Biomedical Engineering Reference
In-Depth Information
(3) Biological treatment includes the aerobic and anaerobic treatment of wastewater by
a mixed culture of microorganisms.
Certain characteristics of wastewater need to be known before treatment. Among themare 1)
physical characteristics, such as color, odor, pH, temperature, and solid contents (suspended
and dissolved solids) and 2) chemical characteristics, such as organic and inorganic
compounds. Major carbon compounds in a typical industrial waste are carbohydrates, lipids
oils, hydrocarbons, and proteins. Other compounds, such as phenols, surfactants, herbicides,
pesticides, and aromatic compounds, are usually in relatively small concentrations (
1 g/L)
but are difficult todegrade by biologicalmeans. Among inorganic compounds present inwaste-
water are nitrogenous compounds (NH 4
and NO 3
), sulfur compounds (SO 2 4
,SO 2 3
,S 0 ,S 2 ,
S 2 2
,andS 2 3
), phosphorus compounds (PO 3 4
, HPO 2 4
,andH 2 PO 4
), heavy metals (Ni 2 þ ,
Pb 2 þ ,Cd 2 þ ,Fe 2 þ ,Cu 2 þ ,Zn 2 þ ,andHg 2 þ ), and dissolved gases, such as H 2 S, NH 3 , and CH 4 .
The carbon content (strength) of a wastewater sample can be expressed in several ways:
biological oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon
(TOC). Normally, a 5-day BOD value is reported. The BOD 5 is the amount of DO consumed
when a wastewater sample is seeded with active bacteria and incubated at 20 C for 5 days.
Since the amount of oxygen consumed is stoichiometrically related primarily to the organic
content of wastewater, BOD is a measure of the strength of wastewater. This stoichiometric
coefficient is not always known, since the composition of the organics is usually unknown.
Also, some nitrogen-containing or inorganic compounds will exert an oxygen demand. If
the only organic compound is glucose, oxygen consumption can be easily related to the
carbon content of wastewater under aerobic conditions.
(12.56)
According to the stoichiometry of this reaction 1.07 g of oxygen is required for the oxida-
tion of 1 g of glucose.
Samples of wastewater need to be properly diluted to obtain an accurate BOD 5 measure-
ment, seeded with active bacteria and incubated at 20 C for 5 days along with an unseeded
blank. BOD 5 is calculated using the following equation:
BOD
C
H
O
6 þ 6
O 2 ! 6
CO
2 þ 6
H
O
6
12
2
5 ¼½ð
DO
Þ t ¼ 0 ð
DO
Þ t ¼5 day sample ½ð
DO
Þ t ¼ 0 ð
DO
Þ t ¼5 day blank
(12.57)
BOD measurements have some shortcomings. This method is applicable only to biodegrad-
able, soluble organics and requires a high concentration of active bacteria preadapted to this
type of waste. Moreover, if organic compounds are refractory, 5 days of incubation may not
suffice, and 20 days of incubation (BOD 20 ) may be required.
COD is a measure of the concentration of chemically oxidizable organic compounds
present in wastewater. Organic compounds are oxidized by a strong chemical oxidant, and
using the reaction stoichiometry, the organic content is calculated. Almost all organic
compounds present in wastewater are oxidized by certain strong chemical oxidants. There-
fore,
the COD content of a wastewater sample usually exceeds the measured BOD
(COD
BOD 5 ).
A typical chemical oxidation reaction is
>
heat
H þ þ 2
O 2 7 !
Cr þ 3
3
CH
O
þ 16
Cr
4
CO 2 þ 11
H 2 O
(12.58)
2
2
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