30 Sewage Microbiology and Treatment

Dr Tejpal Dhewa

Learning objectives:

• To understand the status of sewage in India
• To understand about the types and characteristics of sewage
• To gain knowledge about general sewage treatment processes
• The understand the role of microbial communities in the treatment of sewage

TABLE OF CONTENTS

1. Introduction

2.Sewage: Types and characteristics 2.1 Classification of sewage

2.1.1 Origin

2.2.2 Harmfulness

2.2.3 Contamination stability

2.2.3 Human made

2.2 Characteristics of sewage

2.1 Physical and chemical

2.2. Microbiological

2.2.1 Bacteria

2.2.2 Fungi

2.2.3 Protozoa

2.2.4 Viruses

2.2.5 Algae and cyanobacteria

2.2.6 Metazoa

2.3 Biochemical Oxygen Demand

3.Sewage Treatment: Process overview

3.1 Single Dwelling Unit Treatment process

3.1.1 Outdoor Toilets

3.1.2 Septic Tanks

3.1.3 Imhoff- Tank

3.2 Municipal Treatment Process

3.2.1 Primary treatment

3.2.1.1 Screening

3.2.1.2 Grit chambers

3.2.1.3 Sedimentation

3.2.2 Secondary treatment

3.2.2.1 Trickling filters

3.2.2.2 Activated sludge

3.2.2.3 Oxidation lagoons

3.2.2.4 Sludge digestion

3.2.3 Tertiary treatment

3.2.3.1 Chemical flocculation

3.2.3.2 Final filtration

3.2.3.3 Removal of nutrients

3.2.3.4 Disinfection

3.3 Odor control

3.4 Sludge processing and disposal

3.4.1 Sludge digestion

3.4.2 Composting

3.4.3 Incineration

3.4.4 Sludge disposal

4.Summary

1.Introduction

India is the second most populous country in the world where 17.74% of world’s total population lives (source: http://www.worldometers.info/world-population/india-population/; 2018) and it has only 4% of the world’s freshwater resources that are decreasing in terms of quality and capacity (Source:http://www.livemint.com/Opinion/OqNlJtLZ1K1V05DCkfZKfK/Managing-Indias-freshwater.html). Groundwater sources are being rapidly exhausted and surface water resources are mostly contaminated with sewage that is generated in both urban and rural areas.

In general, sewage or wastewater is polluted water that contains harmful liquid, solid or gaseous compounds introduced into natural water sources or soil which may lead to a contamination of surface or underground waters. Recently, a report on water and wastewater management of India shows that about 62,000 MLD (million litre per day) sewage is produced in urban areas, but only about 23,277 MLD is processed in existing sewage treatment systems (Source: http://www.indiaspend.com/cover-story/70-of-urban-indias-sewage-is-untreated-54844;2016). Hence, only 30% of sewage is processed and remaining 70% sewage is not processed due to lack of adequate sewage treatment facilities. According to Central Public Health & Environmental Engineering Organisation (CPHEEO), Ministry of Urban Development, Government of India, assessments that about 70-80% of total water supplied for domestic purpose gets generated as sewage. It has been estimated that urban centres may cross 120,000 MLD by 2051 and rural population will also generate not less than 50,000 MLD. Central Pollution Control Board (CPCB), New Delhi describe that there are 269 sewage treatment plants

(STPs) in exist in India, but only 231 are operational (Source: https://economictimes.indiatimes.com/treating-wastewater-with-the-help-of-modern-technology/toshibashow_dp/55050324.cms ; 2016). Hence, the existing treatment capacity is just 21% of the present sewage production (Source: http://cpcb.nic.in/status-of-stps/ 2018). Since, there is a large gap between the generation of sewage and its treatment resulted to large scale water contamination. Therefore, sewage treatment is becoming more essential due to shrinking water resources and increasing health and environmental concerns. The modern sewage treatment technologies are designed to provide low cost solution with benefits of environment from the reuse of water.

