Application Guide Sludge Mixers en Rev 1

7/27/2019 Application Guide Sludge Mixers en Rev 1

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Application Guide

Sludge Mixers for Anaerobic Digestion

A member of the SIHI Group


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Sludge Mixers for Anaerobic Digestion

Since the 1950s, there has been a constant increase in water consumption and theamount of wastewater. Municipalities, industries and government are forced to treatsewage more and more intensively. During the process of treating wastewater a greatamount of sludge is produced along the process way. This sludge is full of toxic

substance and full of potential pathogens and have to be treated in a separate way sothat it will be transformed from a aggressive into a harmless by-product of wastewatertreatment plants. Additionally the ratio of organic material will be reduced so that thevolume of the treated sludge will be reduced.

The anaerobic digestion process is a common way to reduce the organic ratio in thesludge by generating useful methane gas (Biogas). The anaerobic organisms used in theconversion of waste solids to methane are very sensitive to changes in processconditions. Therefore, homogeneous mixing is essential for a uniform temperature and

nutrient profile. A reliable and economical system is required to solve problems in largetanks such as deposits at the bottom and the formation of scum and foam on the sludgesurface.

The requirements of a mixing system should meet in order to ensure optimal anaerobicdigestion can be described as follows:

The complete digester content need to be homogenised with the raw sludge (2 - 8%dry solid content) supplied in a continuous process.

Intensive mixing is required to maintain a sufficient exchange of substances.

Avoidance of temperature and material gradients.

Efficient utilisation of the entire digester volume.

The Draft Tube Mixermeets these requirements.


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Pumping Technology For A Better Future

The Wastewater Treatment Plant

The sludge of a wastewater treatment plant (WWTP) will be produced in two areas of theprocess - mechanical treatment and biological treatment of wastewater.

The mechanical treatmentIn the mechanical treatment the so called primary sludge will be separated of the wastewater by usingscreens and sedimentation tanks. It contents of mostly fat and large amounts of biodegradable organicmaterial. Up to 90% of all waste is separated from the wastewater within this process step.

The biological treatmentThe biological treatment of wastewater is necessary to separate dissolved and colloidal organic matter(e.g. proteins) and nutrients. For this step specialized bacteria groups are living in so called aerationtanks. The bacterias use this dissolved matters as energy resource and transform this into CO2, N2,Water and biomass. In settlement tanks the purified water will be separated from these bacteria groups.A part of this bacteria groups will be pumped back into the aeration tanks but a certain part of dead

bacterias (so called exceed sludge or secondary sludge) is pumped into the anaerobic digester.

Sewer

Screen

Grit chamber Primary clarifier

To thebiological

wastewatertreatment

Primary sludge to digester

Aeration tank

To digester

Secondary clarifier

Excess sludge


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The Sludge Process

The sludge process which takes place in the anaerobic digester is also called sludgestabilisation and requires a uniform temperature, pH and material profile within the totalvolume of the digester. It is a biological process, which means living bacteria groups areneeded for this step and any bigger variations of the process conditions would kill the

bacteria groups. The process will stop at once!

The produced Biogas consists up to 75% of methane and will be used as energy resource for the WWTP,additionally the Biogas can upgrade for public gas network. Sludge conversation factor is 1m sludge = upto 23m Biogas in 30 days.

For this process a sufficient and reliable mixing device is required and will be meet by the draft tubesludge mixer.

After the process of anaerobic stabilisation the stabilized sludge will be dewatered by centrifuges up to15 - 30% DSC (Dry Solid Content) and can be used according to the sludge or biosolids quality:

Highest quality biosolids are used in agriculture as soil conditioner and fertilizer

Lower quality biosolids can be used on non-agricultural land

Poor quality biosolids can be disposed of by land filling or incineration

The process of anaerobic digestion isseparated into four steps:

Hydrolysis: In this step polymers likehydrocarbons, fat and proteins are brakendown in smaller dissolved polymers byspecialized bacteria.

Acid formation: The dissolved polymersare now be transformed into organic acids,

alcohol and by-products like water and CO2. Acid phase: Organic acids and alcohol will

now be transformed by a differentspecialized bacteria group into acetic acid.

Methane phase: In this final step the aceticacid is now transformed into CO2, water andmethane - so called Biogas.

Combined sludge

Biogas

Stabilisedsludge


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The Draft Tube Sludge Mixer - MFS

The draft tube sludge mixer is a vertical rotary pump with reversible operation. Basically,it consists of two components - the mixing equipment, including shaft and the pumpingdevice, and the draft tube. The pumping device is inserted and adjusted in the specialupper draft tube bell mouth. The sludge is pumped at a velocity of 3 m/s upward or

downward through the draft tube.

Draft TubeThe draft tube connects the two critical digester zones wherethe requirements are particularly demanding.The danger in the upper zone is that a supernatant sludge layerand scum may build up and even reach the gas bleeder lines.The lower zone is a potential area for the accumulation of heavysolids contained in the primary sludge pumped from thesedimentation tank into the digester. Such deposits canconsiderably reduce the effective zone within the digester andlead to costly cleaning measures.The draft tube is installed on a robust base structure. Lateralstability is achieved by four horizontal bracing ropes. In order toavoid vibration, long draft tubes are additionally braced in themiddle. In the event of rope failure, the remaining three ropeswill still secure the correct position of the draft tube.

