Biological Treatment Process

The biological treatment process involves the use of microorganisms such as bacteria and fungi to convert finely divided colloidal and dissolved carbonaceous organic matter in wastewater into various gases and into cell tissues that are then removed from sedimentation tanks as flocculent settle-able organic and inorganic solids. This process often complements both physical and chemical processes and it is classified as follows.

22.1.4.3.1 Activated Sludge Process

This process is a continuous flow aerobic system involving the mixing of clarified wastewater with an active mass of microorganisms, mainly bacteria and fungi, which eventually aerobically degrade organic matter into CO2, H2O, and other by-products.17-21 It is a system that requires series of tanks and efficient operation of the sludge plans.3

22.1.4.3.2 Aerated Lagoons Process

This process is similar to the activated sludge process; however, it requires a large surface area to cause more temperature effects than that experience in the activated sludge process. The aeration process in this system supplies oxygen to the influent wastewater and the turbulent generated keeps the contents of the basin in suspension. The suspended solids are then removed in a settling tank where the wastewater may further be treated before discharge.2,3

22.1.4.3.3 Others

Due to the focus of this work, other biological processes will also be mentioned. They include trickling filters, coating biological contactors, pond stabilization, anaerobic digestion, and biological nutrient removal.235

22.1.5 Wastewater Characterization

Wastewaters generated from manufacturing plants that produce or use inorganic chemicals vary considerably, depending on raw materials, type of process, and the end product, among others. A screening program is often conducted to determine the presence, concentration, and toxicity of metal ions in such wastewaters. The minimum detection limits for the toxic metals are presented in Table 22.1.

TABLE 22.1

Minimum Detection Limits for Some Toxic Metals

Pollutant Concentration (pg/L)

Antimony 10

Arsenic 10

Beryllium 15

Cadmium 1

Chromium 25

Copper 20

Lead 10

Mercury 0.5

Nickel 25

Selenium 10

Silver 15

Thallium 2

Zinc 1

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001, U.S. Environmental Protection Agency, Washington, DC, 1982.

22.1.6 Effluent Disposal

Treated wastewater effluents are often discharged into the biosphere where water bodies are the largest receivers. This practice requires sound engineering practices in order not to cause any adverse effect on the receiving environment; hence, many scientific and engineering factors are considered to facilitate proper mixing and disposal of the effluent.15 Environmental standards, developed by various environmental agencies, are designed to ensure that the impacts of treated wastewater discharged into receiving water bodies are acceptable. Table 22.2 lists the limitations set by some environmental agencies.

22.2 INORGANIC CHEMICAL INDUSTRIES

Most inorganic chemical industries are aggregates of small facilities where over 300 different chemicals are being produced.22 These chemicals are often of mineral origin, mainly employed at some stages in the manufacture of varieties of chemical and nonchemical products; they are not present in the final products.22 Products such as acids, alkalis, salts, oxidizing agents, industrial gases, and halogens are used as basic chemicals for industrial processes, whereas pigments, dry colors, and alkali metals are mostly employed in manufacturing products.

22.2.1 Classification of Inorganic Chemical Industries

The inorganic chemical industries classification is based on the Standard Industrial Classification (SIC) that assigns the code 281 to industrial inorganic chemicals.23 SIC is a statistical classification standard used for all U.S.-based establishments of Federal economic statistics.

22.2.1.1 General Classification of Inorganic Chemical Industries

The SIC classified the chemicals with codes as stated in Table 22.3 having considered the effluent limitations and pretreatment standard within the inorganic chemicals manufacturing point source.

TABLE 22.2

U.S. EPA, NPDES, and ECEDR for Discharges from Wastewater Treatment Plants

NPDESa

Parameter bod5

Total nitrogend Total phosphorusd

30-Day Average Concentration

7-Day Average Concentration

Percentage of Removalc

85 85

ECEDRWDb Concentration (mg/L)

35-60

10-15

Percentage of Removalc

70-90 70-90

70-80 80

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001, U.S. Environmental Protection Agency, Washington, DC, 1982.

a National Pollutant Discharge Elimination System for secondary wastewater treatment plants. b European Community Environmental Directive Requirements for wastewater discharges. c Removal in relation to influent load. d Limited to sensitivity areas subject to eutrophication.

The SIC 281 category does not include some integrated firms that manufacture other types of chemicals within the same site. Other manufacturing facilities that produce and use inorganic chemicals in their process within the facilities used in producing the SIC 281 group are stated in Table 22.4.

Top U.S. companies with inorganic chemical manufacturing operations are listed in Table 22.5.

