Treatment of Industrial Wastewater
Treatment of Industrial Wastewater
Water is used in the industry for process needs, for cooling, for steam generation, for dust suppression, and several other uses. Industrial wastewater also sometimes called effluent is the aqueous discard which results after the water is used for the above purposes. Industrial wastewater is the result of different substances which get dissolved or suspended in water during its used.
Industrial wastewater means the water or liquid which carries waste from industrial processes. It is distinct from domestic wastewater. It can result from any process or activity of industry which uses water as a reactant or for transportation of heat or materials. Industrial wastewater frequently presents physico-chemical characteristics which need treatment before its release to the environment or the sewer system. Further, a number of different industrial activities contribute to emissions of heavy metals, with the majority of industrial releases originating from metal processing facilities (iron and steel, and non-ferrous metals production). The pressure on the environment from industrial activities is highest where there are large-scale individual chemical or metal manufacturing sites, power plants, or clusters of facilities with relatively small emissions.
Industrial wastewater is a complex area and it cannot be simply characterized. Different industrial activities generate very different types and quantities of effluents. Typical domestic wastewater contains, primarily, organic content. Organic content can be measured with several accepted metrics, namely total organic carbon (TOC), chemical oxygen demand (COD), or biochemical oxygen demand (BOD). In addition to that, urban wastewaters contain nitrogen and phosphorus (the majority as part of the organic matter) and dissolved salts (mostly chlorides). In contrast, industrial wastewaters are much more varied. Some industrial wastewaters are similar to a typical urban effluent, but normally the concentration levels and the substances present in industrial wastewaters are different from those of urban wastewater. Industrial wastewater can hamper the integrity of the collection system by reducing its life-span (e.g., by corrosion, acids, and alkalis) and by causing clogging of the transportation pipes. Industrial wastewater can also impact the mechanical elements of the wastewater treatment plant.
Until the mid of 18th century, water pollution was essentially limited to small, localized areas. Then came the Industrial Revolution, the development of the internal combustion engine, and the petroleum-fuelled explosion of the chemical industry. With the rapid development of different industries, a huge quantity of fresh water is used as a raw material, as a means of production (process water), and for transportation of heat or materials. Several kinds of raw materials, intermediate products, and wastes are brought into the water when water passes through the industrial process. Hence, in fact the wastewater is an ‘essential by-product’ of modern industry, and it plays a major role as a pollution source in the pollution of water environment.
Industrial wastewaters have very varied compositions depending on the type of industry and materials processed. Some of these wastewaters can be organically very strong, easily bio-degradable, largely inorganic, or potentially inhibitory. This means TSS (total suspended solid), BOD and COD values can be in the tens of thousands of milligrams per litre.
Because of the very high organic concentrations, industrial wastewaters can also be severely nutrients deficient. Unlike sewage, pH values well beyond the range of 6 to 8 are also frequently encountered. Such wastewaters can also be associated with high concentrations of dissolved metal salts. The flow pattern of industrial wastewater streams can be very different from that of domestic sewage since the former is influenced by the nature of the operations within a production plant rather than the normal activities encountered in the domestic setting.
Wastewater effluents from typical industrial facilities are classified as (i) industrial effluents with organic compounds, (ii) industrial effluents with organic and inorganic compounds, (iii) industrial effluents containing inorganic compounds, (iv) industrial effluents containing suspended solids, and (v) industrial effluents from activities with heating, cooling, and refrigeration systems from thermal power plants and nuclear power plants. Industrial wastewater contains several constituents as described below and hence the wastewater needs proper treatment.
Suspended solids are the contaminants which are visible to the naked eye and can normally be filtered out of the water using common filter paper. Although there is no hard and fast definition, suspended solids tend to be larger than 1 micrometer to 2 micrometers in size. If the water is left to stand without disturbing it, the suspended solids settle to the bottom of the container over time.
Dissolved solids are contaminants which are not visible to the naked eye and cannot be removed from the water by filtration. The dissolved solids are defined as the materials which are normally smaller than 0.45 micrometers in size. The dissolved solids are either normally soluble substances or inversely soluble substances. The normally soluble substances are those materials which become more soluble with increasing temperature. The inversely soluble substances normally referred to as ‘hardness’ ions and are normally limited to salts of calcium, magnesium, strontium, and barium in water treatment. These materials become less soluble as temperatures increase. These materials form scale on the hot surfaces of boiler or heat exchanger tubes.
Colloidal solids are solids which are not quite small enough to be considered dissolved but not quite large enough to be considered suspended solids. Normally colloidal materials appear as a haze in the water, and it is not be possible to see distinct particles with the naked eye. Colloidal materials are typically within the size range of around 0.45 micrometers to around 2 micrometers. Colloidal solids do not settle out from the water since they are so small that they are greatly affected by their ionic surface charges. A colloidal suspension in water is said to be a stable suspension.
Colour is a type of colloidal suspension. Organic molecules which contribute colour to raw surface water are simply macro-molecules which fall into the smaller colloidal size range. In water, these macro-molecules take on an ionic surface charge which stabilizes them and they cannot settle out.
