Method of using water soluble cationic polymers in membrane biological reactors

This invention concerns the use of water soluble cationic polymers to condition biomass in membrane biological reactors and increase water flux through the membrane.

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.

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.

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)acrylamide, N-t-butyl(meth)acrylamide, N-(2)-hydroxypropyl)methacrylamide, N-methylolacrylamide, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, poly(ethylene glycol)(meth)acrylate, poly(ethylene glycol) monomethyl ether mono(meth)acryate, N-vinyl-2-pyrrolidone, glycerol mono((meth)acrylate), 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, vinyl methylsulfone, vinyl acetate, glycidyl(meth)acrylate, and the like.

“Latex polymer” means an invertible water-in-oil polymer emulsion comprising a cationic polymer according to this invention in the aqueous phase, a hydrocarbon oil for the oil phase, a water-in-oil emulsifying agent and, potentially, an inverting surfactant. Inverse emulsion polymers are hydrocarbon continuous with the water-soluble polymers dispersed as micron sized particles within the hydrocarbon matrix. The latex polymers are then “inverted” or activated for use by releasing the polymer from the particles using shear, dilution, and, generally, another surfactant, which may or may not be a component of the inverse emulsion.

Latex polymers are prepared by dissolving the desired monomers in the aqueous phase, dissolving the emulsifying agent(s) in the oil phase, emulsifying the water phase in the oil phase to prepare a water-in-oil emulsion, in some cases, homogenizing the water-in-oil emulsion, polymerizing the monomers dissolved in the water phase of the water-in-oil emulsion to obtain the polymer as a water-in-oil emulsion. If so desired, a self-inverting surfactant can be added after the polymerization is complete in order to obtain the water-in-oil self-inverting emulsion.

“Dispersion polymer” means a water-soluble polymer dispersed in an aqueous continuous phase containing one or more inorganic/organic salts. Representative examples of low molecular weight cationic polymers prepared by dispersion polymerization of water-soluble polymers in an aqueous continuous phase are found in PCT/US01/09060.

“Solution polymer” means a water soluble polymer in a water continuous solution.

“Dry polymer” means a polymer prepared by gel polymerization. In a gel polymerization process, an aqueous solution of water-soluble monomers, generally 20-60 percent concentration by weight, along with any polymerization or process additives such as chain transfer agents, chelants, pH buffers, or surfactants, is placed in an insulated reaction vessel equipped with a nitrogen purging tube. A polymerization initiator is added, the solution is purged with nitrogen, and the temperature of the reaction is allowed to rise uncontrolled. When the polymerized mass is cooled, the resultant gel is removed from the reactor, shredded, dried, and ground to the desired particle size.

“Reduced Specific Viscosity” (RSV) is an indication of polymer chain length and average molecular weight. The RSV is measured at a given polymer concentration and temperature and calculated as follows: RSV = [ ( η η o ) - 1 ] c

wherein η=viscosity of polymer solution;

η o =viscosity of solvent at the same temperature; and

c=concentration of polymer in solution. As used herein, the units of concentration “c” are (grams/100 ml or g/deciliter). Therefore, the units of RSV are dl/g. The RSV is measured at 30° C. The viscosities η and η o are measured using a Cannon-Ubbelohde semimicro dilution viscometer, size 75. The viscometer is mounted in a perfectly vertical position in a constant temperature bath adjusted to 30±0.02° C. The error inherent in the calculation of RSV is about 2 dl/g. Similar RSVs measured for two linear polymers of identical or very similar composition is one indication that the polymers have similar molecular weights, provided that the polymer samples are treated identically and that the RSVs are measured under identical conditions.

IV stands for intrinsic viscosity, which is RSV in the limit of infinite polymer dilution (i.e. the polymer concentration is equal to zero). The IV, as used herein, is obtained from the y-intercept of the plot of RSV versus polymer concentration in the range of 0.015-0.045 wt % polymer.

Preferred Embodiments

The water soluble cationic polymers of this invention are added to the MBR unit to promote the incorporation of colloidal particles, such as cell fragments and single bacterium, into aggregate or floc structures and/or to increase the porosity of the cake layer. The water soluble cationic polymers may be solution polymers, latex polymers, dry polymers or dispersion polymers.

In a preferred aspect of this invention, the water soluble cationic polymer has a molecular weight of about 10,000 to about 2,000,000.

In another preferred aspect, the water soluble cationic polymer has a cationic charge of 100 mole percent

In another preferred aspect, the water soluble cationic polymer is selected from the group consisting of a polymer of epichlorhydrin-dimethylamine crosslinked with either ammonia or ethylenediamine; a linear polymer of epichlorohydrin and dimethylamine, a homopolymer of polyethyleneimine; polydiallydimethylammonium chloride; homopolymer of DMAEM.H 2 SO 4 ; polymerized triethanolamine/methyl chloride quat, polymerized triethanolamnine and tall oil fatty acid/methyl chloride quat, polyethylenedichloride/ammonia, and modified polyethyleneimine.

In another preferred aspect, the water soluble cationic polymer has a cationic charge of at least 70 mole percent.

In another preferred aspect, the water soluble cationic polymer is a polymer of (meth)acrylamide and one or more cationic monomers selected from diallyldimethylammonium chloride, dimethylaminoethyacrylate methyl chloride quatemary salt, dimethylaminoethylmethacrylate methyl chloride quaternary salt and dimethylaminoethylacrylate benzyl chloride quaternary salt.

In another preferred aspect, the water soluble cationic polymer is diallyldimethylammonium chloride/acryamide copolymer.

