Biodegradation of Nitrilotriacetate in Activated Sludge R. D. Swisher, M. M. Crutchfield, and D. W. Caldwell Inorganic Chemicals Division, Monsanto Co., St. Louis, Mo. 63166
rn Nitrilotriacetate may become an important component in detergent formulations in the future, and thus may become a minor component of domestic sewage. However, no significant effects upon the general environment are to be anticipated, since laboratory studies show substantially complete biodegradation. 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. per liter; the NTA-iron complex was degraded as well as free NTA. The added NTA caused no upsets in the 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 nonspecific method involving chelation of ferric iron.
N
itrilotriacetic acid (NTA) (1) contains three carboxyl groups and a bisic nitrogen in a single rather ~~. small molecule. Strong chelating properties result; in aqueous solution the nitrilotriacetate ion combines stoichiometrically with polyvalent metal ions to give soluble complex ions of such stability that many of the characteristic chemical properties of the free metal ions are no longer exhibited.
/CH?CooH N-CH2COOH
Experinientrrl Moterids
(I)
'CH,CO~H Chelation of the calcium and magnesium ions present in hard water is one of the several functions performed by sodium tripolyphosphate (STP) in detergent formulations, and it has been found advantageous in some circumstances to replace a part of the STP by NTA. Complete replacement does not appear feasible, because some deterioration in performance then results. If use of NTA in detergent formulations became common commercially, the NTA would pass into sewage, and if not degraded therein, the average level might be on the order of several milligrams per liter. The present work was undertaken t o gain some insight into its subsequent fate and the consequent environmental effects which might be anticipated. Meanwhile, Bunch and Ettinger (1967) have reported that NTA is biodegradable under the conditions of the Bunch820 Environmental Science and Technology
Chambers (1967) test: a synthetic medium inoculated with sewage organisms. The present results show that the activated sludge sewage treatment process readily degrades NTA, as evidenced by loss of its chelating properties. Furthermore, the most likely intermediate degradation products would be glycine and glycolic acid; these are not at all exotic, but instead should enter the natural biochemical processes with no difficulty because they are natural metabolic intermediates in living cells. If the first step of the degradation is hydrolytic, these two products should be formed directly. If, on the other hand, the first step is oxidative, they still might appear at a later stage in the fragmentation of the molecule. These considerations suggest that no significant incremental effects are to be anticipated when such sewage enters our environment after normal sewage treatment. The degradability of NTA by the bacterial species which constitute activated sludge also suggests that even when raw sewage is released without treatment, NTA therein will be degraded in the receiving waters and soils via the same biochemical mechanisms, carried out by the naturally occurring bacteria which belong to these same species. Under such circumstances, small amounts of undegraded NTA might temporarily be present, but this should introduce no hazard beyond that of the raw sewage itself, because NTA is substantially nontoxic. Pollard (1966) has reported the absence of any effects by NTA on fish up to at least 100 mg. per liter, and on mammals fed orall!. up to several thousand milligrams per kilogram.
NTA. Hampshire Chemical Division, W. R . Grace & Co., trisodium salt monohydrate, molecular weight 275, said to be about 99 pure. Although introduced into the biological systems in this form, throughout this report weights and concentrations are expressed in terms of the free acid, molecular weight 191. Thns. a 100 mg. per ml. stock solution would be made by dissolving 145.4 grams of the Hampshire product in water and diluting to 1 liter. C l nL,4S (Linear Alkglbenzene Sulfonate). Soap & Detergent Association (SDA), 1-dodecene derived reference LAS No. 2, November 1964, 8 9 . 8 z active. A mixture of 2-, 3-, 4-, 5 - , and 6-phenyldodecane sodium sulfonates made by alkylation of benzene with l-dodecene and sulfonation of the product. Biodegrridariori Procedures
Semicontinuous .4ctivated Sludge. The confirmatory test procedure of the SDA (1965) was followed. using activated
sludge obtained from a plant treating mostly domestic sewage. Briefly the SDA procedure calls for fill-and-draw operation using 1500 ml. of mixed liquor, suspended solids of about 2500 mg. per liter, 23-hour aeration, lin-hour settling, and 1 / 2 hour for withdrawing 1 liter of supernatant effluent and refilling with 1 liter of fresh feed. Standard feed was made from tap water b> adding:
Glucose N broth" Beef extract KZHPO, (NH4)2S04 CI2LAS (active)
Llg.:Liter 130 130 130 130 25 20
N broth consists of tryptose (a peptone) 20 grams, NaCl 5 grams, IGHP04 4 grams, I