BOD is a measure of the quantity of oxygen which is consumed by bacteria during the decomposition of organic matter. Having a safe BOD level in wastewater is necessary for producing quality wastewater. If the BOD level is too high then the water can be at risk for further contamination, interfering with the treatment process and affecting the end product. There are several factors which can contribute to high BOD levels such as nitrates and phosphates present in the wastewater, water temperature, and others. Each factor affects the plant life in the water, such as algae, and in turn also has an effect on the organisms which help decompose water contaminants in the wastewater treatment process. The best quality wastewater treatment occurs in an environment which supports the life of these bacteria while maintaining a controlled population of them so as not to encourage rapid bacterial decomposition, which creates higher BOD levels.
COD is similar to BOD. COD measures the quantity of oxygen which is consumed by the water in the decomposition and oxidation processes, specifically the decomposition of organic matter and oxidation of inorganic matter, or chemicals. COD is an application which is normally used for industrial wastewaters.
Industrial wastewater can be normally divided into two types, namely (i) inorganic wastewater, and (ii) organic wastewater.
Inorganic wastewater – Inorganic wastewater is produced mainly in the coal and steel industry, in the non-metallic minerals industry, and in commercial organizations and industries for the surface processing of metals (iron picking works and electro-plating plants). These wastewaters contain a large proportion of suspended matter, which can be eliminated by sedimentation, frequently together with chemical flocculation through the addition of iron or aluminum salts, flocculation agents and some kinds of organic polymers.
The purification of warm and dust-laden waste gases from blast furnaces, converters, cupola furnaces, refuse and sludge incineration plants, and aluminum works results in wastewater containing mineral and inorganic substances in dissolved and undissolved form.
Organic wastewater – Organic wastewater contains organic industrial waste flow from those chemical industries and large-scale chemical works, which mainly use organic substances for chemical reactions. The effluents contain organic substances having different origins and properties. These can only be removed by special pre-treatment of the wastewater, followed by biological treatment.
The effects which the pollutants have on the water environment can be summarized in the following broad categories.
Physical effects – These include impact on clarity of the water and interference to oxygen dissolution in it. Water clarity is affected by turbidity which can be caused by inorganic (fixed suspended solids, FSS) and / or organic particulates suspended in the water (volatile suspended solids, VSS). The latter can undergo bio-degradation and thereby also have oxidation effects. Turbidity reduces light penetration and this reduces photo-synthesis while the attendant loss in clarity, among other things, adversely affects the food gathering capacity of aquatic animals since they are not be able to see their prey. Very fine particulates can also clog the gill surfaces of fishes and thereby affecting respiration and eventually killing them.
Settleable particulates can accumulate on plant foliage and bed of the water-body forming sludge layers which eventually smother benthic organisms. As the sludge layers accumulate, they can eventually become sludge banks and if the material in these is organic then its decomposition gives rise to mal-odours. In contrast to the settleable material, particulates lighter than water eventually float to the surface and form a scum layer. The latter also interferes with the passage of light and oxygen dissolution. Because of the former, these scum layers affect photo-synthesis. Discharge limits on wastewater or treated wastewater discharges typically have a value for TSS such as 30 milligrams per litre or 50 milligrams per litre.
Several industrial wastewaters contain oil and grease. While some of the latter can be organic in nature, there are several which are mineral oils. Notwithstanding their organic or mineral nature, both types of oil and grease cause interference at the air-water interface and inhibit the transfer of oxygen. Apart from their interference to the transfer of oxygen from atmosphere to water, the oil and grease (particularly the mineral oils) can also be inhibitory.
Unlike domestic sewage, industrial discharges can have temperatures substantially above ambient temperatures. These raise the temperatures of the receiving water bodies and reduce the solubility of oxygen. Apart from this, rapid changes in temperature can result in thermal shock and this can be lethal to the more sensitive species. Heat, however, does not always have a negative impact on organisms as it can positively affect growth rates although there are limits here too, since the condition can may favour certain species within the population more than others and over time biodiversity can be negatively affected.
Oxidation and residual dissolved oxygen – Water-bodies have the capacity to oxygenate themselves through dissolution of oxygen from the atmosphere and photo-synthetic activity by aquatic plants. Of the latter, algae frequently play an important role. However, there is a finite capacity to this re-oxygenation and if oxygen depletion, as a result of biological or chemical processes induced by the presence of organic or inorganic substances which exert an oxygen demand (i.e., as indicated by the BOD or COD), exceeded this capacity then the dissolved oxygen (DO) levels decline. The latter can eventually decline to such an extent that septic conditions occur. A manifestation of such conditions is the presence of malodours released by facultative and anaerobic organisms. An example of this is the reduction of substances with combined oxygen such as sulphates by facultative bacteria and resulting in the release of hydrogen sulphide.
The depletion of free oxygen affects the survival of aerobic organisms. DO levels do not, however, need to drop to zero before adverse impacts are felt. A decline to 3 milligrams per litre to 4 milligrams per litre, which still means the water contains substantial quantities of oxygen, can already adversely affect higher organisms like some species of fish. If inhibitory substances are also present, then the DO level at which adverse effects can be felt, can be even higher than before.
