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APPLICATION OF SEWAGE SLUDGE IN INDUSTRIAL WASTEWATER TREATMENT Comprehensive reference examining activated sludge technologies in industrial wastewater treatment, combining a theoretical framework with practical methodologies Application of Sewage Sludge in Industrial Wastewater Treatment provides a roadmap to the methodologies for the treatment of industrial wastewaters from several major sectors integrating theory and practice, highlighting the importance of sewage sludge technologies in industrial wastewater treatment to clean up the environment from pollution caused by human activities, and assessing the applications of several existing activated sludge techniques and introduces new emerging technologies. All discussion within the text is based on a solid theoretical background. Application of Sewage Sludge in Industrial Wastewater Treatment covers key topics such as: * Issues related to activated sludge treatment, such as biodegradability-based characterization, modelling, assessment of stoichiometric, and kinetic parameters and design * Issues related to industrial pollution control, such as in-plant control, effect of pretreatment, and more * Recently increasing quantity and complexity of toxic effluents, which can be bio remediable for plants and suitable microbes, whether natural or customized for specific purposes * Ecological, profitable, and natural solutions designed to eliminate heavy metals, radionuclides, xenobiotic compounds, organic waste, pesticides, and more This reference provides an essential, one-of-a-kind, integrated approach for environmental microbiologists, biochemical engineers, environmental engineers, effluent treatment plant operators, and biologists and chemists at wastewater treatment plants.
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Edited By
Maulin P. Shah
Applied & Environmental Microbiology Lab Gujarat, India
This edition first published 2024
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Cover
Title Page
Copyright Page
1 Sludge Conditioning, Activation, and Engineering
1.1 Introduction
1.2 Conditioning
1.2.1 Conditioning of Sewage Sludge by Increasing the Rate of Hydrolysis
1.2.1.1 Mechanical Pretreatment
1.2.1.2 Thermal Hydrolysis
1.2.1.3 Chemical Pretreatment
1.2.2 Conditioning for Sludge Dewatering
1.2.2.1 Physical Conditioning
1.2.2.2 Chemical Conditioning
1.3 Activation of Sewage Sludge
1.3.1 Adsorbents Produced by Thermal Carbonization
1.3.2 Adsorbents by Physical Activation
1.3.3 Adsorbents after Chemical Activation
1.3.4 Surface Chemistry of the Sewage Sludge-Based Adsorbent
1.4 Conclusion
References
2 Emerging Issues and Their Solutions Related to the Use of Sewage Sludge in Waste Treatment
2.1 Introduction
2.2 Sewage Characteristics
2.3 Emerging Issues in Sewage Sludge Treatment
2.4 Sludge Treatment Process
2.4.1 Pre-treatment
2.4.2 Thermal Treatment Process
2.4.2.1 Incineration
2.4.2.2 Gasification
2.4.2.3 Pyrolysis
2.4.2.4 Supercritical Water Gasification and Hydrothermal Carbonization
2.5 Sustainable Future Prospective for Thermal Treatment of Stabilization of Sewage Sludge
References
3 A Detailed Overview of Anaerobic Digestion of Sewage Sludge with Different Process Intensification Strategies
3.1 Introduction
3.2 Anaerobic Digestion of Sewage Sludge
3.3 Factors Affecting Sludge Digestion
3.4 Bioreactors for Sludge Digestion
3.4.1 Different Anaerobic Bioreactor Configurations
3.4.2 Membrane Bioreactor for Sewage Sludge Digestion
3.4.3 Pilot-scale Anaerobic Digestion
3.4.4 Full-scale Anaerobic Digestion of Sewage Sludge
3.5 Process Improvement Strategies
3.5.1 Sludge Pretreatment
3.5.1.1 Physical and Mechanical Treatment
3.5.1.2 Chemical Treatment
3.5.1.3 Thermal Treatment
3.5.1.4 Biological Treatment
3.5.2 Effect of Additives
3.6 Conclusion and Future Research Direction
References
4 Potential of Treated Industrial Effluent – Challenges and Opportunities
4.1 Introduction
4.2 Physio-chemical Characteristics of Industrial Water
4.2.1 Characteristics of Industrial Influent and Effluent
4.2.2 Quality of Treated Water
4.3 Potential Reuse of Treated Effluent
4.3.1 End Use of Wastewater
4.3.2 Standards
4.3.3 Benefits Involved
