Activated Sludge Process - 7...

Solids Handling and Disposal...

Wastewater sludge is the mixture of wastewater and settled solids. Descriptive of its source, it may be termed primary, secondary, excess activated, or chemical sludge. From its state, or treatment received, it may be called raw or fresh, digested, elutriated, dewatered, or dried. These are the most common descriptive terms and may be used in combination. Other descriptive terms are Imhoff and septic-tank sludge. As with the liquid portion of wastewater, disposal of the solids contained in the sludge must be accomplished. Also, like the liquid portion, the sludge must generally be subjected to some treatment to so alter its character that it may be disposed of without endangering health or creating nuisances.

Reasons for Sludge Treatment...

Sludge is treated to facilitate its disposal. The various treatment processes have two objectives: (1) to reduce the volume of material to be handled by removal of some or all of the liquid portion, and (2) to decompose the highly putrescible organic matter to relatively stable or inert organic and inorganic compounds from which water will separate more readily. This is called digestion which causes a reduction in the total solids.

Nature of Solids...

Screening, Degritting, and Skimming...

Grit and screenings are waste solids that must be disposed of at wastewater treatment plants along with skimmings and other solids. Fortunately, their volume is very small so disposal is not as complicated as that for other solids collected in the treatment processes.

Screenings are materials in the raw wastewater that are caught on screens having ;

Openings usually 1/2 inch to 2 inches. The screens, placed at the head of the treatment plant, remove materials such as rags, sticks, garbage, and any large solids that may find their way into a collection system. Grit can be described as small inorganic solids that are removed from the wastewater after screening. Examples of grit are sand, silt, gravel, ashes, and coffee grounds. Skimmings consist of all types of floatable material which rise in sedimentation tanks.

Grit Removal : Velocity is slowed in the grit chamber and or air is added to have the grit settle out. While small in volume, it is desirable to remove grit, screenings, and skimmings because these solids cause the following operational problems :

- They plug, wear out, and break pumps and other mechanical equipment.
- They occupy space needlessly in treatment units, particularly digesters.
- They are difficult to remove from treatment units such as digesters and sedimentation tanks.
- They can clog pipes and solids dewatering equipment.
- They can produce odors.
- Interfere with digestion.

Screenings Disposal...

Screenings have a moisture content of about 85 to 95 percent and an organic content of 50 to 80 percent. A sanitary means of disposal is required due to the high organic content. Therefore, these materials are usually buried. Sometimes they are incinerated or ground by shredders into small particles. Burial after draining is the most common means of screenings disposal. The solids are placed in a hole or trench and covered with at least 6 inches of dirt. Sometimes lime and odor-masking chemicals are used to prevent nuisance problems such as odor development and insect breeding. Incineration is possible in a separate unit, in a skimmings incinerator, a refuse incinerator, or a dewatered sludge incinerator. The screenings moisture content before incineration should be reduced to about 60 to 65 percent by drainage, pressing, or dewatering.

Grit Disposal...

Grit is removed at almost all sewage treatment plants even though wastewater collection systems should be separated so theoretically street washings will not be a part of the wastes to be treated.

There are two approaches to grit removal: one advocates grit collection units at the head of the treatment plant; the second advocates the use of hydrocyclones or hydro-degritters to remove grit from the settled solids in the primary sedimentation basins.

Heavy inert particles or grit are selectively deposited in units, installed at the head of treatment plants, by velocity control in simple gravity settling structures or by flotation-classification of the inerts and lighter organics in aeration tanks. Aerated grit chambers have the disadvantage of being a source of odors, so they are not recommended when septic wastewater is expected unless the unit is completely covered to capture gases and thereby reduce odors.

Generally, grit collection units are designed to remove particles having a specific gravity of 2.65 and diameters down to 0.2 mm. The quantity of grit collected normally varies from 1 to 12 cu.ft. per million gallons with an average of 4 cu.ft./mg. The moisture content of grit varies from 14 to 34 percent. Grit is sometimes washed after collection to reduce the organic concentration which may be as much as 50 percent of the total solids.

The nature and quantity of the grit influences the method of ultimate disposal. Because there is often a high concentration of organics, burial is the most common disposal technique. Burial reduces the chance of developing odor, insect, and rodent problems. If solids separation is very efficient and if less than 15 percent volatile solids are included in the grit, it can be disposed of as fill without nuisance. Well-washed grit has been used on sludge drying beds, as a cover for screenings, and as a surfacing material for walks and roadways. A few sewage treatment plants have incinerated grit along with dewatered sludge. Being largely inorganic, most of the grit solids are ultimately discharged with the incinerator ash.

Skimmings Disposal...

The volume of scum or skimmings collected from sedimentation basins or separate skimming tank normally varies from 0.1 to 7 cu.ft. per million gallons of sewage. Wide variations are possible due to industrial discharges to the sewerage system. Skimmings normally have a moisture content of 60 to 90 percent and a volatile solids concentration of 90 to 95 percent. Because skimmings are collected as floating material, they include high concentrations of grease and fibrous trash. The heat value can vary from 8,000 to 18,000 Btu per pound.