The case of high-water temperatures because of the warm discharges is somewhat different. The high temperatures can affect metabolic rates positively (possibly two-fold for each 10 deg C rise in temperature) but high temperatures also reduce the solubility of oxygen in water. This means increasing the demand for oxygen while its availability declines. Because of the impact of DO levels on aquatic life, much importance has been placed on determining the BOD value of a discharge. Typical BOD limits set are values such as 20 milligrams per litre and 50 milligrams per litre.
Inhibition or toxicity and persistence — These effects can be caused by organic or inorganic substances and can be acute or chronic. Examples of these include the pesticides and heavy metals. Several industrial wastewaters do contain such potentially inhibitory or toxic substances. The presence of such substances in an eco-system can bias a population towards members of the community which are more tolerant to the substances while eliminating those which are less tolerant and resulting in a loss of bio-diversity.
For similar reasons, an awareness of the impact such substances have on biological systems is not only relevant in terms of protection of the environment but is of no less importance in terms of their impact on the biological systems used to treat industrial wastewaters. Even successful treatment of such a wastewater does not necessarily mean that the potability of water in a receiving waterbody is not getting affected. For example, small quantities of residual phenol in water can react with chlorine during the potable water treatment process giving rise to chloro-phenols which can cause objectionable tastes and odours in the treated water.
Apart from the organic pollutants which are potentially inhibitory or toxic, there are those which are resistant to biological degradation. Such persistent compounds can be bio-accumulated in organisms resulting in concentrations in tissues being considerably higher than concentrations in the environment and hence making these organisms unsuitable as prey / food for organisms higher up the food chain. While some organic compounds can be persistent, metals are practically non-degradable in the environment.
Eutrophication—The discharge of nitrogenous and phosphorous compounds into receiving water-bodies can alter their fertility. Enhanced fertility can lead to excessive plant growth. The latter can include algal growth. The subsequent impact of such growth on a water-body can include increased turbidity, oxygen depletion, and toxicity issues. Algal growth in unpolluted water-bodies is normally limited since the water is nutrient limiting. While nutrients include macro-nutrients like nitrogen, phosphorous, and carbon, and micro-nutrients like cobalt, manganese, calcium, potassium, magnesium, copper, and iron which are needed only in very small quantities, the focus in concerns over eutrophication is on phosphorous and nitrogen since the quantities of the other nutrients in the natural environment are frequently inherently adequate.
In fresh-waters the limiting nutrient is normally phosphorous while in estuarine and marine waters, it is nitrogen. Treatment of industrial wastewater (or domestic sewage for that matter) can then target the removal of either phosphorous or nitrogen, depending on the receiving water-body, to ensure that the nutrient limiting condition is maintained. Removal of nitrogen can likely be necessary if the wastewater contained excessive quantities. When the nutrient limiting condition is no longer present in the water-body, and when other conditions such as ambient temperature are appropriate, excessive algal growth or algal blooms (e.g., the red tide) can occur.
Apart from aesthetic issues, such algal blooms can affect the productivity of the fisheries in the locale. It is to be noted that not all industrial wastewaters contain excessive quantities of nutrients, macro and micro. This deficiency, if there is, results in process instability and / or the proliferation of inappropriate microbial species during biological treatment of the wastewaters. Bulking sludge is a manifestation of such an occurrence. To address this deficiency, nutrients supplementation is needed. The quantities used is to be carefully regulated so that an excessive nutrients condition is not inadvertently created and these excess nutrients subsequently discharged with the treated effluent. In terms of BOD:N:P ratio, the optimal ratio for bio-treatment is frequently taken as 100:5:1 while the minimum acceptable condition can be 150:5:1.
Pathogenic effects — Pathogens are disease-causing organisms and an infection occurs when these organisms gain entry into a host (e.g., a human or an animal) and multiply therein. These pathogens include bacteria, viruses, protozoa, and helminths. While domestic and medical related wastewaters can typically be linked to such micro-organisms (and especially the bacteria and viruses), industrial wastewaters are not typically associated with this category of effects. The exception to this is wastewaters associated with the sectors in the agro-industry dealing with animals. The concern here is the presence of such organisms in the wastewater which is discharged into a receiving water-body and diseases, if any, are then transmitted through the water.
While several of these organisms can be satisfactorily addressed with adequate disinfection of the treated effluent and raw potable water supplies during the water treatment process, there are those which cannot be dealt with so easily. Two examples of such organisms are Cryptosporidium and Giardia. These belong to the protozoa family. The difficulty is that the infected host does not necessarily shed the organism but is likely also to shed its eggs or oocysts. The latter can unfortunately be resistant to the normal disinfection processes. An outbreak of cryptosporidiosis, a gastrointestinal disease, results in the hosts suffering from diarrhoea, abdominal pain, nausea, and vomiting.
With the above effects in view, industrial wastewater treatment is typically be needed to address at least these parameters namely (i) suspended solids (SS), (ii) temperature, (iii) oil and grease, (iv) organic content in terms of BOD or COD, (v) pH, (vi) specific metals and / or specific organic compounds, (vii) nitrogen and / or phosphorus, (viii) indicator micro-organisms (e.g., E. Coli) or specific micro-organisms.
Industrial wastewater characteristics
The physical and chemical characterization given below is valid for the majority of the wastewaters, both municipal and industrial.
Physical characteristics – The principal physical characteristics of wastewater include solids content, colour, odour, and temperature.
