Activated Sludge Process Control: Review of the basics
The degree of success in operating an activated sludge plant depends on a number of factors, of which two are of major importance: 1) the Food/Microorganism ratio, especially at lower Mean Cell Residence Time (MCRT) values and 2) control of the number of pounds of microbes in the entire secondary treatment process. Differing MCRT values will produce contrasting results in such areas the aeration basin mixed liquor concentrations, the secondary process sludge production and the corresponding sludge stability, the amount of oxygen required to successfully operate the process, the percentage of solids and BOD removed from the waste stream, and the degree of nitrification and denitrification. Generally speaking, increasing the MCRT will increase the aeration basin mixed liquor concentration; the increase in microbes will then increase the amount of oxygen required for maintenance of aerobic conditions in the aeration basins; create more stability in the process itself; generally reduces the quantity of WAS solids; dictate the degree of waste activated sludge (WAS) sludge stabilization; and then it may also reach the point of sustaining a nitrifying population of microbes, and finally actually impact the degree of intentional nitrification. (For a more in depth reading on nitrification and its impacts, please refer to my previous presentation.)

Selectors
In the literature and in our treatment plants we are reading about and using aerobic selectors, anoxic selectors, and anaerobic selectors in various manners, in order to attain a specific process goal(s).
Selectors are being used for several purposes, among which are: 1) filamentous microbe control, and 2) reduction of nitrates, and nitrogen and phosphorous reduction. We will discuss each of these separately.

1) Selectors used for filamentous microbe control:
One of the major problems continually facing operators in the process control of activated sludge treatment plants is that of "bulking sludge." Bulking sludge is described as: a high ratio of the volume of an activated sludge sample in relation to the weight (mass) of that volume of sludge. This value is most often determined by use of the (Sludge Volume Index) SVI calculation. We have found that there is both filamentous and non-filamentous sludge bulking. Filamentous sludge bulking appears to be the most frequent problem. Non-filamentous is usually a result of bound water within the bacterial flocs, that make it less dense, and less likely to settle properly. Bulking sludge results in poor sludge settling characteristics, and subsequent water quality impacts.

A review of the literature reveals that contributing factors for filamentous bulking sludge have included:
Raw wastewater characteristics: nutrient content, pH, fluctuations of the flow rate and loading rates, and the nature of the constituents in the wastewater.

Facility Design: poor mixing of the primary effluent and the return activated sludge (RAS), limited aeration capacity, short circuiting in the aeration basins, and clarifiers that retain settled sludge too long (desire 30 -45 minute sludge detention times).

Operational items: low food/microorganism levels, low dissolved oxygen (DO) levels in the aeration basins, insufficient nutrients, and large swings in the organic loading rates of the aeration basins.

One design and operational response to this problem has been the use of "selectors." Selectors, when used for filamentous control, are all designed for heavy organic loading rates. Basically, the process goal is to assimilate as much of the soluble organics as fast as possible, thereby denying the filamentous microbes this food source. At high BOD concentrations, the floc-forming bacteria have a higher BOD uptake rate than the filamentous bacteria and so the floc-forming bacteria are more competitive for the food source. This action enhances the growth and formation of better settling, floc-forming microbes, and limits the growth rate of the filamentous organisms.

MATH TIME!
Generally speaking, selectors used for filamentous control are designed for an organic loading rate of approximately: 2.25 lb BOD/day/lb MLVSS in the selector.

GIVEN: Primary effluent flow rate is 8.2 MGD, and has 95 mg/L BOD. The anaerobic selector MLSS is 3,500 mg/L, with 76% volatile solids content, with a 0.248 MG capacity. Calculate the number of lb BOD/ day/ lb MLVSS.
( 8.2 MGD )( 8.34 lb/gal )( 210 mg/L ) = 14,361.48 = 2.589 lb BOD/ day/ lb MLVSS.
(0.25 MG)(8.34 lb/gal)(3,500 mg/L)(0.76) 5,546.1

When used for filamentous control, the selectors terms: aerobic selectors, anoxic selectors, and anaerobic selectors are descriptive in their "free or dissolved oxygen" conditions. In all of the conditions we will list for aerobic, anoxic, and anaerobic, the microbes will convert the Volatile Fatty Acids and other "more easily utilized BOD" first, denying the filamentous microbes of the nutrients.

Aerobic: In this selector, an "excess" of dissolved oxygen (greater than 2 mg/L in ALL parts of the selector), the microbes will first convert the easy to "utilize" BOD into energy, water, carbon dioxide (CO2 ), and increase their numbers (cell mass).

Anaerobic: In this selector, there are NO easily utilized forms of oxygen such as free, dissolved oxygen, nitrites and nitrates.

Anoxic: In this selector, there is no dissolved oxygen, but there is usually "chemically bound" oxygen which the microbes use for their metabolic processes such as nitrites and nitrates.

2) Selectors used for nitrification, reduction of nitrates, and nitrogen and phosphorous reduction:

AEROBIC SELECTOR
Aerobic selectors would produce strict aerobic conditions through out the entire selector and subsequent basin(s). After converting the BOD (biochemical oxygen demand) into energy, water, carbon dioxide (CO2 ), and increasing their cell mass, the nitrifier faction of the cell mass will be fully involved in nitrification (if the MCRT, water temperature, etc. supports it). This as presented previously, converts ammonia into nitrates.