2.Sewage: Types and Characteristics

Sewage is a mixture of domestic and industrial wastes plus drainage water from rainfall. It contains human and animal excreta, households washing waters, hospital and scientific research laboratory wastes, petroleum wastes, industrial wastes agricultural wastes and microorganisms. Chemically, sewage contains organic contents including carbohydrates, fats and proteins, inorganic compounds, heavy metal residues etc. The modern technology and life style has changed the sewage characteristics of wastewater. For example, the use of synthetic detergents in place of soaps can adversely affect the indigenous microbial population required for effective sewage treatment. Therefore, the exact types and characterization of sewage is an important aspect in order to evaluate its pollution potential, designing of appropriate sewage processing plant, monitor the effectiveness of treatment plant and stop the pollution of the receiving water body. It also helps to establish a cost-effective waste management system.

2.1 Classification of sewage

Sewage can be categorized into four groups on the basis of origin, harmfulness, contamination stability and human activity:

2.1.1 Origin

Sewage can be grouped into followings types on the basis of their sources of origin:

• Domestic: sewage originates from public toilets, households, plant and animal wastes.
• Industrial: wastewater generates during all types of industrial processes.
• Precipitation: It is a rain and melt waters that contains several atmospheric contaminations (i.e. dust, gaseous substances, microbiological (i.e. bacteria, viruses, fungal and protozoa), surface and streets run-offs.

2.2.2 Harmfulness

Sewage may be classified into following groups on the basis of adverse health effect:

• Directly harmful: substances responsible for direct impact on health.
• Indirectly harmful: lead to a reduction of oxygen in water below the critical organism’s requirement. 2.2.3 Contamination stability

Sewage can be grouped into following groups on the basis of stability of contaminates present in waste water:

• Degradable: organic substances that undergo chemical transformation to form simple substances.
• Non-degradable: substances that are not decomposed by microorganisms and do not yield to any chemical transformation.

2.2.3 Human made

Sewage can be grouped into following categories on the basis of human activities:

• Urban and domestic –source: Houses, food serving facilities and hospitals.
• Rural- source: excess chemical fertilizers from fields, animal farms.
• Industrial- source: various kind of manufacturing and processing industries are the main source of Several toxic compounds released in waste water.
• Radioactive- source: radioactive compounds may be contributed in the sewage by health diagnostic facilities, scientific experiments or research and nuclear reactors. Radioactive substances are hazardous and required special kind of storage methods and disposal protocols.

2.2 Characteristics of sewage

The characteristics and composition of sewage depends upon their source of origin, per capita consumption of water by the population, seasons, places etc. It is significant to understand such information when constructing treatment plants to process sewage. The most important characteristics of sewage can be studied under the following subheadings (Table 1):

2.1 Physical and chemical

Sewage consists of approximately 99% water and 1% inorganic and organic materials in both soluble (i.e. sugars, amino acids, fatty acids, alcohols and inorganic ions) and suspended form (i.e. cellulose, lignocellulose, proteins, fats and several particulate inorganic materials). The amount of suspended solids in sewage is so small that is why it is expressed in parts per million (ppm). Depending on the amount of total solids the sewage can be grouped:

• High strength (> 500 ppm)
• Medium strength (200-500 ppm)
• Weak strength (< 200 ppm)

Though, the physical and chemical composition of sewage largely depends on their sources from they produced. Domestic wastewater or sewage consists of human wastes such as faces, urine, gray water from washing, cleaning and meal preparation. The sewage of towns in our country contains (on an average) 355 ppm biodegradable organic matter, 53 ppm nitrogen (N2), 45 ppm potassium (K) and 17 ppm Phosphorus (P), salts of numerous heavy metals including lead (Pb), zinc (Zn), Chromium (Cr), Nickel (Ni) etc. are also present above permitted levels in wastewater. The odor in sewage is due to presence of H2S, amines, ammonia, organic sulphides, mercaptans etc. In general, the color of fresh sewage is gray and later on the decomposition of organic matters it becomes black.

2.2. Microbiological

A number of microorganisms and parasites are usually present in sewage water. Normally, sewage prior to entering the treatment plant will have 105 to 106 microbes per milliliter. Microorganism comes from two general sources: the soil and sanitary wastes in sewage. These sources contain large microbial population as a living part of organic content and their primary function is decomposition of organic materials under aerobic and anaerobic condition. Most commonly bacteria, protozoa and viruses exist and grow in sewage water and are transferred by the vats with the movement of sewage water during the treatment process. However, the number and type of these microorganisms may vary on the basis of sewage origin. Notably, some microorganisms are used during the secondary treatment of sewage to remove various pollutants and few microorganisms continue on to the tertiary treatment to utilize removal of other pollutants such as phosphorus and nitrogen. During the process of disinfection, most of the microorganisms are removed from water through chlorination and ozonation.

