All Things Water Course I, Activated Sludge

welcome to this session this will be on the an overview of the activated sludge process in wastewater treatment activated sludge is the main secondary biological process that's used in virtually all wastewater treatment plants worldwide this course is not designed to teach you how to design an activated sludge system it's designed more to give you an overview of how the process works some of the important parameters involved so you'll have a familiarity with it as you move forward my name is Russ right I am the chief engineer of municipal products at West tech engineering out of Salt Lake City West Tech is a company that supplies and designs the large process equipment that goes into water and wastewater treatment plants I've been with West Tech for 31 years in a variety of positions and education wise I have my master's degree in civil engineering from the University of Utah as well as several additional classes in wastewater treatment as well as a master's in business administration and so with that let's proceed we're going to talk today about first of all where the activated sludge process fits in a typical wastewater treatment plant flow sheet we will then go over some important wastewater treatment parameters or criteria that deal with the performance of the system we'll do some very brief biochemistry and then we'll get into what a typical activated sludge system looks like how its configured we will move on to talk about some important parameters that are used to operate an activated sludge system and then finally we'll go through some different types of systems in actual treatment plants so typical wastewater treatment plant flow sheet looks something like this there are many variations but this is a very typical flowsheet we start out with screening this is designed to remove rags paper plastic products from the the sewage coming into the plant I even had an operator tell me a mattress came floating down the sewer one day we want to screen this kind of stuff out before we get into the rest of the plant and so there's always some type of a screen at the head of the plant following the screening is grit removal most sanitary sewers are made from concrete pipe and as concrete wears you get sand particles fine grit that moves down the pipe and into the treatment plant that grit is a problem if you run grit through a pump you'll wear the pump out very quickly there are a lot of pumps in a treatment plant that grit can also settle at places in the plant where you don't want it to settle and and cost problems so we take the grit out up front with some simple tanks and mechanisms that remove the grit following that is what we call primary treatment or a primary clarifier and not all treatment plants have primary clarifiers but any treatment plant bigger than about five million gallons a day typically has primary clarifiers all that is is a large basin that the liquid flows into and any solid material in that waste stream will settle to the bottom of the tank where it can be collected and removed and the clear relatively clear liquid then flows on into the rest of the plant that removes about 50 percent of the contaminants in typical sewage and it's obviously very inexpensive all you need is a big tank doesn't take a lot of power or energy and and so it's an important part of just about every treatment process but what do we do with the rest the part that won't settle the part that's dissolved in the waste water or is very very fine particulate that's where the biological secondary process comes in and activated sludge is the main processes used throughout the world to accomplish this I will explain more exactly what's involved in that system but you can see in the diagram where that active agent activated sludge system is located consists of a large aerated basin and a clarifier and a recycle stream following that the liquid has been cleaned quite well and all that's left is disinfection where we kill whatever germs are left using chlorine or ultraviolet light many treatment plants are going to the UV light disinfection now because chlorine is hazardous and difficult to work with and the UV disinfection sort of like giant high-strength tanning beds you run the water through there and it actually will kill the germs that are left in the wastewater on the lower part of the flowsheet you can also see that there's we have to handle the solids that settle the solids go into a thickening stage then into a digestion stage where the goal is to reduce the volume of those solids and then it's d watered and it can be composted to create fertilizer it can be directly applied on agricultural land non-food crops there are a lot of ways that that plants deal with the solids that come off of the treatment process couple of important treatment parameters that we want to talk about the first one we call Bo D that stands for biochemical oxygen demand this is a measurement of the amount of oxygen it takes for microorganisms to break down the organic material in sewage the microorganisms in the natural environment will do that they will feed on any you know any you know a dead animal or any waste or something like that organic matter in the environment you know bacteria break those down and these bacteria need oxygen to live and so if you put some waste material into a stream for sample and the microorganisms begin to feed on that and they use oxygen as they do so they respire like we do they will grow and multiply and if there's enough waste there they will grow and multiply to the point where they suck all of the oxygen out of the water that's bad now you have a dead stream or a dead lake turns black everything and it dies not good that's why this is an important parameter to measure the actual test we use is called a B od5 test and the way the B od5 test is performed is we take a bottle with water in it that has been aerated and we measure the oxygen concentration in that water we add our waste water sample and a seed of microorganisms that will break down that waste we seal it tightly and we put it in a cabinet at 20 degrees centigrade for five days we pull it out we measure the dissolved oxygen concentration we know how much oxygen was used up and that becomes then a measure of the biodegradable organic material that was in that waste water that we put in the bottle okay typical influent Bo D that is the the Bo D that's measured on the influent to a treatment plant is typically around 200 milligrams per liter anywhere between 100 to 350 