The purpose of digestion is to attain both of the objectives of sludge treatment ~ a reduction in volume and the decomposition of highly putrescible organic matter to relatively stable or inert organic and inorganic compounds. Additionally, anaerobic sludge digestion produces a valuable by-product in the form of methane gas (the primary constituent of natural gas, which we can burn for heat or convert to electricity). Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. It is, therefore, anaerobic decomposition. The solid matter in raw sludge is about 70 percent organic and 30 percent inorganic or mineral. Much of the water in wastewater sludge is "bound" water which will not separate from the sludge solids. The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth.
Anaerobic digestion involves many complex biochemical reactions and depends on many interrelated physical and chemical factors. For purposes of simplification, the anaerobic degradation of domestic sludge occurs in two steps. In the first step, acid forming bacteria attack the soluble or dissolved solids, such as the sugars. From these reactions organic acids, at times up to several thousand ppm, and gases, such as carbon dioxide and hydrogen sulfide are formed. This is known as the stage of acid fermentation and proceeds rapidly. It is followed by a period of acid digestion in which the organic acids and nitrogenous compounds are attacked and liquefied at a much slower rate.
In the second stage of digestion, known as the period of intensive digestion, stabilization and gasification, the more resistant nitrogenous materials, such as the proteins, amino-acids and others, are attacked. The pH value must be maintained from 6.8 to 7.4. Large volumes of gases with a 65 or higher percentage of methane are produced. The organisms which convert organic acids to methane and carbon dioxide gases are called methane formers. The solids remaining are relatively stable or only slowly putrescible, can be disposed of without creating objectionable conditions and have value in agriculture.
The whole process of sludge digestion may be likened to a factory production line where one group of workers takes the raw material and conditions it for a second group with different "skills" who convert the material to the end products. In a healthy, well operating digester, both of the above stages are taking place
continuously and at the same time. Fresh wastewater solids are being added at frequent intervals with the stabilized solids being removed for further treatment or disposal at less frequent intervals. The supernatant digester liquor, the product of liquefaction and mechanical separation is removed frequently to make room for the added fresh solids and the gas is, of course, being removed continuously.
While all stages of digestion may be proceeding in a tank at the same time with the acids produced in the first stage being neutralized by the ammonia produced in subsequent stages, best and quickest results are obtained when the over-all pH of 6.8 to 7.4 predominates. The first stage of acid formation should be evident only in starting up digestion units. Once good alkaline digestion is established, the acid stage is not apparent unless the normal digestion becomes upset by overloading, poisonous chemicals or for other reasons. It is critical to the overall process to maintain balanced populations of acid formers and methane formers. The methane formers are more sensitive to environmental conditions and slower growing than the acid forming group of bacteria and control the overall reactions.
The progress of digestion can be measured by the destruction of organic matter (volatile solids), by the volume and composition of gases produced, by the pH, volatile acids, and alkalinity concentration. It is recommended that no on parameter or test be used to predict problems or control digesters. Several of the following parameters must be considered together.
The reduction of organic matter as measured by the volatile solids indicates the completeness of digestion. Raw sludge usually contains from 60 to 70 percent volatile solids while a well digested sludge may have as little as 50 percent. This would represent a volatile solids reduction of about 50 percent. Volatile solids reduction should be measured weekly and trended. Downward trends in volatile solids reduction might mean:
• Temperature too low and/or poor temperature control.
• Digester is overloaded.
• Ineffective mixing of digester contents.
• Grit and/or scum accumulations are excessive,
• Low volatile solids in raw sludge feed.
A well digested sludge should be black in color, have a not unpleasant tarry odor and, when collected in a glass cylinder, should appear granular in structure and show definite channels caused by water rising to the top as the solids settle to the bottom.
For domestic wastewater in a normally operating digestion tank, gas production should be in the vicinity of 12 cu.ft. of gas per day per Lbs of volatile matter destroyed. This would indicate that for a 50 percent reduction of volatile matter, a gas yield of six cu.ft. per Lbs of volatile matter added should be attained. The quantity of gases produced should be relatively constant if the feed rate is constant.
Sharp decreases in total gas production may indicate toxicity in the digester. The gas is usually about 70 percent methane, about 30 percent carbon dioxide and inert gases such as nitrogen. An increasing percentage of carbon dioxide may be an indication that the digestion process is not proceeding properly. In plants with primary and secondary digester, raw sludge is pumped to the primary digester displacing partially digested sludge. The major portion of the digestion with the greatest gas yield is in the primary digester.
