Rernovinn nitrogen from waste water W
w
Carl E. Adams, Jr. Associated Water & Air Resources Engineers. Inc., Nashville. Tenn. 37204 The forms of nitrogen most prevalent in waste waters, arid which require treatment, are ammonia (as N H d - ) , nitrate (as N o 3 - - ) , and organic nitrogen. for example, in the amine form. Many industrial wastes also contain nitrated organics which are extremely difficult to remove. The most practical technology for removing ammonia from waste waters includes biological synthesis and nitrification. ion exchange, air and steam stripping, and chlorination. Nitrates are best treated by biological denitrification and ion exchange. The organic forms of nitrogen may. in some cases. be removed by carbon adsorption, although generally biological synthesis and conversion to other more readily removed forms are more practical. Biological synthesis and nitrification of aminonia Of the available treatment alternatives for ammonia removal. biological synthesis and nitrification are the most widely practiced. either unintentionally or by design. In the destruction and assimilation of organic compounds. tiiologica organisms will require, as nutrients, approximately 4.!5 Ib of nitrogen per 100 Ib of BOD removed. Thus. in estimating nitrogen rernoval through a system by synthesis, only the BOD removed and not that applied should be considered. Since nitrcgen which is removed by synthesis is incorporated. at least temporarily, into the microbial cell. it is eliminated only by removal of excess sludge from the system. A system \Nith poor settleability properties cannot be expected to demonstrate a total remova' of 4 . 5 lb of riitrogen per 1 C O I:) BOD because of the kiigh carryover 3f organisms in the effluent. In this case. mixed media filtration may be required to achieve desired effluent qualit y Also. if thc system has an excessive detention time '31 a verv long s udge age (greater than 6-8 days), nitrogen in the form c;f ammonia or organic nitrclgen may be rsleased back into solution so that once again an assumed removal of 4 5 It) N: 100 Ib BOD removal would not be valid. During biologlcal nitrification. ammonia and some organic nitrogeri forms are converted to nirrate compounds by a specific set of organisms. Nitrosomrrlonas and Nitrobacter. These organisms are strictly aerobic autotrophs 1.e.. they obt;3in carbon from inorganic !sources such as COz or H C 0 2 ~ and , do not employ the same metabolic mechanisms as those organisms which assimilate BOD materials. The nitrifiers reproduce and grow at much lower rates than the BOD-removing organisms and thus compete in t i e same environment. For example, in an activated slucige system a constant concentration of organisms or suspended solids (MLVSS) is desired, so that all excess sludge produced and accumLilated is periodi696
Environvsntal Science & Technology
cally wasted. I f BOD-removing organisms are producing at a rapid rate, they will control the required wastage of sludge from the system. which will generally be much higher than the wasting rate required to maintain an adequate population of nitrifiers. Thus. insufficient nitrification will result unless the wastage rate is lowered to accommodate the nitrifier requirements. The wastage rate is usually controlled to maintain a sufficient "slbdge age" in the system for nitrification.
Where control of nitrogen in wa water is applied Flow, Location
MUNlClPAL Washington, D.C. Tampa, Fla. Alexandria, Va. Salt Creek (Chicago), 111. Cleveland, Ohio Arlington, Va. Madison, W is. Fairfax County, Va. Flint, Mich. Waukegan, 111. Highland Park, 111. Gurnee, 111. Jackson, Mich. Orange County, Calif. Benton Harbor, Mich. Owosso, Mich. Central Contra Costa, Calif. Rosemont, Minn. Et Lago, Texas INDUSTRIAL Farmers Chemical Association Chattanooga, Tenn. Mallinckrodt Chemical, Inc. Raleigh, M.C. Schovall-Schrader Dixon, Tenn.