The total solids in a wastewater consist of the insoluble or suspended solids and the soluble compounds dissolved in water. The suspended solids content is found by drying and weighing the residue removed by the filtering of the sample. When this residue is ignited, the volatile solids are burned off. Volatile solids are presumed to be organic matter, although some organic matter does not burn and some inorganic salts break down at high temperatures. The organic matter consists mainly of proteins, carbo-hydrates, and fats. Between 40 % to 65 % of the solids in an average wastewater are suspended. Settleable solids, expressed as millilitres per litre, are those which can be removed by sedimentation. Normally around 60 % of the suspended solids in a municipal wastewater are settleable. Solids can be classified in another way as well. Those which are volatilized at a high temperature (600 deg C) and those which are not. The former is known as volatile solids, the latter as fixed solids. Normally, volatile solids are organic.
Colour is a qualitative characteristic which can be used to assess the general condition of wastewater. Wastewater which is light brown in colour is less than 6 hours old, while a light-to-medium grey colour is characteristic of wastewaters which have undergone some degree of decomposition or which have been in the collection system for some time. Lastly, if the colour is dark grey or black, the wastewater is typically septic, having undergone extensive bacterial decomposition under anaerobic conditions. The blackening of wastewater is frequently because of the formation of different sulphides, particularly, ferrous sulphide. This results when hydrogen sulphide produced under anaerobic conditions combines with divalent metal, such as iron, which can be present. Colour is measured by comparison with standards.
The determination of odour has become increasingly important, as the general public has become more concerned with the proper operation of wastewater treatment facilities. The odour of fresh wastewater is normally not offensive, but a variety of odorous compounds are released when wastewater is decomposed biologically under anaerobic conditions. There are different unpleasant odours which are produced by certain industrial wastewater.
The temperature of wastewater is normally higher than that of the water supply since the warm municipal water has been added. The measurement of temperature is important since majority of the wastewater treatment schemes include biological processes which are temperature dependent. The temperature of wastewater varies from season to season and also with geographic location. In cold regions the temperature can vary from around 7 deg C to 18 deg C, while in warmer regions the temperatures can vary from 13 deg C to 24 deg C.
Chemical characteristics – The principal chemical characteristics of wastewater include (i) inorganic chemicals, (ii) organic chemicals, (iii) volatile organic compounds (VOCs), and (iv) heavy metals and inorganic species.
In case of inorganic chemicals, the principal chemical tests include free ammonia, organic nitrogen, nitites, nitrates, organic phosphorus, and inorganic phosphorus. Nitrogen and phosphorus are important since these two nutrients are responsible for the growth of aquatic plants. Other tests, such as chloride, sulphate, pH and alkalinity, are performed to assess the suitability of reusing treated wastewater and in controlling the different treatment processes.
Trace elements, which include some heavy metals, are not determined routinely, but trace elements can be a factor in the biological treatment of wastewater. All living organisms need varying quantities of some trace elements, such as iron, copper, zinc, and cobalt, for proper growth. Heavy metals can also produce toxic effects. Hence, determination of the quantities of heavy metals is especially important where the further use of treated effluent or sludge is to be evaluated. Several of metals are also classified as priority pollutants such as arsenic, cadmium, chromium, and mercury etc.
Measurements of gases, such as hydrogen sulphide, oxygen, methane and carbon di-oxide, are made to help the system to operate. The presence of hydrogen sulphide needs to be determined not only since it is an odorous and very toxic gas but also since it can affect the maintenance of long sewers on flat slopes, and since it can cause corrosion. Measurements of dissolved oxygen are made in order to monitor and control aerobic biological treatment processes. Methane and carbon di-oxide measurements are used in connection with the operation of anaerobic digesters.
As regards organic chemicals, over the years, a number of different tests have been developed to determine the organic content of wastewaters. In general, the tests can be divided into those used to measure gross concentrations of organic matter higher than about 1 milligram per litre and those used to measure trace concentrations in the range of 10 to the power -12 milligram per litre to 10 to the power -3 milligram per litre. Laboratory methods normally used today to measure gross quantities of organic matter (higher than 1 milligram per litre) in wastewater include (i) BOD, (2) COD, and (iii) total organic carbon (TOC). Trace organics in the range of 10 to the power -12 milligram per litre to 10 to the power -3 milligram per litre are determined using instrumental methods including gas mass spectroscopy and chromatography. Specific organic compounds are determined to assess the presence of priority pollutants.
The BOD, COD and TOC tests are gross measures of organic content and as such do not reflect the response of the wastewater to different types of biological treatment technologies. It is hence desirable to divide the wastewater into several categories, as shown in Fig 1.
Fig 1 Partition of organic constituents of a wastewater
VOCs such as benzene, toluene, xylenes, tri-chloro-ethane, di-chloro-methane, and tri-chloro-ethylene, are common soil pollutants in industrialized and commercialized areas. One of the more common sources of these contaminants is leaking underground storage tanks. Improperly discarded solvents and landfills, built before the introduction of present stringent regulations, are also significant sources of soil VOCs. Several of organic substances are classified as priority pollutants such as poly-chlorinated bi-phenyls (PCBs), poly-cyclic aromatic, acetaldehyde, formaldehyde, 1.3-butadiene, 1.2-di-chloro-ethane, di-chloro-methane, and hexa-chloro-benzene (HCB) etc.
