Enhanced denitrification with methanol at WWTP Zürich-Werdhölzli

~

Wal. Sci. T~ch. Vol. 33. No. 12. pp. 117-126. 1996. Copyrighl C 1996 IAwQ. Published by Elsevier SCIence LId Prlnled in Greal Brilain. All righls reserved.

Pergamon

PH: S0273-1223(96)00465-9

0273-1223/96 SI"00 + 0'00

ENHANCED DENITRIFICATION WITH METHANOL AT WWTP ZURICH-WERDHOLZLI I. Purtschert, H. Siegrist and W. Gujer Swiss Federal Institute for Environmental Science and Technology (EA WAG) and Swiss Federal Institute of Technology (ETH). CH-8600 Dubendorf, Switzerland

ABSTRACf In coordinalion Wilh the EUoguidelines the large wastewaler treatment plants in Switzerland have to be extended with enhanced nitrogen removal. Due to the existing plant configuration. the low CODIN ralio and dilule wastewater. denitrification supported by an external carbon source instead of extending the plant may be an interesting and cost effective solution for municipal wastewaler treatment. At the waslewater treatment plant Zilrich·Werdhlilzli different experiments were performed with methanol addition to predenitrificalion from March to July 1994. The aim of this work was to evaluate the use of methanol as an alternative to plant extension to achieve a higher nitrogen removal efficiency. Therefore. two parallel denitrifying lanes were investigated. one served for methanol addition experiments and the other as a conlrol. The effect of oxygen input into the anoxic zone due to innuent. return sludge and mlxmg was investigated. too. The results show that nitrogen removal efficiency can be substantially increased as compared to the reference lane. The adaplalion period for methanol degradation was only a few days and the process was relatively stable. Based on total nitrogen in Ihe innow. the average denitrification was SS% with methanol addition and 3S% without methanol. The yield coefficient Y COD was 0.4 g CODX g.1 CODMe' Due to the small net growlh rale of Ihe melhanol degraders the denitrification capacity is relatively low and nitrale peak loads cannol be fully denitrified. Hence. methanol as a carbon source requires more or less constanl dosing. To prevent nitrate limilalion. methanol addition should be controlled by Ihe anoxic nitrate concenlrallons. Copyrighl C 1996 IAWQ. Published by Elsevier Science Lid.

KEYWORDS Wastewater treatment; full scale experiments; methanol; external carbon source; denitrification efficiency; denitrification capacity. NOMENCLATURE lli .. efficiency for removal of substance i (%) YCOD" biomass yield in COD units (M Mol) SRT .. Solids Retention Time (T) SVI .. Sludge Volume Index (0 Mol) Me .. Methanol qMc" methanol degradation rate (M L 3 Tol) 0

= = = =

Q rs return sludge flow (ll T.I) Qin = inflow (L3 T·I) Qout outflow (L3 Tol) Q es = excess sludge flow (L3 T-') rden denitrification rate (M L 03 T") ~den denitrification capacity (-) 117

I. PURTSCHERT et al.

118

INTRODUCfION Due to the eutrophication of the North Sea. the countries situated along the river Rhine agreed to reduce the nitrogen load on the Rhine to 50% until the year 2000. Therefore. the larger wastewater treatment plants in Switzerland have to be upgraded for extended nitrogen removal. Denitrification capacity can either be improved by enlarging the denitrifying volume or adding an external carbon source. Due to dilute municipal Swiss wastewater. low CODIN ratio and lack of readily biodegradable COD, the carbon source often limits denitrification. Therefore, adding an external carbon source can be an interesting solution. Different carbon sources can be used. e.g. acetate or methanol. The advantages of methanol as the carbon source are the relatively low cost and the small sludge production as compared to acetate and other commercial organic compounds. As a C .-compound methanol is not available for all microorganisms. Especially in the anoxic environment. one group of organisms being able to utilize methanol is dominating: Hyphomicrobium sp.• a facultative methylotrophic. denitrifying bacteria (Schlegel, 1992; Nurse. 1980; Sperl & Hoare, 1971: Timmermans & Haute. 1983: Uebayasi & Tonomura. 1976). Due to its low growth rate Hyphomicrobium sp. is typically present in small concentrations in the activated sludge. After starting methanol addition. an adaptation period is needed until Hyphomicrobium sp. becomes enriched. which is a drawback for using methanol as a carbon source (Akunna et al., 1993; Nyberg et al., 1992). The aim of this work was to evaluate the use of methanol at the treatment plant Zilrich-Werdh6lzli (adaptation time, biomass yield, denitrification efficiency and denitrification capacity) as an alternative to plant extension to get a higher nitrogen removal efficiency. FULL SCALE PLANT CONFIGURATION The wastewater treatment plant Zilrich-WerdMlzli treats the wastewater of 600,000 population equivalents. Dry weather flow is approximately 170,000 m 3 d- J. The inflow is split into two lanes partitioning the plant in a lane North and a lane South. The overall plant configuration includes screening, grit removal, primary clarification, biological treatment, secondary clarification. filtration and sludge treatment. Phosphate removal is performed by simultaneous precipitation and flocculation-filtration.