22.2.1.2 Subcategory Classification

As a result of variation shown in toxicity, the evaluation of technologies applicable for discharge control, and treatment by some compounds within the industrial chemicals, the SIC 281 groups are further subdivided into 11 subcategories.23 They are aluminum fluoride, chlor-alkali, chrome pigments, copper sulfate, hydrofluoric acid, hydrogen cyanide, nickel sulfate, sodium bisulfate, sodium

TABLE 22.3

General Classification of SIC

Code Name Example

SIC 2812 Alkalis and chlorine Chlorine, caustic soda, soda, ash, potassium, carbonate, hydrogen, helium, oxygen, nitrogen, chrome pigments

SIC 2813 Industrial gas

SIC 2816 Inorganic pigments

SIC 2819 Industrial inorganic chemicals not classified elsewhere

TABLE 22.4

SIC Category of Manufacturing Plants within SIC 281 Facilities

Code Name

SIC 286 Organic chemical facilities

SIC 287 Fertilizer plant

SIC 26 Paper and pulp mills

SIC 31 Iron and steel mills

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001, U.S. Environmental Protection Agency, Washington, DC, 1982.

dichromate, sodium hydrosulfite, and titanium dioxide. Although these subcategories are further subdivided into 44 subcategories,23 this work will focus on the main 11 subdivisions stated above.

22.3 I NDUSTRIAL PROCESSES, WASTEWATER GENERATION,

AND TREATMENT IN INORGANIC CHEMICALS PRODUCTION

This section describes the major industrial processes of individual inorganic chemicals under the 11 subcategories and the related wastewater generated. It contains the sources of wastewater and typical treatment processes.

22.3.1 Aluminum Fluoride

22.3.1.1 Description and Production Process

Aluminum fluoride is produced when partially dehydrated alumina hydrate reacts with hydrofluoric acids gas. The solid aluminum fluoride produced is cooled with noncontact cooling water prior to further processing, while the gases from the reactor are scrubbed with water to remove unreacted hydrofluoric acid from the gas stream. Aluminum fluoride is mainly used in the production of

TABLE 22.5

Top U.S. Companies With Inorganic Chemical Manufacturing Operations

TABLE 22.5

Top U.S. Companies With Inorganic Chemical Manufacturing Operations

Rank

Company

City

State

1

Dow Chemical Co.

Midland

Michigan

2

Hanson Industrial Co.

Iselin

New Jersey

3

W.R. Grace and Co.

Boca Raton

Florida

4

Occidental Chemical

Dallas

Texas

5

BOC Group Inc.

Murray Hill

New Jersey

6

FMC Corp.

Chicago

Illinois

7

Eastman Kodak Co.

Kingsport

Tennessee

8

Air Products and Chemical Inc.

Allentown

Pennsylvania

9

ARCO Chemical Co.

Newtown Square

Pennsylvania

10

Ethyl Corp.

Richmond

Virginia

cryolite and as flux, particularly in the metallurgy, ceramic, and brazing industries for welding, glazing, and fabrication, respectively.

22.3.1.2 Wastewater Characterization

Generally, water used in the aluminum fluoride industry is employed as noncontact cooling water to cool the products coming out of the reactor. Water is equally used in the scrubber located in the plant to scrub the reacted gases before they are vented to the atmosphere. Wastewater resulting from the scrubbing process is often loaded with hydrofluoric acid, aluminum fluoride, aluminum oxide, and sulfuric acid. Wastewater is also generated in the plant housekeeping practices, which cover floor and equipment washings.

A typical plant production of aluminum fluoride indicating water use and wastewater generation is shown in the flow diagram (Figure 22.2).

Results of waste load found in verification sampling of unit product of aluminum fluoride are given in Table 22.6.

22.3.1.3 Wastewater Treatment Process

Copper, arsenic, chromium, and selenium are the major toxic pollutants associated with the production of aluminum fluoride. Selenium, on the other hand, is not regarded as process-related product because of its presence in the raw material. These pollutants are generally reduced by neutralization with lime followed by settling process in a series of settling ponds. The content of the last pond is given a final pH adjustment before being discharged into the environment or recycled to the plant as may be required (Figure 22.3).

Some innovating treatment technologies may be introduced in the treatment of wastewater generated in the aluminum fluoride industry to make its effluent safer. The ion exchange process can be applied to the clarified solution to remove copper and chromium. At a very low concentration, these two pollutants can be removed by xanthate precipitation.24 A combination of lime and ferric sulfate coagulation will effectively reduce arsenic concentration in the wastewater.

