[Abstract] A simple, tubular biotrickling filter was designed for optimal removal of ammonia and odour in ventilation air from a pig house. The removal and transformation of ammonia was studied in detail by analysis and modelling of chemical gradients through the filter. Good correspondence between measurements and model was obtained by using conventional substrate and inhibition kinetics of ammonium and nitrite oxidizing bacteria. Highest rates of ammonia removal were observed in the central section of the filter. Near the air outlet and water inlet the process was ammonia limited, while high nitrous acid concentrations almost excluded any biological activity near the air inlet and water outlet. Nitrous acid inhibition also stabilized pH at 6.5-7 all through the filter. Being sensitive to both ammonia and nitrous acid the nitrite oxidation process occurred mainly in the filter sections near the air outlet / water inlet, and only 8% of the nitrite was turned into nitrate. Water supply only exceeded evaporation by 20% but modelling indicated that additional watering would have limited effect on filter efficiency. The filter was also robust to varying loading, as a 4-fold increase in ammonia inlet concentration only reduced filter efficiency from 86 to 76%

Andersen, Mathias

Nielsen, Anders M.

Nielsen, Lars Peter

Nielsen, Marie Louise

Repositorio da Universidade da Coruña

263Ammonia transformation in abiotrickling air filterLARS PETER NIELSEN, MARIE LOUISE NIELSEN, MATHIAS ANDERSEN,AND ANDERS M. NIELSENDepartment of Biological Sciences, University of Aarhus, Ny Munkegade 1540,8000 Aarhus C, DenmarkABSTRACTA simple, tubular biotrickling filter was designed for optimal removal of ammonia and odour inventilation air from a pig house. The removal and transformation of ammonia was studied in detailby analysis and modelling of chemical gradients through the filter. Good correspondence betweenmeasurements and model was obtained by using conventional substrate and inhibition kinetics ofammonium and nitrite oxidizing bacteria. Highest rates of ammonia removal were observed in thecentral section of the filter. Near the air outlet and water inlet the process was ammonia limited,while high nitrous acid concentrations almost excluded any biological activity near the air inlet andwater outlet. Nitrous acid inhibition also stabilized pH at 6.5-7 all through the filter. Being sensitiveto both ammonia and nitrous acid the nitrite oxidation process occurred mainly in the filter sectionsnear the air outlet / water inlet, and only 8% of the nitrite was turned into nitrate. Water supply onlyexceeded evaporation by 20% but modelling indicated that additional watering would have limitedeffect on filter efficiency. The filter was also robust to varying loading, as a 4-fold increase inammonia inlet concentration only reduced filter efficiency from 86 to 76%.1 INTRODUCTIONAmmonia emitted from animal facilities is a major contributor to acidification andeutrophication of terrestrial and aquatic environments (McCrory and Hobbs, 2001).Significant reductions of both ammonia and door emissions can be accomplished byuse of biological trickling filters. The good results, however, are not sufficientlyreproducible, and further optimization of design and operation is required for morewidespread application.263264Ammonia is a highly water soluble gas (62 M/atm at 20 °C) that is readilyprotonized to ammonium (NH4+) in water. In biofilters ammonia is oxidized to nitrite(NO2-) by ammonium oxidizing bacteria (AOB) and further to nitrate (NO3-) by nitriteoxidizing bacteria (NOB). The overall removal efficiency is the result of complexinteractions of ammonia load, filter surface area, air and water flow, temperature,nitrifier biomasses, and inhibition kinetics. To resolve this we analyzed chemicalgradients in a simple counter-current biofilter and simulated the results with amathematical model. Long term development of nitrifier biomass and the impact ofnitrification processes on the