/030929
Operation of wastewater treatment plants with nitrification, denitrification, and biological P-elimination under winter conditions
1. Introduction
In recent times is often shown that seasonal increased occurrence of filamentous microorganisms, especially of Microthix Parvicella, in sewage treatment plants with nitrification/denitrification and biological P-elimination lead to increase the sludge volume index ISV and foaming on the aerobic basin zones and in the digesters. The Fraunhofer IGB has by investigations at the wastewater treatment plant of the WWTP Heidelberg noted that the filamentous microorganisms in the denitrification and nitrification zones, cause the foam /1 Kempter, Sternad/. In this lecture will be shown correctly that in case of substrate limitation the filamentous micro-organisms because of their smaller Saturation Constants, despite the lower rate of growth, have an advantage in respect to the flakes formers. That the concentration of dissolved oxygen plays a decisive role, it is equally clear, because on the changing partial pressure of oxygen is based the whole process of biological P-elimination as well as the nitri/denitrification. It is also right the statement on the oxygen transfer across the phase boundary, the more effective processes in winter than in summer periods. The question is, what is cause and what is effect? The filamentous microorganisms cause foaming or take advantage only of the causes that lead to the formation of foam?
2. Biochemical bases of the process
With the operation of these plants is used in order to achieve higher performance an optimization method for the purposes of biochemical strategies for "overproduction" at the metabolic activity of the microorganisms. That means nothing other than that through manipulation of the metabolism of the microorganisms, which they usually acquired in the course of their evolution mechanisms, are used by the enzyme-catalysed reactions technologically, with the goal that both, the process speed is increased as well as the balance of the reactions are moved /2 Ott/. The increase in the speed of the reaction is carried out by increasing the concentration of the substrates and Enzymes.
The flakes forming micro-organisms produce on the cell exterior wall a slimy coating by secretion of exoenzymes, whose composition varies depending on the substrate range. Are carbohydrates and fats present in sufficient concentration, the exoenzyme coating consists mainly of polysaccharide - lipoid - hydrolases, which due to their high molecular weight have the effect as flocculation agents and effecting the flocculation of activated sludge. Predominate as substrate protein substances consisting the slimy coating of exopeptidases and amidases, in order to be able to survive after a switchover on the protein metabolism. The good flocculation properties are thus largely lost.
Substrate deficiency, temperature shift, change of the dissolved oxygen concentration will lead to the accumulation of Nucleosidpolyphosphate, which have an analogue function like the hormonal system of higher organisms and initiate the formation of extracellular and intracellular proteases. Extracellular polysaccharide - lipoid - hydrolases provide by the hydrolysis of low molecular and macromolecular compounds the cells easily assimilable substances, such as glucose and lower alkane acids. This process is particularly used in the phosphate release stage of wastewater treatment plants. The intracellular proteases accelerate mainly the protein turnover and thus have a great significance for the provision of amino acids and hence a selective effect on the growth opportunities. The flakes formers (flakes development through the effect of exoenzymes with polyauxic substrate offer for the development of new sources of external substrate) require higher concentrations of carbohydrates and fats during the logarithmic growth phase whereas the filamentous microorganisms their best development opportunities have in the presence of a sufficient supply of substrate mainly in the form of protein compounds (e.g. from the cell lyse of the flakes formers) and will make do with a lower levels of carbohydrates and fats. This form of cell lyse is carried out after the exhaustion of the external substrate in the secondary clarification tank.
Another effect of the interval change of dissolved oxygen concentration consists in a far-reaching regulatory change in the metabolism of microorganisms, which lead to a reduction of the oxygen demand by hydrolysis of high molecular weight organic compounds emitted by the exoenzyme hydrolases in the anaerobic phase (phosphate release level) and the induction or activation of transport systems for various metabolites. Depending on the delay time of the micro-biological system are set in short intervals anaerobic and aerobic environment states. This is a strengthening of the redox reactions by primary dehydrogenases, which realise the oxidation of organic compounds by dehydration, most commonly in use of nicotinamide adenine dinucleotide (NAD) as hydrogen ion acceptor. This is the reduction of the high molecular weight compounds in a cascade reaction, whereby the product of the initial reaction is the substrate of the following reaction. The preservation of the balance of this process is therefore of vital importance for the stability of the process. The different delay times for high and low water temperatures are often technologically not taken into account adequately.