2.2.1 Bacteria

Sewage treatment vats contains both beneficial and harmful bacteria due to their arrival with sewage water.

• Beneficial bacteria: Certain biofilm forming bacteria including Pseudomonas, Chromobacter, Flavobacterium and Zooglea are present in secondary treatment process (trickling filter phase), considered to be beneficial in the removal of organic content from sewage. In addition, nitrogen removing bacteria like Nitrosomonas and Nitrobacter (nitrifiers), Thiobacillus (denitrifier), Desulfotomaculum and Desulfovibrio (sulfate reducer) are also found in sewage.
• Harmful bacteria: bacteria are responsible for numerous human animal and plant diseases by infecting their host via ingestion of contaminated water or food, fecal-oral route etc. Human pathogens cause waterborne infections. For example, some pathogenic bacteria like Escherichia coli serotype O157:H7 (causes food poisoning and diarrhea), Salmonella typhi (typhoid) and Vibrio cholera (cholera) can be found in sewage due to fecal contamination. E.coli is used as a fecal indicator. During disinfection most of these microorganisms are destroyed and removed from sewage.

2.2.2 Fungi

Fungi are single or multi-cellular eukaryotic organisms found in fixed film processes (i.e. Fusarium aquarductuum, Trichosporon cutaneum, Geotrichium candidumm, Ascoidea rubescenes and Subarromyces splendens) and take part in the removal of carbonaceous materials from sewage.

2.2.3 Protozoa

Protozoa such as amoebae, flagellates and ciliates unicellular eukaryotic organisms are found present throughout the complete sewage treatment process. They mainly feed on organic particulate matter. The most commonly ciliated protozoa such as Vorticella, Carchesium and Opercularia play a very important role in maintaining slime layer in trickling filter systems. In addition, protozoa play a predatory role in controlling bacterial population density.

In general, protozoa are useful in sewage treatment but can be harmful to human beings if ingested. For example, protozoan parasites including Giardia lamblia is a waterborne pathogen found in sewage and its cysts enters the water supply through fecal contamination and causes giardiasis.

2.2.4 Viruses

Sewage may become contaminated by more than 140 types of enteric viruses. Viral pathogen enters into the human body through fecal-oral route, multiplies in the gastrointestinal tract and is excreted in large numbers in the fecal matter of infected host. Following human enteric viruses are commonly found in sewage:

• Enteroviruses: Poliovirus (serotype 3): Paralysis, aseptic meningitis

Coxsackievirus A (serotype 23): Aseptic meningitis, Herpangia, paralysis

Coxsackievirus B (serotype 6): Aseptic meningitis, Pleurodynia, myocarditis

Echovirus (serotype 34): Respiratory infections, aseptic meningitis, diarrhea

Hepatitis A virus: Infectious hepatitis

• Reoviruses (serotype 3) : Respiratory infections
• Rotaviruses (serotype 4) : Gastroenteritis
• Adenoviruses (serotype 41) : Respiratory infections, gastroenteritis and acute conjunctivitis
• Norwalk agent ( serotype 1) : Gastroenteritis
• Astroviruses (serotype 5): Gastroenteritis

These viral pathogens are removed by the process of disinfection during sewage treatment operations. 2.2.5 Algae and cyanobacteria

Certain photosynthetic organisms such as algae and cyanobacteria are found at the surface of a biofilm, which is exposed to sunlight. The most common found genera include Chlorella, Oscillatoria, Euglena, Anacystis and Stigeoclonium. These organisms only play a little role in fixed film processes. Most of them decrease the efficiency of the trickling process due to clogging of the filters.

2.2.6 Metazoa

Metazoa are multicellular eukaryotic organisms that include the phyla of arthropods (tardigrades, crustacean and insects) and worms (rotifers, nematodes, oligochaetes and gastrotricha). Metazoa are often seen in trickling filter process, where these detrivorous and predators organisms feed on the biofilm. In general, such organisms only present in an aged biomass under aerobic and microaerophilic conditions.