removal of organic odorants are two important aspectsthat will not be addressed in this paper.2 MATERIALS AND METHODS2.1 TUBULAR BIOTRICKLING FILTERThe first, small version of the tubular biotrickling filter optimized for studiesof function is shown in Figure 1 (M. Andersen, DK Patent no. 3108_06). Contaminatedair is lead into the lower end of 3 independent tubes being 5.5 meter long and with across-section area of 11 cm2 each. The straight airflow gives a good gas-filter contactin relation to pressure drop and minimizes the risk of clogging. The inner surface ofthe tubes is covered with a thin layer of a fibrous material through which the waterslowly percolates driven by gravity. The biofilm developing on the surface of theconductive layer is thus moisturized with fresh water from inside and in direct contactwith the air stream outside.  This almost eliminates a liquid film diffusion barrier thuspromoting the removal of airborne contaminants that are not easily soluble in water.Details on NH3 load and air and water dynamics during the experimental run aregiven in Table 1.2.2 ANALYTICAL METHODSAmmonia in the air was sampled in acid solution and analyzed byspectrophotometer (Bower and Holm-Hansen, 1980). Water was sampled with filterpaper sticks through the sampling ports, and after dilution NO3- and NO2- was analyzedby HPLC and NH4+ as above. pH was determined by pH sticks. Concentrations ofHNO2 and NH3 were calculated from pH and concentrations of NO2- and NH4+ usingpKa values of 3.4 and 9.4 respectively. Evaporation was calculated from theconcentration gradient of a bromide tracer added to the water supply and analyzed byHPLC.L. PETER NIELSEN, M. L. NIELSEN, M. ANDERSEN AND A. M. NIELSEN265Figure 1. The experimental tubular biotrickling filter. The 5 sample points divide the filterin 4 sections of equal length.AMMONIA TRANSFORMATION IN A BIOTRICKLING AIR FILTER266Table 1.Parameters of the model and the experimental filter. Values apply to one filter tube. The termsAOB and NOB stands for ammonium and nitrite oxidizing bacteria, respectively.Parameter ValueInlet NH3 concentration 8.6 ppmAirflow 19400 L/hWater flow 63 mL/hAir volume per section 4.65 LWater volume per section 23 mLEvaporation, section 3 23 mL/hEvaporation, section 4 30 mL/hAir/water mass transfer coefficient per section 5 mL/sSubstrate limitation factors C/(C+Km)NH3 inhibition factor for AOB e(-C/Ki)Other inhibition factors 1/(1+(Ci/Ki)2)Km of NH3 for AOB 786 μMKm of HNO2 for NOB 114 μMKi of NH3 for AOB 2000 μMKi of NH3 for NOB 168 μMKi of HNO2 for AOB & NOB 1.14 μMAOB capacity 50 x NH3 loadNOB capacity 20 x NH3 loadModel iteration frequency 100/sModel runtime 20 h2.3 MODELThe mathematical model was programmed in JAVA and followed in most aspectsa new, more general biofilter model (Nielsen et al., in preparation). The model filterwas divided in 4 sections according to Figure 1 with fully mixed air and water phasesand fixed biomass. In each iteration the changes of gaseous NH3, total NH4+, totalNO2-, and total NO3- were calculated from air/water NH3 mass transfer, nitrificationrates and water and air transport between sections. Subsequently the pH was calculatedby solving a charge balance equation using a two-step Newton-Raphson approximationas described by Volcke et al. (2005). Inlet ammonia concentrations, air and waterflow and evaporation were set to observed values in the experimental filter (Table 1).Compared to NH3 the concentration of organic compounds in the ventilation air wasan order of magnitude lower and transformations of organics were ignored in thepresent model version. Model parameters, kinetic equations, and initial values areL. PETER NIELSEN, M. L. NIELSEN, M. ANDERSEN AND A. M. NIELSEN267Figure 2. Concentration gradients from the water inlet at the top of the filter to the air inlet at thebottom showing NH3 in the air (a) and NO3-, NH4+, NO2-, HNO2, NH3, and pH in the water (b-g).Lines are model simulation results and symbols are measured averages with standard