Important Contaminants In Waste Water Biology Essay

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Water is a naturally occurring substance and takes up about 70% of the earth's surface. It is used for various activities including consumption as potable water and in food, washing, manufacturing processes and so on. When water has been used up, it becomes wastewater and is discharged into natural water sources such as lakes, rivers, and so on through sewerage systems. To ensure wastewater discharged does not constitute pollution of inland waters it is treated before discharge. Thus, the primary aim of wastewater treatment is to ensure human and industrial wastewater is disposed of such that it does not constitute health hazards to humans or unacceptable damage to the natural environment (Sonune & Ghate, 2004). Wastewater can thus be defined as liquid waste made up of a mixture of both inorganic and organic substances including man-made or synthetic substances (Gray, 2005), resulting from household, commercial and industrial activities in addition to groundwater, surface water and storm water (Sonune & Ghate, 2004). In addition to inorganic and organic substances, wastewater also contains high levels of oxygen utilizing wastes, pathogenic organisms, nutrients and sediments, and in some instances toxic compounds (Sonune & Ghate, 2004) Table 1. According to Gray, wastewater can generally be divided into domestic wastewater also known as sewage, industrial wastewater and municipal wastewater (Gray, 2005), see Table 2 and Figure 1.

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Table : Important contaminants in wastewater

Source: (UN-ESCWA, 2003)

CONTAMINANTS

REASONS FOR IMPORTANCE

Suspended solids (SS)

SS can lead to development of sludge deposits and anaerobic conditions when untreated wastewater

is discharged to the aquatic environment.

Biodegradable organics

These are primarily made up of proteins, carbohydrates and fats. They are commonly measured in terms of BOD and COD. If discharged into inland rivers, streams or lakes, their biological stabilization can deplete

natural oxygen resources and cause septic conditions that are detrimental to aquatic species

Pathogenic organisms

Pathogenic organisms found in waste-water can cause infectious diseases.

Priority pollutants

Pollutants including organic and inorganic compounds, may be highly toxic, carcinogenic, mutagenic or teratogenic.

Refractory organics

Refractory organics that tend to resist conventional waste-water treatment include surfactants, phenols and

agricultural pesticides.

Heavy metals

These are usually added by commercial and industrial activities must be removed for reuse of the waste-water

Dissolved inorganic constituents

Dissolved inorganic constituents such as calcium, sodium and sulfate are often initially added to domestic water

supplies, and may have to be removed for waste-water reuse

Table 2: Types of wastewater, sources and composition

Source: (Aizenchtadt, Ingman, & Friedler, 2008)

TYPES OF WASTES

DESCRIPTION

SOURCES

COMPOSITION

DOMESTIC WASTEWATER

Blackwater

Greywater

this is wastewater discharged from households, commercial and institutional facilities

Toilets

Kitchens, laundry, washing/bathing

Urine, faeces, toilet paper

MUNICIPAL WASTEWATER

Domestic wastewater

Commercial wastewater

Sanitary wastewater

Commercial activity

Inflow and Infiltration

Storm flows/Street washing

Groundwater infiltration

this is derived primarily from domestic activities but includes wastewater discharged from commercial and industrial facilities, infiltration and inflow into sewers and storm runoffs

Blackwater, greywater

Restaurants, workshops, etc

Combined systems, separate systems - cross-connections or illegal connections in fractured pipes and manholes

Sand, grit, hydrocarbons, metals, etc.; animal wastes

INDUSTRIAL WASTEWATER

this is derived from various manufacturing and industrial processes

Manufacturing processes, electrolysis, heavy metals, equipment cleaning, cooling systems

Figure : Sources of wastewater

Source: (Metcalf and Eddy, inc., 2003)

Wastewater treatment plants are made up of different treatment processes or units as shown on Table 3 and are designed to treat wastewaters of known composition and flow rate and ultimately generate effluent of certain quality (Gray, 2005). In the UK the design of wastewater treatment plants adheres to the Royal Commission Standard for effluent of 20:30, that is, the effluent has a biochemical oxygen demand (BOD) of 20mg/l and a suspended solid concentration of 3omg/l (Gray, 2005), see Table 4.