ANAEROBIC SELECTOR
"Anaerobic" by definition, is an environment that is void of all forms of oxygen, especially dissolved oxygen, nitrites and nitrates. (It follows that the microbes that stabilize organics in this environment are referred to as "anaerobic." ) If you look at the bottom left corner of the picture to the left, you will see that there is no air being injected into this part of the aeration basin. The Return Activated Sludge (RAS) and primary effluent come together in this selector. NO AIR is added, hence, an anaerobic selector. From the selector the mixed liquor moves ino the aeration portion of the activated sludge process.

This is a closeup picture of the anaerobic selector above. Note that there is only mixing, with the least amount of oxygen injected as possible to maintain "anaerobic conditions."

In sizing anaerobic selectors, a reasonable value appears to be 10% of the total "aeration basin volume." This would be a volume that would be in an un-aerated, but mechanically stirred, separate tank, compartment, or zone preceding the aerobic zone(s) of aeration basins. Early volumes were up to 20% of the total aeration basin volume, with no reported problems in operation. (Some extremely large selectors have been found to create odors.)

Biological phosphorous removal in anaerobic selectors
Mixers are used in anaerobic zones or compartments, to completely mix the microbes with the waste stream. The use of "air" is counter productive here!

One of the characteristics of anaerobic selectors are PolyP bacteria, which as their name implies, react with the polyphosphates in the waste stream. The microbes expend energy to take up fatty acids and store them most commonly as a chemical called polyhydroxybutyrate (PHB) within the cell walls. They get this energy from the breaking of the energy-rich bonds in long-chained inorganic polyphospates, in an anaerobic environment. When this occurs, orthophosphate is released from the bacterial cell. When the bacterial cell passes from the anaerobic selector to the aerobic zone, the previously released soluble orthophosphate is taken back into the cell to replenish the polyphosphates. The polyhydroxybutyrate is aerobically oxidized into new cells, carbon dioxide, and water. Acinetobacter is an example of such a microbe. The end result is the reduction of the quantity of phosphates in the water, by the cells incorporating it into their cells. When we "waste" the excess activated sludge cells, the phosphates are then also wasted from the secondary system. The amount of phosphorous in the sludge will be dependent upon the amount of BOD and phosphate in the influent and the volume of sludge produced.

NOTE: Nitrates impair the activity of the polyP bacteria and therefore the effectiveness of the anaerobic selector.

NOTE: Readily biodegradable volatile fatty acids (VFA) such as acetic acid, acetate, formic acid, etc., are used by the Poly P organisms in the mixed liquor.

Anaerobic selectors typically produce an activated sludge that has SVI's that are lower than that of an anoxic selector. In 1990 "a northern California" wastewater treatment plant inadvertently discovered an anaerobic selector during the conversion of coarse bubble diffusers to fine bubble diffusers. With the coarse bubble diffusers, the facility was unable to maintain a dissolved oxygen concentration at the entrance to the first aeration basin where the RAS and primary effluent were combined. During the conversion to fine bubble diffusers, additional air was needed elsewhere, and this zone was "abandoned" by turning off the air completely. After a few days the settling characteristics started to improve, and surprisingly, kept improving during the conversion process! After the installation of the fine bubble diffusers, the facility incorporated the "selector zone" into its operational scheme by almost eliminating the air in the first half of the first aeration basin. Minimal air is applied, with bubbles barely detected at the surface of the zone. "Minimal air" is used to mix the contents of this zone. Mechanical, subsurface mixers would naturally enhance this anaerobic zone, instead of using "air."

This facility has typically much lower SVI's than other activated sludge plants, with far fewer filamentous sludge bulking problems.

Anaerobic selectors have not been shown to adversely impact a downstream nitrification process.

ANOXIC SELECTOR

We will define anoxic selectors as a biological environment that does NOT have any dissolved oxygen, but may contain chemically bound oxygen, such as nitrites and nitrates.

Typically anoxic selectors are twice the size of an anaerobic selector.
Anoxic selectors are typically used to remove or reduce nitrates in the waste stream.

Higher MCRT’s will also decrease the quantity of sludge and denitrification rates in the post-aeration anoxic tanks. For pre-aeration anoxic tanks, longer SRTs increase the mixed liquor suspended solids (MLSS) and result in a greater denitrification rate.

Introducing an anoxic zone into the flow scheme provides for denitrification of nitrate. In this zone, operated with no dissolved oxygen (DO), the endogenous oxygen demand of mixed liquor suspended solids (MLSS) plus the carryover of BOD (biochemical oxygen demand) from the anaerobic zone causes denitrification of the nitrate produced in the aerobic zone.

During anoxic conditions, dissolved oxygen is not available to the microorganisms for respiration. Because of this, the oxygen molecules are stripped from the nitrate, causing the production of nitrogen gas(N2) . Carbon dioxide and water are also produced in the process, which results from the degradation of BOD. In addition, a portion of the alkalinity consumed during the nitrification process is restored through the denitrification process. When the mixed liquor flows to the secondary anoxic zones, there will be a relatively small concentration of extra cellular BOD in the wastewater. However, denitrification will still proceed since the microorganisms utilize internal storage products to reduce nitrate (endogenous denitrification).
 

NOTE: Readily biodegradable volatile fatty acids (VFA) such as acetic acid, acetate, formic acid, etc., are used by the organisms in the mixed liquor. Methanol, ethanol, and acetate may be used as an additional carbon source when required in the process stream.

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