welcome back this is part two a four point two water this is our first science understanding desalination is a process used to remove minerals from saline water to produce fresh potable water reverse osmosis and thermal distillation are two widely used methods for desalination will also need to be able to describe the disadvantages of using desalination for the production of potable water this is a picture of the Adelaide desalination plant which is located in lonzell this was fairly recently built so back in 2012 it was built in response to a pretty bad period of drought that South Australia suffered so desalination is a process that is used to remove dissolved salts and minerals from saline or seawater this end results in potable water and potable water essentially means it's safe for human consumption but also human use this can be achieved through one of two ways it can be thermal distillation or reverse osmosis thermal distillation is a process which is a bit more common in the Middle East this is because rainfall is quite low there and they do have a fairly high abundance of fossil fuels which they can use in this process essentially how this works is that sea water is fed through pipes and it is heated that heat could be produced by the combustion of fossil fuels the water then evaporates and eventually it will condense and cool into collection vessels so here we can see the evaporation and then condensation of water into these collection vessels and the water essentially is isolated as pure water one of the concerns is at the bottom we can see that there is a buildup of a concentrated salt solution which we call brine and this is usually safely returned back into the sea through a series of diffusers to ensure that we aren't severely affecting the concentration of the seawater the second method of desalination is through the use of reverse osmosis we can see that in this diagram here in step four what this shows you is that sea water typically undergoes a series of processes before we can carry out reverse osmosis so for example we have intake screens which attempt to prevent any intake of marine life we are floatation units which can allow for the coagulation and flocculation of clay particles and other components and also in step three a gravity sand unit which is essentially designed for filtration this leads into the next science understanding reverse osmosis is a filtration technique whereby water is forced under pressure through a semi permeable membrane you'll need to be able to explain how reverse osmosis produces potable water from saline water before we can talk about reverse osmosis we need to talk about what osmosis is to understand the press also notice what we can see on the left here is a tube which consists of water in between we have this so-called semi permeable membrane to the left we have water with a low concentration of a salt and to the right we've got water with a high concentration of salt this semipermeable membrane allows for small molecules like water to flow through it however it doesn't allow for these components dissolved within the salt to do the same thing what we know is that water will typically flow from an area where it is more concentrated or another way of saying that is that there is a lower concentration of a solute contained within it to a region where there is a lower concentration of water or again in other words a region where there is a higher concentration of solid particles this flow of water from low solute concentration to high solute concentration has to take place through a semipermeable membrane and what it's trying to do is essentially equalize these concentrations on the left and right side so overall we will see the movement of water from the left to the right through the semipermeable membrane until it equalizes the concentration on both sides one other thing to factor in is that osmosis is a passive process so what that means is it doesn't require energy input to do so water essentially naturally flows from low salat concentration to high solute concentration as you can imagine reverse osmosis is essentially going to be the reverse here we can see one side with a low solute concentration on this side what's with a high salat concentration so we can think of seawater what we're trying to do is remove the water from this seawater and end up with a highly concentrated brine solution to do that we have to force water to flow in the opposite direction so in other words we want it to flow against its natural tendencies to do that we would need to supply external pressure which can be quite energy intensive to get water to move from an area of high solute concentration to a region of low solvent concentration as a result on the left side we'll end up with freshwater without this high concentration of salts however this sea water itself like mentioned before will become very very concentrated and like with thermal distillation we would look at needing to find a way to dispose of this so typically this is dispersed back into the ocean and into the Seas through a series of diffusers to try and safely remove it without significantly affecting these salt concentrations in the sea or ocean water reverse osmosis is essentially how the Adelaide desalination plant acts to treat seawater the construction of the desalination plant was relatively controversial and that's because the desalination plants themselves have advantages as well as disadvantages if we look at the advantages first we know that they can produce potable water it also reduces the demands on fresh water another benefit is that it's not dependent on climate so things such as rainfall and it does end up producing a reliable source of fresh water or potable water some downsides however is that they can be extremely energy intensive processes they can be expensive to produce and to maintain the waste products and being heat disposal may be an environmental concern and at times the water inlets can even drag in and kill marine life so it's not good for the local ecosystems this is our last science understanding hypochlorous acid chlorine and high perchlorates oxidizes use for water disinfection explain the effect of pH on the equilibrium between chlorine and water and hydrochloric acid and hypochlorous acid what a disinfection is one of the last steps in producing possible water this typically involves the use of chlorine and its associated compounds such as chloramine and these are used to help disinfect our water suppliers one way to do this is to inject chlorine gas into water the chlorine gas reacts with water and it establishes an equilibrium that's shown by this equilibrium process here over to the right we can see the formation of two key products this one's been broken up into two components we have hard chlorous acid HOCl and hydrochloric acid being expressed as H+ and Cl minus that's because hydrochloric acid is a strong acid which readily ionizes or we can think of it as readily dissociating to produce ions another way of looking at this is to show you how chlorine reacts with water to produce hydronium ions instead of hydrogen ions which we know actually don't exist independently in solution so either-or equation can be used to show this equilibrium process it's important to note that this equilibrium process is going to be pH dependent so we're going to firstly consider this reaction and we're going to look at what happens when the pH is decreased or increased or in other words at low pH and high pH we'll start off with low pH if we were to decrease the pH what that essentially means is that we're going to increase the concentration of hydronium ions in solution because it's an equilibrium process we know that the equilibrium system want to offset this increase in concentration by trying to decrease it and to do that we would see that the equilibrium will favor the back reaction or in other words the equilibrium will shift to the left so that means that more of our products on this side are going to form into chlorine gas and water as a result this increases the concentration of chlorine gas which is actually an undesirable thing on the other hand if we were to increase the pH what this will do is increase the concentration of hydroxide ions and we know that this can react with hydronium to produce water and in doing so will decrease the concentration of hydronium ions so the system itself is going to then favor the forward reaction to try and increase it again or in other words the equilibrium is going to shift to the right and in doing so this is going to increase the concentration of high chlorous acid as well as hydrofluoric acid however it's worth noting that hypochlorous acid is our substance of focus this is the key disinfecting agent and I'll explain why later as a mission before hydrochloric acid is a strong acid so that makes hypochlorous acid a weak acid and we know that hot chlorous acid it is a molecular structure but it has the ability to act as an acid and to lose protons so in the presence of water it can essentially undergo ionization to produce what we call hyper chloride ions that can be represented in this equation here and you can note that this is also an equilibrium process so hatre co reacts with water it produces hard chloride ions as well as hydronium ions like with the previous equation this equilibrium is also going to be dependent on the pH so we can consider what would happen if we were to decrease or increase the pH starting off with a reduced pH so again we know this will increase the concentration of h3o plus on this side here so the equilibrium we can expect will shift to the left and we will end up with an increase in the concentration of HOCl and a decrease in the concentration of co oh minus for increased PHS that's just going to work in the opposite manner so we're going to decrease the concentration of h3o plus the equilibrium is going to shift to the right and in doing so that's going to favor the production of hyper chloride ions remember what I said before that hypochlorous acid is the key disinfecting agent so this is rather undesirable whereas this is desirable because there are two equilibrium processes at play here what it means is that we need to manipulate the equilibrium through pH to try and maintain a high concentration of hypochlorous acid in this graph here we can see that at a pH range of around four to five this is going to ensure that we essentially have a hundred percent of the chlorine available as hypochlorous acid as we increase the pH from this point it will decrease the concentration of HOCl but in doing so it will increase the concentration of hydrochloric Islands that's based on this equation here whereas if we were to further decrease the pH from a pH of four then that will decrease the hypochlorous acid concentration but it will increase the concentration of chlorine and that would be based on this equation here so those effects can be summarized as shown here below before we talk about how hypochlorous acid acts as a disinfecting agent we need to consider these hyper chlorides because these are the active ingredients that we find in many bleaches and disinfectants sodium hypochlorite is typically what we call liquid chlorine because it's a soluble form whereas calcium hypochlorite is typically called solid chlorine because it essentially is in a solid form the benefit of bleaches and disinfectants consisting of sodium and calcium hypochlorite is that they react in water and they end up producing hypochlorous acid we can see that in this equation down here on this slide we can see the two different forms of hypochlorite so granular pool chlorine here we can see the active ingredient is calcium hypochlorite and this white Qing disinfectant we can see zoomed in that it's active ingredient is sodium hypochlorite so that would be in liquid form whereas this would be in solid solid form so the reason why a hard chlorous acid but also chlorine gas and hard chlorides as well as other associated compounds are classified as disinfecting agents is that they typically kill bacteria by their oxidizing action in particular they can cause the oxidation of proteins and enzymes within bacteria that are needed for their own growth they could cause the denaturation of these proteins and enzymes which would then render them ineffective and then that would affect their ability to grow and reproduce out of these we say about hypochlorous acid is the most effective disinfectant and the reason for that is because it is an electrically neutral substance and this means it can penetrate through the cell walls of bacteria bacterial cell walls are typically negatively charged so we can see that high chloride ions would actually have a very hard time penetrating through the cell wall so they would typically have to act on the outside and eventually they can kill bacteria through this oxidizing action to finish up we can see how significant the difference in the ability for say hypochlorous acid and hard chloride ions are in their oxidizing ability this graph shows that it takes approximately one and a half minutes for hypochlorous acid at a concentration of 0.1 parts per million to kill 99% of e.coli bacteria whereas if we had a sample of hyper chloride ions at the same concentration it would take a hundred and twenty minutes to do exactly the same thing and again the reason for that is because the hot chloride ions can only act on the outside of the bacterial cells and so it'll take a really long time to essentially break down the cell wall and cell membrane and then to affect their growth and reproduction so that concludes our work on four point two water thanks for watching and I'll see you guys next time