Activated Sludge Wastewater Treatment Overview
Author's Note:This article will describe the basic systems involved in the activated sludge type of wastewater (sewage) treatment for a larger city, and will walk through the Scranton, Pa., treatment plant to give an overall picture of a specific system. It is intended for the average Wethead -- lots of practical descriptions, a bit of humor, flow-through breakdowns of how the processes affect each other -- and will touch periodically into technical terms.
I apologize in advance for the length. Hopefully when this has been read, you may have a better appreciation of what happens long after that toilet is flushed.
This article does not recommend nor argue against the use of this technology for it's purpose, nor does it condone or condemn any practices or procedures used at the Scranton, Pa., WWTP and its authority or contracted operations company. (That was a CYA for myself in case anybody's wondering!)
This article was originally meant solely for the private use of Dan Holohan's "HeatingHelp.com" and the "Hot Tech Topics" that is a part of that site, and is a small token of this author's appreciation for the wonderful information obtained from there. If Dan wishes to use this elsewhere, he is more than welcome. This will be informal but (hopefully) educational reading for the average visitor.
There will be several sections, including overviews of combined and sanitary collection systems, pumping stations, basic terminology of wastewater pollutants, screening/grit removal, primary and secondary settling, aeration systems, sludge thickening, belt filter press operation, chemical usage, disinfection, laboratory tests for plant monitoring, effluent limits, maintenance, and power usage.
Before you start running away or falling asleep, this will be done with "baby steps" in mind, and will be fairly easy to follow along. (I have done the process description by how the flow will go through the plant as much as possible.) If you want to go crazy with terminology, fluid mechanics, nitrogen cycle issues, etc., please look elsewhere -- there are tons of books and schools on all of these and more. It is probably best to read this in smaller pieces over the course of a day or two, then it actually may be remembered.
IntroductionActivated sludge treatment of wastewater has been around for about 100 years in one form or another, and is probably the largest means of achieving a goal of pollution control of our waterways on a public basis. Nearly every city in this country uses a form of this treatment because of its proven record, relatively simple technology, and its cost effectiveness. Other technologies are making headway into the large plants, and this is a good thing as long as the process is constantly improved at an acceptable cost.
How many people actually know what it is? I would venture to guess that a small percentage of Wetheads could walk through a plant and describe more than half the processes -- and you folks are smarter than the average bear because of your knowledge of pumps and piping systems.
Basically, this technology uses a combination of settling and aeration (yeah, that's how it's spelled) to reduce pollutants. Aeration is the use of lots of air (from large blowers) to feed the microbiology that lives in the plant, and to help with mixing as well. This population of aerobic bacteria ("bugs") is the heart of the system, and must be cared for in the best possible way. Some other forms of treatment use the same items in different ways, while others use more or less.
The removal of solids (settleable and suspended), ammonia (in various forms), various other pollutants, and ensuring a nearly bacteria free and stable water back into our rivers and streams is no small task -- especially when you have millions of gallons of flow (sometimes up to a billion!) to deal with every day through your plant. And here's one thing to remember folks -- you can't shut it off! The "feces flow" never stops, so you can't just shut a valve when there are problems. In other words, you have to deal with whatever comes down the pipe.
I was the superintendent at Scranton's plant for 11 months, and will use that site as reference throughout this article. This plant was designed in the late-1950s/early-1960s from what I was told, and built from 1970-72. Before it was completed, all wastewater from Scranton went directly to the Lackawanna River. Unlike most areas, there was nothing in place before that time (again, this is what I was told). There are still a small handful of original employees working there today, and that is a huge asset.
My background was as an operator of water, industrial and domestic wastewater treatment plants (first learned in the military), and I went to school part-time to earn a degree in Civil/Environmental Engineering. I tested and passed each test for an "A" license in Pennsylvania for water and wastewater treatment operations, and was proud to qualify for the exams based on my experience, not my education. After working in a few jobs as a "typical" engineer (don't go there!), I went to Scranton in March of 2000 to cover for a couple of weeks while the (temporary) superintendent was away. I felt so comfortable there because of the people and the type of plant that I was there until March of 2001, despite the 124-mile commute each way.
