Tag Archives: sewage

Sewage reactor engineering, Stirred tank designs

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Over the past few years, I’ve devoted several of these essays to analysis of first-stage sewage treatment reactors. I described and analyzed the rotating disc reactor found at the plant is Holly here, and described the racetrack,“activated sludge” plug reactor found most everywhere else here. I also described a system without a primary clarifier found near Cincinatti. All of these were effective for primary treatment; soluble organics are removed by bio-catalyzed oxidation:

2 H-C-O-H + O2 –> CO2 + H2O.

A typical plant in Oakland county treats 2,000,000 gallons per day of this stuff, with the bio-reactor receiving liquid waste containing about 200 ppm of soluble and colloidal biomass. That’s 400 dry gallons for those interested, or about 3200 dry lbs./day. About half of this will be oxidized to CO2 and water. The rest (cell bodies) are removed with insoluble components, and applied to farmers fields or buried, or burnt in an incinerator.

There is another type of reactor used in Oakland County. It’s mostly used for secondary treatment, converting consolidated sludge to higher-quality sludge that can be sold or used on farms with less restriction, but it is a type of reactor used at the South Lyon treatment plant, for primary treatment. It is a Continually stirred tank reactor, or CSTR, a design that is shown in schematic below.

As of some years ago, the South Lyon system involved a single largish pond lined with plastic with a volume about 2,000,000 gallons total. About 700,000 gallons per day of sewage liquids went into the lagoon, at 200 ppm soluble organics. Air was bubbled through the liquid providing a necessary reactant, and causing near-perfect mixing of the contents. The aim of the plant managers is to keep the soluble output to the, then-acceptable level of 10 ppm; it’s something they only barely managed, and things got worse as the flow increased. Assume as before, a value V and a flow Q.

We will call the concentration of soluble organics C, and call the initial concentration, the concentration that enters,  Ci. It’s about 200 ppm. We’ll call the output concentration Co, and for this type of reactors, Co = C.  The reaction is first order, approximately, so that, if there were no flow into or out of the reactor, the concentration of organics would decrease at the rate of

dC/dt = -kC.

Here k is a reaction constant, dependent on temperature oxygen and cell content. It’s typically about 0.5/hour. For a given volume of tank the rate of organic removal is VkC. We can now do a mass balance on soluble organics. Since the rate of organic entry is QCi and the rate leaving by flow is QC. The difference must be the amount that is reacted away:

QCi – QC = VkC.

We now use algebra, to find that

Co = Ci/(1 + kV/Q).

V/Q is sometimes called a residence time; for the system. At normal flow, the residence time of the South Lyon system is about 2.8 days or 68.6 hours. Plugging these numbers in, we find that the effluent from the reactor leaves at 1/35 of the input concentration, or 5.7 ppm, on average. This would be fine except that sometimes the temperature drops, or the flow increases, and we start violating the standard. A yet bigger problem was that the population increased by 50% while the EPA standard got more stringent to 2 ppm. This was solved by adding another, smaller reactor, volume = V2. Using the same algebraic analysis, as above you can show that, with two reactors,

Co = Ci/ [(1 + kV/Q)(1+kV2/Q)].

It’s a touchy system, but it meets government targets, just barely, most of the time. I think it is time to switch to a plug-flow reactor system, as used in much of Oakland county. In these, the fluid enters a channel and is reacted as it flows along. Each gallon of fluid, in a sense moves by itself as if it were its own reactor. In each gallon, we can say that dC/dt = -kC. We can thus solve for Co in terms of the total residence time, where t again is V/Q. We can rearrange this equation and integrate: ∫dC/C = – ∫kdt. We then find that, 

      ln(Ci/Co) = kt = kV/Q

To convert 200 ppm sewage to 2 ppm we note that Ci/Co = 100 and that V = Q ln(100)/k = Q (4.605/.5) hours. An inflow of 1000,000 gallons per day = 41,667 gal/ hour, and we find the volume of tank is 41,667 x 9.21 = 383,750 gallons. This is quite a lot smaller than the CSTR tanks at South Lyon. If we converted the South Lyon tanks to a plug-flow, race-track design, it would allow it to serve a massively increased population, discharging far cleaner sewage. 

Robert Buxbaum, November 17, 2019

Kindness and Cholera in California

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California likely leads the nation in socially activist government kindness. It also leads the nation in homelessness, chronic homelessness, and homeless veterans. The US Council on Homelessnesses estimates that, on any given day, 129,972 Californians are homeless, including 6,702 family households, and 10,836 veterans; 34,332 people are listed among “the chronic homeless”. That is, Californians with a disability who have been continuously homeless for one year or cumulatively homeless for 12 months in the past three years. No other state comes close to these numbers. The vast majority of these homeless are in the richer areas of two rich California cities: Los Angeles and San Francisco (mostly Los Angeles). Along with the homeless in these cities, there’s been a rise in 3rd world diseases: cholera, typhoid, typhus, etc. I’d like to explore the relationship between the policies of these cities and the rise of homelessness and disease. And I’d like to suggest a few cures, mostly involving sanitation. 

A homeless encampment in LosAngeles

Most of the US homeless do not live in camps or on the streets. The better off US homelessness find it is a temporary situation. They survive living in hotels or homeless shelters, or they “couch-serf,” with family or friends. They tend to take part time jobs, or collect unemployment, and they eventually find a permanent residence. For the chronic homeless things are a lot grimmer, especially in California. The chronic unemployed do not get unemployment insurance, and California’s work rules tend to mean there are no part time jobs, and there is not even a viable can and bottle return system in California, so the homeless are denied even this source of income*. There is welfare and SSI, but you have to be somewhat stable to sign up and collect. The result is that California’s chronic homeless tend to live in squalor strewn tent cities, supported by food handouts.

Californians provide generous food handouts, but there is inadequate sewage, or trash collection, and limited access to clean water. Many of the chronic homeless are drug-dependent or mentally ill, and though they might  benefit from religion-based missions, Los Angeles has pushed the missions to the edges of the cities, away from the homeless. The excess food and lack of trash collection tends to breed rats and disease, and as in the middle ages, the rats help spread the diseases. 