4.4 Challenges
4.4.1 Technical Aspects
4.4.2 Economic Aspects
4.4.3 Social Aspects
4.4.4 Risk Factor
4.4.5 Policy Frameworks
4.4.6 Possible Solutions
4.5 Future Scope
4.6 Conclusion
References
5 Biological and Microbiological Characteristics of Activated Sewage Sludge
5.1 Introduction
5.2 Biological Properties of Activated Sludge
5.3 Microbial Community Structure of Activated Sludge
5.4 Seasonal Variation of Microbiological Characteristic of Activated Sludge
5.5 Physiological Function and Enzyme Activity of Microbiome in Activated Sludge
5.6 Antibiotic Resistant Gene in Activated Sludge
5.7 Conclusion
References
6 Pre-treatment of Industrial Wastewater
6.1 Introduction
6.1.1 Background
6.2 Importance of Key Industrial Sectors
6.3 Need for Pre-treatment of Industrial Effluents
6.3.1 Pre-treatment Processes
6.3.2 Sustainability Approach
6.4 Classification of Pre-treatment Technologies
6.4.1 Classical Technologies
6.4.2 Hybrid Methods
6.4.3 Categorization of Techniques
6.4.3.1 Thermal Pre-treatments
6.4.3.2 Mechanical Pre-treatments
6.4.3.3 Chemical Pre-treatments
6.4.3.4 Biological Pre-treatments
6.4.3.5 Special Pre-treatments
6.5 Characteristics of Different Pre-treatment Technologies
6.5.1 Biodegradability
6.5.2 Dewaterability
6.5.3 Renewable Energy Production
6.5.3.1 Bio-gas Production
6.5.3.2 Bio-hydrogen Production
6.5.3.3 Bio-ethanol Production
6.5.3.4 Bio-diesel Production
6.6 Pre-treatment of Types of Industrial Waste
6.6.1 Major Industries and their Pre-treatment Technologies
6.6.1.1 Petrochemical Industry
6.6.1.2 Paper and Pulp Industry
6.6.1.3 Coal Manufacturing Industry
6.6.1.4 Textile Industry
6.6.1.5 Polyester Industry
6.6.1.6 Pharmaceutical Industry
6.6.1.7 Olive Oil Industry
6.6.1.8 Fertilizer Industry
6.6.2 Cost Analysis
6.7 Future Prospects
6.8 Conclusion
References
Index
End User License Agreement
CHAPTER 01
Table 1.1 Conditioning for...
Table 1.2 Conditioning for...
Table 1.3 Activation of sewage...
CHAPTER 02
Table 2.1 Chemical composition...
Table 2.2 Summary of different...
CHAPTER 04
Table 4.1 Approximate capacity...
Table 4.2 Discharge quality...
Table 4.3 Discharge standards...
Table 4.4 Discharge standards...
Table 4.5 Treatment technologies...
Table 4.6 Standard limits of...
CHAPTER 05
Table 5.1 Main characteristics...
Table 5.2 Microbial abundance...
CHAPTER 06
Table 6.1 Energy Usage of...
CHAPTER 02
Figure 2.1 Flowchart showing...
Figure 2.2 Comparison of...
CHAPTER 03
Figure 3.1 Overview of a...
CHAPTER 04
Figure 4.1 Conventional...
Figure 4.2 Illustration...
Figure 4.3 Examples of...
Figure 4.4 Illustration...
CHAPTER 06
Figure 6.1 Types of...
Figure 6.2 Schematic...
Figure 6.3 Classification...
Figure 6.5 Microwave...
Figure 6.6 Ultrasonic...
Figure 6.7 Ozone pre-treatment...
Figure 6.8 Coagulation-flocculation...
Figure 6.9 Cannibal...
Figure 6.10 Outline...
Figure 6.4 Thermal...
Cover
Title Page
Copyright Page
Table of Contents
Begin Reading
Index
End User License Agreement
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Joe Cyril Harrish A.M and Hitesh S. Pawar*
DBT-ICT Centre for Energy Biosciences, Institute of Chemical Technology, Matunga, Mumbai, India* Corresponding author
During the wastewater treatment process, sewage sludge is generated in massive amounts. It contains a multitude of toxic substances such as heavy metals, organic contaminants, and pathogens. This sewage sludge has severe negative effects in the environment. The water industry is expecting a wide number of constraints due to stringent regulation [1]. In a wastewater treatment plant (WWTP), the cost of the disposal of sewage sludge alone accounts for about 50% [2]. Thus developing a feasible treatment method for this is crucial for waste management. Some of the conventional techniques usually applied for sludge disposal include: composting, incineration, anaerobic digestion, landfill, and recycling. The definition of sewage sludge is that it is the residue produced from the treatment of wastewater. The sludge is comprised of two fractions which are the primary and secondary sludge. The secondary sludge is generated from the biological treatment system. Chemical sludge results from the treatment of sludge with chemicals [3].