The inhomogeneities of the dissolved oxygen to carry on this way, that the greatest part of the high molecular weight organic compounds by dehydration by means of dehydrogenases and in addition by hydrolytic splitting of bonds by hydrolases convicted in low-molecular compounds. This is compared to the oxidation under homogeneous environmental conditions have a considerable reduction of the oxygen consumption and thus the nitrification of ammonium is accelerated.
A further form of increase of efficiency of biotechnological processes consists in the fixation of micro-organisms and their exoenzymes grew up on so-called carriers, the favourable conditions for the development of slow-growing sessile microorganisms such as nitrosomonas and nitrobacter. Immobilized microorganisms have a higher process speed through the influence of the electric double layer of colloid particles and the activation of exoenzymes. This process is supported by the electrokinetic phenomena of the interfacial effects and the resulting changes in density and composition of the substances in molecular scale perpendicular to the Surface.
A phenomenon that occurs, is that fixed proteolytic enzymes also outside the cell in many cases under certain temperature conditions retain their full biological activity. (This effect was often in recent times to the aeration equipment with flexible membranes observed). Because of the fact that also the hydrolases can be fixed to the carriers, a biogenic activity of enzymes outside the cell in the psychrophile temperature range is achieved, which without carrier material or higher concentrations of water-soluble soaps by lowering the surface tension of the water is not reached. On the surfaces of an active carrier material, there is a dynamic equilibrium between decomposition of organic substances and expulsion of the oxidized products. A dense occupancy with micro-organisms inhibits not this process in case of active carrier material. In the case of large activated sludge flakes occurs therefore not any limitation of the diffusion processes on fixed micro-organisms.
3. The effect of the metabolites and the substances of the autolysis of the cells
3.1 Influence on the flocculation properties and the sludge volume index
A "survival strategy" of the micro-organisms is that its metabolism can extremely quickly adjust to drastic changes in the oxygen and nutrient content, temperature, and other environmental conditions. As a result, they will offer during the aerobic phase in the presence of an appropriate nutrient a stormy construction metabolism. The organic substances in the wastewater are removed and in the form of single-celled-organisms protein and nucleic acid as a cell substrate synthesized. Contains the liquid phase remain sufficiently dissolved phosphates, they are used to prevent overheating of the cell due to high heat release in the metabolic process in the form of polyphosphate accumulator (Valutingranula) as energy-rich phosphoproteins stored in the cell. During this stormy period of growth, are formed endogenous proteases as enzymes for catalytic acceleration of this process in proportional flow to the mass increase of organic substance. As C-sources are used carbohydrates and fat compounds.
First, will intake the in the waste water situated low molecular weight substrates, such as monosaccharides or lower alkane acids. Are exhausted these sources, the flakes-forming microorganisms open up new sources of external substrate, either by using a quasi-hormonal control by mediators, such as Nucleosidpolyphosphaten, is initiated the emission of exoenzyme. At this point in time are mainly hydrolases, which can split high molecular weight carbohydrate and fat compounds, such as esterase or glycosidases /6 Ott/. These polysaccharide-lipoid-hydrolases form on the cell wall a slimy coating, which has the same high molecular weights and has the same characteristics as synthetic flocculants and thus lead to the flocculation of activated sludge. This has an effect of flocculation and therefore this sludge has a low sludge volume index.
At the end of the aeration tank are the organic substrates of wastewater in accordance with the balance of the micro-biological systems of the respective wastewater treatment processes. These balances are available for high-load plants at 40 ... 70 mg/l BOD5, for low-load plants at 15 ... 30 mg/l BOD5 and for the advanced wastewater treatment plants at 6 ... 20 mg/l BOD5. At this point in time, the logarithmic growth phase is terminated. It is followed by the deceleration phase and the stationary phase of growth. Due to the nutrient limitation is triggered in the micro-organisms the as "stringent response" characterised reaction. This means that after a so-called "saving circuit", in which all energy- and material-intensive reactions are inhibited, started the use of the inner reserves and after them started the looking for new external Substrates.
