Activated Sludge Process - 10...


1. Aerobic Digestion...

Aerobic digestion is an extension of the activated sludge aeration process whereby waste primary and secondary sludges are continually aerated for long periods of time. In aerobic digestion the microorganisms extend into the endogenous respiration phase, which is a phase where materials previously stored by the cell are oxidized, with a reduction in the biologically degradable organic matter. This organic matter, from the sludge cells is oxidized to carbon dioxide, water and ammonia. The ammonia is further converted to nitrates as the digestion process proceeds.

Eventually, the oxygen uptake rate levels off and the sludge matter is reduced to inorganic matter and relatively stable volatile solids. The major advantage of aerobic digestion is that it produces a biologically stable end product suitable for subsequent treatment in a variety of processes. Volatile solids reductions similar to anaerobic digestion are possible.


Some parameters affecting the aerobic digestion process are: (1) rate of sludge oxidation, (2) sludge temperature, (3) system oxygen requirements, (4) sludge loading rate, (5) sludge age, and (6) sludge solids characteristics.


Aerobic digestion has been applied mostly to various forms of activated sludge treatment, usually "total oxidation" or contact stabilization plants. However, aerobic digestion is suitable for many types of municipal and industrial wastewater sludges, including trickling filter humus as well as waste activated sludges. Any design for an aerobic digestion system should include: an estimate of the quantity of sludge to be produced, the oxygen requirements, the unit detention time, the efficiency desired, and the solids loading rate.

Air Requirement...

15 - 20 cfm per 1,000 cubic feet of digester capacity is adequate. The air supplied must keep the solids in suspension; this requirement may exceed the sludge oxidation requirement. A dissolved oxygen concentration of 1 to 2 ppm should be maintained in the aerobic digestion tanks.

Detention Time...

Waste activated sludge only, after sludge thickening. 10 - 15 days volumetric displacement time. If sludge temperatures are much less than 60 O F, more capacity should be provided. Primary sludge mixed with waste activated or trickling filter humus. 20 days displacement time in moderate climates.

Tank Design...

Aerobic digestion tanks are normally not covered or heated, therefore, they are much cheaper to construct than covered, insulated, and heated anaerobic digestion tanks. In fact, an aerobic digestion tank can be considered to be a large open aeration tank. Similar to conventional aeration tanks, the aerobic digesters may be designed for spiral roll or cross roll aeration using diffused air equipment. The system should have sufficient flexibility to allow sludge thickening by providing supernatant decanting facilities.

The advantages most often claimed for aerobic digestion are :

- A humus-like, biologically stable end product is produced.
- The stable end product has no odors, therefore, simple land disposal, such as lagoons, is feasible.
- Capital costs for an aerobic system are low, when compared with anaerobic digestion and other schemes.
- Aerobically digested sludge usually has good dewatering characteristics. When applied to sand drying beds, it drains well and redries quickly if rained upon.
- The volatile solids reduction can be equal to those achieved by anaerobic digestion.
- Supernatant liquors from aerobic digestion have a lower BOD than those from anaerobic digestion. Most tests indicated that BOD would be less than 100 ppm. This advantage is important because the efficiency of many treatment plants is reduced as a result of recycling high BOD supernatant liquors.
- There are fewer operational problems with aerobic digestion than with the more complex anaerobic form because the system is more stable. As a result, less skillful labor can be used to operate the facility.
- In comparison with anaerobic digestion, more of the sludge basic fertilizer values are recovered.

The major disadvantage associated with aerobic digestion is high power costs. This factor is responsible for the high operating costs in comparison with anaerobic digestion. At small waste treatment plants, the power costs may not be significant but they certainly would be at large plants. Some investigators have observed that aerobically digested sludge does not always settle well in subsequent thickening processes. This situation leads to a thickening tank decant having a high solids concentration.

Some sludges do not dewater easily by vacuum filtration after being digested aerobically. Two other minor disadvantages are the lack of methane gas production and the variable solids reduction efficiency with varying temperature changes.

2. Anaerobic Digestion...

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.

General Description...

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 is dependent on many interrelated physical and chemical factors. For purposes of simplification, the anaerobic degradation of domestic sludges 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. Methane is an odorless, highly inflammable gas which can be used as a fuel. 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.

Process Indicators...

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.

Volatile Solids Reduction...

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.

Volume and Composition of Digester Gases...

