Introduction

Even though it appears to be in plentiful supply on the earth’s surface, water is a rare and precious commodity, and only an infinitesimal part of the earth’s water reserves (approximately 0.03%) constitutes the water resource which is available for human activities. The growth of the world’s population and industry has given rise to a constantly growing demand for water in proportion to the supply available, which remains constant. Thus, it is necessary to minimize its consumption and it is also necessary to return it back to the environment with the minimum contamination load because of the limited capacity of selfpurification, hence the importance of wastewater treatment process.

During the last two decades large scale environmental initiatives have taken place in Europe and the United States, these have resulted in strict environmental regulations on the industrial emissions for the chemical industry. It has been necessary to invest in cleaner technologies and in treatments that are more effective. On the other hand, numerous chemical companies have installed effluent treatment systems to meet the recently elaborated regulations of the country in which they are settled or to meet the regulations of the countries with which they trade.

The chemical industry comprises the companies that produce industrial chemicals. Basic chemicals or ‘‘commodity chemicals’’ are a broad chemical category including pharmaceutical products, polymers, bulk petrochemicals and intermediates, other derivatives and basic industrials, inorganic/organic chemicals, and fertilizers. The chemical industry is of importance in terms of its impact on the environment.

Chemical industrial wastewaters usually contain organic and inorganic matter in varying concentrations. Many materials in the chemical industry are toxic, mutagenic, carcinogenic or simply almost non-biodegradable. This means that the production wastewater also contains a wide range of substances that cannot be easily degraded. For instance, surfactant and petroleum hydrocarbons, among others chemical products that are being used in chemical industry reduce performance efficiency of many treatment unit operations.

The purpose of this review is to discuss the current wastewater treatment technologies in chemical industry. Because of the specificity of their waste waters, the chemical industry are required either improving the existing waste water treatment processes or developing combinations of various processes. This enables one to emerge with feasible treatment schemes targeting treatment of high strength wastewater.

Technologies to treat chemical industry effluents

In terms of wastewater treatment there are four classifications of treatment. Preliminary treatment involves the removal of large particles as well as solids found in the wastewater. The second classification is primary treatment, which involves the removal of organic and inorganic solids by means of a physical process, and the effluent produced is termed primary effluent. The third treatment is called secondary treatment; this is where suspended and residual organics and compounds are broken down. Secondary treatment involves biological (bacterial) degradation of undesired products. The fourth is tertiary treatment, normally a chemical process and very often including a residual disinfection.

Physico-chemical treatment

Oil –Water Separator–Treatment of oily effluent: Oil and grease (O&G) is a common pollutant in a wide range of chemical industries. Oil refineries, petrochemical plant, chemical plant, textile and food processing industries report high levels of oil and grease in their effluents (with an Oil and grease concentration up to 200,000 mg/l).

Regulations that govern the allowable discharge of oil and grease into municipal treatment plants and surface waters are becoming increasingly stringent.

New facilities are also subject to more stringent discharge limits than existing sources. For example, existing sources discharging produced water are required in the US to limit O&G levels to less than 48 mg/l as compared to new facilities which have to comply with a limit of 29 mg/l.

On the other hand, Oil and grease in wastewater can exist in several forms: free, dispersed or emulsified. The differences are based primarily on size. In an oil–water mixture, free oil is characterized with droplet sizes greater than 150 mm in size, dispersed oil has a size range of 20–150 mm and emulsified oil has droplets typically less than 20 mm. Oil and grease concentrations in wastewater as measured by the recommended test procedures of the US Environmental Protection Agency do not determine the presence of specific compounds, but groups of compounds based on their extractability by a particular solvent. Solvents that are commonly used are freon and hexane. Thus, the term ‘‘oil and grease’’ is fairly broad; it could include animal and vegetable source oils, fatty acids, petroleum hydrocarbons, surfactants, phenolic compounds, napthenic acids, etc.

Conventional approaches to treating oily wastewaters have included gravity separation and skimming, dissolved air flotation, de-emulsification, coagulation and flocculation. Gravity separation followed by skimming is effective in removing free oil from wastewater. Oil – water separators such as the API separator and its variations have found widespread acceptance as an effective, low cost, primary treatment step.