During the saving circuit phase, is inhibited the synthesis of stable ribonucleic acids (rRNS, tRNS) and all amino acid-consuming reactions are reduced or stopped. At the same time, such processes are stimulated, which would lead to an amino acid accumulation. The for this necessary energy will be extracted from the stored phosphorus compounds. In the stationary growth phase begins a gradual death of microorganisms, so that will be equal growth and mortality rates. The decrease in protein synthesis is, at the same time, connected with a strong reduction in the number of ribosomes in the cell. In Escherichia coli, these e.g. approx. 30.000, so about 50% of the cell weight, which after deficiency symptoms can be reduced to approximately 7,000 ribosomes within a short period of time.
Because in this phase the carbohydrates and external fat compounds in wastewater will be used to equilibrium, will produced Exopeptidases (amino- and carboxy peptidases) and amidasen (asparaginase, ATPases, etc.) in order to make able to use the by the start of the cell lyse released substances. The microorganisms turn on protein metabolism and use in addition to their own inner reserves the cell substrate of dead cells. The status of the endogenous breathing is achieved.
The filamentous microorganisms present lower saturation constants in the Monod equation, as the flakes forming. They can use the residual concentration of carbohydrate and fat compounds and will overriding use the substrates of the cell lyse of the flake’s formers. They develop therefore not a slimy coating by secretion of exoenzymes on the cell exterior wall. The, during the autolysis of the cells released purines and polyphosphates act in addition as deflocculants, so that the activated sludge will be very bulky.
A key role in the detection of these "deficiencies" and the intracellular implementation is assigned to the Nucleosidpolyphosphate. The Nucleosidpolyphosphate have a high metabolic lability, causing a rapid fluctuation of the intracellular concentration in response to changes in the environment. They occur during the deceleration phase. Come to these C-sources- or amino acid deficiency still other stress factors such as lack of oxygen, these processes are still triggered faster and more profound metabolic regulatory intervention, such as activation of oxidoreductases (cytochrome oxidases), transferases and different cytochrome systems.
At the stage of the autolysis of the cells, however, are also released the stored organic phosphorus compounds with -P-O-P-bonds (polyphosphates, pyrophosphate) and histidine. These substances are strong chelating agent (metal ion complexes, which means even in the presence of a case-by-case basis remain in solution) and prevent the coagulation of the sludge as a result of their deflocculating effect. Its effect is equal to the in the industrial production known "biologically active detergent additives" for the stabilization of suspensions. They are in extremely low concentrations (1 ... 5 ppm) highly effective. Already at a concentration of 1 ppm lime remains in solution, the for the flocculation effect required CaCO3 - establishment is completely blocked. Iron salts remain ineffective as well.
In the winter time react the for chemical phosphate precipitation admitted metal salts (iron chloride or iron sulphate) during the recirculation with the metabolic products of microorganisms and can result in esterification and in case of displacement of the equilibrium position to saponification and can lead in case of reaction with alkyl residues to sulphonic acids, causing a strong foam formation. The alkali and earth alkaline salts of sulfonic acids are readily soluble in water. These reactions are photo synthesis, which proceed under the influence of visible light and ultraviolet radiation.
3.2 Impact of internal and external recirculation on the composition of the cell substrate
If one considers the return sludge management, it should be noted that the duration of retention in the second clarifier and the pump sumps have an influence on the composition of the cell substrate. In the return sludge is a rapid depletion of dissolved oxygen, so that the micro-organisms switch on the nitrate respiration and the denitrification processes start already in the second sedimentation tanks. A too long retention time in this environment can have a negative influence on the sedimentation processes in low-load systems. In systems with nutrient removal are still so much internal reserve substances present that in this stage an autolysis of cells occurs only if the activated sludge supernormal long time remains at this level.