Some of the constituents of sewage sludge are protein, fats, nitrogen, phosphoric acid, calcium oxide, cellulose, magnesium oxide, potash, etc. [4]. Sludge components include the chemicals used for treatment. The biochemical transformation during treatment will produce fulvi and humic acids in them. Usually, the treatment methods for wastewater for generating sludge are mechanical treatment such as sedimentation, or sometimes biological process or clarifiers for removal of secondary sludge. The secondary sludge contains harsh chemicals. So, neutralization, coagulation, and precipitation of the above compounds are done. Solids which are removed during mechanical pretreatment are the primary sludge which contains a high amount of water, pathogens and they look vile, smell bad and are susceptible, while the secondary sludge is fluffy and yellow in colour, biologically active and dewatering is difficult.
Disintegration is the word for conditioning the sludge by destroying its structure by means of increasing the hydrolysis. Nowadays, more attention is given to the hydrolysis of the sludge. This will in turn increase the biogas production. This biogas can be used as an energy source. Additionally, simultaneous generation of volatile acids during conditioning can be seen. This volatile acid becomes a good carbon source for denitrifiers. The main goal of this type of conditioning is to make the available nutrients accessible to the microbes by disruption of the EPS matrix and cell wall thus aiding hydrolysis (anaerobic digestion) of complex organic molecules of sludge.
During ultra-sonication, periodic waves of compression and rarefaction propagate through the wastewater sludge. Microbubbles generated through ultra-sonication collapse rapidly with a time interval of microseconds inducing cavitation. This sudden violent collapse of microbubbles gives rise to extreme temperature of around 5000 K and pressure of around 500 bars. This cavitation generates robust hydro-mechanical shear forces and highly reactive radicals (H · and · OH). The oxidizing effect of reactive radicals and hydro-mechanical shear forces disintegrates the sludge flocs and releases the cellular material [5–7]. The specific energy output of ultra-sonication is optimized based on the total solid content of the wastewater. The main disadvantage of this method is high energy consumption. Specific energy input determines the solubilization efficiency of the sludge. The specific energy input for disintegration of sludge flocs is around 80 KJ L−1 [8]. The optimal specific energy input based on total solids was around 1000 KJ Kg−1 improves settleability. But for sludge destruction and transformation of insoluble organics into soluble organics, a specific energy input of around 26 000 KJ kg−1 of total solids is needed [9]. But this consumes an enormous amount of energy. Ultra-sonication greatly improves the efficiency of anaerobic digestion of sludge. This in turn increases the biogas production (methane). The ultrasound pretreatment method increases the destruction of volatile solids and biogas production by 15–35% [10].
Microwave irradiation works in the wavelengths between 1 mm to 1 m and oscillation frequencies in the range 0.3‒300 GHz [5, 11]. In theory the damage of sludge flocs can be done in two ways. The first is that dipole rotation under oscillating electromagnetic field generates heat which boils the intracellular fluid and breaks the bacterial cells, and the second way is that changes in dipole orientation of polar molecules breaks the hydrogen bonds and denatures the complex biological molecules [12, 13]. This pretreatment method enhances the solubilization of organic matter in the sludge and increases the biogas production by 50% [14]. This pretreatment method causes rapid lysis of cell residue along with simultaneous disruption of extracellular polymeric substances. The non-thermal effect exerted by disorientation of dipoles has a slight effect on the solubilization of organic matter. But, the mesophilic anaerobic biodegradability of the sludge and biogas production is increased [15]. Although it has its advantages, the microwave irradiation treatment method has no discernible effect on hydrolysis but improves dewaterability and destruction of pathogens responsible for anaerobic digestion.
This is a high-voltage electric field method also known as pulsed electric field. During the sludge disintegration process, a high voltage field is induced by charges and causes the rapid disruption of sludge flocs, this makes the nutrients in the effluent easily accessible for the microbes responsible for fermentation [16]. It increases the soluble organic matter by 110–460% in comparison to conventional digestion systems. It decreases the size of the digesters size 40%. The ratio of soluble COD to the total COD increased by 4.5 times and biogas production increased by 2.5 times [17, 18]. The main advantage of this treatment method, is that it drastically reduces the amount of sludge without affecting the treatment efficiency.