The layout of this plant is very basic, and unlike most larger plants today was mostly manually operated. In other words, the processes have to be monitored by the older convention of having the operators continually watching everything in person or by older types of monitoring. To use an analogy, let me describe it as a large, 25-year-old house that is heated with hot water baseboard with no outdoor reset -- good old-fashioned cast iron w/a tankless coil that may not be the most efficient, but she'll last a long while with some care and won't throw you too many curve balls. Improvements to this system have been on-going since my departure, so these descriptions are for this system in 2000/2001.
Collection Systems and Pumping StationsSewage collection systems are of two basic types -- sanitary and combined. A sanitary collection system is for wastewater only (both industrial and domestic if needed), while a combined system also has storm water in the same system. This last item was the practice until about the 1970s in most places, when the two systems started to separate. Dedicated storm sewer systems (a misnomer in my book ... how about storm drainage systems?) are much better for sewage treatment plants because they keep the heavy flows from rain out of the plant. Most cities -- 772 according to the EPA that serve 40 million people (epa.gov/npdes/home) -- still use combined collection systems but have ordinances for any new construction to separate the two flows. Oldest systems used wood pipes, then terra cotta and cast iron, while new lines are often composites. The larger mains are usually reinforced concrete pipe, although other materials also are used.
Manholes are placed at a maximum of 500 feet apart (I believe that is across the country), and usually they are closer. There are also a myriad of tees, wyes and collection boxes throughout every system. Scranton's system is about 50 total miles, I think, and that's a lot of pipe! The last two decades have seen technology bring in cameras to view the system. This is how most cracks and breaks are found today. The use of little robots to get a videotape of lines sure beats a guy crawling around in there! This is also one way of finding illegal connections and sometimes heavy polluters -- tough to argue a taped recording and samples that show the pollutant levels, eh?
Repairs to these systems used to entail digging out lines for replacement, but today they can often send a re-lining machine through the pipe. This is another wonderful technology that is cool to see.
In Scranton, most of the collection system is combined. Sewage from a typical house flows through a 4-inch pipe while industries use larger ones, then they flow into the street main by gravity until a pump station "lifts" the flow along (or it gets to the treatment plant if the site is close). Depending on the location, one of five stations gets the flow to the plant. Most Wetheads are probably familiar with this stuff to a certain extent.
Because this as a combined system, there are about 50 or so bypass points in the system at Scranton. This is for a heavy rain event to prevent the flow from overloading the plant. Yes folks, when it's raining heavy in an older city, chances are that flush of yours -- although diluted -- might end up in the river. Not good, but certainly better than ruining the treatment plant. I'll explain more on this later.
The reason for this is simple; you simply cannot build a collection system and treatment plant for the highest possible combined system flow it might see. Analogy time: Here, you can't oversize like many boilers and heating systems in a house, because the cost (including the land space) would be astronomical. You would have to size it for up to 100 times the average flow. How about putting a 5 million Btu/hr. boiler in a 1,500-sq. ft. well-insulated home (say 50,000 Btu/hr. heatloss)?
Basically, you design the plant for the maximum sanitary flow; to be more specific, for the hydraulic and/or organic loading (more on this later). Scranton's treatment plant is designed for about 24 million gallons a day (MGD -- and we're not talking beer), and can take more for short periods of time. It has bypassed over 60 MGD at the plant bypass (not the other 50) at times.
Scranton's collection system has five pump stations that keep the flow moving to the plant, with bypass points along the way for preventing overflows. The pump stations vary in size, and normally have two pumps, one for backup. They must be checked and maintained daily, and have trouble alarms that alert the plant operator of problems when unattended. Most pump stations go unnoticed, until there's a problem ("WHEW! What is that smell?").
The fun part of maintenance out in this system is for sewage blockages in the street. Many Wetheads may know of the Vactor trucks that simultaneously clean out and vacuum lines. It's a large truck that has a pump and high-pressure water hose on the front and a vacuum system with a 600-gallon (or so) tank on the back. These things can pull a mixture of sand, rocks and water up 25 feet through a 4-inch hose while rinsing the area at the same time.