2.3 Biochemical Oxygen Demand

Biochemical Oxygen Demand (BOD, also called Biological Oxygen Demand) is the amount of dissolved oxygen needed by microorganisms for the aerobic break down of organic content present in sewage or wastewater at a certain temperature (20oC) over a specific time period (5 days). This demand can vary depending on temperature, nutrient concentration and the availability of enzymes in indigenous microbial populations. The magnitude of BOD, it is an indication of the amount of organic material in the wastewater or sewage. For example, more oxidizable organic matter present means the higher the BOD. The strength of wastewater is stated in the terms of BOD level (high values means that a high level of organic content is present, whereas low value means that little oxidable content is present). In general, the BOD value is expressed in parts per million (ppm) of oxygen consumed by the sewage sample.

The BOD can be used as a measure of the effectiveness of wastewater treatment plants. Since, the aquatic life is adversely affected whenever appreciable amount of strong sewage are discharged into natural water bodies like ponds, lakes, streams etc. because the level of dissolved oxygen (usually 7-8 ppm to 3 ppm) declined quickly to meet out the oxygen demand of effluent. Hence, it becomes difficult for the survival of fish or other aquatic life at below 3 ppm dissolved oxygen. Therefore, BOD level in the treated effluent must be monitored at each step of sewage treatment from starting to the end of process.

3.Sewage Treatment: Process overview

Disposal of sewage without adequate treatment may have objectionable situations such as increased possibility for spreading of pathogenic microorganisms, contamination to natural bodies of water, where depletion of oxygen may detrimental to aquatic life and making unsafe the shellfish to human consumption, offensive odors and build-up of debris etc. Thus, to overcome these conditions treatment of sewage is essential.

The sewage of a city is collected by sewer systems that carry the sewage to location for treatment and disposal. Usually, there are three types of sewer systems (Fig 1):

1. Sanitary sewers: transport domestic and industrial sewage.
2. Storm sewers: carry surface and rain water.
3. Combined sewers: carry off both sanitary and storm sewers.

However, there are several types of sewage treatment processes practiced, but they may be grouped into those applicable to individual dwelling and those used by a community or municipality.

3.1 Single Dwelling Unit Treatment process

Single-unit structures are most suitable for small shopping centres, motels and individual flats or apartments.

3.1.1 Outdoor Toilets

Outdoor toilets are constructed outside the house when plumbing installations is not possible due to any reasons. While constructing these toilets care could be taken to see that it is not access to the flies or vectors, and their drainage and joining water supplies must be separated.

3.1.2 Septic Tanks

Septic tank is an anaerobic digestion tank usually employed for the treatment of sewage discharged from the communities or residential quarters where population is less and sufficient land is available. A septic tank is made of metal or concrete that achieves two goals (Fig 2):

• Sedimentation of solid materials
• Biological degradation of settled solid materials.

As sewage enters the tank, sedimentation of solid materials occurs in the bottom of the tank named sludge which undergo anaerobic decomposition through the action of anaerobic bacteria that degrade solid organic compounds to simpler and soluble compounds. The remaining clear water and gases (i.e. H2S) are allowed to go out through perforated pipes buried in the ground. The digested water percolates into the soil where the soluble organic compounds undergo biodegradation by the diverse microbial population. The sludge from septic tank must be frequently removed to prevent choking of the pipes and may be distributed under the soil surface through a disposal field.

The collection pipelines of septic tanks should be checked regularly to avoid any leakage near residential area as it may contaminate the drinking water pipelines if they damaged. The septic tank device should be Septic tank treatment cannot guarantee elimination of all pathogenic microorganisms.

3.1.3 Imhoff- Tank

Actually, Imhoff- tank is a restructured septic tank and that is normally used to treat larger community sewage. It was designed by German engineer Karl Imhoff (1876-1965) for the reception and processing of sewage with the two objectives:

1. Clarification of sewage by simple settling and sedimentation.
2. Biodegradation of solids materials of sewage

Imhoff tank is consists of two chambers (Fig 3): top chamber (settling compartment) and lower chamber (sludge digestion compartment. The top chamber gets sewage and weightier particles settle into the lower chamber and gradually undergo anaerobic degradation. During microbial action, the gas liberated (i.e. CH4) can be drawn out through a vent, collected and can be used as fuel. The sewage effluent (remaining sewage water) is either allows discharging in larger water body or is subjected to aerobic decomposition. The sludge is from time to time removed, exposed to air or may be used as manure in the crop field. Imhoff tank can remove 40 to 60 % suspended solids from sewage and a BOD reduction about 15 to 35%. It has no mechanical parts and comparatively simple and economical to operate.