During homogenization, the generation of extreme pressure gradient, turbulence, and rapid depressurization creates cavitation and in turn produces strong shearing forces. The sudden pressurization and depressurization breaks the sludge flocs releasing intracellular substances. For sludge disintegration or solubilization, the operating pressure was around 20–80 MPa and the number of cycles was around 1 to 4 [19]. In industry, high pressure homogenization is considered over other pretreatment methods. This is due to ease of operation, high energy-efficiency and low capital cost. This method increases the sludge reduction by 23% and biogas production by 30% [20].
Usually, thermal hydrolysis is the commercially accepted and well established pretreatment method for sludge dewaterability [21, 22]. But, research is still underway for its ability to improve sludge disintegration (hydrolysis). The efficiency of this treatment method relies on the treatment temperature and time. The solubilization of sludge increases with increase in operating temperature. The carbohydrates and protein are affected more than lipids in thermal hydrolysis. The operating temperature for sludge solubilization, should not exceed 190 °C for maximum efficiency. Further, increase in temperature decreases the efficiency as it affects the anaerobic digestion. The biodegradability is weakened with further increases in temperature because high temperature results in production of recalcitrant compounds, i.e. melanoidins. These molecules are difficult to degrade and in addition to this even inhibit the biodegradability of organic matter [23]. The treatment time has less influence on solubilization of sludge in comparison to treatment temperature. This technique greatly reduces the hydraulic retention time, this in turn reduces the reactor size for anaerobic digestion. This pretreatment method is advantageous as it destroys pathogens, removs odor, reduces sludge volume, improves dewaterability, etc.
In this method, the microbial cells in the sludge flocs are deformed by chemicals, this favors enzymatic digestion of organic matter in the sludge. Some of the chemical treatment methods employed are as follows.
This methods shows great promise due to ease of operation, high methane conversion efficiency, simple operation, and it is inexpensive [24]. In acid hydrolysis, HCl, H2SO4, H3PO4, and HNO3 are used in treatment, while for alkali treatment NaOH, KOH, Ca(OH)2, CaO, and ammonia are used. The major benefit of this method is that it can be performed at ambient temperature [2]. The effectiveness of this method relies on the affinity of the organic matter in the sludge to the acid or alkali and also the characteristics of the sludge. Acid treatment is most suitable for hydrolysis of lignocellulosic biomass but has little to no effect on lignin hydrolysis. The theory behind acid treatment is the hydrolysis of hemicellulose liberating monomeric sugars and oligomers from the matrix of cell wall thereby supporting enzymatic digestibility. The drawback of acid treatment is the generation of toxic byproducts such as furfural, hydroymethyl furufural which strongly inhibits fermentative microbes [25]. Other drawbacks include its corrosive nature due to extremely low pH which can affect the reactor and special measures should be taken into account to prevent the above mentioned problem. To overcome this problem, acidic treatment is integrated with thermal treatment.
On the other hand, alkali treatment is best suitable for the breakdown of lignin. The theory behind this treatment is saponification and solvation, which causes depolymerization and breakage of lignin-carbohydrate linkages [26]. It also saponifies the intermolecular ester bonds of xylan hemicellulose, but the degree of solubilizing them is not as great as acid treatment. The additional alkalinity improves the methanogenic activity and stabilizes the process of anaerobic digestion by acting as a buffer system [27]. NaOH is the most effective in enhancing biogas production and increasing the solubilization of the sludge. Although it has its advantages, some of the disadvantages such as NaOH treatment is not compatible with anaerobic digestion. As overloading NaOH blocks the metabolic pathway of anaerobic microbes and decreases the methane productivity. To overcome this problem, alkali treatment is integrated with microwave, ultrasound, and thermal pretreatment systems in order to reduce the alkali consumption and this also increases the methane productivity.
This is the most widely employed peroxidation process as it disrupts cell membrane and disintegrates the sludge flocs resulting in excessive solubilization of sludge. Ozonation is integrated with anaerobic sludge digestion to bypass hydrolysis and increase methane productivity [28, 29].