Awesome! The person that invented this technology should be put on a pedestal. I'm sure you can imagine how these things were done in the old days. It still doesn't make the job easy. The old saying is they must learn two things: "'S##t' flows downhill, and payday is Friday." These folks earn their pay. Besides the pump stations, blockages and bypasses, there are a slew of drains, manholes, broken lines and other fun things to deal with on a daily basis.
The last mile or two of Scranton's collection system flows to the plant completely by gravity. As with nearly all plants, it is located at a low point in the city and close to the river for obvious reasons. As the flow comes to the plant, it is mixed very thoroughly by the velocity of the water. The line gets deeper into the ground to ensure gravity flow, and when it enters the plant it is about twenty feet underground.
TerminologyBefore we begin the process description there are some terms that need to be presented. Bear with me here, it won't hurt too much. This is the necessary pain before we get to the fun of cleaning that "feces flow."
- Hydraulic Loading: This is the actual flow of water. In most plant terms it is millions of gallons per day (MGD).
- Organic Loading: This is the "pollutant" load on the plant, and is in terms of parts per million (ppm) or milligrams per liter (mg/L) and sometimes per billion (ppb). This combines with the hydraulic flow to give the operator an idea of the "real" load on the plant. You could have a low flow with high organics that taxes the treatment system, or high flow with low organics. Or the dreaded high flow/high organics. You could also have low organic loads, which starve the biomass of the plant. This load could be solids, BOD, ammonia and others (each of these will be explained).
- pH: Most Wetheads are familiar with this one. This is the scale that indicates the relative acidity (0-7) or alkalinity (7-14) of the plant.
Most wastewater has a pH around 7.5 to 8.0, but industries vary like crazy. Some have 2.5 while others may have 10.0 and this gets tricky. Typical treatment systems like the pH through the plant to be between 6.5 and 8.0, with the vast majority of plant effluent to waterways having to stay between 6.0 and 9.0. (Remember, 7.0 is neutral with below going acidic and above alkaline.) Each tenth of a point is a double of "strength" of acidity/alkalinity due to the logarithmic scale. This is probably the most recognized, and maybe most important, single measure in water quality. For sewage there are many other factors, but this has one to be kept under control; it changes through the plant's flow as the treatment of the wastewater occurs.
- Solids: There are many types of solids, but let's stick with the single term of total suspended solids (TSS). This is the total solids load on the system after the grit and screenings (rags, sticks, etc.) are removed. Typical influent TSS at Scranton is about 225 ppm, with the effluent being about 5 to 10 ppm, giving a removal of about 90 percent-plus on a regular basis. This number is reduced as the flow goes though the plant with the exception of one area. More on that next.
- Mixed Liquor Suspended Solids: This is the total amount of solids in the aeration tanks -- where the bugs live -- and is MLSS. A better description of the bugs themselves is MLVSS, or "volatile." These are in ppm or mg/L, and depending on the season and other factors can be in the range of 1,200 to 3,200 MLSS in the aeration tanks. Scranton has about 6.4 million gallons of aeration normally used, so this means there are billions (maybe trillions) of microbiological life doing most of the work. That's a lot of bugs! Imagine if they started a union.
You have to closely control the population to control many factors in the treatment system, and this is where many of the issues take place for recycling the flows through the plant. Flow is taken from the bottom of all tanks in the treatment plant, and either pumped through the sludge processing area, back to the headworks, or both. This balancing of flow is vital to the treatment system. The bug population is (mostly) controlled in this manner. By sending some of the population to the sludge process area, you are removing them; regeneration ensures there is always new bugs coming into the system. Too few bugs = bad discharge, and too many bugs = not enough air and starvation/die-off of the population.
- BOD: This is a basic measure of the water's pollution, expressed in ppm (or mg/L). The definition of BOD is "biological/biochemical oxygen demand" (some say biological while most say biochemical). There is also CBOD (carbonaceous) that is a closer measure of the "food" level in the wastewater, but I'll keep things simple. Normally, it is the measure of oxygen consumed after a five day period (BOD5) that originated from the length of time the river Thames flowed between two points, I believe. This test takes five days to get results, so it has its limits. There is also COD (chemical oxygen demand), an excellent measure of industrial pollutants that gives results in a few hours. A much more effective measure in certain situations. High levels of these take too much oxygen out of streams and rivers, killing most of the wildlife.