3.2 Municipal Treatment Process

The municipal sewage treatment systems accomplished using several steps namely including primary or physical treatment, secondary or biological treatment and tertiary or chemical treatment (Fig 4).

3.2.1 Primary treatment

When the sewage receives at a sewage treatment plant, it is first subjected to physical or mechanical means to remove solid impurities floating solids, settling suspensions, oils and fats. In this step, the sewage is passed through a series of filters of graded openings and then allowed to flow through sedimentation unit. Such impurities are concentrated in and collected from sedimentation units called sludge. Later, sludge and remaining watery liquid are processed during secondary treatment. Thus, primary treatment considered to be the preparation of sewage for further treatment and purification.

3.2.1.1 Screening

Screening is the first step in primary se wage treatment, removes the largest solids like boxes, bottles, tires, cans, metals etc. using screen bars. Separated materials may be incinerated, ground up or used for landfill.

3.2.1.2 Grit chambers

Grit chambers are designed to remove smaller solids like pebbles, broken glass, sand, grit etc. from sewage. Such particles must be removed otherwise they may damage to pumps and other equipment or machines used during sewage treatment. Grit chambers are of 3 types:

1. Horizontal grit chamber
2. Aerated grit chamber
3. Vortex grit chamber

3.2.1.3 Sedimentation

It is also called primary settling. It removes smaller particulate material like paper and fecal matter by simple physical settling of matter due to its density, the force of gravity etc. (Fig 5). Certain chemical agents called coagulants and flocculants may be applied to speed up this process by encouraging aggregation of particles. These biosolids or sludge is often treated biologically through anaerobic degradation in a sludge digester.

3.2.2 Secondary treatment

Subsequent primary treatment, the sewage water subjected to the next phase called biological treatment or secondary treatment. In this step, dissolved and suspended biological materials are removed by indigenous microbial decomposition in a managed environment. Biological treatment is carried out in two important phases namely, aerobic phase and anaerobic phase. The aerobic phase includes the digestion of sludge under oxygen by using trickling filters, activated sludge and oxidation lagoons and the anaerobic phase is represented by sludge digestion in the absence of oxygen. At the end of secondary treatment process, microorganisms are required to be separated from the treated water before to discharge or tertiary treatment. During secondary, the BOD is reduced up to 90-95%.

3.2.2.1 Trickling filters

Trickling filters made up of generally 5-10 feet deep fixed bed of rocks, lava, slag, crushed stone, gravel, polyurethane foam or plastic media (Fig 6). The sewage effluent is sprayed over the surface of the bed by using an overhead sprayer which is rotating at a constant speed. The spraying saturates the effluent with O2. The fed surface becomes covered with aerobic microbial population comprising of bacterial species including Sphaerotilus natans, Beggiatoa, Flavobacterium, Achromobacter, Zooglea and Pseudomonas, microalgae, microfungi and protozoa. As the effluent percolates over the media, the aerobic microflora decomposes the organic materials to small soluble molecules. Later, the treated effluent collected at the bottom of the tank is passed through the sedimentation tank. It may be recycled over the bed again if needed. The aerobic digestion of sewage in a trickling filter is very slow process and will take longer time to decompose completely therefore, partially digested sludge may be subjected to tertiary treatment.