referencelane

clarifier

"\."\."\."\. inflow-

~. II~iffft i %

return sludCle

. t-:-i , ~Af

experlmena lane

clarifier

~~d

'1111

- . outflow

..1' ..1' ..1' ..1'

%

&?%iY,%t

i methanol

Figure I. The two investigated lanes of the WerdhOlzli activated sludge plant.

Each lane of the biological treatment consists of 6 parallel aeration tanks and 6 secondary clarifiers. 10 of the 12 tanks are fully aerated and only used for nitrification. For the experiments two parallel tanks of lane North have been equipped with predenitrification. 28% of the total tank volume of each of them has been converted into anoxic zones (Fig. I). The activated sludge tank volumes are: V AI'" V A2 = 700 m 3, V nit .,. 3600 m 3• Vclarilier'" 6000 m3. The two lanes were both sampled in order to have an experimental (methanol

Enhanced denitrification with methanol

119

dosing) and a reference lane. The total sludge age was about 12 days and the return sludge flow was twice the dry weather inflow, which is about 14,000 m3 per day and lane. MATERIALS AND METHODS Twenty-four hour composite samples were taken daily from primary and secondary effluent and analysed for total nitrogen, ammonia, nitrate, nitrite, total phosphorus, ortho phosphate, total COD, dissolved COD and total suspended solids (TSS). Grab samples from influent, anoxic and aerobic zone and return sludge were analysed for oxygen, ammonia, nitrate and nitrite to determine concentration profiles. About three times weekly sludge from the anoxic and aerobic zone was taken to analyse denitrification capacity with and Without methanol in a 7 I batch reactor. Activated sludge was analysed weekly for sludge settlement (SVI) as well as total nitrogen and total phosphorus content of the activated sludge (iN' ip). Microscopic sludge analyses were performed weekly to follow possible changes in microbiological species. Nitrate was measured on line in the two anoxic tanks of the experimental lane. To evaluate the flow characteristics in the anoxic tanks, two tracer experiments with bromide were performed. The dissolved ionic compounds nitrate, nitrite, ammonia and phosphate were determined colorimetrically by Flow Injection Analysis (FlA) after filtration (0.7 ~m). Total nitrogen as well as total phosphorus content were measured in the same way after digestion with K2S20 g• To analyse methanol in the batch experiments. a gas chromatograph (Carlo Erba Vega 6(00) with a Flame Ionisation Detector (flO) was used. It was operated with a capillary column and H2 as carrier gas. The dissolved and the total COD were measured colorimetrically by the HACH method (DRI2000). The Total Suspended Solids (TSS) were filtered by a GFIF filter. The filtered material was dried for 2 hours at 105°C. To observe the sludge settlement. the Sludge Volume Index (SVI) was performed using an Imhoff cone (sludge to water 1 to 1) and a settling time of 30 minutes. Microscopic analysis was carried out by phase-contrast light microscopy. The Dr. Lange online analyser based on ultraviolet absorption analysed the nitrate in the permeate of an ultrafiltration station (PES 0.1 ~m). OPERATING PROBLEMS As the anoxic tanks are completely mixed, dosing to the second anoxic tank has the drawback of losing methanol to aerobic degradation. By dosing the first tank, practically all methanol is used for denitrification but methanol to nitrate degradation is more difficult to estimate due to variable readily biodegradable COD and oxygen input. In the beginning, methanol was added to the second anoxic tank and later to the first one. To evaluate the flow characteristics. a tracer experiment with bromide was performed. The results showed considerable back mixing from the aerobic to the anoxic tank, which prevented nitrogen balances occurring around the anoxic zone. The anoxic and aerobic tanks are separated from one another by plastic walls. The principal water stream flows through a 20 cm opening at the bottom. Aeration and small lateral openings between the plastic wall and the concrete tank wall led to substantial back flow before sealing these lateral openings. A second tracer experiment showed that the improvement was successful. Substantial input of oxygen into the anoxic zone reduces denitrification efficiency. For predenitrification oxygen input is from the inflow, the surface - especially by increased stirring - and the return sludge flow. Figure 2 shows the three contributions relative to the total oxygen input. Oxygen input from the inflow is substantial, particularly during rainy weather due to a 7 m high Archimedian screw pump. Normally the surface input is small. if mixing is properly designed. The critical Part of the oxygen input comes from the return sludge. In the case of an internal recirculation. the oxygen concentrations at the end of the aerobic tank should be near zero. Oxygen input from the return sludge is brought in by screw pumps, overfalls and open channels. Due to frequency control of the screw pump in the experimental lane, the overfall into the pumping pit could be prevented and oxygen input was substantially decreased. This might be the reason why the denitrification efficiency in the experimental lane is slightly higher than in the reference lane, even without methanol addition.

