The process flow sheet or flow diagram is a graphical representation of the sequence in which various unit operations and unit processes are adopted for treatment of sewage at any sewage treatment plant. The design of process flow sheet involves selection of an appropriate combination of various unit operations and unit processes to achieve a desired degree of contaminant removal.
The selection of unit operations and unit processes for the treatment of sewage depends on several factors such as characteristics of raw sewage, degree of purification required, disposal facilities available, cost involved including cost of installation, maintenance and operation, ease of construction and maintenance, benefits that accrue from better environmental sanitation, location, availability of land and topographical conditions.
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The design of process flow sheets in an important step in the overall design of sewage treatment and requires a thorough understanding of the treatment units and associated unit operations/processes along with the mechanisms involved and performance levels attainable under variable conditions. It requires optimization of sewage treatment system coupled with stage wise optimal design of individual operation/process to achieve a design involving minimum cost.
The main contaminants in domestic sewage to be removed are biodegradable organics, as usually measured by BOD, suspended solids and pathogens with the first two having been traditionally considered as the performance indicators for various treatment units. It is generally the objective of domestic sewage treatment plant to produce treated effluent having BOD5 of 30 mg/1 or less and suspended solids of 50 mg/l or less for disposal into inland water bodies.
The process flow sheet of a conventional municipal sewage treatment plant comprises the unit operations of screening, grit removal and primary sedimentation followed by unit process of aerobic biological treatment usually achieved by activated sludge process or trickling filter followed by secondary sedimentation.
The sludges removed by primary and secondary sedimentation are digested anaerobically followed by drying of anaerobically digested sludge on sand sludge drying beds. This process flow sheet is depicted in Fig. 10.1
It is possible to replace the activated sludge process or trickling filter process by low cost treatment devices such as oxidation ditch, aerated lagoon or waste stabilization pond. Such treatment devices obviate the necessity of some of the unit operations and processes like primary sedimentation and anaerobic digestion. Some of these process flow sheets are shown in Fig. 10.2.
With the better understanding of microbiology and biochemistry of anaerobic treatment, it is now possible to treat dilute organic sewage such as domestic sewage directly through anaerobic treatment using recently developed innovative devices such as Upflow Anaerobic Sludge Blanket Reactor (UASBR), Fluid- Bed Submerged Media Anaerobic Reactor (FB-SMAR), Anaerobic Filter (AF) or Static Bed Submerged Media Anaerobic Reactor (SB-SMAR) and Anaerobic Rotating Biological Contactor (An RBC).
It is generally reported that in the case of these anaerobic treatment devices BOD5 removal efficiencies may range from 60 – 80%. Consequently post treatment will generally be required to achieve the prescribed effluent standards. The process flow sheet employing anaerobic treatment devices is depicted in Fig. 10.3.
The efficiency of a treatment plant depends not only on proper design and construction but also on good operation and maintenance. Table 10.9 gives the expected efficiencies of various treatment units as indicated in the Manual on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental Engineering Organisation New Delhi.
Flow diagrams of typical sewage treatment plants incorporating different processes or units are shown in Figs. 10.4, 10.5 and 10.6. The sewage treatment plant shown in Fig. 10.4 consists of Imhoff tanks and low rate trickling filters, and is suitable for small towns or cities.
The sewage treatment plant shown in Fig. 10.5 consists of sedimentation or settling tanks and high rate trickling filters, and is suitable for towns or cities of medium size. The sewage treatment plant shown in Fig. 10.6 consists of sedimentation or settling tanks and activated sludge tanks, and is suitable for large cities, where continuous attendance and supervision is possible.
