Sewage Treatment

The Activated Sludge Process is a natural process and nature offers us a unique solution to treat sewage. The sewage contains high levels of microorganisms present in the food in the right balance. Hence, these microorganisms will consume the largest amount. This allows the quick breakdown of the Biochemical oxygen demand (BOD) levels to more reasonable levels.

The next level would involve heavier and less mobile microorganisms to reduce the BOD levels further until the final acceptable standards are obtained.

We can identify four major sections in the activated sewage plant system. These sections are repeated in the different systems available.

• Collection and anaerobic storage (not always required, depending on the solids removal systems proposed)
• Aeration of the Sludge
• The setting of the sludge removing all solids
• Chlorination and phosphor removal to bring the final effluent up to the required standard.

What is Biochemical Oxygen Demand (BOD)?

BOD determines the quality of water. It is measured by the amount of oxygen required by bacteria and other microorganisms to oxidize the organic matter present in a water sample over a period of 5 days.

The BOD of drinking water should be less than 1. BOD of raw sewage may rise to several hundred.

What is Primary Treatment?

It is a simple and ineffective method of sewage treatment. The process involves the undissolved solids in raw sewage to settle out of the suspension, forming sludge. It removes only one-third of the BOD and none of the dissolved minerals.

Due to the contamination by toxic chemicals present in the industrial waste, efforts made to use the sludge as a fertilizer has been hampered.

What is Secondary Treatment?

In this process, the waste is brought in contact with oxygen and aerobic microorganisms (organisms that can survive and grow in an oxygenated environment). This results in the breakdown of most of the organic matter to harmless substances such as carbon dioxide.

Many treatment plants pass the waste from primary treatment to secondary treatment.

Primary and Secondary treatment together can remove up to 90% of the BOD. Chlorination removes the bacteria from the secondary treatment waste. This is then discharged to surface water.

Advanced Waste Treatment

Primary and Secondary treatment together removes most of the theorganic matter in sewage, lowering the BOD. However, most of the nitrogen and phosphorus in the sewage remain in the waste from

Secondary treatment. The surface water enriched with these inorganic nutrients can cause algae blooms. To avoid this, the third stage of treatment called tertiary treatment is conducted.

Biological Treatment

Biological treatment is the process of utilizing naturally occurring living organisms to degrade, stabilize and destroy organic contaminants. These microorganisms use waste as their source of energy and carbon. Biological treatment technologies are limited to only organic wastes.

All living organisms require a source of energy and carbon to be able to develop and reproduce. Many organisms (autographic) get their carbon from inorganic compounds like carbon dioxide, while other organisms (heterotrophic) use organic compounds to get their carbon.

Microorganisms use Aerobic and anaerobic metabolic pathways to degrade organic waste. During aerobic respiration, the organism utilizes oxygen to break down complex organic compounds, while in anaerobic it does not require oxygen. Microorganisms can gain energy and carbon directly from the contaminated waste or sometimes with the help of another substance they can together break down the contaminated waste.

One of the most important characteristics of waste is its biodegradability. The biodegradability of waste can be measured in the laboratory through BOD (5) / COD tests.

Chemical Treatment

Chemical treatments alter the structure of the waste components to make them less harmful than their original form. The purpose of using chemicals is to immobilize, mobilize for the extraction or to detoxify the contaminant. This treatment may achieve one or all of the tasks.

Emphasis should be given to the following during chemical treatments:-

• The possibility of chemical treatment is dependent on the characteristics of the waste and site. Therefore, a careful study of site hydrology and geology must be conducted.
• Many chemical treatments involve the delivery of a fluid to the subsurface. Precaution must be taken to avoid the relocation of treatment reagents since they can be toxic themselves.
• Chemical treatment can be applied to both organic and inorganic wastes. However, a detailed analysis of the waste must be done, so that the wrong reagents are not mixed with the waste resulting in increased toxicity.

Sequential Batch Reactor

They are industrial processing tanks used for the intensive biological treatment to crud sewage

The Sequential Batch Reactor combines the effectiveness of activated sludge aeration with batch treatment to provide excellent quality, even under extreme loadings.

Benefits and Features

• Main pollutants including ammonia are reduced by 95%.
• Phosphates are reduced by 99%.
• Batching system eliminates peak surges.
• It is completely recyclable.
• All components are easily accessible.
• The sequential bath reactor uses an aeration process known as activated sludge but operates on storage and batch treatment cycle which ensures full treatment of all the effluent.
• Circle sewage enters the primary settlement tank where gross solids settle out and the liquid floating on the surface can be drawn off.
• During this aeration cycle, efficient Oxygen transfer stimulates the bacteria (the active sludge) to feed on the sewage liquor thereby removing most of the organic pollutants. After this, the bubble diffuser is switched off and there is a set period of two hours. During which the dense activated sludge sinks to the bottom, leaving behind the clarified effluent. This is then discharged into the drain.
• The SBR is an advanced biological aeration treatment plant designed to produce a very high quality of final effluent, in addition to overcoming several of the common problems.

 Common Problems faced In Other Treatment Plants

• Heavy flow can force effluent through the plant before it has had sufficient treatment time.
• The fixed film which has a media matrix can suffer from media blockage because of excessive bacteria growth. Replacement and maintenance of the media material may be required at regular intervals.
• The Mechanical and electrical components in the tanks are difficult to maintain and can be a safety risk.
• The liquor is not spread evenly over the available bacteria preventing an adequate Oxygen supply.

How Sequential Batch Reactor overcomes these problems

• The SBR overcomes the peak flow problems by operating on storage and batching system. It stores the effluent at peak times in the primary settlement tank and treats it in small batches throughout the day. This ensures that each batch receives the full treatment time.
• There are no fixed media within the SBR to get blocked up. It operates on the activated sludge principal where the bacteria float in suspension in the reactor and form active mixed liquor.
• There are no moving parts or electrical components hence it’s safe and there’s no maintenance required.
• The bacteria are aerated by the high volume and high-quality air supply and are mixed completely.