Reasons to Choose MBR+NF Technology

8/13/2019 Reasons to Choose MBR+NF Technology

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1. PROBLEMS TO REACH THE TENDER REQUIREMENTS WITH SBRTECHNIQUE

Setting too low feed concentrations

In the bidding too low COD and BOD concentration is determined for the leachate. It isbelieved a COD concentration in the feed of 2.650 mg COD/ l.

We believe that this concentration level for far too low. Based on our experience much higherconcentrations have to be assumed considering the waste characteristics and climaticconditions. In the realized by Wehrle Umwelt leachate treatment plants concentrations below2.650 mg COD /l are only found in old landfills (where separate collection of bio waste,possibly with mechanical-biological treatment of residual waste has been applied).

The leachate concentrations known to us in the dumpsites in Eastern Europe are between5.000 mg/ l and 15.000 mg/l. We therefore recommend a significantly higher concentration forthe design.Since the higher COD concentrations lead to higher sludge production, the required minimumsludge age for nitrification results either a significantly enlarged bioreactor volume or modifiedsolids retention by means of ultrafiltration. As an ultrafiltration additionally increases theprocess stability and lower outlet values are achieved, we have chosen this option.

To comply with the requirements a post treatment by means of a downstream membranesystem (reverse osmosis or nanofiltration) should be foreseen. The biomass retention byultrafiltration offers several other advantages. The solids and BOD free flow after ultrafiltrationprovides optimal benefits for further treatment in the downstream membrane system. The lattergives the opportunity to use membrane elements with a smaller spacer. With the fully liberatedfrom the BOD biology the bio-fouling problem in the membrane post stage is significantlymitigated. This reduces cleaning intervals and leads to a significantly longer membranelifespan. The arguments for an MBR technology over a SBR technology are again summarizedbelow in chapter 6.



2. ADVANTAGES MBR VS. SBR

2.1 Higher Biomass Concentration

The biomass concentration in an MBR is with 15 kg/m³ much higher than in an SBR System(3-5 kg /m³). Therefore the reactor volume of an MBR is far smaller than that of an SBR,usually only 15 – 20 % of the SBR Volume.

2.2 Better COD removal

The Ultrafiltration, which is part of an MBR process, achieves a complete biomass (= 0 solidsor biomass in UF permeate) retention whereas the biomass retention of an SBR is fairly poor.This means that the solid-free effluent of an MBR has a better COD, because

a) The solids themselves are organic particles and contribute to the COD.

b) The complete retention of all biomass in the MBR Systems results in a high concen-tration of specialists which achieve a better COD removal rate.

c) A thin layer in the Ultrafiltration acts itself as screen and retains additionally a part of theCOD

The COD in the effluent of an MBR is 30 to 60 % lower than the effluent from an SBR!

2.3 Higher process stability

There is yet another aspect to the biomass retention of an SBR:In case feed concentrations to the SBR vary greatly, or in case the feed contains toxicsubstances or in case the composition of the feed quality varies considerably, the quality of thebiomass in the SBR changes. The settling behavior worsens and thus the SBR is prone to losesubstantial amounts of biomass (= solids) with the treated effluent.This means a higher COD in the treated effluent (more solids = higher COD). If this lasts forlonger, SBR loses too much of its active biomass and the complete biological processbecomes seriously disturbed of not to a stand-still. In that case only a complete refill of thebioreactor with new sludge can help. Of all biological processes that there are on the market,the MBR is the most stable, most robust process – due to the fact that the biomass iscompletely retained and the adaptation possible of the biomass can be achieved.

2.4 Shorter start up time of the system

Because of the Ultrafiltration and its positive effect on the biomass adaptation, the start-upphase of an MBR is far shorter than that of other biological processes.

2.5 Higher stability against variation of the influent concentration

Again: Because of the Ultrafiltration and its positive effect on the biomass adaptation, an MBRcan handle strong variations in feed flow, feed load and feed composition better than any otherbiological process.

2.6 More flexibility against high salt concentrations

An MBR can handle high salt concentrations of up to 25 g/l stably and without negative side-effects!



SBR are taking place one after the other, the nitrification has to take place within a short timespan = the required air must be inserted in a short time = the blowers must be bigger toprovide sufficient air in a short time span.