The efficiency of sludge solubilization depends on the dosage and amount of ozone introduced in a moderate range [30]. However, ozonation efficiency depends on the reaction kinetics and mass transfer of ozone. The rate of kinetic reaction between sludge mixed liquor and dissolved ozone is average, even though the applied dosage is high [31]. The introduction of a high dosage of ozone can cause complete mineralization of liberated cellular components affecting methane productivity [32]. The main disadvantage of this method is the high capital investment
The microbubble ozonation was introduced to overcome the high capital investment with higher performance. The sludge solubilization was increased from 15‒30% to 25‒40% with the bubble contactor with ozone doses of 0.06‒0.16g O3 g−1 of total solids and ozone utilization also improved from 72 to 99%. In this process, rapid generation of hydroxyl radicals is seen thereby accelerating the solubilization efficiency of sludge [33].
This is a catalytic reaction involving reaction between hydrogen peroxide and the catalyst iron ions(Fe2+). The result generates enormous amounts of highly active radicals (·OH). The hydroxyl radicals generated from this reaction have high oxidative potential of + 2.80 V which is greater than the hydroxyl radicals produced from ozonation and hydrogen peroxide alone. These are effective for the disintegration of sludge flocs (EPS) and disruption of cells [22, 34]. This releases the bound water and intracellular material. Release of bound water enhances dewaterability. In addition to this, the Fenton process enhances methane productivity and sludge solubilization. The efficiency of this process depends on many variables such as treatment time, intial pH and temperature, Fe2+/H2O2, and reagent concentration [22, 35]. This method shows greater efficiency by increasing the soluble COD by five times and methane productivity increased by 75% [7]. The major drawback on Fenton oxidation is that it needs extremely low pH to prevent precipitation and hydrolysis of Fe3+ [2, 36, 37]. Since for Fenton oxidation, enormous amounts of H2O2 and Fe2+ will be present. This will scavenge the hydroxyl radicals.
This is a new and emerging sludge pretreatment technology for efficient dewatering of sludge. In this method, persulfate (S2O82−) molecules will be activated by heat and UV light or by transition metals. This will liberate an extremely strong oxidant which is sulfate free radicals (SO4−). These free radicals have a redox potential of around 2.60 V. These free radicals oxidize and attack EPS and bacterial cells and particularly target aromatic peptides, tryptophan protein, humic, and fulvic acid-like substances of EPS. Then the simultaneous cleavage of the polymeric back-bone of cells results in the rupture of their cells. The major advantage of this method over Fenton oxidation is that sulfate radicals have higher oxidation potential over wide range of pH which makes them cost effective [38].
Sludge dewatering decreases the sludge volume for ease of transportation, energy utilization, and reduction of leachate production in landfill sites on disposal. Some of the conditioning technologies are as follows (Table 1.1).
Table 1.1 Conditioning for increasing the rate of hydrolysis.
s.no
Pretreatment
Condition
Performance
Reference
1
Sonication of activated sludge
3380 KJ kg
−1
42% methane yield, 13% voltalile solid removal
[
39
]
2
Sonication of mixed sludge
5000 KJ kg
−1
35% methane yield
[
40
]
3
Microwave irradiation of activated sludge
14 000 KJ kg
−1
570.7% biogas production
[
41
]
4
Microwave irradiation of thickened sludge
-
106% biogas production, 53.1% volatile solid removal
[
42
]
5
Electrokinetic disintegration of activated sludge
10 kWh m
−2
100% methane production
[
43
]
6
Electrokinetic disintegration of primary sludge
33 kWh m
−3
33% methane production
[
44
]
7
High pressure homogenization of mixed sludge
12000 psi
0.61–1.32 of methane/Ld
43–64% of volatile solids removal
[
45
]
8
High pressure homogenization of concentrated sludge
150 bar
30% gas production
[
46
]
9
Thermal hydrolysis of secondary sludge
134–140 °C
3.4 bar, 30 min
Volatile removal 42%
[
47
]
Sludge dewaterability effectiveness is assessed based on minimizing the compressibility and increasing the permeability of sludge. Some of the porous materials include coal fly ash, wood chips, gypsum, and lignite and rice husk. The addition of porous material to the wastewater in the proportion of 30–50% dry matter can be followed in the wastewater treatment. This treatment method reduces the HRF by 58–88% and compressibility coefficient by 22–90%. Inorganic porous material has an advantage over organic porous material as it reduces the phosphorus content after filtration by the formation of insoluble phosphate. This lessens the negative charges on the sludge and improves sludge flocculation. Mineral based adsorbent has a disadvantage over carbon based adsorbent as it has lower capabilities of improving the permeability of the sludge in comparison. Also carbon based materials do not decrease the calorific value of the sludge. The porous material should not increase the volume of the dewatered sludge and porous material particle size should be more than 10 µm in order to not block the filter medium [48].