Several industries discharge heavy metals, it can be seen that of all of the heavy metals, chromium is the most widely used and discharged to the environment from different sources. Several of the pollutants entering aquatic eco-systems (e.g., mercury lead, pesticides, and herbicides) are very toxic to living organisms. They can lower reproductive success, prevent proper growth and development, and even cause death.
However, chromium is not the metal which is most dangerous to living organisms. Much more toxic are cadmium, lead, and mercury. These have a tremendous affinity for sulphur and disrupt enzyme function by forming bonds with sulphur groups in enzymes. Protein carboxylic acid (-CO2H) and amino (-NH2) groups are also chemically bound by heavy metals. Cadmium, copper, lead, and mercury ions bind to cell membranes, hindering transport processes through the cell wall. Heavy metals can also precipitate phosphate bio-compounds or catalyze their decomposition.
The pollutant cadmium in water can arise from industrial discharges and mining wastes. Cadmium is widely used in metal plating. Chemically, cadmium is very similar to zinc, and these two metals frequently undergo geochemical processes together. Both metals are found in water in the +2-oxidation state. The effects of acute cadmium poisoning in humans are very serious. Among them are high blood pressure, kidney damage, destruction of testicular tissue, and destruction of red blood cells. Cadmium can replace zinc in some enzymes, thereby altering the stereo-structure of the enzyme and impairing its catalytic activity. Cadmium and zinc are common water and sediment pollutants in harbours surrounded by industrial facilities.
Inorganic lead arising from a number of industrial and mining source occurs in water in the +2-oxidation state. Lead from leaded gasoline used to be a major source of atmospheric and terrestrial lead, much of which eventually enters natural water systems. Acute lead poisoning in humans causes severe dysfunction in the kidneys, reproductive system, liver, and the brain, and nervous system.
Mercury is found as a trace component of several minerals, with continental rocks containing an average of around 80 ppb (parts per billion), or slightly less, of this element. Cinnabar, red mercuric sulphide, is the chief commercial mercury ore. Metallic mercury is used as an electrode in the electrolytic generation of chlorine gas, in laboratory vacuum apparatuses, and in other applications. Organic mercury compounds used to be widely applied as pesticides, particularly fungicides. Mercury enters the environment from a large number of miscellaneous sources related to human use of the element. These include discarded laboratory chemicals, batteries, broken thermometers, lawn fungicides, amalgam tooth fillings, and pharmaceutical products. Sewage effluent sometimes contains up to 10 times the level of mercury found in typical natural waters.
Cyanide ion, CN-, is probably the most important of the different inorganic species in wastewater. Cyanide, a deadly poisonous substance, exists in water as HCN which is a weak acid. The cyanide ion has a strong affinity for several metal ions, forming relatively less toxic ferrocyanide, [Fe(CN)6]4-, with iron (II), for example. Volatile HCN is very toxic. Cyanide is widely used in industry, especially for metal cleaning and electroplating. It is also one of the main gas and coke scrubber effluent pollutants from gas works and coke ovens. Cyanide is widely used in certain mineral processing operations.
Ammonia is the initial product of the decay of nitrogenous organic wastes, and its presence frequently indicates the presence of such wastes. It is a normal constituent of some sources of ground-water and is sometimes added to drinking water to remove the taste and odour of free chlorine. Since the ‘pKa’ (The negative log of the acid ionization constant) of the ammonium ion, NH4+, is 9.26, majority of ammonia in water is present as NH4+ rather than NH3.
Hydrogen sulphide, H2S, is a product of the anaerobic decay of organic matter containing sulphur. It is also produced in the anaerobic reduction of sulphate by micro-organisms and is developed as a gaseous pollutant from geothermal waters. Wastes from different industrial plants can also contain H2S. Nitrite ion, (NO)2- , occurs in water as an intermediate oxidation state of nitrogen. Nitrite is added to some industrial processes to inhibit corrosion. It is rarely found in drinking water at levels over 0.1 milligram per litre. Sulphite ion, (SO3)2- , is found in some industrial wastewaters. Sodium sulphite is normally added to boiler feed-waters as an oxygen scavenger 2(SO3)2- + O2 = 2(SO4)2-.
Effluent from industrial sources contains a wide variety of pollutants, including organic pollutants. Primary and secondary sewage treatment processes remove some of these pollutants, particularly oxygen demanding substances, oil, grease and solids. Others, such as refractory (degradation-resistant) organics (organo-chlorides, and nitro compounds etc.), and salts and heavy metals, are not efficiently removed. Soaps, detergents, and associated chemicals are potential sources of organic pollutants. Majority of the environmental problems presently attributed to detergents do not arise from the surface-active agents, which basically improve the wetting qualities of water. The greatest concern among environmental pollutants has been caused by poly-phosphates added to complex calcium, functioning as a builder.
Bio-refractory organics are poorly bio-degradable substances, prominent among which are aromatic or chlorinated hydrocarbons (benzene, bornyl alcohol, bromo-benzene, chloroform, camphor, di-nitro-toluene, nitro-benzene, and styrene etc.). Several of these compounds have also been found in drinking water. Water contaminated with these compounds are to be treated using physical and chemical methods, including air stripping, solvent extraction, ozonation, and carbon adsorption.
