130325
Physiological fundamentals of biological wastewater and sludge treatment
and their application in the operation of wastewater and sludge treatment technologies by practitioners
“Anything found to be true of E. coli must also be true of elephants."
(Jaques Lucien Monod)
Table of Contents
1.. The waste water treatment plant as a complex technology and a complex system together with the sewerage network
2.. What are strategies to achieve an “Overproduction” in the course of microbiological metabolism by use of stress situations?
3.. Which are the main principles which superimpose?
4.. How do use according to the state of art these elements during the operation of the wastewater treatment plant?
5.. What happens in Zone I?
6.. What occurs in Zone II and Zone III?
7.. The metabolism in Zone IV and V
8.. What is the cause for formation of bulky sludge?
9.. What is the reason for formation of foam?
10. How can we control these processes?
11. How do use these elements during the operation of the sludge treatment steps-considering the special attention on MBBR-process and MBR-process?
12. Influence of the sludge thickening and dewatering processes
1.The wastewater treatment plant as a complex-technology and a complex system together with the sewerage network
The wastewater- and sludge treatment is a complex technology of a multitude superimposed balanced processes, whereas the state of balance of these processes will be continual changed by substances, which in the different steps will be formed or deconstruct. A significant influence on the state of balance of these processes have especially the temperature, the boiling points of the alkanes and amines, redoxpotential, C:N-relation and the water hardness, which is occasionally influenced by melting of snow. Particularly with the application of processes for biological elimination of phosphorus and nitrogen compounds from wastewater and intensified after the application of membrane reactors to solid/liquid separation the sludge removal according to the state of the art takes not into account the consideration of the physiological fundamentals of microbiology, of the mass-action law and of the Monod-equation.
Since the beginning of the biological wastewater treatment is removed the produced surplus activated sludge together with the return sludge at the end of the technological chain of wastewater treatment after the solid/liquid separation before the discharge of the treated wastewater into the recipient. This process management was also maintained after the introduction of the biological nutrient removal processes and of the Membrane-Bioreactor- Process (MBR-process). It has always been only being proposed additional process steps to improve these existing disadvantages for the process of increased phosphorus reception in the anaerobic Zone and for the processes in the digester and the sludge dewatering.
The processes of the biological phosphorus- and nitrogen removal use physiological “stress conditions” for the generation of an "overproduction" or "luxury metabolism" of these nutrients. During the treatment of waste water is in the aerobic phase oxidized ammonium to nitrate and as energy reserve stored polyphosphates (Volutin granules).
The phosphate reserves will again have discharged in an anaerobic phase of the microorganisms to prevent overheating and stored low molecular weight fatty acids in the cells. This process required a strong anaerobic milieu without dissolved or as nitrate bounded oxygen. The lowest amount of oxygen has in this zone a negative influence on the effectiveness of this process. Therefore, have been installed different additional process steps to remove the residual oxygen. The effectiveness of the phosphorus removal can be increased by increasing of the concentration of microorganisms in the anaerobic zone.
The oxygen, which is bounded as nitrate will in a phase without dissolved oxygen used of the microorganisms for the metabolism to build up cell substrate. After exhaustion of these energy reserves started an increased autolysis of cells. This autolysis of the cells leads to a serious deterioration of the dewatering characteristic of the surplus sludge by the release of ortho-phosphate, fatty acids, and cell substrate (Purine, Histidine, and other nucleic acids) causing the deflocculating of activated sludge as these substrates act as detergents and chelating agents. The microorganisms switch in the anoxic phase to the nitrate respiration, which is under consideration of the mass-action-law the reason that biochemical bounded water will be in the cells. That water cannot release by use of physical dewatering processes. The released cell substrate changed the state of balance of the carbon-protein-relation into direction of predomination of proteins and leads therefore to the saponification and forming of a sudden foam in soft water while temperature shift in the range of the boiling points of the released amino acids and to significant favourable growing conditions for filamentous microorganisms with the result of producing of bulky Sludge.
If residual oxygen (dissolved or bounded as nitrate) will be carried over into the gravitative thickening reactor or the digester leads the release of ortho-phosphate in the anaerobic digesters to a disadvantage of the operation due to deposition of Magnesium-Ammonium-Phosphate (MAP, Struvite) in the pump stations and the effluent piping of the sludge dewatering centrifuges. To control these difficulties are always necessary new additional process steps in the treatment technology, from what often the benefits of the biological nutrient removal will be lost.