Table 3: Some unit processes in wastewater treatment

Source: (Gray, 2005; UN-ESCWA, 2003)

PROCESS UNITS IN WASTEWATER TREATMENT

Physical Unit process

Balancing

Screening

Sedimentation

Flotation

Hydrocyclone

Filtration

Centrifugation

Reverse osmosis

Ultrafiltration

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Microfiltration

Chemical Unit process

Neutralization

Precipitation

Ion exchange

Oxidation reduction

Adsorption

Table 4: Typical composition of raw sewage in the US and UK

Source: (Gray, 2005)

PARAMETER

US (Mg/)

UK (Mg/l)

PH

7

7

Biochemical oxygen demand (BOD)

250

350

Chemical oxygen demand (COD)

500

700

Suspended solids

250

400

Ammonia Nitrogen

30

40

Nitrate Nitrogen

< 1

< 1

Total Phosphorus

10

15

The treatment method utilized for a particular wastewater flow is dependent on the quantity, composition and temperature of the wastewater (Rulkens, 2008). Municipal wastewater flow fluctuates with disparity in usage and is affected by factors such as climate, community size, standards of living, dependability and water supply, cost of water and supply, conservation requirements, level of industrialization, meter services and so on (Liu & Liptak, 2000); it is thus expected to have broad variations in flow rates (UN-ESCWA, 2003). Unit flow rate data can be utilized in estimating wastewater flow rates from households, commercial and institutional facilities (Liu & Liptak, 2000). Industrial wastewater flow rates vary and are a reflection of the type and size of industry, reliable estimation of industrial wastewater flow rates can be obtained from specific information on each industry (Liu & Liptak, 2000). Wastewater flow rates depict daily, weekly and seasonal patterns comparable to the patterns water usage exhibits (Liu & Liptak, 2000). Table 5 shows information required by engineers when designing a wastewater treatment plant.

Table 5: Information required in designing wastewater treatment plant

Source: (Liu & Liptak, 2000)

TYPES OF FLOW

DESCRIPTION

Average Daily Flow

This is the average flow rate occurring over a 24 hour period based on total annual flow rate data. It is used by engineers to evaluate the capacity of a wastewater treatment plant and in developing flow rate ratios.

Maximum Daily Flow

This is the maximum flow rate that occurs over a 24 hour period based on annual operating data. The data is relevant when designing facilities involving retention time such as equalization basins and chlorine-contact tanks.

Peak Hourly Flow

This is the peak sustained hourly flow rate that occurs during a 24 hour period based on annual operating data. The data obtained is required for designing collection and interception sewers, wastewater pumping stations, grit chambers, sedimentation tanks, chlorine-contact tanks and so on.

Minimum Daily Flow

This is the minimum flow rate occurring over a 24 hour period based on annual operating data. It is important when sizing conduits where solids deposition might occur at low flow rates.

Minimum Hourly Flow

This is the minimum sustained hourly flow rate that occurs over a 24 hour period based on annual operating data. The data is required in determining the possible process effects and size wastewater flow meters, particularly those that pace chemical-feed systems.

Sustained Flow

The sustained or exceeded flow rate value for a particular number of consecutive days based on annual operating data. Data obtained is used in sizing equalization basins and other hydraulic components.

A good wastewater treatment process is one that is capable of efficient elimination or alteration of various kinds of pollutants in a manner that is cost effective, sustainable in terms of chemical and energy use, release of pollutants and the generation of energy and compounds of value including treated water for recycle (Rulkens, 2008). Wastewater treatment methods are a combination of settlement processes and either biological or physicochemical processes. Treatment involves separating solids that can settle, suspend and solubilize from wastewater to form large particles that can be removed via settlement; these large particles form sludge which is treated separately and then disposed (Gray, 2005). Conventional wastewater treatment is a combination of physical, chemical and biological processes and methods of removing solids, organic matter, and in some cases nutrients from wastewater (Sonune & Ghate, 2004). Treatment methods are generally classified into four categories in order of increasing level of treatment: preliminary treatment or pretreatment, primary treatment, secondary treatment and tertiary or advanced treatment (Idelovitch & Ringskog, 1997), Table 6.

