Print Email Download

Design Of Wastewater Treatment Plant Environmental Sciences Essay

Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by industrial or commercial activities prior to its release into the environment or its re-use. It involves mainly three stages, called primary, secondary and tertiary treatment. Activated sludge process can be used to treat the wastewater produces from Oushadi Ayurvedic Pharmaceuticals. Contaminants include oils, particulate solids, materials having high concentrations of biochemical oxygen demand (BOD) and suspended solids (SS). Primary treatment consists of temporarily holding the sewage ,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. One major problem faced by Oushadi was the complete removal of oil from wastewater and this posed a threat to the use of treated water for their boilers. Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by natural, 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 is sometimes defined as anything more than primary and secondary treatment.

Comprising over 70% of the earth’s surface, water is undoubtedly the most precious natural resource that exists on our planet. Recycling of wastewater in industries is therefore a process, which is a necessity rather than a luxury. Wastewater treatment is the process of removing contaminants from wastewater. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a waste stream (or treated effluent) and a solid waste or sludge suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with many toxic organic and inorganic compounds. Wastewater can cause contamination of ground water, lakes, streams and rivers. It wastes and decreases the amount of potable water available on earth, causes an imbalance in aquatic ecosystems and wastes the nutrients contained in wastewater by not recycling them. Many industries use large volumes of water in their manufacturing operations. Because some of this water becomes contaminated, it requires treatment before discharge.

Improvements in determining the effects of industrial waste discharges have led to the adoption of stringent environmental laws, which define the degree of treatment necessary to protect water quality. Discharge permits, issued under the National Pollutant Discharge Elimination System (NPDES), regulate the amount of pollutants that an industry can return to the water source. The permitted quantities are designed to ensure that other users of the water will have a source that meets their needs, whether these needs are for municipal water supply, industrial or agricultural uses, or fishing and recreation. Consideration is given to the feasibility of removing a pollutant, as well as the natural assimilative capacity of the receiving stream. This assimilative capacity varies with the type and amount of pollutant.

Wastewater treatment plants are designed to convert liquid wastes into an acceptable final effluent and to dispose of solids removed or generated during the process. In most cases, treatment is required for both suspended and dissolved contaminants. Special processes are required for the removal of certain pollutants, such as phosphorus or heavy metals. Wastewater can be recycled for reuse in plant processes to reduce disposal requirements. This

practice also reduces water consumption.

Sahradaya College Of Engineering & Technology, Kodakara 1

Design Of Wastewater Treatment Plant

1.1 POLLUTANTS IN WASTEWATER

Organic Compounds

The amount of organic material that can be discharged safely is defined by the effect of the material on the dissolved oxygen level in the water. Organisms in the water use the organic matter as a food source. In a biochemical reaction, dissolved oxygen is consumed as the end products of water and carbon dioxide are formed. Atmospheric oxygen can replenish the dissolved oxygen supply, but only at a slow rate. When the organic load causes oxygen consumption to exceed this resupply, thus dissolved oxygen level drops, leading to the death of fish and other aquatic life. Under extreme conditions, when the dissolved oxygen concentration reaches zero, the water may turn black and produce foul odours, such as the "rotten egg" smell of hydrogen sulphide. Organic compounds are normally measured as chemical oxygen demand (COD) or biochemical oxygen demand (BOD).

Nutrients

Nitrogen and phosphorus are essential to the growth of plants and other organisms. However, nitrogen compounds can have the same effect on a water source as carbon- containing organic compounds. Certain organisms use nitrogen as a food source and consume oxygen.

Phosphorus is a concern because of algae blooms that occur in surface waters due to its presence. During the day, algae produce oxygen through photosynthesis, but at night they

consume oxygen.

Solids

Solids discharged with a waste stream may settle immediately at the discharge point

or may remain suspended in the water. Settled solids cover the bottom-dwelling organisms, causing disruptions in population and building a reservoir of oxygen-consuming materials. Suspended solids increase the turbidity of the water, thereby inhibiting light transmittance. Deprived of a light source, photosynthetic organisms die. Some solids can coat fish gills and

cause suffocation.

Sahradaya College Of Engineering & Technology, Kodakara 2

Design Of Wastewater Treatment Plant

Acids and Alkalies

The natural buffering system of a water source is exhausted by the discharge of acids and alkalies. Aquatic life is affected by the wide swings in pH as well as the destruction of

bicarbonate alkalinity levels.

