Wastewater and Water Treatment Technologies
Wastewater and Water Treatment Technologies
Cheap and plentiful, water was for centuries a production utility which the steel industry took for granted. But in the present scenario, water resources are becoming increasingly scarce due to the growing imbalance between fresh water availability and consumption, hence the access to clean and safe water has become one of the major challenges of the modern society. Water demand is keep on increasing because of (i) increase of the population and migration to drought prone regions, (ii) rapid industrial development and increasing water use per capita, and (iii) climate change leading to changing weather patterns in populated areas. This has resulted into steel industry entering into a new water-constrained era. Further, in the last three decades, concerns about environmental pollution have increased around the world and this has resulted in the promulgation of more restrictive environmental regulations.
The steel industry uses huge quantities of fresh water for a variety of usage which includes cooling, dust suppression, cleaning, temperature control (heat treatment), transport of waste materials (ash, sludge, and scale etc.), and other usages. Water is an essential part of some of the steel plant processes such as water addition to control moisture content of the coking coal blend, pelletizing of sinter mix, making of green pellets during the production of iron ore pellets, production of steam and hence power, and granulation of blast furnace slag etc. Use of large amount of water also generates considerable quantity of wastewater which is to be discharged. Discharges of wastewater from steel industry have been recognized as one of the factors of aquatic pollution. Wastewater generated in the steel plant processes contains many dissolved and undissolved substances and chemicals.
The principal uses of process water by iron and steel plant processes include cooling and cleaning of process off-gases, direct cooling of coke and slag, direct cooling and cleaning of steel, product rinsing, process solution makeup, and direct cooling of process equipment etc. Most of the water used by the iron and steel plant is for non-contact cooling of processing equipment. Water is also used for steam and power generation.
Process wastewater is defined as are any wastewater which comes into direct contact with the process, product, by-products, or raw materials for the production of iron and steel. Process wastewaters also include wastewater from slag quenching, equipment cleaning, air pollution control devices, rinse water, and contaminated cooling water. Sanitary wastewater and storm water are not considered process wastewater. Non-contact cooling wastewater is the cooling water which does not directly contact the process, products, by-products, or raw materials. This wastewater is not considered process wastewater. Non-process wastewater is that generated by non-process operations such as utility wastewaters (water treatment residuals, boiler blow-down, air pollution control wastewaters from heat recovery equipment, and water generated from co-generation facilities), treated or untreated wastewaters from ground water remediation systems, dewatering water for building foundations, and other wastewater streams not associated with production processes.
A discharge of wastewater means the release of treated or untreated waste water into a receiving stream. A discharge can occur from a treatment plant or from an overflow in the collection system. Untreated waste water discharge can create several undesirable conditions. These include (i) oxygen depletion and odour production in the stream, (ii) negative effects on human health due to the presence of pathogenic microorganisms, (iii) sludge and scum accumulations, (iv) eutrophication of water bodies due to the growth of aquatic plants and algae since the waste water can contain certain quantity of nutrients, and (v) production of large quantities of malodorous gases because of the decomposition of the organic compounds present in wastewater. This discharge of wastewater contributes to the pollution of the water bodies of the area if it is not properly treated and made harmless before the discharge. Hence, the treatment of wastewater is a must before leaving the plant premises and its discharge to the natural water bodies.
Methods of wastewater treatment were first developed in response to the adverse conditions caused by the discharge of wastewater to the environment and the concern for public health. Wastewater treatment in the steel industry is quite complex since the nature of waste waters have different characteristics from various processing units of the steel plant.
Pure water consists of 2 parts of hydrogen and 1 part oxygen. In nature, water contains many dissolved impurities. In fact, water is referred to as ‘the universal solvent’ due to its ability to dissolve many substances. Even distilled water and rainfall water are not ‘completely’ pure since they normally contain very low levels of dissolved substances such as ammonia, which are considered impurities. There are dissolved substances found in surface and ground water. As rain falls, nitrogen and other gasses are absorbed. Water, as it travels through the ground, can dissolve substances from the earth such as sodium, calcium, iron, phosphorus, magnesium, and sulphate.
Fresh domestic untreated or raw water has a musty odour, a pH range of 6.5 to 8 and is grayish-brown in colour. The contaminants typically found in untreated water can be broadly classifies into four basic classes namely (i) organic contaminants, (ii) inorganic contaminants, (iii) pathogen, and (iv) other contaminants. The typical contaminants present in wastewater are given below.
Solids – Total solids in the wastewater can be in the form of dissolved solids or suspended solids. Suspended solids can be in the form of colloidal solids (which cannot be settled) or settleable solids. Suspended solids (SS) lead to the development of sludge deposits and anaerobic conditions when untreated waste water is discharged in the aquatic environment. Total suspended solids (TSS) include all particles which pass through a filter. As levels of TSS increase, a water body begins to lose its ability to support a diversity of aquatic life. Suspended solids absorb heat from sunlight, which increases water temperature and subsequently decreases level of dissolved oxygen. Some solids can also be floatable solids. These floatable solids are typically made up of oil or grease particle and make up the scum. Scum is most easily removed by surface skimming equipment.
Biodegradable organics – Biodegradable organics are composed principally of proteins, carbohydrates, and fats. Biodegradable organics are measured mostly in terms of BOD (biochemical oxygen demand) and COD (chemical oxygen demand). BOD, an important measure of water quality measures the amount of oxygen needed by bacteria and other organisms to oxidize the organic matter present in a water sample over period of 5 days at 20 deg C temperature. COD measures all organic carbon with the exception of some aromatics (benzene, toluene, and phenol etc.) which are not completely oxidized in the reaction. COD is a chemical oxidation reaction. High BOD and COD contributes to low oxygen concentrations in water bodies and together adversely impacts the aquatic life of water bodies. The biological stabilization of bio-degradable organics can lead to the depletion of natural oxygen resources and to the development of septic conditions, if discharged untreated to the environment.
Pathogens – Pathogens are micro-organisms which cause, or can cause, disease. Communicable diseases can be transmitted by the pathogenic organisms in wastewater.
Nutrients – Both nitrogen and phosphorus, along with carbon, are essential nutrients for growth. When discharged to the aquatic environment, these nutrients can lead to the growth of undesirable aquatic life. When discharged in excessive amounts on land, they can also lead to the pollution of groundwater.
Critical pollutants – These are organic and inorganic compounds which have characteristics of causing unknown or suspected carcinogenicity, mutagenicity, or high acute toxicity. The presence of these compounds in wastewater is to be minimized for public health reasons and to protect the biological treatment processes.
Refractory organics – These organics tend to resist conventional methods of waste water treatment. Typical examples include surfactants, phenols, and agricultural pesticides. Some of these can be toxic to the biological treatment processes.