In the case that the return sludge is leaded via a return sludge - denitrification stage, the exogenous substrates will be used in the stage of nitrate respiration. In this case the phosphoproteins are hardly reduced. The same is expected when a part of the internal recirculation is as long as directed in this stage, as long as the assimilable substrates are equivalent to the nitrate oxygen and no fault with the phosphate release in the anaerobic stage occurs. The remaining part of the internal recirculation must be after the anaerobic stage lead in the denitrification tank (possibly designed as a "control stage"). Will now lead the denitrified return sludge/waste water mixture in the anaerobic stage, the phosphate rich sludge assimilated in this stage hydrolysed low molecular substrates, such as monosaccharides or lower alkane acids and store them as polyhydroxy alkane acids (mostly polyhydroxy butyric acid) or phosphorus-containing fatty substances (Phosphatide or other phospholipids in the cell /Ott 6/. As energy source will be used the phosphoproteins, which during the aerobic phase were embedded as storage substances to prevent overheating of the cells. In order to maintain the balance of the osmotic pressure of the cells, so that the cell internal pressure is not too large, the protein-bound phosphates were after depolymerisation as ortho-phosphate partially again launched back into the liquid phase (phosphate release). The cells are now again full of fat compounds and can use in the subsequent denitrification zone nitrate oxygen for the respiration. In the ensuing aerobic phase are consumed the stored fatty substances, which are stored in form of polyhydroxy alkane acids and carbohydrates, mostly in the form of glycoside. In the citrate cycle synthesized poly-phosphates are stored as phosphoproteins and the process begins again.
In the case that the denitrification process due to residual concentrations of dissolved oxygen or temperature influence is disturbed, there will be a havy autolysis of the cells. The cell content is rich in polyhydroxy alkane acids, which react to soaps and ester. These processes are strongly influenced by the buffer capacity and the electrokinetic potential of wastewater.
Esterification and its reversal the saponification are equilibrium reactions, which in its results are depend from the quantitative ratio of the reactants. For the ester formation shall be governed by the law of mass action
[acid] * [alcanol] = K
[ester] * [water]
In other words, is increased the counter of this fraction by higher concentrations must also increase the concentration of esters, because the value of the fraction is constant. The reverse process is a saponification, where the esters will be split by water in alcanols and acids. The salts of carboxylic acids are soaps. The alkali and ammonium salts of carboxylic acids are water soluble and react alkaline. Carboxylic acids form with potassium soft soaps, which have also in cold water a good sparkling effect. They form an emulsion with grease and reduce the surface tension of the water, what leads to its washing property and its foaming. The ammonium soaps have the same effect.
With precipitants, such as iron(III)-chloride, calciumclorid or aluminium sulphate are triggered precipitation reactions of carboxylic acids salts. In hard water, where contents dissolved calcium and magnesium salts, will be formed with the calcium and magnesium ions a water-insoluble soap, that does not foam so much. With calcium chloride is caused a spontaneous lime-scale formation in hard water, with hydrochloric acid, for example, can supplanted the carboxylic acid from its salt. The carboxylic acid drops swimming without foaming on the solution.
The boiling points of some alkanes (e.g. 2,2-dimethyl propane) and amines (e.g. dimethylamine) lie in the area of water temperatures in the winter time (14…16°C), which is why can occur temperature-related features. While at higher temperatures, the concentration of these compounds in the water is low, it increased the value below the boiling points of the alkane and amine. This can be to exceed the threshold concentration (threshold effect), which can be triggered the reactions for the synthesis of foaming substances.
4. Requirements for the process control
The control of processes that build on such insufficient and stress conditions, is difficult due to the short interval duration of runnable processes. This is caused by the need to increase the energy metabolism of the microorganisms and the targeted reduction of biomass production through the creation of space- and time-dependent changes in concentration of the substrate and the dissolved oxygen in the various reactor zones under the changing loads and temperature conditions.