Metals

Certain metals are toxic and affect industrial, agricultural, and municipal users of the

water source. Metals can cause product quality problems for industrial users. Large quantities of discharged salts necessitate expensive removal by downstream industries using the receiving stream for boiler makeup water.

The contaminants in wastewater are removed by physical, chemical and biological methods. The specific method are classified as physical unit operations, chemical unit processes and biological unit processes.

1.2 Unit Operations and Process in Wastewater Treatment

TABLE 1.1 Operations and Process in Wastewater Treatment

Contaminant Unit Operations/ Processes Classifications

Suspended solids

Screening Sedimentation Floatation

Filtration

Physical

Biodegradable Organics

Activated Sludge Trickling Filters Rotating Biological

Contactors

Biological

Phosphorus

Lime Coagulation

Chemical

Heavy Metal Chemical Precipitation Chemical

Sahradaya College Of Engineering & Technology, Kodakara 3

Design Of Wastewater Treatment Plant

CHAPTER 2

WASTEWATER CHARACTERISTICS

2.1 WASTEWATER CHARACTERISTICS

To design a treatment process properly, characterization of wastewater is perhaps the most critical step. Wastewater characteristics of importance in the design of a treatment process can be grouped into the following categories:

 Temperature

 pH

 Colour and Odour

 Carbonaceous substrates

 Nitrogen

 Phosphorous

 Chlorides

 Total and volatile suspended solids (TSS and VSS)

 Toxic metals and compounds

 Density

 Oil & grease

 Alkalinity

2.1.1 Temperature:

The temperature of wastewater is commonly higher than that of the local water supply, because of the addition of warm water from household activities. As the specific heat of water is much greater than that of air, the observed wastewater temperatures are higher than the local air temperatures during most of the year and are hotter only during the hottest summer months. Depending on the location and time of the year the effluent temperatures can either higher or lower than the corresponding influent values.

2.1.1.1 Effect of temperature

The temperature of water is a very important parameter because of its effect on chemical reactions and reaction rates, aquatic life, and the suitability of the water for beneficial purposes. In addition, oxygen is less soluble in warm water than in cold water. The increases in rate of biochemical reactions that accompanies an increase in temperature, combined with the decrease in the quantity of oxygen present in surface waters, can often cause serious depletions in dissolved oxygen concentrations in summer months. Optimum temperatures for

bacterial activity are in range from 25 to 35oC.Aerobic digestion and nitrification stops when

the temperature rises to 50oC. When the temperature drops to about 15oC, methane producing bacteria become quite inactive and at about 5oC, the autotrophic nitrifying bacteria practically

Sahradaya College Of Engineering & Technology, Kodakara 4

Design Of Wastewater Treatment Plant

cease functioning. At 2oC, even the chemo heterotrophic bacteria acting on carbonaceous material become essentially dormant.

2.1.2 pH:

The hydrogen ion concentration is an important quantity parameter of both natural waters and wastewaters. The usual means of expressing the hydrogen ion concentration is as pH, which is defined as the negative logarithm of hydrogen ion concentration. The concentration range for the existence of most biological life is quite narrow and critical typically 6 to 9. Wastewater with a extreme concentration of hydrogen ion is difficult to treat by biological means, an if concentration is not altered before discharge, the wastewater effluent may alter the concentration in the natural waters. For treated effluents, discharged to the environment the allowed pH range usually varies from 6.5 to 8.5.

The pH of fresh domestic waste water is slightly more than that of the water supply to the community. However, the onset of septic conditions may lower the pH while the presence of industrial wastes may produce extreme fluctuations.

2.1.3 ODOUR AND COLOUR:

Fresh domestic waste water has slightly soapy and earthy odour and cloudy appearance depending upon its concentration, With the passage of time, the waste water becomes stale, darkening in colour with a pronounced colour due to microbial activity.

2.1.3.1 Odour:

Odours are usually caused by gases produced by the decompositions of organic matter or by substances added to the waste water. Fresh wastewater has a distinctive, somewhat disagreeable odour which is less objectionable than the odour of wastewater which has undergone anaerobic decomposition. The most characteristic odour of stale or septic wastewater is hydrogen sulfide. Odours have been rated as the foremost concern of public relative to wastewater treatment facilities. Within the past few years, the control of odour has become major consideration in the design and operation of wastewater collection, treatment and disposal facilities, especially with respect to public acceptance of these facilities.

Effect of Odours:

The importance of odours at low concentrations in human terms is related primarily to psychological stress they produce rather than to the harm they do to the body. Offensive odours can cause poor appetite, lower water consumption, impair respiration, nausea and vomiting and mental perturbation. Some odorous compounds are toxic at elevated

concentrations.