The API oil – water separator is designed to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute.

The API separator, however, is not effective in removing smaller oil droplets and emulsions. Oil that adheres to the surface of solid particles can be effectively removed by sedimentation in a primary clarifier. Dissolved air flotation (DAF) uses air to increase the buoyancy of smaller oil droplets and enhance separation. Emulsified oil in the DAF influent is removed by de-emulsification with chemicals, thermal energy or both. DAF units typically employ chemicals to promote coagulation and increase flock size to facilitate separation.

Emulsified oil in wastewater is usually pre-treated chemically to destabilize the emulsion followed by gravity separation. The wastewater is heated to reduce viscosity, accentuate density differences and weaken the interfacial films stabilizing the oil phase.

This is followed by acidification and addition of cationic polymer / alum to neutralize negative charge on oil droplets, followed by raising the pH to the alkaline region to induce flock formation of the inorganic salt. The resulting flock with the adsorbed oil is then separated, followed by sludge thickening and sludge dewatering.

Coagulation–flocculation: Most wastewater treatment plant includes sedimentation in their process. The sedimentation also called clarification is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity. Settled solids are removed as sludge, and floating solids are removed as scum. Wastewater leaves the sedimentation tank over an effluent weir to the next step of treatment. The efficiency or performance of the process is controlled by: retention time, temperature, tank design, and condition of the equipment. However, without coagulation/flocculation, sedimentation can remove only coarse suspended matter which will settle rapidly out of the water without the addition of chemicals. This type of sedimentationtypically takes place in a reservoir, sedimentation or clarification tank, at the beginning of the treatment process.

Coagulation-flocculation consists on the addition on the clarification tanks of chemical products that accelerate the sedimentation (coagulants). The coagulants are inorganic or organic compounds such as Aluminium sulphate, Aluminium Hydroxide chloride or high molecular weight cationic polymer. The purpose of the addition of coagulant is to remove almost 90% of the suspended solids from the wastewater at this stage in the treatment process.

Adsorption techniques to treat wastewater: Adsorption is a natural process by which molecules of a dissolved compound collect on and adhere to the surface of an adsorbent solid. Adsorption occurs when the attractive forces at the carbon surface overcome the attractive forces of the liquid.

Granular activated carbon is a particularly good adsorbent medium due to its high surface area to volume ratio. One gram of a typical commercial activated carbon will have a surface area equivalent to 1,000 square meters.

Granular activated carbon: The pollution of water resources due to the indiscriminate disposal of heavy metals has been causing worldwide concern for the last few decades. It is well known that some metals can have toxic or harmful effects on many forms of life. Metals, which are significantly toxic to human beings and ecological environments, include chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), manganese (Mn), cadmium (Cd), nickel (Ni), zinc (Zn) and iron (Fe), etc. This problem has received considerable amount of attention in recent years. One primarily concern is that marine animals which can readily absorb those heavy metals in wastewater and directly enter the human food chains present a high health risk to consumers. Wastewater from many industries such as metallurgical, tannery, chemical manufacturing, mining, battery manufacturing industries, etc. contains one or more of these toxic heavy metals. Industries carries out operations like electroplating, metal/surface finishing and solid-state wafer processing, generate wastewater contaminated with hazardous heavy metals. The concentrations of some of the toxic metals like Cr, Hg, Pb, As, etc. are higher than permissible discharge levels in these effluents. It, therefore, becomes necessary to remove these heavy metals from these wastewaters by an appropriate treatment before releasing them into the environment.

In view of the toxicity and in order to meet regulatory safe discharge standards, it is essential to remove heavy metals from wastewaters/ effluents before it is released into the environment. Conventional methods for the removal of heavy metals include precipitation, coagulation/flocculation, complexation/sequestration.

Application of above-mentioned methods becomes economically unviable for the removal of heavy metals at lower concentrations. Adsorptive treatment using non-conventional adsorbents, such as agricultural and industrial solids wastes, have been used for the removal of heavy metals [5–7]. A number of other materials have also been used to remove heavy metals from wastewater, such as peat, wool, silk, and water hyacinth. Many papers have appeared on preparation of activated carbon from cheaper and readily available materials.