29 Microbiology of industrial effluents and their treatment

Dr Tejpal Dhewa

Learning objectives:

• To understand the status of industrial effluents in India
• To understand about the types and sources of industrial effluents To gain knowledge about General effluent treatment processes
• The understand the role of Microbial communities in the treatment of industrial effluents

TABLE OF CONTENTS

1.Introduction

2.Industrial effluents: sources and characteristics 2.1 Major effluent sources

2.2 Effluent characteristics

2.1.1 Physical

2.1.2 Chemical

2.1.3 Biological

3. Effluent treatment process: an overview

3.1 Pre-treatment

3.1.1 Screening

3.1.2 Equalization

3.1.3 Oil and grease separation

3.1.3.1 API oil-water-separator

3.1.3.2 Parallel Plate Separator

3.1.3.3 Hydrocyclone Oil Separator

3.2 Primary treatment

3.2.1 Chemical process

3.2.1.1 Neutralization

3.2.1.2 Chemical precipitation

3.2.2 Physical process

3.2.2.1 Sedimentation

3.2.2.2 Flotation

3.3 Secondary treatment

3.3.1 Activated sludge

3.3.2 Trickling filter

3.3.3 Oxidation pond

3.4 Tertiary treatment

3.4.1 Filtration

3.4.1.1 Granular media filters

3.4.1.2 Carbon adsorption

3.4.2 Membrane separation

3.4.2.1 Ultrafiltration

3.4.2.2 Reverse Osmosis

3.4.3 Removal of nutrients

3.5 Disinfection and odor control

3.6 Sludge treatment and disposal

3.6.1 Sludge digestion

3.6.2 Composting

3.6.3 Incineration

3.6.4 Disposal

4.Summary

1.Introduction

In India, rapid industrialization and urbanization has been continuously increasing the demand of water. Almost 70% of current consumption is driven by agricultural activities and 15-20 % being each consumed by industries and municipal usage. In urban areas, about 80% of water supplied becomes wastewater due to anthropogenic industrial and commercial activities before to its discharge into the environment or its re-use.

It is estimated that 13,500 MLD (million of litres per day) of industrial wastewater is generated and only 8,000 MLD (about 60%) of industrial wastewater is treated and remaining wastewater is directly goes to natural water resources and caused negative effects to the ecosystem and human’s life (Source: http://cpcbenvis.nic.in/cpcb_newsletter/sewagepollution.pdf ). Contamination of drinking water supplies from industrial by products or wastes is a result of several types of industrial process and disposal methods. Most of the industries required large amounts of water for processing and later, discharge their waste into nearby water sources without removing hazardous chemicals/wastes or improper disposal of wastes in surface soil. Therefore, most of such wastes keep accumulating in the environment and consequently posing challenges in the treatment and management.

The Clean Water Act has standards for the acceptable discharge of a limited amount of contaminates into water bodies (Source: https://www.epa.gov/laws-regulations/history-clean-water-act). However, most of the developing nations industries not following these guidelines properly due to financial concerns and lack of strict regulatory guidelines. Although recent trends in the developed world have been to minimize such production or recycle these waste within the production process by using Zero Liquid Discharge Technology.

2.Industrial effluents: Sources and characteristics

In general, wastewater or effluents are originated from various industries consists of both organic and inorganic substances. The organic waste such as solvents and cleaning fluids, pesticide residues, dissolved residue from fruit and vegetables, lignin from pulp and paper industries etc. Effluent may also contain inorganic waste include metals and brine salts. However, the exact composition and characteristics of effluent depends on their source from they generated. The detailed information of various sources and effluent characteristics are discussed below:

2.1 Major effluent sources

Industries which use large amounts of water in their processes include food processing, chemical manufacturing, steel plants, textile etc. generates certain amounts of wastes during processing, and operation and maintenance of machines are as follows (Table 1).

 2.2 Effluent characteristics

Characterization of effluent is essential for an effective and economical effluent treatment and management. It is useful in the extent of treatment with choice of a suitable treatment method. Later, such information’s are also considered for planning and designing of an efficient effluent treatment plant. For example, physico-chemical parameters (listed in Table 2) of effluents are required to be examine in the inlet and final outlet of Common Effluent Treatment Plants (CETPs).

