PRACTICAL EXPERIENCE WITH ON-LINE MEASUREMENT OF NITRIFICATION INHIBITION OF WWTP INFLUENT

Stražar M., Burica O.,Podbevšek S., Levstek M. Domžale-Kamnik Wastewater Treatment Plant, Študljanska 91, 1230 Domžale, Slovenija

e-mail: strazar@ccn-domzale.si Abstract Inhibition of nitrification is one of the most sensitive methods for detection of harmful effects on the biomass in a WWTP. A NitritoxMonitor (LAR, Germany) which measures nitrification inhibition on line every 30 minutes in the inlet of the WWTP’s biological stage was installed. Comparison with the ISO 9509/1:1989 method showed good correlation. The biomass is increased with a constant ammonia-rich substrate of 2,000 mg N/l, so the repeatability of the method of about 2 % was very good. The principle of the LAR method is respirometric measurement of the oxygen uptake of the autotrophic biomass, which is more sensitive than that of the heterotrophic one. The percentage of nitrification inhibition is given as the difference between the oxygen uptake of the reference and sample. Experience showed that a result of less than 25 % of nitrification inhibition did not result in harmful effects on the WWTP biomass, but a higher percentage evidently increased ammonia nitrogen in the effluent and nitrification could be lost for some days or even months. However, when cyanide (<10 mg/l), organic solvents or heavy metals were present, nitrification inhibition was 100 %. Cyanide, together with high concentrations of organic solvents and/or heavy metals originate from the pharmaceutical and electroplating industries. As the WWTP is sometimes asked to accept different substrates from technological processes, the nitrification inhibition measurement is useful in addition to the biodegradability test for ascertaining if the substrate is suitable for biological treatment, as well as for determination of the acceptable dilution factor. Introduction The Domžale-Kamnik Wastewater Treatment Plant (WWTP) receives about 40 % its input as industrial wastewaters from various industries and occasionally substances are present in the influent which inhibit the nitrification process efficiency so that nitrogen concentrations in the effluent can exceed the permitted limit. Due to the presence of these various substrates the correlation between nitrification inhibition and the presence of special chemicals needed to be defined in order to be able to press the polluters to eliminate inhibitory substances from the incoming wastewater. The influent was tested for inhibition of the hatertotrophic biomass many times and did not show any harmful efect as the heterotrophic biomass is not so sensitive to inhibitors as the autotrophic one. Materials and methods The nitrification inhibition test is based on on-line measurement by a NitritoxMonitor which measures inhibition of nitrification in the WWTP influent every 30 minutes. It is a continuously operating measuring device, which registers the presence of toxic substances in the continuously examined in-put water by recording the change of respiration activity of autotrophic

microorganisms. The method is essentialy a respirometric test, measuring a change in oxygen uptake rate in the liquid phase. The biomass required for the measuring process is cultivated in a small fermentator which is part of the device. The cultivation is done without contact with the sample water and habituation effects are avoided. The metabolic activity of the microorganisms is measured and kept constant during the measurement. Cultivation under determined, selective nutrient and living conditions makes the spectrum of microorganisms well defined, so the measurements have a good reproducibility. The substrate is real incoming wastewater of variable quality and the possible presence of toxic substances which can be inhibitory. At the time of measurement 20 % of biomass, 25 % of substrate and 55 % of aerated water is meixed and the measurement is performed. The recorded change of the oxygen concentration (O2 respiration) of the test water is compared with the oxygen respiration which was determined during the most recent calibration with nintoxic, demineralized water (reference measurement). From the ratio of the oxygen respiration of the test water and reference water, an inhibition result is given as a percentage. If the test water shows the same oxygen respiration as the nontoxic, clean reference water, the toxicitx amounts to 0 %. If there is no oxygen respiration in the test water, this behavior is indicated as total inhibition (100 % toxicity). Results and discussion Nitrification inhibition is detected occasionally and an example of inhibition with its consequences on the effluent is presented in Figure 1. The figure shows that high inhibition of nitrification in the influent lasting about 4 hours is followed by an increase of ammonia nitrogen in the outlet. On a similar occasion of inhibition, wastewater samples before and after inhibition were analysed for cyanide, organic solvents, AOX and pH value. A correlation between nitrification inhibition and pH value, organic solvents and AOX was not proved but the coorelation between nitrification inhibition and cyanide was shown to be strong. Figure 2 shows that at the time of high inhibition (over 50 %), the cyanide concentration was high, around 1 mg/l, whereas at lower inhibition no significant cyanide was detected.

