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DESIGN OF SEWAGE TREATMENT PLANT

UNIT-I

1) Introduction 3-6 1.1 Definitions

1.2 Treatment of sewage

1.3 Sewage compositions

2) Origin of Sewage 7-8 2.1 Sewage origin

2.2 Sewage mixing with rain water

2.3 Industrial effluent

3) Types of Sewage 9 3.1 Domestic sewage

3.2 Industrial sewage

3.3 Storm sewage

4) Types of Sewerage Systems 10 4.1 Combined System

4.2 Separate system

4.3 Partially combined system

UNIT-II

5) Advantages and disadvantages 12

Of the Sewerage System

6) Objectives of the Study 13-14 6.1 Study of the sewage treatment plant

6.2 Physical characterization of the domestic waste water

6.3 Chemical characterization of the domestic waste water

6.4 Biological characterization of the domestic waste water

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UNIT -III

7) Treatment of Waste Water 18-27 7.1 Pretreatment

7.2 Sewage treatment

7.3 Grit removal

7.4 Flow equalization

7.5 Fat & grease removal

7.6 Primary sedimentation

7.7 Secondary sedimentation

7.8 Tertiary sedimentation

7.9 Filtration

7.10 Lagoons or ponds

7.11 Biological nutrient removal

7.12 Nitrogen removal

7.13 Phosphorus removal

7.14 Disinfection

7.15 Fourth treatment stage

UNIT-IV

8) Design of Sewage Treatment Plant 29-31 8.1 Plant Capacity

8.2 Sizing calculation for collection pit

8.3 Sizing calculation of bar screen

8.4 Sizing calculation of aeration tanks

8.5 Check for aeration period/hydraulic retention time

8.6 Sizing calculation for sludge drying beds

8.7 Term used in the model full form

Drawings 32-33

Conclusion 34

References 35

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UNIT-I

-1 Introduction

-2 Origin of Sewage

-3 Types of Sewage

-4 Types of Sewerage Systems

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Introduction -1

1.1 Definitions:

Sewage treatment is the process of removing contaminants from wastewater primarily from household sewage. It includes physical, chemical and biological processes to remove these contaminants and produce environmentally safe treated wastewater (or treated effluent). A by-product of sewage treatment is usually a semi-solid waste or slurry, called sludge, which has to undergo further treatment before being suitable for disposal or land application.

1.2 Treatment of Sewage:

Sewage treatment may also be referred to as wastewater treatment, although the latter is a broader term which can also be applied to purely industrial wastewater. For most cities, the sewer system will also carry a proportion of industrial to the sewage treatment plant which has usually received pretreatment at the factories themselves to reduce the pollutant load. If the sewer system is a combined sewer then it will also carry urban runoff (storm water) to the sewage treatment plant.

The term sewage treatment plant or sewage treatment works in some countries is nowadays often replaced with the term wastewater treatment plant.

Sewage can be treated close to where the sewage is created, which may be called a decentralized system or even an on-site system. Alternatively, sewage can be collected and transported by a network of pipes and pump stations to a municipal treatment plant. This is called a centralized system although the borders between decentralized and centralized can be variable. For this reason, the terms semi-decentralized and semi-centralized are also being used.

1.3 Sewage Compositions:

Pollution in its broadest sense includes all changes that curtail natural

utility and exert deleterious effect on life. The crisis triggered by the rapidly

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growing population and industrialization with the resultant degradation of

the environment causes a grave threat to the quality of life. Degradation of

water quality is the unfavorable alteration of the physical, chemical and

biological properties of water that prevents domestic, commercial,

industrial, agricultural, recreational and other beneficial uses of water.

Sewage and sewage effluents are the major sources of water pollution.

Sewage is mainly composed of human fecal material, domestic wastes

including wash-water and industrial wastes. The growing environmental

pollution needs for decontaminating waste water result in the study of

characterization of waste water, especially domestic sewage. In the past,

domestic waste water treatment was mainly confined to organic carbon

removal. Recently, increasing pollution in the waste water leads to

developing and implementing new treatment techniques to control nitrogen

and other priority pollutants. Sewage Treatment Plant is a facility designed

to receive the waste from domestic, commercial and industrial sources and

to remove materials that damage water quality and compromise public

health and safety when discharged into water receiving systems. It includes

physical, chemical, and biological processes to remove various contaminants

depending on its constituents. Using advanced technology it is now possible

to re-use .sewage effluent for drinking water.

