monitoring of digesters in biogas plants

8/10/2019 Monitoring of digesters in biogas plants

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Analytical control of fermentationprocesses in biogas plants

Introduction Against the background of finite fossilfuels and the contentious use of nuclearenergy, renewable sources of energy aresteadily gaining in importance. Leadingthe way is the use of methane gasobtained from fermentation processesin biogas plants. Plant operators inGermany receive a high return for feedingelectricity generated from renewable rawmaterials into the national grid up to0.18 per kWh.

All plant operators therefore have avested interest in running their biogasplant as efficiently as possible. Ifexcessive amounts of biomass are fedinto such plants however, this may havedrastic economic consequences and mayeven inactivate the biomass, necessitatinga cost-intensive restart. On the other

BIOGAS_INTRODUCTION

hand, long-term underloading of a plantalso has financial consequences, as lesselectricity and heat are generated andrevenue is therefore lost.

A precise and reliable analysis ofthe fermentation processes usingphotometric cuvette tests, easy-to-usetitrators and process measurementtechnology for online monitoring ensuresstable process management costeffectively.

Structure of a biogas plantIn a biogas plant, natural fermentationand decomposition processes producebiogas, which is used to generateelectricity as efficiently as possible [2].

In the first phase the substrate ismade available, stored and treated inaccordance with requirements and

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fed into the bioreactor (Fig. 1). In thesecond phase, anaerobic fermentationprocesses take place in the digester,producing biogas. In the third phase thegas is treated, stored and utilised. Finally,in the fourth phase, the fermentationresidues are utilised (e.g. as fertiliser in theagricultural sector).

The main component of biogas ismethane (about 50-70 %). One cubicmetre of biogas contains about sixkilowatt hours of available energy and isequivalent to about 0.6 litres of fuel oilin terms of its average calorific value [3].

The heat generated during its combustionis fed into the fermentation process asprocess heat or used to heat on-siteliving and working quarters and livestockbuildings or sold to external customers(e.g. operators of local heat networks).

Fig. 1: Design of a typical biogas plant (CHP = combined heat and power) Eluate analysis: Ntot , COD, NH4, heavy metals Digester analysis: NH4, org. acids, COD, acid capacity Online process analysis: pH, temperature, redox and TS

Co-substrates

Solids feeder

Stirrer Stirrer

Main digester Post-fermentation tank Membrane-coveredgas holder

Fermentationresidue storage

Co-substrates

Stirrer

Mixing pit

Buyer: thermal energyHeat

Condensateseparator

Heat

B i o g a s Block CHP unit

Engine Generator

Electricity

Connection togrid

Biogas Biogas

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There are basically two types of plants inwhich wet fermentation processes takeplace: plants that use renewable rawmaterials and co-fermentation plants. Theformer use renewable raw materials suchas maize, grass, complete cereal plantsand grains, sometimes together withmanure slurry.

In co-fermentation plants, substratesof non-renewable raw materials are used,such as residues from fat separators,food residues, flotation oil, industrialwaste products (glycerol or oil sludge)and domestic organic waste. Manureslurry is also usually used.

Biogas is produced by a highlysensitive and complex process. Withoutinstrumentation, biogas plants are oftenunderloaded, i.e. the biomass feed rateis too low, so that electricity generationisn't cost effective. This can resultin substantial financial shortfalls (see

Table 1, [4]). The effects of overloading

are especially drastic. Such unintentionaloverfeeding slows down or stops thebiological fermentation process and maycause a total system breakdown. A cost-intensive plant restart is then necessary.

The anaerobic fermentationprocess

The fermentation of biomass is a fourstep anaerobic digestion process, which

is brought about by the complementaryactivities of several species of bacteria.