Sahradaya College Of Engineering & Technology, Kodakara 5

Design Of Wastewater Treatment Plant

2.1.3.2 Colour:

Historically, the term ‘condition’ was used along with composition and concentration was used to describe waste water. Conditions refer to the age of wastewater, which is determined qualitatively by its colour and odour. Fresh wastewater is usually a light brownish grey colour. However as the travel time in collection system increases, and more anaerobic conditions develop, the color of waste water changes sequentially from grey to dark grey and ultimately black . When the color of wastewater is black, the wastewater is described as septic . In most cases the grey , dark grey and black color of wastewater is due to formation of metallic sulphites which form as the sulphide produced under anaerobic conditions react with metals in the wastewater.

2.1.4 CARBONACEOUS CONSTITUENTS:

Carbonaceous constituents are measured by BOD, COD or TOC analyses. While the BOD has been the common parameter to characterize carbonaceous material in wastewater, COD is becoming more common in most current comprehensive computer simulation design models.

2.1.4.1 Biochemical Oxygen demand:

The BOD test gives a measure of the oxygen utilized by bacteria during the oxidation of organic material contained in a waste water sample. The test is based on the premise that all the biodegradable organic material contained in the wastewater sample will be oxidized to CO2 and H2O, using molecular of oxygen as the electron acceptor. Hence, it is a direct measurement of oxygen requirements and an indirect measure of biodegradable organic matter.

2.1.4.2 Chemical oxygen demand:

The COD test is based on the principal that most organic compounds are oxidized to CO2 and H2O by strong oxidizing agents under acid conditions. The measurement represents the oxygen that would be needed for aerobic microbial oxidation, assuming that all organics are biodegradable.

2.1.4.3 Total Organic Carbon:

The total carbon analyzer allows a total soluble carbon analysis to be made directly on an aqueous sample. In many cases TOC can be correlated with COD and occasionally

with BOD values.

Sahradaya College Of Engineering & Technology, Kodakara 6

Design Of Wastewater Treatment Plant

Table 2.1: Definition of Solids Found in Wastewater

Test Description

The residue remaining after a wastewater

Total Solids (TS)

Total Volatile Solids(TVS)

sample has been evaporated and dried at a specified temperature (103-105oC).

Those solids that can be volatilized and burned off when the TS are ignited (500±50 oC).

oTotal Fixed Solids (TFS) The residue that remains after TS are ignited

(500±50 C).

Portion of the TS retained on a filter with a specified pore size, measured after being dried at a specified temperature (105oC). The

Total Suspended Solids (TSS)

Volatile Suspended Solids (VSS) Settle able Solids

filter used most commonly for the determination of TSS is Whatman glass fiber filter which has a nominal pore size of about

1.58µm.

Those solids that can be volatilized and burned off when the TSS are ignited (500±50 oC).

Suspended solids, expressed as mL/L, that will settle out of suspension with in a specified period of time.

Sahradaya College Of Engineering & Technology, Kodakara 7

Design Of Wastewater Treatment Plant

CHAPTER 3

STAGES OF WASTEWATER TREATMENT

3.1 STAGES OF WASTEWATER TREATMENT

1) Preliminary Treatment: Removal of wastewater constituents such as rags, sticks, floatable grit and grease that may cause maintenance or operational problems with treatment operations, process and ancillary systems.

2) Primary treatment: Removal of a portion of suspended solids and organic matter from the wastewater.

3) Advanced Primary: Enhanced removal of suspended solids and organic matter from the wastewater typically accomplished by chemical addition to wastewater.

4) Secondary treatment : Removal of biodegradable organic matter (in solution or suspension)

5) Secondary with nutrient removal: Removal of biodegradable organics, suspended solids and nutrients. (N2, P or both N2 and P).

6) Tertiary treatment: Removal of residual suspended solids (after secondary treatment)

usually by granular medium filtration or micro screens. Disinfection is also typically a part of tertiary treatment. Nutrient removal is often included in this definition.

7) Advanced Tertiary: Removal of dissolved and suspended materials remaining after normal biological treatment when required for various water reuse applications.

3.2TREATMENT PROCESS FLOWSHEET:

Depending on the contaminants to be removed, an almost limitless number of process combinations can develop using the unit operations and process. The term “flow sheet” is used to describe particular combinations of unit operations and process used to achieve a specific treatment objective. Apart from the analysis of the technical feasibility of the individual treatment methods, the exact flow-sheets configuration will depend on factors such as (1) the needs of the of the client’s needs, (2) the designers past experience, (3) regulatory agency policies on the application of specific treatment methods, (4) the availability of equipment suppliers, (5) what use can be made of existing facilities, (6) the availability of qualified operating personnel, (7) initial construction costs and (8) future operation and

maintenance costs.