Pumping device and Splash DiscThe overhung mixer shaft has a one or two-stage pumping

device and a splash disc mounted to it.The pumping device consists of two opposite helical blades andis adjusted within the upper draft tube bellmouth. The clearancebetween blade edge and digester wall is a few millimetres. Evenin the case of the largest draft tube sludge mixers with a flowrate of 7.200 m/h the clearance does not exceed 15 mm.These small gaps allow a very efficient pumping performance.The pumping device is self-cleaning, i.e. even large plaits,which despite screens and grinders sometimes get into theinterior of digesters, cause no problems.The splash disc has been developed and optimised overseveral decades. During upward operation, it diverts the liquid

flow to the sludge surface. The splash disc distributes thesludge over an area of a diameter of up to 4,500 mm.


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The Mode of Operation

The operation of the draft tube sludge mixer is reversible, i.e. the sludge can be movedupwards or downwards.

Upwards mode of MFSDuring upward operation, the splash disc distributes the sludge overthe sludge surface area softening the supernatant sludge layer anddestroying part of the scum built up during the process. In the sameway the foam build up will be controlled.

Downwards mode of MFS

During downward operation, the softened supernatant sludge layer,the scum and foam will be sucked down the draft tube and remixedwith the digester contents. The high velocity of up to 3 m/s causes astrong jet flow at the lower bell mouth, flushing away any deposits atthe bottom.

Reversible operation ensures the problems in the critical zones are

successfully dealt with and the digester contents are thoroughlymixed. The by-product of destroying these both critical zones theentire volume of the digester will be mixed up sufficiently.


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The Process Requirements

Main process requirements for the mixing device in anaerobic digester:

Customer Need Mixer Requirement Halberg Solution

Sufficient mixing of the entiredigester volume

Specialized digester mixingdevice

Draft Tube Mixer Type MFS

Avoiding of supernatant blanketon the sludge surface

Destroying capacity of mixingdevice or additional breakingdevice

Splash disk of the MFS; in the up-wards mode MFS creates a liquidumbrella to destroy and to breakthe scum blanket

Controlling of foam built up Destroying capacity of mixingdevice or special foamdestroyer

Splash disk of the MFS; in the up-wards mode MFS creates a liquidumbrella to control effectively thefoam built up

Avoiding of accumulation ofsettlements in the bottom areaof digesters

Lifting capacity of the mixingdevice

In the downwards mode the MFSproduce a jet stream in the outlethead of the digester and blows

the bottom cleanReliability and maintenancefree

Robust design andmaintenance friendly

Strong shaft design Strong designed bearings Easy and fast disassembling

without stopping the anaerobicdigestion

Constant flow over theoperation

Optimized clearance betweenthe impeller and tube

Special design of the inlet of thedraft tube with small gap betweenwhich guarantee a continuouslyhigh flow for remove belowarticles

Tested equipment before

delivery

Test bench in production plant Real operation test with test

reports and vibration analyseEnergy efficiency Less required power Specific power consumption from

2 - 5 w/m. Savings in comparison up to

40.000 Euro/year

Local sales and Serviceavailability

Locations all over the word Subsidiary all over the world withspecial local trained sales andservice people for mixers


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All Rights Reserved SIHI Group BV

EUROPE Sterling Fluid Systems (Poland) SIHI Pumps (Taiwan) TaipeiSterling Fluid Systems (Austria) Warszawa Tel. +886 2 2808 4675Wien Tel. +48 (0) 22 335 2480/81 [email protected]. +43 (0) 1 680 050 [email protected][email protected] SIHI Pumps (Korea) Seoul

Sterling Fluid Systems (Romania) Tel.: +82 2 553 2592Sterling Fluid Systems (Belgium) Bucuresti [email protected] Tel. +40 (0) 21 610 7188Tel. +32 (0) 2 481 7711 [email protected][email protected] AMERICAS

Sterling Fluid Systems (Spain) SIHI Pumps Limited (Canada)Sterling SIHI (Bulgaria) Madrid Guelph OntarioSofia-llinden Tel. +34 91 709 1310 Tel. +1 519 824 4600Tel. +359 (0) 2 8228311 [email protected] [email protected]@sterlingsihi.bg

Sterling Fluid Systems (Schweiz) SIHI Pumps Inc. (USA)Sterling Fluid Systems Schaffhausen Grand Island, New York(Czech Rep.) Olomouc Tel. +41 (0) 52 644 0606 Tel. +1 716 773 6450Tel. +420 587 433 651 [email protected] [email protected]@sterling.cz

Sterling Fluid Systems (UK) SIHI Ltda. (Chile)Sterling Fluid Systems (France) Altrincham, Cheshire Quilicura, SantiagoTrappes Tel. +44 (0)161 928 6371 Tel. +56 2 756 5900Tel. +33 (0) 1 34 823 900 [email protected] [email protected]@sterlingfluid.com

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For further address details please visit:www.halberg.com


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Application Guide Sludge Mixers en Rev 1