Activated Sludge Process - 1...

Activated Sludge...

The activated sludge process was developed in England in 1914 and was so named because it involved the production of an activated mass of microorganisms capable of aerobically stabilizing the organic content of a waste. Activated sludge is probably the most versatile of the biological treatment processes capable of producing an effluent with any desired BOD. The process has thus found wide application among domestic wastewater and industrial wastewater treatment. Like the trickling filter, activated sludge is a biological contact process where bacteria, fungi, protozoa and small organisms such as rotifers and nematode worms are commonly found. The bacteria are the most important group of microorganisms for they are the ones responsible for the structural and functional activity of the activated sludge floc. All types of bacteria make up activated sludge. The predominate type of bacteria present will be determined by the nature of the organic substances in the wastewater, the mode of operation of the plant, and the environmental conditions present for the organisms in the process.

Fungi are relatively rare in activated sludge. When present, most of the fungi tend to be of the filamentous forms which prevent good floc formation and therefore decrease the efficiency of the plant. A high carbohydrate waste, unusual organic compounds, low pH, low dissolved oxygen concentrations, and nutrient deficiencies stimulate fungi growths. The other forms of microorganisms present in activated sludge play a minor role in the actual stabilization of the organics in wastewater.

Process Design Consideration...

The activated sludge process is usually employed following primary sedimentation. The wastewater contains some suspended and colloidal solids and when agitated in the presence of air, the suspended solids form nuclei on which biological life develop and gradually build up to larger solids which are known as activated sludge.

Activated sludge is a brownish floc-like substance consisting of organic matter obtained from the wastewater and inhabited by myriads of bacteria and other forms of biological life. Activated sludge with its living organisms has the property of absorbing or adsorbing colloidal and dissolved organic matter. The biological organisms utilize the absorbed material as food and convert it to inert insoluble solids and new bacterial cells. Much of this conversion is a step-by-step process. Some bacteria attack the original complex substances to produce simpler compounds as their waste products. Other bacteria use the waste products to produce sill simpler compounds and the process continues until the final waste products can no longer be used as food for bacteria.

The generation of activated sludge or floc in wastewater is a slow process and the amount so formed from any volume of wastewater during its period of treatment is small and inadequate for the rapid and effective treatment of the wastewater which requires large concentrations of activated sludge. Such concentrations are built up by collecting the sludge produced from each volume of wastewater treated and re-using it in the treatment of subsequent wastewater flows. The sludge so re-used is known as returned sludge. This is a cumulative process so that eventually more sludge has been produced and is available to maintain a viable biological population of organisms to treat the incoming wastes. The surplus, or excess activated sludge, is then permanently removed from the treatment process and conditioned for ultimate disposal.

The activated sludge must be kept in suspension during its period of contact with the wastewater being treated by some method of agitation. The activated sludge process, therefore, consists of the following steps :

- Mixing the activated sludge with the wastewater to be treated (mixed liquor).
- Aeration and agitation of this mixed liquor for the required length of time.
- Separation of the activated sludge from the mixed liquor, in the final clarification process.
- Return the proper amount of activated sludge for mixture with the wastewater.
- Disposal of the excess activated sludge.

Some Basic Calculations ( As Example )...

About Design and Sludge Control...

About Operation and Process Control...

Mixing the Activated Sludge with the Wastewater to be Treated...

The first step in the activated sludge process is to bring the microorganisms in contact with the organics of the wastewater. This is generally accomplished by the rapid mixing of the return sludge with the wastewater at the inlet of the aeration tank. In some cases small mixing chambers are provided, but this is not the common practice.

Aeration and Agitation of Mixed Liquor...

Aeration serves at least three important functions : (1) mixing the returned sludge with effluent from primary treatment, (2) keeping the activated sludge in suspension, and (3) supplying the oxygen to the biochemical reactions necessary for the stabilization of the wastewater. The theoretical oxygen requirement can be computed by knowing the BOD of the waste, the amount of organisms wasted from the system per day and the degree of treatment (whether a nitrified effluent is required). For practical purposes, enough air should be added to the waste to maintain at least two mg/L of dissolved oxygen under all conditions of loading in all parts of the aeration tank. The air requirements for good treatment can be satisfied either by a diffused air system or by mechanical aerators.

