PAPER No.4 : Environmental Chemistry MODULE No.13 : Advanced Waste Water Treatment

Transcription

1 Subject Chemistry Paper No and Title Module No and Title Module Tag 4, Environmental Chemistry 13, Advanced Waste Water Treatment CHE_P4_M13 Dr. S.K. Garg, Principal, Deen Dayal Upadhyaya College, University of Delhi Dr G S Sodhi Asssociate Professor SGTB Khalsa College University of Delhi

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3.Removal of Suspended Solids 3.1 Micro-Straining 4. Removal of Dissolved Solid 4.1 Adsorption 4.2 Reverse Osmosis 4.3 Electrodialysis 4.4. Ion Exchange 5. Nutrient Removal 5.1. Removal of Nitrogen 5.2. Removal of Phosphorous 6. Summary

3 1. Learning Outcomes After studying this module, you shall be able to Know about the various advanced techniques of water purification Learn the concept of electrodialysis, ion exchange, etc. for purification of water. Identify the need for advanced water treatment Learn the concept of nutrients removal 2. Introduction The waste water received from various sources (industrial, domestic, sewage etc.) is purified by a series of steps. The primary and secondary treatment removes most of organic waste (Biological Oxygen Demand) and suspended solids present in waste water. However, up to secondary level of treatment the effluent water is insufficient to meet the quality criteria of drinking water, reusable water for industrial and/or domestic recycle. The effluent from a typical secondary treatment plant still contains mg/l BOD which may be objectionable in some streams. Therefore to meet the quality of effluent provided by secondary treatment, it is essential to carry out some additional treatment, usually called tertiary or advanced treatment. This is done to obtain high quality water as compared to that obtained from secondary treatment processes. The advanced treatment contains the unit operations not normally found in Secondary Treatment. Tertiary treatment removes suspended solids, dissolved organic and inorganic solids and nutrients. 3 Removals of Suspended Solids The removal of suspended solids is done after the secondary treatment has been done. In this process, initially relativelylarge particles are removed by simple settling and filtration processes. After this, the next step is the removal of very small particles for which a special type of filtration technique is used,called as micro-straining. 3.1Micro-Straining For carrying out micro staining, micro strainers are used. Micro strainer is made up of rotating drum supporting a very fine, stainless steel or plastic screen. One end is sealed and the other allows water in and screenings out. This filtration uses very fine wires of stainless steel drawn to diameters barely visible to naked eye. These stainless wires are woven on special high precision looms for producing micro fabrics. The secondary effluent enters into the drum from the centre and flows out through the sides, with the mat of solids accumulating on the screen inside the drum. The drum s top is above the water level therefore it is continuously cleaned by water jets on the outside. The micro fabrics can intercept a large proportion of solids that are smaller than the minute apertures in the woven fabrics. Microorganisms having diameter as low as 7-12 µ can also be removed effectively by this method. The type of impurities removed by micro-straining are paper fibres, humus, fly ash from power stations and microorganisms like diatones and green algae. Other methods that are used to effectively remove suspended solids are coagulation followed by filtration. The water coming out from micro strainers are cleaner and devoid of very small impurity particles.

4 Figure 1 Micro-Strainer 4. Removal of Dissolved Solids After the insoluble particles are removed, next removal is achieved for the soluble impurities in the polluted water. There are inorganic as well as organic dissolved substances present in the polluted water. Several methods are used for removal of organic (adsorption, solvent extraction etc.) and inorganic contaminants (electrodialysis, reverse osmosis and ion exchange etc.) and these are described as under 4.1 Adsorption Adsorption is the most commonly used technique for the removal of both dissolved solids. Adsorption is a surface phenomenon and is based on the surface area available for the adsorbing species. Activated carbon is the most commonly used absorbent on whose surface the impurities get adsorbed. It is prepared by subjecting charcoal to oxidizing steam at high temperature of about 950 C. The water gas is released from the charcoal to develop a very porous structure in the charcoal, thus the adsorption process increases. Therefore activated charcoal is capable of adsorbing wide variety of compounds and when it becomes saturated with impurities, it is heated in vacuum to about 980 C and the absorbates are driven off. The adsorbent could be reused. It is used for the removal of soluble organics such as phenols, chlorinated hydrocarbons, surfactants, small amounts of dyes and objectionable tastes and odour producing substances from the waste water.