Table 6: Methods of treating wastewater

Source: (Gray, 2005)

TREATMENT METHODS

DESCRIPTION

Preliminary treatment/ Pretreatment

Removal and disintegration of gross solids, removal of grit and separation of storm water. Large amounts of oil and grease are also removed here

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Primary treatment

This follows pretreatment and involves removal of substances that can settle and these are removed as sludge

Secondary treatment

Microorganisms are introduced at this stage to oxidize dissolved and colloidal materials

Tertiary/Advance treatment

here the biologically water is retreated to a quality beyond what is obtained with secondary treatment by removing left over BOD, suspended solids, bacteria, toxic substances or nutrients. Chemical methods are employed in this instance

Sludge treatment

Dewatering, stabilization and final disposal of sludge takes place

Figure : A wastewater plant showing various treatment levels

Source: (UN-ESCWA, 2003)

METHODS FOR TREATING WASTEWATER

2.1 PRETREATMENT

The objective of pretreating wastewater is to remove or reduce the size of solids and other substances which may disrupt treatment operations or damage plant equipment. Solids such as wood, glass, clothing, plastics, grit and others such as faeces, oil and grease and even odours are removed during this stage. It involves physical unit processes such as screening, see Tables 3 and 7.

Table 7: Types of screens used for pretreating wastewater

Source: (UN-ESCWA, 2003)

2.2 PRIMARY TREATMENT

This stage of treatment involves the incomplete removal of suspended solids and organic matter using sedimentation and flotation physical processes (Sonune & Ghate, 2004). The objective of this treatment stage is to produce an effluent, primary effluent, appropriate for downstream biological treatment and to separate solids in the form of sludge that can be easily and cost-effectively treated before disposal (UN-ESCWA, 2003). It achieves the removal of about 25-50% BOD, 50-70% of total suspended solids and 65% of oil and grease; it additionally removes some organic nitrogen and phosphorus, heavy metals associated with solids but not colloidal or soluble components (Sonune & Ghate, 2004). See Figures 3 and 4 and Table 8. At present, effluent and water quality standards demand the removal of organic substances from wastewater at greater levels than can be achieved using just primary treatment (Sonune & Ghate, 2004). Secondary treatment processes therefore have to be employed.

Figure : Settling basin with horizontal flow used in sedimentation pretreatment

Source: (Metcalf and Eddy, Inc., 1991)

Figure : An example of a typical flotation tank

Source: (Liu & Liptak, 2000)

Table 8: Quality of raw wastewater and primary effluent at specific treatment plants in California

Source: (Sonune & Ghate, 2004)

2.3 SECONDARY TREATMENT

The purpose of secondary treatment is to further treat primary effluent by removing dissolved and colloidal organics and suspended solids that were missed during primary treatment (Sonune & Ghate, 2004). It is a biological treatment process as it involves the use of microorganisms, Table 3, the various aerobic biological methods employed vary primarily in the way oxygen is supplied to microorganisms and the rate at which organic matter present in the wastewater is broken down or metabolized (Sonune & Ghate, 2004). Figure 5 depicts a trickling filter, the most frequently employed attached-growth biological treatment process for the removal of organic substances from wastewater (UN-ESCWA, 2003). The performance of secondary treatment plant is nearly always measured based on the removal of BOD and SS. If well-constructed, it may remove from 85-95% of BOD and SS from wastewater, nevertheless the BOD test does not take into account all organic substances present on wastewater (Sonune & Ghate, 2004).on average a secondary treatment may contain BOD and COD of 20mg/l and 60-100mg/l respective but is estimated to remove about 65% of COD in wastewater. Hence, the need for a treatment process that can produce high quality effluent when necessary by removing remaining organic substances (Sonune & Ghate, 2004).

