Biological fluidized bed apparatus

Details Biological fluidized bed apparatus Biological fluidized bed apparatus

2002-05-13

2003-03-11

Upton, Christopher (Department: 1724)

Liquid purification or separation

With gas-liquid surface contact means

With separator

C210S188000, C210S195300, C210S262000, C210S617000

Reexamination Certificate

active

06531058

ABSTRACT:

1. BACKGROUND OF THE INVENTION
1.1 Field of the Invention
The invention pertains to a specific design of a biological fluidized bed apparatus to treat wastewater. The invention claimed here is the particular design of the apparatus and its components.
1.2 Description of the Prior Art
Over the last twenty years, extensive research has been done in the US, Europe and Japan to develop various fluidized bed reactor configurations and processes. Important patented contributions in this field have been made by the following:
Rovel et al. (pat: U.S. Pat. No. 4,482,458)
Vogelpohl et al. (pat: U.S. Pat. No. 4,940,546)
Edwards (pat: U.S. Pat. No. 5,441,634)
Yoda et al. (pat: U.S. Pat. No. 4,762,612)
Clark et al. (pat: U.S. Pat. No. 5,942,116)
Love (pat: U.S. Pat. No. 4,530,762)
Klein (pat: U.S. Pat. No. 5,573,671)
Biological fluidized bed reactors have been widely used to remove dissolved and suspended organic matter from high-strength industrial effluents. In this application, the biological beds are comprised of anaerobic bacteria. Anoxic fluidized bed reactors have also been used for the removal of nitrate from industrial and municipal effluents, in which case the biological bed is comprised of denitrifying bacteria.
In the anaerobic application, these reactors convert dissolved and suspended organic matter into methane and carbon dioxide (biogas). The conversion is accomplished by anaerobic bacteria, which grow attached as biofilm to inert media particles in the fluidized bed. The reduction of organic matter in the treated waste is the result of a combination of physical retention of suspended and colloidal organic matter by solids contact flocculation within the biological bed, hydrolysis of the trapped solids by hydrolyzing bacteria present in the anaerobic consortium, and finally biological conversion of dissolved organic matter into biogas by acetogenic and then methanogenic bacteria.
In the anoxic application, these reactors convert nitrate into nitrogen gas. The conversion of nitrate to nitrogen gas is accomplished by denitrifying bacteria that grow attached to inert media particles in the fluidized bed. These heterotrophic bacteria are ubiquitous in most natural waters. In the process of denitrification, nitrate acts as an electron acceptor and an organic substrate as a reduced electron donor. The substrate used as an electron donor and a source of carbon is a readily degradable organic substrate (i.e., methanol, sugars or acetic acid), which is supplemented to the anoxic reactor.
The fluidized bed reactors in use are an attached biofilm fluidized bed process that is based on the concept that large biomass concentrations can be achieved on a large surface area by dense biofilm attachment to an inert small particle size carrier. A large surface area is created by small inert particles in a bed, fluidized by upward flow. The intense mixing occurring in the bed minimizes diffusion limitations and eliminates clogging and short-circuiting problems. These reactors accumulate large amounts of active biomass, and can achieve very long cell detention times. Existing fluidized bed reactors consist of a cylindrical column with flat bottom. These columns contain a bed of small inert particles, 0.4-1.0 mm zeolite, diatomaceous earth, or activated carbon particles. The bed is fluidized by the upward flow through the column of untreated wastewater mixed with recirculated effluent. In existing reactors, the upward flow necessary to achieve bed fluidization is distributed by means of various nozzles or small diameter pipes placed at the bottom of the column. Anaerobic biofilm develops on the inert media and the physical attachment of anaerobic bacteria to the media surface prevents biomass washout. The high fluid shear force resistance of biofilms allows these reactors to be operated at upflow velocities which would otherwise wash out unattached biomass.
Upflow velocities are set by the recirculation flow, according to the fluidization properties of the bio-particles (e.g., inert particles with attached bacterial film). The upflow velocities are sufficient to reach bed expansions as to allow free release of generated gas bubbles. The degree of bed expansion is achieved by controlling the recirculation flow rate of a portion of the effluent in a closed-loop.
Under some conditions the turbulent flow exerts sufficient shear to prevent the development of thick biofilms on the media, which limit mass transfer. The high surface-to-volume ratio of the bulk of the bed inert media (300 to 2500 ft
2
/ft
3
) creates a vast area for the development of microbial biofilm. Approximately 95 percent of the active biomass in a well-operated fluidized bed reactor is attached growth. This fact enables the development of dense but thin biofilms that lead to high concentrations of attached biomass in the bed.
Despite the great advantages that this process offers in terms of high organic loading rates, short hydraulic retention times and low excess solids generation, the use of these reactors has not been extensive due to various design limitations. The most common problems reported in full-scale applications and their effects on operation and effluent quality are:
(a) Inadequate Flow Distribution at the Bottom of the Bed
In current fluidized bed reactor configurations, clogging of distribution nozzles and the existence of “dead zones,” channeling and short-circuiting inside the bed are major disadvantages. Good flow distribution is necessary to achieve uniform and controlled bed expansion and a well-mixed flow pattern inside the bed. Such patterns promote turbulence at the biofilm/liquid interface and enable all the attached biomass to be in contact with the waste.
When a uniform expansion of the bed is obtained, biogas bubbles generated in the bed are evenly released. This avoids the coalescence of small bubbles into much larger bubbles that disrupt the bed as they rise. Such bed disruptions deteriorate the quality of the effluent by releasing solids trapped within the bed.
Although the solution to inadequate bed expansion would appear to be increasing the recirculation flow in order to achieve higher upflow velocity, this presents the disadvantage of washing solids trapped in the bed, by the high interstitial velocities created by the increased flow. These solids deteriorate the quality of the effluent.
(b) Need for Highly Uniform Particle Size Media in Cylindrical Reactor Configurations
In cylindrical, flat bottom reactors, the inert media must have a highly uniform particle size. Typical media materials are zeolite, sand, and activated carbon. Commercially available media are not highly uniform in particle size. A more uniform particle size media has a higher cost, since narrow particle size range sieving produces more wasted material in the for the media manufacturer.
Since the upflow velocity in cylindrical reactors is constant throughout the bed, the existence of various particle sizes affects uniform expansion. Larger particles weigh more and have higher terminal settling velocities, thus, higher upflow velocities are required to keep them suspended. If there is a range in the particle size of the media, at the upflow velocity necessary to expand the large particles, the small particles are carried out of the bed, or over expanded. On the other hand, at an upflow velocity, at which small media particles reach adequate fluidization, large particles remain unexpanded at the bottom of the reactor, creating a plug and hindering uniform expansion.
(c) Inadequate Solids/Gas/Liquid Separation Within the Reactor
Before the treated effluent exits the top of the reactor, suspended solids need to be removed. The rising bubbles above the bed create a drag effect that helps carry suspended solids to the top of the reactor. Gas bubbles also tend to trap suspended solids, which are attracted by the surface tension of the bubbles. In several reactors without an appropriate incorporated solids separation system organic loadings are kept below the reactor's capacity in order to reduce gas generation as a measure to li

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