First discovered as environmental pollutants in 1966, poly-chlorinated biphenyls (PCB compounds) have been found throughout the world in water, sediments, and bird and fish tissue. They are made by substituting between 1 Cl (chlorine) atom and 10 Cl atoms onto the bi-phenyl aromatic structure. This substitution can produce 209 different compounds.
Thermal pollution – Considerable time has elapsed since the scientific community and regulatory agencies officially recognized that the addition of large quantities of heat to a recipient water body possesses the potential of causing ecological harm. There is really significant heat loads which result from the discharge of condenser cooling water from the steam electrical generating plants and equivalent-sized nuclear power reactors. Large numbers of nuclear power plants presently need around 50 % more cooling water for a given temperature rise than that needed of fossil-fuelled plants of an equal size. The degree of thermal pollution depends on thermal efficiency, which is determined by the quantity of heat rejected into the cooling water.
Thermo-dynamically, heat is needed to be added at the highest possible temperature and rejected at the lowest possible temperature if the greatest amount of effect is to be gained and the best thermal efficiency realized. The present and normally accepted maximum operating conditions for conventional thermal power plants are around 500 deg C and 24 MPa, with a corresponding heat rate of 2.5 kWh, 1 kWh resulting in power production and 1.5 kWh being wasted. Plants have been designed for 680 deg C and 34 MPa, however, metallurgical problems have kept operating conditions at lower levels.
Nuclear power plants operate at temperatures of from 250 deg C to 300 deg C and pressures of up to 7 MPa, resulting in a heat rate of around 3.1 kWh. Hence, for nuclear plants, 1 kWh can be used for useful production whereas 2.1 kWh is wasted. Majority of the steam electrical generating plants are operated at varying load factors, and, hence, the heated discharges demonstrate wide variation with time. Hence, the biota (animal and plant life) is not only subjected to increased or decreased temperature, but also to a sudden, or ‘shock’, temperature change.
Increased temperature causes remarkable reduction in the self-purification capacity of a water body and cause the growth of undesirable algae. The addition of heated water to the receiving water can be considered equivalent to the addition of sewage or other organic waste material, since both pollutants can cause a reduction in the oxygen resources of the receiving waters.
In majority of the cases, the increases in temperature are small and probably do not cause biological harm outside the mixing zone. In fact, little data exists to support the claims of extensive heat damage from power plants on the biota. Also, besides entrainment problems, few substantiated fish kills have been reported as a result of power plant operations. The possible effects of heat on fish can be summarized as (i) direct death from excessive temperature rise beyond the thermal death point, (ii) indirect death because of less oxygen available, disruption of the food supply, decreased resistance to toxic materials, decreased resistance to disease, predation from more tolerant species and synergism with toxic substances, (iii) increase in respiration and growth, (iv) competitive replacement by more tolerant species, and (v) sub-lethal effects. While each of these factors can be important at a specific location, the temperature rises typical of majority of the power plants are normally not high enough to be of concern.
Treatment of industrial wastewater
Treatment of industrial wastewater covers the mechanisms and processes used to treat waters which have been contaminated in some way or other because of the use of the water in the industrial activities. The objective of treatment is to remove the dissolved and suspended substances of the wastewater so that treated water can be safely discharged into the environment or can be recycled back in the same process or can be used in a different process. Fig 2 shows different wastewater treatment stages and common techniques used in each stage.
Fig 2 Wastewater treatment stages and common techniques used in each stage
Technologies for treating industrial wastewater are normally divided into the following four categories namely (i) chemical technologies, (ii) physical technologies, (iii) biological technologies, and (iv) membrane technologies.
Major chemical technologies for industrial wastewater are neutralization, precipitation, coagulation, adsorption, and ion exchange.
Neutralization is the adjustment of alkalinity and acidity of wastewater to a neutral value of pH 7 by either acidic or alkaline treatment.
Precipitation is addition of chemicals to wastewater to change the chemical composition of pollutants so that the newly formed compounds settle down during sedimentation. Precipitation is normally used for the removal of heavy metals from the wastewater which are normally precipitated as hydroxides. However, it is necessary to pre-treat the wastewater to remove the substances which interferes the precipitation of the heavy metals.
Coagulation is the use of chemicals to cause pollutants to agglomerate and subsequently settle out during sedimentation. Coagulation is used for the clarification of wastewater containing colloidal and suspended solids. Silica or poly-electrolyte aids in the formation a rapid settling material. Wastewater containing emulsified oils can be clarified by coagulation process. The process is very efficient for colour reduction of the wastewater but less effective for COD reduction.
Adsorption is the use of a chemical which causes certain pollutants to adhere to the surface of that chemical for subsequent removal. Activated carbon or synthetic active surfaces are used for adsorption.
Ion exchange process is normally used for the removal of undesirable anions and cations from the wastewater. Cations are exchanged for hydrogen or sodium and anions are exchanged for hydroxyl ions. Ion exchange resins consist of an organic or inorganic network structure with attached functional groups. Majority of the ion exchange resins used in wastewater treatment are synthetic resins made by the polymerization of organic compounds.