mgd
Type facility
54
Suspended growth system Fixed film system Ion exchange
50 50 30 30
Fixed film system Ion exchange Breakpoint chlorination Nitrification
22.5
17
Ion exchange Nitrification Nitrificatron Nitrification Nitrification Nitrification
15
Ammonia stripping
13 6
Nttrification Breakpoint chlorination
300 60
20 20 18 17
1 06 0.5
Suspended growth system Ion exchange Fixed film system
1
ion exchange
0 12
Biological nitrification
0 036 Air stripping
Note This IS a partial ltsting of full-scale nitrogen control treatment plants under design construction, or operation
The forms of nitrogen most prevalent in waste water, and which require treatment, are ammonia, nitrate, and organic nitrogen; when one or more of these pollutants are present in municipal and industrial effluents, practical technology can provide treatment alternatives
Historically, published data have claimed that a sludge age greater than 4 days in the activated sludge process is adequate for 90% nitrification at 20°C. However, this information was developed for municipal wastes and should never be applied to higher strength industrial discharges without extensive pilot studies. In the absence of severe inhibitors, a single-stage activated sludge system can be properly designed to achieve both BOD removal and nitrification in a single aeration basin. Effluent ammonia levels less than 10 mg/l. should be consistently achieved depending on the variability of the influent ammonia concentration. I f biodegradable inhibitors are present or effluent levels less than 5 mg/l. of ammonia-nitrogen are required, then a two-stage system will probably be necessary. The two-stage system, where BOD is removed in a separate aeration basin-clarifier system, followed by nitrification in a separate aeration basin-clarifier system, is more efficient for nitrification but involves considerable capital expenditures for two sets of clarifiers, which are designed purely on hydraulic constraints. If a two-stage system becomes necessary, the first set of clarifiers should be designed to obtain a desired underflow concentration for recycle-i.e., a mass ' loading of solids flux concept. Strict attention should not be given to effluent suspended solids since these will be taken out in the second set of clarifiers, which should consider effluent clarity-i.e., zone-settling design concept. The theoretical oxygen requirements, based on the biochemical equations of nitrification, have been defined as 4.57 Ib 0 2 required/lb ammonia nitrified. Generally, this oxygen demand may be satisfied by atmospheric molecular oxygen, furnished and dissolved by conventional aeration equipment. However, since the nitrifiers are autotrophic and obtain their carbon requirements from carbon dioxide-bicarbonates, for example-the oxygen contained in these compounds may also be available for metabolism. Thus, depending on the alkalinity of the waste water, the actual oxygen which must be furnished by
aeration equipment may be lower than the theoretical 4.57 ratio. Discounting the ammonia required for BOD removal, the nitrifiers will also utilize a fraction of the available nitrogen for synthesis of cellular components. This ammonia demand is estimated to be equivalent to 0.7 oxygen equivalents; therefore, the theoretical oxygen ratio of 4.57 would be reduced to about 3.9 based solely on influent and effluent ammonia concentrations. Biological nitrification by the activated sludge process has been applied successfully to ammonia concentrations as high as 500 mg/l. with greater than 90% removal with the single-stage system. Two-stage systems have achieved greater than 97% removal with these high influent levels. Since the nitrifiers are biological organisms, they are subjected to the same constraints which limit most biological systems-i.e., temperature, toxicity, shock loading, or pH. Nitrification has generally been found most efficient in the pH range of 7.8-8.3, and inefficient nitrification systems can be improved by adjusting the influent pH level so that the aeration basin contents are within the optimum range. Sodium hydroxide is recommended for the pH adjustment since lime may precipitate calcium carbonate and limit the system from the standpoint of available inorganic carbon, particularly with high ammonia wastes. Lime may be satisfactory for low-strength ammonia waste waters. Compounds which are most noted for toxicity to nitrifiers include heavy metals, cyanides, halogenated compounds, phenols, mercaptans, guanidines, and thiourea. The presence of trace amounts of these constituents may not nullify nitrification, but could lower the rate. The optimum temperature for nitrification is 28-32°C with nitrification ceasing below 5 " ~ .If severe temperatures are anticipated (less than 12-15°C in the aeration basin), then serious consideration should be given as to the feasibility of nitrification. It is possible to preheat the influent waste stream, but this is not a practical solution with most large streams. Nitrifiers are inhibited below aeration basin levels of 3.0 mg/l. of dissolved oxygen (D.O.). Individual nitrifiers are probably not actually inhibited, but the efficiency of the larger flocs is limited owing to the decreased penetration of oxygen at lower D.O. levels. Well-agitated systems in the laboratory achieved comparable results at low D.O. levels (less than 1.0 mg/l.) as did parallel systems at D.O. levels greater than 3.0 mg/l., indicating the response of floc size to dissolve oxygen. Whatever the mechanism, the nitrification efficiency will decrease with low-aeration basin D.O.levels unless sufficient agitation is provided to shear the larger flocs and provide more Volume 7, Number 8, August 1973
697
i v e ; ~T h , ir-, turn may adversely affect the setpfooer:iei 0'the sludge in the final clarifier. Another method of increasing nitrifier efficiency during colder period'; I:, to increase the concentration of orgar i i s t n s by inc,eased solids recycle. A 40% increase of M LVSS conccnt,ation from 2500-3500 rng:/l. should increase the grc
