Lecture 29 : Wastewater recycling

Greetings. So, in the previous class, we had just discussed the recycling of iron and steel making scraps. And before that we had discussed the iron and steel making slags. And now we are at the end of this discussion on iron and steel making industry wastes, where we will be discussing waste water recycling. Now, why exactly is water being described in steel making waste club? So, the idea is that in this collection of wastes that we look at from basically iron and steel making industries, water waste is very much prominent. It does not appear to be prominent unless until we look at the total quantity of water that is used. So, in today's class, we will be discussing about how waste water is generated, in what fronts and what are the variations in chemical compositions. And what we can do to recycle this waste water to make it fit enough to reuse either reuse in the industry itself or if at all it has to be discharged into the water stream, what is the chemical composition treatment that one needs to do before it is released into the environment. Because we know that the different types of metallurgical wastes that we have seen, let us say from aluminum industries, copper industries, zinc industries and even in iron industries. We know that the direct discharge of these wastes is hazardous, but most of these wastes were solid. And let us say zinc dust is basically the is also actually a solid, but it can be released in the atmosphere also, but most of these wastes are solid. What happen if a liquid is discharged and if this liquid is discharged directly into the water source. Now, we are talking of waste water that is generated in iron and steel making industry in various units and the composition of waste water can change depending on where it was basically generated. And one has to think very judiciously as to what is the process that one has to follow to recycle these waste water streams. So, we will just begin, wide range of processes generate waste water streams. This is not just one single stream, these are waste water streams. Now, let us just broadly see one might not consider, but mining waste water is also one might call it mining drains. So, mining waste water there is coking plant waste water that is happening in the metallurgical coke generating unit. Then we have sintering unit. So, we have sintering waste water and of course, these are just pretreatment units. Finally, we are reaching the iron making, iron making waste water. After iron making we will have steel making waste water. Then we have steel rolling because rolling consumes lots of waste water, rolling process consumes lots of water. So, it generates and then we have basically the cold rolling process itself consumes water and that water can contain large amount of metal ions, which can be brought back into the material cycle. So, we see that these are the categories broad categories of waste water that is basically generated in different processes. We know that such is the consumption, what exactly is the overall consumption per ton of steel. So, let us look at that. It has been found that nearly 28.6 meter cube water is used for 1 ton of steel production, which means this is the estimate of waste water generation per ton. Just multiplying it to let us say the world production of steel in a given year would give us the water that is used directly in steel making industries, taking into account the totality of the steel making industries of the world. Now, this is what we are really looking at and this itself is a very large quantity. Of course, this number is not big when we are just considering 1 ton, but consider the complete steel production throughout the world. Then this number becomes very much significant and which is itself a big motivation by really it is a big problem and by really discharging this waste water into let us say nearby river or any other water source is definitely not a good option. So, quantity of steel versus quantity of water used is basically what we know and we know that nearly 90 percent of the water that is used is basically it can get reused by after pretreatment and removal of ions because at times we also have to consider some evaporation loss. So, even if we consider a given quantity of water is used and we have some losses in evaporation, still the water can be recycled. So, that releasing of water is definitely not an option at least untreated water. So, one has to think of treatment so that we can reuse it or else it can be safely discharged only if we are not intending to reuse it. So, that is how we can proceed. Now, we must note that what are really the variations in chemical compositions. We have seen the categories of waste waters and such a classification of waste water and the chemical compositions that we are now about to study, the variations are enormous because these are generated in different sources. So, one really cannot club these waste waters together. These waste waters should be ideally these waste waters should be separately treated. We know that it may not be possible, but the chemical compositions vary a lot. So, we will now discuss the chemical composition. Here, we have table 1, table 2 and table 3 collected from different sources. These are of course, from different categories of steel making of steel making waste water. So, we have steel making unit, we have cold rolling and again we have steel making. With this we can take an estimate that really the chemical composition fluctuates a lot from sample to sample, from unit to unit and of course, from plant to plant. So, if in a steel making waste water sample, we can say we have iron 4.8 milligrams per liter. We can also have different other elements. These are heavy metals. So, we have A S C D C U C R H G N I P B and Z N. Of course, these are present in relatively very less quantities. Of course, mercury is also in micrograms per liter that is the scale for mercury in this specific data. So, if we look at similarly, if we look at cold rolling waste water, we see that this data is milligrams per liter and these are the concentration in ions of ions. We see that here we have sulphates and chloride described here and we have the metal ions on this side. So, we have sodium, calcium, magnesium, iron and we see that sodium is present in very large quantities in cold rolling waste water. So, cold rolling waste water is having sodium sulphate, chlorine, calcium, magnesium and iron and of course, here we see that iron is very less. Again, from a different steel making waste water stream, we see that the concentration of iron F e 2 plus P b 2 plus Z N 2 plus and C R 6 minus C R 6 plus basically hexavalent of chromium. These are present in such quantities again the concentration is given in milligrams per liter and we see that such 309 milligrams per liter is the concentration of iron. So, we see that the concentration of various metal ions can fluctuate by a large margin. So, at times we can have let us say single digit concentration and at times it can go to hundreds. This depends on what is the source of waste water. So, simply identifying that we will recycle this type of waste water is not going to help. One has to precisely monitor what is the concentration of metal ions in water, what source if water sources vary or let us imagine a situation where different water streams are merged. Then, keeping a track of concentration becomes very difficult because then the homogenization of ions may or may not take place and then one has to build a special reaction mechanism where most of the metals can be isolated. Now, that may also become fruitful if we have such technologies developed, but if we have separated sorted streams of waste water and