Sahradaya College Of Engineering & Technology, Kodakara 8

Design Of Wastewater Treatment Plant

The Process Flow Sheet proposed in this project for the wastewater treatment is as shown below :

Figure 3.1: Process Flow Sheet

Table 3.1: LEVELS OF WASTEWATER TREATMENT TREATMENT LEVEL DESCRIPTION

Removal of wastewater constituents Such as

Preliminary

rags,sticks, floatables,grit and grease that may cause maintenance or operational problems with the treatment operations.

Primary Removal of apportion of the suspended solids and organic matters from wastewater.

Enhanced removal of suspended solids and

Advanced Primary

organic matters from wastewater typically accomplished by chemical addition or filtration.

Secondary Removal of biodegradable matters and suspended solids

Secondary with Nutrient removal Removal of biodegradable organics and nutrients.

Tertiary Removal of residual suspended solids usually by granular medium filtration or micro screen

Sahradaya College Of Engineering & Technology, Kodakara 9

Design Of Wastewater Treatment Plant

CHAPTER 4

LITERATURE REVIEW

4.1 SCREENING

The first unit operation generally encountered in wastewater is screening. A screen is a device with opening, generally of uniform size, that is used to retain solids found in the influent wastewater to the treatment plant. The principle role of screening is to remove coarse material from the flow stream that could damage subsequent process equipment, reduce overall treatment process reliability and effectiveness and contaminate waterways.

Fine screens are sometimes used in place of or following coarse screens where greater removal of solids are required to protect process equipment and eliminate materials that may inhibit the beneficial reuse of bio-solids.

All aspects of screening, removal, transport and disposal must be considered in the application of screening devices including

1. The degree of screening removal required, because of potential effects of downstream processes

2. Health and safety of operators as screenings contain pathogenic organism that attract insects

3. Odour potential

4. Requirements for handling transport and disposal

5. Disposal options

Two general types of screens, coarse screens and fine screens are used in preliminary treatment of wastewater. Coarse screens have clear opening ranging from 6 -150mm, Fine screens have openings ranging less than 6mm. Micro screens will generally have screen opening less than 50µ m, are used principally in removing fine solids from treated effluents. Bar rack for the removal of coarse solids.

4.2 GRIT REMOVAL

Removal of grit from wastewater may be accomplished in grit chambers or by the centrifugal separation of solids. Grit chambers are designed to remove grit, consisting of sand, gravels, cinders or other heavy solid materials that have subsiding velocities of specific gravities substantially greater than those of the organic putrescible solids in wastewater. Grit chamber are most commonly located after the bar screen and before the primary

sedimentation tanks.

Sahradaya College Of Engineering & Technology, Kodakara 10

Design Of Wastewater Treatment Plant

Locating grit chamber ahead of wastewater pump when desirable, would involve placing them at considerable depth at added expense. It is therefore deem more economical to pump the wastewater, including the grit to grot chamber located at a convenient position ahead of the treatment plant units, recognising that pumps may require greater maintenance.

4.2.1 Why Grit is removed?

 Prevent wear on pumps

 Accumulation in clarifiers

 Accumulation in aeration tank

 Accumulation in digesters

 Clogging of sludge piping

4.2.2 TYPES OF GRIT CHAMBERS

Grit chambers are provided to (1) Reduce formation of heavy deposits in pipelines, channels and conduits (2) reduce the frequency of digester cleaning caused by excessive accumulation of grit (3) Protect moving mechanical equipment from abrasion and accompanying abnormal wear.

There are three general type of grit chamber: Horizontal flow either rectangular or square configuration; aerated or vortex type. The aeration type consists of a spiral flow aeration tank where the spiral velocity is induced and controlled by the tank dimension in and quantity if air supplied to the unit. The vortex type consists of cylindrical tank, in which the flow enter tangentially vortex flow pattern; centrifugal and gravitational forces causes the grit to separate. Design of grit chamber is commonly based on removal of grit particles having a

specific gravity of 2.65 and wastewater temperature 15.5oC. However analysis of grit

removal data indicates the specific gravity ranges from 1.3-2.7. Horizontal Flow Grit Chambers

In the horizontal flow type the flow passes through the chamber in a horizontal direction and the straight line velocity of flow is controlled by the dimension of the unit, an effluent distribution gate and a weir at the effluent end. Rectangular and square Horizontal

flow grit chambers have been used for many years.