2.12 Cost reduction in the post treatment

In case further treatment steps are required by a client (e.g. if salts reduction is required or aparticularly low COD must be met with), an MBR is at an advantage: because of the maximalCOD removal and the complete solids retention of an MBR, the installations required for anysubsequent treatment step become simpler and more effective = the subsequent treatmentsteps become cheaper + smaller! For example: In case of a post-treatment with membraneTechnology (RO or NF) a sand filter for example is not necessary. Another advantage is thepossibility to work with standard spiral wounded membranes. These kinds of membranes aremuch lower in the CAPEX and in replacement costs. The price for a standardized spiralwounded membrane is around 30 – 50 € per m2. The price for a comparable plate membraneis around 150 € per each m2. In a post RO System the membrane lifetime expectations isaround 1.5 to 2 years, so this argument is highly relevant for the operational cost of the wholesystem.

2.13 Lower wet sludge volume

Up to the lower sludge load in the MBR and up to the higher MLSS the wet sludge Volume issignificant lower than in an SBR System. Any biological process produces „surplus sludge“ =microbes multiply and grow in the reactor, the MLSS increases over time. Therefore sludgemust be removed from any reactor from time to time. Because the MLSS concentration in anMBR is far higher than in an SBR to begin with and because the surplus sludge can bewithdrawn from the pipe with the thickened sludge coming back from the Ultrafiltration, thevolume of surplus sludge of an MBR is far less than that from an SBR system.

2.14 Higher Temperature in winter times

Up to the smaller volume and a higher Biomass activity the MBR Reactor has a highertemperature in winter times. This makes the nitrification more process stabile also in longfrozen phases. The influent effluent heat exchanger gives the heat from the effluent to theinfluent that helps to increase the Temperature in the Bioreactor also. MBR Plants underoperation in North China, South Germany, Austria and Switzerland demonstrate that the MBR

is able to reduce the Ammonia less than 10 mg/l also during winter times. Against this thereare many SBR Reactors with temperature problems during winter time so some leachateplants need heat from the gas motors to increase the temperature.

3. REASONS FOR POST TREATMENT WITH NANOFILTARION

In the tender a too high removal rate of the SBR reactor is expected. There are outletrequirements defined in the tender that a classic SBR system, even with very conservativedesign cannot keep. The following table lists the parameters COD, BOD, and ammonianitrogen. The expected outlet concentrations of the SBR reactor exceed the demanded acc. tothe tender values significantly. Also, the membrane bioreactor will not reach the requirements

in respect to the parameter COD. Fulfillment of the requirements can only be reached with afurther downstream membrane step. A single-stage nanofiltration is to be employed. Thenanofiltration ensures that the requirements for the COD can be also achieved. In principle, a



reverse osmosis would have been possible as well; the reasons for choosing a nanofiltrationare explained in Chapter 8.

Parameter Leachate

Inlet

Effluent values

From To Dischargelimits

SBR MBR MBR+NF

1 Temperature 0-352 pH 6,5 8,5 6,5-8,53 Suspended

mattermg/l 200 500 60

4 BOD 5 mg/l 500 1,750 25 <50 <25 <255 COD mg/l 2,000 2,650 125 <750 <500 <1256 NH4+ mg/l 200 800 3 <50 <3 <37 PO4-P mg/l 0 10 28 Cyanides mg/l 0 2 0,19 Sulphides mg/l 0 5 0,5

10 Sulfites mg/l 0 50 111 Sulphate mg/l 200 2,500 60012 Volatile

phenolsmg/l 0 200 0,3

13 Extractablesubstances

mg/l 0 100 20

14 Tensides mg/l 0 50 0,515 Plumb mg/l 0 1 0,2

16 Cadmium mg/l 0 1 0,217 Chromium total mg/l 0 1 118 Chromium + mg/l 0 1 0,119 Copper mg/l 0 1 0,120 Nickel mg/l 0 1 0,521 Zinc mg/l 0 1 0,522 Manganese

totalmg/l 0 5 1

23 Free chlorine mg/l 0 3 0,2

4. REVERSE OSMOSIS VS. NANOFILTRATION

4.1 Raw wastewater treatment, simple biological pre-treatment or efficientbiological pre-treatment – Which influence does the dimensioning of thebiological treatment have on the membrane plant operation?

WEHRLE generally sees the membrane technology (osmosis and nanofiltration) as post-treatment to an efficient biology. An efficient biology reduces the BOD and the NH 4 below 5mg/l. The biomass separation is carried out by using a membrane. Therefore, no filtratedmatter can pass into the membrane plant. This means that modules with a very highpacking density can be used. Consequently, the investment and operating costs for the

membrane plant can be reduced significantly.Competitors who offer their own membranes and modules reject the efficient membranebiology as pre-treatment since this would make the use of their usually overpriced own



better retention of low-molecular matters the reverse osmosis can also be used withoutbiological pre-treatment for low-loaded or medium-loaded wastewater.