2.1.1 Physical

Industrial effluent contains following types of physical impurities:

• Total solids: total solids are defined as all the matter that remains as a residue upon evaporation as 103 to 105oC (Fig.1). These are composed of floating matter, settable matter, colloidal matter and matter in solution.
• Odours: industrial effluent may contain either odorous compounds or compounds that generate during the process effluent treatment.
• Temperature: the temperature of industrial wastewater is very important factor since it affects the aquatic life and can change the rate of chemical reaction or process during the treatment process.
• Color: the color of industrial effluent varies according to the type of industry from where it is discharged.
• Turbidity: indicate the quality of effluent discharges and natural water in respect to colloidal and residual suspended matter.

2.1.2 Chemical

Industrial wastewater contains following types of inorganic and organic materials:

• Inorganic matters: such as nitrogen, sulphur and heavy metals are produced mainly in the coal and steel industry.
• Organic materials: most organic wastes are generated by pharmaceutical, cosmetic, textile, complex chemical industries, food & dairy industries and leather industries, contaminating water bodies and increases  Biochemical  Oxygen  Demand  (BOD,  also  called  Biological  Oxygen  Demand)  and Chemical  Oxygen  Demand  (COD).  BOD  is  the  amount  of  dissolved  oxygen  needed  by microorganisms for the aerobic break down of organic content present in 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. For example, more oxidable 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 wastewater 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 wastewater 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 wastewater treatment from starting to the end of process. COD is determined within 3 hours by chemical oxidation of organic material using a mixture of chromic and sulphuric acid at high temperatures.

2.1.3 Biological

A number of microorganisms and parasites are usually present in wastewater water. Normally, wastewater 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 wastewater. 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 effluent water and are transferred by the vats with the movement of wastewater during the treatment process. However, the number and type of these microorganisms may vary on the basis of wastewater origin. Notably, some microorganisms are used during the secondary treatment of wastewater 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.

• Bacteria: Wastewater treatment vats contain both beneficial and harmful bacteria due to their arrival with  wastewater.  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 wastewater. In addition,  nitrogen  removing bacteria  like  Nitrosomonas  and  Nitrobacter  (nitrifiers),  Thiobacillus (denitrifier), Desulfotomaculum and Desulfovibrio (sulfate reducer) are also found in wastewater. 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 diarrhoea), Salmonella typhi (typhoid) and Vibrio cholera (cholera) can be found in wastewater due to fecal contamination. E.coli is used as a fecal indicator. During disinfection most of these microorganisms are destroyed and removed from wastewater.

• 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 wastewater.
• Protozoa: such as amoebae, flagellates and ciliates unicellular eukaryotic organisms are found present throughout the complete wastewater 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 wastewater treatment but can be harmful to human beings if ingested. For example, protozoan parasites including Giardia lamblia is a waterborne pathogen found in wastewater and its cysts enters the water supply through fecal contamination and causes giardiasis.
• Viruses: Wastewater 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  wastewater:  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 wastewater treatment operations.

• 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.
• 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 predator’s organisms feed on the biofilm. In general, such organisms only present in an aged biomass under aerobic and microaerophilic conditions.

3.Treatment process: an overview

Industrial effluent contains various materials depending on the industry. Few effluents have oils and grease, some contain toxic material include cyanide, heavy metals etc. Most effluents from food and beverages industries contain biodegradable organic contaminates that is suitable for microbial growth. Effluent from canned fruit and soda beverages contain high percentage of sugar, very low percentage of protein, nitrogen or phosphorous. Hence, biological growth in these effluents is rather weak. Therefore, each industrial effluent needs a specific treatment technology.

To design an appropriate effluent treatment plant, some parameters such as volume of discharged wastewater, physical, chemical and biological characteristics of effluent etc. must be determined. In general, following treatment steps are required to effective treatment of effluent discharged form large scale industries (Fig.2).