- Dissolved Oxygen (DO): This is the dissolved oxygen in the water, and is essential for a healthy river or stream. This is another basic test that has been around for a while and is very important. It has to be at a minimum of 5.0 ppm (mg/L) in the discharge of most plants. Through the treatment system, this also varies considerably and must be monitored by the operators. The bugs require a certain level for survival in addition to the food (waste) and pH. Scranton's plant has it checked at least once every shift in several locations -- including the plant discharge.
- Ammonia: There are entire college courses dedicated to this, and I won't begin to pretend I understand it all. The source of ammonia in wastewater is normally urine, but could also be industrial based (fertilizers, for example). There are various types, but I will keep it simple by using nitrate, nitrite and nitrification. Denitrification is also important, but I will mostly avoid this topic to keep things simpler. Nitrification is the process by which the "bugs" can consume the ammonia products in the water as food, and oxygen (from aeration) is a key ingredient here.
Basically, the bugs consume the ammonia-based wastes using air, and this converts it to nitrate, nitrite, then it is simply nitrogen gas. That's about as simple as I can get on this subject, and hopefully it gets the point across. The nitrogen cycle is one of those big, complicated messes that is hard to understand but extremely important. Basically, it is a constantly revolving cycle that has many inputs and nearly everything affects everything else.
- Alkalinity: Some people think this is the same as pH, but it's simply related. Alkalinity has to be added to this process as nitrification takes place to "buffer" things. The bugs use the alkalinity from the chemical (caustic solution, lime, other hydroxides) to aid this process. If there is little or no alkalinity, the pH would bounce up and down too much. This is pretty much the case in most water systems.
- Fecal Coliform Bacteria: This is probably familiar to some. It is a bacteria that basically indicates the presence of other (bad) ones, and you can count their colonies pretty easily. The basic level of most plant effluents (from the permit) is less than 200 colonies per 100 ml. That is normally less than the receiving water has, and the method of disinfection is normally chlorine at a minimum 30-minute detention (contact) time. This is an old and basic measure, and is used in many different areas -- ocean/beach testing, streams, etc.
- Sludge Thickening/Processing: Makes you want to grab that chocolate pudding just thinking about this one, right? This where the stuff that settles in the tanks is taken, mixed from the various processes, and made thicker to feed to presses that will make it into a "cake." This stuff is about 20 percent to 25 percent solids content when the process is complete, has lime in it to stabilize the sludge, and is the consistency of, well, cake! It then goes to the landfill in Scranton's case.
Most large plants mix this with wood chips to make compost, but Scranton's got shut down years ago. If you ever drive near the Phila. International Airport you can see the composting area on the opposite side of I-95, along with the Southwest Regional WWTP on the same side as the airport. There are piles and piles of "cake" mixed with wood chips as it's sent through long screws that have screens. Much of this finished product ends up as mulch. This is one way of recycling it, and others include fertilizers (check the MSDS on many lawn products; it's a good source of nitrogen) and land application when applied to the soil.
- Coagulation/Flocculation/Settling: This is a term that is more prevalent in the water treatment industry (the "clean" end of the pipe!), but also is part of this process. Coagulation is the sticking together of smaller particles, usually by means of breaking down the electrical charge so they attract better. This is normally done with a chemical -- Aluminum Sulphate (Alum), Ferric Chloride, or polymers. Flocculation is the formation of these from very tiny particles to ones easily seen by the eye (floc). These then settle to the bottom of a tank when the flow is slowed to a certain point.
This process uses a chemical for the coagulant stage in water treatment, but often does not in sewage treatment. Remember, everything you do in the plant may end up in the river. Initial (primary) settling removes things that will settle without help in two to four hours. After the rest of the primary settled flow is "oxidized" (use of oxygen in the process) by the bugs, it usually settles in the secondary clarifiers.