Skimmings are usually disposed of in one of four ways : (1) buried; (2) pumped to digesters; (3) dewatered by mechanical equipment; (4) incinerated. Burial is simple but requires immediate covering and concern for nuisance problems. Disposal to digesters is very common, particularly with completely mixed units. Without thorough digester mixing, skimmings may form a scum layer which leads to operational problems. Dewatering requires careful control to avoid media plugging. Vacuum filter dewatering normally requires prior mixing with other more easily drained materials. Skimmings, however, could be added to a vacuum filter after a sludge precoat has been formed.

Burning skimmings in incinerators is becoming more popular as the volume of material increases with the the increased use of garbage grinders. Separate incineration of skimmings at the source (skimming tank or sedimentation basin) is recommended by some people because it eliminates operational problems associated with pumping grease to a distant incinerator. However, incineration of this highly volatile and high Btu value material can be a problem due to the development of high temperatures. Most conventional incinerators (in excess of 2,000 O F) that can result from burning skimmings without other lower Btu value solids. In addition to incinerator damage from high temperatures, flashing and odors are two problems that may develop from the burning of skimmings, if proper design and operational procedures are not adopted.


The volume of screenings, grit, and skimmings collected at waste treatment plants is fairly small, but proper disposal is important because they are very objectionable materials. Problems involving odors, insects, rodents, and unsightliness can develop if these solids are not correctly handled. Burial, grinding with discharge to raw wastewater, and digestion with adequate mixing have been satisfactory methods of disposal. However, incinerator design should be improved if skimmings and screenings are to be burned without the addition of dewatered sludge.


Sludge may be defined as a semi-liquid waste having a total solids concentration of at least 2500 ppm. It flows, it can be pumped, and it exhibits hindered settling characteristics in gravity settling basins. Sludge handling and disposal includes :

- Collection of the sludge.
- Transportation of the sludge.
- Processing the sludge to convert it to a form suitable for disposal.
- Final disposal of the sludge.

It has been stated that final disposal is accomplished only when the material has been entirely removed from the treatment plant in a manner that is sanitary, permanent, and satisfactory to all parties concerned.

Composition of Sludge...

The quantity and composition of sludge varies with the character of the sewage from which it is removed and depends upon the treatment process by which it is obtained.

The sludge obtained from plain sedimentation tanks is essentially the settleable solids in the raw sewage and is termed raw sludge. It has undergone practically no decomposition and is, therefore, highly unstable and putrescible. Such sludge is usually gray in color disagreeable in appearance, contains bits of garbage, fecal solids, sticks and other debris, and has a foul odor.

The sludge from the secondary settling tank following a trickling filter consists of partially decomposed organic matter. It is usually dark brown and flocculent, more homogeneous in appearance, and has less odor than raw sludge. The excess sludge withdrawn from the activated sludge process is also partially decomposed, is golden brown and flocculent, and has a rather distinct earthy odor. Both sludges, with further decomposition can become septic and cause offensive odors.

Sludge from the chemical precipitation process is usually darker in color. The odor may be objectionable, but not so bad as sludge from plain sedimentation. It will decompose or digest, but more slowly than sludges from other processes. The volume of sludge produced by this process is so great it is not practical to provide digestion facilities; therefore, other treatments are used in preparing it for disposal. Sludge from digestion processes have a distinct, but unoffensive odor which varies depending on the source of the sludge.

Solids Concentration...

The proportion of solids and water in liquid sludge depends on the nature of the solids, its source, whether from primary or secondary settling tanks and the frequency of removal from these tanks. It may vary from one percent in a watery activated sludge to ten percent or more in a concentrated raw or a digested sludge. Concentration is important because the volume occupied is inversely proportional to the solids content.

It is generally desirable to handle the most concentrated sludge possible for the following reasons: To save storage space in a digester or to provide a longer digestion period for solids; to save pumping capacity; to reduce heat requirements in heated digesters because there is less water to be heated; and to reduce heat and power requirements for other types of sludge treatment.

This is for good settling sludge. After 30 minutes if the sludge is sill at 90 %, there will be organics leaving through the clarifier (sludge in clarifiers illustrated above).

Sludge Treatment Methods...

The processes involved in sludge treatment vary from simple gravity thickening to almost complete destruction of solids by incineration. Which process is selected to accomplish the design objectives depends on one or more of the following factors :

- Character of the sludge;  raw, digested, or industrial.
- Land availability.
- Suitability of sludge for disposal by dilution.
- Local possibilities for using sludge as a soil conditioner or fertilizer.
- Climate.
- Capital and operating costs.
- Size and type of wastewater treatment plant.
- Proximity of the plant to residential areas and local air pollution control regulations.

Sludge Processing Alternatives...

Thickening Stabilization Conditioning Dewatering Disposal
Anaerobic digestion
Aerobic digestion
Vacuum filtration
Pressure filtration
Drying beds

Sludge lagoons