3.2.2.2 Activated sludge

Activated-sludge is a process for treatment of domestic and industrial sewage using air and biological flocs composed of bacteria and protozoa that substantially reduce organic materials (Fig 7). This process starts when air being introduced into a sewage that is held in a large aeration tank combined with following aerobic microbial decomposers to develop biological floc that decomposes organic matters into simple soluble molecules, amino acids, ammonia, phosphorus, nitrates, CO2, H2O etc.:

• Bacteria: Escherichia, Enterobacter, Achromobacter, Flavobcaterium, Pseudomonas, Zooglea, Microccocus, Sphaerotilus, Beggiatoa, Thiothrix etc.
• Protozoa: Amoaebe, Spriotrich and Vertcella. 
• Sedentary rotifers
• Filamentous fungi: Geotrichum, Cephalosporium, Penicillum and Cladodsporium.
•  Yeast: very low number

Now the wastewater is passed through a sedimentation tank and after an adequate disinfection it may be used for agricultural purposes. Though, approximately 90% of the organic content of the sewage is digested by this treatment process but the wastewater still contains considerable amount of phosphates and nitrates hence, it requires tertiary or advanced treatment prior to discharge into large body of water. A portion of the settled material (sludge containing bacterial floc) is returned to head of the activated-sludge aeration tank to re-seed the new sewage receiving the tank.

3.2.2.3 Oxidation lagoons

Oxidation lagoon sewage–treatment is suggested for small societies in rural areas where sufficient land is available. In general, oxidation lagoons are also called as oxidation ponds or stabilization ponds and are the oldest of the sewage treatment systems used in most of developing countries due to its low cost. These are usually 2-4 feet deep shallow ponds designed to treat sewage water through the interaction of sunlight, bacteria and algae (Fig 8). In this pond, sewage kept till organic matter is degraded by aerobic and anaerobic process and usually, one to four weeks’ time is required for the complete decomposition of the solids. However, the treatment depends on temperature, aeration time and quality of the sewage. Algae (i.e. Chlorella pyrenoidosa) grow using energy from sun and CO2 and inorganic compounds released by bacteria in water. Oxygen provided from air and also generated during algal photosynthesis satisfies the biochemical oxygen demand (BOD) of sewage and hence, continuing aerobic conditions in wastewater. In these conditions the aerobic microorganisms grow speedily and digest organic content of the sewage. At bottom of the pond, anaerobic bacteria grow at the expense of the products of heterotrophs to release CH4, H2S and N2 in the atmosphere. The sludge deposits in the pond must ultimately be removed. The combine action of sunlight, heat and settling of solids will reduce the number of pathogenic microorganisms in the sewage.

3.2.2.4 Sludge digestion

Sludge digestion decomposes solids accumulated during primary treatment and during secondary treatment in aerobic treatment processes are subjected to deep tanks, where anaerobic microorganisms decomposes organic materials to soluble substances and gaseous products. This process is described in detail under the section of sludge processing and disposal.

3.2.3 Tertiary treatment

Tertiary treatment is an advanced and final step of sewage treatment and is also called as chemical treatment. The main objective of tertiary treatment is to provide final treatment to further improve the effluent quality before it is released to ground, wet lands, sea, river etc. In this step, a high-quality wastewater can be produced that is suitable for several reuses. Following steps are included in tertiary treatment:

3.2.3.1 Chemical flocculation

Chemical flocculation removes most of the remaining particulate matters. Certain chemical agent’s alum and ferrous sulphate may be added in wastewater to speed up this process by encouraging aggregation of particles. These positively charged hydrous oxides, neutralizes the negatively charges compounds present in wastewater.

3.2.3.2 Final filtration

Final filtration removes dried solids that are later incinerated or used as landfill or fertilizer. Filtration may be achieved using:

• Sand filtration: removes residual suspended matter.
• Activated carbon: removes residual toxins.

3.2.3.3 Removal of nutrients

Sewage may have high levels of the nutrients nitrogen and phosphorus. The excessive discharge into the water bodies can lead to accumulation of nutrients, called eutrophication. This process encourages the overgrowth of weeds and algae including blue-green algae, which may cause an algal bloom formation over the surface of water bodies. Later, causing deoxygenation and induce the production of certain algal toxins that contaminate drinking water.