Heavy metals – Heavy metals can be present in the wastewater generated at the various units of the steel plant. These heavy metals are required to be removed if the wastewater is discharged to a water body used for a potable water source. The presence of heavy metals can also impact the recycling of bios-solids (stabilized waste sludge) on farmland.
Dissolved inorganics – Inorganic constituents such as calcium, sodium, and sulphate can be present in wastewater of some units of steel plant. These are to be removed if the wastewater is discharged to a water body used as a potable water source.
Dissolved chemicals – A large range of dissolved chemicals can be present in the wastewater depending on the process. This needs a careful evaluation of the type of contaminant, their concentration, flow, and ease of biodegradability. As little as 1 milligram per litre concentration in the water discharged can give rise to coloured water which affects the aesthetic quality and transparency of water bodies. It also impacts on photosynthesis. Acids and alkalis create low or high pH situations. Some chemicals can be hard to degrade through conventional treatment processes.
Organic contaminants which are derived from chemical compounds contain carbon. These contaminants can be biodegradable, which means that the contaminants can be consumed by bacteria and other micro-organisms. In the process of being consumed, these organics exert an oxygen demand which can be measured as the BOD of the wastewater. Some organic contaminants (refractory organics) are resistant to bio-degradation. Inorganic contaminants are not bio-degradable, but can be nutrients necessary for micro-organisms to live. These are typically chemical compounds (critical pollutants) or metals which are either present in the wastewater as suspended solids or as dissolved inorganics.
Pathogens are disease-causing organisms including bacteria and viruses which can be deposited in the wastewater through human or animal wastes, or from improperly handled hospitals wastes. Proper hygiene is extremely important when working around wastewater. Other impurities can be thermal wastes. Wastewater discharges with thermal wastes can cause a sudden increase in influent flow and temperature. A typical source of thermal waste is non-contact cooling water (heated water where the temperature exceeds stream temperature). Depending on the use of the stream, limits on the temperature of the wastewater can be established to prevent increasing the temperature of the stream and impacting use. Radioactive wastes can come from laboratories and instruments using radioactive sources. It is normally a good practice not to allow the discharge of radioactive wastes into a sewer system.
The various water treatment processes have mainly three objectives namely (i) to confer and preserve the inherent physical, chemical, and biological qualities of water of the intake source which make it suitable for specific uses such as water for drinking and for use in productive processes, (ii) to permit wastewater treatment which protects the public from health risks without causing any damage to the environment, and (iii) to confer and preserve those characteristics of water in its natural environment which are necessary for the conservation and development of the aquatic life and vegetation, and for provision of drinking water for cattle and wild animals, or for recreational and aesthetic purposes.
The treatment of water or wastewater relies on a number of individual unit operations which are combined to make a process, often referred to as a process treatment scheme. The unit operations are all based on a relatively narrow range of governing principles. The same underpinning mechanisms apply to the process whether it is water from the ground, lakes, reservoirs, rivers or the sea which is to be purified for drinking, or whether it is wastewater (i.e. sewage or industrial effluent) which is to be cleansed for safe discharge to the environment.
Conventional waste water treatment technologies improve the quality of wastewater discharged into the environment and restrain polluted waters from contaminating other available clean water resources. However, these treatment technologies do not make wastewater fit for further beneficial uses in communities closer to the points of generation. Innovative and advanced technologies which can further improve the quality of wastewater are needed to overcome this limitation of conventional technologies, and to promote widespread adoption of recycle and reuse practices.
Advanced treatment processes can be biological processes, physico-chemical processes, or a combination of both (hybrid processes). Biological processes to remove nutrient pollutants such as nitrogen and phosphorus provide the platform for further wastewater treatment to reusable quality. Physico-chemical processes such as deep-bed filtration, floating media filtration, and membrane filtration, play a major role among treatment technologies for water reuse. Membrane filtration has significant advantages over other processes since they produce high quality effluent which needs little or no disinfection with minimum sludge generation. The hybrid processes attempt to obtain the benefits of both biological and physico-chemical processes in one step.
Since the reclaiming of wastewater and the introduction of processes for purifying and rendering water potable normally complement the original objective of safeguarding the environment the various processes are considered as belonging to the same field. The water treatment technologies can be categorized into four general areas namely (i) physical methods, (ii) chemical methods, (iii) biological methods and (iv) energy intensive methods.
Physical processes remove solids from wastewater as it flows through screens or filter media, or solids are removed by gravity settling or air flotation. Particles entrapped with air float to the surface and can be removed. Physical methods of wastewater treatment represent a body of technologies which can be referred largely to as solid-liquid separations techniques, of which filtration plays a dominant role. Filtration technology can be broken into two general categories normally conventional and non-conventional. This technology is an integral component of drinking water and wastewater treatment applications. It is, however, but one unit process within a modern water treatment plant scheme, whereby there are a multitude of equipment and technology options to select from depending upon the ultimate goals of treatment. For understanding the role of filtration, it is important to make distinctions not only with the other technologies employed in the cleaning and purification of wastewaters, but also with the objectives of different unit processes.
Chemicals are used in wastewater treatment to create changes in the pollutants which increase the ability to remove them. Changes can include forming floc or a heavier particle mass to improve removal by physical processes. Chemical methods of treatment rely upon the chemical interactions of the contaminants which are needed to be removed from water. The application of chemicals either aids in the separation of contaminants from water, or assists in the destruction or neutralization of harmful effects associated with contaminants. Chemical treatment methods are applied both as stand-alone technologies and as an integral part of the treatment process with physical methods. Normally chemical addition and physical processes are employed together to provide treatment.
Biological treatment processes are systems which use micro-organisms to degrade organic contaminants from wastewater. In wastewater treatment, natural bio-degradation processes have been contained and accelerated in systems to remove organic material and nutrients. The micro-organisms metabolize nutrients, colloids, and dissolved organic matter, resulting in treated wastewater. Excess microbial growth is removed from the treated wastewater by physical processes. Biological processes are the preferred way of treatment as they are cost effective in terms of energy consumption and chemical usage.
Among the energy intensive technologies, thermal methods have a dual role in water treatment applications. They can be applied as a means of sterilization, thus providing high quality drinking water, and / or these technologies can be applied to the processing of the solid wastes or sludge, generated from water treatment applications. In the latter cases, thermal methods can be applied in essentially in the same manner as they are applied to conditioning water, namely to sterilize sludge contaminated with organic contaminants, and / or these technologies can be applied to volume reduction. Volume reduction is a key step since there is a tradeoff ultimately between polluted water and hazardous solid waste. Energy intensive technologies include electro-chemical techniques, which by and large are applied to drinking water applications. They represent both sterilization and conditioning of water to achieve a palatable quality.