J. PURTSCHERT et al.

120

experimental lane

reference lane

10%

15% Urface

41% return sludge

(j

return sludge inflow

61%

44%

(J

dry weather, 100% = 3.7 g02lm3

dry weather, 100%

experimental lane

nference lane

6 30%'1 %surface

rface 29% inflow

= 5.8 g02lmJ

5%

38% ~SUrface

return sludge

return sludge

inflow

inflow 57%

64%

rainy weather, 100% = 7.3 g02lmJ

rainy weather, 100% = 8.3 g02lm3

Figure 2. Distribution of tolal oxygen input to the anoxic reaclors (concentrations relative to 10lal now). ACfIVATED SLUDGE ANALYSIS In both lanes the activated sludge had about the same biological and chemical composition and did not change during the experiments: The CODIfSS ratio iCOD 1.1 (g COD g.1 TSS).the total nitrogen content iN = 5.4% (g N g.1 COD) and the total phosphorus content ip = 2.8% (g P g-I COD) of the total COD. Sludge Volume Indices (SVI) were between 80 and 100 ml g". Microscopic analysis showed that methanol addition did not affect floc condition and the amount of filamentous organisms. The sludge flocs were small to middle sized. irregularly formed and of compact structure. Not many filamentous organisms occurred in the sludge (category 0-1. Eikelboom. 1987). the dominating types were type 021 N and type 0041. /lyphomicrobium sp. organisms often grow as appendages of filamentous organisms (Sperl & Hoare. 1971). Such appendages were observed. especially in the experimental lane. /lyphomicrobium sp. is a small bacteria (= I Jlm. Schlegel. 1992: Uebayasi & Tonomura. 1976) and cannot clearly be identified with a light microscope (magnification: IOOOx).

=

DENITRIFICATION RESULTS AND DISCUSSION Denitrification efficiency The denitrification efficiency is defined as follows: Tlden

=(CN,lol,in - Qe/Q

In

TSSes iN icOD - Nso1 .OUI - TSS out iN icOD) I CN.IOI,1n

The efficiency for removal of substance i is defined as follows: TI.,el

=(C.,lol,in - Ci.IOI.out) I C•. IOI .1n

Enhanced denitrification with methanol

\2\

The difference of the two efficiencies TlN.el and Tlden gives the percentage nitrogen being incorporated into the biomass. After the biomass was adapted to methanol degradation 55% of the total inlet nitrogen was denitrified. 17% incorporated and 28% occurred in the outflow (Fig. 3). In the reference lane only 35% was removed due to denitrification. The nitrogen in the outflow was mainly nitrate. With a high sludge recirculation ratio of Qr/Qin = 3 and methanol addition to the first compartment more than 60% was denitrified. which results in more than 75% total nitrogen removal. Because the return sludge flow cannot be increased above three times the dry weather inflow. nitrate was temporarily limiting denitrification.

reference lane

experimental lane

outflow

28%~ _ denitrification .; :>',

17%

48%

~y~", denitrification ( ) outflow •biomass

55%

-,""-J;

biomass

35%

"

17%

Figure 3. Efficiency of nitrogen removal by denitrification and incorporallon into bIomass during the lime with adaptated btomass. methanol dosing to second compartment A2 and a reCIrculation rallo of QrlQ m =2 (\00'k = total nitrogen mthe mnow). Table I. Average removal efficiency of experimental and reference lane

efficiency nNel nnen nNit nPel nrnnel nT<:<:el

unil % % % % % %

based on total N input total N input total N input total P input total COD input total TSS input

experimental lane 72 55 77 81 87 89

reference lane 52 35 77 83 88 89

Denitrjfication rates due to methanol dc~radatiQn During methanol dosing to the second tank A2 of the experimental lane. daily grab samples were taken from companment A I and with a delay of a hydraulic retention time from A2. Neglecting the small nitrite concentrations. the steady state nitrate balance of A2 yields the following volumetric denitrification rate: rdcn

