How to design an UASB reactor?

Up Flow - Anaerobic Sludge Blanket Reactor (UASB)

About Anaerobic Wastewater Treatment

Anaerobic Wastewater Treatment is a wastewater treatment system using biology that without using of air or oxygen. It aimed to remove organic pollution in wastewater, slurries and sludge. Anaerobic microorganisms convert organic pollutants into a &#;biogas&#; which contains methane and carbon dioxide.1

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Conversion of Organic Pollutants to Biogas by Anaerobic Microorganisms

About Up flow-Anaerobic Sludge Blanket Reactor (UASB)

Up flow anaerobic sludge blanket technology also known as UASB reactor is a form of anaerobic digester which used in wastewater treatment. UASB reactor is a methane-producing digester, which uses an anaerobicprocess and forming a blanket of granular sludge and is processed by the anaerobic microorganisms.2

Concept and Design

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.1

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 Peristaltic 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 biogasesgenerated will be collected as the valuable fuel or for deposal.

The average full-scale design loading of the UASB of 682 full-scale plants surveyed was 10 kg COD/m3.d.

UASB Reactor Height and Area

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.

Gas Liquid Solid (GLS) Separator

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.

Figure 2: Details of the Gas-Liquid-Solid (GLS) Separator

Comparison Between Anaerobic and Aerobic Process

Table 1: Anaerobic vs Aerobic Treatment for kg CODB/d

(For a given biodegradable chemical oxygen demand (CODB) waste load)

Function and Application

• Breweries and beverage industry
• Distilleries and fermentation industry
• Food Industry
• Pulp and paper.

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;
• Less nutrients required;
• System can be shut down for extended periods without serious deterioration; and
• 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; and
• Longer start-up period.
• A proper temperature range is required for the anaerobic process (15oC to 35oC), therefore it is not applicable during cold season in certain countries. (i.e. Canada)
• Some equipment (i.e. pH meter, thermometer etc.) and professional staff is necessary for monitoring the internal condition of the reactor. It is costly.

Case study

Cairo, Egypt

This study is carried out to propose an appropriate treatment technology for wastewater discharged from a flavor production factory. Industrial wastewater discharged from this factory ranges between 50&#;70 m3/d with an average value of 60 m3/d. The major source of pollution in this factory is due to cleaning of the vessels therefore the treatment has been carried out on the end-of pipe wastewater.

The wastewater is characterized by high values of COD, BOD, TSS and Oil and grease , , and 626 mg/l respectively. Primary sedimentation of the wastewater for four hours reduced the COD, BOD, TSS and Oil and grease by 43, 47, 80 and 74%, respectively. For the treatment of the produced wastewater, the biological treatment process such as activated sludge, rotating biological contactor (RBC), up-flow anaerobic sludgebed reactor (UASB) have been selected.

The results from each treatment process proved to be efficient for the treatment of such wastewater. The treated wastewater characteristics are in compliance with the Egyptian law which regulates the discharge ofindustrial wastewater to the sewerage system

Conclusion

In conclusion, up flow Anaerobic Sludge Blanket (UASB) reactor is a form of anaerobic digester that is used in the treatment of wastewater. It&#;s typically suited to dilute waste water streams (3% TSS with particle size >0.75mm).

As we had mentioned earlier, these are the 4 top applications of the reactors:

• Breweries and beverage industry
• Distilleries and fermentation industry
• Food Industry
• Pulp and paper

Together, these four industrial sectors account for 87% of the applications. However, the applications of the technology are rapidly expanding, including:

1.      treatment of chemical and petrochemical industry effluents

2.      textile industry wastewater

3.      landfill leachates

4.      Conversions in the sulfur cycle and removal of metals.

Furthermore in warm climates, the UASB concept is also suitable for treatment of domestic wastewater.

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References

http://en.wikipedia.org/wiki/Upflow_anaerobic_sludge_blanket_digestion

http://www.paques.nl/?pid=43&parentid=41

http://www.springerlink.com/content/j1qunpt/

 

Related Publications

Nanotechnology for Water and Wastewater Treatment - Piet Lens, Jurate Virkutyte, Veeriah Jegatheesan
 Publication Date: Feb - ISBN -

Uncertainty in Wastewater Treatment Design and Operation - Evangelina Belia, Marc B. Neumann, Lorenzo Benedetti, Bruce Johnson, Sudhir Murthy, Stefan Weijers and Peter A. Vanrolleghem (IWA Task Group on Design and Operations Uncertainty - DOUTGroup)
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Benchmarking of Control Strategies for Wastewater Treatment Plants - Krist V Gernaey, Ulf Jeppsson, Peter A Vanrolleghem, John B Copp and Jean-Philippe Steyer
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Anaerobic Reactor

UASB Units

UASB type units are one in which no special media have to be used since the sludge granules themselves act as the 'media' and stay in suspension. UASB system is not patented. A typical arrangement of a UASB type treatment plant for municipal sewage would be as follows:

1. Initial pumping
2. Screening and degritting
3. Main UASB reactor
4. Gas collection and conversion or conveyance
5. Sludge drying bed
6. Post treatment facility

In the UASB process, the whole waste is passed through the anaerobic reactor in an upflow mode, with a hydraulic retention time (HRT) of only about 8-10 hours at average flow. No prior sedimentation is required. The anaerobic unit does not need to be filled with stones or any other media; the upflowing sewage itself forms millions of small "granules" or particles of sludge which are held in suspension and provide a large surface area on which organic matter can attach and undergo biodegradation. A high solid retention time (SRT) of 30-50 or more days occurs within the unit. No mixers or aerators are required. The gas produced can be collected and used if desired. Anaerobic systems function satisfactorily when temperatures inside the reactor are above 18-20°C. Excess sludge is removed from time to time through a separate pipe and sent to a simple sand bed for drying.