In the diffused air system, air under low pressure, generally not more than eight to ten pounds, is supplied by blowers and forced through various types of porous material in plates or tubes installed near the bottom of the aeration tank. As the air is discharged into the liquid phase it breaks up into fine bubbles thus increasing the surface area across which oxygen diffuses from air into the wastewater. The plates or tubes are so located in the aeration tank that a rotary motion is imparted to the wastewater mixture resulting in a considerable amount of air being absorbed from the atmosphere. Diffuser plates may be composed of fused crystalline alumina or high-silica sand. They are set in containers usually made of reinforced concrete. Diffuser tubes are made of similar material, or, more recently, of corrugated stainless steel pipe with multiple outlets and wrapped with saran twisted cord. These are suspended in the aeration tank in sections and can be disconnected above the wastewater surface and removed for cleaning or renewal. When installed on swing joint connections so that they may be brought to the surface of the tank, they are known as "Swing Diffusers".

To prevent clogging of the diffuser plates or tubes, the air supplied to them should be filtered to remove dust, oil or other impurities, and the piping should be of non-corrosive material. There are a number of types of filters available based on different principles which may be used alone or in combination.

Standard Operating Procedures - Aeration and DO Control...

Procedure Frequency Method Range Condition Probable Cause Response
Check DO level Every 2 hours DO - meter or iodometric method Normally 1 to 3 mg/L High Too much aeration Decrease aeration
Satisfactory - Continue monitoring
Low Too little aeration Increase aeration
Check uniformity of aeration pattern in aeration tank Daily Visual observations Uniform mixing, roll pattern, air bubble disbursement Dead spots, uneven roll, improper distribution of air, localized boiling Diffuser malfunction Perform DO profiles, balance air distribution with header valves, pull and check for plugged diffusers

Procedure Frequency Calculation Condition Probable Cause Response
Check air requirement (Diffused aeration) Daily SCF/lb COD or lb BOD removed High Poor O2 transfer or nitrification Check uniformity of aeration, check for nitrification
Satisfactory - Continue monitoring
Low Inaccurate DO, COD, or BOD measurement Recalibrate DO - meter, check lab analysis

Procedure Frequency Calculation Condition Probable Cause Response
Check air requirement (Mechanical aeration) Monthly lb O2/lb COD or lb BOD removed High Low Loading Reduce number of units in operation, check for adequate mixing
Satisfactory - Continue monitoring
Low High loading, insufficient aeration capacity Improve primary treatment, increase number of units in operation

Mechanical Aerators...

Mechanical aerators are of two general types ; surface and turbine. Surface aerators consists of submerged or partially submerged impellers, which are centrally mounted in the aeration tank and which agitate the wastewater vigorously, entraining air in the wastewater and causing a rapid change of the air-water interface to facilitate solution of the air. Another type of surface aerator, more popular in Europe, consists of a paddle wheel or brush, partly submerged in the wastewater, revolving on a horizontal axis. Air is absorbed by surface contact and by droplets thrown through the air by the paddle mechanism.

Turbine aerators are usually upflow types that rely on violent agitation of the surface and air entrainment for their efficiency. A draft tube may be utilized to control the flow pattern of the circulating liquid within the aeration tank. The draft tube is a cylinder with flared ends mounted concentrically with the impeller, and extending from just above the floor of the aeration tank to just beneath the impeller.

Surface aerators are rated in terms of their oxygen-transfer rate or pounds of oxygen per horsepower-hour (hp-hr) at standard conditions. Surface aerators normally have an efficiency from 2 to 4 lb. O2/hp-hr while turbine aerators vary from 2 to 3 lb. O2/hp-hr.

In the activated sludge process, the sludge accomplishes the major part of the removal of BOD from the wastewater being treated in a relatively short period of aeration. It takes, however, a much longer time for the sludge to assimilate the organic matter which it has absorbed. During this time an aerobic environment must be maintained. To effect the most complete treatment of wastewater and the most economical operation in the conventional activated sludge process, an aeration detention time of six to eight hours has been found to be adequate for diffused air aeration and nine to twelve hours for mechanical aeration. Substantially shorter periods are used in some of the modifications of the conventional process. These shorter aeration periods generally result in a lowering of the quality of the plant effluents.