Figure : A view of a trickling filter used in biological wastewater treatment

Source: (Metcalf and Eddy, Inc., 1991)

2.4 ADVANCED TREATMENT

Although both primary and secondary wastewater treatments remove most of the BOD and suspended solids in wastewater, it is becoming increasingly apparent that higher levels of treatment are required as the two mentioned are incapable of protecting receiving waters or providing reusable water for industrial and or domestic purposes (Sonune & Ghate, 2004). Advanced treatment is thus an addition to conventional treatment plants for removing nitrogen, phosphorus, biodegradable organics, nutrients, bacteria and viruses, and toxic substances (UN-ESCWA, 2003). According to Sonune & Ghate, "advanced wastewater treatment can be defined as any process designed to produce an effluent of higher quality than usually achieved by conventional secondary treatment process or containing unit operations not normally found in secondary treatment", see Table 9. The definition therefore includes nearly all units not common in wastewater treatment today (Sonune & Ghate, 2004). Unlike conventional secondary treatment, which to some extent removes BOD and suspended solids, advanced treatment produces effluents that can be reused for industrial process or cooling water and may be recycled to increase the accessibility of domestic water supply (Sonune & Ghate, 2004).

Table 9: Types of advanced treatment

Source: (Sonune & Ghate, 2004)

TREATMENT TYPE

DEFINITION

Tertiary Treatment

Any treatment process in which unit operations are added to the treatment process after the conventional secondary treatment stage. The added unit operation can range from a simple filtration to the addition of several processes for treating suspended solids, nitrogen, phosphorus and so on.

Physicochemical Treatment

A treatment process in which biological and physical-chemical processes are intermingled to obtain effluent of specified quality.

Combined biological-physical and chemical Treatment

This process differs from tertiary treatment in that in tertiary treatment unit processes are added after conventional biological treatment while here biological and physicochemical processes are combined.

2.4.1 MEMBRANE TECHNIQUES

In the past membrane treatment processes were not often utilized in wastewater treatment due to high cost and energy demand of membranes, and lack of experience. However, the manufacture of cheaper polymer-membrane materials having high selectivity and efficient chemical, thermal and mechanical resistance has improved its utilization today (Seo & Vogelpohl, 2009). The method is utilized in purifying and / or concentrating varying types of fluids including water, wastewater, pharmaceutical and chemical products (Sonune & Ghate, 2004). The process is driven by pressure and depends on the membrane's pore size to achieve separation of components based on their pore sizes (Sonune & Ghate, 2004). When compared to conventional physical-chemical treatment methods, its advantages include: reduction in installation space, cost, the use of toxic chemicals (Sonune & Ghate, 2004), sludge reduction and its simplicity (Juang et al., 2007). However, according to Juang et al., membrane processes are limited by the issue of particle blocking, polariztion, membrane surface fouling after continuous use.

2.4.2 DESALINATION

The process of desalination is used to remove dissolved minerals from brackish water, seawater, or treated wastewater (Sonune & Ghate, 2004). It is increasingly being used to obtain potable water from brackish water and seawater, improve the freshwater quality for drinking and industrial activities and in treating industrial and municipal wastewater before discharge or recycle (Sonune & Ghate, 2004; Madwar & Tarazi, 2002). The combination of reverse osmosis with conventional or modern pretreatment technologies are widely used in desalination of wastewater (Madwar & Tarazi, 2002). See Tables 10 and 11 for examples of industrial application of desalination and types of desalination methods.

Table 10: Industries that apply desalination methods

Source: (Madwar & Tarazi, 2002)

INDUSTRIES THAT APPLY DESALINATION METHODS:

Power generation industry

Glass manufacturing industry

Electronics industry

Textile industry

Cooling systems

Construction industry

Metal manufacturing industry

pulp and paper industry

Table 11: Types of desalination methods

Source: (Sonune & Ghate, 2004)

DESALINATION METHODS

DESCRIPTION

Reverse osmosis

Wastewater is passed through a semi permeable membrane under pressure, thereby separating out the salts, Figure 6. The water is pretreated before being pumped through the membrane to exclude particles that may obstruct the membrane pores. The water quality obtained is dependent on pressure, salt concentration of feed water and permeation constant of membranes used.

