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United Nations Environment Programme
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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augmentation
in West Asia>


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.

Technology Description

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 industrial context.

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, suspended solids.

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.

 

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