2. What are strategies to achieve an “Overproduction” in the course of microbiological metabolism by use of stress situations?
First, I like to give the practitioners a very short initiation in the fundamentals of biochemical metabolic pathways of microorganisms to have a better understanding for the multistep reactions in biological treatment systems. That may give a survey of enzymes catalysing the biochemical turnover in living matter. The most biochemical reactions are reversible depending on the supply of reaction partners, removal of the reaction products and the state of balance of these reactions. The technological use of these interrelationships of the metabolic pathways causes in the practice a very big problem as the reactions are superposed and depend on load, retention time, temperature, pH-value, redoxpotential and substrate as well as time interchanges. These practical problems we will discuss later.
A shortage of substrate, temperature shift and changing of dissolved oxygen concentration in wastewater are the reasons for accumulation of nucleosidpolyphosphate in the microorganisms which, like the hormonal system of higher organisms, initiate the formation of extracellular and intracellular proteases. Extracellular polysaccharide-lipoid-hydrolases provide for the cells low-molecular weight, easy assimilable modular units (glucose, alkane acids) by hydrolysis of macro-molecular compounds. The intracellular proteases accelerate mainly the protein turnover and provide amino acids as preconditions for the growth of microorganisms. By means of that they have a selective effect on the growing possibilities.
Inhomogeneity of dissolved oxygen contributes that the most part of macromolecular organic compounds transfer due to dehydration by means of dehydrogenases and by hydrolysis of macromolecular compounds by hydrolases into low-molecular weight units. In contrast with the oxidation under homogeny milieu conditions will be achieved a considerable decreasing of oxygen demand and with that will be accelerated the nitrification of ammonium. (The dependence of the electrokinetic potential we will see later).
On the changing of partial pressure of dissolved oxygen and the type of organic substances is based the entire process of biological P - removal, as well as the nitrification/denitrification process. To achieve higher efficiency, it must be used an optimization in the biochemical strategies to reach an "overproduction" of the metabolic activity of the microorganisms. This will occur by technologically manipulation of the metabolism regulations which the microorganisms have acquired during their evolution. Therefore, we have the technological possibility to increase or decrease the concentration of substrate and enzymes with the goal for the enzyme-catalysed reactions both increase the process speed and shift the balanced conditions of the reactions (supply of reactions partners, removal of reaction products and location of equilibrium).
A directly influence on the temperature (temperature shift) will be usually not possible, but we can have an influence on the seasonal constrained effects of the temperature changes. The state of balance during the cascade reactions are also depend on the temperature and has a very important consequence for the stability of such processes and the electrokinetic potential of redox reactions with their influence on the oxygen transfer through the membranes, which are in winter more effective as in summer periods. Therefore, the seasonal temperature changes (8°C to 24°C) during spring and autumn causes if they reach ~ 14…16°C a change of the balanced reaction of the ester formation with the result of production of water-soluble potassium liquid soap or ammonium soap. These create with fat in soft water emulsions which decrease the surface tension. This is the reason for formation of foam during these periods. In the case of more hard water (containing a high concentration of calcium- and magnesium-ions) will be created a water-insoluble soap, which not causes foam.
To note is also the carry-over of dissolved oxygen by the recirculation processes of the different technologies (secondary sedimentation tanks; membrane bioreactors; multistep feed processes) or the different left-over of organic substances of the different treatment processes because of the balanced conditions (high sludge load; low sludge load; extended aeration; MBR, biofilm carrier process).
In case that a tails assay of dissolved oxygen or/and temperature shift disturbs the denitrification, process occurs an intensified autolysis of cells. The cell substrate is reach on poly-hydroxy-alkane-acids, which also react to esters and soaps. These reactions are influenced by the buffer capacity and the electrokinetic potential of the wastewater and have a very intensive effect on the balanced reactions of esterification and saponification while they change the quantitative proportion of the reaction partners (mass-action law). We will discuss this in detail later.