The present study comprises the study on quality of domestic waste water that is discharged from the city Chandigarh and Panchkula, through the kitchen outlets and bathroom effluents. The study includes characterization tests for pH value, acidity, alkalinity, chloride, residual chlorine, turbidity & DO. When designing a new sewage works or extending an existing one, the design should be based on the flow rate and sewage strength. Estimation of the flow rate is therefore a critical factor in design. If an existing works is to be extended and the flow records and plant performance data of the plant are available, then careful assessment of the rate of flow, daily and seasonal fluctuations in flow rate and sewage strength and operational data for the various process units can be used to determine the design parameters of the

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extended plant. However if no reliable data are available, as is often the case in designing new works, then estimates have to be made. This chapter provides guidelines in estimating flow for design purposes. This is achieved by establishing the size of the population for the contributing area to the works, thereby making it possible to obtain a reasonable estimate of likely flows to the works. It is necessary to have good estimates of existing or likely wastewater flows and biological loads over the period encompassed by the design. When the design is an extension or refurbishment of an existing plant one is usually able to access some data regarding flows and Wastewater strengths as a basis for process sizing, but for new plants, in areas which are still in the process of planning or development, it is necessary to make a number of design assumptions. In the case of residential areas or developments where there is no existing data, the best estimates of flow and load are usually based on population projections. The developers of the residential areas or estates will have an overall plan of the number of stands and for smaller developments may even have knowledge of the size of houses to be built, or the number of bedrooms in the flats or apartments planned. From this it should be possible to obtain a reasonable estimate of the likely population and in turn from this, an estimate of likely wastewater flows and biological loads. Population and flow projections for areas served by a wastewater treatment plant should be made before sizing of treatment processes and piping. Where possible, designs should be based on a 10-year design period for any one phase of construction. However shorter periods or staged developments often need to be implemented to match predicted growth patterns. In considering staged development the ultimate development of the collection area should be assessed to determine how the layout for the plant may appear if the area is fully developed. The population per dwelling unit depends on a number of factors. Obviously the size of the house is relevant and a four-bedroom house is likely to have a higher occupancy than a one or two bedroom unit.

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The area is also relevant. A holiday area is likely to be much closer to being fully occupied in season than a development which houses permanent residents. Income level and family size are also important. Many lower income areas have larger families than high income areas and populations of 6 to 8 persons per household are common in poorer areas compared to 4 persons per unit in a high income area.

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Origin of Sewage -2

2.1 Sewage Origin: Sewage is generated by residential, institutional, commercial and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, and sinks draining into sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and black water is becoming more common in the developed world, with treated greywater being permitted to be used for watering plants or recycled for flushing toilets.

2.2 Sewage mixing with rainwater:

Sewage may include storm water runoff or urban runoff. Sewerage systems capable of handling storm water are known as combined sewer systems. This design was common when urban sewerage systems were first developed, in the late 19th and early 20th centuries. Combined sewers require much larger and more expensive treatment facilities than sanitary sewers. Heavy volumes of storm runoff may overwhelm the sewage treatment system, causing a spill or overflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport storm water. Backups of raw sewage can occur if excessive infiltration/inflow (dilution by storm water and groundwater) is allowed into a sanitary sewer system. Communities that have urbanized in the mid-20th century or later generally have built separate systems for sewage (sanitary sewers) and storm water, because precipitation causes widely varying flows, reducing sewage treatment plant efficiency.

As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require storm water to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for storm water include retention basins, wetlands, buried vaults with various kinds of media filters, and vortex separators (to remove coarse solids).

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2.3 Industrial effluent:

In highly regulated developed countries, industrial effluent usually receives at least pretreatment if not full treatment at the factories themselves to reduce the pollutant load, before discharge to the sewer. This process is called industrial wastewater treatment. The same does not apply to many developing countries where industrial effluent is more likely to enter the sewer if it exists, or even the receiving water body, without pretreatment.

Industrial wastewater may contain pollutants which cannot be removed by conventional sewage treatment. Also, variable flow of industrial waste associated with production cycles may upset the population dynamics of biological treatment units, such as the activated sludge process.

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Types of Sewage -3

This modern water carriage sewerage system not only helps in removing the domestic and industrial waste water, but also helps in removing storm water drainage. The runoff resulting from the storms is also sometimes. Sewage system has been divided into three category.