Design Approach

Size of Reactor: Generally, UASBs are considered where temperature in the reactors will be above 20°C. At equilibrium condition, sludge withdrawn has to be equal to sludge produced daily. The sludge produced daily depends on the characteristics of the raw wastewater since it is the sum total of (i) the new VSS produced as a result of BOD removal, the yield coefficient being assumed as 0.1 g VSS/ g BOD removed, (ii) the non-degradable residue of the VSS coming in the inflow assuming 40% of the VSS are degraded and residue is 60%, and (iii) Ash received in the inflow, namely TSS-VSS mg/l. Thus, at steady state conditions,

SRT= Total sludge present in reactor, kg
         Sludge withdrawn per day, kg/d

     = 30 to 50 days.

Another parameter is HRT which is given by:

HRT= Reactor volume, m3
         Flow rate, m3/h

     = 8 to 10 h or more at average flow.

The reactor volume has to be so chosen that the desired SRT value is achieved. This is done by solving for HRT from SRT equation assuming (i) depth of reactor (ii) the effective depth of the sludge blanket, and (iii) the average concentration of sludge in the blanket (70 kg/m3). The full depth of the reactor for treating low BOD municipal sewage is often 4.5 to 5.0 m of which the sludge blanket itself may be 2.0 to 2.5 m depth. For high BOD wastes, the depth of both the sludge blanket and the reactor may have to be increased so that the organic loading on solids may be kept within the prescribed range.

Once the size of the reactor is fixed, the upflow velocity can be determined from

Upflow velocity m/h = Reactor height
                                     HRT, h

Using average flow rate one gets the average HRT while the peak flow rate gives the minimum HRT at which minimum exposure to treatment occurs. In order to retain any flocculent sludge in reactor at all times, experience has shown that the upflow velocity should not be more than 0.5 m/h at average flow and not more than 1.2 m/h at peak flow. At higher velocities, carry over of solids might occur and effluent quality may be deteriorated. The feed inlet system is next designed so that the required length and width of the UASB reactor are determined.

The settling compartment is formed by the sloping hoods for gas collection. The depth of the compartment is 2.0 to 2.5 m and the surface overflow rate kept at 20 to 28 m3/m2-day (1 to 1.2 m/h) at peak flow. The flow velocity through the aperture connecting the reaction zone with the settling compartment is limited to not more than 5 m/h at peak flow. Due attention has to be paid to the geometry of the unit and to its hydraulics to ensure proper working of the "Gas-Liquid-Solid-Separator (GLSS)" the gas collection hood, the incoming flow distribution to get spatial uniformity and the outflowing effluent.

Physical Parameters

A single module can handle 10 to 15 MLD of sewage. For large flows a number of modules could be provided. Some physical details of a typical UASB reactor module are given below:

Reactor configuration Rectangular or circular. Rectangular shape is preferred Depth 4.5 to 5.0 m for sewage. Width or diameter

To limit lengths of inlet laterals to around 10-12 m for facilitating uniform flow distribution and sludge withdrawal.

Length As necessary. Inlet feed gravity feed from top (preferred for municipal sewage) or pumped feed from bottom through manifold and laterals (preferred in case of soluble industrial wastewaters). Sludge blanket depth 2 to 2.5 m for sewage. More depth is needed for stronger wastes. Deflector/GLSS This is a deflector beam which together with the gas hood (slope 60) forms a "gas-liquid-solid-separator" (GLSS) letting the gas go to the gas collection channel at top, while the liquid rises into the settler compartment and the sludge solids fall back into the sludge compartment. The flow velocity through the aperture connecting the reaction zone with the settling compartmentt is generally limited to about 5m/h at peak flow. Settler compartment 2.0-2.5 m in depth. Surface overflow rate equals 20-28 m3/m2/d at peak flow.

Process Design Parameters

A few process design parameters for UASBs are listed below for municipal sewages with BOD about 200-300 mg/l and temperatures above 20°C.

HRT

8-10 hours at average flow (minimum 4 hours at peak flow)

SRT 30-50 days or more Sludge blanket concentration (average)

15-30 kg VSS per m3. About 70 kg TSS per m3.

Organic loading on sludge blanket 0.3-1.0 kg COD/kg VSS day (even upto 10 kg COD/ kg VSS day for agro-industrial wastes). Volumetric organic loading 1-3 kg COD/m3 day for domestic sewage (10-15 kg COD/m3 day for agro-industrial wastes) BOD/COD removal efficiency Sewage 75-85% for BOD. 74-78% for COD. Inlet points Minimum 1 point per 3.7-4.0 m2 floor area. Flow regime

Either constant rate for pumped inflows or typically fluctuating flows for gravity systems.

Upflow velocity About 0.5 m/h at average flow, or 1.2 m/h at peak flow, whichever is low. Sludge production 0.15-0.25 kg TS per m3 sewage treated. Sludge drying time Seven days (in India) Gas production Theoretical 0.38 m3/kg COD removed. Actual 0.1-0.3 m3 per kg COD removed. Gas utilization Method of use is optional. 1 m3 biogas with 75% methane content is equivalent to 1.4 kWh electricity. Nutrients nitrogen and phosphorus removal 5 to 10% only.

 

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