Sahradaya College Of Engineering & Technology, Kodakara 11

Design Of Wastewater Treatment Plant

Typical Design Information for Horizontal Flow Grit Chamber

Table 4.1

Item

Detention time

Horizontal velocity

Settling velocity for removal of .21mm material

Settling velocity for removal of .15mm material

SI

Unit

Range

Typical

s

45-90

60

m/s

.25-.4

.3

m/min

1-1.3

1.15

m/min

.6-.9

.75

%

30-40

36

%

25-50

30

Head loss in a control section as % depth in channel

Added length allowance for inlet and outlet turbulence

Horizontal- Velocity Grit Chambers:

They are controlled by either a

 Parshall flume

 Proportional weir

Parshall fume is used more widely due to less head loss than the weir. The flume and weir are also used to measure flow rates. In the chamber, a constant horizontal velocities is maintained by proper cross-sectional geometry of the chamber.

 Horizontal velocity must be adequate to keep the organic matter in suspension.

 Horizontal velocities should be sufficient so as to prevent scouring of settled grit along the bottom of the channel. Values of .23-.38m/s are common.

4.3 FLOW EQUALIZATION

Process for water treatment works best with uniform conditions. Shock to the bioprocesses in the form of sudden change in the concentration of nutrients can upsets. If the concentrations or flow rates of the waste vary greatly, dosages for treatment must be constantly be readjusted. Flow equalization is a method used to overcome the operational problems caused by flow rate variation, to improve the performance of downstream process, and to reduce the size and cost of t downstream treatment facilities.

Flow equalisation is damping of flow rate variations to achieve a constant or nearly constant flow rate and can be applied in a number of different situations depending on characteristics of collection system. There may be aeration both to keep the fluid from

becoming anaerobic and smelly and to biodegrade some of the organic compound present.

Sahradaya College Of Engineering & Technology, Kodakara 12

Design Of Wastewater Treatment Plant

The principle applications are for the equalisation of:

1. Dry weather flow to reduce the peak flow and loads.

2. Wet weather flow in sanitary collection systems experiencing inflow and infiltration.

3. Combined storm water and sanitary system flows.

In the line arrangement all of flow passes through equalisation basin. This arrangement can be used to achieve considerable amount of concentration and limits its divert into equalisation basin. Although the pumping requirements are reduced in this arrangement, the amount of constituents concentration damping is considerably reduced.

The principle benefits that are cited as derived from application of flow equalisation are:

1. Biological treatment is enhanced, because shock loadings are eliminated or minimised, inhibiting substances can be diluted and pH can be stabilised.

2. The effluent quality and thickening performance of secondary sediment tank

following biological treatment is improved consistency in solid loading.

3. Effluent filtration surface area requirements are reduced, filtered performance is improved and more uniform filter back wash cycles are possible by lower hydraulic loading.

4. In chemical treatment, damping of mass loading improves chemical feed control and process reliability.

5. Often the rest of the plant designed with a smaller equipment( less capital investment) because of this improvement in performance.

Disadvantage of flow equalisation include

 Relatively large land areas are required.

 Equalisation facilities may have to be covered for odour control near residential area.

 Additional operation and maintenance required

 Capital cost is increased.

4.4 PRIMARY SEDIMENTATION

The objective of treatment by sedimentation is to remove readily settle able solids and floating materials and thus reduce the suspended solids content. Primary sedimentation is used as a preliminary step in the further processing of the waste water. Efficiently designed and operated primary sedimentation tanks should remove from 50-70% of the suspended solids and from 25-40% of the BOD.

Sedimentation tanks have also been used as storm water retention tanks which are designed to provide a moderate detention period (10-30mins) for overflows from either combined sewers and storms sewers. The purpose of sedimentation is to remove a substantial portion of organic solids that otherwise would be discharged directly to the receiving waters. Sedimentation tanks have also been

used to provide detention periods sufficient for effective disinfection of such overflows.

Sahradaya College Of Engineering & Technology, Kodakara 13

Design Of Wastewater Treatment Plant

4.4.1DESCRIPTION

Almost all treatment plants are mechanically cleaned sedimentation tanks of standardized circular or rectangular design. The selection of the type of sedimentation unit for a given application is governed by the size of the installation, by rules and regulations of local control authorities, by local site conditions and by the experience and judgment of the engineer. Two or more tanks have to be provided so that the process may remain in operation when one tank is out of service for maintenance and repair work.