Fig.2: Generalized scheme of effluent treatment. Source: Author and ILLL

In India, small scale industries are in majority that may not have enough money to afford cost of such modern wastewater treatment plant. The Central Pollution Control Board (Govt. of India), New Delhi has set-up several Common Effluent Treatment Plants (CETPs) for cluster of small scale industries. The treatment methods adapted in CETPs includes dissolved air floatation, dual media filter, activated carbon filter, sand filtration and tank stabilization, flash mixer and primary and secondary clarifiers and sludge drying beds. Large solid materials and settable solids are removed during primary treatment using bar screen, grit chamber and sedimentation tank. Later, the treated industrial wastewater from CETPs discharged in nearby rivers or water sources, for example, about 133 MLD effluents of 10 CETPs of Delhi used to disposed in Yamuna River.

3.1 Pre-treatment

Pre-treatment of influent is carried out in the following steps:

3.1.1 Screening

Screening is the first step in primary effluent treatment, removes the largest solids like boxes, bottles, tires, cans, metals etc. using screen bars (Fig.3). Separated materials may be incinerated, ground up or used for landfill.

3.1.2 Flow equalization

The hydraulic velocity or flow rate of entering effluent is carried out in flow equalization tank. The main objective of this tank is to overcome the operational problems that arise from flow variations and improve the performance of treatment process at each stage, allowing adequate time for physical, chemical and biological process. It is designed for sufficient mixing of solids (adjustment of concentration etc.) and preventing their sedimentations. In addition, it also provides adequate aeration to avoid odor problems. Sometimes, this tank may also be used as an emergency tank (temporary storage) to equalize effluent in case of any process failure during the treatment process.

3.1.3 Oil and grease separation

Oil discharged from small-scale industries can be separated from contaminated or open water surfaces through various skimming devices. These devices are considered cheap and efficient way to separate oil, grease and other hydrocarbons from effluent and desired level of water purity can be obtained. In addition, skimming is also an economic method to separate most of the oil prior using chemical process or membrane filters. However, grease skimming includes higher viscosity hydrocarbons. Therefore, skimmers must be fitted with heaters which keep grease fluid for release. The floating forms of grease like solid clumps, mats etc. can be enhanced using mechanical apparatus or aerator can be used to expedite removal.

3.1.3.1 API oil-water-separator

The effluent released from large-scale industries including chemical plants, oil and petrochemical refineries and natural gas processing plants usually have huge amounts oil and suspended solids. These industries remove oil and suspended solids from their effluent using a device known as API oil-water-separator (Fig.4), which is designed according to the standards given by the American Petroleum Institute (API). The design of API separator is based on the Stock’s Law (the specific gravity difference between the oil and wastewater and between the suspended solids and wastewater). The suspended solids settle to the bottom of the separator as sludge whereas the oil rises to the top of the separator and purified wastewater is the middle layer (between the oil and sludge layer. Later, the oil layer is skimmed off and consequently either re-processed or disposed of. The purified water layer is subjected to further treatment. At last, the sediment is removed by a chain and scraper and a sludge pump.

3.1.3.2 Parallel Plate Separator

The parallel plate separators are similar to API oil-water-separator except tilted parallel plate assemblies that provide more surface area for suspended oil droplets to join into larger globules (Fig. 5). Though these separators also depend upon the specific gravity between the water and the suspended oil but the parallel plates enhance the degree of oil-water removal. This equipment needs comparatively less space than API separator to attain the same degree of removal.

3.1.3.3 Hydrocyclone Oil Separator

Hydrocyclone oil separators operate on the process where effluent enters the cyclone chamber and is revolved under high centrifugal forces up to 1000 times the force of gravity, resulting the oil droplets and water separated and are released through two different nozzles (in opposite direction) of cyclone chamber (Fig. 6).

3.2 Primary treatment

When the effluent receives at an effluent 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 effluent 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 effluent for further treatment and purification.

