ACTIVATED SLUDGE TREATMENT PROCESS

Activated sludge refers to a mass of microorganisms cultivated in the treatment process to break down organic matter into carbon dioxide, water, and other inorganic compounds.

The activated sludge process has three basic components:

A reactor in which the microorganisms are kept in suspension, aerated, and in contact with the waste they are treating

Liquid-solid separation A sludge recycling system for returning activated sludge back to the beginning of the process.

There are many variants of activated sludge processes, including variations in the aeration method and the way the sludge is returned to the process.

While many activated sludge treatment works have been built in developing countries, very few work as well as intended. Activated sludge can be appropriate where high removal of organic pollution is required, funds and skilled personnel are available for operation and maintenance, and land is scarce or expensive. Since activated sludge requires the continuous operation of oxygen blowers and sludge pumps, a steady energy supply is a key requirement. The system usually needs some form of pre-treatment, such as screening and primary sedimentation.

Technical Features and Requirements

There is a vast literature on the design of various forms of the activated sludge treatment process. General considerations include wastewater characteristics, local environmental conditions including temperature, possible presence of toxic or other inhibitory substances will the process receive industrial effluents, oxygen transfer requirements and reaction kinetic.

Operation of the activated sludge process requires more operator control than the other treatment processes discussed. The operator must adjust aeration, return rates and waste rates to maintain the balance of food, organisms and oxygen. Operators must observe operation of the aeration basin to check on mixing pattern, type and amount of foam (normally small amounts of crisp white foam), colour of activated sludge (normally dark, chocolate brown), and odours (normally musty or earth odour). In regard to the settling tank, observations include flow pattern (normally uniform distribution), settling, amount and type of solids leaving with the process effluent (normally very clean).

In process control operations, sampling and testing are important. Testing may include settle ability testing to determine the settled sludge volume; suspended solids testing to determine influent and mixed liquor suspended solids, return activated sludge solids, and waste activated sludge concentrations; determination of the volatile content of the mixed liquor suspended solids; dissolved oxygen and pH of the aeration tank, BOD and/or COD of the aeration tank influent and process effluent; and microscopic evaluation of the activated sludge to determine the predominant organism.

Advantages of Activated Sludge Disadvantages of Activated Sludge

Efficient removal of BOD, COD and nutrients when designed and professionally operated according to local requirements. The process itself has flexibility and numerous modifications can be tailored to meet specific requirements such as for nitrogen removal. Activated sludge is the best documented and most widely used form of secondary wastewater treatment.

Expensive in terms of both capital and O&M costs, requires a constant energy supply, needs trained operators who can monitor the system and react to changes immediately, and the availability of spare parts and chemicals may be an obstacle. The track record of activated sludge plants in the developing world is very poor, and few operate as designed or intended.

In addition, support equipment, including return pumps, waste pumps, flow measurement devices for return and waste, as well as equipment to provide aeration (mixers and/or blowers) is also required.

Primary effluent is mixed with return activated sludge to form mixed liquor. The mixed liquor is aerated for a specified length of time. During the aeration the activated sludge organisms use the available organic matter as food producing stable solids and more organisms. The suspended solids produced by the process and the additional organisms become part of the activated sludge. The solids are then separated from the wastewater in the settling tank. The solids are returned to the influent of the aeration tank (return activated sludge). Periodically the excess solids and organisms are removed from the system (waste activated sludge). Failure to remove waste solids will result in poor performance and loss of solids out of the system over the settling tank effluent weir.

To obtain desired level of performance in an activated sludge system, a proper balance must be maintained between the amounts of food or organic matter, organisms that are activated sludge and

oxygen. There are a number of factors that affect the performance of an activated sludge treatment system. These include:

I. TemperatureII. Return rates

III. Amount of oxygen availableIV. Amount of organic matter availableV. Ph

VI. Waste ratesVII. Aeration time

VIII. Wastewater toxicity

Rotating Biological Contactors (RBC)

Rotating biological contactors (RBC), also called rotating biological filters, are fixed-bed reactors consisting of stacks of rotating disks mounted on a horizontal shaft. They are partially submerged and rotated as wastewater flows through. They are used in conventional wastewater treatment plants as secondary treatment after primary sedimentation of domestic grey- or blackwater, or any other biodegradable effluent. The microbial community is alternately exposed to the atmosphere and the wastewater, allowing both aeration and assimilation of dissolved organic pollutants and nutrients for their degradation.

Advantages Disadvantages

High contact time and high effluent quality (both BOD and nutrients)

High process stability, resistant to shock hydraulic or organic loading

Short contact periods are required because of the large active surface

Low space requirement Well drainable excess sludge collected in

clarifier Process is relatively silent compared to

dosing pumps for aeration No risk of channelling Low sludge production

Continuous electricity supply required (but uses less energy than trickling filters or activated sludge processes for comparable degradation rates)

Contact media not available at local market

High investment as well as operation and maintenance costs

Must be protected against sunlight, wind and rain (especially against freezing in cold climates)

Odour problems may occur Requires permanent skilled technical

labour for operation and maintenance

Technical Features and Requirements

The surface of the disk is covered with a biological slime similar to that on the media of a trickling filter. RBC units are usually installed in a concrete tank so that the surface of the wastewater passing through the tank almost reaches the shaft. This means that about 40% of the total surface area of the disks is always submerged. The shaft continually rotates at 1 to 2 rpm, and a layer of biological growth 2 to 4 mm thick is soon established on the wetted surface of each disk. The organisms in the slime assimilate (remove) organic matter from the wastewater for aerobic decomposition. The disk continues to rotate, leaving the wastewater and moving through the air. During this time, oxygen is transferred from the air to the slime. As the slime re-enters the wastewater, excess solids and waste products are stripped off the media as sloughing. These sloughing are transported with the wastewater flow t a settling tank for removal.

Typically, a single contactor is not sufficient to achieve the desired level of treatment, so a group of contactors are used in series. Each individual contactor is called a stage and the group is known as a train. Most RBC systems consist of two or more trains with three or more stages in each. One major advantage of the RBC system is the level of nitrification that can be achieved if sufficient stages are provided.

 

During operation, observations of the RBC movement, slime colour, and appearance are helpful in determining system performance; that is, they can indicate process conditions. If the unit is covered, observations are usually limited to that portion of the media that can be viewed through the access door. The following may be observed:

Grey, shaggy slime growth - indicates normal operation

Reddish brown, golden shaggy growth - nitrification

White chalky appearance - high sulphur concentrations

No slime - severe temperature or pH changes

 

In regard to typical performance, a well-maintained, properly operated RBC typically produces a high quality effluent with BOD at 8-95% and Suspended Solids Removal at 85-95%. The process may also reduce the levels of organic nitrogen and ammonia nitrogen significantly if designed for this purpose.

Operation and Maintenance

Large-scale RBCs are often covered to protect them from cold temperatures, rain, wind and sun. Sometimes, artificial aeration is required to keep the process aerobic when the systems are covered. During operation, the system must be supervised by professional operators. Maintenance includes lubrication of moving parts, motors and bearings; replacing seals, motors, servicing bearings, and cleaning the attached-growth media. The discs may be also checked for debris accumulation, ponding and excessive or not sufficient biomass accumulation.

Although fixed film units such as RBC and trickling filters are operation and maintenance intensive, they do not require seeding with bacterial cultures and the start-up phase is therefore considerably shorter. However, it takes 6 to 12 weeks for the biofilm to establish for a good treatment performance.