The first step is hydrolysis (Fig. 2).First of all, long chain substances,carbohydrates, proteins and fats are

broken down into smaller fragmentssuch as simple sugars, glycerol, fattyacids and amino acids. In the secondstep (acidification, acidogenesis),fermentative microorganisms convertthese products into short chain fattyacids such as acetic acid, propionic acidand butyric acid. Lactic acid, alcohols,hydrogen and carbon dioxide are alsoformed. The third stage of acetic acid

formation (acetogenesis) combinesthe prior acidification with methaneformation. The starting substrates area number of final products from theacidification phase, i.e. short chain

fatty acids, propionic acid, polymersubstrates (carbohydrates, fats,proteins) and butyric acid. Togetherwith lactic acid, alcohols and glycerol,these substances are converted by theacetogenic microorganisms into aceticacid, hydrogen and CO 2. In the finalstep, methane is formed. The methanebacteria produce biogas, which containsup to 70 % methane.

All the described processes runalmost simultaneously in a biogasplant. They are in a sensitive state ofequilibrium, which is dependent on thepH and temperature. Changes may havea negative effect and drastically disruptthe total biogenic process.

Fig. 2: Anaerobic digestion steps resulting in the production of biogas [5]

Polymer substrates(carbohydrates, fats, proteins)

Fragments and dissolved polymers

Biogas

Hydrolysis

Acetogenesis

Methanogenesis

H2 CO2 Org. acids Alcohols Acetic acid

Acidification

Acetic acid

Lost income due to underloadingUnit Plant A

(underloading)Plant B

(comparison)Energy yield % 80 90Energy production per year kWh 3504000 3942000Energy revenue (9.9 ct/kWh) /a 346896 390258Renewable raw materials bonus (6 ct/kWh) /a 210240 236520Bonus for utilisation of heat (2 ct/kWh) /a 70080 78840Revenue per year /a 627216 705618Revenue per month /a 52268 58802

Tab. 1: Loss of income due to underloading, as shown by a 500 kWel. biogas power plant


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ensure the trouble free operation of abiogas plant.

Process monitoring in practice The suitability of the process probes wastested in a biogas plant in north Hesse,Germany. The parameters temperature,total solids, pH and redox weremeasured [7].

pH/Redox measurement The digital pHD electrode used to measurepH and redox is fully encapsulated sothat it does not come into contact withthe fluid being measured. A special,soil resistant salt bridge forms thedirect contact to the fluid to enable themeasurements to be made. In contrastto conventional membrane basedelectrodes, this electrode can be usedfor very long periods even in fluids witha high particulate content, e.g. digesterwater. The intervals between cleaning

are especially long. Electrode poisoning,e.g. by any H 2S that may be present, is

COD The chemical oxygen demand is theamount of oxygen required to oxidisethe oxidisable components of thefermentation substrate. All oxidisableorganic compounds are totallychemically oxidised to CO 2 and H 2O.COD is a reliable indicator of the energypotential of a fermentation substrate.

Organic acids/Fatty acidsLow molecular fatty acids, in particularacetic acid, propionic acid and butyricacid are formed during the first andsecond steps of the fermentationprocess. If the fermentation processis proceeding efficiently, the values forthese compounds, expressed as anequivalent amount of acetic acid, liebetween 500 and 3,000 mg/l. In thiscase, the processes in the digester, theproduction of acid by hydrolysis and thedegradation of acid by methanisation are

in equilibrium. If the acid concentrationrises above 10,000 mg/l, the pHusually falls below 7. According toBischofsberger [6], a drop in pH to6.4, at an acetic acid concentrationof 1,000 mg/l, reduces the methaneconcentration by half. Acetic acidconcentrations below 1,000 mg/l shouldbe determined by the titrimetric VOA/

TAC method.

AmmoniumDuring the fermentative degradationof protein rich substrates in particular,e.g. grass silage or chicken droppings,high concentrations of ammonium ionsmay be generated. Ammonium existsin a pH dependent equilibrium withammonia, which is toxic to the bacteria.If the pH increases the equilibrium shifts,favouring ammonia. Regular checks ofthe ammonium content of the digester

prevented and dilution of the electrolytes isavoided.Figure 4 shows examples of time-coursecurves obtained from pH and redoxmeasurements. Through appropriatepositioning in the control zone of a pumpstation, the conditions in the digesteror post-digester can be accuratelydetermined and monitored. During thetest period (8 months), the pH readingsremained in the range from 6.9 to 7.4. Inthe post-digester the average was 7.38,with a minimum of 7.20 and a maximum of7.50. These results confirm that the pH ina digester is not constant, but undergoesfluctuations. A similar result was obtainedfrom the readings of the redox potential.