depending on the type of community of whether their industries contributing or not that's a very typical kind of a number okay the u.s. national minimum effluent standard on the far end of the treatment plant says that a treatment plant cannot put water into a receiving body that is more than 30 milligrams per liter of Bo D on a 30-day average 45 for a seven-day average now that's the u.s. national minimum part of the Clean Water Act of 1972 but there are many state and local standards that are considerably tighter than that and so the the activated sludge process is is helpful because from that process typically we can get Bo D down typically in the single digits in milligrams per liter or parts per million so it's very effective process and it does a good job let's move on to TSS or total suspended solids this is a measure of the particulate matter in the waste water what we do is we run the wastewater sample through a point four five micron filter in a standard procedure and whatever stays on the filter paper is considered suspended solids obviously there's some very refined solids that do go through but we consider those dissolved because they're so small that essentially they are you dry that the solids on that filter paper weigh it now you have the suspended solids concentration for that wastewater and again the typical influent coming into a sewer treatment plant in the United States is somewhere around 200 milligrams per liter there's a range there again depending on the the type of community and the industries and so on there in the in the community again the u.s. national minimum standard is 30 milligrams per liter suspended solids going out of the treatment plant in a 30-day average over seven days forty-five milligram per liter and again much tighter in in certain areas of the country depending on the water body that's going into typical TSS coming out of a treatment plant that's using an activated sludge process is in the single digits so again very effective in removing the particulate matter from the waste stream okay let's look at very briefly some biochemistry and this is not very technical but basically we want to talk about what goes on and what the microorganisms do as they remove this B OD or this organic matter from the wastewater the when there is oxygen present the microorganisms convert this organic material to carbon dioxide ammonia and new microorganisms obviously they're reproducing and growing those substances are fairly harmless now ammonia has some problems and we'll talk about ammonia later but this is the way this is the the basic process that goes on now if you try to write a chemical equation this is not a balanced chemical equation because there's all kinds of different organic chemicals that you'd have to to include in that but basically on the left side of the equation we have Co HN meaning some type of organic chemicals combined with oxygen in the presence of microorganisms you get from that co2 ammonia and that's an approximate chemical formula for bacteria new microorganisms now I said this was the equation the bio chemical equation for removing Bo D where is the bo d in this equation well remember Bo d stands for biochemical oxygen demand meaning it's the oxygen that's absorbed in the biodegradation of this organic matter so in a sense it represents the organic matter it's a surrogate for actually trying to measure all of these different organic chemicals in their concentrations you could also say well it is the oxygen that's being consumed by that in the wastewater industry they're kind of used simultaneously and we often talk about removing Bo Diaz if Bo D were a substance it's really a surrogate for all this organic matter the urine the feces the food products that are ground up in your disposal and everything else that ends up in the sewer that's organic in nature and can be degraded by these microorganisms so we talked about removing Bo D or how much Bo Diaz in the wastewater a lot of times the professionals are really meaning how much organic material is there that we need to get rid of okay all right let's talk about a typical activated sludge system and what that looks like this is just a very simple diagram basically we have a large tank we call an aeration Basin and we have a clarifier which is a settling Basin our flow comes in to the aeration Basin fills that up now in an aeration Basin we have logically some kind of aeration system we either have diffusers on the floor that bubble air up through the liquid or we have some kind of mechanical device that stirs and churns up the liquid and gets oxygen and air to dissolve in that water so that there's plenty of oxygen for the microorganisms to do their job from that point the liquid goes into the settling Basin or clarifier and the microorganisms actually settle to the floor of the clarifier and the liquid that comes out the top is like I said typically single digit in Bo D and suspended solids and so the clear liquid flows out the top but we take the solids that settle in the bottom of that clarifier and we recycle them we call it return activated sludge into the aeration Basin so the sludge that settles in the clarifier is recycled into the aeration Basin and activated to to work on the contaminants that come into the system so the when we keep doing that until we have maybe 30 times the amount of microorganisms in the system than we had in the waste stream that was coming into the plant in the first place so we can degrade this organic matter 30 times faster than if you were to put it in a stream that's the whole goal is to use natural biological forces in nature to clean the water but do it quickly and in a confined space and that's the reason we have to add the air to the to the tank is because with that many microorganisms they're using a lot of oxygen and we have to add extra oxygen rather than just what is going to dissolve in the water through the atmosphere and so we have to provide that extra aeration now this is not just a mass balance of solids in and solids out because these microorganisms are growing and multiplying and so there's significant amount of solids that are actually generated by this process it's new microorganisms that are growing they're absorbing this organic matter and growing new new cells so you have to remove some from the system that's what we call wasps or waste activated sludge the solids that are wasted from the system and disposed of let's talk about some operating parameters in activated sludge these are not terribly difficult systems to