errors (n=3).AMMONIA TRANSFORMATION IN A BIOTRICKLING AIR FILTER268listed in Table 1. Many different kinetic constants are found in the literature, but thegeneral patterns of the model output were quite robust to variations in kinetic constants.The chosen values were mainly derived from Anthonisen et al. (1976). Notice that theuncharged species NH3 and HNO2 and not NH4+ and NO2- were considered the realspecies for substrate uptake and inhibition of nitrifiers (Anthonisen et al., 1976).Chemical equilibrium constants for the inorganic nitrogen and carbon species at 20 oCwere obtained from table values.3 RESULTS3.1 CHEMICAL GRADIENTSOf the water supplied 86% evaporated in the two lower sections (Table 1).Good correspondence between measured and modelled chemical gradients wasobtained after proper adjustment of ammonium oxidation capacity and the results areshown together in Figure 2. Ammonia content of the air declined throughout the filterto an outlet concentration of 1 ppm corresponding to 85% overall removal. Most ofthe removal, 78%, occurred in the two central sections while the upper and lowersection only accounted for 21 and 1% respectively. Concentrations of NO2- and NH4+increased down the filter with final steep increases up to about 300 mM in theevaporation zone (Figure 2c-d). Concentrations of NO3- were much lower and bothmeasured and simulated data showed that virtually all nitrite oxidation occurred in theupper two sections (Figure 2b). The discrepancy between simulated and measuredNO3- concentrations could be ascribed to under estimation of the nitrite oxidationcapacity. Despite the absence of any chemical buffer or pH regulation, the pH valuesremained between 6.5 and 7 (Figure 2g). By comparison with the Km and Ki valuesthe concentrations of free NH3 and HNO2 indicated strong substrate limitation of AOBin the upper sections and strong HNO2 inhibition of both AOB and NOB in the lowersection (Figure 2e-f, Table 1).4 DISCUSSIONThere are many ways to examine the regulation of ammonia transformations in abiological airfilter, and some interesting points can actually be derived without runningany models or experiments.One point is that if the maximum capacity of the ammonium oxidizer biomassexceeds the NH3 load, the process will have to be restricted accordingly by NH3limitation and/or inhibitors. As long as the overall NH3 removal efficiency is good, ahigh inhibitor level is therefore not an indicator of a critical filter situation but ratherL. PETER NIELSEN, M. L. NIELSEN, M. ANDERSEN AND A. M. NIELSEN269an indicator of excess nitrifier biomass. In the present case the maximum capacity inthe model was set to 50 times the NH3 load, and therefore the kinetics somehow hadto reduce ammonium oxidation to less than 2% of the capacity. The results showedthat this was accomplished mainly by HNO2 inhibition in the lower half of the filterwith concentrations up to 170 times the inhibition constant, and by NH3 limitation inthe upper end of the filter with concentrations 15-80 times lower than the Km value(Figure 2e-f, Table 1). The point that inhibitor level and substrate limitation is essentiallydetermined by the biomass/loading ratio has other perhaps counter-intuitiveimplications; including that inhibitor level must decrease significantly with higherNH3 loading but not with enhanced watering. Model perturbations indeed showedthat a 6 times increase in NH3 inlet concentration only reduced filter efficiency from86 to 76% (Figure 3a).Another general point is that in time only half of the NH3 taken up in a biofilterwill be oxidized, while the other half will remain in solution as NH4+ (Smet et al.,2000). This is simply because NH4+ in reality is the only cation available to balancethe produced anions NO2- and NO3- when concentrations are up in tens to hundreds ofmM N. The measurements confirmed this by perfect charge balances, i.e. 310 mMNH4+ versus 320 mM NO2- + NO3- in the outlet (Figure 2). Another way to address thegeneral point of half-way