The MBR unit combines two basic processes: biological degradation and membrane separation-into a single process where suspended solids and microorganisms responsible for biodegradation are separated from the treated water by a membrane filtration unit. See Water Treatment Membrane Processes , McGraw-Hill, 1996, p 17.2. The entire biomass is confined within the system, providing for both control of the residence time for the microorganisms in the reactor (sludge age) and the disinfection of the effluent.

In general, influent enters the bioreactor, where it is brought into contact with the biomass. The mixture is pumped from the bioreactor and then, under pressure, filtered through a membrane. The permeate is discharged from the system while the entire biomass is returned to the bioreactor. Excess sludge is pumped out in order to maintain a constant sludge age, and the membrane is regularly cleaned by backwashing, chemical washing, or both.

Membranes used in the MBR unit include ultra- and microfiltration, inner and outer skin, hollow fiber, tubular, and flat, organic, metallic, ceramic, and the like. Preferred membranes for commercial application include hollow fiber with an outer skin ultrafilter, flat sheet (in stacks) ultrafilter and hollow fiber with an outer skin microfilter.

The wastewater may be pretreated before entering the MBR. For example, a bar screen, grit chamber or rotary drum screen may be used to achieve coarse solids removal. Suspended solids that are not removed during course solids removal may be removed using a sedimentation tank or a clarifier, called a “primary clarifier.” In some cases coagulants such as ferric chloride or Epi-DMA and an anionic flocculant, such as a co-polymer of acrylic acid and acrylamide, are used in the primary clarifier to remove additional solids. In industrial plants where synthetic oils are present in the untreated wastewater, such as an oil refinery, pretreatment to remove oil is accomplished in units such as the inclined plate separator and the induced air flotation unit (IAF). Often, a cationic flocculant, such as a co-polymer of DMAEM and AcAm, is used in the IAF unit to increase oil removal. Also, excess phosphate is sometimes precipitated in the bioreactor by the addition of metal salts such as ferric chloride, so that the phosphate does not pass through the membrane and into the final effluent.

Depending on the ultimate use of the water and the purity of the MBR permeate, the clarified wastewater may also be subjected to post treatment. For instance, in water reclamation where treated wastewater is ultimately recharged into an aquifer used as a source for drinking water, the permeate may be further treated with reverse osmosis (RO) to reduce the dissolved mineral content If the water is to be recycled into a process, then the requirements of that process may necessitate further treatment of the permeate for removal of recalcitrant organics not removed by the MBR. Processes such as nanofiltration or carbon adsorption might be used in these cases. Finally, all biologically treated wastewater must be disinfected prior to discharge into a receiving stream, generally by addition of sodium hypochlorite, although this is not required for discharge into a municipal sewer.

The cationic polymers are introduced into the aeration basin/bioreactor by various means, for example by dosing into the wastewater feed line ahead of the bioreactor or by dosing directly into the bioreactor.

In all cases, the polymer should be thoroughly mixed with the mixed liquor in the bioreactor to maximize adsorption. This may be accomplished by feeding the polymer into an area of the bioreactor where an aeration nozzle is located. So-called “dead” zones in the bioreactor having little to no flow should be avoided. In some cases, a submerged propeller mixer may be needed to increase mixing in the basin, or the sludge can be re-circulated through a side arm loop.

Solution polymers can be dosed using a chemical metering pump such as the LMI Model 121 from Milton Roy (Acton, Mass.).

The recommended cationic polymer product dosage, based on mixed liquor in the bioreactor, is about 25 to about 1000 ppm, at MLSS (mixed liquor suspended solids) of approximately 1-2%. If the MLSS is lower than 1%, a proportionately lower dosage of polymer may be used. The polymer can be periodically pumped directly to the biorcactor mixed liquor or into the wastewater feed line. If polymer is fed continuously to the wastewater feed, then dosage would be considerably lower, about 0.25 to about 10 ppm. Polymer may be pumped intermittently (“slug fed”) or continuously to the wastewater.

Overdosing polymer may result in reduced biological activity and organics removal in the bioreactor. For this reason, a low polymer dosage should be used initially: for example about 25 to about 100 ppm in mixed liquor. Additional polymer can then be fed to increase flux while maintaining biological activity. Permeate TOC (total organic carbon), COD (chemical oxygen demand), or BOD (biological oxygen demand) can be monitored to ascertain biological activity.

Alternately, ajar test can be conducted with samples of mixed liquor. Using a four paddle mixer, the sample jars are dosed with sequentially higher amounts of polymer; one jar is left untreated. After mixing, the samples are allowed to sit forseveral hours, so that the solids can settle to the bottom of the jar. The turbidity of the water on top of the settled solids (supernatant) is measured to ascertain the effectiveness of the polymer dosage. A turbidimeter from Hach Company (Loveland, Colo.) could be used. A dosage that gives lower turbidity in the jar than the untreated sample will increase flux in the MBR.

In the event of a polymer overdose, dosing of cationic polymer should be halted until biological activity returns to normal levels. It may also be necessary to discharge more biosolids from the bioreactor to assist in recovery of bioactivity. Addition of bioaugmentation products containing appropriate bacteria may also be helpful in recovering activity after polymer overdose.

The foregoing may be better understood by reference to the following Examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.

Representative cationic polymers of this invention are listed in Table 1. Polymers B and C are from Ciba (Tarrytown, N.Y.); Polymers M and N are from BASF (Mount Olive, N.J.). All other polymers are from ONDEO Nalco Company, Naperville, Ill.