The average value was -392 mV, withfluctuations between -250 and -476 mV.

These values show that fermentationprocesses must be properly understoodand closely monitored to ensure optimaland therefore profitable operation of a

biogas plant.

Online measurement of pH and redox potential

Fig. 4: Measurements from the pressure pipe line of a biogas plant digester in north Hesse,Germany

Redox [mV] pH

Redox potential pH


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BIOGAS_PROCESS MONITORING

Process monitoring in practice

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Total solids (TS)

The time course curves shown inFigure 5 were produced from some ofthe comparative measurements carriedout in a biogas plant over a period ofsix months. The total solids contentwas determined precisely by a SOLITAXhighline sc probe, which uses a colourindependent combined infrared dualscatter light measurement method. Inparallel to the online measurements,the TS content was determinedgravimetrically in an external referencelaboratory. Due to the varying particlesize distribution in the sludge, a plant-specific correction factor has to be usedto calibrate a TS probe. The determined

TS values were in the range from 3.8to 10 % and the results obtained in thelaboratory were slightly higher thanthe equivalent results of the onlinemeasurements carried out on site. Both

time course curves accurately reflected

the fluctuations in the TS content withinthe fermentation process and thereforeenable the plant to be monitored andprecisely controlled.

Monitoring digesters withphotometric tests

The most important parameters in thefermentation process, which can bemonitored using chemical or photometricmethods, are the organic acids formedas intermediates, the COD and theammonium concentration. Until now,the necessary measurements havebeen carried out in external servicelaboratories, with the associated highcosts and sometimes considerabledelays between sampling and theavailability of the results.

A late response to negativeprocesses in the digester may enableconsiderable inhibition of the biogasproductivity to occur, even to the extentthat the biomass may be inactivated,bringing the total plant to a halt.

For this reason, it is advisable tomonitor the digester directly and asquickly as possible with photometric

cuvette tests, which have been regardedas the state of the art in wastewatermonitoring for several years.

The suitability of photometriccuvette tests for monitoring organicacids, ammonium and COD wastested on numerous samples from aco-fermentation biogas plant (1.5 MW el. )in Lower Saxony. Reference analyseswere carried out by the contractlaboratory of a nearby mechanicalbiological waste treatment plant. Adecisive factor for the assessment wasthe comparability of the results to thoseobtained using corresponding standardmethods and the consistency of theresults obtained from diluted and spikedsamples.

Samples taken from a digester mayremain biologically active. The possibilityof post sampling formation of aceticacid in the samples cannot therefore beexcluded.Fig. 6: Photometric measuring station: HT 200S

thermostat, cuvette test, DR 2800 photometer

Comparative measurement Total solids

Fig. 5: Comparative measurement of digester and post digester samples: SOLITAX highline versusgravimetric laboratory analysis (filterable substances)

TS [%] Laboratory analysis SOLITAX highline sc

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For the comparability of the results of thetwo methods it was especially importantthat the period between on-site analysis(using the cuvette test) and laboratoryanalysis (using the standard methodwas as short as possible). Besidesthe samples from the co-fermentationbiogas plant, numerous samplesfrom the mechanical-biological wastetreatment plant were analysed.

In addition, the concentrations of theorganic acids in digester samples fromvarious biogas plants were analysedusing ion chromatography (IC) and thefast photometric test. The sum of theindividual components determined usingIC was compared with the total organicacids determined with the photometrictest.

Sample preparationColourless, clear solutions are requiredfor photometric tests. For this reason

it is important that the preparation ofthe digester samples, which have ahigh particulate content, is carried outcarefully and in line with good practice.

