 Okay, so we might choose not to think about this. But where did last night's dinner go when you flushed it down the toilet? While this may gross you out, the answer to this question talks about one of the most critical pillars of public health and civil engineering – wastewater treatment plants. There are two major parts in this treatment process – the primary treatment and the secondary treatment. The first one, or the primary treatment, is all about getting rid of the physical things like rocks and pebbles and stuff. The second one, the secondary treatment, that focuses on removing all the organic matter in the water with the help of microbes. Let's dive a little deeper into each one of these processes and see how they actually work out. When you flushed down last night's dinner, it got collected by the sewer pipes. These pipes are a part of an intricate network called the sewage system, which collects waste water from all kinds of establishments. All of this collected waste water, or sewage, passes through bar screens. Bar screens, which are these things right over here. Now bar screens are these vertical bars which trap all the larger, solid stuff, like plastic, cans, bottles, wood, tissues, etc. We commonly call this step the pre-treatment step – pre-treatment. So this step, where it gets trapped at the vertical bars or at the bar screens, this is called the pre-treatment stage. The primary treatment begins right after this. First, the waste water is moved to something called a grit chamber. This right here, this is the grit chamber. Now this chamber, it removes all the rocks and sand and tiny pebbles from the water. And this is important because if you don't remove these things, they might go ahead and clog or destroy the pumps which are involved in this machinery. All the rocks and pebbles which are separated out in the grit chamber, that is the grit, it gets removed through this outlet right over here. So this is where the grit is removed from. This is the grit. The waste water is then sent off to something called a clarifier aka a settling tank. Now since this clarifier is involved in primary treatment, we also call this the primary clarifier. Now here, all the suspended solids in the water, they will sediment or they will settle down and over here it will form this layer which is called the primary sludge. Now there are other types of wastes too other than the suspended solids like soaps and oils and grease and all of that will kind of float up because they are so less dense than water, much much less denser than water. So it just floats right on top and this layer on top of the water is called scum. So this is the scum. Once both of these things are removed, the sludge is removed with the help of a scraper and the scum is removed with the help of a skimmer. Now once these things are removed, the water, the waste water, slightly treated waste water is now sent off to the next major part of the treatment process which is the secondary treatment. Secondary treatment or biological treatment hinges on the activity of microbes. The slightly treated water from earlier which is also called the effluent, it's called the effluent which is the water that is entering from here, this effluent now goes into these aeration tanks. So this is an aeration tank, aeration tank. So the purpose of this aeration tank is to blow air bubbles into the water which is done with the help of this air compressor over here, an air compressor which is constantly blowing air into air bubbles into the water and that increases the levels of oxygen. So the main thing is to increase the level of oxygen in the water. The already present aerobic bacteria and fungi in the water thrive in all of this oxygen and digest the organic matter present. It's like in all you can eat buffet for them. The microbes they grow so exponentially that they end up forming these things called flocks which are nothing but masses of bacteria interwind with fungi forming a mesh or a network of some sort. This mixture of waste water and flocks is agitated constantly as more and more air bubbles are blown into it and this continues till the water quality has improved. But how will you tell that the water quality has improved or not? I mean we can't just go ahead and use this water and check it for ourselves, so how? Well there is something that indicates the quality of the water. It is called BOD, biological or biochemical, so you can call it either, so it's biological or biochemical oxygen demand, BOD. So which is essentially the amount of oxygen that microbes need to break down all the organic matter present. So if these microbes need a whole lot of oxygen that means there is a whole lot of organic matter to break down. Which means if BOD is high then the organic content in the water, the organic matter in the water is also high that means it's more polluted. So if the BOD is high then the water quality is super low. That's what BOD indicates, if there's a higher BOD it means the water quality is super low because the organic matter in the water is also pretty high. So we let our microbes do their thing, have their buffet till the BOD of the water has reduced significantly. Once we are happy with our water quality we send this entire mixture into another clarifier or settling tank and since this is a part of the secondary treatment this becomes the secondary clarifier or settling tank. And this layer of bacteria and fungi that sediments or settles down this is called the activated sludge. Now why activated? That's because this layer or this sludge is steaming with live microbes which are extremely active and they can actively break down all of this organic matter in the water. Because of how active these microbes are a portion of it, a very small portion of it is actually pumped back into the aeration tank to increase the growth of microbes. So you can see that there's this one pipe that leads back to the aeration tank. This is the small portion of activated sludge which is being pumped back into the aeration tank. Back to our treated water. So this on top the bluish part that you can see this is our almost treated water. Now this water we are going to go ahead and disinfect it with the help of UV or chlorine. So this yellow bottle that you can see over here this is the disinfectant. Now sometimes we use chlorine sometimes you can use UV to make sure that you know there are no microbes present in the final water that is going to be released into the environment. So this water that comes out in the end this water is completely devoid of any remaining microorganisms or pathogens for that matter. Once this water is fully treated it is left it is released back into the environment. This is the red arrow over here which is difficult to see because I wrote on top of it. So let me reduce the size of this just a little bit okay so yeah so now you can see it clearly so this red arrow means that we are releasing the treated water the completely treated water back into the environment. Sometimes we also go a step ahead and treat this already treated water a little bit more and we also use it for drinking purposes. So this water is treated to such an extent that you can go ahead and consume it I mean how crazy is that toilet water drinking water that's pretty crazy in my opinion if you ask me. Okay so now one question remains is that what really happened to this sludge right so we had some sludge in our primary treatment and there was this activated sludge in the secondary treatment what is going to happen to this sludge so if you are thinking that we just let this sludge go as it is that then you'll be very wrong because all of this sludge this remaining the one from primary treatment and the activated sludge all of these things they go into another part of the whole process and this is called the sludge digester where you will digest the sludge with the help of anaerobic bacteria anaerobic bacteria now anaerobic bacteria will go ahead so if the sludge is deposited over here in like heaps of some sort and these anaerobic bacteria will go ahead and break down this sludge that we have over here and when they do something like that when they break down the sludge it is going to release certain gases so if I say that the gases are released from here then we're going to have methane hydrogen sulfide and carbon dioxide so when anaerobic bacteria breaks down all of that sludge these are the gases which is produced and together this gas is called biogas which is a fuel of huge huge importance to us it's a fuel that is used extensively in a lot of rural areas and it's pretty helpful to us now we wouldn't get this biogas had we used aerobic bacteria instead to digest this sludge also like plus the aerobic bacteria if they break down the sludge we would they would end up generating a lot of waste which is very very difficult to get rid of or to dispose of in comparatively the waste that is generated by anaerobic bacteria is very very less and it's easily taken care of compared to the aerobic bacteria and which generate a whole lot more difficult undisposable waste compared to this one so anaerobic bacteria does a fabulous job they digest off all of the sludge whatever waste remains after this breakdown is disposed of in landfills so we just went from last night's pasta or noodle or whatever we had for dinner all the way up to drinking water and that is one heck of a journey in my opinion and if we didn't have these wastewater treatment plants then it would have been a huge concern to public health so I believe that wastewater treatment plants are really really underrated and we don't really talk much talk about it often possibly because of how gross it can be but it is extremely important and I hope you understand that as well.