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Environmental fate & pathways

Additional information on environmental fate and behaviour

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Endpoint:
additional information on environmental fate and behaviour
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Meets generally accepted scientific standards, well documented and acceptable for assessment. In this study the substance Trisodium Nitriloacetate Monohydrate was used and in the field 'Study result type' the indication 'read-across from supporting substance' is made. The difference between the substance Trisodium Nitriloacetate Monohydrate and Trisodium nitrilotriacetate persist only in the water molecules attached to the substance Trisodium Nitriloacetate Monohydrate. As the experiments with Trisodium nitrilotriacetate monohydrate were conducted in aqueous solution, the additional water molecules do not influence the outcome of the study to any extent. The differences in molecular weight between and Trisodium nitrilotriacetate and Trisodium nitrilotriacetate monohydrate were considered in recalculation the respective concentrations for Trisodium nitrilotriacetate. Therefore, this study is not regarded as a 'real' read-across. Therefore, this entry is not listed in the read-across report attached to this dossier.
Cross-reference
Reason / purpose:
reference to same study

Data source

Reference
Reference Type:
publication
Title:
Biodegradation of Nitrilotriacetate in Activated Sludge
Author:
Swisher, R.D.; Crutchfield, M.M.; Caldwell, D.W.
Year:
1967
Bibliographic source:
Environmental Science & Technology, Vol. 1, pp. 820-827

Materials and methods

Principles of method if other than guideline:
A laboratory test was conducted to determine the biodegradation of nitrilotriacetate in activated sludge. Both fill-and-draw (24-hour cycle) and continuous-flow (6-hour and 3-hour retention) activated sludge units were used, fed synthetic sewage containing NTA at levels from 20 to 500 mg/l (throughout the report weights and concentrations are expressed in terms of the free acid (>molecular weight 191); respective concentration range for Na3NTA (> molecular weight 257): 27 - 673 mg/l).
NTA was determined colorimetrically.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
- Name of test material (as cited in study report): nitrilotriacetate (NTA)
- Analytical purity: about 99 %


Results and discussion

Any other information on results incl. tables

Effects of NTA on sludge process

The functioning of the laboratory activated sludge units was checked as described below by examination of LAS removal, chemical oxygen demand (COD) removal, sludge synthesis, and sludge settling. Effects were negligible for the most part, even at NTA levels up to 200 mg/l. There was no indication that any difficulties in sewage treatment plant operation might result from very low concentrations which might be met in the field.

All feeds contained 20 mg/l of C12LAS in addition to the organic nutrients, and the effluents were monitored daily by a modified Hellige methylene blue test to detect any undegraded LAS. All effluents, from control and NTA units alike, almost always showed less than 0.1 mg/l of methylene blue-actives substances (MBAS), indicating that the NTA did not interfere with the biodegradation action of the sludge.

In the activated sludge process a certain fraction of the organic nutrient, perhaps 10-50 %, is converted to new sludge – i.e., bacterial cells – instead of being oxidized completely to CO2abd water, depending on the exact operating conditions. Under the conditions of the present work the amount of newly synthesized sludge is estimated at 20 to 39 % of the nutrient fed. This is greater than the loss of sludge in the effluent due to incomplete settling, resulting in a gradual increase in mixed liquor suspended solids and necessitating occasional removal of solids if the level is to be held in the desired range. Throughout this work there was no significant difference between the control units and the NTA units with respect to the net increase in solids, except when NTA was fed at 500 mg/l as the sole carbon source, discussed below.

Finally, there were no significant effects of NTA on the settling rate of the sludge at the concentrations observed.

 

NTA as the sole carbon source

Pending the development of the analytical method for NTA a semicontinuous activated sludge unit was put into operation, starting with fresh sludge and the standard feed plus an added 80 mg/l of NTA. During the next 3 months the NTA level was raised to 160, 150, 350, and finally 500 mg/l, with corresponding reduction in the amount of standard feed down to zero, and operation was continued at the final level for another 6 months. During this period the NTA served as the sole source of organic carbon, except for the small amount supplied by the 20 mg/l of LAS.

Operation of the unit was smooth throughout, and LAS removal was essentially complete. From the MBAS effluents observed and from the production of sludge the authors conclude that there is no doubt that the NTA can serve as the sole source of organic carbon for the nutrition of the sludge, and hence that NTA is biodegradable.

NTA biodegradation rates

Extensive biodegradation of NTA was clearly evident. The degradation rate of NTA in the semicontinuous operation corresponds to a half-life of about 4 to 6 hours. From this one might anticipate perhaps 50 to 75 % degradation in a continuous flow unit with 6-hour retention time, and 25 to 50 % degradation in a 3-hour unit. In actual fact, the degradation is well over 90 % at either rate, once acclimation has occurred. Thus the biodegradation rate in the continuous flow systems correspond to a half-life considerably less than 3 hours and is considerably faster than in the semicontinuous units.

Besides, the authors were able to demonstrate that NTA biodegradation required prior acclimation.

Applicant's summary and conclusion

Conclusions:
The present results show that the activated sludge sewage treatment process readily degrades NTA, as evidenced by loss of its chelating properties. Furthermore, the authors proposed glycine and glycolic acid to be the most likely intermediate degradation products. Both intermediates should enter the natural biochemical processes with no difficulty because they are natural metabolic intermediates in living cells.
These considerations suggest that no significant incremental effects are to be anticipated when such sewage enters the environment after normal sewage treatment. The degradability of NTA by the bacterial species which constitute activated sludge also suggest that even when raw sewage is released without treatment, NTA herein will be degraded in the receiving waters and soils.
Executive summary:

The results of the laboratory study conducted indicate that no significant effects upon the general environment are to be anticipated, since substantially complete biodegradation of NTA could be demonstrated. Both fill-and-draw (24-hour cycle) and continuous-flow (6-hour and 3-hour retention) activated sludge units were used, fed synthetic sewage containing NTA at levels from 20 to 500 mg/l. The added NTA caused no upsets in normal functioning of the units. NTA can serve as the sole source of organic carbon for activated sludge, demonstrated by operation of a fill-and-draw unit for 6 months under such conditions. NTA analyses were by a non-specific method involving chelation of ferric iron.