- Laboratory Testing: The lab is the "eyes" of the inner-workings of the plant. Tests for pH, solids, ammonia, nitrate, nitrite, alkalinity, fecal coliform, BOD, COD, DO (dissolved oxygen) and more are performed on the influent, primary (settling) effluent, aeration system, secondary effluent, chlorine contact tanks, sludge thickening and final effluent. The results of your tests on the plant's effluent are part of your reports that are sent to the state and federal agencies. The final effluent is the most important factor in the system but the way to improve it is to know what's going on inside the plant based on lab results and your own experiences and instincts.
Besides testing for the parameters mentioned, daily tests are done on the settling characteristics, and a microscope is used to check on the "bugs." You can see changes in how active they are, see the balance of young and older ones, and also check the floc particles for density (higher density = better settling). There are many different "bugs" and I'll keep it simple by saying you want to keep a balance of them (hopefully more good than bad). The variety keeps the population more resistant to toxic shocks (mostly from industry).
- Odors: This item is very important for many reasons -- including keeping the surrounding neighborhoods happy. The experienced operator uses all of his or her senses to check the plant, but a good nose is essential, and, actually, the plant doesn't smell as bad as you may think if it's running well. There are many different odors through the plant, and each process area has it's own unique smell. Changes in odors can indicate problems with aeration, bug die-offs, chemical slugs into the plant, ammonia problems, etc.
Odor control systems are placed throughout the plant, and are especially important in the headworks (influent) building, sludge processing and sludge storage areas. When possible, a lot of plants cover everything possible and direct all airflows though odor treatment systems. Scranton's WWTP (that's wastewater treatment plant for you non-"terd-herders") has all of the large tanks outside (primary and secondary settling, and aeration) with everything else inside and contained.
The Treatment SystemThe flow through Scranton's WWTP is as follows:
- 1.Headworks -- including screening, grit removal and main pump station;
2.Primary Clarification -- this is the first real settling area;
3.Secondary Treatment (Aeration) -- this is where most of the pollution is removed by microbiological action. Basically, it's the "bug house;"
4.Secondary Clarification -- final settling tanks;
5.Chlorine Contact/Disinfection -- remove pathogens, and "polishing" settling/skimming;
6.Final Effluent -- this is the one that is the bottom line -- where your flow goes back to nature. It flows to the Lackawanna River, and eventually to the Chesapeake Bay.
Headworks BuildingThe first place the flow sees is the headworks building. Before the flow enters the building, there is a plant bypass that can be used for heavy rains. If it is needed, a 48-inch valve is opened to send flow directly to the river. The valve is located a few feet above the influent gate to the plant, so the flow has to be at a certain level to go directly to the river. There is a meter here for monitoring the amount of by-pass, and it is reported monthly to both the Pa. Department of Environmental Protection and the EPA (more on this later).
As the flow enters the building, it is first greeted by a manual bar screens. This is simply a set of bars about 1-1 1/2 inches apart that prevents large items from entering the plant. It holds cans, bottles, sticks and (the operator's favorite) rats. These seem to come into the plant during rains because they make nests in the storm sewer area of the system, then drown when it rains enough. To make matters worse, they are completely bald and sometimes skinned due to the rough ride in the reinforced concrete pipe. Needless to say, this is not the best of jobs for the operator when he has to rake the bar screens. There are two screens, with one used for normal flow and a second brought in for heavy flow.
There is considerable venting and exhaust fans in this building, as you can imagine. There is also monitoring of the gasses as the flow enters the plant (Hydrogen Sulfide and petroleum distillates are of particular importance). Very large plants have extremely sensitive monitoring, and these are mostly due to the industries that bring their wastewater to the treatment plant. Until about 30 years ago, most industry simply dumped anything to the plants, but now close monitoring of the effluents is the norm, and many have their own treatment systems that then discharge to the local plant.
The next treatment step is the mechanical bar screens. These are motor-operated screens that can pick-up things down to items smaller than a cigarette butt. (This is where any paper money flowing into the plant is collected and returned to its rightful owner. There is one operator in particular who always thinks he is going to find a big wad of cash here someday -- I hope he does!)
