Aerobic Solids Digestion Process

The aerobic digestion process is a treatment process that utilizes aerobic microbes to stabilize the solids. The microbes digest solids from primary sedimentation processes, and those from secondary treatment processes like those attached to the microbial flocs from activated sludge and biofilters (trickling filters). Due to the length of time that the solids remain under aeration, the long solids retention time allows for the microbes to feed off of the cell contents of other dying/decaying microbes under digestion. This is referred to as "endogenous respiration" or "endogenous stabilization." There will be an "inert faction" between 20 and 25% by weight, in the resulting stabilized solids. This inert faction will consist of fine inorganic solids, organic solids, and cell components that will not be degradable by the process. (It may be beneficial to think of an "aerobic digester" as an "activated sludge aeration basin with a much higher concentration of microbes.")

Aerobic digestion does NOT produce methane as a by-product. In anaerobic sludge digestion this "digester gas" is most often utilized for fueling the digester boilers that heat the digester sludge contents and other buildings, fuel engines that produce electricity for the facility, and may even power facility vehicles! The aerobic digestion process is actually energy intensive, as it requires additional energy (ie electrical power) to power the blowers that supply air and oxygen to the aerobic digester. Often, the temperature variability among the seasons of the year produces variability's in the operating performance of these digesters. Since the process is temperature dependent (at least in the extremes) it should therefore be covered and heated in cold climates to keep the contents above 60 deg F.

The process may have an additional disadvantage in the sludge dewatering category. It appears that in some facilities, the stabilized sludge to be subsequently dewatered is composed of minute biological clumps. These are extremely high in number, smaller in size, and have far greater surface areas per unit volume than primary sludge.... meaning "very difficult to dewater!" There are also aerobic digesters that produce a product that is easy to dewater, but it seems that these have an associated primary clarification process and a secondary treatment process that does not produce a high percentage of the total aerobic digesters loading.

The process is easy to control. It usually has lower ammonia concentrations, as I have seen 30mg/L ammonia in an aerobic digester decant and up to around 1,000 mg/L in our anaerobic digesters. I have seen lower BOD concentrations in the return stream from aerobic digesters than in anaerobically digested sludge; and aerobic digesters have a volatile solids reduction that is comparable to anaerobic digestion. Few odors are experienced if properly designed and operated. Explosive gases (methane) are not produced (there are two view points regarding the production of methane!).

Aerobic digesters may be operated as batch operations or as continuous flow-thru operations. I have seen both type of operations operate quite well. In either case, my personal preference would be to include provisions for the process to concentrate the solids in the aerobic digesters. This may be accomplished by one of two methods: 1) Solids are routinely pumped from the digester to a gravity thickener or a belt thickener to thicken the solids and return them to the digester (the decanted water is returned to the plant headworks for treatment.) or 2) Turn off the aeration system for a short period of time, allow the solids sufficient time to settle (like in a clarifier) and then decant the clear liquid back to the treatment plant headworks. In this manner, you are able to take advantage of the solids stabilization and reduction in solids and control the concentration of solids under digestion better. (This option clearly gives the operator more control over the treatment process by separating the hydraulic detention time from the solids retention time.) The supernatant is usually returned to the treatment plant headworks for treatment with the incoming raw wastewater. In some facilities they have the option of returning it straight to the trickling filters or the activated sludge aeration basins. Many times this turns out to be an internal shock load to this process. I suggest close monitoring of the dissolved oxygen concentration in the aeration basins, across the trickling filters, etc. for adverse impacts.

The solids loading is typically in the range of 0.02 to 0.15 lb VSS/cuft/day, depending on the type of sludges being fed, the solids detention time, the hydraulic detention time, and the temperature of the digester? One reference (1) recommends a volatile solids loading rate of 0.1 to 0.3 lbs volatile solids/ day/cubic foot of digester volume. .

In one facility, the raw wastewater passes through coarse barscreens (1/2 inch bar spacing), and then passes thru an aerated grit system enroute to fine screens instead of primary clarifiers. The wastewater then enters the aeration basins of the activated sludge process. The mixed liquor flows from the aeration basins into the secondary clarifiers. The Return Activated Sludge (RAS) returns to the aeration basins. The waste activated sludge (WAS) is pumped directly into the aerobic digester, which has decanting capability. It is this decanting capability that allows the operators to thicken the solids as the volatile solids are reduced. The operator taking me on the tour responded to my inquiry on the difficulty of dewatering by stating, "I've tried everything to form a good dewatering floc except Elmer's Glue!" While they routinely get about 7% to 9% solids by dry weight, off their belt press, the polymer costs are also a little higher per dry ton than facilities with anaerobic sludge digesters processing primary sludge and activated sludge. Their plant and the aerobic digester operate very well. They are not over-aerating, but I have also seen some success in allowing the solids to bio-floculate.

At our wastewater treatment facility, we have found that we are unable, at any cost or any method to dewater anaerobically digested sludge immediately after it has passed thru a fine mesh sludge strainer. We have an easy time and excellent dewatering results when that same sludge is passed thru the strainer, sent to a sludge lagoon, and dewatered the next day or two! This may also be true at facilities utilizing aerobic digesters, if the solids were allowed to bio-floculate after digestion.

My personal recommendation is to be brave and try different dewatering polymers from time to time, and you may get lucky and find one that will work best for your facility. You should not have to purchase a full drum or such for a trial. If the supplier/vendor REALLY believes their product will work, then THEY should supply you with a FREE 5 gallon pail for testing purposes. I sometimes think some of these vendors/suppliers are making a living out of doing just expensive chemical trials! OK, it's just my editorial comment, now back to the subject!

