Q1 Qo 01 02 Ql3 Oa 05 06 07 Q8 Q9 1x5 11

Fig 3.12 The Lineweaver-Burke plot for identifying the type of inhibition of sodium ion concentration for nitrite oxidizing bacteria (Krittiya 1984).

Hassan et al. (1988) evaluated the performance of a packed-bed biological reactor in the presence of inhibitors, following either complete or partial modes of competitive, non-competitive, mixed or uncompetitive inhibition. For all types of inhibition, it was found that an increase in the inlet substrate concentration reduces the steady-state conversion in the reactor. The increase in the value of the parameter l/K,, which indicates the specific action of the inhibitor, increases the conversion for the partially competitive and non-competitive inhibition mode, while it reduces that for product inhibition.

Substances inhibitory to nitrifying bacteria or nitrification.

Some research has been carried out by microbiologists on the effect of specific organic and inorganic compounds on pure cultures of Nitrifiers. Table 3.17 show the results presented by Blum and Speece (1991) for nitrosomonas toxicity due to organic compounds for IC50 concentration of less than 20 mg/l.

More compounds have been found to be inhibitory to ammonia oxidation by Nitrosomonas species than to nitrite oxidation by Nitrobacter species. No explanation for this has so far been given in the literature.

Most inhibitory compounds in a waste water treatment plant are present in the range of mg/l and even some in the range of ja.g/1, and may, therefore, be difficult to detect analytically when they are present in waste water.

Only a few studies have been made on nitrification inhibition in activated sludge; the most complete one was made by Tomlinson et al. (1966). Five of the compounds included in the list are among the compounds most used by industry. Two of these, chloroform and phenol, are general inhibitors of bacterial metabolism.

Most of the very potent inhibitors in the nitrification process are sulphur-containing compounds; they can act as metal-chelating compounds, and thus inhibit enzymes requiring metals for activation (Dixon et al. 1964; Downing et al. 1964).

No reports have been found on inhibition of ammonia oxidation induced by aliphatic or aromatic amines. Hockenbury and Grady (1977) pointed out that the inhibitory effect of nitrogen-containing compounds was caused by competition with ammonia for the active site on an enzyme, although no supporting evidence has been given in the literature. Likewise, compounds, similar in structure to nitrite, have been hypothesized to be inhibitory because of their competitive effects, although only few

Table 3.16 Different types of inhibition models.

^ypes of nhibition

Mechanisi!

Rate expression

Michaelis-Menten form

Uncompetitive

Noncompetitive

"m

Substrate

k2E0S

k2Eo

where Kc & K'c are the dissociation constants for ES and SES respectively

S 1+P/Kr where = k2EQ (E0 = initial enzyme concentration), Kg = (k^ + k2)/k1

This is the simplest mechanism, other mechanisms could be hypothesized which would lead to alternative rate expressions data have been presented by Hockenbury and Grady (1977).

An investigation conducted by Hockenbury and Grady (1977) concluded that p-Nitrobenzaldehyde, p-nitroaniline and n-methylaniline were all inhibitors of nitrite oxidation by Nitrobacter species when present in a concentration of at least 100 mg/l. Dodecylamine, aniline and n-methylaniline were potent inhibitors of ammonia oxidation by Nitrosomonas species, causing 50% inhibition at concentrations of less than 1 mg/l. Aniline, ethylenediamine, hexamethylenediamine and monoethaniolamine are commonly used organic substances, known to inhibit ammonia oxidation by Nitrosomonas species. Ammonia exerts substrate inhibition on its own oxidation, and the inhibition of ammonia oxidation by aniline, dodecylamine and ethylenediamine is niether competitive nor non-competitive. Hockenbury and Grady (1977) proposed that it is related to substrate inhibition. The inhibitory effect of aniline, dodecylaniline and ethylenediamine increases as the concentration of ammonia nitrogen in the medium is increased. The results presented by Hockenburg and Grady (1977) are shown in Table 3.18. The results are divided into two levels, compound concentrations yielding 50 and 75 % inhibition of the nitrifying culture.

Table 3.19 show the ammonium nitrogen and nitrate nitrogen concentration range for nitrobacter inhibition as function of pH at 20°C. The results are in accordance with knowledge of the ionisation of both ammonium nitrogen and nitrate nitrogen.

Neufeld etal. (1986) presented different equations for the inhibition of phenolic compounds on the nitrification and discussed the influence of free cyanide and complexed cyanide compounds on the nitrification kinetic.

Figure 3.13 shows that even small amounts of free cyanide in solution inhibit the biological rate of nitrification. The relationship of the maximum reaction rate Vmax and the free cyanide concentration was found to follow the equation:

where [CN"] is the free cyanide concentration in mg/l at pH = 8,0. It is important to know the actual pH in the waste water environment and correct the [CN"] to pH = 8 using the proposed equation.

Table 3.17 Inhibitory effect of organic compounds with an IC^ value of less 20 mg/l, on pure cultures of Nitrifiers.