All these four technology groups can be combined in water treatment, or they can be used in select combinations depending upon the objectives of water treatment. Among each of the general technology classes, there is a range of both hardware and individual technologies which can be selected. The selection of not only the proper unit process and hardware from within each technology group, but the optimum combinations of hardware and unit processes from these four groups depends upon such factors as (i) the cleanliness requirements of the final water effluent from the plant, (ii) the quantities and qualities of the effluent water which is to be treated, (iii) the physical and chemical properties of the pollutants needed to be removed or rendered neutral in the effluent water, (iv) the physical, chemical, and thermodynamic properties of the solid wastes generated from treating water, and (v) the cost of treating water, including the cost of treating, processing and dumping of the solid wastes.
The treatment of steel industry wastewater needs a variety of strategies to remove different types of contaminants. These strategies consist of (i) solid removal, (ii) oil and grease removal, (iii) removal of bio-degradable organics, (iv) activated sludge process, (v) trickling filter process, (vi) treatment of toxic materials, (vii) treatment of acids and alkalis, and (viii) treatment of other organics. The wastewater treatment units in a steel plant are also known as effluent treatment plants (ETP).
The objectives of the ETP are (i) to ensure discharge of good water quality to the natural environment, (ii) to remove pollutants most efficiently and at the lowest cost, (iii) to avoid and / or minimize other environmental impacts such as odour creation, gas emission, noise production, and solid disposal, (iv) to produce treated water for reuse and recycling, and (v) to recover salts if economically viable. While planning an ETP, the requirements to be considered are (i) desired outgoing effluent quality or permit requirements to comply with national, state, local and / or organizational guidelines, (ii) effluent volume needing treatment, (iii) capacity of the production shop, (iv) complexity of the technology, ease of operation, adaptability, reliability and robustness, and energy requirements, (v) capital and operational costs, (vi) available land area, and (vii) mass of sludge generation and disposal requirements.
Treatment of wastewater produced by the various shops of the steel plant normally takes place in an effluent treatment plant purposely built in the area of production. The treatment of the wastewater carried out in such ETPs confers such characteristics to the wastewater so that it can be safely discharged from the plant to the water bodies or can be recycled back to the process either completely or partially.
Wastewater from the various process of the steel plant depending on its characteristics is subjected to different treatment options. The different combination of the treatment processes of physical, chemical, biological, and energy intensive technologies are used for the removal of solids, organic matter, and sometimes, nutrients from wastewater.
The wastewater treatment processes (Fig 1) of the steel plant falls into same four groups as described earlier. The treatment is carried out at four levels namely preliminary, primary, secondary, and tertiary levels. These levels describe the different degrees of treatment of wastewater. These processes are described as below in brief.
Fig 1 Wastewater treatment processes
The objective of preliminary treatment is the removal of materials which can cause blockages, clogging of downstream equipment and equipment abrasion. Preliminary treatment is normally carried out before the wastewater is sent to the ETP and for improving the performance of the ETP. In this treatment, the coarse solids and other large materials from the wastewater are removed. The removal of these materials is essential for enhancing the operational and maintenance efficiency of the subsequent treatment units. In this treatment of wastewater, a number of unit processes are used to eliminate the undesirable characteristics of wastewater. These normally include (i) control of the odour, and (ii) such operations as pre-aeration, coarse solid grinding, and removal of large materials using screens and grates etc. Many a times the removal of oil and grease as well as pH correction is also carried out.
It is the first step in the wastewater treatment process or the second step after the preliminary treatment. Primary treatment follows preliminary treatment and involves physical and chemical treatment to correct the pH from alkaline conditions to a pH near neutralization, and physical settling of suspended solids in primary clarifiers to reduce the BOD and SS load on downstream processes. Overall, the adoption of primary clarification units represents fewer problems on the downstream biological process operation. For example, there is a lower quantity of oil and grease and biomass accumulation in the biological reactor, minimizing possible settlements in the tank and reducing the tendency to ‘non- filamentous’ bulking of activated sludge biomass, etc. Primary treatment in most cases uses coagulation – flocculation processes to improve solids separation.
Overall, the adoption of primary clarification units represents fewer problems on the down-stream biological process operation. The objectives of primary treatment are the removal of settleable organic and inorganic solids by sedimentation and removal of floating materials by skimming. During the primary treatment around 35 % to 55 % of the total inward BOD, around 55 % to 75 % of the total SSs, and around 70 % of the oil and grease are normally removed. Some amounts of few organic phosphorus and organic nitrogen as well as heavy metals associated with solids are removed during the primary sedimentation but the colloidal and dissolved constituents are not affected.
During the primary treatment, the physical separation of suspended solids from the wastewater using primary clarifiers is the carried out. The TSSs and the associated BOD levels are reduced in this treatment process and the waste is prepared for the next step of the wastewater treatment. The removal of the settleable organic and inorganic solids by the sedimentation and skimming of materials is the main objective of this treatment step.
Primary treatment involves various physico-chemical processes and ensures a satisfactory performance of the subsequent treatment processes. The main process used in the primary treatment is sedimentation while the auxiliary processes used are fine screening and flocculation and floatation. The flocculation is normally preceded by a chemical treatment usually with lime, alum, or a proprietary chemical. The main purpose of this treatment is to remove metals by precipitation and also to remove some related colloidal BOD to generate chemical sludge. The primary treatment applies coagulation-flocculation processes to improve solids separation. Some of these processes are described below.
Flocculation – It is a physico-chemical process which helps to encourage the aggregation of viscous colloidal and delicately separated suspended matters by mixing physically and by aiding chemical coagulant. This process consists of a rapid mixing tank and a flocculation tank. The wastewater stream mixes with the coagulants in a rapid mix tank and is then passed through the flocculation basin and in the flocculation basin a slow mixing of waste occurs which allows the particles to be collected in the form of more settleable and heavier solids. A better mixing is facilitated with the help of a diffused air or the mechanical paddles. The natural organic polymers, inorganic electrolytes, and synthetic poly-electrolytes are the various different types of chemicals used for the coagulation. Depending upon the characteristics and the chemical properties of the contaminants, the specific chemicals are selected.
Sedimentation – The main purpose of primary sedimentation is to allow separation of the solid and liquid phase fractions in the wastewater. It removes the readily settleable solids using the gravity. The solids are mainly organics as well as the floating material such as fats, oils, and grease. The settled solids are known as primary sludge. Hence, the process reduces the SS content of the influent wastewater. Even though the volume of primary sludge is only around 2 % of the total influent wastewater volume, it makes up around 30 % to 40 % of the organic load received (expressed as COD) and some 40 % to 60 % of the SS loading. The baffles and oil skimmers to remove the greases and floating solids are included in the sedimentation chamber and also there can be mechanical scrapers for removal of sludge from the bottom of the chamber.
The efficiency of solids removal depends on the characteristics of the sedimentation tank or clarifier. A sedimentation tank is a device which includes inlet baffling for the dissipation of energy, a quiescent zone for particulate settling, and mechanical means for the removal of settled solids, and low flow velocity to the outlet.