Total homelessness by state, 2018; California leads the nation. The better off among these individuals do not live on the streets, but in hotels or homeless shelters. For most, this is a short term situation. The rest, about 20%, are chronically homeless. About half of these live on the streets without adequate sewage and water. Many are drug-dependent.

The first major outbreaks of the homeless camps appeared in Los Angeles in August and September of 2017. They reappeared in 2018, and by late summer, rates were roughly double 2017’s. This year, 2019, looks like it could be a real disaster. The first case of a typhoid infected police officer showed up in May. By June there were six police officers with typhoid, and that suggests record numbers are brewing among the homeless.

To see why sanitation is an important part of the cure, it’s worth noting that typhoid is a disease of unclean hands, and a relative of botulism. It is spread by people who go to the bathroom and then handle food without washing their hands first. The homeless camps do not, by and large, have hand washing stations. and forced hygiene is prohibited. Los Angeles has set up porta-potties, with no easy hand washing. The result is typhoid epidemic that’s even affecting the police (six policemen in June!).

rate od disease spread.
R-naught, reproduction number for some diseases, CDC.

Historically, the worst outbreaks of typhoid were spread by food workers. This was the case with “typhoid Mary of the early 20th century.” My guess is that some of the police who got typhoid, got it while trying to feed the needy. If so, this fellow could become another Typhoid Mary. Ideally, you’d want shelters and washing stations where the homeless are. You’d also want to pickup the dirtier among the homeless for forced washing and an occasional night in a homeless shelter. This is considered inhumane in Los Angeles, but they do things like this in New York, or they did.

Typhus is another major disease of the California homeless camps. It is related to typhoid but spread by rodents and their fleas. Infected rodents are attracted to the homeless camps by the excess food. When the rodents die, their infected fleas jump to the nearest warm body. Sometimes that’s a person, sometimes another animal. In a nastier city, like New York, the police come by and take away old food, dead animals, and dirty clothing; in Los Angeles they don’t. They believe the homeless have significant squatters rights. California’s kindness here results in typhus.

Reproduction number and generation time for some diseases.

The last of the major diseases of the homeless camps is cholera. It’s different from the others in that it is not dependent on squalor, just poor health. Cholera is an airborne disease, spread by coughing and sneezing. In California’s camps, the crazy and sick dwell close to each other and close to healthy tourists. Cholera outbreaks are a predictable result. And they can easily spread beyond the camps to your home town, and if that happens a national plague could spread really fast.

I’d discussed R-naught as a measure of contagiousness some months ago, comparing it to the reproductive number of an atom bomb design, but there is more to understanding a disease outbreak. R-naught refers merely to the number of people that each infected person will infect before getting cured or dying. An R-naught greater than one means the disease will spread, but to understand the rate of spread you also need the generation time. That’s the average time between when the host becomes infected, and when he or she infects others. The chart above shows that, for cholera, r-naught is about 10, and the latency period is short, about 9 days. Without a serious change in California’s treatment of the homeless, each cholera case in June will result in over 100 cases in July, and well over 10,000 in August. Cholera is somewhat contained in the camps, but once an outbreak leaves the camps, we could have a pandemic. Cholera is currently 80% curable by antibiotics, so a pandemic would be deadly.

Hygiene is the normal way to prevent all these outbreaks. To stop typhoid, make bathrooms available, with washing stations, and temporary shelters, ideally these should be run by the religious groups: the Salvation Army, the Catholic Church, “Loaveser and Fishes”, etc. To prevent typhus, clean the encampments on a regular basis, removing food, clothing, feces and moving squatters. For cholera, provide healthcare and temporary shelters where people will get clean water, clean food, and a bed. Allow the homeless to work at menial jobs by relaxing worker hiring and pay requirements. A high minimum wage is a killer that nearly destroyed Detroit. Allow a business to hire the homeless to sweep the street for $2/hour or for a sandwich, but make a condition that they wash their hands, and throw out the leftovers. I suspect that a lot of the problems of Puerto Rico are caused by a too-high minimum wage by the way. There will always be poor among you, says the Bible, but there doesn’t have to be typhoid among the poor, says Dr. Robert Buxbaum.

*California has a very strict can and bottle return law where — everything is supposed to be recycled– but there are very few recycling centers, and most stores refuse to take returns. This is a problem in big government states: it’s so much easier to mandate things than to achieve them.

July 30, 2019. I ran for water commissioner in Oakland county, Michigan, 2016. If there is interest, I’ll run again. One of my big issues is clean water. Oakland could use some help in this regard.

How to avoid wet basements

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My house is surrounded my mulch — it absorbs enough rainwater that I rarely have to water.

Generally speaking water gets to your basement from rain, and the basic way you avoid wet basements is by providing some more attractive spot for the rainwater to go to. There are two main options here: divert the water to a lake or mulch-filled spot at least 8 feet away from your home, or divert it to a well-operated street or storm drain. My personal preference is a combination of both.

At right I show a picture of my home taken on a particularly nice day in the spring. Out front is a mulch-filled garden and some grass. On the side, not shown is a driveway. Most of the rain that hits our lawn and gardens is retained in 4 inches of mulch, and waters the plants. Four inches of mulch-covered ground will hold at least four inches of rainwater. Most of the rain that hits the house is diverted to downspouts and flows down the driveway to the street. Keeping some rainwater in the mulch means you don’t have to pay so much to water the trees and shrubs. The tree at the center here is an apple tree. I like fruit trees like this, they really suck up water, and I like the apples. We also have blueberries and roses, and a decorative pear (I like pears too, but they are messy).

In my opinion, you want some slope even in the lawn area, so excess rainwater will run to the sewers and not form a yard-lake, but that’s a professional preferences; it’s not always practical and some prefer a brief (vernal ) lake. A vernal lake is one that forms only in the spring. If you’ve got one, you may want to fill it with mulch or add trees that are more water tolerant than the apple, e.g. swamp oak or red cedar. Trees remove excess water via transpiration (enhanced evaporation). Red Cedars grow “knees” allowing them to survive with their roots completely submerged.

