One of the more serious problems facing the wastewater treatment field is that of solids treatment, and the subsequent disposal of the treated solids. It is the goal of this section to describe the techniques of anaerobic sludge digestion in general terms. As stated in Part I, anaerobic sludge digestion is carried out in the absence of free oxygen by organisms we call "anaerobic" micobes. In this section we will outline the process itself.
The principal goal is to reduce the amount of organic material in the sludge that the environment would have to stabilize. We find that it is far more beneficial to reduce the organic matter in our treatment plant prior to disposing of the solids in the environment.
The sludge digestion treatment process is utilized to help
accomplish the following goals:
1) Reduce the volume of the sludge to dispose of.
2) Reduce the amount of organic (volatile) solids in the sludge.
3) Render the sludge less obnoxious and more easily disposable.
4) To make it dewater more easily.
5) To convert some of the solids into useable energy.
6) To accomplish the above in an economic and controllable manner.
Primary sludge contains food garbage from sinks, fecal solids, egg shells, fine sand particles, some rubber products, etc. The secondary sludge is principally microorganisms (that may be wasted to a sludge thickening process such as the dissolved air floatation thickener for concentration prior to being introduced into the digester), that are also capable of causing environmental problems if released into the receiving waters (such as a creek).
These organics, consisting of the microbes and any organics accompanying them, or "food" therefore, is broken down by the bacteria in the digester until the amount of digestable organics is almost depleted. At this point the solids in the sludge is no longer as "strong" a problem as it was in the beginning. Also to be considered here is the amount of water in the sludge. By removing water PRIOR to sludge digestion, there is even less volume to be handled...ie/ if we have the following below, you will see the benefits in concentrating solids:
weight of solids in pounds..... 2 2 2
per cent solids................ 2 5 10
pounds of water removed...... 60 80
gallons of sludge........... 11.7 4.7 2.2
(Note: as we reduced water, and hold the weight of solids at two (2) pounds, we see the corresponding percentage of solids increase.)
The anaerobic sludge digestion is carried out in the absence of free oxygen by organisms we call "anaerobic". About 70% of the solids are organic and 30% are inorganic or mineral. Raw sludge solids have water "bound" to it, but as the organisms breakdown the chemical structures, the water is released . These organisms first attack the weak chemical compounds, like the sugars. This first stage has been referred to as the "Acid Production Stage". This acid production stage proceeds very rapidly. The saprophytic bacteria attach themselves to the solids, or lie very near to them and secrete their digestive enzymes onto the food to digest it. The digested food is then absorbed through the cell walls of the bacteria for life processes. During this acid producing stage there is almost no change in the organic matter. A lot of acids are produced, and this causes the ph to drop below the neutral value of 7. When starting up a digester, or when a digester is over-fed, these acids may easily drop the pH below 6, and then this low pH inhibits the next stage. The next stage is called the "Gasification Stage". In this stage the reaction is of two parts. The first is that the organic acids and the nitrogenous compounds are attacked and liquified at a much slower rate. This reaction is the acid digestion stage. during this part the pH rises from the 5.1 to the 6.8 level....because the acids are being consumed. The second part of the gasification stage is that of "Intensive Digestion, Stabilization, and Gasification. " In this part, the more resistant materials, such as the proteins, amino-acids, etc., are attacked and digested. The pH will then rise to the 7.4 level (if no more acids are being produced to off-set this). Large volumes of methane gas are produced....about 70% of the digester gas will be methane (it will burn) and about 29% will be carbon dioxide (it does not burn)...with the remaining 1% being other gases and water vapor.
It is to be remembered that all of the steps and stages are taking place AT THE SAME TIME. As new sludge is fed, sludge that has been in the digester for many days, may be just completing the final step. If we over feed the digester, then we may upset the balancing reactions.....because the new "feeding" of organics will produce acids, in proportion to the amount of organics fed, that may inhibit the methane producing reaction to take place. The gasification stage takes place very slowly, in relation to the faster acid production stage. Think of the whole thing as a production line...where a little is done here and then the product moves to the next stage and so on. If we allow one step to over-run the others, then the whole production line falls down due to the overload! We can also cause problems by not feeding the digester enough organics, but we usually do not experience this underfeeding problem as nearly as often as "overfeeding"! It is possible to overfeed one primary digester, and then starve one or two others, but a careful balance of solids loading will prevent that from occurring. This is often due to an instrumentation and control problem, or due to inattention if done manually between the operators on shift.
