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 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.

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.