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Environmental fate & pathways

Biodegradation in water: screening tests

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Endpoint:
biodegradation in water: screening tests
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
The biodegradation of cyanuric acid in representative anaerobic systems was described. Primary settled domestic sewage to which cyanuric acid was added was allowed to become, or was made, anaerobic to examine cyanuric acid levels. Anaerobic mixed liquor containing 14C-cyanuric acid tracer solution; anaerobic nutrient broth with mixed inocula from sewage plant effluent and muds and soils, which also contained added cyanuric acid, were studied. 14CO2 evolved was carried by N2 through a bubble trap containing NaOH solution. This solution plus rinsings were analysed for 14C activity by liquid scintillation counting. Approximately 2 × 106 dpm were used per experiment.
GLP compliance:
no
Oxygen conditions:
anaerobic
Inoculum or test system:
other: anaerobic sewage, mixed liquor and nutrient broth
Details on results:
Anaerobic sewage:
If primary settled domestic sewage to which cyanuric acid was added was made anaerobic the cyanuric acid concentration was reduced by 25 – 50% in 48 h, and complete disappearance of the cyanuric acid was observed within 72 – 96 h. The total Kjeldahl nitrogen after the three week incubation rose from 65 μg/mL to 209 μg/ml, and the ammonia nitrogen rose from 52 μg/ml to 191 μg/mL. As the nitrogen equivalent of 430 μg of cyanuric acid per ml is 140 μg/mL, all of the ammonia and Kjeldahl nitrogen increase was accounted for by conversion of cyanuric acid.

Anaerobic mixed liquor:
The 14C was evolved as 14CO2 at 4% within 7 h, 11% (total) within the next 17 h and 82% (total) in 17 days. Essentially, no 14C from cyanuric acid was synthesized into biomass. However, in one repeat of the experiment, the 14CO2 evolution was 50% in 8 days, 71% in 13 days and in a third repeat of the experiment, the 14CO2 evolution was 93% in 6 h. Therefore mixed liquor activity was very variable.

Anaerobic nutrient broth:
The 14CO2 in the effluent gasses contained 80% of the radioactivity initially added.

Conclusions:
Cyanuric acid biodegrades readily under a wide variety of natural conditions, and particularly well in anaerobic activated sludge and soils. CO2 and ammonia are the initial hydrolytic breakdown products.
Endpoint:
biodegradation in water: screening tests
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
Biodegradation of cyanuric acid in an aerobic system is described. Primary settled domestic sewage to which radiolabelled cyanuric acid was added, was used. 14CO2 evolved was carried by air through a bubble trap containing NaOH solution
GLP compliance:
no
Remarks:
Predates GLP
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge (adaptation not specified)
Details on inoculum:
Activated sludge was maintained in the laboratory in continuous-flow units of the Eckenfelder-Ford type at 2 g/l solids content using either raw or pasteurised domestic sewage as feed. An automated draw-fill activated sludge unit was operated in the laboratory on an 8 h cycle: a 7h period of stirred aeration, a 55 min settling period, a 2 min period for drawing off the upper two thirds of the volume and 3 min for refilling with refrigerated domestic sewage. . The sewage reservoir was filled manually every 4 – 6 days. In this unit, air was introduced into the mixed liquor via a fritted-glass dispersion tube.
Parameter:
% degradation (DOC removal)
Value:
100
Sampling time:
8 h
Details on results:
Cyanuric acid exerts no biological oxygen demand in aerobic media. In highly aerobic media cyanuric acid resists biodegradation. The ability to degrade cyanuric acid was rapidly gained and lost by bacteria grown in aerated medium when the dissolved oxygen was lowered and raised. Organisms which degrade cyanuric acid multiply in both aerobic and anaerobic conditions and do not require any acclimatisation to be active for cyanuric acid decomposition. The degradation activity is turned on and off, with a time lag of a few minutes, when the environment is made anaerobic or aerobic.
With 1 to 3 μg of dissolved oxygen per ml, good removal of cyanuric acid occurs in continuous flow laboratory-aerated sewage units if the residence time is at least 6 h. At uncharacteristically high dissolved oxygen, the removal is poorer but considerable if the residence time is longer
Conclusions:
In highly aerobic media cyanuric acid resists biodegradation. Anaerobic growth in sewage degrades cyanuric acid.

Description of key information

Supporting information as cited in the literature has been provided which demonstrates that the biodegradation of cyanuric acid is dependent on the specific microorganisms present.  

Key value for chemical safety assessment

Additional information

Normally, cyanuric acid degrades very slowly under aerobic conditions since the majority of aerobic microorganisms do not possess the genes to produce the specific enzymes required to degrade cyanuric acid. As a result, cyanuric acid shows minimal biodegradation in standard screening tests conducted at atmospheric oxygen levels and with no acclimation.

 

However, cyanuric acid can be degraded much more rapidly if: 1) specific fungal or bacterial strains are present which contain the genes/enzymes required, 2) the microorganisms have been acclimated to cyanuric acid, and 3) organic nutrients are present for the microorganisms. Furthermore, since bacteria degrade cyanuric acid in order to obtain ammonia for use in synthesis of biomass, the biodegradation of cyanuric acid is normally more pronounced if the ambient conditions are nitrogen-limited, but will be less likely to occur if more readily degraded sources of ammonia-nitrogen are present.