At the end of the biological basin are limited the organic substrates of the waste water according to the balanced conditions of the microbiological systems of the respective wastewater treatment processes. Now start the microorganisms an as “stringent response” called reaction. This means that after a so-called "saver mode" or “lag phase”, in which all energy and structural intense reactions are inhibited, begins the use of the inner reserves and it is kept a look toward new external substrate.
An additional possibility to increase the efficiency of biotechnological processes can be reached by fixation of microorganisms and their exoenzymes on carriers. With that will reach favourable conditions for the development of slowly growing sessile microorganisms like Nitrosomonas and Nitrobacteria. Immobilized microorganisms have a higher process velocity due to inducement of the electrical double layer of the colloidal particles (higher density and more effective composition of the substances on the cell-membranes). A phenomenon that thereby occurs in many cases under certain temperature conditions (in the psychrophilic range ≤ 10°C) is that fixed proteolytic enzymes keep outside of the cells their full biogenic activity. These strategies must be considered in the complex of the influence of sewer network, the mutual treatment steps in the treatment plant as a complex technology and also regarding the main principles of superposition.
3. Which are the main principles which superimpose?
3.1 All life processes follow a natural logarithm function (ln x), which can be divided into four phases:
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Phase -A maintenance metabolism
- Phase- B process of growing - construction metabolism
- Phase-C lag - Phase (temperature sensitive)
- Phase-D autolysis of cells
3.2 Dissolved oxygen in wastewater and bounded Oxygen in the cells:
- anaerobic environment (use of oxygen bounded on P as PO4)
- anixic environment (use of oxygen bounded on N as NO3)
- Aerobic environment (dissolved oxygen in wastewater)
- inhomogeneity of dissolved Oxygen
4. How do use according to the state of art these elements during the operation of the wastewater treatment plant?
4.1 Design, calculation and operation of a WWTP starts with the analysis of the network and the influence of faecal matter from septic tanks:
Sewer System
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Influence of seasonal variations
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Pumping stations
- Retention time - flow time
- Water hardness (also temporary in case of melding of snow)
- BOD:COD ratio and C:N:P ratio
- Impact of Biofilm on the pipe wall and NO3 content in wastewater
Faecal matter
- often partially digested
- Carbon source depending on maintenance Quality of the septic tanks
- BOD:COD ratio and C:N:P ratio
4.2 The biological tank is divided into five zones:
Its starts with zone 1- phosphorus release tank equipped with agitators. There work in accordance with section 3.1 the phase A (maintenance metabolism) and D (Autolysis of cells). The Operation Control has to consider
the concentration of the easy degradable low-chain organic substances (monosaccharide and alkane acids)
the anaerobic environment (use of oxygen bounded as PO4 and disability by NO3 and dissolved oxygen
Minerals supply (potassium content, magnesium content, calcium ions, and sodium content)
the ph-value and redox potential
biogenic activity of enzymes for proteolysis external of the cells, dynamic balance of degradation of organic compounds, and rejection of the oxygenated products
The next tank is the anoxic zone 2 and 3 - pre-denitrification equipped with Mixers (zone 3 will be optional used as an anoxic and aerobic zone and is eqipped with mixers and an aeration system). There works in accordance with section 3.1 the phase A (maintenance metabolism). The Operation Control has to consider
the concentration of the easy degradable low-chain organic substances (monosaccharide and alkane acids)
anoxic enviroment (use of oxygen bounded on N as NO3)
Nitrogen compounds (reduction of NO3 to N)
the ph-value and redox potential
esterification and its inversion saponification
balanced conditions in cascade reactions
dynamic balance of degradation of organic compounds and rejection of the oxygenated products
5. What happens in Zone I?
The zone I has an anaerobic environment, the redox potential shall be very negative. Into the zone I flows the wastewater which is characterized by the sewage system and will be mixed depend on the treatment process with the recirculation of sludge or the internal recirculation of wastewater from biological treatment process. That means depend on the length and structure of the sewerage (seasonal fluctuation, activity of the sewer slim, COD : BOD-relation, retention time, and inhomogeneity of the velocity) the wastewater contents more or less easy assimilable low-molecular substances. Depend on the characteristic of the aeration process varied also the concentration of carbon sources in the recirculated treated wastewater as well as in the recirculated sludge (according to the balanced conditions of BOD5 = 40 to 70 mg/l for high sludge load, 15 to 30 mg/l for low sludge load and 6 to 20 mg/l for nutrient removal processes). The processes are depending on the pH-value, redox potential, and concentration of phosphates in the recirculation sludge. In case of biofilm technology will be the biogenic activity of enzymes for proteolysis also external of the cells wherefore will be increased the dynamic balance of degradation of organic compounds.
