A Low Cost Open Tank Pure Oxygen System
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The 61.3% mass reduction achieved in ten days with the 4.5% sludge represents a
doubling of the digestion rate normally maintained with 0.8% return activated sludge. A
second advantage in digesting thickened sludge is the possibility of reducing the digestion
space requirements by 80 - 90%. The planned full scale application of this technology
(50 tons/day of waste activated sludge) at Metro Denver will be unique for this high
concentration of solids.
Future plans include investigating the feasibility of pure oxygen digestion of combined
primary and waste activated sludges. Past attempts to aerobically digest primary sludge
have not met with success due in part to very high oxygen demands as well as clogging
problems associated with high grit concentrations.
Combined high rate digestion of both primary and secondary sludges in one unit
operation could result in significant savings in both capital and operations expenditures.
The second major objective of the joint Metro-Martin project is to provide data on
accelerated bio-oxidation using the high efficiency diffuser in the activated sludge
process.
Since its inception, the activated sludge process has been subjected to continued
development and modification in an attempt to improve the effluent quality or effect
economies of operation.
The ideal system would combine all of the following features:
1) low oxygen/air gas volumes per pound BOD removed
2) low sludge production per pound BOD removed
3) short detention times in both aerators and clariflers
4) high BOD loading capabilities per 1000 ft
3 of aerator capacity
5) high percentage removals of BOD, COD, and suspended solids
6) low capital and operations costs per volume of sewage treated.
To date no single modification based on conventional air diffusion technology has
even begun to approach in its entirety the "ideal" activated sludge system, as defined
above. The "conventional" system is an intermediate compromise. Loadings range
between 0.2 to 0.4 lb. BOD 5 /day/lb. VSS and an intermediate amount of waste sludge
per lb. BOD removed is produced.
The "step" aeration modification is able to hold a slightly higher inventory of mixed
liquor solids, but no significant savings are achieved in air requirements or sludge
production compared to the conventional system.
"Contact" stabilization permits higher volumetric loadings than the conventional
process but usually requires additional capital investment for reaeration tanks and special
sludge thickening units.
The various "high rate, short term" modifications achieve high loading factors and
short detention times at the expense of treatment efficiency (% BOD removal). Relatively
large volumes of sludge are produced per unit weight of BOD removed. The
"Biosorption" process can reduce the absorptive stage to less than thirty minutes with
BOD removal rates exceeding 90%, but large quantities of sludge are produced.
In recognition of the fact that waste sludge disposal has assumed paramount
importance in most inland municipal treatment plants, because of dewatering difficulty,
lack of convenient disposal sites and high costs of chemical conditioning, the concept of
"total oxidation", also referred to as "aerobic digestion" or "extended aeration" has
become increasingly popular in recent years.
Most total oxidation plants are designed for no intentional wasting of sludge from the
393
The 61.3% mass reduction achieved in ten days with the 4.5% sludge represents a
doubling of the digestion rate normally maintained with 0.8% return activated sludge. A
second advantage in digesting thickened sludge is the possibility of reducing the digestion
space requirements by 80 - 90%. The planned full scale application of this technology
(50 tons/day of waste activated sludge) at Metro Denver will be unique for this high
concentration of solids.
Future plans include investigating the feasibility of pure oxygen digestion of combined
primary and waste activated sludges. Past attempts to aerobically digest primary sludge
have not met with success due in part to very high oxygen demands as well as clogging
problems associated with high grit concentrations.
Combined high rate digestion of both primary and secondary sludges in one unit
operation could result in significant savings in both capital and operations expenditures.
The second major objective of the joint Metro-Martin project is to provide data on
accelerated bio-oxidation using the high efficiency diffuser in the activated sludge
process.
Since its inception, the activated sludge process has been subjected to continued
development and modification in an attempt to improve the effluent quality or effect
economies of operation.
The ideal system would combine all of the following features:
1) low oxygen/air gas volumes per pound BOD removed
2) low sludge production per pound BOD removed
3) short detention times in both aerators and clariflers
4) high BOD loading capabilities per 1000 ft
3 of aerator capacity
5) high percentage removals of BOD, COD, and suspended solids
6) low capital and operations costs per volume of sewage treated.
To date no single modification based on conventional air diffusion technology has
even begun to approach in its entirety the "ideal" activated sludge system, as defined
above. The "conventional" system is an intermediate compromise. Loadings range
between 0.2 to 0.4 lb. BOD 5 /day/lb. VSS and an intermediate amount of waste sludge
per lb. BOD removed is produced.
The "step" aeration modification is able to hold a slightly higher inventory of mixed
liquor solids, but no significant savings are achieved in air requirements or sludge
production compared to the conventional system.
"Contact" stabilization permits higher volumetric loadings than the conventional
process but usually requires additional capital investment for reaeration tanks and special
sludge thickening units.
The various "high rate, short term" modifications achieve high loading factors and
short detention times at the expense of treatment efficiency (% BOD removal). Relatively
large volumes of sludge are produced per unit weight of BOD removed. The
"Biosorption" process can reduce the absorptive stage to less than thirty minutes with
BOD removal rates exceeding 90%, but large quantities of sludge are produced.
In recognition of the fact that waste sludge disposal has assumed paramount
importance in most inland municipal treatment plants, because of dewatering difficulty,
lack of convenient disposal sites and high costs of chemical conditioning, the concept of
"total oxidation", also referred to as "aerobic digestion" or "extended aeration" has
become increasingly popular in recent years.
Most total oxidation plants are designed for no intentional wasting of sludge from the