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 pound of volatile matter destroyed. This would indicate that for a 50 percent reduction of volatile matter, a gas yield of six cu.ft. per pound 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. 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. 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.


pH measures the hydrogen ion concentration of the sludge and indicates if the sludge is acid or alkaline. Generally, the pH must be maintained between 6.5 to 7.5 to promote methane gas formation. Decreases in pH mean possible digester upset. Normally, however, the decreases in pH occur very rapidly and hence pH gives little advance notice of trouble. A low pH indicates that an upset has already occurred.

Volatile Acids and Alkalinity Ratio...

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 1,500 and 6,000 mg/L (as calcium carbonate).

The ratio between the volatile acids and the bicarbonate alkalinity concentrations is an excellent process indicator. Normally if ;

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.

Environmental Factors...

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 + 2O F per day. Pumping excessive quantities of thin sludge can cause significant decreases in digester temperature. Thin, dilute sludges 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 55O F, about 90 percent of the desired digestion is completed in about 55 days. As the temperature increases, the time decreases, so that at 75O F the time is cut to 35 days, at 85O F to 26 days, and at 95O F to 24 days. The theoretical time for sludge digestion at 95O F is one half that at 60O F. Of course, the figures are average, not exact figures for all sludges of varying composition. These digestion times may be materially reduced in digesters provided with efficient mixing of thickened sludge.

Food Supply...

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 twice that needed in heated digesters.

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 before it becomes septic.

Mixing and Seeding...

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. This mixing serves several purposes. The incoming raw solids are intimately mixed with the actively digesting sludge :

- Promotes contact between food and organisms.
- Scum formation is reduced.
- The transmission of heat from internal coils or other heating devices to the sludge is improved.
- Distributes alkalinity throughout tank, aiding in pH control.
- Continuously disperses and dilutes inhibitory materials in feed sludge.

There are a number of methods or combination of methods whereby proper mixing is attained. These include :

- Stirring by rotating paddles and scum breaker arms.
- Forced circulation of sludge and/or supernatant by pumps or by draft tubes with impeller.
- Discharge of compressed sludge gas from diffusers at the bottom of the digestion tank.

Mixing may be either intermittent or continuous, but however effected it provides all working organisms their proper food requirements and helps maintain uniform temperature. Intermittent mixing allows separation and removal of supernatant from a single stage digester. With continuous mixing the digestion proceeds at a higher rate throughout the entire tank, thus reducing the tank capacity needed. Such continuous mixing requires a second digester or storage tank into which digesting sludge may be moved to make room for fresh sludge in the first digester and to make possible separation and removal of supernatant in the secondary digester.

Absence of Toxic Materials...

The presence of various inorganic and organic substances in a digester can cause inhibition or toxicity. Heavy metals, light metal cations, oxygen, sulfides and ammonia are potential toxic materials.

The best method of controlling digester toxicity is to prevent the toxic materials from entering the system. A strong industrial waste ordinance which is enforced is the ideal solution. The cause of toxicity should be identified. Temporary relief from toxicity may be achieved by: (1) increasing the mixing the dilute and disperse the toxic material; (2) uniform feeding of sludges rather than slugs; (3) decreasing the feed to the digester; (4) diluting the sludge; (5) pumping in an active seed sludge if the digester is only partially inhibited.

3. Thermal Stabilization...

Thermal stabilization is a heat process by which the bound water (water associated with sludge) of the sludge solids is released by heating the sludge for short periods of time.

Exposing the sludge to heat and pressure coagulates the solids, breaks down the cell structure, and reduces the hydration and hydrophilic (water loving) nature of the solids. The liquid portion of the sludge can then be separated from the solid by decanting and pressing.

4. Chemical Stabilization...

Chemical stabilization is a process whereby the sludge matrix is treated with chemicals in different ways to stabilize the sludge solids.

Chlorine Stabilization...

Stabilization by chlorine addition has been developed and is marketed under the registered trade name "Purifax". The chemical conditioning of sludge with chlorine varies greatly from the more traditional methods of biological digestion or heat conditioning. First, the reaction is almost instantaneous. Second, there is very little volatile solids reduction in the sludge. There is some breakdown of organic material and formation of carbon dioxide and nitrogen; however, most of the conditioning is by the substitution or addition of chlorine to the organic compound to form new compounds that are biologically inert.

Lime Stabilization...

The lime stabilization process can be used to treat raw primary, waste activated, septage and anaerobically digested sludges. The process involves mixing a large enough quantity of lime with the sludge to increase the pH of the mixture to 12 or more. This normally reduces bacterial hazards and odor to a negligible value, improves vacuum filter performance and provides satisfactory means of stabilizing the sludge prior to ultimate disposal.