The microorganisms have a very high capability of conversions and adaptations to extreme metabolic situations by production of enzymes. In single cells the enzymes are estimated up to 5,000 – 10,000 types.
We remind us: During the aerobic phase the microorganisms undertake, if they have a sufficient food and nutrient supply, a whirlwind construction metabolism. Are carbohydrates and fats in a sufficient concentration in fluid, then the exoenzymes slim will be mainly from polysaccharide-lipoid-hydrolases. The organic substances of wastewater will be decomposed in a cascade reaction (the substrate of the following reaction will be result of the previously reaction) and synthesized as cell substrate in the form of one-cell protein, nucleic acid, and phosphoprotein as well as in case of “overproduction” as inorganic polyphosphate like Volutin granules as energy reserve and triglyceride as fat reserve. These hydrolases act due to their high molecular weight like industrial produced flocculants und causes the formation of flocs in the activated sludge.
If the cells then in the lag and autolysis phases, because of food shortage, cell substrate like protein and amino acid ejaculate into the fluid are predominant proteins and aminoacids as substrate available. In this case the exoenzymes slim altered to exopeptidasen and amidasen to have the possibility to use the own internal reserves and the cell substrate of the cell lyses as well as the easily assimilable low-chain alkane acids (mostly polyhydroxybutyricacid) and glucoses for survival and quickly to switch to the protein metabolism. The inorganic polyphosphate energy reserves are now no more necessary and will to prevent an overpressure in the cells spout as ortho-phosphate into fluid. Caused by the cell lyses the cells spout also purine (compound found in nucleic acids) and polyphosphates into the wastewater acting additional as deflocculates and the spouted histidine acting as chelating agents with the result that the cells lose widely the good flocculation properties.
cells out of zone 1 cells out of zone 4 → into zone 1
6. What occurs in Zone II and Zone III?
The zone II has an anoxic environment, i.e. no dissolved oxygen is present. The redox potential shall be negative and bounded oxygen of the nitrates will be used for pre-denitrification. Necessary are here easy degradable low-chain organic substances like monosaccharide and alkane acids. The sludge recirculation and the internal wastewater recirculation have a high effect on the composition of the cell substrates. The different treatment technologies require different types of recirculation. Have we first a look on the classical activated sludge process. Here influenced the retention time in the secondary sedimentation tanks and in the pump sumps in the way that quickly decreases the dissolved oxygen concentration. Therefore, the microorganisms will switch on the nitrate respiration for the metabolism of the, during the retention time in the anaerobic zone I, stored fatty substrates (alkane acids).
In case of MBR process the internal recirculation will directed into the Zone II. The effect is like the direction of the activated sludge through a recirculation sludge denitrification step. That means the endogenous substrates will be used during the retention time in the anoxic zone by nitrate respiration. The phosphor-proteins of the cell substrate and the polyphosphate will hardly reduce.
If the denitrification process would be disturbed by tails assay of dissolved oxygen or by temperature shift (boiling points of alkanes and amines), results an increasing of autolysis of cells. The cytoplasm is reach on polyhydroxyalkanacids, which depending on the buffer capacity and the electrokinetic potential of the wastewater react to soaps and esters (formed by an acid with an alcohol like glycol, e.g. triglyceride).
In case of increasing of autolysis of cells we have also an increasing of the concentration of protein and amino acids. If the concentration of protein and amino acids is predominant during this phase, the filamentous microorganisms would have their favourable growing capabilities. Sufficient protein compounds they can use by the cell substrate of flocculants after the autolysis due to shortage of carbohydrates or/and temperature shift.
With that microorganisms, which can use a smaller concentration of C-sources and can use amino acids and protein in their metabolism, will have the possibility for a mass growth. As the filamentous microorganisms have a less saturation Monod-constant Ks as the flocculants will be performed a bulky sludge from filamentous microorganisms.