Volatile acids (mainly acetic acid) are generated by the acid forming bacteria as a result of the initial breakdown of the sludge solids. The volatile acids concentration indicates digestion progress and is probably the best warning sign of trouble. In a well operating digester, the volatile acids concentration should be measured weekly and remain fairly constant. Sudden increases in volatile acids means digester trouble. During periods of digester imbalance, volatile acids should be measured daily.
Bicarbonate Alkalinity indicates the buffering capacity of the sludge, the ability to keep the pH constant, and the ability to neutralize acids. Normally, the bicarbonate alkalinity varies between 1500 and 6000 mg/L (as calcium carbonate). The ratio between the volatile acids and the bicarbonate alkalinity concentrations is an excellent process indicator. Normally if the ratio of volatile acids concentration (mg/Liter) to bicarbonate alkalinity (mg/Liter) < 0.25, the digester is operating properly. A rising volatile acids to bicarbonate alkalinity ratio means possible trouble. Sometimes either decreasing the sludge feed to digester or resting the digester will correct the problem.
Since digestion is accomplished by living organisms, it is desirable to provide an environment in which they are most active and carry on their work in the shortest time. The environmental factors involved are moisture, temperature, availability of proper food supply, mixing and seeding, alkalinity, and pH. To these might be added the absence of chemicals toxic to the organisms. Moisture is always adequate in wastewater sludge.
It has been found that sludge digestion proceeds in almost any range of temperature likely to be encountered, but the time taken to complete digestion varies greatly with the temperature. Also rapid changes in temperature are detrimental. Digester temperature should not vary more than _+ 2° F per day. Pumping excessive quantities of thin sludge can cause significant decreases in digester temperature. Thin, dilute sludge with a high moisture content also waste digester space and reduce solids retention time. The methane forming organisms are extremely sensitive to changes in temperature. At a temperature of 55° F, about 90 percent
A popular figure for sludge from average domestic sewage is an expected gas yield of one cu.ft. per capita per day. Industrial wastes, depending on their character may raise or lower this figure materially.
of the desired digestion is completed in about 55 days. As the temperature increases, the time decreases, so that at 75° F the time is cut to 35 days, at 85° F to 26 days, and at 95° F to 24 days. The theoretical time for sludge digestion at 95° F is one half that at 60° F. Of course, the figures are average, not exact figures for all sludge of varying composition. These digestion times may be materially reduced in digesters provided with efficient mixing of thickened sludge.
The proper amount of food must be provided for the digester organisms. This is in the form of volatile sludge solids from the various wastewater treatment units. The total volume of raw sludge pumped to the digester, the rate at which it is pumped, and the degree to which it is made available to all of the different groups of organisms are vital factors in efficient digester operation. If too much sludge is added to a digester, the first, or acid stage, predominates to such an extent that the environment becomes unfavorable for the organisms responsible for the second stage of digestion, the balance of the whole digestion process is upset, and the digester is said to be overloaded. If this is due to unbalanced plant design whereby the digester capacity is too small in relation to the sludge producing units, the only solution is to provide additional digester capacity. There are, however, other factors which can upset the balance of the digestion process and which are under the control of the operator. In heated digesters, failure to maintain uniform temperatures in the digester within the proper range will upset the digestion process. Adding fresh solids in large volumes at widely separated intervals or removing too much digested sludge at one time will result in temporary overloading. Avoid shock loadings of solids. In unheated digestion tanks similar conditions are to be expected seasonally and during winter months digester organisms are almost dormant, so that with the advent of warm weather there is in the digester an excessive accumulation of almost raw sludge solids. This, together with the normally slower digestion in unheated tanks, necessitates storage capacity
The organisms in a digester are most efficient when food is furnished them in small volumes at frequent intervals. Fresh sludge solids should therefore be pumped to the digester as often as practical, at least twice a day for the smallest plants and more frequently where facilities and operators' attention are available. This, of course, fits in with the proper schedule of removing sludge from settling units
In starting a digester unit, quickest results can be obtained by putting in it at the start some digested sludge if this is obtainable from another digester or a nearby plant. In this way all stages of digestion can be started almost simultaneously instead of by successive stages. This seeding supplies an adequate number of organisms of the methane forming type to consume the end products of the first stage and in this way the unit will "ripen" in the shortest time. After normal operation has been established, seeding of the fresh solids as added to the digester by mixing them with the digesting sludge greatly improves the rate of digestion.
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