RECTANGULAR TANKS

Rectangular sedimentation tanks may use either chain and flight solids collectors or travelling bridge type collectors. The solids settling in the tank are scraped solids hoppers in small tanks and transfers troughs in large tanks. In every long unit two collection mechanisms can be used to scrape solids to collection points near the middle of the tank length. Where possible, it is desirable to locate solids pumping facilities close to the collection hoppers.

4.4.2 SEDIMENTATION TANK PERFORMANCE

The efficiency of sedimentation basins with respect to removal of BOD and TSS is reduced by

1. Eddy currents formed by inertia of incoming fluid

2. Wind induced circulation cells formed in uncovered tanks

3. Thermal convection currents

BOD AND TSS REMOVAL R=

Where

R - expected removal efficiency t- detention time

a, b – empirical constants

4.4.3 DETENTION TIME

The bulk of finely divided solids reaching primary sedimentation tanks is incompletely flocculated but is susceptible to flocculation. Flocculation is aided by eddying motion of the fluid within tanks and proceeds through the coalescence of fine particles at a rate that is a function of their concentration and of the natural ability of the particles to coalesce upon collision. Coalescence of a suspension of solids becomes more complete as time elapses, thus detention time is a consideration in the design of sedimentation tank. Normally primary sedimentation tanks are designed, to provide 1.5-2.5hrs of detention based

on the average rate of waste water.

Sahradaya College Of Engineering & Technology, Kodakara 14

Design Of Wastewater Treatment Plant

4.4.4 SCOUR VELOCITY

To avoid the resuspension or stirring of resuspended particles horizontal velocity through the tank should be kept considerably low. The following equation gives critical

velocity.

( )

VH=[ ]

Where VH - Horizontal velocity that will just produce scour, LT-1 (m/s)

K - Constant that depends on type of material being scoured s – Specific Gravity of Particles

g- Acceleration due to gravity d- Diameter of Particles

f- Darcy-Weisbach friction factor(Unit less)

4.5 REMOVAL OF OIL

The discharge of wastewater-s containing fats, oils and greases (FOG's) to the sewer system is regulated by most treatment plants. In setting discharge limits for these types of materials, a distinction is generally made between those materials of biological origin (animal fats, vegetable oils, etc.), which generally result from the processing of foods, and those of a mineral origin (hydrocarbon solvents, gasoline, lube oils, paraffin, etc.), which generally result from industrial manufacturing processes. Since the biodegradability of the hydrocarbon type wastes from industrial sources, service stations and vehicle wash areas, is less than that for the material resulting from food processing operations, the limits set by agencies are usually more stringent for the hydrocarbon or petroleum derived material. The testing for the oil and grease content of a wastewater is usually performed using either Freon or hexane as an extracting solvent in accordance with EPA approved procedures. Absorption on a silica gel column of the more polar biological FOG's is generally used to determine the percent of the hydrocarbon fraction of the extracted fats, oil and greases in the wastewater sample.

The presence of oily wastes in wastewater can be in several forms. The material can either be in a free, emulsified or dissolved state. Of the three forms, the treatment to remove either emulsified or dissolved oil is generally more complex and expensive. In many instances, chemicals, such as detergents or other solubilising agents, have been added to induce the oil to remain in the emulsified form and drastic steps must be taken to break the emulsion before the oil can successfully be removed. Pre-treatment for the removal of free floating oil in wastewater streams is usually accomplished by taking advantage of the specific gravity difference between the organic material and water. The waste stream or wastewater batch is discharged into a separator unit where the water and oil have a chance to separate and the oil is given an opportunity to float to the surface. Removal of the floating oil is then

accomplished through either skimming or allowing it to drain into a waste oil holding tank.

Sahradaya College Of Engineering & Technology, Kodakara 15

Design Of Wastewater Treatment Plant

The most frequently used type of separator is the API (American Petroleum Institute) type, which can remove up to 60 to 99% of the free oil in a waste stream. Should there be excessive discharges of oil and grease to sewerage systems, problems may occur with the clogging of sewers and pumping plants and with the interference of biological treatment processes.