The samples from the digester andpost-fermentation tank were diluted inthe ratio 1:20. To do this, the tip of asingle-use pipette tip was cut off andthe appropriate volume was added stepby step (25 ml, 5 x 5 ml) to a 500 mlround flask. The diluted sample was thensplit up and passed through a 0.45 mpolycarbonate membrane filter undera pressure of 6 bar in the referencelaboratory. The standard analyses werecarried out on the filtrate. For eachphotometric cuvette test, 20 ml of thefiltered sample was centrifuged in a tablecentrifuge for 10 minutes at 13,500rpm, providing approximately 20 mlclear and almost colourless centrifugate,with which the cuvette tests were thencarried out.

Dilution series for LCK 365 Organic acids

Fig. 7: Dilution series for checking the plausibility of the measurements of organic acids in a postdigester sample

Acetic acid equivalents [mg/l]

Dilution

Plausibility of the measurements There are two relatively simple methodsof determining the plausibility of the

results: dilution and spiking [8]. For thefirst of these, a dilution series is preparedfrom the measured original sample. Ifany interference is present in the originalsample, its effect is weakened as thelevel of dilution of the sample increases.

The concentration of the original samplecan be calculated from the measurementresults obtained from the dilutions. Thepresence of any interference is revealedby a significant deviation between theconcentration of the undiluted originalsample and the concentrations calculatedfrom the measurements of the dilutionseries. If there is virtually no deviation, nosample specific interference is present.

The spiking method requires steadilyincreasing concentrations of a standard tobe added to the measured sample. Theconcentration determined by measuring

the spiked samples, (corrected to takeaccount of the dilution factor), shouldcorrelate to the known concentration of

added standard.Figure 7 shows the results obtained

from a dilution series prepared from a postdigester sample from a biogas plant duringthe course of long-term comparativemeasurements. Except for the resultfrom the 1:10 dilution, all the results arewithin the range of tolerance, which ishigher for samples with a high particulatecontent than for samples that contain noparticles. This is a consequence of theunavoidable dilution error. The cause of thelow result obtained from the 1:10 dilutionis unknown, but may be associated withthe above mentioned uncertainty of thevolume reduction. The presence of anyinterference can be excluded, as thefollowing dilution steps did not confirm thistrend.


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BIOGAS_COMPARATIVE MEASUREMENTS

Comparative measurements

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The spiking test was carried out usinga sample from the post digester of aco-fermentation biogas plant. Glacial

acetic acid was used as the standard,which was diluted with distilled waterto a concentration of 1,000 mg/l. Thelowest and highest spiking values werethose of the distilled water and thedilute solution of glacial acetic acid. Theplot of measured values against theexpected values gives a straight line witha correlation of 0.999. This almost linearrelationship indicates that no interferenceis present in this sample (see Figure 8).

Comparative measurements photometric and standardmethods

The comparative measurements fromthe photometric cuvette tests and thecorresponding standard methods werecarried out in the operations laboratoryof a mechanical biological wastetreatment plant and the on-site referencelaboratory.

The organic acids were measuredusing the German standard methodDEV H21. The operations laboratory

used the LANGE cuvette test LCK 365Organic acids [9]. The hydrolysis neededin order to determine the acetic acidequivalents was carried out by heatingthe sample for 10 minutes at 100 C ina preheated dry thermostat. The clearand colourless sample was then cooledto room temperature before beingevaluated with the cuvette test. As withall photometric methods, particlesand strong colours can influence themeasurement result and must thereforebe removed if necessary, e.g. by passingthe sample through a 0.45 m filter ordiluting it still further.

The ammonium concentration ofa sample from a post digester wasdetermined with the LANGE cuvettetest LCK 302 [10] and the photometricDEV E5 method.

The COD (chemical oxygen demand)was determined with the German

standard method DEV H44 and theLANGE cuvette test LCK 514 [11]. Thetwo methods use the same reagentsbut differ in terms of the digestion time.

A special high-temperature thermostat(HT 200S) reduces the usual 2 hourdigestion period to 15 minutes.