Low frequency ultrasound generates acoustic cavitation through mechanical shear forces. This disrupts the cell membrane and liberates intracellular organics into the aqueous medium. This helps in disintegration of the biological flocs and homogenization of sludge thus dewatering of the sludge happens. The homogenized sludge has sponge-like properties. Although, it is debatable based on studies, ultra-sonication may have negative effects on dewaterability [49–51].
Hydrodynamic cavitation is another process using the venturi or orifice plate in the flowpath of liquid which in turn increases the velocity of the flow during reduction of the cross-sectional area. This increase in velocity decrease the local pressure resulting in cavitation. The cavitation produced by this is similar to ultra sonication. The critical parameter in the hydrodynamic cavitation is the optimization of flow regime for positive effects [52].
Freezing/thawing is an effective way of sludge dewatering. During this process, sludge is first frozen at a temperature below abpit ‒20 to ‒15 °C which results in the formation of ice crystals and this is followed by thawing at room temperature. Sludge particles were excluded from the ice crystals and water molecules were trapped in the ice crystals. While thawing, the concentrated solid was segregated from the water. The continuous freezing/thawing cycle causes crystallization of intracellular water and its volume expands. It is accompanied by swelling of cells and lessens the bound water content which will result in heavy sludge dehydration. The only limiting factor in implementation of this technology is high energy consumption [52–54]
The integration of mechanical compression with electrical treatment enhanced the sludge dewatering and reduces the water content by up to 50%. The sludge particles usually have amphoteric groups which makes them negatively charged. This drives them towards the cathode/anode based on their charges. The concentration gradient of ions in the water phase is created by electrically driven movement of hydrated ions and enhances the water diffusion in sludge. In the permeable polar plate, water is removed from the sludge through water osmosis. Other ways of removal in this process include electrolysis of water. These processes further enhance the dewatering of sludge [55].
By altering the physico-chemical properties of the sludge by the use of chemicals for improving sludge dewaterability is referred gto as chemical conditioning. Some types of chemical conditioning methods are enzymatic treatment, coagulation/flocculation, advanced oxidation process, and acid/base treatment (Table 1.2).
Table 1.2 Conditioning for dewatering of sludge.
s.no
Pretreatment
Hypothesis
limitation
Reference
1
Porous material addition
van der Waals forces
Disposal cost
[
63
]
2
Sonication
Disintegration of bioflocs, cell rupture for releasing of water
Specialized equipment, high capital cost
[
63
]
3
Thermal treatment
Bioflocs disintegration/rupturing of cell for releasing water
High capital cost, difficulty to scale up
[
63
]
4
Freezing/thawing
Homogenous ice crystal formation and cell swells to release moisture
High capital cost, difficulty to scale up
[
63
]
5
Electrical treatment
Electromigration, electrophoresis, electro osmosis, electrolysis,
High capital cost, specialized walls for anticorrosion
[
63
]
6
Acid/base treatment
Regulation of surface charge and cell rupture
cororsiveness
[
63
]
7
Advanced oxidation process
Lysis of EPS and cell rupture
Robust pH control
[
63
]
8
Enzymatic treatment
EPS degradation
Harsh conditions and long duration process
[
63
]
Aluminium and ferric salts are the most commonly used inorganic salt coagulants. In comparison, aluminium salts are more effective than ferric salts which result in rapid settling. However, due to low cost, rapid settling velocity, flexible pH, lower toxicity, and better effects at low temperature ferric salts are preferred over aluminium salts. Some of the disadvantages of aluminium salts include high consumption as most of the aluminium ions will be consumed by soluble-EPS for chelation and for protein-like substances ferric ions show more affinity than aluminium ions. The drawback of using ferric salts is that it may corrode the walls of the equipment [56, 57].
The polymerized form of aluminum ions show more efficiency in coagulation in comparison to the monomeric form of aluminium as it forms sludge flocs with small particles with low compressibility resulting in denser flocs. This is due to the fact that multivalent inorganic compounds have charge density and large molecular mass and this could be the reason for balance of charge even at low dosage [58].
The adsorption-bridging reaction on the particles of sludge is the principle behind the macromolecular polymers known as flocculants. This improves the solid‒liquid separation by creating a density difference between the flocs and the