For many homes, the trick to avoiding a flooded basement is to get the water away from your home and to the street or a retention area.

When it comes to rain that falls on your hose, one option is to send it to a vernal lake, the other option is to sent it to the street. If neither is working, and you find water in your basement, your first step is to try to figure out where your rainwater goes and how it got there. Follow the water when it’s raining or right after and see where it goes. Very often, you’ll discover that your downspouts or your driveway drain into unfortunate spots: spots that drain to your basement. To the extent possible, don’t let downspout water congregate in a porous spot near your house. One simple correction is to add extenders on the downspouts so that the water goes further away, and not right next to your wall. At left, I show a simple, cheap extender. It’s for sale in most hardware stores. Plastic or concrete downspout pans work too, and provide a good, first line of defense agains a flood basement. I use several to get water draining down my driveway and away from the house.

Sometimes, despite your best efforts, your driveway or patio slopes to your house. If this is the case, and if you are not quite ready to replace your driveway or patio, you might want to calk around your house where it meets the driveway or patio. If the slope isn’t too great, this will keep rainwater out for a while — perhaps long enough for it to dry off, or for most of the rainwater to go elsewhere. When my driveway was put in, I made sure that it sloped away from the house, but then the ground settled, and now it doesn’t quite. I’ve put in caulk and a dirt-dam at the edge of the house. It keeps the water out long enough that it (mostly) drains to the street or evaporates.

A drain valve. Use this to keep other people’s sewer water out of your basement.

There is one more source of wet basement water, one that hits the houses in my area once a year or so. In our area of Oakland county, Michigan, we have combined storm and sanitary sewers. Every so often, after a big rain, other people’s rainwater and sanitary sewage will come up through the basement drains. This is really a 3rd world sewer system, but we have it this way because when it was put in, in the 1900s, it was first world. One option if you have this is to put in a one-way drain valve. There are various options, and I suggest a relatively cheap one. The one shown at right costs about $15 at Ace hardware. It will keep out enough water, long enough to protect the important things in your home. The other option, cheaper and far more hill-billy, is to stuff rags over your basement drains, and put a brick over the rags. I’ll let you guess what I have in my basement.

Robert Buxbaum, June 13, 2019

We don’t need no stinking primary clarifier

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Virtually every sewage plant of Oakland County uses the activated sludge process, shown in the layout below. Raw sewage comes in, and goes through physical separation — screening, grit removal, and a first clarifier – settling tank before moving to the activated sludge oxidation reactor. The 1st clarifier, shown at left below, removes about half of the incoming organics, but it often stinks and sometimes it “pops” bubbles of fart. This is usually during periods of low flow, like at night. When the flow is slow, it arrives at the plant as a rotting smelly mess; it’s often hard to keep the bubbles of smell down.

Typical Oakland Sewage plant, activated sludge process with a primary clarifier.

Typical Oakland County Sewage treatment plant, activated sludge process with a primary clarifier.

The smell is much improved in the oxidation reactor, analyzed here, and in the 2nd clarifier, shown above at right. Following that is a filter, an ultraviolet cleanup stage, and the liquids are discharged to a local river. In Oakland county, the solids from the two clarifiers are hauled off to a farm, or buried in a landfill. Burial in a landfill is a costly waste, as I discuss here. The throughputs for most of these treatment plants is only about 2-3 million gallons of sewage per day. But Oakland county can produce 500 million gallons of sewage per day. The majority of this goes to Detroit for treatment, and sometimes the overflow is dumped rotting and smelly, in the rivers.

A few months ago, I visited the Sycamore Creek Wastewater facility outside of Cincinnati. This is an 8 million gallon per day plant that uses the “extended aeration process”, shown in the sketch below. I noticed several things I liked: the high throughput (the plant looks no bigger than our 2-3 million gallon plants) and the lack of a bad smell, primarily. The Sycamore Creek plant had an empty hole where the primary clarifier had once been. Lacking this clarifier, the screened sewage could not sit and pop. Instead it goes directly from grit removal to the oxidation reactor, a reactor that looks no bigger than in our plants. This reactor manages a four times higher throughput, I think, because of a higher concentration of cellular catalyst. Consider the following equation derived in a previous post:

ln C°/C = kV/Q.

Here, C° and C are the incoming and exit concentrations of soluble organic; k is the reaction rate, proportional to cellular concentration, V is the volume of the reactor, Q is the flow, and ln is natural log. The higher cellular concentration in the extended aeration plant results in an increased reaction rate, k. The higher the value of k, the higher the allowed flow, Q, per reactor volume, V.

The single clarifier at the end of the Sycamore Creek plant does not look particularly big. My sense is that it deals with a lot more sludge and flow than is seen in our 2nd clarifiers because (I imaging) the sludge is higher density, thus faster settling. I expect that, without the 1 clarifier, there is extra iron and sulfate in the sludge, and more large particles too. In our plants, a lot of these things are removed in the primary clarifier. Sludge density is also increased, I think, because the Cincinnati plant recycle a greater percentage of the sludge (I list it as 90% in the diagram). Extra iron in the reactor also helps to remove phosphates from the water effluent that flows back to the river, an important pollution concern. Iron phosphates are insoluble, and thus leave with the sludge. In Oakland county’s activated sludge plants, it is typical to add iron to the reactor or clarifier. In Cincinnati’s extended aeration plant, I’m told, iron addition is generally not needed.

Typical Oakland Sewage plant, activated sludge process with a primary clarifier.

Cincinnati sewage treatment plant, extended aeration process with no primary clarifier.

