Suspended solids
30 mg/L
Other than high residues of nitrogen and phosphates in the effluent, treatment
plant operators are content with a BOD of 20 mg/L and suspended solids of 30 mg/L
or thereabouts. The July 1977 effluent guidelines for publicly owned secondary
treatment plants issued by the Environmental Protection Agency under the authority
of the 1972 amendments to the Federal Water Pollution Control Act [1] specify the
maximum monthly average effluent BOD and suspended solid requirements to be
30 and 45 mg/L, respectively. One should not overlook also the presence of
refractory organics in the effluent that could eventually exert an oxygen demand
on the receiving water. Tertiary treatment processes will be needed to polish the
effluent in order to achieve the goal of zero pollution discharge.
1.2 Principles of Biological Oxidation
In the biological oxidation of wastewater, both synthesis and oxidation occur. Many
groups of activated sludge microorganisms take part in carrying out the process. The
important groups are described in Table 3.1.
In an activated sludge system that is properly operated, biological flocs are
produced that incorporate most of the important groups of microorganisms. Metazoa
and the fast moving protozoa are not part of the biological flocs because they can
break away from them. Nevertheless, metazoa and protozoa constantly graze on
biological flocs and are consequently found together in a highly stabilized wastewater. Although the ecosystem of an activated sludge process is complex, some
general principles of biological oxidation can be applied [2–10].
1.2.1 Physical Adsorption
It has been observed that a fast initial removal of organics usually occurs when the
wastewater is contacted with activated sludge in an aeration tank. This initial
removal can be accomplished in a few minutes. The removal rate depends upon
the wastewater characteristics and the volatile solid concentration of the activated
sludge. The removal is interpreted as an adsorption phenomenon and was reported as
early as 1939 by Ruchhoft [2].
The initial adsorption removes primarily discrete and colloidal particles. The
adsorbed organic matter is subsequently oxidized or used in the synthesis of cellular
components. It is important to recognize that, once adsorbed onto the biological
flocs, the removal of organics from the wastewater is complete since the flocs are to
be separated in the final clarifier. However, initial adsorption has little or no practical
effect on dissolved organics. This explains why a contact stabilization process can
take full advantage of initial adsorption only when it is applied to the treatment of
wastewater containing large amounts of colloids and easily adsorbed solids [11, 12].
3 Biological Processes
79
30 mg/L
Other than high residues of nitrogen and phosphates in the effluent, treatment
plant operators are content with a BOD of 20 mg/L and suspended solids of 30 mg/L
or thereabouts. The July 1977 effluent guidelines for publicly owned secondary
treatment plants issued by the Environmental Protection Agency under the authority
of the 1972 amendments to the Federal Water Pollution Control Act [1] specify the
maximum monthly average effluent BOD and suspended solid requirements to be
30 and 45 mg/L, respectively. One should not overlook also the presence of
refractory organics in the effluent that could eventually exert an oxygen demand
on the receiving water. Tertiary treatment processes will be needed to polish the
effluent in order to achieve the goal of zero pollution discharge.
1.2 Principles of Biological Oxidation
In the biological oxidation of wastewater, both synthesis and oxidation occur. Many
groups of activated sludge microorganisms take part in carrying out the process. The
important groups are described in Table 3.1.
In an activated sludge system that is properly operated, biological flocs are
produced that incorporate most of the important groups of microorganisms. Metazoa
and the fast moving protozoa are not part of the biological flocs because they can
break away from them. Nevertheless, metazoa and protozoa constantly graze on
biological flocs and are consequently found together in a highly stabilized wastewater. Although the ecosystem of an activated sludge process is complex, some
general principles of biological oxidation can be applied [2–10].
1.2.1 Physical Adsorption
It has been observed that a fast initial removal of organics usually occurs when the
wastewater is contacted with activated sludge in an aeration tank. This initial
removal can be accomplished in a few minutes. The removal rate depends upon
the wastewater characteristics and the volatile solid concentration of the activated
sludge. The removal is interpreted as an adsorption phenomenon and was reported as
early as 1939 by Ruchhoft [2].
The initial adsorption removes primarily discrete and colloidal particles. The
adsorbed organic matter is subsequently oxidized or used in the synthesis of cellular
components. It is important to recognize that, once adsorbed onto the biological
flocs, the removal of organics from the wastewater is complete since the flocs are to
be separated in the final clarifier. However, initial adsorption has little or no practical
effect on dissolved organics. This explains why a contact stabilization process can
take full advantage of initial adsorption only when it is applied to the treatment of
wastewater containing large amounts of colloids and easily adsorbed solids [11, 12].
3 Biological Processes
79