Major physical technologies for the industrial wastewater are screening, clarification and sedimentation, floatation, and air stripping.
Screening is the removal of coarse solids out of wastewater by using a straining device.
Clarification and sedimentation of wastewater is a common and essential process in industrial wastewater treatment plants. Clarifiers consist of tanks or basins which hold wastewater for a period of time, allowing solids or other materials suspended in the water to settle to the bottom.
Floatation is carried out with the help of small air / gas bubbles injected into the wastewater. The air / gas bubbles cause pollutant particles in the wastewater to rise to the surface for subsequent removal. Floatation process is normally used for oil separation. Free oil is floated to the surface of the tank and then skimmed off.
Air stripping is the removal of VOCs and semi volatile organic compounds from wastewater with the use of air flow.
Biological technologies consist of biological treatment which is a more natural wastewater treatment process than other wastewater treatment methods. Micro-organisms feed on the complex materials present in the wastewater and turn them into simpler substances, preparing the water for further treatment. The main objective of the biological technologies is to reduce the BOD level. Major biological technologies for the industrial wastewater are air activated sludge process, high purity activated sludge process, aerated pond / lagoon process, trickling filter process, rotating biological process and oxidation ditch process.
Air activated sludge process is an aerobic process in which bacteria consume organic matter, nitrogen, and oxygen from the wastewater and grow new bacteria. The bacteria are suspended in the aeration tank by the mixing action of the air blown into the wastewater.
High purity activated sludge process is an aerobic process which is similar to air activated sludge process except that in place of air, pure oxygen is injected into the wastewater.
Aerated pond / lagoon process is an aerobic process which is similar to air activated sludge process. In this process mechanical aerators are used either to inject air into the wastewater or to cause violent agitation of the wastewater and air in order to transfer oxygen to the wastewater.
Trickling filter process is a fixed film aerobic process in which a tank containing media with a high surface to volume ratio is used. Wastewater is discharged at the top of the tank and trickles (percolates) down the media. Bacteria grow on the media utilizing the organic matter and nitrogen from the wastewater. Fig 3 shows schematics of trickling filter process.
Fig 3 Schematics of trickling filter process
Rotating biological process is a fixed film aerobic process similar to the trickling filter process except that the media is supported horizontally across the tank of the wastewater. The media upon which the bacteria grow is continuously rotated so that it is alternately in the wastewater and the air.
Oxidation ditch process is similar to the activated sludge process. Physically an oxygen ditch is ring shaped and is equipped with mechanical aeration devices.
Membrane technologies are becoming increasingly important in treatment of industrial wastewater. With the help of these technologies, it is possible to remove particles, colloids, and macro-molecules, so that wastewater can be disinfected. Membrane technologies are normally classified according to the size range of the separated species. Major membrane technologies for the industrial wastewater are reverse osmosis, nano-filtration, ultra-filtration, and micro-filtration.
Reverse osmosis process is used to separate dissolved salts and small organics.
Nano-filtration process is used for selective demineralization of water or concentration of organic solutions. The process is used for separation of antibiotics.
Ultra-filtration process is used to separate emulsions, colloids, macro-molecules, or proteins.
Micro-filtration process is used to separate small particles, large colloids, and microbial cells.
Characteristics of wastewater from industrial sources vary with the type and the size of the facility and the on-site treatment methods, if any. Because of this variation, it is frequently difficult to define typical operating conditions for industrial activities. Options available for the treatment of industrial waste water are summarized briefly in Fig 4. For introducing a logical order in the description of treatment techniques, the relationship between pollutants and respective typical treatment technology is normally taken as reference. These are (i) removal of suspended solids and insoluble liquids, (ii) removal of inorganic, non-biodegradable or poorly degradable soluble content, and (iii) removal of biodegradable soluble content.
Fig 4 Range of wastewater treatments in relation to type of contaminants
Industrial wastewaters can be very different from sewage in terms of their discharge patterns and compositions. Notwithstanding this, several industrial wastewater treatment plants, for example and like sewage treatment plants, use biological processes as key unit processes in the treatment train. Given the variations in wastewater characteristics, ensuring these biological processes and upstream / downstream unit processes are appropriately designed, it presents a great challenge. The problems intensify when information on the wastewaters and their treatment is lacking.
Specific treatment processes – Because of the characteristics of the process specific waste, there is not a standard design to treat all the industrial wastewaters. Each facility needs a design specific to the process at hand. Chemical operations, in conjunction with different physical operations (settling, coagulation-flocculation, and grease and oil removal etc.), have been developed for the treatment of industrial wastewater. Different chemical processes developed for wastewaters are (i) chemical precipitation, (ii) chemical neutralization, and (iii) oxidation and reduction processes.
Chemical precipitation consists of adding chemicals to form particulates from dissolved solids and facilitate their removal by sedimentation. Chemical precipitation is used for the removal of phosphorous, heavy metals, and reduction of hardness. Heavy metals can be precipitated as metal hydroxides through the addition of lime [Ca(OH)2] or caustic soda (NaOH). For example (Cu)2+ + 2NaOH = Cu(OH)2 + 2(Na)+, and (Cu)2+ + Ca(OH)2 = Cu(OH)2 + (Ca)2+.