3.1 Domestic Sewage

3.2 Industrial Sewage

3.3 Storm Sewage

3.1 Domestic Sewage:

The wastewater from residences and institutions, carrying body wastes primarily feces and urine, washing water, food preparation wastes, laundry wastes, and other waste products of normal living, are classed as domestic or sanitary sewage.

3.2 Industrial Sewage:

Liquid-carried wastes from stores and service establishments serving the immediate community, termed commercial wastes, are included in the sanitary or domestic sewage category if their characteristics are similar to household flows. Wastes that result from an industrial processes such as the production or manufacture of goods are classed as industrial wastewater, not as sewage.

3.3 Storm Sewage:

Surface runoff, also known as storm flow or overland flow, is that portion of precipitation that runs rapidly over the ground surface to a defined channel. Precipitation absorbs gases and particulates from the atmosphere, dissolves and leaches materials from vegetation and soil, suspends matter from the land, washes spills and debris from urban streets and highways, and carries all these pollutants as wastes in its flow to a collection point.

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Types of Sewerage Systems -4

Sewerage system has been divided into three categories, namely it has been listed below

4.1 Combined System

4.2 Separate System

4.3 Partially Separate System

4.1 Combined System:

When the drainage is taken along with the sewage then the system is called combined system. In the modern days a separate system is generally preferred to a combined system although each individual case should be decided separately.

4.2 Separate System:

When the drainage and sewage are taken independently of each other through two different sets of conduits then the system is called separate system.

4.3 Partially Combined System:

Sometimes a part of drainage system especially that originating from the roofs or paved courtyards of buildings is allowed to be admitted into the sewers and similarly sometimes the domestic sewage coming out from the residence or institutions, etc. is allowed to be admitted into the drains, the resulting system is called partially combined system.

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UNIT-II

-5 Advantages and Disadvantages of Sewerage Systems

-6 Objectives of the Study

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Advantages and disadvantages of -5 Sewerage System

Advantages: Capable of removing 97% of suspended solids.

Biological nitrification without adding chemicals.

Oxidization and nitration achieved.

Solids ad liquids separation.

Removes organics.

Cost effective.

Easily maintained mechanical work.

Self-sustaining system.

Disadvantages: Cleaning is hassle.

Most plants need at least three tanks.

Temperature changes affect the tank greatly.

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Objectives of the Study -6

The principal objective of waste water treatment is generally to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. An environmentally-safe fluid waste stream is produced. No danger to human health or unacceptable damage to the natural environment is expected. Sewage includes household waste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. Sewage also includes liquid waste from industry and commerce. The objectives of the study are:

Design of the sewage treatment plant.

Physical, chemical and biological characterization of the domestic waste water from the city Panchkula.

Waste water samples from the kitchen effluent and the bathroom waste of the city of Panchkula of the residence. The following physical characteristics were studied:

Odor

Taste

Color

Floatables

Turbidity The following chemical characteristics were studied:

Total Iron

Copper

Zinc

Potassium

Lead

Aluminum

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For determination of inorganic non-metallic constituents we determined the:

Alkalinity

Acidity

Chloride

Residual Chlorine

Sulphate

Ph. of the sample

Biochemical Oxygen Demand Dissolved Oxygen

6.1 Physical characteristic of waste water: Odour: It depends on the substances which arouse human receptor cells on coming in contact with them. Pure water doesn’t produce odour or taste sensations. Thus waste water which contains toxic substances has pungent smell which makes it easy to distinguish. Odor is recognized as a quality factor affecting acceptability of drinking water. The organic and inorganic substance contributes to taste or odor. The ultimate odor tasting device is the human nose. The odor intensity is done by threshold odor test.

Taste: The sense of taste result mainly from chemical stimulation of sensory nerve endings in tongue. Fundamental sensations of taste are, by convention more than by research evidence, salt, sweet, bitter, and sour. The rating involves the following steps: a) Dilution series including random blanks is prepared. b) Initial tasting of about half the sample by taking water into mouth and

holding it for several seconds and discharging it without swallowing. c) Forming an initial judgment on the rating scale. d) A final rating made for the sample. e) Rinsing mouth with taste and odor free water. f) Resting.

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Color: Color in water results from the presence of natural metallic ions such as Fe or Mg, humus and peat materials, planktons and weeds. It is removed to make water suitable for general and industrial applications. After turbidity is removed the apparent color and that due to suspended matter is found out. Tristimulus, Spectroscopic and Platinum cobalt method is used.