4.5 Retention of ammonia-nitrogen (NH 4-N)

Ammonia nitrogen (NH 4-N) and ammonia (NH 3-N) are in dissociation equilibrium. Thenanofiltration neither retains NH 4-N nor NH 3-N. An osmosis can retain NH 4-N but NH 3-Nonly to a very small extent. In order to ensure retention, a pH reduction and, consequently,an adjustment of the equilibrium towards NH 4-N are necessary. If the pH values arebetween 6.0 and 6.5, retention of NH 4-N of > 90 % is possible.

4.6 Retention of nitrogen oxides (NO 2-N and NO 3-N)

A nanofiltration is not able to retain the NO 2-N molecule. The retention of NO 3-N is between30 and 60 %. An osmosis membrane is able to retain both compounds very well.

4.7 pH adjustment (acid dosing) in nanofiltration and reverse osmosis

In both processes, the pH should be reduced in order to avoid inorganic precipitation on themembrane. For the nanofiltration, depending on the concentration of the inorganic matter,the adjusted pH value is between 6.7 and 7.0. For an osmosis membrane, the pH value isusually adjusted at a slightly lower level (pH between 6.5 and 6.8).If there are high ammonia nitrogen concentrations in the wastewater, the reverse osmosishas to consist of several steps (two-step or three-step) or the pH has to be further adjusted(pH between 6.2 and 6.5).It has to be considered that the specific acid consumption is determined by the buffercapacity in the wastewater. For a lot of wastewaters, the high ammonia concentration iscrucial for the buffer capacity of the system. If the ammonia nitrogen is not decomposed ina biological pre-treatment, a significantly higher acid consumption to adjust the pH valuesmentioned above is necessary. Osmosis plants without nitrification as pre-treatment andwith disposal of the concentrate back to the landfill body are to be given special attention.By concentrating the ammonia in the circuit landfill – membrane plant and back, thenecessary acid dosing and, consequently, the increase of the salinity of the landfill body isbecoming higher and higher.

4.8 Yield factor of nanofiltration and reverse osmosis

Depending on the characteristics of the wastewater, the yield of the nanofiltration isbetween 85 and 95 %. The yield of an osmosis plant depends on the plant pressure and onthe concentration of the inorganic matter. For the treatment of saline wastewaters, such asleachate, the yield factor of an osmosis plant is between 55 and 70 %. A yield factor of 80% can only be achieved for very low-loaded leachate.

4.9 Membrane cleaning

Since the nanofiltration membranes in wastewater treatment can only be used after anefficient biological pre-treatment, the NF membrane surface is significantly less polluted. A

chemical cleaning is done every 2 to 6 weeks. An osmosis plant for raw leachate has to becleaned once a week. If there is an efficient biological pre-treatment, the cleaning intervalscan be up to 3 weeks.



4.10 Pressure in nanofiltration and reverse osmosis

Due to the low salt concentrations in the concentrate, the pressure in the nanofiltration

plants is significantly lower than in osmosis plants. The pressure for a nanofiltration ofleachate is between 12 and 25 bar. Depending on the membranes and the plantconfiguration, the pressure of the osmosis technology used for leachate treatment is up to50 bars/ 75 bars /90 bars /120 bars.

4.11 Energy consumption in nanofiltration and reverse osmosis

Due to the lower pressure, the energy consumption of the nanofiltration is at 2 to 3.5kWh/m3 inlet. For osmosis plants, depending on the number of steps and on the plantpressure, the energy consumption is at 6 to 15 kWh/m3 inlet.

4.12 Membrane lifetime and membrane replacement costs

Due to the less frequent chemical cleaning, the operating lifetime of nanofiltrationmembranes is between 2 and 4 years. For osmosis membranes without biological pre-treatment, the operating lifetime is between 0.5 and 1.5 years. The membrane operatinglifetime for osmosis plants with efficient biological pre-treatment is between 1.5 and 2.5years.

4.13 Colour retention with nanofiltration and reverse osmosis

The colour of the treated wastewater is very important for certain applications of industrialwastewater treatment. By using a process combination of membrane bioreactor withnanofiltration post-treatment, complete colour retention is achieved. The colour of thenanofiltration permeate is similar to the colour of reverse osmosis permeate.