The solids pumped to the aerobic digester should be as concentrated as reasonable. This is especially important if the aerobic digester does not have a decanting capability or other method of increasing it's solids concentration. Trickling filter and activated sludge is typically less than 1% solids (and there might not be anything you can reasonably do to increase this), and primary sludge should be concentrated to about 3 - 5% prior to sludge digestion if possible. An example of wasting effectively in an activated sludge plant: most operators would waste from the Return Activated Sludge (RAS) piping system, than from the aeration basin to increase the concentration. This will assist the operator in achieving longer solids retention times in the aerobic digester. (Solids Retention Time, days = lbs solids in digester/solids fed to digester, days)

In some literature references, a correlation exists between temperature and sludge age. Usually I see solids retention times between 10 to 15 days for activated sludge and trickling filter sludges, and an additional 5 to 8 days added to this range when primary sludge is also fed to the digester. (One near my location uses 15 days during the summer, and 20 days during the winter to achieve a volatile solids reduction of about 45% year 'round.) As the temperature of the contents decrease, the operator may need to increase the solids detention time (sludge or solids residence time) to achieve the same volatile solids reduction, in about the same amount of time. There are limits to this, though. Conversely, as the temperature of the contents rises, the stabilization (conversion) of the solids usually occurs in a shorter solids retention time (as opposed to the "hydraulic detention time".) The literature search reveals that when temperatures drop below 16 deg C (about 60 deg F) we most always require longer detention times to maintain the same volatile solids reduction values. Typically most aerobic digesters achieve VS reductions between 40 and 50%. Expect lower values if you have an aerobic process feeding that has a high volatile solids reduction already. An example is an "extended aeration" activated sludge oxidation ditch, with a high MCRT like 15 to 20 days. There will obviously be fewer pounds of volatile solids to reduce in the aerobic digester (goes for anaerobic digesters also!)

But in some facilities operators are convinced that temperature changes impact the operation of their aerobic digester, while others with about the same ambient temperatures say it does not. The reality in warmer climates, like ours in California, may be somewhere in the middle. For those that say it does, they tell me "This is in part almost always related to seasonal temperature changes." The process operates quite well in the summer, and then "slowly decays as winter rolls in." Some operational strategies that you may try are: increasing the solids retention time during the cold periods, thereby having longer periods of time for stabilization to take place; conserving heat in the process by pumping raw, more concentrated solids to the process over longer periods of time and/or insulating all pipes relevant to the process like sludge lines and air lines from the blowers.

It is important to have a completely mixed digester, with a dissolved oxygen concentration of at least 1.0 mg/L in all areas of the digester. A literature search reveals that most secondary sludges, like activated sludge, require amount 2.3 to 2.5 lbs of oxygen to stabilize 1 pound of volatile sludge solids. It also appears that a range of 1.5 to 2.0 pounds of oxygen are required to stabilize one pound of volatile solids from a primary sedimentation sludge. Maintaining a proper DO will also minimize odor impacts. If you also wish to convert ammonia into nitrate you will have to increase the dissolved oxygen concentration to above 2.5 mg/L. Some are forced to approach 3 mg/L to assist the nitrifiers in the conversion.

An expected problem among aerobic digesters that nitrify with poorly buffered sludges is pH control. The optimal pH is above 6.5. We have discussed previously the conversion of ammonia, and ammonia groups that exist as part of organic compounds, into nitrate. In the Operator Notebook chapters on nitrification (Part 1), we see where this conversion results in the creation of free hydrogen ions (H+). These hydrogen ions (H+) are the basis of acidity, and if the buffering capacity is insufficient to neutralize all of them, the pH will drop as the acidity increases. (See Operator Notebook on Alkalinity). Most all of the aerobic digesters that will be expected to nitrify are therefore designed with pH control equipment of some type (allow for the addition of a base like sodium hydroxide (NaOH), or a buffering compound). It is usually not used when the sludge has sufficient alkalinity to maintain a pH above 6.5

Another problem that you may have to contend with is foaming. This may be related to low dissolved oxygen concentrations, organic loading rates that are outside of the proper range, or the tranfer - growth of nocardia microbes from the activated sludge process. If the source of the problem is in your activated sludge aeration basins, try to minimize its downstream impacts on your digester. Rod H., yours truly, and others have had some moderate success in using surface sprays on the aeration basin pushing the foam into "trap boxes" where we then dose the foam with a chlorine bleach solution. I say bleach, as we the concentration is around that of bleach, 5%. When you skim this foam, living, from the aeration basins and pump it into your digester, you are just moving it to another place where it can cause you more grief. That is why many of us try to minimize its impact at the source. Foaming of the contents may occur. In one case I am told by the operators that is seasonal, marking the fall and spring seasons of each year. They "live thru it" by skimming off the foam and chlorinating the foam heavily with sprays before sending it back to the headworks of the plant.

Filamentous microbial populations may develop in the aerobic digester, much like in the activated sludge process. The controls are similar to those used in the activated sludge process: control of the dissolved oxygen concentration, organic loading, etc.

In closing, while there is a fair amount of data out there on aerobic digestion, I think that a careful, detailed, survey of a very larger percentage of existing aerobic digesters would be very helpful. Comparisons of above grade to below grade vs/temperatures/volatile solids reduction variability; concrete vs/steel tanks; percentage of primary vs/secondary solids in volatile solids reduction; compare the "sludge age" of secondary solids sent to the aerobic digester and then to dewatering vs dewatering ease; etc. would assist others in the design and operation of aerobic digesters.

1) "Sludge Stabilization" Manual of Practice No FD 9, WEF, Washington DC (1985)
2) "Process Design Manual for Sludge Treatment and Disposal" EPA 625/1-79-011 (1979)
3) "Wastewater Engineering Treatment, Disposal, Reuse" Metcalf and Eddy, Inc. McGraw-Hill, Inc. New York (1979)

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