PROCESS SELECTION
3.1 Introduction
Following feasibility studies, the next stage in the design project is the evaluation and comparison of the alternative process routes for manufacture of the chemical, the treated water in this case. The selection of an appropriate process is an important decision, thus all the subsequent work depends upon this choice. Although the selection can be changed or modified at a later stage, this is at least before the plant is built, such a decision results in a serious waste of time and money. However, probably not such a waste as building an uneconomic or unsafe plant!
In this section, the different technologies are presented and qualitatively analyzed to reduce their number to one process based on specified criteria; qualitative analysis will then be carried on the three processes out of which one process will be selected. This process should be the most economical and suitable process.
3.2 Process Selection Factors
The design of treatment facilities will be determined by feasibility studies, considering all engineering, economic, energy and environmental factors. All legitimate alternatives will be identified and evaluated by life cycle cost analyses. Additionally, energy use between candidate processes will be considered. For the purpose of energy consumption, only the energy purchased or procured will be included in the usage evaluation. All treatment process systems will be compared with a basic treatment process system, which is that treatment process system accomplishing the required treatment at the lowest first cost. The selection and design of the water treatment processes to be used at a particular facility are dictated by practicability, reliability, flexibility, and overall economics.
3.3 Water Treatment Systems
There are three types of water treatment technological system in use; they are
1. Conventional water treatment plant without pretreatment.
2. Conventional water treatment plant with pretreatment.
3. Membrane bioreactor or Membrane-coagulation reactor.
3.3.1 Conventional Water Treatment Plant without Pretreatment
Addition of coagulant
Bar Screen
Disinfection
To Storage and Distribution
Figure 3.1 Block diagram of conventional water treatment plant without pretreatment.
Figure 3.1 is a block diagram for a conventional water treatment plant. The combination of the first 3 steps primarily removes colloids (including some micro-organisms) and natural organic matter (NOM). Sand filtration is a polishing step that removes much of the colloidal material remaining after step 3 (sedimentation).
3.3.2 Conventional Water Treatment Plant with Pretreatment
Addition of Chlorine Addition of coagulant
Travelling Screen
Bar Screen Disinfection
To Storage
and Distribution
Figure 3.2 Block diagram of conventional water treatment plant with pretreatment.
In the above block diagram, this technology of treating water has pretreatment stages (i.e. travelling screen, aeration, oxidation, and presedimentation).
3.3.3 Membrane-coagulation reactor
Disinfection
Portable
Water Storage
Surface Rapid Mix
Water Coagulation Filtration Membrane
Unit
Bar Screen
Aeration
Sludge
Figure 3.3 Flow diagram of membrane – coagulation reactor
The MCR incorporates flocculation, sedimentation and filtration in 1 reactor instead of 3, suggesting the potential for substantial savings in space and capital costs. The potential water quality benefits arise because the membranes may block a substantial fraction of the small colloids, low molecular weight NOM, and microorganisms that do not sediment and pass through conventional sand filters. Reduction in chemical usage is less certain but may result because of the MCR system’s ability to retain even small flocs. As explained in Table 2.8, advantages and disadvantages of membrane separation, significant limitations in the use of membranes are the disposal of the waste (concentrate) being generated, fouling of membranes, cost of membrane replacement after every five years, and increased energy cost-electricity, lack of a reliable low-cost method of monitoring the integrity of low pressure membrane processes, and finally the fact that it works best on groundwater or low solids surface water
3.4 Reliability
Unless the treatment plant can be taken out of service for a period of time for maintenance and repair work, two or more of all essential items, such as pumps, settling basins, flocculators, filters, and chemical feeders must be provided. The degree of importance of each item must be evaluated on a case-by-case basis, considering that safe water has to be supplied at all times. Also if there is a definite possibility that lengthy power outages will occur, installation of emergency generating facilities at the water treatment plant should be contemplated.
