Treatment of Industrial Wastewater

Abstract

In industrialized countries, there is a great diversity of the specificities of manufacturing processes and means used for industrial wastewater treatment. The developments that these processing problems can allow are immense, given the many production sectors. Faced with this situation, we have developed in this work the essential ideas concerning the problems of the treatment of very particular effluents from industrial establishments, the conditions to be met by the discharges and the different treatment methods: primary, secondary and tertiary. Examination of these types of treatment allowed us to divide industrial effluents into four categories.

1. Introduction

In a number of regions, industrialization is developing which requires a call for labor and the formation of large groups of people, at the same time many buildings, houses, shops, and factories are being built [1]. The resulting discharges of domestic and industrial wastewater are abundant and the region is therefore naturally affected by pollution. The gross pollution produced by industry must be equivalent to a population of 90 million. It is currently estimated that 1/5th of the industrial water is connected to public networks, the 4/5th being isolated and probably treated inside the plant [2].

Collective sewerage is the preferable solution, as far as possible, for urban wastewater and, under certain conditions, for industrial effluents, which by their qualitative characteristics, are likely to transit through the public network [3]. All mixed water can then be treated by the community’s wastewater treatment plant. It is then said that this is a mixed treatment [4].

First, we must be aware that industrial development is changing rapidly in terms of quality and quantity. Attention should be paid to the quality characteristics of industrial effluents and their nature by classifying wastewater and waste liquids:

• Wastewater is used for washing appliances and machines, refrigeration…; may contain chemicals;

• Waste liquids result from manufacturing, such as acidic substances from emptying metal pickling tanks or toxic chemical solutions.

Wastewater can often be treated more easily than waste liquids, however, these can contain interesting products to recover. The growing interest in the recovery of industrial waste and by-products is linked both to the energy crisis, the reduction of the world’s raw material resources, and for many countries the need for increasingly expensive imports, and finally the legislation, which is becoming very strict concerning the protection of nature and the environment [5].

Each production sector can work to reduce pollution and optimize waste. To this end, it can:

• In particular, recovering settling sludge with a view to saving energy, composting or recycling raw materials;

• Detoxify certain sludge materials through incineration or biodegradation.

2. Source of effluent

Effluent from mills is of two kinds:

• Domestic effluents including storm water and water from the sanitary services and canteens of the establishment;

• Effluents of a clearly industrial nature originating from workshops and due to the plant’s own activity are wastewater of different origin requiring either specific treatment, mixing, or recycling (mineral washing water, cooling water, etc.).

Public sewers under the same conditions must receive the former as water of identical nature from the other funds of the agglomeration; of course, this presupposes the existence within the establishment of a separative network.

The second part of the effluent can be received in the sewer system under certain conditions, pre-treatment, with the agreement of the community and sanitary services or be refused in the public network.

3. Conditions to be met by wastewater discharges

3.1 Evacuation routes

There are various ways of discharging this wastewater, including:

• Public sewerage networks provided downstream of a treatment plant;

• Natural environments used directly or through a public sewerage system without a treatment plant;

• Specially constructed engineered absorbent wells (filter wells);

• Soil used by application under certain conditions for natural purification, but does not include deep geological layers.

Releases of industrial effluents to the natural environment must meet the requirements summarized in Table 1.

Discharge of industrial effluents into the natural environment	1st case: Discharges far from city water supply intakes, beaches, shell beds			2nd case: Discharges near the water intakes of town beaches, bank of shells
	Industrial pollution-load
	Weak	Important	Preponderant
a1) pH-General case	5,5 < pH <8,5	5,5 < pH <8,5	5,5 < pH <8,5	5,5 < pH <8,5
a2) pH in the case of lime neutralization	5,5 < pH <9,5	5,5 < pH <9,5	5,5 < pH <9,5	5,5 < pH <9,5
b) Maximum temperature	30°C	30°C	30°C	30°C
c) Cyclic hydroxyl compounds and their halogenated derivatives	Prohibited	Prohibited	Prohibited	Prohibited
d) Substances likely to promote the manifestation of odors, flavors and abnormal coloring in natural waters used for human consumption	Prohibited	Prohibited	Prohibited	Prohibited
e) Substances liable to ignite	Prohibited	Prohibited	Prohibited	Prohibited
f) Total suspended solids	100 mg/L	50 mg/L	30 mg/L	30 mg/L
g) Substances capable of causing the destruction of fish downstream of the discharge	Prohibited	Prohibited	Prohibited	Prohibited
h) Radiation hazard substances	Prohibited	Prohibited	Prohibited	Prohibited
i) Additional requirements for physical criteria: Conductivity Decantable materials
j) BOD5	200 mg/L	100 mg/L	40 mg/L	40 mg/L
k) Total Kjeldahl Nitrogen (TKN)	6 mg/L in N; 80 mg/L in NH4	30 mg/L in N; 40 mg/L in NH4	10 mg/L in N; 15 mg/L in NH4	10 mg/L in N; 15 mg/L in NH4
l) Additional requirements for chemical criteria: Dissolved oxygen, oxidability to potassium permanganate, Phosphates
m) Dilution d (ratio spill in 24 h between the flow of the watercourse and the flow of the effluent) according to the duration of the industrial discharge spill in 10 h	d ˃ 300; d ˂ 720	150 ˂ d ˂ 300; 360 ˂ d ˂ 720	d ˂ 150; d ˃ 360