dedicated units then we can recycle these waste water appropriately. Now, application of technology, the recycling technology for waste water and the technologies that we are about to discuss let us say reverse osmosis or ion exchange resin beds or nanofiltration units. If these technologies are available then these can be clubbed one after the other and waste water in general can be processed, but such facilities have to be developed. So, that efficient removal of these heavy metals from the waste streams is achieved. So, from this table what we see is that in most of the waste water samples we have iron and we have other heavy metals like chromium, zinc and in some cases copper and lead, magnesium, sodium these are present. So, one can devise a strategy of precipitating or removing these metal ions and then if at all these salts can be separated out further pH balance can be done. So, that these can be reused in the plant itself. So, one has to think of wide range of chemical composition wide range of chemical composition for available for chemical composition for chemical composition dictates the recycling process. So, if we know that this that wide range is available we just directly focus on the available chemical composition and then directly just work around the given waste water samples. Now, what are the recycling strategies recycling processes? We know that the chemical composition has to be the key because we have a wide range of waste waters and why do we really need to recycle why recycle waste water. So, we know that the quantity is enormous just need to multiply the numbers here. So, x meter cubed in this case it is 28 meters cubed into the total steel. So, that is how we could just say that it acts it is actually not and then we know that chemical composition has heavy metals and it is not safe to discharge. So, environmental concern if that was an engineering concern this is an environmental concern and it is very hazardous hazardous waste. Now, how do we really go about the recycling process itself? So, conventional processes the conventional processes are basically the precipitation chemical precipitation. So, if we are using sodium carbonate or calcium carbonate that is basically the chemical precipitation so that we are able to precipitate some salts. So, what we might get is some salts or we can have chemical reactions. If we are targeting some specific product from waste waters then we can have chemical reactions. This can also be aiming removal of chemical of these heavy metal ions that. So, why we are recycling? Because we want to bring it out. So, removal of heavy metals as compounds products. So, this can be done conventional processes involve the precipitation or we can also have some sort of oxidation. So, oxidation or reduction reaction that can help us in removal of these products. Now at times these might work, but now the chemical variation itself in waste water is enormous one has to think of devising other strategies also on top of them. So, we will now look at those strategies which are beyond the regular precipitation strategy. So, one can think of use of ion exchange or reverse osmosis, but we will have this different strategy where in let us say we have feed we have membrane nano filtration membrane and this gives us permeate rejected stream of waste water. So, nano filtration membrane is generating two types of feeds. So, after filtration they reject of the nano filtration membrane unit is basically released that is being processed. So, mixing tank is present here and this mixing tank is basically picking up reject of waste water stream coming from n f membrane and this is basically organic solvents. Organic solvents that can be miscible with water this leads to generation of solution, solution of water and organic solvents and we have a settling tank. So, we had a mixing tank now we had a settling tank and after that we have the removal of salts. So, here after the mixing of organic solvents we let it move to the settling tank where there is the ejection of salts and we also have the movement of solution to a vacuum solution and after condenser what we get is basically treated water and one stream is actually going up to the organic solvents. So, basically this is the organic solvent stream that is recycled here in the condenser. What we see here is that we have a distillation we have a wide variety of products actually. So, what are important products here? First step is basically permeate of n f membrane nano filtration membrane. The second one is basically the salts that is generated in the settling tank. The third one is the organic solvents. So, these are basically cycled and the treated water which is what we really wanted. So, such type of operations is this is just one method by which we can club different types of operations one after the other and remove salts, but it is lot of investment to you know to remove salts. So, we have to even think of such recycling strategy. One has to be very sure about what we really wish to remove and to what extent because we know that most of these processes have their own advantages and disadvantages and their limitations as to which as to what is the limit of metal extraction. Now, if we are able to achieve the desired chemical composition after these processes, then it is acceptable else one really is in a dilemma what type of processes should be used to achieve the chemical composition. Now, what are the other processes used for waste water recycling? So, what are the other processes? Here we are just going to jot down some of the processes that people have explored in waste water recycling and what are the key points that they wish to put up for addressing the waste water issue. So, some of the processes are electrochemical reactions. We look at electrochemical reactions that is oxidation. We can have chemical oxidation. We have photo degradation with H 2 O 2 addition and this is basically H 2 O 2 addition and UV radiation. We have electro coagulation. Similarly, we have precipitation. Now, we know that precipitation is a conventional route, but still people have tried different methods. Precipitation of specific products let us say hydrates, hydroxides and sulphides. Now, adsorption of various products of let us say various heavy metal ions has been explored using lot many different materials. So, we will just write adsorptions using rice husk or kaolinite clay or coke breeze or nickel aluminum LDH layer double hydride layer double hydroxide. So, LDH NILLDH and so on so forth. So, lot many adsorption reagents materials have been used for the adsorption reactions wherein people have tried to use materials as adsorbents and the heavy metal ions or the material of that of their choice are adsorbed. In turn, leading to the waste water recycling and of course, there is the use of microalgae. So, one of the microalgae could be Arthrospera maxima, A maxima and such microalgae have been used for the remediation of waste water. Now, the degree of success for various types of processes are varied and this again depends upon sample to sample processing conditions to from one type of process to the other and what exactly was the chemical composition such data is available and one can really devise an ideal strategy for waste water recycling. If we are able to club these strategies one after the other, but again it depends upon the economic and technological constraints that are present in the industry. So, now we are at the end of this metallurgical waste unit of iron and steel making and now in the upcoming classes we will be focusing on electronic wastes, wherein we will be discussing different types of electronic wastes and the general methods of recycling such wastes. Thank you.