Activated Sludge Process

It is the most reliable and commonly used technology. The basic process involves aeration, microbial growth in aeration basin, sludge re-circulation after sedimentation from secondary clarifies and to get desirable quantity of MLSS/microbes. The process was first reported by Arden & Lockett in 1914.
Process Description Activated Sludge Process: 

• Preliminary Treatment:
• Primary Treatment:
• Secondary Treatment (Biological Process)
• Tertiary Treatment (Filtration &  chemical Treatment)

Preliminary Treatment: 

Generally effluent holds large objects like plastic, paper, wood, metals, sand & grit which may interfere in treatment process by the clogging of pipeline & pumps etc. By removing such objects from effluent we can minimize the problem. Preliminary treatment is designed to remove such impurities/object from effluent wastewater. The stages of preliminary treatment are as under- 

1. Screening Grit 
2. Removal Oil & Grease 
3. Skimming 

Screening: 
The first unit of operation in any Effluent/Sewage Treatment Plant. Screening removes objects like paper, plastic, wood, rags, metals and many other floating materials to prevent the clogging and damage of pipeline & equipment’s. The types of screen depend on the industry & quality of floating materials. There are two types of screens-

Coarse screen:
Coarse screens are used to remove large floating materials like plastic, polyethene rags, wood, metals & debris from influent. Coarse screen have opening in between of 6 mm to 150 mm (0.25 inch to 6 inch).

Fine Screen: are used to remove smaller size objects which may cause problem during operation. The opening sizes ranges in between 1.5mm to 6 mm (0.06 inch to 0.25 inch). These screens are used after coarse screens.

2. Grit Removal:
Industrial / Domestic Effluent contains inorganic solids such as pebbles, cinder, sand & slit that are heavier (higher specific gravity) than the biodegradable solids. Removal of these substances is essential because if they enter in main treatment scheme, they will clog the pipe line and damage the pumps. Reduction of volume of equalization tanks are the common issue because these substance get deposited in the equalization basin. 
Many types of grit removal facilities are available some are-

• Aerated grit Chamber
• Vortex type grit removal system
• Detritus tank
• Flow grit chambers (Commonly used and reliable) 

3. Oil & Grease removal:
Oil & grease comes from the leakage or spillage during the replacement/filling of oil & grease in the machineries which are used in industries. Some industries like food & beverage and hotel industries have large potential of oil & grease pollution because they use huge quantity of edible and it comes in effluent during washing of manufacturing equipment’s. Oil & grease are hazardous material, if oil and grease mixed into water bodies, it forms oil layer on the surface of water and reduces the light penetration into water and photosynthesis. It further hinders oxygen transfer from atmosphere to water and this leads to decrease the dissolved oxygen level at the bottom of the water and adversely affects survival of aquatic life.  Oil & grease can be removed by the construction of grease trap & also by installing mechanical oil skimmers. Grease traps are a chamber having baffle wall and an inlet & outlet chambers. It provides the retention time by which the floating oil & grease accumulates at the top. There are various technologies are available for O & G skimming, we can select as per our need. Some of the common conventional methods include flotation, gravitational methods, chemical treatment, dissolved air flotation (DAF). Oil Skimmers and use of membranes.

2. Primary Treatment: 

After preliminary treatment the effluent further treated through this system. It includes following stages-
1. Equalization & pH Control
2. Primary Clarification

1.Equalization & pH Control: 

Equalization of effluent is necessary before start the feed for further treatment. For domestic effluent equalization is not needed because there is no variation in pH or having neutral pH. The pH & quality of industrial effluent changes every hour due to the production process. So equalization is highly recommended in industrial effluent plant. 

An equalization tank is a simple rectangular tank having coarse bobble diffuser for mixing of incoming effluent. The capacity of equalization tank is directly proportional to the incoming effluent quality. Generally 4-6 hours of retention time is needed for equalization. The main function of equalization tank are-

• Act as buffer. 
• To collect the incoming effluent having fluctuations in COD, BOD & TSS.
• To provide the retention time & equalize the COD, BOD, TSS & other parameters.
• During peak hours it helps to maintain the designed/standard feed rate to primary settling   tank.
• Equalize the effluent to prevent shock load to aeration tank 
• Neutralization of pH of effluent

Acid & alkali are used to neutralize the pH, if pH is less then alkali is used & acid is used for neutralization of high pH. 

2. Primary Settling Tank or Primary Clarifier: 

The primary clarifier is a physical treatment unit which is responsible for removing solids before biological treatment system/aeration tank. It is widely used, most common & cost effective solution to remove the solids from effluent after equalization. The scum is removed from the surface by skimmers (it is not available in all clarifier/optional) and the settleable solids (sludge) settled down at the bottom by gravity settling and  sludge get accumulated in sludge hopper by the help of scapper. The clarified effluent/ supernatant come out from clarifier through a weir and move towards aeration tank for biological treatment. The primary clarifier can remove 90-95% of settelable solids and 40-60% of suspended solids. Settling of solids directly impact on the BOD load of effluent, means removal of solid reduce the BOD. It can observe that after primary clarified effluent has 25-50% less BOD than inlet to primary clarifier. (The reduction in BOD, TSS & Settelable solids depends on the design of clarifier.)

The sludge can be removed through sludge removal system which includes gate valve, pump with motor and pipeline from hopper to sludge collection tank or at sludge drying beds. 

3.Secondary Treatment/Biological Treatment: This is the heart of the treatment system, It includes aeration tank & secondary clarifier for settling, recirculation of Activated sludge &  drainage of excess sludge to sludge drying beds/sludge holding tank.

Aeration tank: 

An aeration tank is a holding tank having air injection facilities inmixed liquor (Mixed liquor is mixer of effluent & activated sludge). Activated sludge contains colonies of naturally occurring bacteria, protozoans, metazoan & other microorganisms. These microorganisms are responsible for the breakdown & digestion (microbial digestion) of organic pollutants and nutrient present in effluent. Surface aerators & diffused aeration are the most common tecchnologies for injection of air. The diffused aeration is most acceptable technology due to its efficiency & less electric unit consumption.

The aeration tank is always have a settling tank (usually known as "final clarifier" or "secondary settling tank") which helps to settle down the biological floc's (sludge). The design of secondary clerifier is same as primary clarifier, it has only one difference i.e., sludge re-circulation system, which recirculate the sludge from secondary clarifier to aeration tank to maintain the desired level of Mixed Liquor Suspended Solid (MLSS). 

3.Tertiary Treatment :  Tertiary treatment has two stages-

1.Filtration:
2.Disinfection:

Filtration: 

The filters are used for the removal of suspended matter, as well as floating and sinkable particles. Two types of filters are utilized in this process;

Dual Media Filter:  

It is a combination of quartzite (sand) and anthracite stones in a vessel. The  effluent enters through inlet point & passes through the sand & anthracite media. The effluent flows from top to down wards, and during this process the suspended particles get trapped. Backwashing is required  to avoid chocking of media (Sand & anthracite). Backwashing is a process in which  water or water + air feed to filter from bottom to upwards, to remove the trapped substance which may chock the media. 

Activated Carbon:

Activated carbon is used to purify, police & for odour removal of treated effluent. The Granular activated carbon known as a highly adsorbent material used to remove tiny suspended matter along with odour from treated effluent. It is also known a miracle filter media by many researchers because of its unique ability to remove odours, colour, chlorine and volatile organic chemicals, pesticides and trihalomethanes (a group of suspected carcinogens). Activated carbon has a large surface area to absorb the contaminants present in treated effluent.

Disinfection:

Disinfection of treated effluent is very essential before the utilization or discharge, because it contains so many harmful microbes and pathogens too. These pathogens may harm human, animals and plants also. We can use chlorine,  hypo & ozone for disinfection of effluent. UV is also a very good measures for disinfection.

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