Total solids: It refers to matters suspended or dissolved in water and waste water. Solids affect the water or effluent quality adversely in a number of ways. Water with highly dissolved solids are not palatable and may cause physiological reaction in transient consumer. A limit of 500 mg dissolved solids/L is desirable for drinking waters. Evaporation method is used to separate total solids and their weight is found out.

Floatables: One important criterion for evaluating the possible effect of waste disposal into surface water is the amount of floatable material in the waste. It is important because it accumulates on the surface and may contain pathogenic bacteria and viruses. Two general types of floating matters are found. a) Particulate matters like grease balls. b) Liquid component capable of spreading as thin visible film over large

areas.

Turbidity: Clarity of water is important in producing products destined for human consumption and in many manufacturing uses. It is caused by suspended matter such as clay, silt and finely divided organic and inorganic matter, soluble colored organic compounds. Turbidity is an expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample. The standard method for determination of turbidity has been based on the Jackson candle Turbiditimeter and Nephlometer.

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6.2 Chemical characteristic of waste water: Chemical characteristics of water state the presence of metals their treatment, the determination of inorganic non-metallic constituents and the determination of organic constituents. Here goes a brief description of all the experiments we have performed.

6.3 Biological characteristic of waste water: Water quality has a key role in deciding the abundance, species composition, stability, productivity and physiological condition of indigenous populations of aquatic communities. Their existence is an expression of the quality of the water. Biological methods used for evaluating water quality include the collection, counting and identification of aquatic organisms. Most microorganisms known to microbiologists can be found in domestic wastewater like Bacteria, Protozoa, Viruses, and Algae. Planktons, Periphyton, Macro-python, Macro-invertebrates, Fish, Amphibians and Aquatic reptiles are the biotic group of interdependent organism. Wastewater contains vast quantities of bacteria and other organisms. Aerobic bacteria break down organic matter in the presence of available oxygen. Anaerobic bacteria disintegrate organic matter which is shut off from free oxygen, such as in the interior of a mass of feces or a dead body. The products of anaerobic decomposition have an extremely nauseating odor. Matter in which this condition exists is said to be septic. A multitude of the bacteria in wastewater are coliform bacteria: those found in the digestive tract of normal humans. It is these comparatively few pathogenic organisms that pose the greatest public health hazard. Waste water which is not properly treated may eventually find its way into a community water source and spread waterborne diseases.

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UNIT-III

-6 Treatment of Waste Water

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Treatment of Waste Water -7 Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Treating wastewater has the aim to produce an effluent that will do as little harm as possible when discharged to the surrounding environment, thereby preventing pollution compared to releasing untreated wastewater into the environment.

7.1 Pretreatment:

Pretreatment removes all materials that can be easily collected from the raw sewage before they damage or clog the pumps and sewage lines of primary treatment clarifiers. Objects commonly removed during pretreatment include trash, tree limbs, leaves, branches, and other large objects.

Fig: - Pretreatment

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The influent in sewage water passes through a bar screen to remove all large objects like cans, rags, sticks, plastic packets etc. carried in the sewage stream. This is most commonly done with an automated mechanically bar screen in modern plants serving large populations, while in smaller or less modern plants, a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and flow rate. The solids are collected and later disposed in a landfill, or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process.

7.2 Sewage Treatment:

Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.

Fig: - Sewage Treatment

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Primary Treatment: It consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment. Some sewage treatment plants that are connected to a combined sewer system have a bypass arrangement after the primary treatment unit. This means that during very heavy rainfall events, the secondary and tertiary treatment systems can be bypassed to protect them from hydraulic overloading, and the mixture of sewage and storm water only receives primary treatment.

Secondary Treatment: Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.

Tertiary Treatment: Tertiary treatments sometimes defined as anything more than primary and secondary treatment in order to allow rejection into a highly sensitive or fragile ecosystem. Treated water is sometimes disinfected chemically or physically prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

7.3 Grit removal:

Pretreatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is

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desirable at larger plants. Grit chambers come in 3 types: horizontal grit chambers, aerated grit chambers and vortex grit chambers. The process is called sedimentation.

Fig: - Great Removal

7.4 Flow equalization:

Clarifiers and mechanized secondary treatment are more efficient under uniform flow conditions. Equalization basins may be used for temporary storage of diurnal or wet-weather flow peaks. Basins provide a place to temporarily hold incoming sewage during plant maintenance and a means of diluting and distributing batch discharges of toxic or high-strength waste which might otherwise inhibit biological secondary treatment including portable toilet waste, vehicle holding tanks, and septic tank pumpers. Flow equalization basins require variable discharge control, typically include

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provisions for bypass and cleaning, and may also include aerators. Cleaning may be easier if the basin is downstream of screening and grit removal.