The extended aeration part of the above process refers to the secondary sludge oxidizer, the continuously stirred tank reactor, or CSTR shown at lower right above. The “CSTR” is about 1/5 the volume of the main oxidation reactor and about the size of a clarifier. Oxidation in the CSTR compliments that in the main oxidizer removing organics, making bio-polymer, and improving (I think) the quality of the sludge that goes to the farms. Oxidation in the CSTR reduces the amount of sludge that goes to the farms. The sludge that does go, is  less-toxic and more concentrated in organics and minerals. I’m not sure if the CSTR product is as good as the product from an anaerobic digester, or if the CSTR is cheaper to operate, but it looks cheaper since there is no roof, and no (or minimal) heating. This secondary oxidizer is very efficient at removing organics because the cellular catalyst concentration is very high – much higher than in the main oxidizer.

During periods of high load, early morning, the CSTR seems to serve as a holding tank so that sludge does not build up in the clarifier. Too much sludge in the clarifier can start to rot, and ruin the effluent quality. The way you tell if there is too much sludge, by the way, is through a device called the “sludge judge.” I love that name. The Cincinnati plant used a centrifugal drier; none of our plants do. The Cincinnati plant had gap the bubble spots of the main oxidizer. This is good for denitrification, I’m told, an important process that I discuss elsewhere.

The liquid output of their clarifier (or ours) is not pure enough to be sent directly to the river. In this plant, the near-pure water from the clarifier is sent to a trickling filter, a bed of sand and anthracite that removes colloidal remnants. Some of our plants do the same. I suspect that the large surface area in this filter is also home to some catalysis: last stage oxidation of remaining bio-organics. On a regular basis, the filter bed is reverse-flushed to remove cellular buildup, slime, and send it to the beginning of the process. The trickling filter output is then sent to an ultraviolet, bacteria-killing step before being released to the rivers. All in all, I suspect that an extended aeration process like this is worth looking into for Oakland County, especially for our North Pontiac sewage treatment facility. That plant is particularly bad smelling, and clearly too small to treat all its sewage. Perhaps we can increase the throughput and decrease the smell at a minimal cost.

Dr. Robert E. Buxbaum, December 18, 2018. I’m running for water commissioner of Oakland county, MI. If you like, visit my campaign site. Here are some sludge jokes and my campaign song.

A rotating disk bio-reactor for sewage treatment

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One of the most effective designs for sewage treatment is the rotating disk bio-reactor, shown below. It is typically used in small-throughput sewage plants, but it performs quite well in larger plants too. I’d like to present an analysis of the reactor, and an explanation of why it works so well.

A rotating disc sewage reactor.

A rotating disk sewage reactor; ∂ is the thickness of the biofilm. It’s related to W the rotation rate in radians per sec, and to D the limiting diffusivity.

As shown, the reactor is fairly simple-looking, nothing more than a train of troughs filled with sewage-water, typically 3-6 feet deep, with a stack of discs rotating within. The discs are typically 7 to 14 feet in diameter (2-4 meters) and 1 cm apart. The shaft is typically close to the water level, but slightly above, and the rotation speed is adjustable. The device works because appropriate bio-organisms attach themselves to the disk, and the rotation insures that they are fully (or reasonably) oxygenated.

How do we know the cells on the disc will be oxygenated? The key is the solubility of oxygen in water, and the fact that these discs are only used on the low biological oxygen demand part of the sewage treatment process, only where the sewage contains 40 ppm of soluble organics or less. The main reaction on the rotating disc is bio oxidation of soluble carbohydrate (sugar) in a layer of wet slime attached to the disc.

H-O-C-H + O2 –> CO2 + H2O.

As it happens, the solubility of pure oxygen in water is about 40 ppm at 1 atm. As air contains 21% oxygen, we expect an 8 ppm concentration of oxygen on the slime surface: 21% of 40 ppm = 8 ppm. Given the reaction above and the fact that oxygen will diffuse five times more readily than sugar at least, we expect that one disc rotation will easily provide enough oxygen to remove 40 ppm sugar in the slime at every speed of rotation so long as the wheel is in the air at least half of the time, as shown above.

Let’s now pick a rotation speed of 1/3 rpm (3 minutes per rotation) and see where that gets us in terms of speed of organic removal. Since the disc is always in an area of low organic concentration, it becomes covered mostly with “rotifers”, a fungus that does well in low nutrient (low BOD) sewage. Let’s now assume that mass transfer (diffusion) of sugar in the rotifer slime determines the thickness of the rotifera layer, and thus the rate of organic removal. We can calculate the diffusion depth of sugar, ∂ via the following equation, derived in my PhD thesis.

∂ = √πDt

Here, D is the diffusivity (cm2/s) for sugar in the rotifera slime attached to the disk, π = 3.1415.. and t is the contact time, 90 seconds in the above assumption. My expectation is that D in the rotifer slime will be similar to the diffusivity sugar in water, about 3 x 10-6 cm2/s. Based on the above, we find the rotifer thickness will be ∂ = √.00085 cm2 = .03 cm, and the oxygen depth will be about 2.5 times that, 0.07 cm. If the discs are 1 cm apart, we find that, about 14% of the fluid volume of the reactor will be filled with slime, with 2/5 of this rotifer-filled. This is as much as 1000 times more rotifers than you would get in an ordinary constantly stirred tank reactor, a CSTR to use a common acronym. We might now imagine that the volume of this sewage-treatment reactor can be as small as 1000 gallons, 1/1000 the size of a CSTR. Unfortunately it is not so; we’ll have to consider another limiting effect, diffusion of nutrients.

Consider the diffusive mass transfer of sugar from a 1,000,000 gal/day flow (43 liters/sec). Assume that at some point in the extraction you have a concentration C(g/cc) of sugar in the liquid where C is between 40 ppm and 1 ppm. Let’s pick a volume of the reactor that is 1/20 the normal size for this flow (not 1/1000 the size, you’ll see why). That is to say a trough whose volume is 50,000 gallons (200,000 liters, 200 m3). If the discs are 1 cm apart, this trough (or section of a trough) will have about  4×10^8 cm2 of submerged surface, and about 9×10^8 total surface including wetted disc in the air. The mass of organic that enters this section of trough is 44,000 C g/second, but this mass of sugar can only reach the rotifers by diffusion through a water-like diffusion layer of about .06 cm thickness, twice the thickness calculated above. The thickness is twice that calculated above because it includes the supernatant liquid beyond the slime layer. We now calculate the rate of mass diffusing into the disc: AxDxc/z = 8×10^8 x 3×10-6 x C/.06 cm = 40,000 C g/sec, and find that, for this tank size and rotation speed, the transfer rate of organic to the discs is 2/3 as much as needed to absorb the incoming sugar. This is to say that a 50,000 gallon section is too small to reduce the concentration to ln (1) at this speed of disc rotation.

