Environmental Engineering FOURTH EDITION

90 ENVlRONMEN'IAL ENGINEERING

BOD

0 5 10 15 20

Day

Figure 5-5. Long-term BOD. Note that BODdt here includes both ultimate carbona-
ceous BOD (Lo) and ultimate nitrogenous BOD.

ammonia (NH3; NH: in ionized form), which is further decomposed into nitrite (NO,)

and nitrate (NO;):

NH2 - CO NH2 + H20 + 2NH3 + C02 ammonification (5.11)

NH: + 1$02 + NO, + 2H' + H20 nitrification, step 1 (5.12)

NOT + $02 + NO:. nitrification, step 2 (5.13)

Note that the first step, ammonification, does not require oxygen; it can be done by a

wide variety of aerobic and anaerobic plants, animals, and microbes.

The BOD curve is thus divided into nitrogenous and carbonaceous BOD areas.

The ultimate BOD, as shown in Fig. 5-5, includes both nitrogenous and carbonaceous

BOD. For streams and rivers with travel times greater than about 5 days, the ultimate

demand for oxygen must include the nitrogenous demand.

Although the use of BODdt (carbonaceous plus nitrogenous) in dissolved oxygen

sag calculations is not strictly accurate, the ultimate BOD may be estimated as

BODdt = a(BOD5) + b(TKN), (5.14)

where TKN is the total Kjeldahl nitrogen (organic nitrogen plus ammonia, in ma),

and a and b are constants.

The state of North Carolina, for example, has used a = 1.2 and b = 4.0 for cal-

culating the ultimate BOD, which was then substituted for the ultimate carbonaceous

BOD (Lo) in the dissolved oxygen sag equation.