CONTENTS

Chapter No Page No

Chapter 1

INTRODUCTION

1.1 GENERAL

1.2 AIM

1.3 DATA AVAILABLE

Chapter 2

LITERATURE REVIEW

1.4 METAL PLATING

1.5 NICKEL PLATING

1.6 CHROME PLATING

1.7 PROCESSES GENERATING WASTE WATER

1.8 CHARACTERISTICS OF WASTE WATER

1.9 METHODS OF CHROME AND NICKEL REMOVAL

1.10 SEWAGE TREATMENT

Chapter 3

SCOPE OF WORK

REFERENCES

CHAPTER 1

INTRODUCTION

1.1 GENERAL

The metal finishing process often produces undesirable byproducts or wastes including air emissions, wastewater, and hazardous and solid wastes. These wastes predominately result from organic halogenated solvents, ketones, aromatic hydrocarbons, and acids used during the surface preparation stage of the process and from metals (primarily present in the form of dissolved salts in the plating baths) used during the surface treatment stage. Cyanide, used in many plating baths, is also a pollutant of concern. Following Table provides a summary of these pollutants and their sources.

Process Inputs and Pollution Generated (EPA 1995b)

Process Material Input Air Emission Process Wastewater

Solid Waste

Surface PreparationSolvent Degreasing and Emulsion Alkaline and Acid Cleaning

Solvents Emulsifying

agents Alkalis Acids

Solvents (associated with solvent degreasing and emulsion cleaning only)

Caustic mists

Solvent Alkaline Acid wastes

Ignitable Wastes

Solvent wastes

Still bottoms

Surface FinishingAnodizing Acids Metal ion-

bearing mists Acid mists

Acid wastes Spent solutions

Wastewater treatment sludges

Base metals

Chemical Conversion Coatings

Dilute metals Dilute acids

Metal ion-bearing mists

Acid mists

Metal salts Acid Base wastes

Spent solutions

Wastewater treatment sludges

Base metals

Electroplating Acid/alkaline solutions

Heavy metal-

Metal ion-bearing mists

Acid/alkaline Cyanide

Metal Reactive

bearing solutions

Cyanide-bearing solutions

Acid mists Metal wastes wastes

Plating Metals (e.g., salts)

Complexing agents

Alkalis

Metal ion-bearing mists

Acid mists

Cyanide Metal wastes

Cyanide Metal wastes

Other Metal Finishing Techniques (including polishing, hot dip coating, and etching)

  Metal fumes Acid fumes Particulates

Metal Acid wastes

Polishing sludges

Hot dip tank dross

Etching sludges

Scrubber residues

Wastewater

The rinsing process is the primary source of waste generated in metal finishing operations. Rinsing removes plating solutions or cleaners from the work piece. Rinse waters often contain low concentrations of process chemicals carried by the work piece into the rinse (also known as dragout).

Sources of wastewater that are typically treated on site include:

1. Cleaning rinsewater 2. Plating rinsewater 3. Tumbling and burnishing rinsewater 4. Exhaust scrubber solution

Wastewater that is typically regulated but not treated includes:

Non-contact cooling water Steam condensate Boiler blowdown Stormwater

To meet air emission regulations, vapors and mists, which are emitted from process baths, are controlled by exhaust systems equipped with mist collection and scrubbing systems. This treatment process generally produces a metal hydroxide sludge that must be managed as a

hazardous material. Once treated, wastewaters are discharged to a sewer authority or to a body of water (EPA 1995b).

Solid and Hazardous Waste

Metal finishers periodically discharge process baths when they lose their effectiveness because of chemical depletion or contamination. Accidental discharges of these chemicals also can occur (e.g., when a tank is overfilled). These concentrated wastes are either treated on site or hauled to an off-site treatment or recovery facility. In general, the sources of hazardous and solid wastes at a plating shop include:

Spent plating baths Spent etchants and cleaners Strip and pickle baths Exhaust scrubber solutions Industrial wastewater treatment sludge, which can contain materials such as cadmium,

copper, chromium, nickel, tin, and zinc Miscellaneous solid wastes such as absorbants, filters, empty containers, aisle grates, and

abrasive blasting residue Solvents used for degreasing

Pollution Control Board Regulations

The Central Pollution Control Board (CPCB), statutory organisation, was constituted in September, 1974 under the Water (Prevention and Control of Pollution) Act, 1974. Further, CPCB was entrusted with the powers and functions under the Air (Prevention and Control of Pollution) Act, 1981.