Based on the above calculation, I’m inclined to increase the speed of rotation to .75 rpm. At this speed, the rotifer-slime layer will be 2/3 as thin 0.2 mm, and we expect an equally thinner diffusion barrier in the supernatant. At this faster speed, there is now 3/2 as much diffusion transfer per area because the thinner diffusion barrier, and we can expect a 50,000 liter reactor section to reduce the initial concentration by a fraction of 1/2.718 or C/e. Note that the mass transfer rate to the discs is always proportional to C. If we find that 50,000 gallons of tank reduces the concentration to 1/e, we find that we need 150,000 gallons of reactor to reduce the concentration of sugar from 40 ppm to 2 ppm, the legal target, ln (40/2) = 3. This 150,000 gallons is a remarkably small volume to reduce the sBOD concentration from 40 ppm to 2 ppm (sBOD = soluble biological oxygen demand), and the energy use is small too if the disc bearings are good.

The Holly sewage treatment plant is the only one in Oakland county, MI using the rotating disc contacted technology. It has a flow of 1,000,000 gallons per day, and has a contactor trough that is 215,000 gallons, about what we’d expect though their speed is somewhat higher, over 1 rpm and their input concentration is likely lower than 40 ppm. For the first stage of sewage treatment, the Holly plant use a vertical-draft, trickle-bed reactor. That is they drizzle the sewage-liquids over a slime-coated packing to reduce the sBOD concentration from 200 ppm to before feeding the flow to the rotating discs. My sense of the reason they don’t do the entire extraction with a trickle bed is that the discs use far less energy.

I should also add that the back-part of the disc isn’t totally useless oxygen storage, as it seems from my analysis. Some non-sugar reactions take place in the relatively anoxic environment there and in the liquid at the bottom of the trough. In these regions, iron reacts with phosphate, and nitrate removal takes place. These are other important requirements of sewage treatment.

Robert E. Buxbaum, July 18, 2017. As an exercise, find the volume necessary for a plug flow reactor or a stirred tank reactor (CSTR) to reduce the concentration of sugar from 40 ppm to 2 ppm. Assume 1,000,000 gal per day, an excess of oxygen in these reactors, and a first order reaction with a rate constant of dC/dt = -(0.4/hr)C. At some point in the future I plan to analyze these options, and the trickle bed reactor, too.

Sewage jokes, limericks, and a song.

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I ran for water commissioner (sewer commissioner) of Oakland county, Michigan last year, lost, but enjoyed my run. It’s a post that has a certain amount of humor built-in. If you can’t joke about yourself, you’ve got no place in the sewer. So here are some sewage jokes, and poems, beginning with an old favorite; one I used often in my campaign:3b37b9cab2d27693de2aa7004a3d90ef

Why was Piglet staring into the toilet?
He was looking for Poo.

Last week someone broke into the police station and stole all the toilets. The cops are still searching. So far, they have nothing to go on.paperwork

On administration: In life as on the toilet, the job isn’t done until the paperwork is finished.

Speaking of toilet paper: do you know why Star Trek is like toilet paper? They both go past Uranus and capture Klingons. I wrote an essay on Toilet paper — really. 

Here’s my campaign song and video. It’s sung by Art Carney (I’ve no rights, but figure they’ve expired). The pictures are of me, my daughter, and various people we met visiting sewage treatment plants around the county. Great men and a few great women who don’t mind getting their hands dirty. 

septic12

The Turd Burglar, We’re No.1 in the No. 2 business. What a motto!

And now for sewage Limericks:

There once was a man named McBride.
Who fell in the sewer and died.
The same day his brother
Fell in another,
And they were interred side by side.

There is a double intent in that Limerick, in case you missed it

By the sewer she lived, by the sewer she died. Some said t’was disease, but I say, Suicide

sewage treatment

sewage treatment plant in Pontiac, MI — the county’s largest.

How do you describe a jocular sewage joker? pun gent.

Life is like a sewer, what you get out of it is what you put into it (Tom Lehrer). And sometimes it stinks.

Robert E. Buxbaum, June 4, 2017. There is just one more sewage joke I know, but I thought I’d leave it out. It concerns the sewage backup at the prom. Unfortunately, the punchline stinks.

pee in the shower and other water savers

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Do you want to save the planet and save money at the same time? Here are some simple tips:

The first money and planet saver, is to pee in the shower. For those who don’t have a lawn, or who don’t water, your single biggest water cost is likely the toilet. Each person in your household will use it several times per day, at roughly 1.6 gallons per flush. In Oak Park, Michigan the cost of water is 1.5¢/gallon, so each flush costs you, roughly 2.5¢. If you pee in the shower every morning, you’ll save yourself about one flush per day, or 2.5¢. Over the course of a year you’ll have used about 500 gallons less, and will have saved yourself somewhere between $5 and $10. Feel good about yourself every morning; the effort involved is truly minimal.

Related to peeing in the shower, I should mention that many toilets leak. A significant part of your water bill can often be cut by replacing the “flapper valve on the inside of your toilet tank, and/or by cleaning the needle fill valve. To see if you need this sort of help, put a few drops of food dye in the toilet when you leave in the morning. If the color is largely gone by the time you get back, the toilet is leaking the equivalent of a few volumes per day, that is at least as much water as is flushed. If the color goes faster, or you hear the tank refill when no one used it, you’re leaking more. Check the flapper and replace it if it’s worn — it’ll cost about $3 — and check the needle-fill valve. They don’t work forever. Cleanliness is near godliness.

Mulch is good, this is too much concentrated by the tree trunk. Use only 2 inches and spread it out to save water and weeding.