4.5.1 TEST METHODS FOR OIL AND GREASE

The test procedures used to measure oil and grease concentrations in wastewater do not determine the presence of specific substances, but groups of substances that can be extracted from a sample using a particular solvent. The sixteenth edition of Standard Methods for the Examination of Water and Wastewater provides for the use of three test procedures to determine oil and grease concentrations in wastewater samples. These procedures include (1) the partition-gravimetric method which involves the extraction of dissolved and emulsified oil and grease using trichlorotrifluoroethane, (2) the partition-infrared method which uses an extraction process identical to(1)method together with infrared detection methods and (3) the Soxhlet extraction method which is based on an acidification of the sample, separating the oils from the liquid by filtration and extraction using trichlorotrifluoroethane. The above test methods have occasionally been used interchanqeab1y under the assumption that they give comparable results.

4.5.2 CONTROL TECHNOLOGY FOR THE REMOVAL OF OIL AND GREASE

The control techno1ogy for oil and grease removal varies in complexity, although the basic processes involve the collection and recovery of oils and the removal of undesirable pollutants before discharge to a receiving system.

4.5.2.1GRAVITY OIL-WATER SEPARATION

When a free oil or dispersed oily water mixture is brought to a relatively quiescent state and given sufficient time, the oil droplets will coalesce and eventually separate from wastewater, forming a continuous floating oil layer which may be skimmed off. Dissolved Air Flotation Oil-Water Separation Dissolved air flotation (DAF) devices utilize the gravity separation concept for the removal of oil and grease from wastewater but tend to be more effective in removing the dispersed oil mixture because the buoyancy differential is enhanced by induced small air bubbles. Coagulant aids such as polyelectrolytes are commonly used to promote agglomeration of the oil-bearing matter into large flocs which are more easily

removed

Sahradaya College Of Engineering & Technology, Kodakara 16

Design Of Wastewater Treatment Plant

4.5.2.2 INDUCED AIR FLOTATION OIL-WATER SEPARATION

The principle of the IAF is that an intimate mixture of air and mineralladen liquid is forced through nozzles which provide the separating action necessary to create millions of bubbles. The bubbles are then disseminated throughout the flotation chamber. Oil and suspended solids attached to the air bubbles are carried to the surface of the water where they form a froth. A skimmer paddle sweeps the oil and solids-laden froth into an overflow chamber.

4.5.2.3 ULTRAFILTRATION REMOVAL OF OIL AND GREASE

Carbon adsorption or membrane filtration using reserve osmosis treatment is very effective to remove dissolved and emulsified oils. The concept of ultrafiltration is based on the sieving action of a membrane retaining molecules larger than the membrane pores. Reverse osmosis uses a semipermeable membrane to filter dissolved matter using very high pressures; an extremely high quality feed is required for the efficient operation of reverse osmosis facilities. The effluent from these operations contains essentially no oil and grease. However due to the large capital and operating costs associated with these devices, they are utilized very infrequently.

4.5.2.4 BIOLOGICAL TREATMENT

Biological treatment is generally effective in degrading dissolved oils and other types of stabilized emulsions which cannot be destabilized by chemical coagulants. However, a biological system is only effective on highly dilute oil-contaminated wastewaters because

Sahradaya College Of Engineering & Technology, Kodakara 17

Design Of Wastewater Treatment Plant

mineral-based oils are adsorbed by the microorganisms faster than they can be metabolized. In activated sludge systems, the adsorbed oil tends to damage sludge settling characteristics and cause system failure, anaerobic digestion Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen and is widely used to treat wastewater. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich digestate can be used as fertiliser. The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Methanogens, finally are able to convert these products to methane and carbon dioxide

4.6 ACTIVATED SLUDGE PROCESS

The basic activated sludge treatment process consists of the following 3 basic components.

1. A reactor in which the microorganism responsible for the treatment are kept in suspension and aerated.

2. Liquid solids separation, usually in sedimentation tank.

3. A recycle system for returning solids removed from the liquid solid separation unit back to the reactor.

Numerous process configurations have evolved employing these components. Important features of the activated sludge process are the formation of flock & settle able solids that can be removed by gravity settling in sedimentation tanks. In most cases, the activated sludge process is employed in conjunction with physical and chemical processes that are used for the preliminary and primary treatment of wastewater, and post treatment including disinfection and possibly filtration.

Historically most activated sludge plants have received waste water pretreated by primary sedimentation. Primary sedimentation is most efficient at removing settle able solids; whereas the biological processes that are essential for removing soluble, colloidal and particulate organic substance. For application such treating waste water from smaller sized communities, primary treatment is often not used as more emphasize is placed on simpler and less operators intensive treatment methods. Primary treatment is omitted in areas of the world that have hot climate where odour problems from primary tanks can be significant. For these applications, various modifications of conventional activated sludge processes are used, including

sequencing batch reactors, oxidation ditch systems, aerated lagoons, or stabilization ponds.