Table 1.

Requirements for discharges of industrial effluents to the natural environment.

3.2 Discharge requirements

According to the general requirements, it is stipulated that in all cases, liquid industrial discharges must have the following characteristics [6]:

1. pH between 5,5 and 8,5 (if the effluent is neutralized by lime, the pH may be between 5,5 and 9,5);

2. Temperature less than or at most equal to 30°C;

3. Absence of hydroxylated cyclic compounds from the phenol series and their halogen derivatives;

4. Absence of any coloring substance or substance likely to give off foul odors and abnormal flavors;

5. Absence of any substance likely to ignite directly or indirectly after mixing with other effluents, or to release toxic vapors;

6. Absence of floating materials or any substance harmful to the proper functioning of the evacuation and treatment works;

7. No substances that may affect aquatic wildlife downstream of the point of discharge;

8. No risk of radiation to the neighborhood;

9. There are also additional requirements for physical criteria, such as conductivity, decantable materials in cm³/L in 2 h;

10. Chemical or biochemical criteria such as nitrogen and dissolved oxygen in mg/L, BOD₅ in mg/L, and oxidability to potassium permanganate in mg/L.

It should be added that some wastewater has high concentrations of phosphates. Phosphorus removal is essential to prevent eutrophication of lakes.

4. Industrial wastewater treatment methods

From the point of view of terminology, we say “purification” when it comes to domestic or rainwater wastewater, but we say “treatment” when dealing with industrial wastewater; these two words each represent a particular specialization.

Since industrial wastewater does not have the characteristics of a domestic-dominated effluent, it is often necessary to impose pre-treatments or treatments on manufacturers. For this wastewater, after settling for 2 hours:

• COD greater than 750 mg/L;

• COD/BOD₅ report 2,5;

• Suspended solids ˃ 300 mg/L.

The processes of pretreatment and treatment of industrial wastewater have a large number of points in common with urban water treatment processes. Thus, the techniques used in the treatment of industrial effluents obviously include those used in urban wastewater, but they also include processes that are more specific to them. The latter processes include settling, sieving, flocculation, flotation, electrocoagulation, ion exchange, lagooning [7], physicochemical processes [8], and reverse osmosis.

All these considerations lead to consider two treatment hypotheses:

• The first is that of mixed treatment, that is to say that industrial effluents are discharged into an urban wastewater network and that an appropriate pre-treatment deemed essential; very often, industrial waste effluents and urban wastewater are located in the same area, at first glance, it may be interesting to carry out their respective purification using a common structure. But the problem is not so obvious for several technical, operational and financial reasons that are interpreted at the level of reciprocal responsibilities. The operation of the public wastewater treatment plant must remain as permanent as it was initially, so the contribution of the industrial effluent must not profoundly upset the nature of the wastewater to be purified.

• The second is the case of a major industrial establishment discharging its effluents directly into the natural environment, treatment is ordered by the health regulations of the department and the municipality.

Each of these two eventualities must be examined [9]. The definition of any treatment must take into account [10]:

• From the drainage system of public sewers and the possible separation of effluents, it is often useful to isolate certain effluents, this is necessary when the flow of industrial wastewater contains a high concentration of toxic materials;

• The nature of the effluents, that is to say the daily volume, the minimum and maximum hourly flow rate, the continuous or discontinuous manufacture of plant products, the extent and periodicity of the pollution peaks, and the possibility of separation of the discharge circuits;

• Of the nature of all pollutants, without forgetting one, otherwise the proper functioning of the station will be disrupted;

• Optimum choice between the various treatment systems.