nitrification is to consider pH: If the nitrifiers tried to generatejust 1 mM NO2- + NO3- more in excess of NH4+ the anion excess would have to bebalanced by H+, thus implying a pH drop to around 3 which would stop the bacterialong before. In the present filter with significant NO2- accumulation, the immediateeffect of lowering pH was the protonization of NO2- to form the highly inhibitoryHNO2. In that way kinetics of HNO2 inhibition of ammonium oxidation served as aFigure 3. Modelled NH3 removal efficiency as a function of inlet NH3 concentration (a)and water supply (b). All other parameters are as shown in Table 1.AMMONIA TRANSFORMATION IN A BIOTRICKLING AIR FILTER270biological pH buffer ensuring that pH nowhere dropped below 6.5, despite thecontinuous uptake of a strong base, NH3, and conversion to a moderately strong acid,HNO2 (Figure 2). In other filters with well-established nitrite oxidation and thereforeaccumulation of NO3- in place of NO2- pH may drop below 5, as the HNO2 block is notoperating (L.B. Guldberg, unpublished).In biological airfilters there must be some water run off removing the generatednitrogen salts. Not only in order to sustain NH3 removal but also to avoid emission ofthe detrimental gasses NO and N2O as observed in poorly watered biofilters (Trimbornet al., 2003). The cost of handling the wastewater, however, makes it relevant to considerhow the run off can be minimized. Of the 63 ml supplied every hour to each filter tube53 ml evaporated in the lower part of the filter and only 10 ml drained off. This meansthat microorganisms in the upper half of the filter were blessed with more than 6times the flow of water that eventually was discharged wastewater. The lowest sectionessentially served as a waste condenser with poor biological conditions due to nitrousacid accumulation. Osmotic stress might have been another important limiting factorat these high salinities (Smet et al., 2000). Model perturbations (Figure 3b) showedthat the overall removal efficiency would only increased from 85% to 95% followinga doubling of the water supply to 126 mL/h and thereby the generation of about 7times more wastewater ((63 mL + 10 mL)/10mL). On the other hand a reduction ofthe water supply by 16% to 53 mL or less would leave no wastewater and the filterwould stop working.  The major ambition with the modelling studies partly presentedhere is actually to develop better algorithms for optimization of water supply andbiomass management in response to varying ammonia and door loading, air flow,temperature, humidity, etc.5 ACKNOWLEDGEMENTSThe Danish Ministry of Food, Agriculture and Fisheries funded this work.REFERENCESAnthonisen, A.C., Loehr, R.C., Prakasam, T.B.S. and Srinath, E.G. (1976) Inhibition of nitrificationby ammonia and nitrous-acid. J. Wat. Pollut. Cont. Fed. 48(5): 835-852.Bower, C.E. and Holm-Hansen, T. (1980) A salicylate-hypochlorite method for determining ammoniain seawater. Can. J. Fish. Aquat. Sci. 37(5): 794-798.L. PETER NIELSEN, M. L. NIELSEN, M. ANDERSEN AND A. M. NIELSEN271Smet, E., Langenhoven, H.V. and Maes, K. (2000) Abatement of high concentrated ammonia loadedwaste gases in compost biofilters. Wat. Air Soil Poll. 119: 177-190.Trimborn, M., Goldbach, H., Clemens, J., Cuhls, C. and Breeger, A. (2003) Endbericht zum DBU-Forschungsvorhaben Reduktion von klimawirksamen Spurengasenin der Abluft vonBiofiltern auf Bioabfallbehandlungsanlagen (AZ: 15052). ISBN 3-933865-30-1, BonnerAgrikulturchemische Reihe 14.Volcke, E.I.P., Van Hulle, S., Deksissa, T., Zaher, U. and Vanrolleghem, P.A. (2005) Calculation ofpH and concentrations of equilibrium components during dynamic simulation by means ofa charge balance. BIOMATH Technical Report. Ghent University, Belgium. pp. 63.AMMONIA TRANSFORMATION IN A BIOTRICKLING AIR FILTER

Ammonia transformation in a biotrickling air filter

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Ammonia transformation in a biotrickling air filter

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