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2.5.2 Industrial Effluent Treatment
Because domestic wastewater does not exhibit great variability in regard to its content
of organic and inorganic materials, its treatment is generally similar
throughout large regions. In contrast,
the content of industrial effluents varies from one industry to another, and
therefore requires varying treatment, based on the specific industry producing
it. The specifications for releasing
various types of industrial effluents into the public sewer network or to
surface watercourses is not only determined by the concentration of biological
oxygen demand (BOD), chemical oxygen demand (COD) and suspended solids, but
also by the content of organic and inorganic elements, which vary from country
to country. In general, therefore,
selecting a treatment method for industrial effluents depends primarily on the following factors:
- Identifying the various pollutants present in the effluent;
- Characterizing the effluent;
- Regulating the sewers and separating the waste streams;
- Selecting the treatment technology
based on the different available physical, chemical or biological treatment capabilities.
Industrial effluents (primarily from factories and plants) contain various materials,
depending on the industry. Some
effluents contain oils and grease, and some contain toxic materials (e.g.,
cyanide). Effluents from food and
beverage factories contain degradable organic pollutants. Some effluents contain, while others lack,
nutritious materials suitable for microbiological growth. Effluents of canned fruit and soda beverages,
for example, contain high percentages of sugar, very low percentages of
protein, and little nitrogen or phosphorus. Biological growth in these effluents, therefore,
is rather weak. Thus, every industrial effluent (and
pollutants) require a specific treatment technology.
In order to design a suitable method for treating an industrial effluent, the
following major parameters must be determined:
- Daily wastewater volume;
- Maximum and minimum water discharges;
- Chemical characteristics of the water used in the industry;
- Continuous and intermittent manufacturing stages;
- Intensity and periods of pollution peaks;
- Possibility of separating waste streams;
- Possibility to carry out local or partial treatment, or recycling;
- Probability of secondary pollution incidents,
even if slight or occasional, that can worsen treatment plant operation
(appearance of glues, fibers, oils, sand, etc.).
In general, when a treatment plant is designed, these
parameters must be determined through analysis of the different manufacturing
processes in the industrial facility, and comparison of the results with
information from other similar industrial facilities.
When a treatment plant is constructed for a factories, the
quantity of pollutants in the effluent must be precisely determined through
continuous and routine analysis of the water, and compared with chemicals used in the factory.
Despite the fact that every pollutant requires a certain
treatment technology, these different technologies include the following stages:
- Separate treatment – for the purpose of
separating materials that require special treatment.
This is important when the wastewater contains high
concentrations of BOD, COD, H2S, NH4 or poisonous
materials. It may be more costly to
dispose of the water than to treat it in certain ways;
- Preliminary treatment – useful for all
industries and important for food and agricultural industries. It includes (1) grit removal in some
cases (iron and steel foundries, rainwater and sandpits), (2) oil removal for
hydrocarbons and oils, and (3) equalization of liquid flow and pollutant load.
- Physical-chemical treatment -- this
treatment can be an intermediate or final stage, based on the type of
treatment. It is conducted for (1)
settling of poisonous minerals or salts, (2) removal of oils in emulsions, and
other suspended substances, (3) clarification and dilution of colloidal BOD and
COD concentrations (the pH has to be maintained in a realtively narrow range
depending on the nature of the used process, either settling, crystallization
or absorption), and (4) floatation units for oil and fiber removal.
The physical-chemical treatment is preceded, or followed, by other methods such as
(1) electric neutralization, (2) oxidation or reduction, and (3) degassing or stripping.
- Biological treatment -- the use of this
treatment depends on the biodegradable contents of the wastewater.
- Removal of non-biodegradable material
-- the biological purification process is considered on of the best methods for
reducing the BOD concentration and the COD resulting from decayed organic
compounds of different types (solvents, aromatic materials, hydrocarbons).
- Industrial sludge – although organic
sludge exists in some cases, the sludge is primarily inorganic in nature. Industrial wastewater contains large
quantities of sludge, resulting particularly from physical-chemical treatment
methods. All the methods used in
treating activated sludge from wastewater treatment plants can be sued in the
Some treatment processes for effluents from different industries are described herein.
- Treatment of Food Industy Effluents
Food industry effluents are generally characterized by a high content of
biodegradable pollutants. Suspended
substances constitute a large part of these pollutants, in addition to the high
BOD and COD concentrations of the effluents.
These effluents are treated with biological methods as a primary treatment.
The nitrogen and phosphorus concentrations
in the effluents also are reduced. The
removal of suspended substances is sometimes done via a settling process after
adding coagulation materials to the effluent.