7.5 Fat and grease removal:

In some larger plants, fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as a froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal.

7.6 Primary Sedimentation:

In the primary sedimentation stage, sewage flows through large tanks, commonly called pre-settling basins, primary sedimentation tanks or primary clarifiers. The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Grease and oil from the floating material can sometimes be recovered soap making.

7.7 Secondary Sedimentation:

Secondary sedimentation is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and protozoa consume biodegradable soluble organic contaminants and bind much of the less soluble fractions into floc.

Some secondary treatment methods include a secondary clarifier to settle out and separate biological floc or filter material grown in the secondary treatment bioreactor.

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7.8 Tertiary Sedimentation:

The purpose of tertiary sedimentation is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.) More than one tertiary sedimentation process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called effluent polishing.

7.9 Filtration:

Sand filtration removes much of the residual suspended matter. Filtration over activated carbon, also called carbon adsorption, removes residual.

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Fig: -Filtration

7.10 Lagoons or Ponds:

Lagoons or ponds provide settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as Daphnia and species of rotifers greatly assist in treatment by removing fine particulates.

7.11 Biological Nutrient Removal:

Biological nutrient removal (BNR) is regarded by some as a type of secondary treatment process and by others as a tertiary treatment process. Waste water may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to the environment can lead to a buildup of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae, and blue-green algae. This may cause an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the

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algae by bacteria uses up so much of the oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus.

7.12 Nitrogen removal:

Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate, followed by denitrification, the reduction of nitrate to nitrogen gas.

Fig: Nitrogen Removal

Nitrogen gas is released to the atmosphere and thus removed from the water. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the waste water, organic matter or an added donor like methanol. The sludge in the denitrification tanks must be mixed well. Over time, different treatment

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configurations have evolved as denitrification has become more sophisticated.

7.13 Phosphorus removal:

Every adult human excretes between 200 and 1000 grams of phosphorus annually. Studies of United States sewage in the late 1960s estimated mean per capita contributions of 500 grams in urine and feces, 1000 grams in synthetic detergents, and lesser variable amounts used as corrosion and scale control chemicals in water supplies.

Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate-accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells. When the biomass enriched in these bacteria is separated from the treated water, these bio solids have a high fertilizer value. Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron or lime. This may lead to excessive sludge production as hydroxides precipitates and the added chemicals can be expensive.

7.14 Disinfection:

The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low.

Chlorination remains the most common form of waste water disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the

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natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically de-chlorinated, adding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation.

7.15 Fourth treatment stage

Micro pollutants such as pharmaceuticals, ingredients of household chemicals, chemicals used in small businesses or industries, environmental persistent pharmaceutical pollutant (EPPP) or pesticides may not be eliminated in the conventional treatment process (primary, secondary and tertiary treatment) and therefore lead to water pollution. Although concentrations of those substances and their decomposition products are quite low, there is still a chance to harm aquatic organisms. They mainly belong to the group of environmental persistent pharmaceutical pollutants. Techniques for elimination of micro pollutants via a fourth treatment stage during sewage treatment are being tested in Germany, Switzerland and the Netherlands. However, since those techniques are still costly, they are not yet applied on a regular basis. Such process steps mainly consist of activated carbon filters that adsorb the micro pollutants

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Unit-IV

-8 Design of Sewage Treatment Plant

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Design of Sewage Treatment Plant -8

8.1 Plant Capacity:

Average water supply per day = 423000 lit (Say)

= 0.423 mld*

Average sewage generated per day = 85% of supplied water

= 0.85*0.423=0.36 mld

= 360 kld** Average sewage generated per hour=360/24=15 cum/hr Peak factor = 3 Design flow capacity (maximum) = 13 x 3=45 cum/hr *mld – Million liter per day **kld – Kilo Liter per day

8.2 Sizing Calculation for Collection Pit: Retention time required = 4 h Average design flow = 15 m3/h Capacity of collection sump = 4 x 15=60 m3

Assume liquid depth = 5 m Area required for collection pits = 60/5 = 12 m2

Let it is a circular tank r = 1.93m Volume of the pit provide = π/4 x 4 x 4 x5 = 62.8 m3