Considering the reliability factor, the membrane-coagulation reactor is just one and its cost is high; therefore, it will not be easily replaced in case of damages or faults. Also, due to the cost of disposing the waste being generated and excessive fouling that may occur, conventional method will therefore be better than membrane separator. Since membrane – coagulation reactor works best on groundwater or low solids surface water, at times when there is high load on the surface (river) water, it cannot adequately treat the water. Membrane – coagulation reactor is a new technology and the technological knowhow of the unit will be a challenge; since operators will have to be trained on how to operate it, and this might be a quite tasking on the community.
Due to the above reasons, it is best to choose the conventional method of water treatment.
3.5 Choice of Conventional Process
Since the conventional approach has been selected ahead of membrane – coagulation reactor, then we choose between the two conventional methods.
The conventional water treatment plant with pretreatment has screening, aeration, oxidation and presedimentation units. The functions of the pretreatment units are briefly outlined below.
3.5.1 Screening
Surface waters require screens travelling screen (of size 0.375 inch) or strainers for removal of material too small to be intercepted by the coarse rack bar screen (of size 1 ̶ 3 inch)
3.5.2 Aeration
In water is brought into contact with air for the purpose of transferring volatile substances to or from water. These volatile substances include oxygen, carbon dioxide, hydrogen sulfide, methane and volatile organic compounds responsible for tastes and odor. Aeration may also be useful in oxidizing iron and manganese, oxidizing humic substances that might form trihalomethanes when chlorinated, eliminating other sources of taste and odor, or imparting oxygen to oxygen deficient water.
3.5.4 Presedimentation
Presedimentation is used to remove excessively heavy silt loads prior to the treatment processes. This unit is very important for a water source of high turbidity. It is accomplished without the use of coagulant chemicals.
The above pretreatment processes thus have advantages as it has been outlined above; therefore, the conventional water treatment process with pretreatment is selected.
3.6 Selection of Conventional Units
3.6.1 Selection of Pretreatment Units
Screening: Bar screen (coarse particles), and travelling screen (fine particles).
Aeration: Multiple – tray tower aerators with perforated distributors should be designed to provide a small amount of head, and it is efficient.
Prechlorination: Use of Sodium hypochlorite for Chlorine source
Presedimentation: Sedimentation basin
3.6.2 Selection of Chemical Treatment Units
Roughing filters are often used in place of coagulation/ flocculation, and sedimentation for water treatment. Coagulation/ flocculation are selected ahead of roughing filters because it is more effective for a larger population.
Coagulation: Rapid Mix basin. Aluminum sulphate (alum) and polyphosphate fluoride act as coagulants for the treatment.
Flocculation: Flocculation basin.
Sedimentation: Sedimentation basin contains lamella plate settlers.
3.6.3 Selection of Filtration Unit
There are basically three types of filters employed in conventional water treatment: they are slow sand filter (gravity), rapid sand filter (gravity), and pressure filter.
Table 3.1 Selection Matrix of Filtration Methods
Criteria Method Slow sand filter Rapid sand filter Pressure filter
Is it cost effective? Yes Yes Yes
Is it energy efficient? Yes Yes No
Is it chemical free? Yes Yes Yes
Can it be easily maintained? Yes Yes Yes
Overall is it suitable? Yes Yes No
Both slow sand filter and rapid sand filter are suitable for use, but since the population water is been treated for is more than 5,000 people; thus slow sand filter cannot be used. Rapid sand filter is thus selected.
3.6.4 Selection of Disinfection Method
Table 3.2 Selection Matrix of Disinfection Methods
Criteria Method Chlorination Ultraviolet Disinfection Ozonation
Is the output drinkable? Yes Yes Yes
Is it cost effective? Yes Yes No
Is it energy efficient? Yes No No
Is it chemical free? No Yes Yes
Can it be easily maintained? Yes No No
Overall is it suitable? Yes No No
Chlorination is thus chosen as the disinfection method for the water treatment.
The block diagram of the process selected is thus shown below.
Addition of Lime, Soda-ash Addition of Alum
Travelling Screen
Bar Screen Addition of Calcium hypochlorite
To Distribution
and Storage
Figure 3.4 Block diagram of process flow.