The reason for this equipment is to prevent these items from getting in the pumps, controls and small piping of the plant and causing problems. Like the manual bar screens, these are parallel and can be used together when needed. Huge plants have very large screens. I visited the Northeast Philly plant, and you could put a Cooper Mini in those boxes, and they go about 20 feet up and down. All solids collected from the manual and mechanical bar screens are rinsed, drip-dried and then the contents are vacuumed (with a Vactor truck) to haul to the landfill.
The next process at Scranton and most other plants is the grit removal system. There are two channels that keep the flow moving at a certain velocity. Fast enough to keep the majority of solids from settling out, but the grit and sand will be scraped off the bottom of the chamber and pumped to a grit clarifier. These grit pumps take a serious beating -- pumping this abrasive mixture 24/7, at high head and flow. The grit clarifiers settle the grit out and a screw takes it up a ramp into a truck for the landfill. The water from the process is recycled back into the headworks building.
The flow now comes into the main pump area. This is called a wetwell, for obvious reasons. The Scranton pumps are 125 horsepower pumps, four in total, that are about 11,000 gpm each. There are two that have a simple speed adjustment, one that runs off a VFD and electronic level control, and the fourth pump is one of the original constant speed models that's still kickin' -- 30 years and counting.
Normal flow is for the VFD pump, with one of the adjustables coming in and out as needed. All flow through the main part of the plant is from this location only, so it is critical that the level is monitored well and that the pumps are kept as constant as possible. The wetwell is a large box with a cone bottom, and has about six recycle lines coming in from all over the plant. Although the flow from the collection system may be up to 24 MGD, this wetwell could easily see flows twice that high for hours at a time, depending on tanks draining, processes recycling, and plant problems.
This recycling of water occurs all through the plant, and is an integral part of the process. The flows through the plant vary from each area, depending on how much "recycle" is added to the normal flow. In general, this is one of the keys to troubleshooting the hydraulic problems in a treatment system. The plant does not like changes in characteristics -- including flow -- and the operator must try to maintain a constant state if possible.
And you Wetheads thought balancing a residential hydronic system was tough. Try a system that has 15 variables at eight different locations with dozens of pumps!
The rest of the flow through the main part of the plant will be all outdoors from here, and the tanks that will be mentioned total about 15 million gallons. Needless to say, this takes up a large area of the site. The flow time for Scranton's WWTP from headworks to final effluent is usually between about 12 and 28 hours. Most plants of this type try to maintain a "detention time" somewhere near this amount. Other technologies will vary this considerably, depending on the application.
Primary ClarifiersThis is the first true settling area of the plant, and is also called primary treatment. This area is where solids will settle and be taken to the sludge thickeners. Floating debris is also removed here and sent to the sludge process area as well. The first section of these tanks has a cone-shaped bottom with a sump. Scrapers run the length of the entire settling basin and push the sludge into the sump along the bottom and skim along the top to the opposite end. A sludge "blanket" is checked to ensure the layer of sludge is kept at permissable levels -- about 1-4 feet or so. The sludge pumps have to be adjusted based on the blanket level, sludge thickening tanks conditions, total sludge (plant-wide) conditions, and the influent flow characteristics. This process removes about 20-40 percent of the solids, and about 10-20 percent of BOD and ammonia.
Remember the part about trying to control your flow through the plant? Some of it can be done here by bringing tanks into service, then emptying them, to keep the flow at a decent level. This is known as flow equalization, and in smaller plants there are usually tanks for this very purpose. As the plant influent flow rises, these tanks can be filled. We used to keep two full all of the time and bring the other two in and out as needed. By equalizing flow, you are helping the process maintain a relatively steady state. This is especially helpful during heavy rains to avoid any excess bypassing. I am proud to say that in my tenure at this plant, we had extremely low incidences of bypassing when compared to the plant's history. The operators used to get a bit crazy running around and turning four to eight large valves at a time, but they put up with me pretty well.
Aeration TanksThe flow then travels to the aeration tanks. This is the "bug house" and is the heart of this type of treatment system. Take care of the bugs and they will take care of the poop! This means that you have to try and maintain consistent levels of pH, alkalinity, flow, food, air (oxygen), solids, ammonia, etc.