- Treatment of Textile Industry Effluents
The pollutant content of textile industry effluents typically contains (1) natural
and artificial treated fibers, (2) dyes, and (3) used materials (especially coloring materials).
The resultant effluents are treated in two main stages, including (1)
chemical-physical treatment -- for removing suspended, coloring and dyeing
materials, neutralizing the water pH, and removing phenol by using activated
carbon after oxiding it with ozone, and (2) biological treatment – for removing
the remaining organic material.
- Treatment of Oil Refinery Effluents
The basic constituent of oil refinery wastewaters are hydrocarbons. Other materials include (1)
organic materials (phenol, alcohols), (2) sulfuric compounds (sulfide, mercaptan,
sulfate), (3) sodium salts, and (4) suspended solids (sand, etc.).
Treatment of the wastewater is done in two main stages, including (1) physical-chemical
treatment – for separating oils, adding coagulation material, and separating
the resulting deposits in the settling basins (this stage can remove between
70-90% of the oils in the effluent), (2) biological treatment – for biological
removal of the remaining oils. Because
of the toxicity of sulfate compounds to biological organisms, these compounds
are sometimes removed from the water before it enters the biological
ponds. The resulting sludge is dewatered and incinerated in special incinerators.
- Treatment of Metal Surface Coating Industry Effluents
Some materials are used to coat metals, primarily to prevent corrosion of the
metal. The water resulting from this
industry typically contains (1) high concentrations of toxic metals, and (2)
diluted rinse water.
The resulting wastewater is treated with physical-chemical methods. This treatment mode of considered
one of the most important, due to the toxic metals in the water that must be removed.
The effluents also exhibit a large
variability in regard to the chemical materials used during the manufacturing process.
Extent of Use
Industrial wastewater treatment has become an important issue in the countries of the West
Asia region, due primarily to the increasing environmental considerations that
are beginning to become a priority in the region.
Industries have long discharged their effluents into nearby river
courses, public sewers, fields or the sea, resulting in a number of
environmental and health problems. The
increasing environmental awareness has led to issuing of obligatory regulations
and definite specifications for effluent quality in virtually every country in
the region before it can be discharged into public sewers or surface
watercourses. Further, within the
framework of rational water use in industry, particularly to reduce water
losses, technological advancements have facilitated the reuse of
properly-treated industrial drainage water.
Thermal plants for power generation are considered the largest use of
such treated water, requiring about 2 liters/kilowatt hour of generated
power. Considering the large quantity
of electricity produced, it is estimated that millions of cubic meters of
treated water are used for this purpose.
It is noted that treated effluent reuse has received high attention in most of the
countries in the West Asia region that have endorsed it. They also have endorsed the World Health
Organization specifications for determination of the quality of industrial
wastewater effluents that can safetly be discharged into public sewer systems.
Operation and Maintenance
In contrast to domestic wastewater, the chemical and biological content of
industrial effluents varies on the basis of the industry producing them. Thus, the associated operation
and maintenance problems of the associated treatment facilities varies
accordingly. The treatment technology
for the leather industry effluents, for example, is more complicated than for
other industries, due primarily to the large water volumes used in this
industry (approximately 5 m3/100 kg of dewatered leather). The wastewater from this industry
is very polluted, containing proteins, fats, hair remains, wool, toxic materials
(sulfide, chrome), and high BOD concentrations (reaching up to 700-900 mg/L),
thereby being more complicated to treat, for example, than effluents from the
beverage industry. Thus, treating
leather industry effluents requires skilled technicians to operate and maintain
the required treatment technology.
In all cases, it is noted that as an indication of proper operation and
maintenance, the treated industrial drainage water must comply with certain
conditions to be released into the public sewer network, surface watercourses,
or the sea, including (1) minimum concentration of BOD, and (2) little, or no,
Discharge of industrial effuents into public sewer systems also requires the water to
have several additional characteristics, including (1) does not affect the
safety or health of sewer network workers, (2) does not affect the sewer pipes,
and (3) does not affect the treatment processes in water treatment plants.
In addition, the required specifications of industrial effluents released into
public sewers include the following:
- BOD concentration not exceeding approximately 20 mg/L;
- Suspended solids concentration not exceeding approximately 600 mg/L;
- No release of grease, oils or nitrogen
compounds (due to their impacts on the treatment process);
- No release of explosives or toxic
materials (due to their potential impaccts on the safety and health of the sewer network workers;
- Sulfate concentration should be less
than 100 mg/L, and the pH value should be between 6-10 (to protect the sewer pipes).
It is noted that a continuous monitoring program is necessary to ensure the
quality of the treated water.