Esterification and the reverse reaction saponification are equilibrium reactions, where the results are depending on the quantitative ratio of the reactants. For the esterification applies the mass-action law:
That is, is the counter of this fraction increased by higher concentrations must also increase the concentration of the esters, since the value of the fraction is constant. The reverse process is a saponification, that is, the esters in the wastewater will divide in alkanol and acid if the acid concentration will be decreased by alkaline solutions and -OH groups, which will be performed during the denitrification process.
The boiling points of alkanes and amines are in the range of the temperature shift during the winter period, which is why increases the concentration of these compounds in wastewater. In case the threshold concentration will be exceeded can also be initiated in soft water the syntheses of foam capable substances.
The salts of the carboxylic acids are soaps. The alkali and ammonium salts of carboxylic acids are water-soluble and react alkaline. Carboxylic acids form soft soaps with potassium, which even in cold water have a good foaming capability. They form an emulsion with grease and set down the surface tension of water (ζ-potential), on what is based their foaming capability. The same effects have the ammonium soaps.
In hard water, which contains calcium and magnesium ions, will be performed with these ions water insoluble soaps, which not have the capability to foam. Using precipitants, like ferric chloride or aluminium sulphate, will perform the precipitation of carboxylic salts.
7. The metabolism in Zone IV and V
The zones IV and V have an aerobic environmental with dissolved oxygen in the wastewater. During polyauxic food supply, i.e. the stepwise adaptation of organisms to changing culture conditions by cascade reaction, the microorganisms have the capability of conversions and adaptations and can use also external macromolecular compounds by production of enzymes. If there is a high concentration of macromolecular carbohydrate and fat during the logarithmic growing phase the microorganisms produce exoenzymes, which are localized at the cell membrane, to start the cascade reactions by flocculation and they work together like a consortium. They start a vehemently construction metabolism using endogenous proteases. These proteases influence the catalytic acceleration and will be produced in a proportional amount to the growing of the biomass. The source of nitrogen, which is necessary for the formation of biomass, is the NH4 supply in the wastewater. Surplus ammonium will be oxidized to NO2 and NO3 in the wastewater.
Contains the wastewater still dissolved phosphates are these stored into the cells as energy reserve in the form of inorganic polyphosphate like Volutin granules to prevent an overheating of the cells. The preservation of the state of balance of this process (i.e. temperature, oxygen partial pressure, food, and nutrient supply) is therefore of vital importance for the stability of the process. The different lag phases for high and low temperature of the wastewater are often technologically not sufficient noted.
8. What is the cause for formation of bulky sludge?
In case of substrate limitation (limitation of carbon sources) have filamentous microorganism’s despite of the lower growth rate an advantage compared with flocculants as they have a less saturation constant in the MONOD-equation. If in the system is a limitation of carbohydrates and fat it will start the so called “stringent reaction”. That means that after a “stand-by mode” or “lag phase”, during this will be inhibited all energy- and construction metabolism intensive reactions like amino acid using reactions. It follows the autolysis of cells. That will be intensified by temperature shift and is the reason that in the system accumulates a higher concentration of proteins and amino acids. With this we have the best advance stipulation for the development of filamentous microorganisms, which have a higher efficiency in case of protein metabolism.
Monod-equation:
Where:
μ is the specific growth rate of the microorganisms
μmax is the maximum specific growth rate of the microorganisms
S is the concentration of the limiting substrate for growth
Ks is the "half-velocity constant"—the value of S when μ/μmax = 0.5
μmax and Ks are empirical coefficients to the Monod equation. They will differ between species and based on the ambient environmental conditions.
In some applications, multiple terms of the form [S/(Ks + S)] are multiplied together where more than one nutrient or growth factor has the potential to be limiting (e.g. organic matter and oxygen are both necessary to Heterotrophic bacteria).
The filamentous microorganisms don’t produce flake forming exoenzymes slimes. The during the autolysis of the cells into the wastewater spouted purine (compound found in nucleic acids) and polyphosphates acting additional as deflocculates and the spouted histidine acting as chelating agents with the result that the sludge will be very bulky. These agents we can compare with industrial produced detergents to stabilize suspensions (washing powder). They are in extremely low concentrations (1 ... 5 ppm) highly effective. Already in a concentration of 1 ppm remains lime in solution. The CaCO3 – formation, which is necessary for the flocculation, is totally blocked. Iron salts remain just as ineffective. These phenomena are often observed at underloaded treatment plants and/or during the periods with temperature shift.