4.1 Changes in concentration of dissolved oxygen
The creation of short-periodic changes in concentration of dissolved oxygen, their interval duration is adapted on the delay time of the micro-biological system, is particularly important for the release of biopolymers of the cells of microorganisms of activated sludge, as well as to increase the synthesis of exoenzymes and increase their activity and varies with the day and week chart courses of stress and with the seasonal temperature trend. In this control process must watch to prevent the inhibition of the activity or the synthesis of exoenzymes due to increasing concentration of metabolites /2 Ott/. A significant influence on this process on biofilm reactors has the fluidization of the carrier material. This effects in the anoxic and anaerobic reactor zones by means of a closed blanket on the surface the prevention of the entry of air oxygen through the air/wastewater interphase boundary into the wastewater. This will not require to cover these reactors. On the other hand, the micro-organisms and exoenzyme are fixed on the carrier material and must be fluidized to ensure a permanent boundary renewal.
4.2 Increasing the concentration of dry matter
Despite constant degradation of the organic share is to ensure an increase in the concentration of dry substance in the system. This will be obtained by the fixation of the micro-organisms and the exoenzyme on the carrier and, on the other hand, by controlling of the external and the internal recirculation. In the anoxic zones is by agitators to ensure that occurred a closed surface by means of a grain filter acting as a floating layer from the carrier material and, on the other hand is to ensure the fluidization of the carrier with the necessary velocity on the bottom. The fluidization of the carrier material in the aerobic zone must be ensured through the formation of a loop flow by the aeration equipment. In the anaerobic reactor zone is to focus as much as possible carrier material. In this stage is the all-important hydrolysis processes through the carrier fixed micro-organisms, which are necessary for the biological phosphate elimination process and perpetuation of the denitrification even under unfavourable conditions of the C : N ratio.
4.3 Control of the effluent concentrations and pH-value
In the microbial oxidation of ammonium to nitrate via nitrite involved bacteria groups have relatively low growth rates, which will be strongly inhibited caused by high concentrations of NH4+, NO2- and NO3-. Only in a very narrow range of pH is to expect a complete nitrification /3 Ott/.
At high concentrations of these compounds and at low temperatures, can one not expect, as a result of lower saturation values of the gases, a self-regulation of this system. The operation is aggravated by the fact that the nitrification by the formation of H+-ions even negative intervenes in the buffer system. In contrast to the Nitrification leads the Denitrification by the formation of OH--ions to an increase in the buffering capacity and the pH-value. It is therefore to make sure a sufficient dilution by the external and internal recirculation. The balance between nitrification and denitrification must therefore always, not only from an energy point of view, be respected in order to prevent a collapse of the system.
A further essential influence on the nitrification has the relation between the organic load of wastewater and the nitrogen concentration in the inflow. The lower the quotient BSB : N at the influent of the plant is, the stronger will affect load fluctuations and temperature conditions. The freight fluctuations are particularly pronounced with excess of carbon compounds during the day and shortage during the night hours /Peukert 4/.
On the carrier material in the anaerobic reactor zone settle acetogene bacteria. The coarse waste water substances, which are held back in the carrier material layer will be hydrolysed by this "biofilm" and thus will be ensure, also in times of low flow, always a sufficient carbon supply and obtain an equalization of the BOD : N - relationship. By the upstream denitrification stage is reached a positive influence on the pH-value in the nitrification stage. The prerequisite is, however, the procedural decoupling of the metabolism of the microorganisms in the biofilm of the anaerobic reactor zone from metabolism of the microorganisms in the activated water-sludge suspension, which circulates through all the reactor zones.