It serves as a field formation and also provides technical services to the Ministry of Environment and Forests of the provisions of the Environment (Protection) Act, 1986. Principal Functions of the CPCB, as spelt out in the Water (Prevention and Control of Pollution) Act, 1974, and the Air (Prevention and Control of Pollution) Act, 1981, (i) to promote cleanliness of streams and wells in different areas of the States by prevention, control and abatement of water pollution, and (ii) to improve the quality of air and to prevent, control or abate air pollution in the country.

In order to protect the environment from pollution CPCB has laid emission standards for different industries. Following are the standards applicable for electroplating industry.

Environmental Standards

Effluent ELECTROPLATING INDUSTRY : WASTEWATER DISCHARGE STANDARDS Parameter

Concentration not to except, mg/l (except for pH and temperature)

pH 6.0 to 9.0

Temperature should not exceed 5°C above the ambient temperature of the receiving body

Oil & grease 10 Suspended solids 100 Cynaides (as 'CN') 0.2 Ammonical nitrogen (as N) 50 Total residual chlorine (as Cl2) 1.0 Cadmium (as Cd) 2.0 Nickel (as Ni) 3.0 Zinc (as Zn) 5.0Chromium as Cr Hexavalent 0.1 Total 2.0 Copper (as Cu) 3.0 Lead (as Pb) 0.1 Iron (as Fe) 3.0 Total Metal 10.0

1.2 AIM OF PROJECT

Aim of this project is to design Sewage and Effluent Treatment Plant for a Chrome-Nickel Electroplating Industry.

1.3 DATA AVAILABLE

1 Type of Industry : Chrome Nickel Electroplating Industry

2 Location : Bangalore3 Cr- Bearing Wastewater : m3/d4 Concentration of Cr : ppm5 Ni-Bearing Wastewater : m3/d6 Concentration of Ni : ppm7 Sewage : m3/d

CHAPTER 2

LITERATURE REVIEW

2.1 METAL PLATING

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solder ability, to harden, to improve wear ability, to reduce friction, to improve paint adhesion, to alter conductivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish. Thin-film deposition has plated objects as small as an atom, therefore plating finds uses in nanotechnology.

2.2 Nickel Plating

In industries normally two methods are used for nickel plating:

1 Electro Nickel Plating

2 Electro less nickel

1 Electro Nickel Plating Process

Electro nickel plating, also known as nickel electro-deposition, is becoming an increasingly popular process for a variety of different manufacturing applications. Electro nickel plating is a process that uses an electrical current to coat a conductive material, typically made of metal, with a thin layer of nickel. Other metals used for electroplating include stainless steel, copper, zinc, and platinum.

Pre-treatment Process for Electro Nickel Plating

Proper pre- and post-treatment of the base product has a direct correlation to the quality and deposition rate of electro nickel plating. To help ensure uniform and quality adhesion, chemical or manual preparation includes the following three steps:

Pre-treatment surface cleaning: Surface cleaning entails eliminating contaminants through the use of solvents, abrasive materials, alkaline cleaners, acid etch, water, or a combination thereof.

Surface modification: Modifying the exterior of the base product improves adhesion through processes such as striking or metal hardening.

Post-treatment surface cleaning:

Performing finishing operations, such as rinsing, end the electroplating process.

2 Electroless nickel plating

Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metals or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2•H2O) which reacts with the metal ions to deposit metal. The alloys with different percentage of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.

Electroless nickel plating is an auto-catalytic reaction used to deposit a coating of nickel on a substrate. Unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. This plating technique is to prevent corrosion and wear. EN techniques can also be used to manufacture composite coatings by suspending powder in the bath.

Electroless nickel plating has several advantages versus electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, can deposit on non-conductive surfaces.