=(Qm+Qr~) (CN03 .A \ - CN03 .A2 ) I VA2

(g NOrN m· 3 d· l )

The denitrification rates due to hydrolysis of paniculate COD in the reference and in the experimental lane are about equal. The difference between the total respiration with methanol addition and the respiration rate of the reference lane illustrates the activity of lIypllOmicrobium sp. due to anoxic methanol degradation. For comparison. respiration due to methanol degradation as well as methanol addition was related to total activated sludge COD (kg COD m· 3 reactor) (Figure 4): rden.COD =I (rden.e~p - rdcn.ref) 2.86 g COD g'\ NOrN} I COD~ludge (g COD N.den kg') CODsludge d· l )

I. PURTSCHERT el 1I1.

122

The data in Figure 4 demonstrate that a substantial amount of the added methanol gets lost to the aerobic zone. since rden,COD S qMe,COD (1-YCOD) with YCOD

=biomass yield coefficient from batch experiments =0.4 g COD Xg-I CODMe (s. below). 140

A"erllg' Temperature: 14.5"C

16.O"C

120

~

i~

exp

17.5"C

100 80 60

40 20 0 1.3.-18.4.

19.4.-2.6.

3.6.-21.6.

Figure 4. Average respiration rates (rdcn) in expenmental (exp) and reference (reO lane; periods with various dosing (qMe); based on lotal sludge COD. Nitro~en

profiles

Nitrogen profiles were observed along the whole activated sludge plant. Samples were taken from the inflow. the return sludge. the anoxic tanks AI and A2 as well as the inlet and the outlet of the aerated tank. Figure 5 shows a typical experiment in both lanes during the time of methanol dosing. The goal of these profiles was to obtain detailed information on the behaviour of the nitrification and the denitrification process. Nitrogen mass balances from such profiles yield valuable information about the quality of full scale experimental data. e.g. profiles taken during heavy rains have not been representative due to strong inflow fluctuations and dilution effects. Figure 5 shows complete nitrification in both lanes. The nitrite concentrations were always smaller than I g N m- 3. Due to constantly higher denitrification. the nitrate concentration in the return sludge was lower in the experimental than in the reference lane. In both lanes denitrification in the first anoxic tank AI was equal (1.2 in ref. lane and 1.3 g N m- 3 in expo lane). From the difference of the total inorganic nitrogen in the effluent (nit2) of the reference and experimental lane a nitrate elimination of more than 4 g NOrN m- 3 inflow was observed due to methanol addition (AN. Fig. 5). Since methanol was added to compartment A2 of the experimental lane this corresponds to the difference in nitrate elimination of the second anoxic compartments multiplied with the ratio of flow through the reactor relative to inflow: (1.7 - 0.5 g N m- 3 flow) • (Qm + Qrs)/Qin = 3.8 g N m· 3 inflow. Methanol addition to nitrate elimination ratio was about 5.5 g CODMe g" N which is substantially higher than observed from batch experiments where 4.8 g CODMe g.1 N were used (see below). As demonstrated in the results above a substantial fraction of the methanol was lost to the aerobic zone due to hydraulic short cuts and reactor concentration needed for methanol degradation. The anoxic zone should therefore be divided into two zones and methanol added to the first zone.

Enhanced denitrification with methanol

reference lane

16

experlmeriallane

14

----------f

12

~10

i

8

Z

4

123

dN

.... 6 2 O~""4J~4-l""~""'-+i-"""'.I..--4-"'...-.L"""""4-Jiolll.il~oI4l"""'"

Figure 5. Nitrogen profiles in the experimental and reference lane (sampling locations s. Fig. I). In the experimental lane methanol (21 i CODMe m- 3 inflow) was added to companment A2. Operating parameters: SRT =12 d. Qm =12500 m3 d· l • Qrs =28000 m3 d· l • T =18·C. Yield coefficient Batch experiments with activated sludge from the anoxic and aerobic zone were performed to determine the yield coefficient YCOD of the methanol degraders as well as the aerobic and anoxic biomass activity.

methanol degradation

55 50 aerobic 45 0 anoxic § 40 ~ 35 .. 30 ~ 25 20 70 80 90 100 110 120 130 140

i

---------time [min]

