Up Flow - Anaerobic Sludge Blanket Reactor (UASB)

 UASB reactor is based on the so-called three-phase separator, which enables the reactor to separate gas, water and sludge mixtures under high turbulence conditions. This allows for compact, cheaper designs. 

 The reactor has multiple gas hoods for the separation of biogas. As a result the extremely large gas/water interfaces greatly reduce turbulence, making relatively high loading rates of 10 – 15 kg/m3.d possible. Separation in the UASB reactor requires only 1.0 meter of height, which prevents flotation effects and, consequently, floating layers.

Generally, during the treatment of UASB reactor, the substrate passes through an expanded sludge bed which containing a high concentration of biomass first. After that, the remaining part of substrate passes through a less dense biomass which named the sludge blanket.

 The influent is pumped to the UASB reactor from bottom of it by pressure pump. The influent move upwards and get contact with the biomass in sludge bed, then continue to move upwards and the rest substrates act with the biomass again in the sludge blanket which has a less concentration of biomass compared with the sludge bed below. 

 The volume of sludge blanket must be sufficient to conduct the further treatment to wastewater by-passed from the lower layer of sludge bed by channeling. At the same time, it will help to ensure a stable effluent quality. A 3 phases (Gas-Liquid-Solid or GLS) separator located above the sludge blanket to separate the solid particles from the mixture (gas, liquid, and solid) after treatment and hence allowing liquid and gas to leave the UASB reactor. 

After the treated wastewater will be collected by the effluent collection system via number of launders distributed over entire area discharging, to main launder provided at periphery of the reactor. And the biogas generated will be collected as the valuable fuel or for deposal.

The average full-scale design loading of the UASB of 700 full-scale plants surveyed was 10 kg COD/m3.d

example : a stream of 2000 m3.d with COD 6000 is treated what is the volume in M3 for the reactor ?

COD= 6 kg M3 ,Stream = 2000 m3 .d total COD day = 12,000 kg COD .d  then it will need 12,000 divided by 

10 kg cod m3.d = 1200 M3 .

 To reduce the plan area and to reduce the cost of land, GLS separator and influent distribution arrangement etc. the reactor should be as high as possible. And the height of the sludge bed should be sufficient to minimize the channelling and to make sure the liquid up flow velocity within the maximum permissible limits (1.2 – 1.5 m/h). Therefore, the height of the sludge bed should be at least about 1.5 to 2.5 meters and hence the height of the reactor should be restricted to 4 meters to provide convenient accommodation for sludge bed, sludge blanket and 3 phases separator. As the standard mentioned, the maximum height of the reactor is around 8 meters but the applicable height in common usage is between 4.5 and 6 meters.

In addition, the sludge bed occupies 30 to 60% of the total reactor volume, 20 to 30% of the total volume is provided for sludge blanket and GLS separator occupies remaining 15 to 30% of the total volume.

Example : a Diameter  of 20 meter is provided for one reactor , 4 reactors will be built in parallel . 

height of sludge = 2.0 meter  then : 3.14X 20X20/4X2=628 M3 , this value provide 50% of the total reactor volume . then a 30%  sludge blanket is equal to 377 M3 and the rest is 251 m3 for GLS Separators 

Given this results the height of each section is 

sludge bed (50% ) - 2 m

sludge Blanket(30%) -  1.2 m

GLS (20%) - 0.80 m 

Total Height = 4 m 

Providing 2 reactors on a squire  Field  40X 40 then total volume will be 4 X =5026 m3  

at this volume 5026 M3 X 10 Kg COD /M3 = 50, 260 Kg COD.d 

Cheese Dairy effluent loading - 6 kg COD /m3  then  50,260 / will allow 8376  M3/day 

Cow manure effluent loading - 25 kg COD /m3  then 50,260 will allow 2010 M3/day 

waste water from slaughter  house effluents 60 kg COD /M3  then 50,260 allow 837 m3/day 

 the main objective of this design is to facilitate the sludge return without help of any external energy and control device. The function of the GLS separator is to provide enough gas-water interfaces inside the gas dome, sufficient settling area out side the dome to control surface overflow rate; and sufficient aperture opening at bottom to avoid turbulence due to high inlet velocity of liquid in the settler, to allow proper return of solid back to the reactor. Due attention has to be paid to the geometry of the unit and its hydraulics, to ensure proper working of the GLS separator. 

1.  Power consumption | UA = 1.5 KW | Aerobic=65 KW
2. Net Biosolids Produced | UA =15-100 | Aerobic=200-600
3. Energy Produced | UA=150 KW | Aerobic - None 

Advantages

•  During the treatment process a amount of valuable biogas energy will be produced which can be collected for other usage 
•   Much less bio-solids waste generated compared with aerobic process because much of the energy in the wastewater is converted to a gaseous form and resulting in very little energy left for new cell growth .
•  A low energy requirement for the treatment process 
•  System can be shut down for extended periods without serious deterioration .
•  Can handle organic shock loads effectively 

Disadvantages

•  Anaerobic treatment cannot achieve surface water discharge quality without post-treatment  
•  Reduced sulphur compounds are produced, which need to be properly addressed in terms of corrosion, odour and safety; especially while dealing with cow manure .
• it  needs Longer start-up period  , some time very long depend on the type  feed

• A proper temperature range is required for the anaerobic process (25oC to 35oC), therefore it is not applicable during cold season in certain countries.
•  professional staff is necessary for monitoring the internal condition of the reactor. It is costly. and some time not available in certain countries. 

data

•  the average effluents from the dairy  not including whey which is sent for protein recovery is - 2700 M3/day 5 days per week 
•   The Biological load if no treatment for fat removal prior to reactor treatment is COD =6000 or 6.0 kg/cubic meter BOD = 2.95 Kg/cubic meter  TSS=2.3 Kg/cubic meter and Fat at 0.8 Kg /cubic meter .
• Majority of effluent is coming from cleaning of the dairy vessels and some from cleaning of milk trucks and Machines . 

•  2 digestors each 5000 m3 converted to  UASB  with following treatment by SBR

Requirements 

•  To reduce COD/BOD of Inlet  by 65 % before SBR or exit of COD 2.1 and BOD -1.03  
•  To allow 2700 M3/day treatment in 10,000 M3 reactors  
• to restrict operation cost in terms of energy required  to 200 KW/day 
• to restrict PH adjustment cost to less then 0.05 $ / M3 on entrance. 
• To reduce COD and BOD after SBR to values accepted by  Israel  water Authorities .