4.1 Pretreatment (case of mixed treatment) homogenization basins

If it is accepted that the treatment of urban wastewater is generally satisfactory through the application of biological processes, the treatment of the mixture must be possible without complications for the municipality. It is then necessary to ensure that the extreme characteristics of the effluents are known and respected and that the dissolved oxygen (O₂) content is always sufficient. It is also essential to specify the dilution rate of domestic wastewater and industrial water, in order to regulate flows, by creating “homogenization basins”. It is also essential that the normal operation of the public network and treatment plant is not disrupted.

Physical pretreatment devices such as desalination, screening, sieving, degreasing or de-oiling can also be installed in the plant.

It is also possible to clarify raw water pH between 6,5 and 8,5 to reduce BOD₅ and COD. Prior neutralization by lime or NaOH soda is essential if the pH of the raw water is outside the 6–9 interval. De-oiling is very important because the greases reduce the water-air exchanges necessary for biological purification and clog the pipes.

In some cases, the elimination of toxic substances by flocculation-oxidation processes by chemical means is required.

It is reasonable to note that the implementation of the joint treatment of industrial effluents and urban wastewater only seems possible if there is a good understanding between the industrialist and the communal community.

4.2 Treatment processes specific to industrial effluents (case of release into the natural environment)

As in the case of urban waste water, these special treatments for industrial waste water make it possible to separate, on the one hand, the treated water and discharged into the natural environment and, on the other hand, to collect a certain number of wastes in the form of sludge. This design can be modified if the manufacturer intends to recover a significant number of reusable products. It is therefore understandable that this situation justifies the diversity of specific treatment processes for industrial waste effluents [11].

4.2.1 Preliminary treatments

They include:

• Screening, desalination by settling in ponds or by centrifugation or by cyclonation;

• Degreasing using the grease separator;

• The de-oiling by means of oil interceptors or by flotation, the latter process consisting of producing, in an effluent containing fine particles of oil in suspension, an upward flow of gaseous bubbles capable of capturing the particles in passing, and drive them to the surface where they are mechanically skimmed. Flotation can be improved in some cases by the development of commonly applied aeroflotation or electro-flotation; in the latter process, gaseous bubbles are produced by the electrolysis of effluent [12].

4.2.2 Primary treatment

These processes are available:

• Settling by gravity with or without coagulation and flocculation, and settling by separation by overflow in an upward circulation of the effluent;

• Effluent electrolysis flotation (electroflotation);

• Sieving by passing water through sieves with more or less fine meshes;

• Centrifugation with coagulation of suspended matter;

• The so-called stripping process for the elimination of gases (phenol, nitrogen) and the elimination of odors;

• Neutralization of too acidic or too alkaline waste water, chemically (lime or sulfuric acid, soda);

• Physical and chemical elimination of SS (coagulation-flocculation).

4.2.3 Secondary treatments

These include, but are not limited to:

• Chemical processes that make it possible to obtain recoverable substances; physico-chemical treatments, such as coagulation-flocculation, seem by far the most efficient processes with a simple implementation, they eliminate a multitude of substances, but they represent high reactive expenditure;

• Natural biological processes such as spraying on permeable soils, aerated or not;

• Artificial biological processes by high-load percolating bacterial beds and, in some cases, activated sludge processes (Figure 1), biological disks, oxidation channel, and stabilization basin are all means to be used within the limits of the nature of the effluents.

Figure 1.

Schematic of an industrial wastewater treatment plant using activated sludge.

4.2.4 Tertiary treatments

Tertiary treatments, which are also called advanced purification or finishing purification, are a complement of wastewater treatment to regenerate these waters and adapt their new qualities to the use that we want to do. In any case, tertiary treatment plant must use as a source of water to treat wastewater after preliminary, primary and secondary treatments. On average, the three successive treatments above eliminated 80–98% of biochemical oxygen demand (BOD₅) and suspended matter [13]. They have reduced, but not eliminated, microbial contamination. However, they left many mineral salts and did not eliminate many undesirable substances.

It should be noted that approximately one secondary effluent has the following characteristics (Table 2):

COD	From 50 to 150 mg/L
Total organic carbon	From 20 to 50 mg/L
Azote (in NH3)	From 5 to 30 mg/L
Color	pale straw
Odor	variable
Turbidity	From 30 to 60 Jackson units

Table 2.

Characteristics of a secondary effluent.