4.4 Control of the metabolic activity of the microorganisms
During the transition from aerobic to anaerobic reactor zones changes the contents of the cell substance of the micro-organisms is fundamental. In the aerobic zone are polyphosphates, so-called "Valutingranula" stored in the cell, whose concentration increases with a constant change of environmental conditions (phosphate sludge). In the anaerobic phosphate release zone will be depolymerised and ejected these polyphosphates and with the time are stored ever larger fat pads (lower alkane hydroxy acids), which under anoxic and in particular aerobic conditions milieu will be assimilated. If the cycles of the polyphosphate and fat storage will be inhibited, for example at low temperatures, will result an excessive fat storage concentration in the cells. After exceeding a threshold concentration results with that a dispersion stabilization by fatty acid condensation products in the sludge. This excessive concentration of endogenous fat storage substances lead to metabolic problems. The transition to the protein degradation is greatly delayed, the esters - soap - balance shifted. The result of a strong aeration of this sludge results in a high forming of foam /3 Ott/ and the possibility of use of non-degraded protein compounds to the rapid development through the filamentous bacteria, because these will no longer be inhibited in their development by the reduction of the flakes formers.
Furthermore, will be used as assimilable substrate in case of too low external substrate supply the substrates from the lysed cells, such as oligopeptides and amino acids. With this will be lost a triggering factor for the initiation of the Exoenzym formation (amino acid deficiencies). The processes of elimination of phosphorus and denitrification can collapse.
The extracellular proteases have the effect of biogenic flocculants, which are comparable in terms of molecular weight and the flocculation effectiveness with the synthetic flocculants. If their production is inhibited, loses the activated sludge its good flocculation properties. Particular importance have in this regard, next to pyrophosphates and purines ,the histidine. This amino acid is used only in the growth phase. It is an integral part of the active centre of many enzymes, where they are used for the detection and formation of negatively charged substrates and anionic cofactors. Their special position is underlined by the fact that it is the only proteinogenic amino acid, which is effective in the physiological pH-range 6...8 as buffering capacity. This amino acid will perform very stable complex formation with metals. This may result in a return solution from heavy metals and alkaline earth metals from the Sludge.
The metabolic chelating agent release by complex formation the Ca2+-ions from the lipopolysaccharide layer that surrounds the microorganisms. They can resolve even heavy metals, which are needed for the load balancing of the to be hydrolysing substrates, such as also negatively charged polysaccharides. This makes histidine and some of its biosynthesis precursors after exceeding a threshold concentration as deflockulants. The sludge flocks dissolve and the sludge volume index is rapidly increasing. After the fall of the dominant flakes formers have certain filamentous microorganisms the opportunity for rapid growth, using the amino acids of the lysed cells. Connected with the end of the stationary growth phase of the flakes formers is a shift of the pH-value of the activated sludge in the alkaline range, which is then negatively charged. This change of the electrode potential has in addition to the influence of the molecule size a major impact on the migration velocity of the particles in the electric field and affects the lysis of the cells. The acid capacity decreases with insufficient denitrification on the critical value of 1.5 mmol/L. Under these conditions, there must taken measures to increase the acid capacity.
Literatur:
/1/ Dr.-Ing. Werner Sternad, Dr. Brigitte Kempter: Schaumbildung in der biologischen Abwasserreinigung - Ursachen und Gegenmaßnahmen
Fraunhofer-Institut für Grenzflächen- und Bioverfahrenstechnik Stuttgart
/2/ Dr.-Ing. Peter Ott: Neue Verfahren der Wasserbehandlung auf der Grundlage der Biotechnologie,
Wasserwirtschaft – Wassertechnik 8 (1988)
/3/ Dr.-Ing. Peter Ott: Treatment plants for biological nitrogen removal from landfill leachate
Paper 2 given at German-Chinese Special Waste Seminar; University Beijing; Dec. 1994
/4/ Dr. V. Peukert: Mit Aufwuchsträgern gegen Stickstoff und Phosphat
Institut für Wasserwirtschaft, Berlin
/5/ Dr.-Ing. Peter Ott: Using autochthonous enzymes in compact reactors for sludge stabilisazation and thickening in sewage treatment plants
Report given at World Congress "Biotechnology 2000" ,Moscow 1988
/6/ Dr.-Ing. Peter Ott Behandlung von Abwasserschlämmen, die bei der biologisch-chemischen Phosphatfällung anfallen
Wasserwirtschaft – Wassertechnik 7 (1988)