Electrodialysis (ED)

ED uses ion-permeable membranes and electricity to effect separation of salts from wastewater. The membranes are placed in a stack and those that permit cations are alternated with those that permit only anions through. Electric current supplied serves to drag ions through the membrane concentrating them between each membrane. Reversing the direction of electric current aids to prevent fouling of membranes

Ion exchange (IX)

IX resins are used to replace unwanted ions in water (wastewater) with desired ions, as the water passes through the resin. The frequency at which resins are replaces is dependent on the concentration of dissolved solids in the wastewater. It is used in municipal wastewater treatment to remove calcium and magnesium ions and in producing pure water for industrial processes.

Freeze desalination

This method is based on the fact that as seawater freeze, ice crystals are formed from pure water leaving behind dissolved salt and other minerals. It uses less energy in concentrating a greater range of wastewater compared to distillation. Conventional freezing process involves the following steps:

Precooking feed water

Crystallization of ice into slush

Separation of ice from brine

Washing the ice

Melting the ice

Modern methods are looking into reducing the number of steps particularly the washing step.

Figure : A reverse osmosis system

Source: (Sonune & Ghate, 2004)

BIOLOGICAL AND CHEMICAL TREATMENT METHODS

3.1 PHYSICO-CHEMICAL METHODS

Chemical methods of treating wastewater involve the use of chemical reactions to cause changes in wastewater, Table 12. They often lead to a net increase in dissolved substances in wastewater which is significant if the wastewater is to be recycled. Chemical methods are usually used with physical unit operations and biological methods (UN-ESCWA, 2003). In past years the primary reason for treating wastewater was to produce an effluent that met regulatory standards for the discharge of effluent into inland waters; which could be achieve with conventional biological processes (Rulkens, 2008). However, the primary purpose for treating wastewater today is largely to produce recyclable water and recover important compounds and energy from wastewater; advanced physic-chemical treatment methods are now utilized in achieving a treatment process that is environmentally sustainable (Rulkens, 2008). Some advantages of chemical treatment methods include (Rulkens, 2008; Leentvaar et al., 1978):

Improving the capability of biological treatment processes.

Recovering valuable substances and energy from wastewater.

Attaining high quality necessary for the use of effluents obtained from treating wastewater.

Removing salt (desalinate) from brackish/ salty water and seawater.

Treating concentrated liquid and sludge by products of wastewater treatment processes.

Reducing installation space.

Better meeting of peak loading.

Insensitivity to toxic compounds in wastewater.

Immediate start-up and shut-down.

Table 1: Chemical treatment processes for treating wastewater

Source: (Amudaa & Amoo, 2007; MacCrehan, Bedner, & Helz, 2005; Macauley, Qiang, Adams, Surampalli, & Mormile, 2006; Lema, Omil, & Suarez, 2009; UN-ESCWA, 2003)

METHODS

DESCRIPTION

Chemical Precipitation

Coagulation/Flocculation

Compounds such as ferric chloride are added to wastewater to enable suspended colloidal particles aggregate into larger readily settleable flocs (Amudaa & Amoo, 2007). it may be used for pretreating industrial wastewater prior to discharge into municipal sewerage, primary treatment of urban wastewater and so on (Lema et al., 2009) , see Figure 7.

Adsorption with activated carbon

Adsorption involves the adhesion of a gas, liquid or soluble substance to a surface, in this instance activated carbon. This method usually follows biological treatment and is used to remove left over soluble organic substances and other small particles (UN-ESCWA, 2003), see Figure 8.

Disinfection

Disinfection of wastewater involves the destruction or removal of pathogenic microorganisms present in wastewater, to reduce the risk of human exposure (UN-ESCWA, 2003; Macauley et al., 2006), Table 12. Methods used include:

Chemical substances such as chlorine, ozone and so on

Radiation

Heat, light

Filtration, sedimentation and so on

Dechlorination

Chlorine used in disinfecting wastewater is toxic to aquatic organisms, thus when used in disinfection it is removed by the process of dechlorination before effluent discharge into surface waters. Reducing agents such as sulfite (SO4 or HSO-3) is added in excess to enable dechlorination (MacCrehan et al., 2005).