Sahradaya College Of Engineering & Technology, Kodakara 18

Design Of Wastewater Treatment Plant

4.6.1 COMPLETE-MIX ACTIVATED –SLUDGE PROCESS

Effluent from the primary sedimentation tank and recycled return activated sludge are introduced typically at several points in the reactor. Because the tank contents are thoroughly mixed, the organic load, oxygen demand and substrate concentration are uniform through out the entire aeration tank and the F/M ratio is low. Care should be taken to assure that the CMAS reactor is well mixed and that the influent feed and the effluent withdrawal points are selected to prevent short circuiting of untreated or partially treated waste water. The complete Mix Reactor is usually configured in square rectangular or round shapes. Tank dimensions depend mainly on the size type and mixing pattern of the aeration equipment. The design concepts are the design SRT, the selection of kinetic and stoichiometric coefficients and the application of appropriate mass balances.

4.6.1.1Fundamentals of Process Analysis Control

The purpose of this analysis is to introduce the basic considerations involved in process design, process control measures, operation problems associated with activated sludge process and activated sludge selector process.

4.6.1.2 Process Design Considerations

In the design of activated sludge process consideration must be given to

1. Selection of reactor type

2. Applicable kinetic relationship

3. Solid retention time and loading criteria to be used

4. Sludge production

5. Oxygen requirement and transfer

6. Nutrient requirements

7. Other chemical requirements

8. Settling characteristics of bio solids

9. Use of selectors

10. Effluent characteristics

4.6.2.3 Selection of Reactor Type

Important factors that must be considered in the selection of reactor types for activated sludge process include

1. Effects of reaction kinetics

2. Oxygen transfer requirement

3. Nature of waste water

4. Local environmental conditions

5. Presence of toxic or inhibitory substance in the influent waste water

6. Costs

7. Expansion to meet future treatment needs

Sahradaya College Of Engineering & Technology, Kodakara 19

Design Of Wastewater Treatment Plant

4.6.3 KINETIC RELATIONSHIPS

Kinetic relationships are used to determine biomass growth and substrate utilization and to design process performance.

4.6.3.1 Selection of Solid Retention Time And Loading Criteria

Certain design and operating parameters distinguish one activated sludge process from another. The common parameters used are the solids retention time (SRT), food to biomass ratio F/M also known as food to microorganism ratio and the volumetric organic loading rate. While the SRT is the basic design and operating parameter, the F/M ratio and volumetric loading rate provides values that are useful for comparison to historical data and typical observed operating conditions.

4.6.3.2 Solids Retention Time

The SRT in effect represents the average period of time during which the sludge has remained in the system. It is one of the most critical parameters for activated sludge design as SRT affects the treatment process performance, aeration tank volume, sludge production and oxygen requirements. For BOD removal, SRT values may range from 3-5 days, depending on mixed liquor temperature. At 18-25®C an SRT value close to 3 days is desired where only BOD removal is required to discourage nitrification of the associated O2 demand.

4.6.3.3 Food to Microorganism Ratio

A process parameter commonly used to characterize process designs and operating conditions is the food to microorganism (biomass) ratio (F/M). Typical values for BOD.F/M ratio reported in the literature vary from 0.04g substrate/g biomass.d to 1g substrate/g biomass.d for high rate process. BOD.F/M ratio is usually evaluated for systems that where designed based on SRT.

4.6.3.4 Volumetric Organic Loading Rate

Volumetric organic loading rate is defined as the amount of BOD or COD applied to the aeration tank volume per day. It is expressed in kg BOD or COD /m3.d. Higher volumetric organic loading rates generally result in higher required oxygen transfer rates per unit volume for the aeration system.

4.6.4 Sludge Production

In the design of the sludge handling facility depends on prediction of sludge production by the activated sludge process. If the sludge handling facilities are under size, the treatment process performance maybe compromised. That is sludge will accumulate in an under size sludge handling facility and eventually the sludge capacity of the activated sludge system will be exceeded and the excess solids will exit to the secondary clarifier, potentially violating discharge limits. The quantity of sludge produced can be estimated using the

equation given below.

Print Email Download

Share This Essay

Did you find this essay useful? Share this essay with your friends and you could win £20 worth of Amazon vouchers. One winner chosen at random each month.

Request Removal

If you are the original writer of this essay and no longer wish to have the essay published on the UK Essays website then please click on the link below to request removal:

Request the removal of this essay.


More from UK Essays

Paid Writing Services

Free Content

About UK Essays

Order Now

Instant Price