Mulch is good, this is too much concentrated by the tree trunk. Use only 2-3 inches and spread it out from the trunk to save water and weeding without attracting bugs.

If your valve is leaking and you decide to replace it, you may want to replace with a variable flush valve. Typically, there are two options: a big vale for big flush (1.6 gal) and a small valve for small flush (1 gal or less). These are widely used in Europe. You can make up for this cost rather quickly at 1.5¢/gallon.

The next big issue is lawn-care. If you water your lawn and flowers daily, you’ve likely noticed that you pay about $300/month for water in the summer: a lot more than in the winter, or than your lazes-faire neighbor in the summer. Every $150 of summer-excess, water bill you pay represents about 10,000 gallons applied to your lawn. That’s a cubic foot, or 1¢ to 2¢ of water applied per ft2 per month for typical watering. While many sites advise that you can save by adding a rain barrel, I disagree. Rain barrels are costly, ugly, and are a lot of work ago plumb in. And each barrel only holds 55 gallons of water, 82¢ worth when full. You do a lot better, IMHO by putting down an inch or two of mulch around your flowers and vegetables. This mulch requires no work and will keep you from needing to water these areas for the 3-4 days after every rainfall. A layer of 1″ to 2″ will help your soil hold 0.5 to 1 gallon of water per square foot. At typical prices of mulch and water, this will pay for itself in 1-2 years and will help you avoid weeding. Mulch is a far better return than the rain-barrels that are often touted, and there’s far less effort involved. Buy the mulch, not the barrel, but don’t put down too more than 2″ on flowers and vegetable. Trees can take 3 -4″; don’t use more. Avoid a mulch mountain right next to a tree, it causes the roots to grow weird, and provides a home for bugs and undesirable anaerobic molds.

A little more work than the above is to add a complete rain garden or bioswale. Build it at the bottom of any large incline on your property, where the water runs off (It’s likely a soggy swamp already). Dig the area deeper and put, at the bottom of the hole, a several-inch layer of mulch and gravel. Top it off with the soil you just removed, ideally raising the top high enough that, if the rain garden should fill, the water will run off to the street. Plant in the soil at the top long-rooted grasses, or flowers, vegetables, or water-tolerant trees. You may want to direct the water from your home’s sump pump here too (It can help to put a porous pipe at the bottom to distribute this water). If you do this right, you’ll get vegetables or trees and you won’t have to water the garden, ever. Also, you’ll add value to your property by removing the swampy eyesore. You’ll protect your home too, since a major part of home flooding comes from the water surge of sump water to the sanitary sewer.

Robert E. Buxbaum, April 14, 2017. I ran for water commissioner, Oakland County, MI, Nov. 2016. Among my other thoughts: increased retention to avoid flooding, daylighting rivers, and separating the sanitary from the storm sewers. As things stand, the best way to save money on water– get the same deal the state gave to Nestle/ Absopure: they pay only $200/year to pump 200 gal/minute. That is, they pay only 1/3000 of what you and I pay. It helps to have friends in government.

Most flushable wipes aren’t flushable.

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I’m a chemical engineer running for Oakland county water resources commissioner, and as the main job of the office is sewage, and as I’ve already written on the chemistry, I thought I might write about an aspect of the engineering. Specifically about toilet paper. Toilet paper is a remarkable product: it’s paper, compact and low in cost; strong enough to clean you, smooth on your bum, and beyond that, it will disintegrate in turbulent water so it doesn’t clog pipes. The trick to TP’s dry strength and wet-weakness, is that the paper pulp, wood cellulose, is pounded very thin, yet cast fluffy. For extra softness, the paper is typically coated with aloe or similar. Sorry to say, the same recipe does not work for wet-wipes, paper towels or kleenex (facial tissues); all of these products must have wet-strength, and this can cause problems with sewer clogs.

Patent 117355 for perforated toilet paper claimed it as an improved wrapping paper.

Patent 117,355 for perforated toilet paper on a roll. It’s claimed as an improved wrapping paper.

Before there was toilet paper, the world was a much sadder, and smellier place. Much of the world used sticks, stones, leaves, or corn cobs, and none of these did a particularly thorough job. Besides, none of these is particularly smooth, or particularly disposable, nor did it fall apart — not that most folks had indoor plumbing. Some rich Romans had plumbing, and these cleaned themselves with a small sponge on the end of a stick. They dipped the sponge end in water for each use. It was disgusting, but didn’t clog the pipes. I’ve seen this in use on a trip to Turkey 25 years ago — not in actual use, but the stick and sponge was there in a smelly bucket next to the hole in the ground that served as the commode.

The first reasonably modern toilet was invented in 1775 by Alexander Cummings, and by 1852 the first public flush toilets were available. The design looked pretty much like it looks today and the cost was 1¢. You got a towel and a shoe-shine too for that penny, but there was no toilet paper as such. Presumably one used a Roman sponge or some ordinary, standard paper. A popular wipe, back in the day was the Sears-Roebuck catalog. It came free to most homes and included a convenient hole in the corner allowing one to hang it in and outhouse or near the commode. It was rough on the bum, and didn’t fall apart. My guess is that it clogged the pipes too, for those who used it with flush toilets. The first toilet-specific paper wasn’t invented till 1859. Joseph Gayetty, an American, patented a product from pulverized hemp, a relatively soft fiber, softened further with aloe. This paper was softer than standard, and had less tendency to clog pipes.

Toilet paper has to be soft

Toilet paper is either touted to be soft or strong; Modern Charmin touts wet strength, while Cottonelle touts completeness of wipe: ‘go commando.”

The next great innovation was to make toilet paper as a perforated product on a roll. These novelties appear as US Patent #117,355 awarded to Seth Wheeler of Albany, NY 25 July 1871 (Wheeler also invented the classic roll toilet paper dispenser). Much of the sales pitch was that a cleaner bum would prevent the spread of cholera, typhoid, and other plagues and that is a legitimate claim. As the  market expanded, advertising followed. Some early brands touted their softness, others their strength. Facial tissues, e.g. Kleenex, were sold specifically as a soft TP-like product that does not fall apart when wet. Sorry to say, this tends to go along with clogged toilets; do not flush more than one kleenex down at a flush. Kleenex is made with the same short fibers and aloe as toilet paper, but it contains binders (glue) to give it wet-strength. My guess is that Charmin is made the same way and that it isn’t great on your plumbing.