Among the tertiary treatment processes likely to be used are:

1. Filtration on materials such as clinker, gravel and sand and possibly also active coals, with the aim of reducing suspended matter, avoiding clogging: filtration is recommended for better adaptation of groundwater recharge;

2. Chemical, physical or biological purification of the following processes:

   • Lime precipitation;

   • Artificial aeration;

   • Lagoon systems, where available land permits;

   • Finally sterilization for the rapid destruction of pathogenic microorganisms, especially if the effluents come from hospitals or slaughterhouses.

5. Technological advancements

5.1 Possibility of wastewater treatment using pure or oxygen-enriched air

Until recent years, atmospheric air has been the only source of oxygenation used directly in biological wastewater treatment plants. In fact, if water is brought into contact with a large quantity of bacteria, the degradation of organic matter is rapid; but, at the same time, the demand for oxygen increases considerably.

Therefore, a closed biological reactor fed with pure oxygen allows a very high purification efficiency. The dissolution energy expenditure is considerably reduced compared to the conventional provisions. The residence time in the oxygen activation tank is reduced to 1,33 h. The elimination rate of BOD5 is 88–94%, which of COD reaches 84% and that of suspended matter: 97%.

As for the purified effluent, the BOD5 decreases to 14 mg/L; the COD does not reach 90 mg/L and the suspended solids do not exceed 12 to 15 mg/L; it is easy to understand that all this is important from the point of view of the protection of the receiving environment.

Of course, if the supply of atmospheric oxygen is free, it must be agreed that the use of pure oxygen is expensive. The essential advantages of using pure oxygen are not sought economically, but much more in terms of flexibility, operational safety and ease of operation. Environmental stresses are very favorable to processes using pure oxygen.

5.2 Other possible solutions

Possible solutions involve types of treatment that can be classified as follows:

• Physical: settling, filtration, sieving, micro-filtration;

• Chemicals: lime-based, flocculation, nitrogen and phosphorus extraction;

• Biological: lagoon, activated sludge, discharge to soil;

• Bacteriological: by use of chlorine, chlorine gas, chlorine dioxide, chlorine bromine mixture, or ozone, by adsorption using activated charcoal or fly ash and by ultraviolet radiation.

These difficult and expensive types of treatment are currently implemented, in very specific cases. But the principle of water regeneration can be applied on a large scale, if one wishes to ensure better protection of the natural environment, or reduce the prohibitive supply of water to the inhabitants of a city, when this work requires the costly construction of a very long network.

6. Industrial sludge

Industrial sludge is most often predominantly mineral, but in some industrial branches, it is predominantly organic. Urban sewage sludge treatment does not apply to industrial sludge with some special modifications adapted to thickening or mechanical dewatering [14].

Sludge thickening is practiced by settling and very variable loads of 50 to 700 kg of solids per m² and decimeter thickness are often encountered.

Sludge dehydration does not justify thermal conditioning, which is why chemical conditioning is used using calcium carbonate, synthetic polyelectrolytes or simply sawdust. Filter strips [15] are suitable for the proper dewatering of carbonate sludge or fibrous materials.

Sludge from physicochemical purification is more abundant than sludge from biological purification and can be used as follows:

• Non-toxic mineral sludge may be spread outdoors on land;

• Toxic mineral sludge must be stored in controlled landfills on impermeable waterproofed soils (corroi of clay in specialized centers);

• Fermentable organic sludge, must be stabilized before spreading or incinerated;

• Oil-rich sludge is incinerated;

• Finally, sludge can be taken back for recovery of raw materials if possible.

7. Areas for future research and potential advancements

7.1 Problems and composition of sludge

The effluents admitted to a wastewater treatment plant emerge mainly as sludge, organic or mineral pollution (dissolved or suspended in purified water) and various gases.

Domestic and industrial wastewater treatment technology results in the production of large quantities of sludge during the various phases of purification and primary and secondary treatment.

Sludge production is related to the pollution load of the raw water and the proper functioning of the plant and the treatment system; it is recognized that the activated sludge process generally results in high sludge production.

Today, we must consider that the disposal of sludge must be thought and resolved within the framework of the recovery of urban waste, which allows the reduction of expenses. Indeed, sludge treatment represents an investment of 30 to 50% of the construction cost of the wastewater treatment plant with very significant operating expenses (50% on average).

The primary and secondary sludge in mixture most often contain 95% water and sometimes up to 98%. Translated into dry sludge, 0.8 kg per kg of BOD5 was eliminated (0.3 kg for primary sludge and 0.5 kg for secondary sludge). In other words, the dry matter content is in the range of 40 to 60 kg per m3 of sludge.