5 4.2. Reverse Osmosis Reverse osmosis (RO) process is the reverse of the osmosis process and is used to remove dissolved impurities like salts form industrial waste water and has become more popular at homes for water purification. This results in reduction of concentration of dissolved solids, including a variety of ions, metals and suspended particles such as asbestos. Osmosis is the natural process, which involves the flow of solvent from the less concentrated solution or pure liquid into the more concentrated solution across a semi permeable membrane. The pressure, which is exerted by the solvent, as it passes through the semipermeable membrane is known as osmotic pressure. On the contrary, reverse osmosis, as the name suggest, is the reverse of osmosis. Here the direction of flow of solvent (water) is from the high concentration solution (i.e impure water) to low concentration (pure water). To carry out the RO process, pressure needs to be applied. The contaminated water is placed above a semi permeable membrane and subjected to high pressures. Pure water flows down the semi permeable membrane and solute molecules (impurities) which are larger than the pore size of the membrane are retained by the membrane. Pressure applied in reverse osmosis is of several order of magnitude in excess of the pressure in the osmosis process known as osmotic pressure. Figure 2a and 2b show the process of osmosis and reverse osmosis respectively. This process is effective in removal of the particles in the size ranges form 10-4 to 10-2 µm and operates at efficiency greater than 90%. Osmosis Figure 2a: The phenomenon of osmosis

6 Figure 2b: The phenomenon of reverse osmosis RO membranes are effectively non porous and significantly reduce total dissolved solids, heavy metals, organic pollutants, viruses, bacteria, and other dissolved contaminants, but there is a limit to which the mechanical pressure can be put on the solution. High pressure tends to tear the semi permeable membrane. The materials most commonly used for the filtration are cellulose acetate, cellulose butyrate and polyamide. An RO membrane can typically produce gallons of water per day and it depends on the several factors, including membrane type and condition, operating conditions and degree of impurity in waste water. 4.3 Electrodialysis Electrodialysis is basically an electrochemical process involving migration of ions through ion selective semi permeable membrane. Electrodialysis is a membrane desalination process and uses an electric current for the passage of water across the membrane unlike the RO process which uses external pressure. It is an efficient method for removal and concentrating salt. The process is based on the principle that the dissolved salts as impurities are either positively or negatively charged and hence migrate to opposite charge electrodes when electric field is applied. Therefore it uses two types of ion selective membrane: cation membrane, which allows transfer of cations only and anion membrane which allows transfer of anion only. Figure 4 represents the working of an electrodialysis cell. A typical electrodialysis system consists of an electrolytic cell divided into three compartments, using two semi permeable membranes. Parallel channels are constructed by alternating membranes (anionic and cationic) between the electrodes. On passage of electric current through the waste water or saline water, the negatively charged ions migrate to the anode and positively charged ions migrate to cathode. Therefore the solutions in alternate channel are likely to become more concentrated and in remaining become more dilute, i.e ion free pure water.

7 Figure 3: The phenomenon of Electrodialysis Using electrodialysis method, around 50% to 94% of the dissolved solids are removed from waste water containing 12,000 mg/l of total dissolved solids. The limitation of electrodialysis is that it cannot be used effectively for the removal of organic molecules because as they are larger in size than inorganic molecules, they get clogged in the tiny pores of semi permeable membrane. The other drawback is that it requires high energy for desalination of concentrated feed solutions Ion Exchange Ion exchange technique is used to remove metals which cause hardness, manganese and iron containing salts. Ion exchanger consists of a cation exchanger coupled with anion exchanger. Ion exchange resins are cross linked, long chain organic polymers with a microporous structure. The ion exchange properties of ion exchanger depends upon the functional groups attached to the chain. Water is first passed over the cation exchanger which are capable of exchanging its H + ions with cations of the impure water and acidic resins containing basic groups (-NH 2, -OH) are capable of exchanging their anions with the anions of impure water. Resin SO!! H! + M! Resin SO!! M! + H! Resin N! OH! + X! Resin N! X! + OH! H! + OH! H! O