Aeration systems are usually dark brown in color, with a musty/earthy smell. A darker color indicates "older" bugs -- they don't work as hard so get rid of 'em! A lighter color indicates a thin biomass or too many young bugs. Young bugs eat a lot, but don't settle very well. There is normally a small amount of foam and /or bubbles floating on top, and the solution is in constant "boil." In other words, it is always rolling around to keep it well-mixed. Sometimes foaming is a problem, especially in the fall when too many of the "bad" bacteria (filamentous, among others) comes to town and gets tangled through the biomass. These seasonal changes can be partially avoided by disinfecting at the proper times, but this also affects the biomass in a bad way if you're not careful.
The flow set-up in Scranton's aeration system is called "plug flow." This means that all of the flow from the primary settling tanks is introduced at the front of the aeration tanks. Variations of this are step feed (split the flow along the first half of the tanks) and anoxic zone feed (areas of no air combined with others).
The bugs at the front of the tanks in Scranton are seeing a high food/low air/higher pH environment, while the end of the tanks has lower food/higher air/medium pH. The front also has the return flow of bugs, which are generally very hungry and in need of air. They are returning from the final clarifiers, and have not been "fed" for a few hours. The flow rate is the same at both ends. This is one of the few areas that does not have recycle piping in Scranton's plant. An alkaline chemical is added here for nitirification.
As the flow goes though the aeration system the bugs are breaking down the waste, using oxygen to "breathe" and also using up alkalinity provided by the chemical. If all is well, the bugs will settle in the final clarifiers, and about 90 percent of the BOD and Ammonia is removed. If the bugs had enough to eat and are "fat and happy," they will settle very well in the final clarifiers.
If there is a lot of oils and grease that are still not removed, or the bugs are shocked from toxic loading, or there's too many or not enough bugs (not enough/too much food), or the pH/alkalinity/air is not right, or the hydraulic load is too high or low, etc., there will be problems. Many of the problems are reflected in the final clarifiers, but are a symptom of aeration problems.
Secondary ClarifiersThe secondary clarifiers is the place where you really start to visually see the job done by the plant. The flow from the aeration system at the front is brown, and if you're doing things right, it is nearly crystal clear at the back.
Problems with settling here include high flow, high organic load (not enough reduction by the aeration system), "pin floc" -- which is small particles that do not settle and overflow -- turbid (cloudy) effluent from low pH or high flow or settling too fast (old bugs), emulsified greases/oils (sounds appetizing doesn't it?), and denitrification. This last item is rather difficult to simply explain, but let's just say that the nitrogen gas released from nitrification/denitrification processes should happen almost entirely in the aeration system, not here. If this last occurs, you get big chunks of sludge "turtles" rising to the surface. Not a pretty site when the inspectors come around. Sorry, but I couldn't totally avoid this topic as promised earlier.
The flow balancing (again) plays an important role in final clarification. The detention time for settling must be kept within a small window -- too fast and no settling, too slow and denitrification, possible biomass die-offs, and sludge build-up. The bugs that settle here are constantly fed back to the aeration system with a portion plus the skimmings to the sludge processing area.
At the end of the final clarifiers, there is also a certain portion of water removed for "plant water" at the Scranton facility. This is used for providing pressurized water for the chlorine system, fire protection, certain mixing and recycle functions, and sprays for the tank areas.
Chlorine Contact TanksThe effluent from the final clarifiers flows to the disinfection tanks. Basically long settling tanks that hold the flow for a minimum of 30 minutes of chlorine contact. Any bugs that are still around are greeted with a serious shot of chlorine. I can imagine death being quite painful - and for all the work they did! An utter lack of respect, but it's the law to have only a very small number of fecal coliform bacteria in the effluent. Final "polishing" settling and skimming is also done here.
In recent years there has been a push to limit the chlorine residual in most plant discharges, due to wildlife diminishment, and the possibility of trihalomethanes (THM's) being formed in rivers. This last item is a carcinogen that is the result of chlorine in contact with certain biological matter -- tree leaves and such I believe.