Flocculation and sedimentation tanks can be rectangular, circular, or inclined plate (Lamella), the selection of which is based on local site conditions, area available, and experience of the design team. Ideally, two or more tanks are needed. Rectangular and Lamella tanks use less land area than circular tanks and are useful where land availability is lesser.
Rectangular tanks have straight flow patterns to increase flocculation (in chemically aided sedimentation) and reduce retention time. The water enters at one end, passes through an inlet baffle arrangement and traverses the length of the tank to the effluent weirs and trough. They are designed to have a length: width ratio of 3:1 to 5:1 providing a large effective settling zone closely resembling ideal conditions, and a bottom slope of 1 %. A mechanical scraper at the bottom moves collecting the settled sludge into a collection zone. The sludge is then pumped out subsequently.
In circular clarifiers (Fig 2), the flow pattern is radial. To achieve the radial flow pattern, wastewater is introduced in the majority of designs in the centre or sometimes around the periphery of the tank. In the centre design, the wastewater is transported through a pipe and central baffle known as the ‘centre well’ and flows radially towards a weir which runs around the circumference of the tank. The centre well has a diameter typically between 15 % to 25 % of the total tank diameter and height of 1 m to 2.5 m. The quiescent settling zone is to be large enough to meet the overflow rate and depth requirements for discrete and flocculant settling.
Treated water is discharged over v-notched weir plates. The floor is sloped to aid in sludge concentration and removal. Sludge is removed using mechanical rakes. Typical detention time in a sedimentation tank is 2 hours to 3 hours. Suspended solids removal is 45 % to 55 %.
Fig 2 Schematic of a circular sedimentation tank
Dissolved air flotation – Air bubbles are used in this process. They are needed to raise the suspended particles in wastewater upto surface level so that the suspended particles can be easily collected and removed. The bubbles of air introduced in the wastewater are mainly attached to the particles aiding it to float. The suspended solids, dispersed oil and greases from the oily wastewater and some other effluents can be removed by the process of dissolved air flotation (DAF).
For removal of oil and grease, dissolved air flotation is well suited especially where the specific gravity of suspended solids is close to 1.0. DAF process uses pressurized air to release micro air bubbles (10 micrometers to 50 micrometers in diameter) which attach to the particles, making it easy for the free oil particles to rise to the surface and then be skimmed off. DAF process is very effective in the removal of oil and grease since oil does not naturally settle, having a specific gravity less than that of the water. When the oil is present in the emulsified form, it needs chemicals to destabilize the oil emulsion layer.
The pressurized water flow can be the entire inflow of wastewater, part of the inlet flow, or water already treated by the process (effluent). This results in dissolved air flotation to be three types of usable process, called full flow, partial flow or recirculated flow respectively. Fig 3 shows a schematic of the DAF process. The most common DAF application for wastewater treatment is a recirculated flow system, as it needs less equipment for pressurization (lower energy consumption), it avoids pump abrasion problems, and prevents the formation of colloids and emulsions within the pumping system.
DAF process can reduce oil concentrations to 10 mg/litre to 25 mg/litre as long as the influent concentration is not greater than 500 mg/litre. DAF process operates at higher hydraulic loading rates than gravity sedimentation systems and hence detention times are shorter by 15 minutes to 30 minutes. This allows the DAF process to be more compact and has a smaller footprint. DAF process systems are available in circular or rectangular configurations.
Fig 3 Schematic of the dissolved air flotation process
In retention tank the wastewater is pressurized and contacted with air. The super-saturated and pressurized water is passed through a pressure-reducing valve to the bottom of the floatation tank. The super-saturated air begins to come out in the form of fine bubbles from the solution, as and when the pressure starts releasing. The air bubbles attached with the suspended particles and trapped in sludge flock float over the surface and these floats are always swept from the surface and the mud is then collected from the bottom of the tank. The oil removal efficiency of the DAF process can be increased by the addition of certain coagulants.
Chemical treatment processes – The chemical treatment can be used, preferably before biological treatment as it removes the toxic chemicals which can kills the micro-organisms and or at any stage in the treatment process as and when it is necessary. Chemical treatment processes are described below.
Dissolved solids removal – Dissolved solids can be removed through a number of different methods namely (i) conversion to suspended materials, normally using chemicals to precipitate the contaminant as a solid or gas, to allow them to be removed by physical separation, (ii) adsorption onto a solid material, which can either be suspended or fixed as a bed, such as powdered or granular activated carbon, (iii) rejection using dense membrane processes, such as reverse osmosis or nano-filtration, or (iv) conversion to relatively innocuous end products.
Conversion necessarily involves chemistry or biochemistry, and the chemical reaction can be either reduction/oxidation (redox) or non-redox. Many chemical and biochemical processes operate by oxidation, the end products in the case of organic pollutants normally being carbon dioxide, nitrate and water. Examples of chemical reduction include the quenching of excess chlorine using bisulphite or the biochemical reduction of nitrate to nitrogen, the latter being referred to as ‘denitrification’. There also exist many important non-redox chemical processes, such as pH adjustment or precipitation of alkaline earth salts such as calcium carbonate or sulphate.
Neutralization – There is a wide range of pH of the untreated wastewater and it is not so easy to treat the wastewater with such type of varying range of the pH value. To optimize the treatment efficiency the neutralization process is used to adjust the pH value. To reduce the pH value sulphuric or hydrochloric acids can be added and to raise the pH value, dehydrated lime or sodium hydroxide alkalis can be added. Normally the process of neutralization is carried in a rapid mix holding tank or in a tank used for equalization. To control the pH of the discharge in order to meet the standards, the process of neutralization can be carried out at the end of the treatment.
Precipitation – The process of precipitation is carried out in two steps for the removal of the metal compounds from the stream of the wastewater. The mixing of precipitants with the wastewater and allowing a formation of the insoluble metal precipitants is the first step of the precipitation process. The removal of the precipitated metals from the wastewater through clarification and filtration is carried out in the second step and then the resulting sludge is being treated in a proper manner, and after treatment, it is recycled or disposed off. The important parameter to be considered in a chemical precipitation is pH controlling.
The solubility of metal hydroxides increases towards higher or lower pH and this is amphoteric in nature. Thus, for the precipitation of hydroxide for each metal, there is an optimum value of pH. As there is normally more than one metal in wastewater, hence, it is very much difficult to select the optimum treatment chemical and the pH control becomes more difficult and also it involves a transaction between the best possible removals of two or more metals. Lime, sodium hydroxide, soda ash, sodium sulphide and the ferrous sulphate are the various different chemicals used for the process of precipitation. The process for the effective removal of the metals like antimony, arsenic, chromium, copper, lead, nickel and zinc is normally the hydroxide precipitation and for removing mercury, lead, copper, silver, cadmium etc. sulphide precipitation is used. Fig 4 shows flow diagram of chemical treatment process for metal removal.