4.14 Disposal of concentrate from nanofiltration and reverse osmosis plants backto the landfill

Should a disposal of concentrate resulting from osmosis plants back to the landfill body beallowed, it has to be considered how the wastewater constituents should be removed fromthe landfill body circuit and for which of them a further increase in concentration can be

expected during the operation of the plant. If these aspects are not taken intoconsideration, the concentrate disposal can have a negative influence on the operation ofthe membrane plant which might even have to be turned down after several years due totoo high costs. The following substance groups should be considered:

• Biodegradable carbon (BOD and partly COD)• Non-biodegradable carbon• Nitrogen compounds• Chlorides• Sulphates• Carbonates



The biodegradable compounds are well reduced biologically in the anaerobic bioreactor,i.e. the landfill, on a long-term basis. There is no increase in concentration of those. Thelandfill as anaerobic bioreactor, however, cannot reduce nitrogen. Without nitrification,stripping or another technical reduction, the concentrations will rapidly increase and,

consequently, have a negative influence on the operating costs. Sulphates and carbonatesas bivalent compounds will rapidly be reduced in the landfill body since it has to beexpected that the leachate contains sufficient counter ions. The chlorides are a veryproblematic issue since they are difficult to reduce in the landfill body. This is indicated bythe very rapid increase in conductivities and the chloride concentrations in the circuit landfill

– membrane plant and back. This increase could have the following negative influences onthe operation of the membrane plant:

• Yield reduction• Increase in acid consumption• Shorter membrane cleaning intervals and membrane operating time

With a process combination MBR with nanofiltration as post-treatment, the followingsubstance groups are removed from the circuit:

• Biodegradable carbon (BOD and partly COD) (in the membrane biology)• Non-biodegradable carbon (if necessary flocculation and precipitation or,

respectively, activated carbon)• Nitrogen compounds (in the membrane bioreactor)• Chlorides (discharge via the permeate)• Sulphates (discharge via the permeate/precipitations in the landfill body)• Carbonates (discharge via the permeate/precipitations in the landfill body)

A process combination of MBR with nanofiltration post-treatment and, if necessary, aphysical-chemical concentrate treatment enables an inclusion to the landfill body withouthaving to fear negative influences.

4.15 Treatment of concentrate from nanofiltration and reverse osmosis plants

Concentrates from nanofiltration plants with biological pre-treatment are loaded withpersistent organic (“hard COD”) and inorganic matter. Organically bounded- and ammonianitrogen only exist in low concentrations since they have been removed to a large extent inthe biological pre-treatment. In order to treat the COD from the concentrates of NF plants,different process technologies can be used. In the mid-1990s, concentrates were for thefirst time treated by using chemical oxidation and/or activated carbon in Germany. At themoment, three plants are being operated with activated carbon treatment in Germany. Theactivated carbon has proved itself because of its simple operation which is less susceptibleto malfunctioning. Due to the high COD concentrations, high loading on the activatedcarbon is achieved. Since 2005, several plants with flocculation and precipitation steps forthe treatment of concentrate are being operated in Asia. The reduction of the COD in theNF concentrate does not achieve the concentration levels of an activated carbon plant.However, 50 to 60 % of the non-easily degradable COD compounds can be reduced. Theconcentrates treated by using activated carbon can be completely led back into the biologydue to the high COD elimination degree. Depending on the COD removal, the concentratestreated by using flocculation and precipitation can be partly led back into the biology. If the



reduction of the “hard” COD is incomplete, either a powdered carbon dosing has to becarried out or a rest of the concentrates has to be disposed back to the landfill body. Due tothe difference in pH value between the pH value of the concentrate (6.0 to 7.0) and the pHvalue in the landfill body (7.5 to 8.0), the COD compounds are fixed by precipitation of the

bivalent sulphate ions in the landfill body. Apart from the described possibilities ofconcentrate treatment, the nanofiltration concentrates can be further concentrated to aminimal volume by osmosis plants with higher pressure levels.

In leachate treatment, the concentrates from osmosis plants have the followingcharacteristic: The inorganic matter is concentrated up to the limits of their solubilityproduct. Due to the high salt concentrations and the precipitation issue, physical-chemicalprocesses to reduce the organic compounds in osmosis concentrates cannot be used.

Another possibility to further reduce the concentrates is the thermal treatment byevaporation. This technology is used when the disposal of the concentrates back to thelandfill body is not approved. Due to the high operating costs and the very problematicoperation, only few evaporation plants to reduce the concentrate quantities are beingoperated in Europe. Evaporation plants do not achieve a stable discharge, either. Thehighly concentrated, liquid evaporation rest has to be further disposed of.

Reasons to Choose MBR+NF Technology

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