Paper towels and most baby wipes are worse to flush than Kleenex. They are made with lots of binder and they really don’t fall apart in water. Paper towels should never be flushed, and neither should most baby wipes, even brands that claim to be ‘flushable.” When flushed, these items tend to soak up fat and become fat bergs – the bane of sewer workers everywhere. There is a class action law suit against flushable wipe companies, and New York City is pursuing legislation to prevent them from claiming to be flushable. Still, as with everything, there are better and worse moist-wipe options. “Cottonelle” brand by Kleenex, and Scott flushable wipes are the best currently. In a day or less they will dissolve in water. These products are made with binders like kleenex, but the binder glue is a type that dissolves in any significant amount of water. As a result, these brands fall apart eventually. For now, these are the only flushable brands I’d recommend flushing, and even then I suggest you only flush one at a time. In tests by Consumer Reports, other brands, e.g. Charmin and Equate flushable wipes do not dissolve. These manufacturers either have not quite figured out how to make dissolvable binders, or they can’t get around Kleenex’s patents.

Robert Buxbaum. October 10, 2016. If you live in Oakland County, MI, vote for me for water commissioner. Here’s my web-site with other useful essays. I should mention Thomas Crapper, too. He invented the push-button flush and made some innovations in the water cistern, and he manufactured high-end commodes for Parliament and the royal family, but he’s irrelevant to the story here.

A plague of combined sewers

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The major typhoid and cholera epidemics of the US, and the plague of the Al Qaeda camps, 2009 are understood to have been caused by bad sewage, in particular by the practice of combining sanitary + storm sewage. Medieval plagues too may have been caused by combined sewers.

Combined sewer system showing an rain-induced overflow, a CSO.

A combined sewer system showing an rain-induced overflow, a CSO.

A combined system is shows at right. Part of the problem is that the outfall is hard to contain, so they tend to spew sewage into the lakes and drinking water as shown. They are also more prone to back up during rain storms; separated systems can back up too, but far less often. When combined sewers back up, turds and other infected material flows into home basements. In a previous post, “follow the feces,” I showed the path that Oakland county’s combined sewers outfalls take when they drain (every other week) into Lake St. Clair just upstream of the water intake, and I detailed why, every few years we back up sewage into basements. I’d like to now talk a bit more about financial cost and what I’d like to do.

The combined sewer system shown above includes a small weir dam. During dry periods and small rain events, the dam keeps the sewage from the lake by redirecting it to the treatment plant. This protects the lakes so that sometimes the beaches are open, but there’s an operation cost: we end up treating a lot of rain water as if it were sewage. During larger rains, the dam overflows. This protects our basements (usually) but it does so at the expense of the drinking water, and of the water in Lake St. Clair and Lake Erie.

As a way to protect our lakes somewhat Oakland county has added a retention facility, the George W. Kuhn. This facility includes the weir shown above and a huge tank for sewage overflows. During dry periods, the weir holds back the flow of toilet and sink water so that it will flow down the pipe (collector) to the treatment plant in Detroit, and so it does not flow into the lake. Treatment in Detroit is expensive, but nonpolluting. During somewhat bigger rains the weir overflows to the holding tank. It is only during yet-bigger rains (currently every other week) that the mixture of rain and toilet sewage overwhelms the tank and is sent to the river and lake. The mess this makes of the lake is shown in the video following. During really big rains, like those of August 2014, the mixed sewage backs up in the pipes, and flows back into our basements. With either discharge, we run the risk of plague: Typhoid, Cholera, Legionnaires…

Some water-borne plagues are worse than others. With some plagues, you can have a carrier, a person who can infect many others without becoming deathly sick him or herself. Typhoid Mary was a famous carrier of the 1920s. She infected (and killed) hundreds in New York without herself becoming sick. A more recent drinking water plague, showed up in Milwaukee 25 years ago. Some 400,000 people were infected, and 70 or so died. Milwaukee disinfected its drinking water with chlorine and a bacteria that entered the system was chlorine tolerant. Milwaukee switched to ozone disinfection but Detroit still uses chlorine.

Combined sewers require much larger sewage treatment plants than you’d need for just sanitary sewage. Detroit’s plant is huge and its size will need to be doubled to meet new, stricter standards unless we bite the bullet and separate our sewage. Our current system doesn’t usually meet even the current, lower standards. The plant overflows and operation cost are high since you have to treat lots of rainwater. These operation costs will keep getter higher as pollution laws get tougher.

In Oakland county, MI we’ve started to build more and more big tanks to hold back and redirect the water so it doesn’t overwhelm the sewage plants. The GWK tank occupies 1 1/2 miles by about 100 feet beneath a golf course. It’s overwhelmed every other week. Just think of the tank you’d need to hold the water from 4″ of rain on 900 square mile area (Oakland county is 900 square miles). Oakland’s politicians seem happy to spend money on these tanks because it creates jobs and graft and because it suggests that something is being done. They blame politics when rain overwhelms the tank. I say it’s time to end the farce and separate our sewers. My preference is to separate the sewers through the use of French drains or bio-swales, and through the use of weir dams. I’m running for drain commissioner. Here’s something I’ve written on the chemistry of sewage, and on the joy of dams.

Dr. Robert E. Buxbaum, July 1-Sept 16, 2016.

The chemistry of sewage treatment

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The first thing to know about sewage is that it’s mostly water and only about 250 ppm solids. That is, if you boiled down a pot of sewage, only about 1/40 of 1% of it would remain as solids at the bottom of the pot. There would be some dried poop, some bits of lint and soap, the remains of potato peelings… Mostly, the sewage is water, and mostly it would have boiled away. The second thing to know, is that the solids, the bio-solids, are a lot like soil but better: more valuable, brown gold if used right. While our county mostly burns and landfills the solids remnant of our treated sewage, the wiser choice would be to convert it to fertilizer. Here is a comparison between the composition of soil and bio-solids.

