in the biofilm as such.     

通过控制泥龄实现亚硝酸盐型同步硝化反硝化

采用序批式活性污泥法处理人工配制的城市生活污水，通过控制泥龄成功地实现了亚硝酸盐型同步硝化反硝化，曝气过程中NO2^-N／NOx^-N值始终保持在84．48％以上，曝气结束时大约有80．39％的氨氮通过同步硝化反硝化途径被去除。在适宜的曝气量下，利用排泥的方法控制反应器内适宜的泥龄，可以实现稳定的亚硝酸盐型同步硝化反硝化。     

Simultaneous nitrification/denitrification in a biofilm airlift suspension (BAS) reactor with biodegradable carrier material

Simultaneous nitrification and denitrification in one reactor has been realized with different methods in the past. The usage of biodegradable biocompounds as biofilm carriers is new. The biocompounds were designed out of two polymers having different degradability. Together with suspended autotrophic biomass the biocompound particles were fluidized in an airlift reactor. Process water from sludge dewatering with a mean ammonium nitrogen concentration of 1150 mg L⁻¹ was treated in a two stage system which achieved a nitrogen removal of 75%. Batch experiments clearly indicate that nitrification can be localized in the suspended biomass and denitrification in the pore structure of the slowly degraded biocompounds. Images taken with CLSM prove the concept of the pore structure within the biocompounds, which provide both a heterotrophic biofilm and carbon source.     

Nitrification in rotating disc systems—I: Criteria for transition from oxygen to ammonia rate limitation

Identification of the rate limiting substrate and half-order rate constants are presented on the basis of biofilm kinetics for triple substrate conditions (organic matter, ammonia and oxygen) in a rotating biological disc system. These findings have been verified by pilot-scale experiments. The transition from ammonia rate limitation to oxygen rate limitation occurs at a bulk ammonia-nitrogen concentration to bulk oxygen concentration ratio of approx. 0.4. Accordingly, oxygen is the rate limiting substrate. For most practical purposes the experiments showed that the oxygen limited half-order reaction constant in the presence of simultaneous organic matter degradation is reduced compared to nitrification alone by a factor which is a function of the growth of the heterotrophs and the nitrifiers in the biofilm.     

The effect of reactor hydraulics on the performance of activated sludge systems—II. The formation of microbial products

A new model involving the concept of soluble residual microbial products formation was used to investigate the effect of reactor hydraulics on the substrate removal efficiencies of activated sludge systems. Strong experimental evidence in the literature suggests that what is measured in most studies is not the remaining portion of the influent degradable substrate, but organic matter of microbial origin which is residual, at least for the operating conditions considered. An appropriate simulation approach was formulated to account for the formation of these products, by a simple mechanistic modification of the newly proposed task group model. This model showed no practical difference between the performances of completely mixed and plug flow activated sludge systems, because they produced almost equal amounts of these microbial products, under similar operating conditions.