Sedimentation Process and Design of Settling Systems

Sedimentation Process and Design of Settling Systems
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Primary treatment also called primary sedimentation is a sanitation technology that removes suspended solids and floating organic material called scum to reduce the suspended solids load for subsequent treatment processes. The removal of pathogens during primary treatment is not high; therefore, downstream treatment will require further pathogen removal technologies to meet discharge or reuse guidelines. It is not expected that discrete pathogens and indicator organisms are removed by settling during conventional primary treatment.

However, they may be removed when attached to particles. In ordinary primary sedimentation systems that are properly designed and operated, the reduction of all types of pathogens and fecal indicators can be expected to be between 0 and 1 log 10 units.

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With chemically-enhanced primary treatment and advanced primary treatment also referred to as high-rate clarification processes, helminth egg removal can be 1 to 3 log 10 and virus, bacteria, and protozoa removal can be from 1 to 2 log Screenings and grit, if not removed at the beginning of a wastewater treatment plant, can impair downstream treatment processes and damage equipment e.

Figure 1 shows where preliminary treatment is used within the sanitation service chain.

Figures 2 through 5 show examples of manual and mechanized preliminary treatment systems in operation. Figure 1. Locations where preliminary treatment is used within the sanitation service chain.

Figure 2. Screenings are removed by hand and should be buried or incinerated onsite. Grit, also removed manually, should be buried or stored onsite depending on quantities produced. Figure 3. An excavation for disposal of screenings at a wastewater stabilization pond system in Masaya, Nicaragua.

Lime is placed on top of the materials for odor control and partial disinfection. Figure 4. Note the facility is fenced to prevent unauthorized entry. Figure 5. A mechanized preliminary treatment system at the Cuzco, Peru wastewater treatment facility. The photos show the covered bar rack top left and mechanical disposal of screenings top right and the bottom photo the aerated grit chamber with mechanical disposal directly into trucks.

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As is often the case in developing countries, the screenings and grit are disposed in a poorly operated landfill and there is a high risk of pathogen release to the environment. As Feachem et al. Screenings and grit will be expected to contain high concentrations of pathogens, and this has occasionally been reported in the literature Marin et al. Several investigators have also noted elevated concentrations of airborne microorganism indicators bioaerosols and detection of antibiotic resistant genes in enclosed mechanized preliminary treatment facilities as shown in Table 1 Heinonen-Tankski et al.

Figure 6 illustrates the inputs and outputs for preliminary treatment processes.

Primary treatment of wastewater

Heinonen-Tankski et al. As discussed above, preliminary treatment will have little effect on pathogen removal in the liquid wastestream.

Sedimentation rate

There is little that can be done to enhance the removal of pathogens in the liquid wastestream in preliminary treatment. This is especially important in developing countries where open dumps are common and it cannot be assumed materials will be buried offsite in a sanitary landfill.

In this case, treatment plant designs should include onsite disposal of screenings and grit as was shown in Figures 2 through 5. Operation and maintenance manuals should include detailed discussion of the safe handling and disposal of screenings and grit, and operators should receive proper training. Design of large-scale mechanized facilities should consider the possibility for airborne pathogens. Therefore, they should consider: 1 enclosed preliminary treatment units that are designed with proper ventilation and air filtration and 2 outdoor preliminary treatment units that are covered.

No literature data were found on pathogen concentrations in scum, but it can be assumed to have significant concentrations and should be handled accordingly. Primary sedimentation is a form of centralized or semi-centralized wastewater treatment and is an integral part of conventional wastewater treatment primary and secondary treatment as developed historically and practiced today Figures 7 and 8. Primary sedimentation tanks can be rectangular or circular, and typically operate with a hydraulic detention time of 1.

The settled primary sludge solids, which are highly putrescible, must be continuously removed from the bottom of the sedimentation tank and stabilized, usually by anaerobic digestion and less frequently by aerobic digestion see Chapter on Sludge Management.


Typical performance data for the removal of total suspended solids TSS and biochemical oxygen demand BOD 5 in primary sedimentation tanks are shown in Figure Primary effluent requires downstream secondary treatment for further removal of organic matter, usually aerobic technologies e. Figure 7. Locations where primary sedimentation is used within the sanitation service chain.

Wastewater sedimentation

Figure 8. Primary sedimentation within the framework of conventional primary and secondary treatment of wastewater and sludge management. Figure 9. A hopper-type rectangular sedimentation tank where settled solids are removed by hydrostatic pressure left photo is the tank under construction; right photo is the tank in operation Panajachel, Guatemala photo reproduced with permission of Stewart Oakley.

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Figure A circular primary sedimentation tank discharging to a trickling filter. The removal of pathogens during primary treatment is not high, therefore downstream treatment will require pathogen removal technologies in addition to organic matter removal to meet discharge or reuse requirements. Imhoff tanks are primary sedimentation tanks that include additional volume for settled sludge storage and anaerobic digestion.

They are designed for small flows and are still commonly used in developing countries. As shown in Figures 12 and 13, an Imhoff tank consists of: 1 a V-shaped settling compartment, 2 a large compartment underneath the settling compartment for settled sludge storage and digestion, and 3 separate compartments for biogas venting and scum removal.

Preliminary Treatment and Primary Sedimentation

Imhoff tanks are used in small communities because of low investment costs and simple operation and maintenance Tilly et al. As with primary sedimentation tanks, the effluents from Imhoff tanks require further treatment before discharge or reuse. The sludge withdrawn from Imhoff tanks, which should be well digested before withdrawal, is typically dewatered in sludge drying beds. A section and plan view of an Imhoff tank.

The sludge digestion compartment can be as deep as 6 m to store and digest primary sludge. Figure from Tilly et al. An Imhoff tank at completion of construction. This tank has a depth of 6 m and is below the groundwater table; as a result a curtain drain was installed to divert groundwater flow.

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Saylla, Peru photo reproduced with permission of Stewart Oakley. Tilly et al. Historical information on pathogen fate in primary treatment and wastewater treatment systems is available in Feachem et al. Primary sedimentation tanks receive untreated wastewaters that typically have received pretreatment screening and grit removal. Typical concentrations of pathogens in influent wastewaters are provided in the Introduction chapter. The outputs from primary sedimentation processes include primary effluent and primary sludge, both of which require further treatment for stabilization and pathogen removal: primary effluent is typically treated by aerobic secondary processes e.

Conventional primary sedimentation processes are designed specifically for suspended solids removal and any removal of viral, bacterial, protozoan or helminth pathogens is incidental to the design objectives.

Theoretical section (boring)

This book is designed to serve as a comprehensive source of information of sedimentation processes and design of settling systems, especially as applied to . Download Citation on ResearchGate | Sedimentation Process and Design of Settling Systems | This book is designed to serve as a comprehensive source of.

A summary of the most important factors for removal of the different pathogen types is presented in Table 2. The retention in the settling floc can be due to adsorption to surfaces or entrapment within the matrix of the settling floc particles Jimenez et al. Mechanisms: Removal of pathogens is very low and only occurs by sedimentation through pathogen retention in settling flocs. South, buy sedimentation process and design of settling systems; the First of the god; Abydos, Assiut. May Osiris carefully are with his likely according.

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