12

denitrification

~------------

~

~1O ~

8

0

6

zoJ, z

4

0

M~

20 40 60 80 100 120 140 time [min]

Figure 6. Batch experiments: Aerobic and anoxic methanol degradation (left) and nitrate profile under anoxIC conditions with and without methanol addition (right). Figure 6 shows a typical experiment in which both nitrate and methanol decrease linearly. Since in an aerobic environment not only Hyphomicrobium sp. degrades methanol, aerobic is higher than anoxic tnethanol degradation. From anoxic experiments in which either methanol or nitrate were completely degraded the saturation constants KMe and KN03 are assumed to be smaIl (below I glm3). Methanol to nitrate degradation was: 2.86gCODg- I NO -N _-=-_-=:...--_,,-3= 4.8 g COD Me g-I N03-N = 3.2 g Me g.1 N03-N 1- YCOD From more than 30 parallel batch experiments the yield coefficient YCOD under aerobic and anoxic Conditions results in 0.4 g COD x g.1 COD Me . In laboratory experiments with pure cultures often a smaller "COD (0.2-0.3) is found (Nurse, 1980: Uebayasi & Tonomura, 1976). Chudoba tl al. (1989) determined a "COD for methanol of 0.35-0.37. Timmermans & Haute (1983) describe the denitrification with methanol a

I. PURTSCHERT ~, at.

124

a function of pH. There a YCOD of about 0.4 corresponds to pH =7 which was the pH of the activated sludge in our experiments. Nyberg et al. (1992) observed in full scale experiments an utilization of methanol above the theoretically expected amount, too. Adaptation period To determine the adaptation period for anoxic methanol degradation, batch experiments were performed about three times per week (Fig. 7). Since the adaptation period observed was only a few days, methanol addition was switched to the reference lane from day 65 to 80 where an equally short adaptation period was found. Because Hyphomicrobium sp. is also growing on other substrates and under aerobic conditions (Uebayasi & Tonomura, 1976; Sperl & Hoare, 1971) and due to the long solids retention time and warm temperature, it might already be present in the activated sludge before methanol is added. methanol addition:

280

methanol addition to experimental lane, A2

ref. A2

up. AI

240

~

280 240

200

II

160 120

~

80 40

o

40

o

I

4

0

8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 time [d)

Figure 7. Aerobic and anoxic methanol degradalion observed from batch experiments (T = 20"C). Between day 65 and 80 methanol addition was switched to the reference lane. In both lanes adaptation periods for anoxic methanol degradation were only a few days. During methanol addition to A2 and AI the methanol added was related to the sludge mass in A2 and total anoxic volume. respectively. Denitrification capacity The denitrification capacity is defined as follows: ~en

= qMe,max I qMe,aver = methanol degradation in batch reactor I full scale methanol degradation

From the average anoxic methanol degradation qMe,aver (calculated from the methanol addition) in the full scale plant and the maximum methanol degradation rate in the batch experiments qMe,max the denitrification capacity was estimated (Table 2). Only data with neither methanol nor nitrate limitations are considered, except by methanol addition to A I. Then, methanol was limiting denitrification and qM,aver was calculated from the methanol added divided by the total anoxic volume.

Enhanced denitrification wilh methanol

125

Table 2. Denitrification capacity by methanol addition to first and second compartment. ~den is calculated from the maximum to the average methanol degradation. qMe was related to total sludge COD. SRT tot = 12 d. Vtot = 5000 m3 V anox

SRTanox

date

m3

26.4.-3.6. 13.7.-20.7.