Pretreatment

Before performing electroless nickel plating, the material to be plated must be cleaned by a series of cleaning chemicals such as bases and acids, this process is called the pre-treatment process. Failure to remove unwanted "soils" from the part's surface would result in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove the chemical that adheres to the surface. De greasing removes oils from surface; acid cleaning removes scaling. Activation is done with a weak acid etch, or nickel strike, or, in the case of non-metallic substrate, a proprietary solution. After the plating process, plated materials must be finished with an anti-oxidation or anti-tarnish chemical (trisodium phosphate or chromate) and pure water rinsing to prevent unwanted stains. The rinsing materials must then be completely dried off or sometimes baked off to obtain the full hardness of the plating film.

The pre-treatment required for the deposition of nickel and cobalt on a non-conductive surface usually consists of an initial surface preparation to render the substrate hydrophillic. Following this initial step, the surface is activated by a solution of a noble metal, e.g., palladium chloride. Silver nitrate is also successfully used for activating ABS and other plastics. Electroless bath formation varies with the activator. The substrate is now ready for nickel deposition.

Chrome Plating

Hard Chrome Plating

Hard chrome plating is chrome plating that has been applied as a fairly heavy coating (usually measured in thousandths of an inch) for wear resistance, lubricity, oil retention, and other 'wear' purposes. Some examples would be hydraulic cylinder rods, rollers, piston rings, mold surfaces, thread guides, gun bores, etc. 'Hard chrome' is not really harder than other chrome plating, it is called hard chromium because it is thick enough that a hardness measurement can be performed on it, whereas decorative chrome plating is only millionths of an inch thick and will break like an eggshell if a hardness test is conducted, so its hardness can't really be measured directly.

Hard chrome plating is almost always applied to items that are made of steel, usually hardened steel. It is metallic in appearance but is not particularly reflective or decorative. Hard chrome plating is not a finish that you would want on a wheel or bumper.

TREATMENT OF PLATING WASTEWATER

Metal Finishing Industry Wastes Recovery techniques are treatment methods used for the purpose of recovering or regenerating process constituents that would otherwise be discarded. Included in this group are the following:1. Evaporation2. Ion exchange3. Electrolytic recovery4. Electro dialysis5. Reverse osmosis

Solids removal techniques are used to remove metals and other pollutants from process wastewaters to make these waters suitable for reuse or discharge. These methods include the following:1. Hydroxide and sulfide precipitation2. Sedimentation3. Diatomaceous earth filtration4. Membrane filtration5. Granular bed filtration6. Peat adsorption7. Insoluble starch xanthenes treatment8. Flotation

Three treatment options are used in treating common metals wastes:

1. The Option 1 system consists of hydroxide precipitation followed by sedimentation. This system accomplishes end-of-pipe metals removal from all common metals-bearing wastewater streams that are present at a facility. The recovery of precious metals, the reduction of hexavalent chromium, the removal of oily wastes, and the destruction of cyanide must be accomplished prior to common metals removal.

2. The Option 2 system is identical to the Option 1 treatment system but with the addition of filtration devices after the primary solids removal devices. The purpose of these filtration units is to remove suspended solids such as metal hydroxides that do not settle out in the clarifiers. The filters also act as a safeguard against pollutant discharge should an upset occur in the sedimentation device. Filtration techniques applicable to Option 2 systems are diatomaceous earth and granular bed filtration.

3. The Option 3 treatment system for common metals wastes consists of the Option 2 end-of pipe treatment system plus the addition of in-plant controls for lead and cadmium. In-plant controls would include evaporative recovery, ion exchange, and recovery rinses. In addition to these three treatments, there are several alternative treatment technologies applicable to the treatment of common metals wastes. These technologies include electrolytic recovery, electro-dialysis, reverse osmosis, peat adsorption, insoluble starch xanthenes treatment, sulfide precipitation, flotation, and membrane filtration.

PRECIOUS METALS

Precious metal wastes can be treated using the same treatment alternatives as those described forth treatment of common metals wastes. However, due to the intrinsic value of precious metals, every effort should be made to recover them. The treatment alternatives recommended for precious metal wastes are the recovery techniques of evaporation, ion exchange, and electrolytic recovery.