Figure 9 shows the time coursecurves of the almost simultaneouscomparative measurements obtainedwith the cuvette test and the standardmethod. Due to the predefinedmeasuring range of the cuvette testsand the relatively high concentration ofthe measured parameters, the samplessubjected to the photometric tests hadto be diluted. The COD samples andthe samples for the determination of theorganic acids were diluted in the ratio1:20, centrifuged and measured. A fewammonium samples still had some slightintrinsic colour after being centrifuged,and therefore had to diluted still further

(1:50). The comparative measurements

were carried out over a period of severalmonths. In the case of the ammoniummeasurements and the determinationof the organic acids, the correlationbetween the measurements obtainedwith the two methods was very similarin terms of their accuracy. Both cuvettetests are therefore suitable for monitoringthese very important control parametersin biogas processes.

The results of the COD methodsdiffer from each other somewhat, i.e.when they are plotted, one curve isabove the other. They neverthelessaccurately reflect the changes inconcentration of the organic load in thedigester and post digester. In this case aplant specific correction factor should beintroduced.

Spiking test with acetic acid

Fig. 8: Spiking test to check the plausibility of the measurements of organic acids in a post-digester sample

E x p e c

t e d v a

l u e o n

b a s i s o

f d i l u

t i o n

/ s p

i k i n g

[ m g

/ l ]

Dist. water Dilution/spikin Linear (dilution/spiking)

Standard

Average value of the analysis [mg/l]

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Comparative measurements cuvette test and standard analysis

Fig. 9: Comparative measurements photometric cuvette test (red) and standard method (blue) post digester sample from a biogas plant

[mg/l]

COD

NH4-N

Org. acids

Ion chromatography (IC)measurements

The cuvette test for organic acidsand the standard method (steamdistillation and titration of the acetic acidequivalents) both determine the sumof the individual substances. They donot give the concentrations individualsubstances, e.g. propionic acid, butyricacid or acetic acid. In order to obtainthe concentrations of these substancesand to clarify whether the sum of theirconcentrations correlates with theresults obtained using the cuvette testand steam distillation, a number ofdigester samples were also subjectedto ion chromatography and the resultswere compared with those of the fastphotometric test.

In view of the lower total solidscontent of the samples (clearly below10 %), sample preparation was mucheasier than for the previous comparative

measurements. Simple filtration througha folded filter usually sufficed.

The measurements were carried outby Ingenieurbro Stahmer, Bremen. Itanalyses up to 100 digester sampleseach week for ammonium nitrogen,COD and organic acids. The ionchromatogram for sample I shows a totalof four individual organic substances inthe digester sample: acetate, lactate,propionate and n-butyrate. The sumof the individual concentrations is2,234.73 mg/l organic acids (Tab. 2).

The photometric test gave a sum of2,530 mg/l. The difference in the resultsfor this sample is therefore 13 %. Giventhe already mentioned dilution errorassociated with substances containinga high level of solids, this is a very goodvalue. The following ion chromatographicanalyses gave much smaller differences.

Ion chromatographic analysis

Fig. 10: Ion chromatogram of a digester sample showing the individual components acetate,lactate, propionate and N-butyrate


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BIOGAS_MEASUREMENT TECHNOLOGY

Measurement technology for biogasplants

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Self cleaning SOLITAX highline sc probe fordetermination of the solids content

Wiper

Sample con-taining solids

These results demonstrate that theresults obtained using the cuvette testfor the determination of organic acidscorrelate well with the results of the ICstandard method and that the sum ofthe individual substances determinedusing the IC method also correlates very

well with the result of the photometrictest and the steam distillation.

Summary The degradation processes incommercially operated biogas plantstake place in a sensitive microbiologicalsystem. High, profitable gas yields and a

pHD sc sensor for pH or redox measurement

Safety fitting for mounting the SOLITAXhighline sc

fermentation product that can be readilyused for agricultural purposes canonly be obtained if the plant process iscontrolled.

The described online measurementtechnology for metering pH, redox,total solids and temperature, together

with the photometric cuvette tests usedin operations laboratories, allow costeffective, precise, real time monitoring ofthe fermentation process so that biogasplants can be efficiently controlled. Therequired investment is relatively low,compared with the risks associated withan uncontrolled fermentation process.

