Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2000-05-11
2002-10-22
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S629000, C210S199000, C210S220000, C210S221200, C210S620000
Reexamination Certificate
active
06468429
ABSTRACT:
TECHNICAL FIELD
An apparatus and method for controlling hydraulic flow, mixing and gas transfer characteristics in long vertical shaft bioreactors.
FIELD OF INVENTION
This invention relates to long vertical shaft bioreactors for the aerobic biological treatment of wastewater and aerobic digestion of biodegradable sludges. In particular, the invention relates to an apparatus and method to improve control of hydraulic flow, inter-zonal mixing and gas transfer characteristics in such bioreactors.
BACKGROUND
Long vertical shaft bioreactor systems are well known in the prior art. For example, U.S. Pat. Nos. 5,645,726 and 5,650,070, Pollock, which issued on Jul. 8, 1997 and Jul. 22, 1997 respectively, relate to bioreactors adapted for the treatment of biodegradable sludge and wastewater. These bioreactors comprise a circulatory system that includes at least two long substantially vertical side-by-side or coaxial chambers (i.e. a downflow and an upflow chamber) in communication with each other at their upper and lower ends. In particular, the upper ends of the chambers are connected through a surface basin and the lower ends communicate in a common mix zone located immediately below the lower end of the downflow chamber. A “plug flow” zone with no recycle is located immediately below, and communicates with, the mix zone. As used in this patent application “plug flow” refers to net downward migration of solid particles from the mix zone toward an effluent outlet located at the lower end of the reactor. The net downward migration may include some local back mixing.
The wastewater or sludge waste to be treated is caused to circulate repeatedly through and between the downflow and the upflow chambers, the surface basin and the mix zone. A portion of the circulating flow is directed to the plug flow zone and is removed at the lower end thereof as effluent.
Normally the waste-containing liquor comprising biomass is and microbes, referred to as “mixed liquor”, is driven through the circulatory system by the injection of an oxygen-containing gas, usually air, into either or both of the mix zone and the plug flow zone. Typically, in a reactor for the treatment of wastewater, the air is injected 5-10 feet above the bottom of the reactor and, optionally, a portion of air is also injected immediately below the lower end of the downflow chamber. The deepest air injection point divides the plug flow zone into a quasi plug flow zone with localized back mixing above the deepest point of air injection, and a strict plug flow zone with no mixing below the deepest point of air injection. The influent wastewater is introduced into the upflow chamber a short distance above the lower end of the downflow chamber. At start-up, air is injected at depth through the influent line into the upflow chamber thus causing liquor circulation between and through the upflow and downflow chambers in the nature of an air lift pump. Once circulation has started, all the air injection is diverted to the mix zone and/or plug flow zone. Bubbles rising out of these zones are entrained in the upflow chamber and excluded from the downflow chamber (because the downward flow of liquor in the downflow chamber exceeds the rise rate of the bubbles). Thus all the air bubbles are transferred to the upflow chamber and stable circulation is maintained.
Usually the surface basin is fitted with a horizontal baffle at the top of the upflow chamber to force the mixed liquor to traverse a major part of the basin and release spent gas before again entering the downflow chamber for further treatment. A zone of turbulence is created at the lower end of the downflow chamber by the turn-around velocity head as the circulating flow reverses from downward to upward flow. This mix zone is not well defined but typically is between 15-25 feet deep. A portion of the mixed liquor in the mix zone flows downwardly into the top of the plug flow zone in response to an equal amount of treated effluent being removed from the lower end of the plug flow zone into an effluent line as discussed above.
Reaction between waste, dissolved oxygen, nutrients and biomass (including an active microbial population), substantially takes place in an upper circulating zone of the bioreactor defined by the surface basin, the upflow and downflow chambers and the mix zone. The majority of the contents of the mix zone circulate upwardly into the upflow chamber. In this upflow chamber undissolved gas, mostly nitrogen, expands to help provide the gas lift necessary to drive circulation of the liquor in the upper part of the reactor. The spent gas is released from the liquor as it traverses the horizontal baffle in the surface basin. The plug flow zone located below the upper circulating zone provides a final treatment or “polish” to the mixed liquor flowing downward from the mix zone to effluent extraction at the lower end of the reactor. The injected oxygen-containing gas dissolves readily under pressure in the liquor in the plug flow zone where there is localized back mixing resulting in a slow net downward movement of liquor. Undissolved gas (bubbles) migrate upward to the very turbulent mix zone under pressure. The gas to liquid transfer in this zone is very high reaching overall reactor oxygen transfer efficiencies in excess of 65%. The products of the reaction are carbon dioxide and additional biomass which, in combination with unreacted solid material present in the influent wastewater, forms a sludge (or biosolids).
Long vertical shaft bioreactors designed for aerobic treatment of wastewater and sludge are generally similar. However, wastewater treatment bioreactors typically require a much smaller plug flow zone. Additionally, sludge treatment bioreactors preferably include two different aeration distributors for injecting air into the reaction vessel at two separate locations, namely in both the mix zone and the plug flow zone as described above.
The principal products of aerobic digestion of sludge biosolids in the mesophilic temperature range (up to approximately 40° C.) are carbon dioxide, nitrate nitrogen, and reduced sludge mass. The principal products of aerobic digestion in the thermophillic temperature range (approximately 45° C.-70° C.) are carbon dioxide and ammonia.
While existing long vertical shaft bioreactors, such as those described in U.S. Pat. Nos. 5,645,726 and 5,650,070, are useful in the treatment of wastewater and sludge biosolids, they exhibit several shortcomings which limit their commercial effectiveness. When such prior art bioreactors are designed to accommodate a wide range of loads and flows, the mix and plug flow zones may become over sized resulting in a loss in hydraulic and oxygen transfer efficiency under some operating conditions. As a compromise, prior art bioreactors are typically optimized for one condition - usually average load and flow. Unfortunately average conditions only occur briefly two or three times a day in a typical municipal waste treatment plant operating under diurnal loading conditions.
Under increasing loads and flows, where the air rate must be increased to satisfy the greater biological air requirement, the efficiency of the reactor is compromised. This is due to the increase in the circulation rate resulting from the increase in air rate. Increasing the circulation rate actually lowers the dissolved gas concentration, lowers the respiration rate of the microbes and increases hydraulic head losses, as explained in further detail below.
Hydraulic Considerations
When the circulation velocity in the upper circulating zone of the bioreactor increases, the mixing time at maximum pressure for the air and water decreases. Furthermore, for any given air rate, increasing the liquor velocity and therefore the liquor volume flowing past the point of air injection, dilutes the concentration of available air per unit volume of liquor. This reduces the saturation potential of air in water. Normally the downflow chamber is approximately one quarter the cross-sectional area of the reactor body so that an increase in liquor flow ve
Noram Engineering and Constructors Ltd.
Oyen Wiggs Green & Mutala
Prince Fred
Simmons David A.
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