The reduction in chlorine effluent residuals has led to many variations of disinfection. UV lighting is becoming more popular, as well as Ozone. These two are popular in the water treatment industry as well, and are widely used in Europe. They provide nearly instantaneous kill and leave virtually no residual, although the energy costs are high. Another method of reducing the residual is to add a chemical at the end of the contact tanks to offset chlorine (sodium bisulfite and others are popular). This, in my opinion, is simply making things more complicated and adding to the possibility of shocking the receiving stream but it is gaining in popularity due to its simplicity to add on to existing systems. Just imagine that chemical -- if it can eat chlorine, I don't want that near my discharge!
So now the nice clean, relatively pathogen-free water has hit the river. It has plenty of D.O. (dissolved oxygen) for the marine life, and the pH is usually about 6.2 to 6.5. A bit low, but costly to maintain much higher. Let's look at the "supporting" processes through the plant.
Sludge ThickeningThis area is where all of the sludges through the plant are mixed, and "thickened" to a point of about 3-5 percent or so at the Scranton facility. Just as the rest of the plant, this is a fairly low- tech operation that involves dissolved air filtration. This is a misnomer of sorts because there really isn't a "filter" involved, it is simply taking the whole settling process and turning it upside down.
As the sludges enter the mixing area, they are combined with plant water, a polymer chemical, and dissolved air. The air forces the formed floc particles to float, and the sludge is scraped off the top. The water at the bottom is the "clear" water that cycles back to the headworks building (another variation for flow balancing).
One note about sludges. They will vary in texture, odor and color very quickly in Scranton's case. I would assume this to be true in other similar plants as well. This is due to many factors including the flow of sludge to the thickeners from each plant area, and the characteristics of each. In general, sludge from the primary clarifiers is much more "treatable" (easier to process), but you have to have a ratio of secondary sludge and skimmings that is decided by the entire treatment process. Basically, this part of the plant is lowest on the totem pole for respect, and because the cake that leaves here does not enter the river, it takes often a backseat to the rest of the system.
That doesn't mean this area is not important. The last thing you want is a full sludge tank that will not run through the presses very fast, and smells really bad to boot!
After the sludge is thickened it is sent to the storage tank (about 350,000 gallons), and is kept aerated until it is ready for the belt presses. When the sludge flows to the presses, it is mixed with a second polymer and an odor-reducing chemical (potassium permanganate). The presses squeeze the sludge through a cloth and a set of rollers. Hydraulics are real helpful here. The discharge from the presses mix with powdered lime in a blade mixer (pug mill) and Voila! there's your cake. It is conveyed to a truck and driven to the landfill.
The key to this step of treatment is balancing the mix of polymer and sludge flow to how the sludge is "pressing." In other words, it has to have a certain texture before it can be pressed. Too much grease/oil, bad mix of secondary sludge, too much/not enough polymer, rolling too fast on the presses all can lead to poor cake quality. The final product must be at least 18 percent solids, and have a pH of at least 8.5, I believe.
Plant RecycleThe control of flow is done throughout the plant. The two major areas of recycle are return sludge and sludge processing. The return sludge is taken from the secondary clarifiers and returned to the aeration system, while the sludge processing area takes sludge from both sets of clarifiers and the effluent (cleaner part) is returned to the headworks area. Other recycle areas include all tank drains, belt press effluent, plant water and grit clarifier effluent.
Besides the flow coming into the plant, all of these items also effect the flow balance as well as the water quality. Careful monitoring of most of these areas helps the operator when troubleshooting the system.
The main reason for sludge return is to control the bug population. Return sludge is also sent to the sludge process area as needed to get rid of some biomass. In the summer, the biomass needs to be reduced due to their higher efficiency while in the winter they have to be increased (cold water slows them down). Along with this balance is the balance to try and reduce the pollutant levels.
ConclusionI hope this gives a rough idea of the workings of an activated sludge wastewater treatment plant, in particular the one located in Scranton, Pa.
I would suggest that people interested in this technology go out and visit their local plant. Most larger towns and cities have tours. Just remember to wash your hands BEFORE and after using the bathroom, and don't touch too many things in certain areas.