operate but there's some important things that need to be watched so that the the system performs properly the first one we'll talk about is the mixed liquor suspended solids concentration mixed liquor is called that because it's a mixture of the raw feed coming in and the returned sludge from the clarifier so it's mixed to form this liquor and it has a concentration of solids typically in an activated sludge system that's anywhere from 2,000 to 4,000 milligrams per liter of solids most of which is actually the bacteria that are doing the work to break down the organic material in the waste if you can run at a higher mixed liquor concentration you don't need as big a volume because you can pack them in tighter and treat in a smaller footprint but the maximum concentration you can practically get by with is limited by a couple of things air requirements that is if you pack them in too tight you cannot physically get enough air dissolved in the water to keep all the bugs alive and active the second problem is in your settling Basin if you have too high a concentration of solids it will not settle as rapidly and you'll need immense clarifiers to try and settle a very very thick or very very high concentrated solids and so that limits the the concentration that you can build this aeration Basin concentration up to and like I said typically it's between two and four thousand milligrams per liter dissolved oxygen concentration this is obviously critical as I've talked about these microorganisms respire and they need the oxygen dissolved in the water to work correctly it has to be sufficient to really degrade all the organic matter or be OD in the waste usually we set it at about two milligrams per liter so we have an oxygen sensor in the aeration Basin at all times and we keep the level in that basin at about two milligrams per liter so there's a little bit excess oxygen that's always there for the microorganisms to do their work if you set it too high there's a couple problems first of all it wastes energy you're spending a lot of energy churning up the liquid or blowing air into the liquid and that's a very expensive part of the waste treatment process is this aeration and so you don't want to set it too high on the other hand oh and also if it's too high you create an environment that favors a certain type of bacteria that we call filamentous bacteria most of the bacteria that do this be OD removal for us are small round or oval shaped bacteria like you see in your lab microscope if you've taken any biology classes or anything like that but there are other types of bacteria that are filamentous they look like tree branches or feathers with long filaments they like the environment with a lot of oxygen and so will grow more of those if we have too much oxygen in the system and they don't settle well in the clarifier it's like dropping a feather it doesn't drop nearly as fast as a rock very similar analogy so we don't want the oxygen too high in the system on the other hand we don't want too too low either if it's too low you're not going to get the beauty removal you need because the bugs would call them bugs the microorganisms can't can't remove the organic material and if you get three low it will go anaerobic which to most of us means it's smelly okay it's gone rotten and it stinks you're going to get hydrogen sulfide and other gases that the neighbors around the treatment plant or not going to appreciate okay hydraulic retention time is the time the liquid stays in the system pretty straightforward typically four hours to 24 hours depending on the process and how much contaminants we need to remove simple equation the hydraulic retention time is the volume of the reactor or the aeration Basin divided by the flow rate Q and that gives us an average time that the liquid is in the system that's a simple parameter but it's important if you have say you have a storm and your city is one of those unlucky cities that has combined sewers that means the storm sewers and the sanitary sewers go to the same pipes you have a big storm you suddenly get a great increase of flow through your treatment plant and if that hydraulic retention time is not long enough it's not long enough for the bugs to eat the organic material that's in the water so you do have to watch that and size the basin so that you have enough retention time for the for the microorganisms to do their job even more important in this type of system though is the solids retention time how long do these microorganisms stay in the system now since we're recycling them back from the clarifier over and over again the solids retention time is much longer than the liquid or the hydraulic retention time we also call this the mean cell residence time because these are bacterial cells that we're talking about typically is 2 to 8 days for simple Bo D removal if we get into nutrient removal processes which is another class that we'll talk about it's even longer but for typical beauty removal most wastewater plants this solids retention time is between 2 and 8 days and it needs to be long enough that we allow the organisms to reproduce grow if our SRT is shorter than the reproductive cycle of the microorganisms they'll be out of the system before they reproduce and we can't get the concentration we need to do the job so this is a very important parameter that we have to design for this is the equation it's a little complicated but it's it looks a little complicated but really isn't the SRT is basically just the volume of the reactor times the concentration of solids in the reactor which gives you a mass of solids divided by two things we have the the waste sludge that's leaving the system that's represented by the Q so W times the X sub W X being the concentration in those wasted solids Q being the flow rate of the wasted solids and the solids leaving the system in the effluent of the clarifier which we hope is very small but to be technically correct we have to include an equation in practical circumstances a lot of plants don't even use the effluent solids in this equation when they calculate their s RT because it's pretty negligible compared to 2,000 milligrams per liter in the aeration Basin and you may be getting five out of the clarifier not five thousand five and so all we're doing is we're saying here's the total mass of solids and here's the amount that's going out so that's what this equation represents finally another parameter that's important is what we call the food to mass ratio this is a ratio of the B OD coming into