The composition of soil and the composition of bio-solid waste. biosolids are like soil, just better.

The composition of soil and the composition of bio-solid waste. biosolids are like soil, just better.

Most of Oakland’s sewage goes to Detroit where they mostly dry and burn it, and land fill the rest. These processes are expensive and engineering- problematic. It takes a lot of energy to dry these solids to the point where they burn (they’re like really wet wood), and even then they don’t burn nicely. As shown above, the biosolids contain lots of sulfur and that makes combustion smelly. They also contain nitrate, and that makes combustion dangerous. It’s sort of like burning natural gun powder.

The preferred solution is partial combustion (oxidation) at room temperature by bacteria followed by conversion to fertilizer. In Detroit we do this first stage of treatment, the slow partial combustion by bacteria. Consider glucose, a typical carbohydrate,

-HCOH- + O2 –> CO2 + H2O.    ∆G°= -114.6 kcal/mol.

The value of ∆G°, is relevant as a determinate of whether the reaction will proceed. A negative value of ∆G°, as above, indicates that the reaction can progress substantially to completion at standard conditions of 25°C and 1 atm pressure. In a sewage plant, many different carbohydrates are treated by many different bacteria (amoebae, paramnesia, and lactobacilli), and the temperature is slightly cooler than room, about 10-15°C, but this value of ∆G° suggests that near total biological oxidation is possible.

The Detroit plant, like most others, do this biological oxidation treatment using either large stirred tanks, of million gallon volume or so, or in flow reactors with a large fraction of cellular-material returning as recycle. Recycle is needed also in the stirred tank process because of the low solid content. The reaction is approximately first order in oxygen, carbohydrate, and bacteria. Thus a 50% cell recycle more or less doubles the speed of the reaction. Air is typically bubbled through the reactor to provide the oxygen, but in Detroit, pure oxygen is used. About half the organic carbon is oxidized and the remainder is sent to a settling pond. The decant (top) water is sent for “polishing” and dumped in the river, while the goop (the bottom) is currently dried for burning or carted off for landfill. The Holly, MI sewage plant uses a heterogeneous reactors for the oxidation: a trickle bed followed by a rotating disk contractor. These have higher bio-content and thus lower area demands and separation costs, but there is a somewhat higher capital cost.

A major component of bio-solids is nitrogen. Much of this in enters the form of urea, NH2-CO-NH2. In an oxidizing environment, bacteria turns the urea and other nitrogen compounds into nitrate. Consider the reaction the presence of washing soda, Na2CO3. The urea is turned into nitrate, a product suitable for gun powder manufacture. The value of ∆G° is negative, and the reaction is highly favorable.

NH2-CO-NH2 + Na2CO3 + 4 O2 –> 2 Na(NO3) + 2 CO2 + 2 H2O.     ∆G° = -177.5 kcal/mol

The mixture of nitrates and dry bio-solids is highly flammable, and there was recently a fire in the Detroit biosolids dryer. If we wished to make fertilizer, we’d probably want to replace the drier with a further stage of bio-treatment. In Wisconsin, and on a smaller scale in Oakland MI, biosolids are treated by higher temperature (thermophilic) bacteria in the absence of air, that is anaerobically. Anaerobic digestion produces hydrogen and methane, and produces highly useful forms of organic carbon.

2 (-HCOH-) –> CO2 + CH4        ∆G° = -33.7 Kcal/mol

3 (-HCOH-) + H2O –> -CH2COOH + CO2 +  2 1/2 H2        ∆G° = -21.9 kcal/mol

In a well-designed plant, the methane is recovered to provide heat to the plant, and sometimes to generate power. In Wisconsin, enough methane is produced to cook the fertilizer to sterilization. The product is called “Milorganite” as much of it comes from Milwaukee and much of the nitrate is bound to organics.

Egg-shaped, anaerobic biosolid digestors.

Egg-shaped, anaerobic biosolid digestors, Singapore.

The hydrogen could be recovered too, but typically reacts further within the anaerobic digester. Typically it will reduce the iron oxide in the biosolids from the brown, ferric form, Fe2O3, to black FeO.  In a reducing atmosphere,

Fe2O3 + H2 –> 2 FeO + H2O.

Fe2O3 is the reason leaves turn brown in the fall and is the reason that most poop is brown. FeO is the reason that composted soil is typically black. You’ll notice that swamps are filled with black goo, that’s because of a lack of oxygen at the bottom. Sulphate and phosphorous can be bound to ferrous iron and this is good for fertilizer. Generally you want the reduction reactions to go no further.

Weir dam on the river dour. Used to manage floods, increase residence time, and oxygenate the flow.

Weir dam on the river Dour in Scotland. Dams of this type increase residence time, and oxygenate the flow. They’re good for fish, pollution, and flooding.

When allowed to continue, the hydrogen produced by anaerobic digestion begins to reduce sulfate to H2S.

NaSO4 + 4.5 H2 –>  NaOH + 3H2O + H2S.

I’m running for Oakland county, MI water commissioner, and one of my aims is to stop wasting our biosolids. Oakland produces nearly 1000,000 pounds of dry biosolids per day. This is either a blessing or a curse depending on how we use it.

Another issue, Oakland county dumps unpasteurized, smelly black goo into Lake St. Clair every other week, whenever it rains more than one inch. I’d like to stop this by separating the storm and “sanitary” sewage. There is a capital cost, but it can save money because we’d no longer have to pay to treat our rainwater at the Detroit sewage plant. To clean the storm runoff, I’d use mini wetlands and weir dams to increase residence time and provide oxygen. Done right, it would look beautiful and would avoid the flash floods. It should also bring natural fish back to the Clinton River.

Robert Buxbaum, May 24 – Sept. 15, 2016 Thermodynamics plays a big role in my posts. You can show that, when the global ∆G is negative, there is an increase in the entropy of the universe.