COMPLEXED METAL WASTES

Complexes metal wastes within the metal finishing industry are a product of electro less plating, immersion plating, etching, and the manufacture of printed circuit boards. The metals in these waste streams are tied up or complexes by particular completing agents whose function is to prevent metals from coming out of solution. This counteracts the technique used by most conventional solids removal methods. Therefore, segregated treatment of these wastes is necessary. The treatment method most suited to treating complexes metal wastes is high-pH precipitation. An alternative method is membrane filtration16, which is primarily used in place of sedimentation for solids removal.

TREATMENT OF CHROME PLATING WASTEWATER

HEXAVALENT CHROMIUM

Hexavalent chromium-bearing wastewaters are produced in the metal finishing industry in chromium electroplating, in chromate conversion coatings, in etching with chromic acid, and in metal finishing operations carried out on chromium as a basis material. The selected treatment option involves the reduction of hexavalent chromium to trivalent chromium either chemically or electrochemically. The reduced chromium can then be removed using a conventional precipitation-solids removal system. Alternative hexavalent chromium treatment techniques include chromium regeneration, electro dialysis, evaporation, and ion exchange.

Chromium Reduction

There are three treatment methods applicable to wastes containing hexavalent chromium. Wastes containing trivalent chromium can be treated using chemical precipitation and sedimentation, which is discussed below. The three methods applicable to the treatment of hexavalent chromium use the following:

1. Sulfur dioxide2. Sodium metabisulfite3. Ferrous sulfate

Hexavalent chromium reduction through the use of sulfur dioxide and sodium metabisulfite has found the widest application in the metal finishing industry. It is not truly a treatment step, but a conversion process in which the hexavalent chromium is converted to trivalent chromium. The hexavalent chromium is reduced through the addition of the reductant at a pH in the range 2.5 to 3 with a retention time of approximately 30 to 40 min.

Ferrous sulfate has not been as widely applied. However, it is particularly applicable in facilities where ferrous sulfate is produced as part of the process, or is readily available. The basis for this technology is that the hexavalent chromium is reduced to trivalent chromium and the ferrous iron is oxidized to ferric iron.

Chromium Precipitation

Chromium precipitation is accomplished through the addition of a chemical reagent to form metal precipitants, which are then removed as solids in a sedimentation step. The options available to a facility as precipitation reagents are lime [Ca(OH)2], caustic (NaOH), carbonate (CaCO3 and Na2CO3), sulfide (NaHS and FeS), and sodium borohydride (NaBH4). The advantages and disadvantages of these reagents are summarized in the following:

1. Limea) It is the least expensive precipitation reagent.b) It generates the highest sludge volume.c) The sledges generally cannot be sold to smelters or refiners.

2. Caustica) It is more expensive than lime.b) It generates a smaller volume of sludge.c) The sledges can be sold to smelter and refiners.

3. Carbonatesa) These may be used for metals where solubility within a pH range is not sufficient to meet treatment standards.

Lime is the least expensive reagent, but it generates the highest volume of residue. It also generates a residue that cannot be resold to smelters and refiners for reclaiming because of the presence of the Calcium ion.

Caustic is more expensive than lime, but it generates a smaller volume of residue. One Key advantage to caustic is that the resulting residues can be readily reclaimed. Carbonates are particularly appropriate for metals where solubility within a pH range is not sufficient to meet a given set of treatment standards. The sulfides offer the benefit of achieving effective treatment at lower concentrations due to the lower solubility’s of the metal sulfides. Sodium borohydride has application where small volumes of sludge that are suitable for reclamation are desired.

TREATMENT OF NICKEL PLATING WASTEWATER

Nickel removal from wastewater is achieved by means of a precipitation treatment. The options of chemicals that can be used are same as Chromium Precipitation. Using NaOH leads to minimum sludge generation during precipitation. pH of wastewater will be maintained at 10.5 to achieve maximum Nickel precipitation.

Summary of Conventional Chemical Treatment Method (EPA, 1995)

Treatment Method

Treatment Chemical

Usage ratio

Optimum pH Minimum Reaction

Time (min)

Comments

Hexavalent chromium reductionAcid method

Sodium metabisulfite

1.5:1 pH = 3.0-3.5 5 Reaction time dependent on pH.

Sulfur dioxide 1:1 pH = 3.0-3.5 5 Reaction time dependent on pH.

Ferrous sulfate8:1 pH =3.0-3.5 5 Reaction time

dependent on pH.