700 1400

d J.7

time

methanol addition compartment

A2 Al

CJMe.aver

CJMe.max

20 DC

Il

COD kll,l COD d,l

16-20 DC Il

COD kll'· COD d,l

64±8 82±5

3.4

79±24 76±3

~

-

0.8 1.1

Table 2 shows very low denitrification capacities. If methanol is added to the first anoxic compartment denitrification due to methanol addition can only be increased by 10-20% during nitrate peak. loads (see below). Due to the small growth rate of Hyphomicrobium sp. (Uebayasi & Tonomura. 1976; Chudoba et al.. 1989) and the relatively high decay rate (Chudoba et al., 1989: 20-30% of the maximum growth rate) denitrification is limited by biomass. At low temperature Hyphomicrobium sp. might be washed out if methanol is only added to the second anoxic compartment (qM,max!qM,aver < I). If methanol is added excessively. Hyphomicrobium sp. will be cultured anoxically and aerobically and wash out will be prevented. But more methanol is needed per kg NOrN denitrified. Otherwise, the anoxic solids retention time has to be enlarged. Qn-Ijne nitrate measyrementS Due to the variation of the CODIN ratio in the primary effluent the nitrate concentration in the anoxic zone varied strongly (Fig. 8). During the weekends the influent CODIN ratio was low (5 g COD/g N) which results in an increased anoxic nitrate concentration. During working days the CODIN ratio was about 7 g COD/g N which led temporarily to nitrate limitations. Methanol addition or sludge recirculation should therefore be controlled by the nitrate concentration in the final compartment of the anoxic zone. Figure 8 shows that nitrate peak loads cannot be fully denitrified due to the low denitrification capacity (see above). Therefore. methanol requires more or less constant dosing. For peak. load denitrification other organic compounds. e.g. acetate (Isaacs et al., 1994) are more suitable.

nitrate-online concentrations, week 23, 6.6. to 12.6.94 II 10

.....

i..... Z

9 8 7 6 5 4 3

2 I 0

Mon

Tue

Wed

Thu

Fri

Sat

Sun

Mon

Figure 8. Nitrate on-line concentrations in the first and second anoxic compartment A 1and A2 during one week (Monday, 0.00 10 Sunday, 24.00). Methanol was added to A2. CONCLUSIONS Methanol addition can substantially increase denitrification efficiency. At the treatment plant ZUrichWerdh~lzlj total nitrogen removal could be improved from 52 to 72%. The adaptation period of the anoxic

I. PURTSCHERT et 01.

126

methanol degrader Hyphomicrobium sp. was only a few days. possibly due to the long solids retention time (12 days) and the warm temperature (I 6-1 8°C). To prevent nitrate limitations. methanol addition should be controlled by the nitrate concentration in the final compartment of the anoxic zone.

Hyphomicrobium sp. is a slowly growing organism with a biomass yield YCOD of 0.4 g CODx g'\ COD Me' Since growth of Hyphomicrobium sp. is mainly limited to anoxic methanol degradation. steady state estimations illustrate a low denitrification capacity. In winter small anoxic solids retention times could therefore be critical and methanol as compared to acetate addition is not suitable to degrade nitrate peak loads. Denitrification with methanol requires a more or less constant dosing and a relatively high anoxic SRT. Methanol addition seems to have no effect on sludge settling. floc conditions and growth of filamentous organisms. Oxygen input due to inflow. return sludge. internal recirculation. mixing and hydraulic short cuts reduce denitrification efficiency substantially.

REFERENCES Akunna, J. C., Bizeau, C. and Molella, R. (1993). Nitrate and Nitrite Reductions with Anaerobic Sludge using various Carbon Sources. Wat. Res., 27,1303-1312. Chudoba, J., Albokova, J. and Cech. J. S. (1989). Detennination of Kinetic Constants of Activated Sludge Microorganisms responsible for Degradation ofXenobiotics, Wat. Res.• 23,143\-1438. Eikelboom D. H. and van BUljsen, H. J. J. (1987). Handbuch flir die mikroskopische Schlammuntersuchung, 2. Aunage. F. Hinhammer Verlag. Milnchen. Isaacs, S. H.• Henze. M.• Soeberg, H. and Kilmmel M. (1994). External Carbon Source Addition as a Means to Control an Activated Sludge Nutrient Removal Process. Waf. Res.• 28, 511-520. Nurse. G. R. (\980). Denitrification with Methanol: Microbiology and Biochemistry, Wat. Res., 14.531-537. Nyberg, U., Aspegren, H., Andersson, B., Jansen, J. la Cour and Villadsen, I. S. (\992). Full-Scale Application of Nitrogen Removal with Methanol as Carbon Source. Wat. Sci. Tech., 26. 1077-1086. Schlegel, H. G. (1992). Allgemeine Miktobiologle, Georg Thieme Verlag, Stullgan. Sperl, G. T. and Hoare, D. S. (1971). Denitnficauon with Methanol: A Selecllve Enrichment for Hyphomicrobium Species, Journal of Bacteriology, 108. 733-736. Timmennans. P. and Haute, A. van. (1983). Denitrification with Methanol: Fundamental Study of the Growth and Denitrification Capacity of Hyphomicrobium. Waf. Res., 17. 1249-1255. Uebayasi, M. and Tonomura, K. (1976). Denitrification by Hyphomicrobium Capable of Utilizing Methanol, Journal of Fermentation Technology, 54, 885-890.