the plant per day to the mass of microorganisms that's there you can you can liken this to the amount of food that you eat every day divided by your weight somebody who weighs 200 pounds can eat more in the way of calories and food that somebody who weighs 100 pounds same is true in microorganisms this is measured in pounds of B OD per day divided by pound of biomass or solids in the reactor normal range is 0.2 to 0.5 you are I couldn't eat 20 to 50% of our body weight in food every day but microorganisms do they have much higher metabolisms than you or I do which is lucky because otherwise we'd be drowning and waste in the world right but if we we can't go too far out of this range or we have problems if it's too high then the microorganism simply can't eat at all they can't handle it all and you won't remove all of it from the waste stream and you also get what we call viscous bulking which basically I like into fat bugs these microorganisms have eaten too much and it actually changes their specific gravity that's not really fat they don't get fat but it's analogous to that and they don't settle well in the clarifier and they even float sometimes and that doesn't work because you take the clear liquid off the top of the clarifier but if it's too low again you have problems with these filamentous organisms that are more efficient at getting what little food is there and so they tend to predominate and again you have poor settling in the clarifier so you have to keep that food to mass ratio in a fairly stable range for this system to work properly let's look at different types of activated sludge systems in a practical standpoint actually in plant designs the most common for particularly larger plants is a diffused aeration system where we bubble air from the floor up through the waste water here's a picture of a a tank that has a bunch of diffusers on the floor of the tank it looks something like this and there are many varieties of diffusers they can be designed for course bubbles which do a better job of mixing but not so great a job of oxygen transfer or fine bubbles which are do a much better job of dissolving oxygen into the liquid but don't mix quite as well and there are many many variations and shapes and sizes and and designs that people come up with but basically they all just bubble air into the water and you need a blower or a compressor to push that air through the pipes and through these diffusers into the waste water the other type that's commonly used is some type of mechanical surface aeration here you can see we have a vertical turbine aerator and it's splashing and turning the water as it does that the air gets entrained in the water and it dissolves oxygen into the water this is what one of those looks like when it's not submerged water levels about there it's only at the surface but it acts to mix and aerate the liquid this is a example of a plant that's using those mechanical surface areas you can see it's just a big rectangular Basin with six cells each one with one of those mechanical aerators in the center this is actually not terribly common typically in rectangular tanks we use the diffused air systems and the mechanical aerators are saved for a type of design we call an oxidation ditch that I'll show to you in just a minute this is a horizontal brush aerator this device spins around its axis horizontal axis there and it's partially submerged so those paddles go through the water and basically do the same thing as the other one they simply turn the surface up so they get a lot of foam and churning and air entrained in the liquid and it also imparts a velocity to the liquid so that you can mix a tank this is the oxidation ditch I was talking about this is the most common application or the common type of tank that's used with mechanical aerators in this system that the tank looks sort of like a racetrack it's an oval and the influent comes in one end and you have either a vertical turbine aerator at the center of the circle at one end that will spin or you put one of those horizontal brush aerators on one side and it acts to move the liquid around this racetrack and aerate it at the same time and of course we have to then go to the clarifier and return our activated sludge back to the basin this is a photo of some oxidation ditches I chose this photo because it actually has both types of mechanical aerators on the right we have four vertical turbine aerators in the ditches on the Left we have six horizontal brush aerators obviously two different phases of development of a plant and expansion that occurred quick summary activated sludge is the most common secondary biological process used in wastewater treatment it includes an aerated tank and a clarifier following and the sludge that settled in the clarifier is recycled back into the aeration basin to increase the amount of microorganisms to treat the wastewater we talked about some treatment parameters that measure the effectiveness of the treatment process those were Bo D biochemical oxygen demand remember that's the amount of oxygen that's used up in breaking down that organic material and then T SS which is just a measure the particulate solids in the wastewater we talked about some important operating parameters mixed liquor suspended solids that's just a measurement of the concentration of solids in the aeration Basin the dissolved oxygen concentration is critical to good growth of the microorganisms the hydraulic retention time is important to give the microorganisms time to break down the waste the solids or tension time is important to make sure we have enough microorganisms in the system and finally the food to mass ratio is just a measure of how many microorganisms we have versus their food supply and we have to keep those in the right balance I mentioned a couple types of activated sludge systems we have diffused air systems which can be coarse bubble or fine bubble and can accommodate basically any tank shape you want because you can simply put the diffusers on the floor you can have a very odd shaped tank sitting in a corner of a wastewater treatment plant if you want to expand your plant you can put diffused air in the bottom of that and use it as an aeration Basin the other kind is mechanical surface aeration which is less expensive typically in power cost but it's most often used in an oxidation ditch type of a design with that I thank you and have a good day you

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Length: 32min 12sec (1932 seconds)

Published: Fri Jan 23 2015