Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2002-01-04
2004-04-20
Drodge, Joseph (Department: 1723)
Liquid purification or separation
Processes
Treatment by living organism
C210S615000, C210S623000, C210S631000, C210S639000, C210S651000, C210S729000, C210S734000, C210S735000
Reexamination Certificate
active
06723245
ABSTRACT:
TECHNICAL FIELD
This invention concerns the use of water soluble cationic polymers to condition biomass in membrane biological reactors and increase water flux through the membrane.
BACKGROUND OF THE INVENTION
Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This aerobic biological process is generally known as the “activated sludge” process in which micro-organisms consume the organic compounds through their growth. The process necessarily includes sedimentation of the micro-organisms or “biomass” to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) in the final effluent. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Membranes coupled with biological reactors for the treatment of wastewater are well known, but are not widely practiced. In these systems, ultrafiltration (UF) or microfiltration (MF) membranes replace sedimentation of biomass for solids-liquid separation. The membrane can be installed in the bioreactor tank or in an adjacent tank where the biomass is continuously pumped from the bioreactor tank and back producing effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier. More importantly, MBRs (membrane biological reactors) de-couple the biological process from the need to settle the biomass, since the membrane sieves the biomass from the water. This allows operation of the biological process at conditions that would be untenable in a conventional system including: 1) high MLSS (bacteria loading) of 10-30 g/L, 2) long sludge retention time, and 3) short hydraulic retention time. In a conventional system, such conditions could lead to sludge bulking and poor settleability.
The benefits of the MBR operation are low sludge production, complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, no problems with sludge bulking, and small footprint. The disadvantages are aeration limitations, membrane fouling, and membrane costs.
Membrane costs are directly related to the membrane area needed for a given volumetric flow through the membrane, or “flux.” Flux is expressed as liters/hour/m
2
(LMH) or gallons/day/ft
2
(GFD). Typical flux rates vary from approximately 20 LMH to about 100 LMH. These relatively low flux rates, due largely to fouling of the membranes, has slowed the growth of MBR systems for wastewater treatment.
Membrane fouling can be attributed to two key components in the feed: proteins and colloidal/particulate material. The MBR membrane interfaces with so-called “mixed liquor:” water with aggregates of bacteria or “flocs”, free bacteria, protozoan, and various dissolved metabolites and cell components. In operation, the colloidal solids and dissolved organics deposit on the surface of the membrane. Colloidal particles form layers on the surface of the membrane, called a “cake layer.” Cake layer formation is especially problematic in MBRs operated in the “dead end” mode where there is no cross flow; i.e., flow tangential to the membrane. Depending on the porosity of the cake layer, hydraulic resistance increases and flux declines
In addition to cake formation on the membrane, small particles can plug the membrane pores, a fouling condition that may not be reversible. Compared to a conventional activated sludge process, floc (particle) size is reportedly much smaller in typical MBR units. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases resistance and decreases flux.
Therefore, there is an ongoing need to develop improved methods of conditioning the biomass in MBR units, particularly methods that reduce fouling of the membranes. This reduction in fouling, and the associated increase in membrane flux, permits the use of smaller systems, with a concomitant reduction in capital costs, or alternatively, increases treated wastewater volumetric flow from an existing system, with a corresponding reduction in cost of operation.
SUMMARY OF THE INVENTION
Polymeric water soluble coagulants and flocculants have not been used in MBR units, as it is generally understood that excess polymer fouls membrane surfaces, resulting in dramatic decreases in membrane flux. However, we have discovered that using water soluble cationic polymers in the MBR process can increase membrane flux by as much as 200 to 500 percent, while leaving virtually no excess polymer in the treated wastewater at the effective dose.
Accordingly, in its principal aspect, this invention is directed to a method of conditioning the biomass in a membrane biological reactor comprising
(i) adding to the biomass an effective amount of at least one water soluble cationic polymer to form a mixture of water and coagulated and flocculated suspended solids; and
(ii) separating the coagulated and flocculated suspended solids from the water by filtering through an ultrafiltration or microfiltration membrane.
DETAILED DESCRIPTION OF THE INVENTION
Definitions of Terms
As used herein, the following abbreviations and terms have the following meanings: AcAm for acrylamide; DMAEA·BCQ for dimethylaminoethylacrylate benzyl chloride quatemary salt; DMAEA·MCQ for dimethylaminoethylacrylate methyl chloride quaternary salt; Epi-DMA for epichlorohydrin-dimethylamine; DADMAC for diallyldimethylammonium chloride; pDADMAC for poly(diallyldimethylammonium chloride); and PEI for polyethyleneimine.
“Cationic polymer” means a polymer having an overall positive charge. The cationic polymers of this invention are prepared by polymerizing one or more cationic monomers, by copolymerizing one or more nonionic monomers and one or more cationic monomers, by condensing epichlorohydrin and a diamine or polyamineor condensing ethylenedichloride and ammonia or formaldehyde and an amine salt.
“Cationic Monomer” means a monomer which possesses a net positive charge. Representative cationic monomers include dialkylaminoalkyl acrylates and methacrylates and their quaternary or acid salts, including, but not limited to, dimethylaminoethyl acrylate methyl chloride quaternary salt, dimethylaminoethyl acrylate methyl sulfate quaternary salt, dimethyamninoethyl acrylate benzyl chloride quaternary salt, dimethylaminoethyl acrylate sulfuiric acid salt, dimethylaminoethyl acrylate hydrochloric acid salt, dimethylaminoethyl methacrylate methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl sulfate quatemary salt, dimethylaminoethyl methacrylate benzyl chloride quaternary salt, dimethylaminoethyl methacrylate sulfuric acid salt, dimethylaminoethyl methacrylate hydrochloric acid salt, dialkylaminoalkylactylamides or methacrylamides and their quaternary or acid salts such as acrylamidopropyltrimethylammonium chloride, dimethylaminopropyl acrylamide methyl sulfate quaternary salt, dimethylaminopropyl acrylamide sulfuric acid salt, dimethylaminopropyl acrylamide hydrochloric acid salt, methacrylamidopropyltrimethylammonium chloride, dimethylaminopropyl, methacrylamide methyl sulfate quaternary salt, dimethylaminopropyl methacrylamide sulfuric acid salt, dimethylaminopropyl methacrylamide hydrochloric acid salt, diethylaminoethylacrylate, diethylaminoethylmethacrylate, diallyldiethylammonium chloride and diallyldimethyl ammonium chloride. Alkyl groups are generally C
1-4
alkyl.
“Nonionic monomer” means a monomer which is electrically neutral. Representative nonionic monomers include acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-isopropyl(meth)acrylami
Collins John H.
Salmen Kristine S.
Breininger Thomas M.
Drodge Joseph
Martin Michael B.
Nalco Company
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