3.2.1 Chemical process

Following chemical processes required before their discharge or subject to further treatment:

3.2.1.1 Neutralization

Industrial effluents usually contain acidic or alkaline compounds which need neutralization prior to release or treatment. For effluent that are intended to discharged to natural water bodies should have a pH in between 6.0 to 9.0 (as specified by waste regulatory agencies) while effluent inflowing to biological treatment process should be maintained between 6.5 to 9.0. This pH range is suitable for microbial growth that is required to decompose organic materials. Acidic effluent is normally neutralized with waste alkaline streams like lime, ammonia, dolomite, caustic soda etc. Lime is most extensively used as alkaline material for neutralization due to its low cost and easy availability. However, lime may be slow to react with acid and may produce insoluble precipitates. Alkaline effluent normally needs treatment with waste acidic streams of hydrochloric or sulphuric acid.

3.2.1.2 Chemical precipitation

Chemical precipitation in industrial effluent involves the addition of certain chemicals to alter the physical state of dissolved and suspended solids and aid their removal by sedimentation. In general, chemical precipitation process (Fig.7) is used for improving effluent treatment plant performance because this process can remove 80 to 90% of bacteria, 50 to 80% of BOD and 80 to 90% of total suspended solid matter.

• Coagulation: It takes place in rapid mix or flash mix basins which are very quick. The primary function of rapid mix basin is to disperse the coagulant so that it contacts all of the effluent water. For example, alum, lime, ferrous sulfate, ferric sulfate and ferric chloride are most frequently used as coagulant.

• Flocculation: the aim of flocculation is to develop aggregates or flocs from the finely separated matter. The flocculation of effluent by either mechanical or air agitation.

3.2.2 Physical process

Following physical processes required before their discharge or subject to further treatment:

3.2.2.1 Sedimentation

It is also called primary settling (Fig 8). 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. Certain chemical agents called coagulants and flocculants may be applied to speed up this process by encouraging aggregation of particles. These biosolid or sludge is often treated biologically through anaerobic degradation in a sludge digester.

3.2.2.2 Flotation

Flotation is a unit setup used to remove solid or suspended particles in a shorter time from an effluent. The removal is achieved by introducing air bubbles or fine gases in the effluent (liquid phase). The bubbles attach to the particulate materials and the floating (buoyant) force of the combined particle and gas bubble is great enough to cause the particles to rise to the surface to form a scum blanket, which is separated by using various skimming devices. Heavy solids including gravel that settles down are collected in a central sludge for separation. Flotation is relatively more efficient process than sedimentation.

• Air flotation: in such system, air bubbles are generated by introducing the gas phase directly into the liquid phase through a rotating impeller through spargers (Fig 9 A & B).

• Vacuum flotation: it consists of saturating the effluent with either directly in an aeration tank or by allowing air to enter on the suction side of an effluent (Fig.10).

3.3 Secondary treatment

Subsequent primary treatment, the effluent 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%.

3.3.1 Activated sludge

Activated-sludge is a process for treatment of domestic and industrial effluent using air and biological flocs composed of bacteria and protozoa that substantially reduce organic materials. This process starts when air being introduced into an effluent 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 effluent 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 containg bacterial floc) is returned to head of the activated-sludge aeration tank to re-seed the new effluent receiving the tank (Fig.11).

3.3.2 Trickling filter

Trickling filters made up of generally 5-10 feet deep fixed bed of rocks, lava, slag, crushed stone, gravel, polyurethane foam or plastic media. The effluent is sprayed over the surface of the bed by using an overhead sprayer which is rotating at a constant speed (Fig 12). 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 effluent 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.

3.3.3 Oxidation pond

Oxidation pond effluent–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 effluent 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 effluent water through the interaction of sunlight, bacteria and algae (Fig. 13). In this pond, effluent 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 effluent. 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 effluent and hence, continuing aerobic conditions in wastewater. In these conditions the aerobic microorganisms grow speedily and digest organic content of the effluent. 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 effluent.