Treatment Method

Treatment Chemical

Usage ratio

Optimum pH Minimum Reaction

Time (min)

Comments

Metal precipitationHydroxide Hydrated lime Variable pH = 7.0-10.0 20 Optimum pH

varies depending on metal to be removed.

Sulfide Caustic soda Variable pH = 7.0-10.0 20 Optimum pH varies depending on metal to be removed.

Ferrous sulfide/sodium sulfide

5:1 pH = 8.0-9.0 15 Polishing after hydroxide precipitation when complexing agents present.

Carbonate Sodium bicarbonate Variable Variable 15 Advantageous for lead ,cadmium, nickel removal.

Insoluble starch xanthate

Cross linked starches

5-10:1 pH >7.0 Instantaneous

Polishing after hydroxide precipitation when complexing agents present.

Relation between pH and % nickel precipitated

pH Adjusted to % Nickel precipitated7.5 6.48.0 64.08.5 94.59.0 99.0

The optimum pH required for precipitation of metals can be found out by following graph:

TREATMENT SCHEME OF CHROME PLATING WASTEWATER

The hexavalent form is very soluble at all pH values and is quite toxic to microbial growth. Chromium Reduction of hexavalent chromium to trivalent is conducted in a baffled reaction chamber utilizing quadravalent sulfur as the reducing agent.

The reaction zone maintains a pH of less than 3 with at least 5 minutes reaction time and a mixing rate of 2.5-3.0 tank turnovers per minute. Since the Chrome Neutralizer effluent contains Sodium Meta Bi-Sulphite (SMBS), it will reduce Cr VI into Cr III. The effluent containing trivalent chromium will then be pumped into the Flash Mixer. Alkali, Coagulant and polyelectrolyte will be mixed in the effluent in flash mixer for pH adjustment and coagulation. pH required for optimum Cr precipitation is 7.5. The effluent rich in coagulants will then be transferred into Flocculation tank followed by clarifier. Cr sludge will precipitate in the clarifier and the clarified water will flow into the filter feed tank by gravity.

TREATMENT SCHEME OF NICKEL PLATING WASTEWATER

Nickel removal from wastewater will be achieved by means of a hydroxide precipitation treatment using NaOH as it leads to minimum sludge generation during precipitation. pH of wastewater will be maintained at 10.5 to achieve maximum Nickel precipitation.

Nickel bearing wastewater will first be collected in an equalization tank for flow equalization. This effluent will be transferred to a flash mixer where chemicals will be added. The chemically enhanced effluent will then be transferred to a flocculation tank equipped with a slow speed agitator. Slow agitation will help to form consistent floc which will settle in clarifier. The flocculated solution is further transferred to a clarifier where nickel hydroxide precipitates out as sludge.

SEWAGE

Characteristics of Sewage

Sewage or wastewater is the waterborne human, domestic and farm wastes. It may include industrial effluent, subsoil or surface waters. Human wastes include faecal material. Domestic wastes include food wastes and wash water.

Chemical Characteristics of Sewage

A typical raw sewage contains about 99.9% water. The remaining 0.1 % is made up of about 70% organic and 30% inorganic solids. The solid content occurs in both suspended and dissolved forms. The inorganic components include ammonia, chloride, grit, salts and metals.

These inorganic chemicals are actually those which are present in the water supply initially. Metal industries and mines also contribute to the inorganic. Organic compounds may be either nitrogenous compounds such as proteins and amino acids or non nitrogenous compounds such as carbohydrates and lipids.

Organic substances are mainly the contribution of plant and animal wastes which are of different composition. For example, animal sewage is relatively high in proteins and lipids. Plant materials have a high content of cellulose and lignin.

Cellulose in sewage mainly comes from the cell wall of plants and plant products. Paper, cotton and certain other plastic products contain cellulose. Since cellulose cannot be digested in human digestive system, major proportion of cellulose in the human diet is excreted as undigested waste. Hemicelluloses, pectin, starch and lignin are the other carbohydrate materials found in sewage.

Microbiological Characteristics of Sewage

The composition of sewage varies depending upon the source of wastewater. This also causes variation in the microbial flora of sewage. Almost all groups of microorganisms, algae, fungi, protozoa, bacteria and viruses are present. Raw sewage may contain millions of bacteria per ml.