The process consists of one or two stirred mixing tanks, where the precipitant agents are added, a sedimentation tank to separate the precipitates, and storage tanks for the chemical agents. If precipitates are colloids a coagulation-flocculation unit is needed. Fig 5 shows schematics of chemical precipitation process.
Fig 5 Schematics of chemical precipitation process
Neutralization of wastewater is carried out for the removal of excess acidity or alkalinity by treatment with a chemical of the opposite composition is termed neutralization. The pH adjustment of wastewater is needed to (i) fulfill the discharge limit requirements, (ii) optimize operating range for other treatment processes such as biological degradation, and (iii) control the aggressiveness of wastewater with respect to corrosion.
Acidic wastewaters can be neutralized with a number of basic chemicals such as caustic soda (NaOH), lime [Ca(OH)2], sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and ammonia (NH3). Alkaline wastewaters are less problematic than acidic wastewaters but nevertheless frequently need a treatment. Alkaline wastewater is treated with combustion gases, concentrated acids such as sulphuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3) or phosphoric acid (H3PO4) and carbon dioxide (CO2). Fig 6 shows schematics of neutralization process.
Fig 6 Schematics of neutralization process
Reduction- oxidation (Redox) processes are used for the chemical oxidation or chemical reduction. These involve use of oxidants or reductants to bring about a change in the chemical composition of a compound or group of compounds to less harmful or hazardous compounds. Fig 7 shows schematics of reduction of chromium process.
Fig 7 Schematics of reduction of chromium process
The reactions which are taking place in the reaction tank of the reduction of chromium process are (i) Na2S2O5 + H2O = 2NaHSO3, (ii) 2H2CrO4 + 3NaHSO3 + 3H2SO4 = Cr2(SO4 )3 + 3NaHSO4 + 5H2O, and (iii) Na2Cr2O7 + 3NaHSO3 + 5H2SO4 = Cr2(SO4)3 + 5NaHSO4 + 4H2O. The reaction which is taking place In the neutralization tank is Cr2(SO4)3 + 6NaOH = 2Cr(OH)3 + 3Na2SO4.
For the oxidation of industrial wastewaters, the chemical oxidizing agents used are ozone (O3), hydrogen peroxide (H2O2), permanganate (MnO) and chlorine (Cl2). The oxidation of industrial wastewaters is carried out for (i) disinfection, (ii) oxidation of iron and manganese, (iii) control of odorous compounds, and (iv) removal of ammonia.
For the reduction of industrial wastewaters, the chemical reducing agents used are ferrous sulphate (FeSO4), sodium bicarbonate (NaHCO3), and sulphur dioxide (SO2). The reduction of industrial wastewaters is carried out for removal of metals. Chemical reduction of metals normally results in products (hydroxides and sulphides) which can be treated more easily in down-stream treatment facilities such as chemical precipitation.
In considering the application of the chemical processes, it is important to note that one of the inherent disadvantages associated with the majority of the chemical unit processes, as compared with physical or biological unit operations, is that they are additive processes. In chemical processes something is added to the wastewater to achieve the removal of something else. As a result, there is always a net increase in dissolved constituents in the wastewater.
Biological anaerobic treatment – Anaerobic biological processes are very attractive for wastewaters with a high organic load and a constant quality. These processes are used for treatment of wastewaters from several industries. The application of these processes has become increasingly important in recent years.
Bio-degradability of wastewater depends on the levels of BOD and COD. When BOD / COD level is equal to or greater than of 0.5, the wastewater is easily treatable by biological means. When BOD / COD level is less than 0.3, the wastewater is partially treatable by biological means and acclimated micro-organisms is needed. When BOD / COD level is less than 0.1, biological treatment is very unlikely to be of benefit. Non-bio-degradable or hardly bio-degradable compounds can affect biological treatment since they are resistant to bio-degradation (recalcitrant) and, they inhibit the metabolic pathways and growth of micro-organisms, consequently, hampering the degradation of easily bio-degradable compounds. Fig 8 shows schematics of anaerobic biological process.
Fig 8 Schematics of anaerobic biological process
When comparing with aerobic processes, anaerobic processes produce lower quantity of sludge. Anaerobic processes result in lower biomass production, hence, sludge processing and disposal costs are reduced greatly. However, this implies more start-up time to develop necessary biomass. Anaerobic processes have low energy consumption since there is no energy need for air or oxygen supply to the reactor, but only for efficient stirring. Anaerobic processes produce low production of an energy-rich gas. Methane, can be recovered and be used as a low-quality fuel. Further, the excess sludge produced from anaerobic processes is stable, whereas the sludge from aerobic process is unstable (high potential for putrefaction and the production of odours).
Industrial wastewater treatment practices are broadly defined as either physical / chemical or biological treatment technologies. Within these two categories there are numerous technologies, known as unit operations and unit processes, which can be used to treat a specific type of wastewater.
The type of wastewater treatment system designed for an industrial facility is to be based on the characteristics of the wastewater and the needed characteristics of the treated wastewater. Wastewater characteristics are a function of the type of industry and its specific manufacturing or production processes and the manner. The treatment approach is to be based on discharge levels permitted by the regulatory authorities prior to discharge to a surface water body.
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