Five-day final initial [BOD ]=[DO] – [DO] Depending

Five-day BOD (BOD5)

The BOD test
is a standardized test that 5 provides information regarding the organic
strength of wastewater. The amount of oxygen consumed in a sample within a
five-day period is measured under carefully controlled and standardized
conditions. Generally, the five-day period is not long enough for complete
oxidation, but it provides sufficient time for microbial acclimation (lag-phase
growth as seen during the first day in Figure 2) and for substantial
(approximately 40 to 80 percent) oxidation. The five-day period has been widely
retained, having its historical roots in early water quality studies when it
was determined that no stream in England had a travel time of greater than five
days to the ocean. The BOD , expressed as mgO 5 2 L-1 (or equivalently as
parts per million, ppm), is the difference between the initial dissolved oxygen
(DO) measurement and the corresponding (final) measurement made on the fifth
day of incubation.

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5 final
initial BOD =DO – DO

Depending on
the nature of the sample, it is either diluted or microbially seeded and
additional nutrients are added. In these cases the equation is slightly
modified. Since this test has been used for regulatory purposes for several
decades, a wealth of information for a large number of effluent types is
available. Unfortunately, little kinetic information can be derived from this
test, and it provides little insight for modeling purposes.

 

Ultimate BOD (UBOD)

The ultimate
biochemical oxygen demand (UBOD) is a parameter that quantifies the oxygen
required for the total biochemical degradation of organic matter by aquatic
microorganisms. UBOD and the rate of oxygen consumption are frequently used
in mathematical models to predict the impact of an effluent on receiving bodies
such as lakes and rivers. The rate of oxygen consumption is therefore often
determined along with the UBOD value in the analytical test. A distinction is
made between the carbonaceous oxygen demand (CBOD) and nitrogenous oxygen
demand (NBOD) during the measurement as well as in many water quality models.
Both CBOD and NBOD contribute to the overall UBOD, but the values and rates of
oxidation differ.

 

Carbonaceous Oxygen
Demand (CBOD)

CBOD is the
oxygen consumed during the oxidation of carbonaceous compounds to carbon
dioxide (CO ) and other oxidized end products. Reduced organic carbon ranges 2
in form from labile (highly biodegradable, e.g., sugars) to almost refractory
(e.g., cellulose) compounds. In reality, the oxidation of organic carbon
consists of a series of biochemical reactions mediated by a variety of
microorganisms feeding on either the substrate or other microorganisms involved
in the oxidation process. Formulations of the CBOD breakdown, however, are
described using simplified oxidation kinetics. The most common equation to
describe the oxidation reaction is a first-order dependency on the CBOD
concentration,

where CBOD
is carbonaceous biochemical oxygen demand remaining, usually in mgO2 L-1, k is
the first-order reaction rate constant, usually d-1, and DO is dissolved
oxygen concentration in mgO2 L-1. This equation can be integrated resulting in
the following: 0 CBOD CBOD kt e? = × (5) where 0 CBOD is the initial CBOD
concentration while t is time in days. The ultimate CBOD can be estimated by
running the experiment until all of the organic carbon is oxidized. However,
this can take between 20-50 days or in some cases much longer. This method has
several limitations that will be discussed later. Modifications of the test
have been proposed to achieve results faster; two such methods are the Thomas
Slope Method and an approximation using the BOD5. The estimate based on the
BOD5 value is based upon the exponential (first-order) nature of oxygen
demand. The ultimate carbonaceous oxygen demand is than Ultimate-CBOD = BOD5 ×
(1 – e-kt) -1 (6) – -1 Ultimate-CBOD = BOD (1- ) 5 kt × e where BOD5 is
the biochemical oxygen demand that is exerted over the five day period.

The value of
the reaction rate constant, k , is determined experimentally or from tabulated
values. Readily degradable wastes (e.g., domestic wastewater) will have higher
(faster) coefficients (0.3 to 0.7 d-1) whereas less readily degradable sources
(e.g., river water) will have lower rates (0.1 to 0.2 d-1). An assumption when
estimating the ultimate CBOD value is that nitrogenous compounds are inhibited
and do not contribute to the overall oxygen consumption.

 

 

 

Nitrogenous Oxygen Demand (NBOD)

NBOD is the
oxygen consumed during the oxidation of nitrogenous compounds (mainly NH3 ) to
nitrate with nitrite being an unstable intermediate. Different from CBOD, only
two classes of bacteria are believed to be responsible for the oxidization of
reduced nitrogen. These bacteria (nitrifiers) are surface-based (associated
with suspended solids), and therefore are usually only present in the water in
low concentrations. In Figure 2, the NBOD does not become discernable until
approximately 7 days of incubation have occurred. This is not uncommon as the
nitrifiers typically have slower growth rates and do not flourish until the
food supply for the heterotrophic (CBOD consuming) microbes has diminished
(i.e., as BOD exerted approaches the ultimate CBOD and the CBOD remaining
approaches zero). The standard analytical test may result in incorrect results
because the growth of nitrifiers on the surface of the sample bottle, known as
bottle effects, can artificially enhance the nitrification. For this reason, a
short-term measurement (1 to 3 days) is suggested to estimate NBOD. An
accurate method to measure NBOD, is to track the ammonia (or total Kjeldahl
nitrogen, TKN, as a surrogate) concentration over a 1 to 3 day period. NBOD
(and rate of oxygen consumption) is estimated using the stoichiometric value of
4.57, although a lower value has also been used since some of the nitrogen is
consumed for cell maintenance. The rate of oxygen demand in samples can thus be
calculated:

3 n 3 DO NH
4.57 4.57 NH d d k dt dt =× =×

where n k is
nitrification rate (typically d-1). Integrating and solving the above equation
results in n

 3 30 NH NH k t e? = ×

Attempts
have been made to measure the CBOD and NBOD rates simultaneously, but this
often results in an incorrect BOD value due to bottle effects.