Figure : A once-through chemical treatment system

Source: (Liu & Liptak, 2000)

Figure : A Granular activated carbon contactor

Source: (Metcalf and Eddy, Inc., 1991)

Table12: Characteristics of common disinfecting agents

Source: (Metcalf and Eddy, Inc., 1991)

3.2 BIOLOGICAL METHODS

Biological treatment of wastewater involves the use of microorganisms to breakdown organic wastes in wastewater (Aytimur & Atalay, 2004); it is usually used in combination with physical and chemical processes (Rulkens, 2008). Biological processes are categorized into the following: aerobic processes, anoxic processes, anaerobic processes, combined processes, pond processes (UN-ESCWA, 2003), Table 13 gives a summary of some of the commonly used methods. The following are advantages of biological treatment processes (Rulkens, 2008; El-Bestawy et al., 2005; Chan et al., 2009):

Lower cost of treatment and absence of secondary pollution.

Ability to easily remove phosphorus and nitrogen, which aid algae growth

Ability to treat nearly all wastewater having BOD/COD ratio of 0.5 or more.

Ability to easily convert compounds containing sulfur into H2S, SO2-4 or sulfur.

Ability to convert ammonia and nitrogen containing compounds into nitrogen gas.

Table 13: some biological processes used in wastewater treatment

Source: (Aytimur & Atalay, 2004; Liu & Liptak, 2000; Metcalf and Eddy, Inc., 1991; Qasim, 1999; UN-ESCWA, 2003)

METHODS

DESCRIPTION

Activated-sludge

Is an aerobic process conducted in a continuous-flow aeration tank containing activated microorganisms, activated-sludge, able to breakdown organic matter present in wastewater in the presence of oxygen. Gram negative bacteria including Nitrogen and carbon oxidizers, floc and non-floc formers and so on are mainly used for this process. A drawback to this process is sludge bulking due to the absence of phosphorus, nitrogen, trace elements, and fluctuations in pH, temperature and dissolved oxygen, see Figure 9.

Trickling filter

This consists of a bed of highly permeable media, made of rocks and plastic packing material, onto which microorganisms are attached and through which wastewater is percolated. Organic substances within wastewater are degraded by adsorption onto the slime layer formed by the microorganisms, see Figure 5 and 10.

Activated Lagoon

An activated lagoon is a basin within which wastewater is treated either based on flow-through or solids recycling. Its biology similar to activated-sludge process but differs in that its large surface area may cause more temperature effects than is seen with traditional activated-sludge processes, see Figure 11.

Stabilization pond

This is a shallow basin used to treat wastewater without solids return due to complete mixing. It is normally classified as aerobic, anaerobic or aerobic-anaerobic depending on the type of biological activity occurring in it, see Figure 12.

Nitrification-Denitrification

In nitrification, Nitrosomonas bacteria and nitrobacter are used to oxidize ammonia and nitrogen into nitrite and then nitrate. It can be achieved by suspended-growth or attached-growth processes such as trickling filters and rotating biological contactors, see Figure 13.

Denitrification removes nitrogen as nitrate by converting to nitrogen gas under anoxic condition. It can be achieved using suspended-growth or attached-growth processes such as plug-flow activated-sludge and a column reactor containing media on which bacteria can grow.

Phosphorus removal

The process of removing phosphorus from wastewater is based on exposing microorganisms to alternating anaerobic and aerobic conditions. This leads to stress and makes the microorganism increase their uptake of phosphorus. A/O, PhoStrip and sequence batch reactor processes are used to achieve phosphorus removal, see Figures 14 and 15.

Figure : A flow diagram showing an activated sludge process

Source: (UN-ESCWA, 2003)

Figure : A flow diagram for trickling filters

Source: (UN-ESCWA, 2003)

Figure : A flow diagram for aerated lagoons

Source: (UN-ESCWA, 2003)

Figure : A flow diagram for stabilization ponds

Source: (Liu & Liptak, 2000)

Figure : The configuration of a rotating biological contactor

Source: (Qasim, 1999)

Figure : A/O process for removing phosphorus in wastewater

Source: (Metcalf and Eddy, Inc., 1991)

Figure : PhoStrip process for phosphorus removal in wastewater

Source: (Metcalf and Eddy, Inc., 1991)