3.1 Introduction
Following feasibility studies, the next stage in the design project is the evaluation and comparison of the alternative process routes for manufacture of the chemical, the treated water in this case. The selection of an appropriate process is an important decision, thus all the subsequent work depends upon this choice. Although the selection can be changed or modified at a later stage, this is at least before the plant is built, such a decision results in a serious waste of time and money. However, probably not such a waste as building an uneconomic or unsafe plant!
In this section, the different technologies are presented and qualitatively analyzed to reduce their number to one process based on specified criteria; qualitative analysis will then be carried on the three processes out of which one process will be selected. This process should be the most economical and suitable process.
3.2 Process Selection Factors
The design of treatment facilities will be determined by feasibility studies, considering all engineering, economic, energy and environmental factors. All legitimate alternatives will be identified and evaluated by life cycle cost analyses. Additionally, energy use between candidate processes will be considered. For the purpose of energy consumption, only the energy purchased or procured will be included in the usage evaluation. All treatment process systems will be compared with a basic treatment process system, which is that treatment process system accomplishing the required treatment at the lowest first cost. The selection and design of the water treatment processes to be used at a particular facility are dictated by practicability, reliability, flexibility, and overall economics.
3.3 Water Treatment Systems
There are three types of water treatment technological system in use; they are
1. Conventional water treatment plant without pretreatment.
2. Conventional water treatment plant with pretreatment.
3. Membrane bioreactor or Membrane-coagulation reactor.
3.3.1 Conventional Water Treatment Plant without Pretreatment
Addition of coagulant
Bar Screen
Disinfection
To Storage and Distribution
Figure 3.1 Block diagram of conventional water treatment plant without pretreatment.
Figure 3.1 is a block diagram for a conventional water treatment plant. The combination of the first 3 steps primarily removes colloids (including some micro-organisms) and natural organic matter (NOM). Sand filtration is a polishing step that removes much of the colloidal material remaining after step 3 (sedimentation).
3.3.2 Conventional Water Treatment Plant with Pretreatment
Addition of Chlorine Addition of coagulant
Travelling Screen
Bar Screen Disinfection
To Storage
and Distribution
Figure 3.2 Block diagram of conventional water treatment plant with pretreatment.
In the above block diagram, this technology of treating water has pretreatment stages (i.e. travelling screen, aeration, oxidation, and presedimentation).
3.3.3 Membrane-coagulation reactor
Disinfection
Portable
Water Storage
Surface Rapid Mix
Water Coagulation Filtration Membrane
Unit
Bar Screen
Aeration
Sludge
Figure 3.3 Flow diagram of membrane – coagulation reactor
The MCR incorporates flocculation, sedimentation and filtration in 1 reactor instead of 3, suggesting the potential for substantial savings in space and capital costs. The potential water quality benefits arise because the membranes may block a substantial fraction of the small colloids, low molecular weight NOM, and microorganisms that do not sediment and pass through conventional sand filters. Reduction in chemical usage is less certain but may result because of the MCR system’s ability to retain even small flocs. As explained in Table 2.8, advantages and disadvantages of membrane separation, significant limitations in the use of membranes are the disposal of the waste (concentrate) being generated, fouling of membranes, cost of membrane replacement after every five years, and increased energy cost-electricity, lack of a reliable low-cost method of monitoring the integrity of low pressure membrane processes, and finally the fact that it works best on groundwater or low solids surface water
3.4 Reliability
Unless the treatment plant can be taken out of service for a period of time for maintenance and repair work, two or more of all essential items, such as pumps, settling basins, flocculators, filters, and chemical feeders must be provided. The degree of importance of each item must be evaluated on a case-by-case basis, considering that safe water has to be supplied at all times. Also if there is a definite possibility that lengthy power outages will occur, installation of emergency generating facilities at the water treatment plant should be contemplated.