Thus Area of the pit provided = 12.6 m2

8.3 Sizing Calculation of Bar Screen: Peak discharge = 45 m3/h Average discharge = 15 m3/h Average velocity @ average flow isn’t allowed to exceed 0.8 m/sec Average spacing between bar 20 mm The velocity = 0.3*60=18 m/h/ m2

Cross sectional area required = flow/velocity = 45/18 = 2.5 m2

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Liquid depth required= 1 m Velocity through screen at the peak flow = 1.6 m/sec Clear area = 2.5/1.6 = 1.3 No. of clear spacing = 1.3/0.02 = 65 Width of channel = (65 x 20) + (67 x 6) = 1702 mm Width of screen = 1700 mm

8.4 Sizing Calculation of Aeration Tank: Bod in the feed sewage = 100 ppm No. of aeration tank = 2 Average flow = 360/2 = 180 kld Total bod load to the aeration tank = 15 x 24 x 100 = 36 kgs Let mlss = 2000 mg/l, f/m=0.15 Volume of tank required = (Q x bod load) / (fm x mlss) = (180 x 100)/0.15 x 2000 = 60 m3

Assume liquid depth = 3.5 m Area = 60/3.5=17.143 m2

Tank size provided = 4.5 x 4.5 x 3.7 So, Volume of tank = 75 m3

8.5Check for Aeration Period/Hydraulic Retention Time: Hydraulic retention time t = 75 x 24/180 = 10 h So, the tank retention time is more than the required time.

8.6 Sizing Calculation for Sludge Drying Beds Maximum design flow rate = 45 m3/h, 360 kld Total feed suspended solid = 250 ppm Total outlet suspended solid = 50 ppm Load to the clarifier = 250-50 = 200 ppm Sludge generated per day = 360x 200/1000 = 72 kg/day Solid content in the feed= 3% Specific gravity of the sludge= 1.015 Volume of sludge= ((72/0.03)/ (1000x1.015)) = 2.36 m3

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For Rourkela weather condition, the beds get dried out about 7 days No. of cycles per year = 365/7 = 52 cycles Period of each cycles = 7 days Volume of sludge per cycle = 2.36 x 7 = 16.55 m3/cycle Spreading a layer 1m per cycle, Area of bed required = 16.55/1 = 16.55 m2

So area of 16.55 m2 for 2 drying beds is provided for the above system Hence, Sludge Drying Bed has dimensions of 4.5 m x 4.5 m x 1 m of two numbers.

8.7 Terms used in the Model Full Form CW Collection Well CBD Coarse Bubble Diffusion AT Aeration Tank PCT Primary Clarifier Tank SCT Secondary Clarifier Tank SDB Sludge Drying Bed

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Drawings

Fig: - Top view of Sewage Treatment Plant

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Fig: -Layout of Sewage Treatment Plant

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Conclusion

The average ranges of physical, chemical and biological characteristics of waste water quality are experimented and found out.

The pH ranges from 7.8 to 8.01. The Turbidity ranged from 10 to 120. The value of Turbidity was found to be within the permissible limit. The Chloride and Alkalinity were in the range of 3.5 to 120 mg/l and 15

to 80 mg/l respectively. The Total Iron content was in the range of 0 to 3 mg/l. The Zinc content was in the limits of 0.1 to 2 mg/l. Copper content ranged from 0 to 0.2 mg/l. Potassium was present in the limits of 2 to 12 mg/l. The parameters studied resemble the waste water quality.

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References

A STUDY ON THE WATER QUALITY OF THE CITY.

IS: 3025 (PART 10) – 1984 METHODS OF SAMPLING AND TEST (PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER, PART 10 - TURBIDITY.

IS: 3025 (PART 15) – 1984, METHODS OF SAMPLING AND TEST (PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER, PART 15 - TOTAL RESIDUE (TOTAL SOLIDS — DISSOLVED AND SUSPENDED).

IS: 3025 (PART 16) – 1984, METHODS OF SAMPLING AND TEST (PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER, PART 16 - FILTERABLE RESIDUE (TOTAL DISSOLVED SOLIDS).

IS: 3025 (PART 21) - 1983, METHODS OF SAMPLING AND TEST (PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER, PART 21 - TOTAL HARDNESS).

IS: 3025 (Part 51) – 2001, METHODS OF SAMPLING AND TEST (PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER, PART 51 – CARBONATE AND BICARBONATE.

IS: 3025 (Part 22) – 1986, METHODS OF SAMPLING AND TEST (PHYSICAL AND CHEMICAL) FOR WATER AND WASTE WATER, PART 22 - ACIDITY.