For the treatment of wastewater, the C:N-ratio is crucial for the type of the metabolism. The microorganisms with a small saturation constant Ks of the Monod-equation (filamentous bacteria) are predominant the flocculants bacteria in the case of a high protein- and aminoacids concentration in wastewater and can in case of low C-concentration change in the protein metabolism and so start a mass-growing of filamentous microorganisms, which is the reason for bulky sludge. This will be intensified by increased autolysis of cells at the end of the anoxic zone III. This situation is exacerbated in the case of soft wastewater, even in the case of only occasionally mixture with melted snow water. If the wastewater temperature then still changed in the range of the boiling points of alkanes and some amines are given the preconditions for the formation of alkaline- and amminosoaps, which is why in soft water starts a spontaneous foaming. The fact that the sludge from this zone is taken, is the prerequisite for the possibility to control accordingly the sludge load to prevent these phenomena.
In the case of hard water, when a sufficient concentration of calcium- and magnesium-ions is present, can directed the liquid phase of the sludge thickening and the sludge dewatering directly into the anaerobic and/or anoxic reactor zones, as in these cases water insoluble soaps are formed.
The OH-groups, which are released in the anoxic zone by denitrification, would have been overlaid the concentration of alkane acids and thus would lead to a saponification according to the mass-action-law. By the necessary process control is neither the anaerobic zone loaded by caring over of nitrate and dissolved oxygen nor with that the phosphorus release disturbed, nor caused an excessive autolysis of cells by the OH-groups of the denitrification in the anoxic zone.
In the case of the feeding of the liquid phase of the sludge thickening and dewatering can be reduced accordingly the anaerobic recirculation of the wastewater from the anoxic zone into the anaerobic zone. A significant increase in the effectiveness of the phosphorus elimination is achieved when, instead of the liquid phase, a part of the thickened sludge is directed into the anaerobic zone I and the liquid phase will be directed into the aerobic zone IV.
The operating principle is in MBR-process characterised by the fact that in the bioreactor can be adjusted a concentration of dry substances (MLSS) from 6 to 20 g/L according to the selected sludge loading. The concentration of MLSS is in the range of the concentration of surplus sludge in conventional activated sludge plants. The solid/liquid separation is carried out by aerated Micro- or Nanomembrane, which is why the volume of the membrane chamber will be calculated to the aerobic reactor volume. The surplus sludge is taken from the anoxic zone III and has the lowest nitrate concentration. The return sludge is removed in the membrane chamber. From the end of the aerobic zone V will be taken the internal recirculation of the wastewater and directed into the anoxic zone II. The microorganisms of this mixture have a very high phosphorus concentration, which will be not reduced, as long as the nitrate is present, during the stay in the anoxic zone. As the chemically bound oxygen of the nitrates is used in that zone by the microorganisms for the respiration can remove from this zone the surplus sludge without to perform problems during the following sludge treatment steps. From this zone is also directed the wastewater for the anaerobic wastewater recirculation into the anaerobic zone I. In that zone will used from the microorganisms the on phosphorus chemically bounded oxygen as an energy source for the storage of fatty acids, and the ortho-phosphate will spout into the wastewater.
A possible variant is that instead of a separate anaerobic recirculation the liquid phase of the sludge thickening and/or of the sludge dewatering will be directed into the anaerobic Zone I. This is especially advantageous for hard water with sufficient calcium and/or magnesium ionic concentrations. So that will be not required separate construction, equipment, pumps, and piping. In case of soft wastewater would it be possible that the as result of the autolysis of the cells released ortho-phosphates, fatty acids, and cell substrate (purine, histidine, nucleic acids) in the case of an increase in the pH-value as a result of the released Hydroxy-groups lead to saponification and thus lead to foaming. The released cell substrate act as deflocculates and chelating agents. For this reason, must the process have controlled by a redox potential measurement and the aerobic zone IV has according to decrease (control stage) and the sludge load to control by removal of surplus activated sludge. By the anaerobic recirculation is largely suppressed the autolysis of the cells and thus not create favourable conditions for a mass growing of filamentous microorganisms.