8 The overall outcome is the removal of cationic and anionic impurities present in water and the resultant is the demineralized water (Figure 4). Figure 4: Ion Exchanger Both the types of resins can be easily regenerated. Cation exchange resins are regenerated by passing a dilute solution of HCl through them. On the other hand, anion exchange resins are regenerated by passing a dilute solution of NaOH through them. The advantage of ion exchange process is that highly acidic or alkaline water can be treated by this process and this process removes hardness of water. 5. Removals of Nutrients The removal of nutrients (nitrogen containing compounds and phosphate) is very important because excess of nutrients causes eutrophication Nitrogen Removal Nitrogen exists in a variety of forms in waste water. During the secondary treatment of waste water microorganisms decompose organically bound nitrogen to ammonia. In water the NH 3 exists in two forms one is as ammonia gas and other is as ammonium ion. Increasing the ph the ammonium ions are converted into NH3 gas. Dissolved ammonia is then expelled in the atmosphere. NH!! + OH! NH! +H! O

9 The process of nitrogen removal by ammonia stripping is based on the above reaction. It is used to low the ammonia content of waste water. Lime or caustic soda is added to the waste water until the ph reaches and at such high ph, NH4+ ions get converted into ammonia gas.figure 5 illustrates the ammonia stripping tower (counter current), which draws air through openings at the bottom, as waste water is pumped to the top of a packed tower. Free ammonia is stripped from water droplets falling into the air stream and discharged to the atmosphere. This method is effective in the removal of NH3 between mg/l. Nitrification Denitrification A second method to remove nitrogen involves the utilization of aerobic bacteria to convert ammonium ions into nitrates, which on denitrification, followed by an anaerobic stage in which different bacteria convert nitrates to nitrogen gas. In the nitrification step, ammonia is first converted into nitrites by nitrosomonasand further oxidation of nitrite to nitrate occurs by nitrobacter. However the rate of these reaction are slow and require long detention time in the aeration tank as well as sufficient amount of dissolved oxygen. 2NH!! + 3 O! 2NO!! + O!!"#$%&%'%()&!"#$%&'(#)$! 2NO!! + H! O + 4 H 2NO!! The second step of this process is denitrification (reduction of nitrates to nitrogen gas) by anaerobic bacteria like pseudomomas. 2NO!! + CH! OH organic matter!"#$%&'&()# N! +CO! + H! O For reduction to occur, the dissolved oxygen (DO)of the water body should be zero and the energy is needed for this reaction which is derived from the organic matter like methanol Removal of Phosphorous Phosphorous in the waste water mainly exists in the forms of orthophosphate. It could be removed biologically and chemically. The chemical method involves the addition lime and alum. Calcium hydroxide combines with phosphate in alkaline medium to form calcium hydroxyapatite, which is insoluble in water and precipitated out. Alum also forms insoluble precipitate with phosphate as shown in reaction!! Al! (SO! )! + 2 PO!!! 2AlPO! +3 SO!! 5Ca(OH)! + 3 HPO!!! 2Ca! (OH)(PO! )! + 3 H! O + 6OH

10 In biological method, the sludge from sedimentation tank is subjected to anaerobic conditions where microorganisms assimilate organic matter (as well as phosphorus) at a very high rate. Once the microorganisms have adsorbed the phosphorous, they are removed as waste activated sludge, thus carrying with them high concentrations of phosphorous. 5. Summary To meet the quality of effluent provided by secondary treatment, it is essential to carry out some additional treatment, usually referred to as advanced treatment. Several methods are employed in advanced waste treatment to includethe removal of suspended solids, dissolved solids, plant nutrients and toxic substances. The suspended solids are removed by the process of filtration, coagulation and microstraining. The dissolved solids and ions are removed by the methods like ion exchange method, reverse osmosis and electrodialysis etc. The nitrogen and phosphorous are removed from the waste water to prevent the eutrophication. There are two general methods used for the removal of nitrogen from waste water: the physical method (ammonia stripping) and biological method (nitrification and denitrification). Chemical precipitation using lime is used to remove phosphorous from waste water.