Fig 4 Flow diagram of chemical treatment process for metal removal
The secondary treatment process involves disintegration or decomposition of the suspended and dissolved organic substances present in the waste water using microorganisms. The activated sludge process (ASP) and the biological filtration methods are the mainly used biological treatment processes. The biological treatment process which is the mainly used for the secondary treatment process is based on the micro-biological action to decay the organic suspended and dissolved wastewater. The microbes can be used for the natural compound, both as a source of carbon sources and as an energy sources.
For removal of organic pollutants, the most efficient secondary treatment process is biological treatment. It primarily employs microbes naturally present in wastewater to break down organic contaminants. Some inorganic compounds like ammonia, cyanide, sulphide, sulphate and thio-cyanate are also biologically degradable. Biological processes can be broadly classified as (i) aerobic in which microbes which are used need oxygen to grow, (ii) anaerobic in which microbes which are used grow in the absence of oxygen but uses other compounds such as sulphate, phosphate or other organics present in the wastewater other than oxygen, and (iii) facultative in which microbes which are used can grow in the presence or absence of oxygen.
Aerobic processes consist of a biological reactor with a controlled amount of biomass and a clarifier for separation of the biomass from the final effluent. Aerobic processes need higher energy inputs and produce greater amounts of sludge compared to anaerobic systems. As an example, for the same 100 kg COD load entering the aerobic treatment plant, the energy needed is 100 kWh for aeration and produces 30 kg to 60 kg of sludge with the outlet effluent COD load of 2 kg to 10 kg. In the anaerobic treatment plant for the same 100 kg COD load, the sludge production is only 5 kg, or six to twelve times less, and produces 40 cum (cubic meter) to 45 cum of biogas which can be converted to produce 382 kWh of electricity. However, the outlet water COD is twice that of the aerobic plant, and hence of a lower quality.
Hydraulic retention time – It is the average time in the aeration basin equivalent to the volume of the basin divided by the average flow and expressed as hours. The hydraulic retention time is required to be sufficiently long to remove the prerequisite BOD and is dependent on the type of the biological treatment system. It can range from 0.5 hours to 120 hours. The lower the hydraulic retention time the quicker the wastewater reaches the outlet.
Mixed liquor suspended solids (MLSS) – Suspended solids level is one of the most important control parameters in biological wastewater treatment processes. It is not only directly related to sludge settling properties and effluent quality, but also related to food / micro-organism ratio which is in turn related with all aspects of sludge properties. MLSS represents the total suspended solids including bacteria, dead biomass, and higher life forms, irrespective of biological activity. The organic portion of MLSS is represented by ‘mixed liquor volatile suspended solids’ (MLVSS) which represents the biomass. MLSS is controlled by the sludge wasting rate. Typical MLSS are dependent on the process type. The more concentrated is the MLSS, the smaller is the equipment footprint and hence the popularity of membrane bioreactors (MBRs) in space constrained locations. MLVSS is 0.75 MLSS.
Food to microorganism (F/M) ratio – It is a term used for expressing the organic loading of an activated sludge process. F/M is a critical factor in process design and operation, especially in determining the aeration basin volume. F/M range is around 0.5 to 1.5. For conventional plants, F/M of 0.2 to 0.5 is aimed for. In biological treatment plants operating at high F/M loads (0.8 to 1.5), the rate of treatment increases at the cost of poor settlability of the sludge. Processes operating at low F/M loads (0.05 to 0.2) are associated with slow BOD removal rates but with good sludge settling. However, the system can be easily upset by a spike load of organics.
Sludge age – It is also known as ‘mean cell residence time’ (MCRT) and ‘solids retention time’ (SRT). It is calculated as the total quantity of sludge in the aeration tank and clarifier divided by the daily sludge losses through waste activated sludge and effluent. Sludge age can vary from 0.5 day to 75 days in low-growth rate systems. Sludge age is an indication of F/M ratios. Shorter times are indicative of high F/M ratios and longer times are indicative of low F/M ratios. Sludge age is expressed by the equation ‘sludge age = sludge mass in (aeration tank + clarifier) / daily sludge losses’.
The quality of sludge age can be determined using a microscope at 100x magnification. Daily microscopic analysis can prevent problems. Micro-organisms considered important in biological treatment are bacteria, fungi, algae, protozoa, rotifers, and worms. The presence of higher life form indicator organisms normally correlates to plant performance. They can indicate if the sludge is young, medium, or old. Good settling sludge is characterized by the presence of protozoa such as stalked ciliates and suctorians and normally is golden brown in colour (sewage treatment plants). Low sludge age is characterized by the absence of stalked ciliates and predominance of free swimming ciliates such as paramecium (these expend a lot of energy in swimming) and high BOD slugs by the absence of higher life forms. Old sludge is characterized by the presence of many worms (nematodes) or rotifers.
Another useful indicator is the ‘sludge volume index’ (SVI). Sludge is poured to a 1 litre graduated cylinder and the percentage of settled sludge in 5 minute intervals is noted for 30 minutes. SVI is expressed in ml/g. It is a reliable troubleshooting test. SVI values can vary from 30 ml/g to 400 ml/g. Values below 150 indicate good sludge settling and above this indicate sludge bulking. Other key variables which affect the operation of the biological reactor are given below.
Oxygen requirement – Oxygen is needed for the decomposition of organic matter. The concentration depends on organic matter consumption, endogenous respiration demand and total nitrification of TKN (total Kjeldahl nitrogen) oxidation. Typical oxygen concentration in an aeration tank is 2 mg/l to 4 mg/l. The higher values are maintained for nitrogen removal. Above this, electricity is wasted.
Sludge production (sludge yield) – The decay of biomass produces sludge. For conventional industrial systems, sludge production can be as low as 0.15 kg / kg BOD, such as in coke making.
Sludge recirculation rate – A portion of the sludge produced is recirculated to promote the production of more sludge in the aeration tank. It is the ratio between the sludge recirculation volumetric flow and treatment volumetric inflow. In any case, the capacity of the sludge recirculation system is not to be less than 200 % of the daily average total inflow.
Nutrient requirements (C:N:P ratio) – Besides carbon, hydrogen, and oxygen biomass needs nitrogen, phosphorous, and micro-nutrients such as iron, calcium, magnesium, copper, zinc and so on. Most industrial wastewaters lack N and P which is to be added (in the form of urea, super-phosphate or ammonium phosphate) to maintain optimal microbial growth conditions. The minimum C:N:P ratio needed for optimal microbial growth in the in aerobic processes is 100:5:1, and anaerobic processes is 330: 5: 1.
The most common biological processes are described briefly below.