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Components Concentration [mg/l] Organic acidsFast test

[mg/l]

Difference [%]

Sample IAcetateLactatePropionaten-ButyrateTotal

1768.781.63

296.488

2234.73 2530 13Sample II 1227.45 1340 9Sample III 3937.93 4130 5Sample IV 1941.69 2070 7

Tab. 2: Ion chromatography results for individual components and the results of the photometric fasttest


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DR 2800 spectrophotometer with various water

analysis reagents

Dry thermostat for all standard digestions

LCK 365 cuvette test LCK 365 for determiningorganic acids

Measuring station for laboratory analysisDR 2800 Compact, powerful spectrophotometer with a wavelength range from

340 to 900 nm for routine analysis and user applications; barcodereader (IBR) for automatic evaluation of cuvette tests; backlit graphicdisplay with touchscreen menu guidance; mains and battery operation

LT 200 Dry thermostat for standard and special digestions; preprogrammed fordigestion for the analysis of COD, total N, total P, TOC, organic acidsand metals

Cuvette tests Ready-to-use reagents for maximum user safety; highly precise;approved methods; more than 80 parameters and measuring ranges

Installation for online measurement in central pumping lineSOLITAX highline sc Stainless steel probe with combined infrared absorption scattered light

process for in-pipe measurement of coloured liquids and sludge.Measuring range: 0.001....150.0 g/l TSArt. no. LXV 424.99.00200Accessories: Safety fitting LZX 337 (can be mounted/dismounted with-out emptying the pipe)

pHD sc pH Rugged, digital differential pH probeProcess probe; material: PEEKMeasuring range: 0...14 pHArt. no. DPD2P1.99Accessories: insertion mount for pHD, stainless steel, 6136800

pHD sc ORP Rugged digital differential redox probeProcess probe; material: PEEKMeasuring range: -2000...2000 mV Art. no. DRD2P5.99Accessories: insertion mount for pHD, stainless steel, 6136800

SC 1000 Controller A SC 1000 digital controller system consists of a single LXV 402 displaymodule and one or more LXV 400 probe modules. The controller ismodularly configured in accordance with the customers specificationsand can be expanded by additional measuring stations, sensors, inputs,outputs and BUS interfaces at any time. Each module controls up toeight sensors

AlternativeSC 100 controller Controls up to two sensors


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Literature

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BIOGAS_LITERATURE

[8] Analytische Qualittssicherung inder Betriebs-Analytik, Anwendungs-bericht Ch. No. 52, HACH LANGE,1997

[9] Working procedure for the LCK 365organic acids from HACH LANGE

[10] Working procedure for cuvette testLCK 302 Ammonium, HACH LANGE

[11] Working procedure for cuvette testLCK 514 COD, HACH LANGE

Literature[1] Erneuerbare-Energien-Gesetz (EEG)[2] Nawaro kommunal, Ausgangsmateri-

alien fr die Biogasnutzung, 2004.[3] Wikipedia, freie Enzyklopdie, 2005[4] Instrumentation, Control and Auto -

mation for full-scale manure-basedbiogas systems, Dr. J. Wiese * andDr. M. Haeck, IWA publications 2006

[5] Biogasanlagen in der Landwirtschaft, AID Infodienst Verbraucherschutz,2005

[6] Bischofsberger, W. et al.: Anaerob-technik, Heidelberg 2005

[7] Dr. Jrgen Wiese, EnerCess GmbH,Robert-Bosch-Str. 7, 32547 BadOeynhausen, Tel. 05731/15339-0,Fax 05731/15339-30,email [email protected],Internet www.enercess.de

Automatic titrator for the volumetricdetermination of organic acids

Cuvette test LCK 114 for the determinationof COD

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www.hach-lange.com up to date and secure, with down-loads, information and e-shop.

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Quality assurance, complete withstandard solutions, instrumentchecks and test solutions.

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monitoring of digesters in biogas plants

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