• During sewage treatment, following treatment processes are used to remove nitrogen and phosphorus. Nitrogen removal is accomplished by chemical or biological method. The biological methods for nitrogen removal are more cost effective and hence, used most widely. The activated sludge process is modified in such a way so that nitrogen is removed from wastewater through the biological oxidation of nitrogen from NH3 to NO3- (nitrification) assisted by Nitrosomonas spp., followed by the reduction of NO3- to N2 (denitrification) facilitated by a wide diversity of bacteria. Later, gas is released to the environment and hence removed from the water.
• Phosphorus can be removed either biological or chemical process. The biological process used known as enhanced biological phosphorus removal, where polyphosphate-accumulating organisms (specific bacteria), are selectively enriched and accumulate large quantities of phosphorus within their cells. Later, such bacteria are separated from the treated water and these bio solids may be used in the crop field due to its high fertilizer value. Phosphorus can also be removed by chemical precipitation using salts of iron like FeCl3, alum or lime. These chemicals are incorporated to the sewage water, which allows the smaller particles with phosphorus to group into bigger masses which then are separated through sedimentation tank. The chemical process of phosphorus removal is easier to operate and comparatively more reliable than biological phosphorus removal. Phosphate-rich sludge may be stored in landfill or sold to fertilizer companies.

3.2.3.4 Disinfection

The main objective of disinfection in the treatment of sewage is to significantly reduce the number of microorganisms in the water to be released back into the environment for the later use of irrigation, bathing, drinking etc. The success of disinfection is depending on followings:

1. The quality of water being treated (i.e. pH, turbidity etc.): turbid or cloudy water will be treated less effectively.
2. Type of disinfectant being applied
3. Dosage of the disinfectants: concentration and contact time.
4. Other environmental variables: Temperature, moisture etc.

The most widely used disinfectants include chlorine, UV light and ozone. These disinfectants can kill microorganisms. Chlorination remains the most common form of waste water disinfection due to its long-term history of the success and low cost but one drawback is that chlorination of residual organic material can produce chlorinated-organic compounds that may be harmful or carcinogenic to the animal or human systems. In addition, residual is toxic to aquatic flora and fauna. Therefore, the final effluent should be chemically dechlorinated before discharged back into the environment.

Ultraviolet (UV) light can be used as an alternative of chemical compounds like iodine or chlorine. In general, UV radiation is capable in the destruction of the nucleic acid (DNA/RNA) of microbial cells and making them incapable of multiplication and growth. In this process, as no chemicals are used and therefore, final treated water has no adverse impact on biological system that later consume it. The key shortcoming of UV disinfection is the need for regular lamp replacement and maintenance as well as continuous monitoring on target microorganisms that are not protected from the UV light. It has been reported that solids existing in the final treated water may shield microorganisms from UV radiation.

Ozone (O3) is generated on-site as required. It is very unstable and highly reactive and oxidizes most organic matters that come in contact with, thus killing numerous pathogenic microorganisms. The disadvantages include high cost of ozone generation and maintenance equipment and need of skilled operators.

3.3 Odor control

During sewage treatment operations, odors are normally emitted by the anaerobic breakdown of organic materials containing nitrogen and sulfur. However, defective or improperly designed sewage treatment process can be the source of ordors. In general, the most odor causing compounds are:

• Inorganic gases:H2S
• Mercaptans: mercaptans, organic sulfides, polysulfides and thiophenes etc.
• Other compounds: Organic acid, phenol and p-cresol.

These compounds are volatile, relatively low molecular weight and usually identified either using a panel of at least five people or by gas chromatography.

Since such odor causing compounds are harmful and therefore, should be treated before their release in the environment. The various treatment methods are used in odor control including use of activated  carbon reactors, ozone contactors, soil/compost filters and wet scrubbers. In most of the urban sewage treatment plants, wet scrubbers are used which may contain an oxidizing agent like chlorine, hydrogen peroxide, ozone and potassium permanganate that remove more than 90% of gases. Later, the treated air is discharged in the environment.

3.4 Sludge processing and disposal

Sludge generated during various sewage treatment processes must be treated and disposed of in a safe and effective way because it contains numerous pathogenic microorganisms and toxic materials including heavy metals that are detrimental to biological systems. The sludge treatment varied according to the amount of solids produced and sewage treatment site specific conditions. However, the most common treatment options are sludge digestion by aerobic and anaerobic method, composting and incineration.