Aerobic processes – activated sludge process – Biological processes, employing aerobic biomass in suspension, have traditionally been known as activated sludge processes. The ASP was developed in the United Kingdom in the early 1900s for the treatment of the domestic sewage and it has since been adapted for removing biodegradable organics in industrial wastewater. The ASP and its variants are capable of treating biodegradable wastewater of moderate strength (10 mg/l to 1,000 mg/l BOD) to high strength (greater than 1,000 mg/l BOD). The ASP does not remove heavy metals or TDS. Some contaminants such as cyanide, and heavy metals such as chromium and mercury, present in the wastewater act as inhibitors for the proper functioning of the ASP as well as other biological processes. ASPs have been categorized, according to the mass loading design, in three groups namely (i) low load activated sludge (extended aeration, oxidation ditches, etc.), (ii) medium load (or conventional), and (iii) high loading.
The ASP involves blending settled primary wastewater or equalized influent with a culture of micro-organisms into a fluid called ‘mixed liquor’. This mixed liquor is passed through an aeration tank which provides an adequate oxygen rich environment for the microbes to eat and stabilize the organic matter in water. Mixing brings oxygen and food to micro-organisms allowing the micro-organisms to clump together whilst preventing floc settling in the aeration tank. The process produces ‘waste activated sludge’ (WAS) consisting of microbes and excess microbial matter. The solids and treated wastewater are separated in a secondary clarifier or other solids separation step such as a membrane bioreactor (MBR). Here the majority of the WAS is returned to the aeration tank as returned activated sludge (RAS) to maintain the microbial population in the aeration tank, as well as ensuring that the activated sludge is old enough to degrade COD and aromatic hydrocarbons. The remainder is removed and undergoes thickening.
The secondary clarifier has the dual purpose of clarifying the wastewater as well as concentrating the sludge. The process is sensitive to pH fluctuations, where a high or low pH can upset the system and cause overloading of the clarifier. Fig 5 shows a schematic of the activated sludge process and membrane bioreactor process.
Fig 5 ASP and MBR processes
Nitrogen containing compounds are toxic to aquatic life, deplete oxygen in the receiving waters, adversely affect public health and reduce the potential for water reuse. Hence, nitrogen containing compounds are removed, if deemed excessive, by nitrification and then denitrification processes. Organic nitrogen is converted to ammonia, then converted to nitrite, which is further oxidized to nitrate and finally to gaseous nitrogen. Denitrification consumes alkalinity and needs to be sufficient so as not to depress the pH. It requires 7.14 mg/l of bicarbonate alkalinity for each 1 mg/l of ammonia nitrogen removed. Oxygen also needs to be maintained at concentrations closer to 4 mg/l for denitrification. The process control is normally customized for each effluent treatment system depending on wastewater characteristics and for optimal operation.
Aerobic processes – oxidation ditch process – This process which has been developed in the 1950s in the Netherlands, is a variant of the ASP and is a special form of extended aeration. The shape of the oxidation ditch is like a ring. Wastewater, micro-organisms and activated sludge is mixed in a continuous loop ditch in order to complete nitrification and denitrification reactions. The oxidation equipment consists of ditch body, aeration mixers and inlet and outlets. Given its long hydraulic retention time of 20 hours to 36 hours, low organic loading and long sludge age compared to conventional ASP, equalization, primary sedimentation, and sludge digestion tanks are omitted.
Oxidation ditch has many advantages in that it provides (i) low energy consumption, (ii) low maintenance, (iii) ease of operation, low capital expenditure, (iv) less sludge due to long extended solids retention time, and (v) resistance to shock loads and hydraulic surges due to long hydraulic retention time. The disadvantages are that the effluent suspended solid quality is inferior to the ASP process and needs a large land area.
Aerobic processes – sequencing batch reactor – The sequencing batch reactor (SBR) process differs from the other ASPs. It is a batch process. The principle is that all of the process steps of ASP, i.e. primary settling, biological oxidation and secondary settling take place in a single tank. The process steps are filling, react, settle, draw, and idle. SBR is compact and has low capital expenditure. It is used when land area is scarce since it needs only one tank to fulfill the aeration and clarification steps. It is also used to treat nitrogen and phosphorous. Standard cycles are normally 4 hour to 6 hours long, resulting in 4 to 6 reaction cycles per day. Compared to the conventional ASP, it is resistant to shock loading, flexible operation due to adjustment of run time and low sludge production.
Trickling filters – These filters, developed in the 1890s, are an example of a fixed film biological process compared to the ASP which is a suspended process. A trickling filter consists of bed of coarse material, such as rounded rocks (25 mm to 100 mm in diameter), crushed stone, wooden or plastic slats and plastic rings over which wastewater is discharged from moving spray distributors or fixed nozzles. The filter media provides a large amount of surface area for the micro-organisms to cling and grow a jelly like bio-film of around 10 mm thickness. In the outer portions of the bio-film (0.1 mm to 0.2 mm) the aerobic bacteria break down the organic matter. When the bio-film becomes very thick it falls off and a new bio-film layer forms. Modern trickling filters use plastic media over rocks since they weigh less and because of it, filter media can be upto 6 m in depth compared to 3 m in depth for rock filters, allowing taller filters using less land area.
The filter effluent is recycled to minimize drying of the filter media, improve filter efficiency, and reduce odour potential. Sometimes, two filters are assembled in series to handle strong wastewater. The sprays rotate at 2 revolutions per minute (rpm) to 5 rpm and a typical wetting rate is 0.6 cum/hour to 2.4 cum /hour. When the wetting rate is too low, the water does not penetrate the depth of the filter bed uniformly causing channeling and acts as an incubator for flies, as well as creating odour problems. Low rate filters operate on natural ventilation, whereas high rate filters require forced draft fans to provide adequate ventilation.
The trickling filter is followed by a secondary clarifier. Trickling filters are classified according to the organic and hydraulic loads such as low rate, intermediate, high rate, roughing filter and super high rate. The advantages of a trickling filter are (i) lower energy requirements than ASPs, (ii) simple operation with no issues of MLSS inventory control and sludge wasting, (iii) better recovery from shock toxic loads, (iv) no problems of bulking sludge in secondary clarifiers, (v) compact and suitable for place where land is scarce, (vi) less equipment maintenance needs, effective in treating high concentration of organics dependent on type of media used, and (vii) better sludge thickening properties. The disadvantages are organic loading levels, that the effluent water quality (in terms of BOD and TSS) is lower than ASP and can need further treatment, odour problems, flies, prone to plugging of filter media and at low temperatures natural ventilation systems do not operate that well.
Moving bed bioreactor – Moving bed bioreactor (MBBR) was developed in the 1980s by Kaldnes in Scandinavia. The MBBR process is a more modern fixed film process in which the micro-organisms grow on plastic media. The media are made from high density polyethylene or polypropylene with a diameter of 13 mm to 25 mm, and hence have a large surface area which helps the biomass to grow inside the surface and are in constant motion due to the compressed air which is blown from under the tank. The process has been applied in a variety of industrial wastewater treatment applications in aerobic and anaerobic modes with or without denitrification depending on the mode of mixing.