7.2 Sewage sludge incineration and energy recovery

Organic matter forms the calorific value of sewage sludge. The lower calorific value is 4000–4500 kcal/kg organic matter for fresh sludge and 3500–4000 kcal/kg for digested sludge. The main purpose of incineration of urban or industrial sludge is the elimination of organic components by oxidation, and this is therefore valid from a certain organic matter content.

Thus, given the evaporation of the residual water contained in the sludge, it appears that this process of operation is interesting if:

• In assets, sufficient useful heat is available in the fuel materials;

• Passively, the heat input required for water evaporation is expected.

In other words, the self-combustion of sludge depends on the drying of the cakes and the organic matter content.

The self-burning limit is between 60 and 70% humidity. It should be added that to prevent the release of intolerable odor-producing volatile matter into the fumes released into the atmosphere, combustion must be increased to 800°C. Of course, at such temperatures, there is destruction of all pathogens.

7.3 Agricultural sludge recovery by composting

One of the main items of expenditure on wastewater treatment is the disposal of sludge produced.

The mixed treatment of pre-hydrated sludge carried out simultaneously, with the composting of household waste (atmospheric composting or accelerated composting in reactors) constitutes an interesting solution from the point of view of national and agricultural economy, because it contributes to the contribution of humogenic product to the land, in the same way, and even better than conventional manure. This common composting method therefore allows substantial savings for public authorities.

The main difficulty lies in the high water content of the sludge; the humidity of the sludge-garbage mixture ensures the best aerobic fermentation must not exceed 50 to 55%. This handicap can be overcome by using sludge free of excess water through special filters before adding it to the garbage.

7.4 Trend for agricultural sludge recovery in liquid form

At the outlet of the sewage works, the sludge is in a very liquid form containing about 1 to 2% dry matter. In this state, the sludge is spread on the agricultural land by means of tanker truck or per ton slurry and a group of pump motor.

However, it should be noted that good soil suitability for spreading often occurs during a short period of the year, in autumn, which does not always allow good environmental protection.

In order to be successful in the disposal of sewage sludge by agricultural spreading, it is necessary to know the following data:

• The origin, composition, and volume of sludge to be disposed of on agricultural land;

• Terrain, soil and subsoil characteristics, and the presence of groundwater;

• Regional rainfall patterns and climate;

• The nature of the crops and the needs of the fertilizer elements;

• The study of odor inconveniences in the vicinity of populated areas and roads;

• Finally, the implementation of information for farmers who may be interested in the project.

When these studies of evaluation of practical and economic conditions are positive, it is permissible to say that the sludge disposal system by spreading is interesting for the municipality and for the farmers.

8. Industrial pollution elements and treatment methods

Table 3 shows lists of some most important elements of industrial pollution and the appropriate treatment methods.

Polluting elements	Modes of treatment
Fats, oils, tar, sand	Insoluble but decantable materials
Fibers, suspended solids	Physical, by flocculation of raw water or reverse osmosis
Biodegradable elements	Biological treatments by aeration
Toxic elements, phosphates, sulphides	Treatments by precipitation or physico-chemical
Dyes, detergents	Treatment by adsorption
Alcohol, phenols, H2S	Stripping treatment
Acid salts and bases	Ion Exchange Processing

Table 3.

Polluting elements and appropriate treatment methods.

9. Types of industries and appropriate treatments

Industrial wastewater is so diverse that specialists have agreed to draw up a list of very characteristic industries grouped by pollution analogy, which makes it possible to gather the means of treatment of these particular effluents. Table 4 provides information on some industries [16, 17].