22.3.2 Chlor-Alkali

22.3.2.1 Description and Production Process

Chlorine, hydrogen, caustic soda, and sometimes caustic potash are coproducts of the electrolysis of saturated aqueous solutions of sodium chloride called brine. The overall chemical reaction is given as

The pulp and paper industry, the plastic industry, and water treatment plants are the major industries using chlorine in large quantity. Chlorine is also an essential raw material in the manufacture of vinyl chloride, chlorinated ethers, and other inorganic and organic chemicals. Chlorine is commonly produced in electrolytic cells where energy in the form of direct current is supplied to drive the reaction. The mercury cell process, the diaphragm cell process, and the membrane cell process are the three types of electrolytic process used for the manufacture of chlorine, caustic soda, and hydrogen from brine; however, mercury and diaphragm cells have large industrial application. Each electrolytic cell consists of two electrodes, anode and cathode, in contact with the electrolyte (brine solution). The method employed to separate and prevent the mixing of the chlorine gas and sodium hydroxide is what distinguishes one cell from another.25

22.3.2.1.1 Mercury Cell Process

The electrolyzer and the decomposer are the two main sections of a typical mercury cell. The electrolyzer is slightly inclined steel trough through which a thin layer of mercury flows over the

Water

Vent vo

Ki Ki

Water

Vent

FIGURE 22.2 A typical plant production of aluminum fluoride indicating water use and wastewater generation.

FIGURE 22.2 A typical plant production of aluminum fluoride indicating water use and wastewater generation.

TABLE 22.6

Summary of Waste Loadings Found in Aluminum Fluoride Verification Data

Pollutions

Antimony

Arsenic

Beryllium

Cadmium

Chromium

Copper

Lead

Nickel

Mercury

Selenium

Zinc

Maximum Waste Loadings (kg/Mg)

0.000005

0.002

0.000002

0.0002

0.005

0.001

0.0002

0.003

0.00005

0.002

0.002

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. Note: 1 kg/Mg = 1 kg/106 g.

Scrubber water

Treated water

FIGURE 22.3 General wastewater treatment process flow diagram at an aluminum fluoride plant.

Treated water

FIGURE 22.3 General wastewater treatment process flow diagram at an aluminum fluoride plant.

bottom to form the cathode of the cell. The saturated brine flows through the troughs above the mercury and parallel graphite or recently developed titanium-coated ruthenium. Titanium oxides form the anode on top of the brine.25 Electric energy flowing through the cell decomposes the brine to chlorine at the anode where it moves upward through gas extraction slits in the cell covers. The sodium ions are absorbed by the cathode to form an amalgam, a mixture of sodium and mercury, which is processed in decomposer cells to generate sodium hydroxide, hydrogen, and reusable mercury.

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4

22.3.21.2 Diaphragm Cell Process

The diaphragm cell consists of multiple electrolytic cells having the anode plates and cathodes mounted vertically and parallel to each other. The cathodes, often flat hollow perforated steel structures that are covered with asbestos fibers, serve as the diaphragm that prevents the mixing of hydrogen and chlorine and back diffusion of hydroxide (OH-) ions from the cathode to the anode. Brine fed into the cell is decomposed to approximately half of its original concentration to produce chlorine gas at the anode and hydrogen and sodium hydroxide at the cathode.

The chlorine gas is drawn off from above the anodes while the hydrogen from the top of the cathode is cooled to remove water and other impurities before being subjected to further use.26 The concentrated sodium hydroxide is settled and stored. The diaphragm cell process does not require a brine purge to prevent sulfate buildup or treatment to remove entrained chlorine gas that is peculiar to the mercury cell.26 Consumption of relatively low electricity and ability to process less pure brine are the advantages this process possesses over the mercury cell process. However, the chlorine gas from the diaphragm process is always contaminated with oxygen, water, salt, and sodium hydroxide, while the caustic soda, equally produced, contains chlorides and they must be processed further to bring them to a usable standard.27

22.3.21.3 Membrane Cell Process

This type of electrolytic cell consists of anodes and cathodes that are separated by a water impermeable ion-conducting membrane. Brine is fed through the anode where chlorine gas is generated and sodium hydroxide solution collects at the cathode. Chloride ions are prevented from migrating from the anode compartment to the cathode compartment by the membrane and this, consequently, leads to the production of sodium hydroxide, free of contaminants like salts. The condition of the membrane during operation requires more care. They must remain stable while being exposed to chlorine and strong caustic solution on either side; they must allow, also, the transport of sodium ions and not chloride ions.