Benefits of MBBR are that it is good for high organic loading applications, improved settling characteristics, no need for sludge recirculation from secondary clarifier thereby making it a ‘once through’ process, compact and low footprint compared to the ASP process and has modular construction. It can also retrofit existing ASP systems, needs fewer operational controls than ASPs, and contains fewer mechanical and instrumentation controls compared to a MBR system. A typical hydraulic retention time for MBBR is 2 hours to 3 hours, compared to 12 hours to 24 hours for ASPs. Disadvantages of MBBRs compared to the ASP are that it needs a higher oxygen concentration, the need for improved influent wastewater screening, and additional hydraulic profile head losses due to flow through the media screening devices.
Membrane bioreactor – Though external membrane bioreactors were originally developed in the 1960s, they became popular only after the development of the immersed (submerged) MBRs in the late 1980s. The lower operating cost of the submerged MBR configuration and the decreasing cost of the membranes have made MBRs a popular choice for domestic and industrial wastewater treatment. MBRs are used for industrial wastes with BOD of 5,000 mg/l to 40,000 mg/l with BOD ranges of 200 mg/l to 600 mg/l. Fig 6 shows the principle of membrane bioreactor.
Fig 6 Principle of membrane bioreactor
The quality of the final effluent from a conventional ASP unit is highly dependent on the hydrodynamic conditions in the clarifier and settling characteristics of the sludge. This leads to variable performance. As a result, large clarifiers are needed with long residence times. The MBR process was developed to remove these disadvantages of conventional ASPs.
MBRs are a hybrid with two interdependent treatment processes: biological treatment and membrane treatment (Fig 5). It is similar to a conventional ASP in that both have mixed liquor solids in suspension in an aeration tank. The difference in the two processes lies in the method of separation of bio-solids. In the MBR process, the membranes create a solid barrier to bio-solids based on micro-filtration (MF) with a pore size of 0.6 micrometers, or ultra-filtration with a pore size of 0. 04 micrometers, and hence are not subject to gravity settling characteristics of the solids. Thus, a MBR unit brings aeration, clarification, and filtration in a single step with MLSS concentrations reaching 20,000 mg/l or higher resulting in a smaller footprint than conventional ASP units.
MBRs provide a final effluent quality independent of sludge conditions with higher removal of organics and persistent pollutants, and nutrients with COD removal of 98 % and suspended solids removal efficiency of 100 %. The high quality effluent produced is ideal for reuse applications. Another feature of MBRs is the long sludge age. However, this also contributes to fouling of membranes. Moreover, MBR units can be installed directly to a reverse osmosis (RO) plant, bypassing the need for an ion exchange or other equipment to protect a membrane plant provided the hardness or scaling compounds are not excessive.
There are two types of MBR configurations namely immersed and side-stream. Immersed systems are more common in large industrial units, whereas side-stream is limited to smaller units. There are also differences in the membrane employed from hollow fibre, flat plate, and tubular. Immersed MBRs use hollow fibre or flat plate whereas tubular membranes are used in side-stream MBRs. MBR produces an equivalent treatment level to an activated sludge process followed by micro-filtration or ultra-filtration.
Despite the advantages of MBRs, there are still challenges in using MBRs in industrial applications. The advantages of MBR are (i) 25 % lower footprint, (ii) replaces the clarifier and gravity filter of conventional systems, (iii) ideal for land constrained sites and lower hydraulic retention time of 4 hours to 8 hours. MBR provides impermeable barrier for solids producing highest quality effluent with BOD less than 5 mg/l and turbidity of less than 0.1 NTU (Nephelometric turbidity unit). Membrane fouling is one of the major challenges which results in reduced performance and frequent cleaning or membrane replacement leading to increased maintenance and operating costs. All MBRs require a minimum of fine screens of 3 mm. Sludge produced can be difficult to dewater. Sludge retention time is independent of hydraulic retention time. High sludge age of 15 days to 140 days can be obtained. It has modular expandability, less odour, and flexible operation with less susceptible to upsets. The process can be automated.
Secondary clarifiers – The purpose of the clarifier is twofold. One is to thicken the solids after biological treatment and then settle them out. The second is to produce a clear effluent of the settled solids. Clarifiers in activated sludge systems are to be designed not only for hydraulic overflow rates, but also for solids loading rates. This is because both clarification and thickening are needed in activated sludge clarifiers. Of the process variables the most important is sludge age or mean cell residence time. Another important control parameter is the solids loading rate which is defined as the required surface for suitable sludge thickening in the bottom of the unit (compression zone). The clarifiers are either of rectangular design or of circular design.
Conventional secondary treatment frequently is not sufficient to meet the required effluent quality standards to discharge water to surface water bodies. The effluents can need tertiary processes so as to complete solids and organic matter removal, for colour reduction or recalcitrant compounds degradation, nutrient reduction, and disinfection. The persistent contaminants which the secondary treatment is not able to remove are removed by the tertiary treatment process. These processes are classified as ‘tertiary treatments’, as they are installed after secondary treatment, but some of them, like oxidation processes, can be also placed before biological treatment to improve the bio-degradability of recalcitrant compounds.
Before the treated wastewater is reused, recycled, or discharged to the environment, the tertiary treatment process is used as a final cleaning process cleaning process to improve the quality of the wastewater. For the removal of nutrient (nitrogen and phosphorus), removal of toxin [pesticides, VOC & metals], and for the polishing of the effluent like BOD & TSS, the tertiary treatment processes are used. These processes are the extension of conventional secondary biological treatment process for the further stabilization of the substances which demands oxygen in the wastewater, and also to remove the nitrogen and phosphorus.
The physical and chemical separation techniques like activated carbon adsorption, flocculation or precipitation are the process involved in the tertiary treatments. The most common tertiary treatment applications are filtration and disinfection and where applicable ammonia and phosphorous removal. Ammonia is toxic to fish and phosphorous causes algal blooms.
Filtration – Filtration is a separation process which consists in passing a solid–liquid mixture through a porous material (filter media) which retains the solids and allows the liquid filtrate to pass through. Granular media polishing filters are used the removal of suspended solids for the removal of suspended solids in the 5 mg/l to 50 mg/L range. The most common filters are the multimedia filters. The quality of the filtrate depends on the size, surface charge, and geometry of both suspended solids and filter media, as well as on the water analysis and operational parameters. Based on media filters can be categorized as (i) single media (sand or anthracite), (ii) dual media (sand and anthracite), and (iii) multimedia (garnet, sand, and anthracite).
The most common filter media in water treatment are sand and anthracite. The effective grain size for fine sand filter is in the range of 0.35 mm to 0.5 mm, and 0.7 mm to 0.8 mm for anthracite filter. In comparison to single sand filter media, dual filter media with anthracite over sand permit more penetration of the suspended matter into the filter bed, thus resulting in more efficient filtration and longer runs between cleaning. The design depth of the filter media is a minimum of 0.8 m. In the dual filter media, the filters are normally filled with 0.5 m of sand covered with 0.3 m of anthracite.