A further possible variant is that also in the classical activated sludge process with nutrient removal the surplus sludge will be removed from the anoxic zone III. It is beneficial in this case to combine the thickening with the anaerobic recirculation so that the liquid phase will be used as anaerobic recirculation in the anaerobic zone I. If soft waste water is treated (also temporary during the period of melting of snow) and the sludge will be thickened by gravitation thickening, is it advantageous the liquid phase to lead into the aerobic zone IV.
A further optional possibility is that the surplus sludge separately or together with primary sludge or screenings from micro sieve in a single stage or two-stage digestion process will be conditioned and hygienized.
We will show now only on the process combinations biological nutrient removal with aerobic stabilization and with digestion.
For the first combination we have two variants:
12. Influence of the sludge thickening and dewatering processes
In recent years, are in some sewage treatment plants, which use the above-mentioned process combination a deterioration of the dewatering properties of the digested sludge and deposits in the supernatant liquid pumps and supernatant liquid pipes observed. To remedy this phenomenon, we can often see an incorrect diagnosis of the causes. For example, it is explained:
- The increased phosphorus content would cause the increased chemical bonded water of the digested sludge. With that would decrease the DS-content of the dewatered sludge and increase the demand of flocculants.
- The dissolution of phosphorus in the digester leads to an increase in the phosphate concentration. By the recirculation of the supernatant liquid in the bioreactor will decrease the effectiveness of the P-removal.
- The increased phosphorus concentration would lead to crystal formation as magnesium-ammonium-phosphate (MAP, Struvite).
- MAP and calcium phosphate causes in case of pH-value over 7.2 (by degassing of CO2) deposits in the discharge pipes of the digester and in the supernatant pumps.
What happens really in the digester? The dissolution rate for phosphorus is between 15 and 23 % and leads to a phosphate concentration in the supernatant liquid of 200 to 750 mg/L. A greater proportion is crystallized in the digester as magnesium-ammonium-phosphate. Therefore, it is often accused that the supernatant liquid would exceed the capacity of the biological P-removal. In this case would not consider that higher phosphorus concentrations will be considerable faster removed than lower concentrations. For example, if the inflow would have a concentration of 9 mg/L would it increase to ~ 11 mg/L. The capacity of the p-removal would not significant changed.
What will be the real cause for the deterioration of the dewatering characteristic of the digested sludge? We will remember on the physiological fundamentals of the biological wastewater treatment and can see that the reason for this phenomenon will be in the process step before the digester. It will be especially evident when the sludge is gravitative thickened and it is extra counterproductive in the case of common gravitative thickening of primary sludge and surplus sludge. The reason will be the start of the nitrate respiration and the with this joint biochemical incorporation of water in the cells, which cannot be dewatered with physical methods.
A remedy action is therefore only the separate thickening of primary sludge and the mechanically surplus sludge thickening!
Depending on the wastewater composition occur in fact deposits of magnesium-ammonium-phosphate and calcium phosphates in the effluent pipes of the digesters, intermediate basins, and supernatant pumps. The prevention is possible by
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dosing of FeClSO4-solution also in case of a good functioning biological P-removal before the secondary sedimentation with the goal to improve the SVI and the sulphur content in the biogas.
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reducing of deposits of calcium phosphates by dosing of mixture of FeClSO4-solution and citric acid (anti scaling solution) before the centrifuges.
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protection of supernatant pumps and supernatant pipes by coating with polyurethane
Considering the microbiological pathways, as demonstrated on the following diagram, and explained before, we can also see that in each case the common thickening of primary and surplus activated sludge in gravity thickener has as result the worst dewatering characteristic.
Therefore, the practitioner should consequent require for the thickening processes of surplus activated sludge a mechanical technology and the dewatering without intermediate storage basin to prevent the start of the nitrate respiration during that process.
The resulting dewatering characteristics are independent of the dewatering process as sludge drying beds, belt filter presses or centrifuges; they shall be formed during the wastewater treatment and the thickening processes.
For demonstration we will have a look on the following diagram, which shows the interdependence of the performance of sludge drying beds on the specific filter cake resistance:
