UASB : Upflow Activated Sludge Blanket Reactor

In line with our endeavor to introduce latest technologies related to Water & Waste water treatment, We have developed The Upflow Anaerobic Sludge Blanket Reactor [UASB] is a single tank process in an anaerobic centralized  or decentralized industrial  wastewater or black water  treatment system achieving high removal  of organic pollutants. Wastewater enters the reactor from the bottom  and flows upward. A suspended sludge blanket filters and treats the wastewater as the wastewater flows through it. Bacteria living in the sludge break down organic matter by anaerobic digestion, transforming  it into biogas. Solids are also retained by a filtration effect of the blanket.  The upflow regime and the  motion of  the gas bubbles allow mixing without mechanical assistance.  Baffles at the top of the reactor allow gases to escape and prevent an outflow of the sludge blanket. As all aerobics treatment, UASB requires a post treatment to remove pathogens, but due to a low removal of  nutrients, the effluent water as well as the stabilized sludge can be used in agriculture. 

Basic Principle :

UASB Reactors are constructed out of concrete or another watertight material and can be designed in a circular or rectangular way.  Wastewater is pumped from the bottom into the reactor where influent suspended solids and bacterial activity and growth lead to the formation of sludge. The sludge blanket is comprised of microbial granules (1 to 3 mm in diameter) , i.e.,  small agglomerations of microorganism  that because of their weight, resist being washed out in the up flow.  The microorganisms in the sludge layer degrade organic compound. As a result, Gases (Methane & Carbon Dioxide i.e, biogas) are released. The rising bubbles mix the sludge without the assistance of any mechanical parts. Upstream velocity  and settling speed of  the sludge is in equilibrium and forms a locally rather stable, but suspended sludge blanket. Sloped walls deflect material that reaches the top of the tank downwards. The clarified effluent is extracted  from the top of the tank  in an area above the sloped walls. A gas liquid solids separator separates gases from the treated wastewater  and the sludge.

 

After several week of use, larger granules of sludge  form which, in turn , act as  filters for smaller particles as the effluent rises through the cushion of sludge. Because of  the upflow regime, granule-forming organism are preferentially accumulated as the others are washed out. Fortunately,  These bacteria are also more efficient for biogas production than flocculated  biomass.

UASB do not require an energy consuming aeration system and can be constructed much simpler than aerobic treatment. They convert the organic matter into biogas which can be recovered. The nutrient rich effluent can be used for agricultural irrigation. Sludge, even is partially stabilized  and can be used as an organic soil fertilized after composting or drying.

UASB reactors can treat all type of high strength Wastewater. It can be used at large scale or decentralized treatment system for domestic & industrial wastewater.

Design Consideration :

UASB Reactor are constructed out of concrete or another watertight material and can be designed in a circular or rectangular way.

Critical Elements For The Design  Of UASB Reactor Are :

• Influent Distribution System
• The Gas Solids Separator
• The Effluent Withdrawl Design
• The Gas That Rises To The Top Is Collected In A Gas Collection Dome And Can Be Used As Energy For Cooking , Heating or Other, But Scrubbing Before Is Required.
• If  The Biogas Is Converted To Electricity, The Heat Produced As A By-product Can Be Reused To Heat The Reactor, Favoring Anaerobic Digestion.
• To Maintain The Reactor Well Mixed And Allowing The Formation Of Granules And A Good Contact Of The Active Sludge Blanket And The Influent Is Equally Distributed In The Bottom Before Moving Upwards.

The Main Influencing Parameter Are :

1. pH Value : The pH Value Need To Be Between 6.3  and 7.85 To Allow Bacteria Responsible For Anaerobic Digestion To Grow. The pH Value Is Also Important Because  At pH Values, Ammoniac [NH4+] Dissociates To NH3 Which Inhibit The Growth Of Methane Producing.
2. Temperature : For An Optimal Growth Of These Bacteria And Thus A Optimal Anaerobic Digestion, The Temperature Should Lie Between 35 To 38 Degree Celsius Below This Range, The Digestion Rate Decreases By About 11% For Each 1 Degree Celsius And Below  15 Degree Celsius The Progress Is No Longer Efficient, Although Bacterial Activity Can Still Be Noticed At Temperature Less Than 10 Degree Celsius.
3. COD Loads : Influent Should Have Concentration Of Above  250 mg COD/Lm, As For Lower Rates, Anaerobic Digestion Is Not Beneficial .  Optimum Influent Concentration Are Above 400 mg COD/L And An Upper Limit Is Not Known.
4. Hydraulic Retention Time : HRT Should Not Be Less Than 2 Hours. Anaerobic Microorganism , Especially Methane Producing Bacteria Have A Slow Growth Rate. At Lower HRT, The Possibility Of  Washout  O Biomass Is More Prominent. The Optimal HRT Lies Within 2 to 20 Hours.
5. Upflow Velocity : The Upflow Velocity In UASB Is An Important Design Parameter As The Process Play With The Balance Of Sedimentation And Upflow. Sludge Should Not Be Washed Out, Minimum Speed To Be Maintained To Keep The Blanket In Suspension And Also For Mixing. An Upflow Velocity Of  0.7 to 1 m/h  Must Be Maintained To Keep The Sludge Blanket In Suspension. Primary Settling Is Usually Not Require Before UASB
6.  Health Aspects/Acceptance : The Operator Should Take Proper Health & Safety Measures While Working In Plant, Such As Adequate Protective Clothing. Effluent And Sludge Still Pose A Health Risk And Should Not Be Directly Handled.

 

The single tank aeration design unit, incorporating a bar screen, a specially designed tank with hybrid fluidized media, a lamella plate clarifier, and tertiary treatment. The bar screen removes larger floating matter and suspended particles. The media significantly increases the surface area for bacterial growth. Air is supplied through surface aerators. The bacteria oxidize the organic matter present in the sewage. The oxidized sewage overflows in the lamella settler. Suspended particles in the treated waste water settle, with a part of the settled sludge sliding back to the aeration tank. Treated water overflows into a chemical contact tank for chlorination (disinfection), after which Multigrade pressure filtration followed by activated carbon wherein the treated water is disinfected & color / smell if any is removed.

Treated effluent from Aeration tank overflows to the lamella plate settler, where finer bio-mass and suspended solids are settled. The settled solids slide back into the Aeration tank, thus reducing the need of a re-circulation pump. Excess bio-mass is periodically removed. The sludge can be taken to Sludge drying beds or can be formed into a cake by using a centrifuge.

Ozonation / UV / Electro Chlorinator May Be Offered As An Alternate To NaOCl[Chlorination]

Clarified water overflows into a chemical contact tank for flocculants / coagulant dosing if required before clarification. Now the treated water is stored in a filter feed tank / chlorine contact tank (Existing) then filtered by passing through a Multi-grade sand filter. The clarified water passes through a Multigrade pressure filtration unit there by reducing the total suspended solids of the water to < 10 levels. This is followed by Activated Carbon followed by Ozonization which would have complete disinfection, COD / BOD reduction to our desired values, colour removal & smell removal. The treated water at the outlet of Ozonization will have a BOD < 15 - 16 ppm, COD < 100 ppm & TSS < 10 ppm.

Advantages  Using Upflow Anaerobic Sludge Blanket Reactor [ UASB ] With Aeration :

 

• Significant Reduction In Space Requirement Due To High Surface Area & Loading Rate Of Media.
•  High Reduction Of BOD
• Can Withstand High Organic And Hydraulic Loading Rates
• Low Sludge Production
• Partial Aeration System Is Required
• Effluent Is Rich In Nutrient And Can Be Used For Agriculture Irrigation
• Low Land Demand Can Be Constructed Underground And With Locally Available Material
• Reduction Of   Methane Gas & Carbon Dioxide.
• Reduced Power & Operation cost
• No Moving Parts Less Maintenance