3.4 Tertiary treatment

Tertiary treatment is an advanced and final step of effluent 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:

3.4.1 Filtration

Following different types of filters are used to remove fine impurities before their discharge or subject to further treatment:

3.4.1.1 Granular Media Filter

Granular media filters are widely used in effluent treatment for the removal of both organic and inorganic suspended solids. These filters can operate either by pressure (pressure filter) or by gravity flow (gravity filters). The most common filters are two (dual) and three media filters. A common design for a two or dual media (0.5mm sand layer below a 0.9 mm anthracite layer). In case of three media filter, 30 to 40 mesh garnet layer below the sand layer. However, specialty filters could use different media with different effective sizes. During this filtration, solids are seized by the bed and finally have to be removed by scrubbing and backwashing (Fig 14 A & B).

Source: Author and ILLL

3.4.2 Membrane separation

Membrane separation techniques are used to remove very fine particles from wastewater, to desalinate water and currently, membranes have been developed to separate organics from water. Membranes are made up of several types of materials but all have a consistent pore size that will allow particles or molecules of a given size to pass through the membrane and will prevent molecules or particles of size larger than the pore size from passing through. In general, membrane separation includes ultrafiltration (UF) and reverse osmosis (RO).

3.4.2.1 Ultrafiltration

Ultrafiltration is a type of membrane filtration in which hydrostatic pressure forces a water or liquid against a semipermeable membrane of 0.1 to 0.001 µ pore sizes. Normally in this process, high molecular weight substances, colloidal materials organic and inorganic polymeric molecules are removed from liquid or wastewater. However, low molecular weight organics and ions like sodium, calcium and chloride are not removed by UF membranes (Fig.16).

3.4.2.2 Reverse Osmosis

An industrial reverse osmosis is a very effective wastewater treatment process for reducing up to 99% of dissolved salts, organics and other fine particles by forcing water under pressure through a semipermeable membrane. This process is termed reverse osmosis due to it is opposite to natural osmotic pressure of water (Fig.17).

3.4.3 Removal of nutrients

Effluent 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 effluent 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 to known as enhanced biological phosphorus removal, where polyphosphate-accumulating organisms (specific bacteria), are selectively enriched and accumulate large quantities of phosphorous 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 effluent water, which allows the smaller particles with phosphorous 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 phosphorous removal. Phosphate-rich sludge may be stored in landfill or sold to fertilizer companies.

3.5 Disinfection and odor control

The main objective of disinfection in the treatment of effluent 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.

During effluent treatment operations, odors are normally emitted by the anaerobic breakdown of organic materials containing nitrogen and sulfur. However, defective or improperly designed effluent treatment process can be the source of odors. 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 effluent 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.

3.6 Sludge treatment and disposal

Sludge generated during various effluent 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 effluent treatment site specific conditions. However, the most common treatment options are sludge digestion by aerobic and anaerobic method, composting and incineration.

3.6.1 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:

• 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 effluent 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 (Fig. 18).

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.

3.6.2 Composting

Composting is most often suitable for small scale effluent 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 effluent 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.

3.6.3 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 effluent 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.

3.6.4 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. Several methods can be applied to reduce water content including lagooning in dry beds that forms dry cakes, pressing of effluent with mechanical filters, thickened by centrifugation that separates solid and liquid and rarely incineration (Fig. 19 A & B).

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 of by disposal in land fill or liquid injection to barren land.

4. Summary

Effluent is a water-carried waste in suspension or solution, generated from various sources mostly from commercial activities or industrial. The status of effluent 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 and cyanide. Therefore, effluent treatment is becoming more essential to reduce their adverse impact on biological systems and modern effluent treatment technologies are designed to provide minimum liquid discharge with benefits of environment from the reuse of water. In general, effluent 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. Effluent treatment consists of four stages: pretreatment: water conditioning, and separation of oils; primary treatment: pH adjustment, 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 mg/liter) 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 mg/liter or removal of over 95% of BOD and TSS. In addition, it also removes pollutants including nitrogen, phosphates and certain no biodegradable organics, ions and macromolecules Finally, disinfected and clarified wastewater is discharged as per the Clean Water Act standards for the acceptable discharge of a limited amount of contaminates into water bodies or re-used in industrial processing. However, treated water may be used for drinking purpose by using recent advanced treatment technologies (i.e. membrane filtration including

ultrafiltration and reverse osmosis). Sludge may be used in crop field as fertilizer or incinerated/landfill if contains toxic chemicals.