The bacterial group comprises mainly the soil borne organisms, Bacillus subtilis, Bacillus megaterium, Bacillus mycoides, Pseudomonas fluorescens, Achromobacter spp. and Micrococcus spp. Bacteria of intestinal origin also occur in sewage in large numbers.

Mostly these are harmless organisms. Examples of this type are Escherichia coli, and other coliforms, Proteus and Serratia species. Potential pathogens include enterococci (Streptococcus faecalis) and Clostridium perfringens. Pathogenic bacteria which cause serious illness like Vibrio cholerae, Salmonella typhi, Salmonella paratyphi and Shigella dysenteriae may also occur in sewage.

Viruses which are released in the faeces from infected host are also occasionally found in sewage, for example, poliomyelitis virus, infectious hepatitis virus and Coxsackie virus. Bacteriophages also occur in comparatively large numbers. During treatment process the microbial flora may be dominated by the corresponding physiological groups.

TYPICAL CHARACTERISTICS OF RAW SEWAGE ARE AS FOLLOWS:

Sr. No. Parameters Concentration

1. Color Hazy

2. Raw Sewage Temperature Ambient

3. pH 7-8.5

4. TSS 200-350 ppm

5. Biological Oxygen Demand – 5-day (BOD5)

200-350 ppm

6. Chemical Oxygen Demand (COD) 500-600 ppm

7. Oil & Grease 10 ppm

TREATMENT METHODS:

Primary Treatment:

AEROBIC TREATMENT:

CHAPTER 3

SCOPE OF WORK

1. Study of Wastewater generated in the industry

2. Selection of Treatment Scheme

3. Design of Treatment Plant (Effluent And Sewage)

CHAPTER 4

REFERENCES

1. Manual of Sewerage and Sewage Treatment -CPHEEO Govt of India

2. Wastewater Engineering – Treatment and Reuse – Metcalf & Eddy

3. A thesis on DEVELOPMENT OF AN INTEGRATED ENVIRONMENTAL ACTION PLAN FOR CHROMALLOY (THAILAND) LTD. by Harnpon Phungrassami (Asian Institute of Technology, School of Environment, Resources and Development, Bangkok, Thailand, August 2001)

TABLE 10.1

UNIT OPERATIONS/PROCESSES, THEIR FUNCTIONS AND DEVICES

USED FOR DOMESTIC WASTE WATER TREATMENT

Sr. No. Unit Operations And Process Functions Treatment Devices

1 Screening Removal of large floating and Settleable solid

Bar racks and screens of various descriptions

2 Grit removal Removal of inorganic suspended solids

Grit chamber

3 Primary sedimentation Removal of organic and inorganic settleable solids

Primary sedimentation tank

4 (a) Aerobic biological suspended growth processes

Conversion of colloidal, dissolved and residual suspended organic matter into settleable bioflocs and stable inorganics

Activated sludge process units and its modification, waste stabilization ponds, aerated lagoons

(b) Aerobic biological Attached growth processes

Conversion of colloidal, dissolved and residual suspended organic matter into settleable bioflocs and stable inorganics

Trickling filter, rotating biological contactor

5 Anaerobic biological growth process

Conversion of organic matter into CH4 & CO2 and organic relatively stable organic residue

Anaerobic filter, fluid bed submerged media anaerobic reactor, upflow anaerobic sludge blanket reactor, anaerobic rotating biological contactor

6 Anaerobic stabilization of organic sludges

Same as above Anaerobic digestor

Table 10.3

EXPECTED EFFICIENCIEC OF VARIOUS TREATMENT UNITS

Process Percentage Reduction

SS BOD Total Coliform

1 Primary treatment (Sedimentation) 45-60 30-45 40-60

2 Chemical Treatment 60-80 45-65 60-90

3 Secondary Treatment

(i) Standard trickling filters 75-85 70-90 80-90

(ii) High rate trickling filters (a) Single stage (b) Two stage

75-85 90-95

75-80 90-95

80-90 60-90

(iii) Activated sludge plants 85-90 85-95 70-96

(iv) (a) Stabilization ponds (Single cell) (b) Stabilization ponds (Two cell)

80-90 90-95

90-95 95-97

90-95 95-98