Industries	Effluent characteristics	Appropriate types of treatment
Agricultural and food industries	Significant and biodegradable pollution; Releases variable over time; Rapidly fermentable substances; Substances causing clogging; Tendency to acidification or organic dominant	Essential preliminary treatments: screening, sieving, desalination, degreasing; Provide for homogenization basins; High-efficiency biological purification processes using selected bacteria; Low-load cleaning processes
Manufacture of paper and paperboard	Cellulose pulp feedstocks are biodegradable substances; Waste leaching is very polluting and contains chemicals that are not very biodegradable (carbonates, sulphides) or toxic (phenols, cyanides). High water consumption requires intensive recycling	Preliminary treatment: screening, screening, flotation, primary and secondary settling; Acid neutralization; Activated sludge treatment; Natural biological secondary treatment in stabilization ponds with aeration by floating turbines; Tertiary treatment in lagoon
Iron and steel industries	Highly polluting effluents by separable suspended solids, but current technical developments tend to result in significant quantities of dissolved mineral substances and chemical materials (carbonates, sulphides) or toxic (phenols, cyanides). The very high consumption requires intense recycling.	Primary treatments with settling; Secondary treatments mainly chemical (neutralization) and decyanurization with chlorine for the elimination of toxic materials; Treatment focuses on pollution prevention and product recovery
Textile industries (natural or artificial fibers)	Wastewater receives chemical products (soaps, acids, lime carbonates, soda sulphate) from oxidants (chlorine, hydrogen peroxide), organic matter (starch, albumin), detergents, toxics; The multitude of washes promotes dilution.	Preliminary treatments essential by sieving, Primary treatments (settling, degreasing); Chemical secondary treatments most often and biologically in some cases; Stabilize pH changes; Interesting combination of physicochemical and biological processes; For artificial fibers (viscose), the treatment corresponds to that of a chemical plant effluent.
Indications on some industries (suite)
Automotive and surface treatment industries	Currently, the pollutants are quite well known, they are made up of dissolved mineral substances, by organic bodies in emulsion or in solution, more or less biodegradable. Effluents are often recycled.	Primary treatments for the demineralization of heavy substances; Physico-chemical treatments; Chemical neutralization; oxidation of cyanide compounds; De-oiling, flocculation, flotation, ion exchangers. Organic products can be fixed by adsorbents such as activated charcoal.
Refining and petrochemicals	Oil pollution is considerable. Petroleum refinery water is either manufacturing water (waste liquids) or cooling water (wastewater).; They are loaded with oil that can be partially eliminated physically—chemically. They are toxic and have a penetrating odor. They contain more or less emulsified oils and hydrogen sulphide, as well as detergents.	The multiplicity of substances likely to be encountered in these waters requires treatment in two or three stages, depending on the nature and concentration of the pollutants.; Preliminary screening and de-oiling treatments; Primary flocculation accelerated settling treatment; Removal of smelly gases by stripping; Secondary treatment by biological dephenolization, bacterial beds filled with plastic materials, activated sludge with high load; Tertiary treatment by filtration on activated coal, sludge settling, ozonation; Aerated lagoon in basins equipped with aeration turbines.
Radioactive effluents	Liquid effluents are only a small part of radioactive waste.; They include cooling water from radio-active ore (uranium) extraction and processing facilities and nuclear reactors.; The pollution generated is very particular and dangerous. It manifests itself in two forms: irradiation (radiation exposure) and contamination (fixation of radio-elements on living tissue).	Radioactivity cannot be destroyed; Treatment consists of limiting the doses corresponding to the limits not to be exceeded, to be sure not to harm human health; In plants where radio elements are handled or used, liquid effluents must first be divided into separate networks according to their concentration of radioactive substances; Precipitates are formed giving sludge; the adsorbent power of charcoal is also used; ion exchangers such as clay or synthetic resins are also used. Decontamination of radioactive materials involves solid residues and liquid products; It is possible, under certain conditions, to incinerate radioactive sludge, otherwise, they are kept in reinforced concrete crates

Table 4.

Indications on some industries.

The examination of the appropriate types of treatment in Table 4 divides industrial effluents into four categories:

Category 1: Industrial effluents of a dominant organic nature make biological purification possible; this mainly concerns the agricultural and food industries.

Category 2: Industrial effluents likely to be purified by the biological sector, but containing toxic products; this is the case of the leather industry.

Category 3: industrial effluents whose treatment can be carried out by physical or chemical treatment, but can be improved by biological purification, this is the case of effluents from the textile industry.

Category 4: industrial effluents of no interest in biological purification; this is the case for the coal, refining and petrochemical industries, the automotive and surface treatment industries, as well as the steel and metallurgical industries and establishments dealing with radioactive liquid elements.

10. Conclusion

The specificity of industrial manufacturing requires to note today the increasing importance of toxic materials contained in the discharges of industrial effluents, from the agricultural and food industries as well as the refining and petrochemical industries (Table 4).

However, these toxic materials, very diverse, are excessively dangerous for the natural environment and difficult to treat.

This is why the treatment methods to be implemented make the application of specific techniques of organic and mineral chemistry adapted to each manufacturing process, which justifies the reason why these treatment plants, very particular to industrial effluents, are integrated into the manufacturing plant itself.