A LOW COST OPEN TANK
PURE OXYGEN SYSTEM FOR
HIGH RATE TOTAL OXIDATION
D.B. COHEN
Operations Research Chemist, Metropolitan Denver Sewage Disposal
District No. 1, Colorado, U.S.A.
Activated sludge plants for waste water treatment, particularly in developing countries,
are uncommon primarily because of the high capital as well as operating costs required
for air compression equipment and excess sludge disposal.
For example, the Metropolitan Denver Sewage Disposal District No. 1 raised $17
million in 1961 to construct a 117 MGD plant serving a population of approximately
850,000. Over half the capital outlay ($9 million) went into the construction of the
aerator and compressor facilities. Two-thirds of the annual operations budget ($4 million)
is spent on concentrating, dewatering, and disposal of the sewage sludge at a cost of
approximately $60 per ton.
These high capital and operating costs are a direct consequence of the very low oxygen
transfer efficiencies that can be obtained from present day conventional aeration
technologies.
The three principal oxygenation methods presently employed in activated sludge
plants include: diffused compressed air, dispersed compressed air (spargers) and
mechanical surface aerators.
The limiting factor for each of these methods has been the cost of maximizing the
total gas/liquid interfacial area. Oxygen transfer efficiencies for the various methods vary
between 3-15%, with energy requirements ranging between 1-5 pounds 0 2 /HP/HR. Most
of the transfer efficiencies reported were determined in clean water, and that after
correction for the alpha factor (which compares the transfer coefficient in waste water
with that in clean water), these efficiencies are reduced by 25-40%.
During the past two decades many investigators have studied the feasibility of
substituting pure oxygen for compressed air as a means of increasing the efficiency of the
activated sludge process. Budd and Lambeth (1) in 1969 reported oxygen absorption
efficiencies as high as 25%, using pure oxygen. Okun (2) concluded that the substitution
of oxygen for air in this process would not be economically feasible because of the low
transfer efficiencies inherent in standard diffuser design.
McKinney and Pfeiffer (3), however, concluded that if an efficient means of overall
oxygen utilization (90%) could be achieved, it would be possible to 1) reduce power
required per unit of 0 2 transferred, 2) reduce or eliminate periods of zero oxygen
concentration, 3) reduce plant size and capital investment, 4) increase plant performance
(final effluent quality), 5) increase rate of stabilization of sewage sludge, 6) increase
organic and volumetric loadings (#BOD/1000 ft
3 ) and 7) increase capacity of overloaded
plants without additional aerator facilities.
The first breakthrough in this field was announced in 1970 by Union Carbide
Company. A three MGD pilot study at Batavia, New York (4) demonstrated that by
hermetically sealing six aeration basins in series and providing them with oxygen gas
circulation and recompression, the relatively inefficient Sparger system could be made to
provide oxygen utilizations in excess of 90%. The higher driving force of the pure oxygen
system (6-10 mg/liter D.O.) also resulted in a net decrease in waste activated sludge
389
PURE OXYGEN SYSTEM FOR
HIGH RATE TOTAL OXIDATION
D.B. COHEN
Operations Research Chemist, Metropolitan Denver Sewage Disposal
District No. 1, Colorado, U.S.A.
Activated sludge plants for waste water treatment, particularly in developing countries,
are uncommon primarily because of the high capital as well as operating costs required
for air compression equipment and excess sludge disposal.
For example, the Metropolitan Denver Sewage Disposal District No. 1 raised $17
million in 1961 to construct a 117 MGD plant serving a population of approximately
850,000. Over half the capital outlay ($9 million) went into the construction of the
aerator and compressor facilities. Two-thirds of the annual operations budget ($4 million)
is spent on concentrating, dewatering, and disposal of the sewage sludge at a cost of
approximately $60 per ton.
These high capital and operating costs are a direct consequence of the very low oxygen
transfer efficiencies that can be obtained from present day conventional aeration
technologies.
The three principal oxygenation methods presently employed in activated sludge
plants include: diffused compressed air, dispersed compressed air (spargers) and
mechanical surface aerators.
The limiting factor for each of these methods has been the cost of maximizing the
total gas/liquid interfacial area. Oxygen transfer efficiencies for the various methods vary
between 3-15%, with energy requirements ranging between 1-5 pounds 0 2 /HP/HR. Most
of the transfer efficiencies reported were determined in clean water, and that after
correction for the alpha factor (which compares the transfer coefficient in waste water
with that in clean water), these efficiencies are reduced by 25-40%.
During the past two decades many investigators have studied the feasibility of
substituting pure oxygen for compressed air as a means of increasing the efficiency of the
activated sludge process. Budd and Lambeth (1) in 1969 reported oxygen absorption
efficiencies as high as 25%, using pure oxygen. Okun (2) concluded that the substitution
of oxygen for air in this process would not be economically feasible because of the low
transfer efficiencies inherent in standard diffuser design.
McKinney and Pfeiffer (3), however, concluded that if an efficient means of overall
oxygen utilization (90%) could be achieved, it would be possible to 1) reduce power
required per unit of 0 2 transferred, 2) reduce or eliminate periods of zero oxygen
concentration, 3) reduce plant size and capital investment, 4) increase plant performance
(final effluent quality), 5) increase rate of stabilization of sewage sludge, 6) increase
organic and volumetric loadings (#BOD/1000 ft
3 ) and 7) increase capacity of overloaded
plants without additional aerator facilities.
The first breakthrough in this field was announced in 1970 by Union Carbide
Company. A three MGD pilot study at Batavia, New York (4) demonstrated that by
hermetically sealing six aeration basins in series and providing them with oxygen gas
circulation and recompression, the relatively inefficient Sparger system could be made to
provide oxygen utilizations in excess of 90%. The higher driving force of the pure oxygen
system (6-10 mg/liter D.O.) also resulted in a net decrease in waste activated sludge
389