0

10

20

30

40

50

60

70

80

90

100

26.3.2002 28.3.2002 30.3.2002 1.4.2002 3.4.2002 5.4.2002 7.4.2002

date

inhi

bitio

n of

nitr

ifica

tion

(%)

0

5

10

15

20

25

30

35

40

45

50

N-N

H4

efflu

ent (

mg/

l)

inhibition in the influent N-NH4 effluent

high inhibition

enlarged ammonia nitrogenin the effluent

Figure 1. The percentage of nitrification inhibition in the influent and the resulting ammonia concentration in the effluent.

0

10

20

30

40

50

60

70

7:12 9:36 12:00 14:24 16:48 19:12 21:36 0:00 2:24 4:48 7:12 9:36 12:00

hour

NI i

nhib

ition

(%)

0

0,2

0,4

0,6

0,8

1

1,2

CN

-tota

l (m

g/l)

NI inhibition CN-total

Figure 2. Inhibition of nitrification and total cyanide concentration in the incoming wastewater.

As the possible industrial sources of cyanide in the studied WWTP influent can only originate from the pharmaceutical or electroplating industries, the presence of heavy metals was also studied. On the occasion when nitrification inhibition was high (around 100 %), the high cyanide concentration (>8 mg/l) was accompanied by high concentrations of zinc (4.7 mg/l) and nickel (>8 mg/l). In Table 1 the percentage of nitrification inhibition and the heavy metal concentrations is presented. Organic solvents were not present in measurable quantities.

Table 1. Analytical parameters and nitrification inhibition in February 2004.

sampling time (2 h samples) pH

NI inh. Cu Zn Cr Ni Pb Fe

N-Kjel.

CN tot.

CN free COD BTX AOX LKCH

unit % mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l

23.2.2004 14:00 7,5 13 0,06 0,32 0,03 0,03 <0,02 1,85 23,3 0,04 0,04 392 <0,03 0,09 <0,02

23.2.2004 16:00 8,0 100 0,58 1,69 0,02 1,88 <0,02 1,79 20 8,8 7,1 484 <0,03 0,07 <0,02

23.2.2004 18:00 8,1 96 0,70 4,66 0,27 2,27 0,02 1,67 22,7 8,4 6,9 335 <0,03 0,09 <0,02

23.2.2004 20:00 7,4 35 0,10 0,62 0,05 0,11 0,07 3,12 17,1 0,2 0,14 288 <0,03 <0,02 <0,02

23.2.2004 22:00 7,4 24 0,05 0,31 0,02 <0,03 0,02 1,50 17,3 0,02 0,02 415 <0,03 <0,02 <0,02

Legend: BTX = benzene, toluene, xylene AOX = sum of halogenated organics LKCH = chlorinated hydrocarbons The correlation of nitrification inhibition with heavy metal concentration and the presence of cyanide leads to the conclusion that the source of inhibitory substances in the case presented is probably the electroplating industry and not the pharmaceutical industry, as the presence of organic solvents was not detected. As the WWTP receives the demands from different poluters to accept the special wastewater the inhibition of nitrification was tested on different special substates. The example of highly biodegradable substrate and its inhibition percentage is given in Table 2. From the test we can conclude that the maximum precentage of added special wastewater can be less than 0,2 % to achieve the dillution which is not harmfull to biomass any more.

Table 2. Nitrification inhibition of special biodegradable substrate at different dilutions.

quantity of sample added Nitrification inhibition

% % 12,5 100

5,0 1002,5 851,3 660,6 450,2 <20

biodegradability 94%Conclusions The on-line test of nitrification inhibition is of great importance when detecting the inhibitory impact of input wastewater on the autotrophic biomass, since the method presented is quick and enables continious checking of the substrate. The results can help the operator to anticipate problems of inhibition and try to avoid the collapse of biomass activity by quick intervention. The test is helpful in making decisions about the acceptability of special substrates for the WWTP. The examples given illustrate the possibilities to detect and analyse substrates which can inhibit the nitrification process and to reveal why the efficiency of nitrogen removal is not as good as expected. References Grady L. C. L., Daigger G. T., Lim H. C. 1999. Biological Wastewater Treatment, 2. izdaja, Marcel Dekker, USA. Hvala N., Vre�ko D., Burica O., Stražar M., Levstek M. 2002. Simulation study supporting wastewater treatment plant upgrading. Water Science & Technology, Vol. 46, No. 4-5. Orhon D., Artan N. 1994. Modelling of the activated sludge systems, technomic, USA. Stražar M., Burica O. 2002. Primerjava dveh tehnoloških postopkov �iš�enja odpadne vode: suspendirana biomasa in biofilm. �iš�enje odpadnih voda 2002, Biološko središ�e v Ljubljani, 13-14. junij 2002 LAR Analytic & Umwelt Messtechink GMBH. 1999. NitritoxMonitor Online Toxicity Analyser Using Nitrifiers. Operational Manual. Document NTX-02:E2199.