Organic Compound ICS0 Concentration mg/L

Table 3.17 Inhibitory effect of organic compounds with an IC^ value of less 20 mg/l, on pure cultures of Nitrifiers.

4-Aminophenol

0,07

3-Chlorophenol

0,20

2-Aminophenol

0,27

2-Bromophenol

0,35

2,3-Dichlorophenol

0,42

2,3,6-Trichlorophenol

0,42

1,3-Dichloropropene

0,48

5-Chloro-1-pentyne

0,59

2,3-Dichlorophenol

0,61

1,3-Dichloropropene

0,67

Chlorobenzene

0,71

4-Chlorophsnol

0,73

2,4-Dichlorophenol

0,79

Trichloroethylene

0,81

4-Bromophenol

0,83

1,1-Dichloroethane

0,91

2,3,5,6-Tetrachlorophenol

1,30

1,1,2,2-Tetrachloroethane

1,40

1,1,2-Trichloroethene

1,90

2,2,2-Trichloroethanol

2,00

4-Nitrophenol

2,60

2-Chlorophenol

2,70

3,5-Dichlorophenol

3,00

2,3,5-Trichlorophenol

3,90

2,4,6-Tribromophenol

7,70

Resorcinol

7,80

2,4,6-Trichlorophenol

7,90

Pentachloroethane

7,90

2,6-Dichlorophenol

8,10

1,1,1,2-Tetrachloroethane

8,70

1,2,4,5-Tetrachlorobenzene

9,80

2-Nitrophenol

11,00

Benzene

13,00

1,5-Dichloropenthane

13,00

1,2,3,4-Tetrachlorobenzene

20,00

Source: Blum and Speece (1991)

Table 3.18 Inhibitoriy effect of organic and inorganic compounds in pure Nitrobacter culture on the nitrification process.

Compound Concentration (mg/l)

at approximately 75% inhibition

Acetone"1"

2 000

Allyl alcohol

19,5

Allyl chloride

180

Allyl isothiocyanate

1,9

Benzothiazole disulfide

38

Carbon disulfide"1"

35

Chloroform"1"

18

o-Cresol

12,8

Di-allyl ether

100

Dicyanidiamide

250

Diguanide

50

2,4-Dinitrophenol

460

Dithio-oxamide

1,1

Ethanol"1"

2 400

Guanidine carbonate

16,5

Hydrazine

58

8-Hydroxyquinoline

72,5

Mercaptobenzothiazole

3,0

Methylamine hydrochloride

1 550

Methyl isothiocyanate

0,8

Methyl thiuronium sulfate

6,5

Phenol"1"

5,6

Potassium thiocyanate

300

Skatole

7

Sodium dimethyl dithiocarbamate

13,6

Sodium methyl dithiocarbamate

0,9

Tetramethyl thiuram disulfide

30

Thioacetamide

0,53

Thiourea

0,076

Trimethylamine

118

+ln the list of industrially significant chemicals.

+ln the list of industrially significant chemicals.

Table 3.18 (continued)

Compound Concentration (mg/l) at approximately 50% inhibition

Dodecylamirie < 1

Aniline < 1

n-Methylaniline < 1

Ethylenediamine 15

Napthylethylenediamine-di-HCI 23

2,2 Bipyridine 23

p-Nitroaniline 31

p-Aminopropiophenone 43

Benzidine-di-HCI 45

p-Phenylazoaniline 72

Hexamethylene diamine 85

p-Nitrobenzaldehyde 87

Triethylamine 127

Ninhydrin > 100

Benzocaine > 100

Dimethylgloxime 140

Benzylamine >100

Tannic acid > 150

Monoethanolamine > 200

Source: Hockenburg and Grady (1977)

Compound

Inhibition Concentrations

Phenol Vitamins: Riboflavin Thiamine Amino acids: L-Lysine L-Threonine L-Histidine L-Valine L-Arginine L-Methionine 2-Chloro-6-trichloromethyl-pyridine Diethyldithiocarbamate Methyl Blue Tannin

Tannin derivatives

100 mg/l

4 mg/l 10 mg/l 10"5 M 10"4 M 10"6 M 10"8 M

Source: Shrama and Ahlert (1976)

Table 3.19 Ammonium Nitrogen and Nitrate Nitrogen Concentration Range for Nitrobacter Inhibition as function of pH (T = 20 0 C).

pH

NH* - N Range, mg/l

NO 2 - N Range, mg/l

6,0

210-2100

30 - 330

6,5

70 - 700

88- 1050

7,0

20-210

260 - 3320

7,5

7 - 70

8,0

2 - 20

g NHg/g VVS day g NHg/g VVS day

Fig 3.13 The influence of [CN"] on the nitrification rate. After Neufeld et al. (1986).

Complexed cyanide was also found to cause a decrease in the maximum reaction rate for nitrification processes in accordance with the following equation:

where [CN] is the complexed cyanide concentration in mg/l.

Using thiocyanate, Fig. 3.14 shows that a plot of Vmax versus the thiocyanate concentration yield a constant reaction rate up to a thiocyanate level of about 236 mg/l. Above this value the reaction rate declined according to the following equation:

0 0

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