The membrane cell produces a very pure caustic soda solution and consumes less energy unlike the mercury and diaphragm processes.24 Also, it poses less pollution risk to the environment unlike

TABLE 22.7

Main Characteristics of the Different Electrolysis Processes

Component

Cathode

Mercury Cell

Mercury flouring over steel

Diaphragm/membrane None

Anode Titanium with RuO2 or TiO2 coating

Cathode product Sodium amalgam

Decomposer/evaporator 50% NaOH and H2 from product decomposer

Electricity consumption 3300 kWh per ton Cl2

Diaphragm Cell

Membrane Cell

Steel or steel coated with Steel or nickel with a activated nickel nickel-based catalytic coating

Asbestos or polymer modified Ion exchange membrane asbestos

Titanium with RuO2 or TiO2 Titanium with RuO2 or TiO2

coating coating

10-12% NaOH with 15-17% 30-33% NaOH and H2 NaCl and H2

50% NaOH with 1% NaCl and 50% NaOH with very little solid salt from evaporation salt

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001, U.S. Environmental Protection Agency, Washington, DC, 1982.

the presence of mercury and asbestos pollutions in the case of mercury and diaphragm cells, respectively. However, like the diaphragm cell process, the chlorine gas produced must be further processed to remove oxygen and water vapor, while the caustic soda produced must be evaporated to increase its concentration. The process also requires very high-purity brine that invariably makes this process very expensive27 (Table 22.7).

22.3.2.2 Wastewater Characterization

General water use in this industry is for noncontact cooling, cell washings, tail gas scrubbing, equipment maintenance, and general area washdown. Wastewater streams from mercury cell facilities mainly come from the chlorine drying process, brine purge, floor sump, and cell. The tail gas is also water scrubbed; although often reused as brine, it contributes to the wastewater stream. The wastewater stream from the diaphragm cell facilities emanates from the borometric condenser during caustic soda evaporation, chlorine drying, and from purification of the salt recovered from the evaporators28 (Table 22.8).

TABLE 22.8

Summary of Contents of Waste Contents of the Three Cells Used in Chlor-Alkali Industry

Waste

Brine mud Cell room waste

Chlorine condensate

Sulfuric acid

Tail gas scrubber liquid

Caustic filter washdown

Source

Cell

Leak, spill, cell wash water

Graphite anodes cooler

Scrubber Scrubber Cell

Content

Mercury, other solids

Asbestos fibers, dissolved chlorine, dissolved hydrogen, sodium chloride lead, chlorinated organic compound Lead, chlorinated organic compound, such as methylene chloride and hexachlorinated benzenes Mercury, asbestos fibers, chlorinated hydrocarbons Hypochlorite

Mercury or asbestos fibers, dissolved salts

This wastewater stream contains lead (Pb) salts and chlorinated hydrocarbons generated from corrosion of the anodes as well as asbestos particles generated as a result of degradation of the diaphragm with use. Wastewater is also generated from the scrubber where the chlorine is wet scrubbed and from the ion exchange resin used to purify the brine solution. These wash water often contains dilute hydrochloric acid with small amounts of dissolved calcium magnesium and aluminum chloride. Like in other cells, the scrubber water also contributes to the wastewater stream.

Flow diagrams of a typical plant production of chlor-alkali indicating water use and wastewater generation are given in Figure 22.4.

Results of raw waste load found in verification sampling of unit product of chlor-alkali are given in Table 22.9.

22.3.2.3 Wastewater Treatment Process

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5).

Prominent among toxic pollutants found in the diaphragm cell are arsenic, chromium, copper, lead, nickel, and zinc, as shown in a typical verification sampling in Table 22.8. Chlorinated hydrocarbons are generated from corrosion of the anodes and reaction of the chlorine with process-exposed rubber. Most of the metals are removed by sulfide or carbon precipitation, while the asbestos is treated with a chemical and the resulting flocs are removed by filtration. The spent caustic solution is also neutralized using a chemical before being discharged. The chlorinated hydrocarbons are removed by the use of vacuum or a steam stripper and sometimes carbon adsorption (Figure 22.6).

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28

22.3.3 Chrome Pigments

22.3.3.1 Description and Production Process

Pigments are commercially classified according to their colors, but they are scientifically classified according to the inorganic compounds coming together as the base elements. A variety of chrome pigments are available in commercial quantities and they include chrome yellow, chrome orange, molybdate chrome orange, anhydrous and hydrous chromium oxide, zinc yellow, and iron blues, which are manufactured in different plants or the same plant within a factory. Chromium forms the base element for these types of chrome pigments. They are widely used in the production of paints, printing ink, floor covering products, paper, ceramics, cements, and asphalt roofing.

Soda ash

Brine purification

Scrubber H2

i

k

Cl2

Brine dechlorination

Legend Waste stream sampled

Collection tank

Mercury abatement

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