In industrial applications, filters are housed in steel pressure vessels where the interior is epoxy coated, with interior manifolds for distribution of water and an under drain system for collection of filtrate and backwashing.
As the filter vessel for pressure filtration is designed for pressurization, a higher-pressure drop can be applied for higher filter beds and / or smaller filter grains and / or higher filtration velocities. The design filtration flow rates are normally 10 m/h to 20 m/h and the backwash rates are in the range of 40 m/h to 50 m/h. The available pressure is normally about 2 bars to more than 4 bars.
For feed waters with a high fouling potential, flow rates of less than 10 m/h and / or second pass media filtration are preferred. If the flow rate has to be increased to compensate for one filter which goes out of service, the flow rate increase is to be gradual and slow to prevent the release of previously deposited particles.
During operation, influent water to be filtered enters at the top of the filter, percolates through the filter bed, and is drawn off through the collector system at the bottom. Periodically, when the differential pressure increase between the inlet and outlet of the pressure filter is 0.3 bars to 0.6 bars, the filter is backwashed and rinsed to carry away the deposited matter. Backwash time is normally about 10 minutes. Before a backwashed filter is placed back into service, it is to be rinsed to drain until the filtrate meets the specification. Backwash rates when excessive leads to loss of filter media.
Variations of the deep rate filtration are high rate filtration which operates at much faster inlet flow rates. Aside from media filters, other types of filters are disc filters and cartridge filters. These are also used to protect membrane filtration systems. Disc filters made from pleated cloth media have very high flow rates and a small footprint, producing very high quality water suitable for reuse applications and do not need extensive backwashing.
Some advanced water treatment processes are also used as the tertiary treatment. These processes are applied to the conventional treated wastewater to improve the quality upto a degree suitable for various applications of recycle and reuse including the potable reuse. The additional tertiary treatment processes are different membrane treatment processes like micro-filtration, ultra filtration, nano-filtration, other processes like reverse osmosis, advanced oxidation processes, and additional disinfection processes like ozonation and the use of ultraviolet radiation. Some of these advanced processes are described below.
Membrane technology – Membranes are a popular choice for water reuse applications since their advent in the 1960s. Costs of membrane systems have reduced dramatically and, coupled with technological advances in membrane design, membrane options and operating limits, the range of applications in water and wastewater treatment is increasing rapidly. In pressure driven membrane filtration, membranes separate the components of a fluid under pressure. The membrane pores, being extremely small, allow the selective passage of solutes. The popularity of membrane processes arises from the fact that they are effective in the removal of both dissolved and suspended solids. A wide range of materials like cellulose acetate, polyamides, poly- sulfones, poly-propylene, nylon, poly-acrylonitrile, poly-carbonate, polyvinyl alcohol, poly-tetra-fluoro-ethylene, ceramic, and metal composites are basically used to produce the membranes. The membrane pore size is the parameter for the degree of selectivity of a membrane. On the basis of the pore size, there are four types of pressure driven membranes. Micro-filtration and ultra-filtration are low pressure applications given their larger pore size. Nano-filtration needs medium pressure, and ‘reverse osmosis, given the smaller pore size, needs significant pressure to push the solute through the membrane.
Advanced oxidation processes – Advanced oxidation processes (AOPs) are defined as processes which involve generation and use of powerful but relatively non-selective hydroxyl radicals in sufficient quantities to be able to oxidize the majority of the complex chemicals present in the effluent water. The AOPs show specific advantages over conventional treatment alternatives since they can eliminate non-biodegradable organic components and avoid the need to dispose of residual sludge. After fluorine ([V (volts) = -3.06], hydroxyl free radicals (OH-) have the highest oxidation potential (V = -2.86). In the AOP process, OH – radicals are generated which in turn react with organic molecules to generate CO2 and water. AOPs can be classified into two groups, non-photochemical AOPs and photochemical AOPs. Photochemical means a light source is needed. Normally ultra violet (UV) light is used as the photo-chemical source. Low pressure UV lamps have a wavelength of 254 nm. Maximum ozone absorption takes place at a wavelength of 253.7 nm. Of the non-photo-chemical technologies, those most prevalent in the treatment are Ozonation, Ozone/ (H2O2) and Fenton’s reaction.
Ozonation – Discovered in 1785, Ozone (O3) is a widely applied strong oxidizing agent (-2.07V) for disinfection of potable water and wastewater, decolourization, odour removal, organics degradation and cyanide destruction, etc. O3 at room temperature is a bluish pungent gas, sparingly soluble in water, highly corrosive, toxic and explosive when the concentrations in air exceed 20 %. As a germicide, it is 3,125 times faster than chlorine. Ozonation efficacy is increased with high pH and temperature. When O3 dissolves in water or wastewater it can remain as the O3 molecule (at pH more than 7 and slower reaction) or decompose (pH more than 8) producing the hydroxyl free radical (OH-) which is a 35 % stronger oxidizing agent than O3. Both reactions occur simultaneously and hence reaction kinetics strongly depends on the characteristics of the treated wastewater (e.g. pH, organic concentrations, presence of foaming agents and surfactants, ozone concentration and temperature, etc.). A pH of 8 to 10 is most suitable for oxidation of organic compounds. Ozone is sparingly soluble in water and rapidly decreases with increasing temperature. At a temperature of 20 deg C, 100 % ozone solubility in water is 570 mg/l. The preferred temperature ranges from 25 deg C to 50 deg C.
A simplified reaction mechanism of ozone at a high pH is given by the equation 3 O3 + H2O = 2OH radical + 4 O2. Molecular O3 is a very selective oxidant. It only reacts with certain compounds and for this reason it can be applied in low dosages for industrial wastewater applications. It can also inhibit or destroy the foaming properties of residual surfactants as well as oxidizing a good portion of the COD. Thus, O3 improves the overall biodegradability of the effluent by converting recalcitrant compounds to easily digestible compounds and can be applied upstream or downstream of a biological treatment plant. The residual oxygen in the vent gas can be recycled back to the secondary biological treatment plant, reducing aeration requirements. Another advantage is that it does not increase sludge mass.
O3 can be applied in the gaseous form and because of its unstable nature it needs to be generated on-site from air or pure oxygen using UV radiation, electrochemistry or corona discharge generators. O3 leak detectors are to be installed to give audible and visible warnings and shut down the generators in the event of a leak. O3 is deactivated in the presence of high concentration of salts. A variation of this process is the O3/H2O2 process. Developed to reduce the O3 concentrations, the H2O2 acts as a catalyst enhancing the capability of O3 to produce more OH radicals. At a low pH, H2O2 reacts very slowly with O3 and at a high pH (alkaline conditions) reacts rapidly.