RESULTS OF SLUDGE DIGESTION
We will have destroyed about 50% of the volatile matter (food) in the sludge; we will have made some of the water release from the solids; and we have stabilized the solids for disposal. For a normal domestic sludge, such as ours, we have made about 12 cuft of gas for every pound of volatile matter destroyed....or about one cuft of gas per person per day in our city.
pH: the best value is that of 7.0, with anything between 6.8 and 7.4 acceptable.( Our primary digesters are most always in the 7.0 to 7.4 range.) In this area all of the organisms can perform at their best rates. It also shows that the acids from the first stage are balanced out by the alkaline production in the acid digestion step, and by the material of the intense gasification step providing a BUFFER> By buffer, we mean that it will help cushion any change in the pH action. It is best not be use pH is a control parameter. Gas production and digester gas composition, volatile acids, alkalinity, and the VA/Alkalinity ratio, will change prior to a change in pH. (The chicken is out of the coupe and go on by the time the pH changes!)
The sludge must be withdrawn from the primary clarifiers (sedimentation) tanks before it becomes septic (decomposes), and should be as dense as possible to keep down unnecessary water storage space. (Water has to be heated, and also reduces the time that the solids have in the digester to be digested.) The sludge should be pumped at short, frequent intervals. This yields a more uniform digestion rate. The temperature of the digester contents should remain within 2 to 3 degrees of the temperature selected. Do your very best to stay within one(1) degree, or make any changes at one (1) degree per day. The type and percentage of the different species of bacteria in the digester is determined by the temperature. If you have fluctuating temperatures, then you will not be able to establish an optimum microbial population. The digester stirring system must be operational to insure that the cold, newly introduced sludge, is mixed with the warm older solids and the bacteria. It is a far better practice to heat the new sludge PRIOR to introduction into the digester, if possible. Remember, that it is the rate of methane fermentation that controls the overall rate of sludge stabilization. The methane forming bacteria grow at a relatively slow rate...with generation times from two to 20 days being reported. These bacteria are also very substrate specific, and many different species are required to ferment the methane....each species has a different growth rate...and therefore the methane is very time dependent. If we do not provide enough time for the methane fermentation, i.e./ short detention times, then the volatile acids will accumulate in the system. In this case we would only convert the solids to a dissolved solids matter...we will not have decreased the organic content. It should also be noted that not all of the organic matter in the sludge is microbially decomposable. About 40% of the sludge is non-degradable.
Volatile Acids commonly found during
Common Name Formula
Formic acid HCOOH
Acetic acid CH3 COOH
Propionic acid CH3 CH2 COOH
Butyric acid CH3 CH2 CH2 COOH
Valeric acid CH3 CH2 CH2 CH2 COOH
Isovaleric acid (CH3) 2 CHCH2 COOH
Caproic acid CH3 CH2 CH2 CH2 CH2 COOH
(I have yet to figure out how to create superscript and subscript on this page!!)
The primary acids produced are acetic, propionic, and butyric acids. Smaller quantities are formic, valeric, isovaleric, and caproic. Approximately 70% of the methane produced during the anaerobic digestion is formed from acetic acid, and about 15% results directly from the propionic acid in the tank. The remainder of the methane is formed from the fermentation of formic acid, from the reduction of carbon dioxide by hydrogen, and from fermentation of long fatty acid chains.
Some facilities have one or more secondary digesters following the primary digesters. The secondary digester may have a floating top, that stores digester gas. That gas which is not burned in the boilers to heat the digester, or used to fuel the cogeneration or "Cogen" engine that generates electrical power, is usually burned at a waste gas burner. The secondary digester is not heated, nor stirred and very little improvement in the solids destruction or gas production takes place. In the secondary digester, the sludge that went through the primary digester treatment is allowed to stand still...which allows the solids to separate from the water. (In order for this to be effective, there is usually two or three times the secondary digester capacity than primary capacity. This is a VERY expensive in terms of tankage. We find that most installations install dewatering equipment that uses the primary digester as the sludge source). The water is still high in organic solids, as those solids that are still bound to the water have not been rendered in a stable condition. The water that flows from the secondary digester is called the supernatant. The supernatant is then returned to the plant's wastewater treatment stream for treatment. The sludge is pumped from the bottom of the secondary digester for further dewatering.
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