Considering the reliability factor, the membrane-coagulation reactor is just one and its cost is high; therefore, it will not be easily replaced in case of damages or faults. Also, due to the cost of disposing the waste being generated and excessive fouling that may occur, conventional method will therefore be better than membrane separator. Since membrane – coagulation reactor works best on groundwater or low solids surface water, at times when there is high load on the surface (river) water, it cannot adequately treat the water. Membrane – coagulation reactor is a new technology and the technological knowhow of the unit will be a challenge; since operators will have to be trained on how to operate it, and this might be a quite tasking on the community.
Due to the above reasons, it is best to choose the conventional method of water treatment.
3.5 Choice of Conventional Process
Since the conventional approach has been selected ahead of membrane – coagulation reactor, then we choose between the two conventional methods.
The conventional water treatment plant with pretreatment has screening, aeration, oxidation and presedimentation units. The functions of the pretreatment units are briefly outlined below.
3.5.1 Screening
Surface waters require screens travelling screen (of size 0.375 inch) or strainers for removal of material too small to be intercepted by the coarse rack bar screen (of size 1 ̶ 3 inch)
3.5.2 Aeration
In water is brought into contact with air for the purpose of transferring volatile substances to or from water. These volatile substances include oxygen, carbon dioxide, hydrogen sulfide, methane and volatile organic compounds responsible for tastes and odor. Aeration may also be useful in oxidizing iron and manganese, oxidizing humic substances that might form trihalomethanes when chlorinated, eliminating other sources of taste and odor, or imparting oxygen to oxygen deficient water.
3.5.4 Presedimentation
Presedimentation is used to remove excessively heavy silt loads prior to the treatment processes. This unit is very important for a water source of high turbidity. It is accomplished without the use of coagulant chemicals.
The above pretreatment processes thus have advantages as it has been outlined above; therefore, the conventional water treatment process with pretreatment is selected.
3.6 Selection of Conventional Units
3.6.1 Selection of Pretreatment Units
Screening: Bar screen (coarse particles), and travelling screen (fine particles).
Aeration: Multiple – tray tower aerators with perforated distributors should be designed to provide a small amount of head, and it is efficient.
Prechlorination: Use of Sodium hypochlorite for Chlorine source
Presedimentation: Sedimentation basin
3.6.2 Selection of Chemical Treatment Units
Roughing filters are often used in place of coagulation/ flocculation, and sedimentation for water treatment. Coagulation/ flocculation are selected ahead of roughing filters because it is more effective for a larger population.
Coagulation: Rapid Mix basin. Aluminum sulphate (alum) and polyphosphate fluoride act as coagulants for the treatment.
Flocculation: Flocculation basin.
Sedimentation: Sedimentation basin contains lamella plate settlers.
3.6.3 Selection of Filtration Unit
There are basically three types of filters employed in conventional water treatment: they are slow sand filter (gravity), rapid sand filter (gravity), and pressure filter.
Table 3.1 Selection Matrix of Filtration Methods
Criteria Method Slow sand filter Rapid sand filter Pressure filter
Is it cost effective? Yes Yes Yes
Is it energy efficient? Yes Yes No
Is it chemical free? Yes Yes Yes
Can it be easily maintained? Yes Yes Yes
Overall is it suitable? Yes Yes No
Both slow sand filter and rapid sand filter are suitable for use, but since the population water is been treated for is more than 5,000 people; thus slow sand filter cannot be used. Rapid sand filter is thus selected.
3.6.4 Selection of Disinfection Method
Table 3.2 Selection Matrix of Disinfection Methods
Criteria Method Chlorination Ultraviolet Disinfection Ozonation
Is the output drinkable? Yes Yes Yes
Is it cost effective? Yes Yes No
Is it energy efficient? Yes No No
Is it chemical free? No Yes Yes
Can it be easily maintained? Yes No No
Overall is it suitable? Yes No No
Chlorination is thus chosen as the disinfection method for the water treatment.
The block diagram of the process selected is thus shown below.
Addition of Lime, Soda-ash Addition of Alum
Travelling Screen
Bar Screen Addition of Calcium hypochlorite
To Distribution
and Storage
Figure 3.4 Block diagram of process flow.
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