3.4.1 Sludge digestion

Sludge digestion is a biological process where solid organic materials decomposed either anaerobically or aerobically into stable substances. These processes reduce the amount of organic matter and also number of pathogenic microorganisms present in the solids materials. The digested is unobjectionable and having plant growth potential. Anaerobic digestion is a bacterial process that is performed in the absence of oxygen. The sludge digestion process can be carried out using following processes (Fig 9):

• Thermophilic digestion: sludge is digested in large tanks at a temperature of about 55oC. It is more expensive due to high heating cost and operation cost.
• Mesophilic  digestion:  most  widely used due to  low operation cost  and domestic sludge digestion is carried out in septic tanks (at a temperature about 36oC) that usually retain the sewage up to two days and able to reduce 35 to 40% BOD.

Aerobic Treatment Units (ATUs) can be designed to increase the BOD reduction using a combination of aerobic and anaerobic treatment systems in the septic tank. Another common method of treating sludge is mesophilic anaerobic digestion method where sludge is detained for minimum two weeks to allow hydrolysis, abiogenesis and methanogenesis. At the end of this process, sugars and complex proteins are converted into more simple molecules like H2O, CO2 and CH4.

The main advantage of anaerobic digestion is the production of mixture of gases including CH4 and CO2 (biogas) which can be collected and used in boilers for heating, in generators for electricity production and cooking of food in rural areas where liquefied petroleum gas is not easily available.

• Aerobic digestion: It is similar to activated sludge process, where bacterial cells rapidly consumes organic materials and convert into CO2 under the aerobic conditions. It can be achieved by using jet aerators or bubble diffuser systems to oxidize the sludge. However, plugging is commonly a problem due to sediment deposition into the smaller air holes.

3.4.2 Composting

Composting is most often suitable for small scale sewage treatment plants. It is also an aerobic process that involves mixing of dewatered sludge with cheap carbon sources like wood chips, saw dust etc. In the presence of sufficient oxygen, this mixture undergo decomposition in thermophilic temperature range where bacteria digest sewage solids along with provided carbon source and generate a large amount of heat. After 2-3 weeks, the sludge has been transformed to humus type material that can use as a soil conditioner.

3.4.3 Incineration

Incineration of sludge is less frequent practice due to environmental pollution concerns (i.e. air emissions) and requirement of fuel such as fuel oil or natural gases to burn sludge. To combust sewage sludge, fluidized bed incinerators and stepped multiple hearth incinerators are often used. In addition, co-firing in community or municipal waste-to-energy plants is frequently done due to low operation cost.

3.3.4 Sludge disposal

If liquid sludge is produced then it is subjected to further treatment that makes it suitable for final disposal. Commonly, sludge is dewatered to reduce the volumes so that it can be transported off-site for safe disposal (Fig 10). Several methods can be applied to reduce water content including lagooning in dry beds that forms dry cakes, pressing of sewage with mechanical filters, thickened by centrifugation that separates solid and liquid and rarely incineration.

The final disposal of sludge depends upon the source of origin. For example, sludge originated from domestic wastes may be discharged into larger water bodies if it is liquid or if solid than sold to local farmers (used as fertilizer). The sludge produced from commercial or industrial areas is contaminated with toxic materials including heavy metals can be disposed off by disposal in land fill or liquid injection to barren land.

4.Summary

Sewage is a water-carried waste in suspension or solution, generated from various sources including sanitary, industrial, agricultural and surface runoff. The status of sewage generation and treatment capacity in urban centers seems insufficient and unsuitable and therefore, surface water sources are heavily contaminated with toxic chemicals including heavy metals. Hence, sewage treatment is becoming more essential to reduce their adverse impact on biological systems and modern sewage treatment technologies are designed to provide low cost solution with benefits of environment from the reuse of water. In general, sewage treatment is a process of removing physical, chemical and biological contaminants from wastewater with the objective to produce an environmentally safe fluid waste (treated water) and solid waste (treated sludge) suitable for disposal or reuse as fertilizer. Generally, sewage treatment consists of three stages: primary treatment: physical removal of coarse solids, about 30% removal of BOD and 60% removal of Total Suspended Solids (TSS); secondary treatment: decomposition and reduction of organic content as well as TSS (25-30 ppm) by biological activities and reduction in BOD level and TSS not less than 85%, pH between 6.0 and 9.0 and tertiary treatment: advanced treatment, removal of BOD and TSS to less than 9 ppm or removal of over 95% of BOD and TSS. In addition, it also removes pollutants including nitrogen and phosphates. Finally, disinfected and clarified wastewater is discharged into nearby natural water bodies (Table 2).