SYSTEMS AND METHODS FOR UPGRADING CONVENTIONAL ACTIVATED SLUDGE PLANTS
1. A wastewater treatment process comprising steps of:
- biological treatment to produce mixed liquor;
separation of a first portion of an effluent from the mixed liquid through a filtration membrane; and
,separation of a second portion of an effluent from the mixed liquid through a secondary clarifer.
A conventional activated sludge wastewater treatment plant is upgraded by adding one or more of a membrane filtration unit, a membrane aerated biofilm reactor (MABR) unit, and a screen. The membrane filtration unit is added between a process tank and the secondary clarifier. The membrane filtration unit extracts treated effluent at a rate up to 25% of the influent flow rate. The plant is not converted into a membrane bioreactor (MBR) since the MLSS concentration is still less than 4,000 mg/L. The membrane-aerated biofilm reactor (MABR) unit is added to a process tank of the plant and provides attached growth biological treatment. The screen extracts solids from water flowing to the process tanks. When optionally used in combination, the added units and processes increase the capacity of the primary separation, biological processing and secondary separation functions of the plant. The influent flow rate may be increased.
- 1. A wastewater treatment process comprising steps of:
biological treatment to produce mixed liquor; separation of a first portion of an effluent from the mixed liquid through a filtration membrane; and
separation of a second portion of an effluent from the mixed liquid through a secondary clarifer.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
- 9. A wastewater treatment process comprising steps of:
biological treatment of primary effluent attached and suspended growth biological processes to produce mixed liquor; separation of a treated effluent from the mixed liquid; return of activated sludge to the biological treatment; and
one or more of screening influent in parallel with a primary clarifier and screening return activated sludge.
- 10. A modified conventional activated sludge wastewater treatment plant comprising:
one or more process tanks or other biological reactors; a secondary clarifier; and
one or more of a) a filtration membrane, b) a screen in parallel with a primary clarifier, c) a screen configured to remove solids from return activated sludge, and d) an attached medium in the one or more process tanks or other biological reactors.
- View Dependent Claims (11, 12, 13, 14, 15)
This specification relates to treating wastewater such as sewage or industrial wastewater using an activated sludge process.
The activated sludge process is a biological treatment process that is standard practice in many countries of the world. In a conventional activated sludge wastewater treatment plant (WWTP), wastewater passes through one or more biological process tanks maintained under various states of oxidation and mixing. Organisms grow in suspension in the process tanks. The combination of wastewater and organisms is called mixed liquor. The mixed liquor is separated in a secondary clarifier to produce a treated effluent and activated sludge. A portion of the activated sludge (return activated sludge or RAS) is recycled to one or more of the process tanks. Another portion of the activated sludge (waste activated sludge or WAS) is wasted. The recycle of activate sludge causes the retention time of the organisms to be greater than the hydraulic retention time of the plant. The mixed liquor suspended solids (MLSS) concentration is typically less than 4000 mg/L. Optionally the wastewater passes through a primary clarifier before being treated in the process tanks. The primary clarifier produces primary sludge and primary effluent. The primary effluent flows to the process tanks.
This specification describes systems of methods that can be added to a conventional activated sludge (CAS) plant to upgrade it. The upgraded plant may produce effluent of a higher quality or treat wastewater at a higher rate or both. The various systems and methods described herein can be used individual or in any combination of two or more of them.
In one example, a membrane filtration unit is added between the process tanks and the secondary clarifier. At that point, biological treatment is essentially complete, or at least essentially as complete as it will be in the secondary clarifier. The membrane filtration unit extracts treated effluent from the mixed liquor before secondary clarification. The treated effluent extracted through the membrane filtration unit may be mixed with the treated effluent from the secondary clarifier. Extracting treated effluent as permeate from the membrane filtration unit reduces the hydraulic loading rate, or both the hydraulic and solids loading rates, of the secondary clarifier depending on whether solids rejected by the membrane are sent to the secondary clarifier or to the return activated sludge (RAS) line. The amount of treated effluent extracted by membrane filtration is preferably lower than 25% of the influent flow rate. The plant is not converted into a membrane bioreactor (MBR) as the system is run under CAS operating conditions (e.g., MLSS <4,000 mg/L).
In another example, a membrane-aerated biofilm reactor (MABR) unit is added to the plant, for example by being immersed in a process tank. The MABR unit adds biological treatment by attached growth to the conventional suspended growth.
In another example, one or more screens, for example micro-screens, are added to extract solids from water flowing in or to the process tanks. In one option, a micro-screen is added in parallel with a primary clarifier. In another option, a portion of the RAS is screened before being returned to the process tanks. Using either or both of these methods reduces the solids loading of the process tanks.
When used in combination, the units described above and their corresponding processes increase the capacity of the primary separation, biological processing and secondary separation functions of the plant. The influent flow rate to the plant may be increased.
Conventional activated sludge (CAS) is a common wastewater biological treatment process. A CAS typically has three treatment steps although the primary treatment step described below can optionally be omitted. Pre-treatment removes larger particles with mechanical means such as coarse screening, grit removal and oil & grease flotation. Primary treatment, typically in a clarifier, removes suspended solids including some organic matter by physical separation. One or more biological reactors removing organic matter (e.g., COD/BOD) using microorganisms, typically under aerobic conditions in at least one reactor. The biological reactors may also include multiple zones or tanks, optionally with one or more recycle loops between them, where the environmental conditions are controlled (i.e between aerobic, anoxic and anaerobic conditions) to favor different biological pathways to remove nutrients such as nitrogen and phosphorous. Secondary treatment, typically in a clarifier, separates the mixed liquor suspended solids (MLSS) from the final effluent, recycles a portion as return activated sludge (RAS) and wastes a portion (WAS) to control the sludge retention time (SRT).
The wastewater treatment plant (WWTP) 10 shown in
CAS plants often need to be upgraded or expanded. Upgrading is needed when the treatment objectives or effluent regulations become more stringent and the level of treatment achieved by the plant is not sufficient. Expansion is needed when the flow rate and/or pollutant concentration of the influent wastewater increases. Upgrading and expanding a CAS plant can be complex and expensive as it involves adding tankage and mechanical equipment. In many cases, the CAS plant is located at a site where there is very little room available.
A method of upgrading a CAS plant can involve adding one or more products to the CAS plant. These products target the three treatment steps (primary separation, biological treatment, secondary separation) described above. They can be used individually or in combinations of two or more of together. One type of product involves a micro-screen, alternatively called a micro-sieve, or other screen, which may be added to complement primary treatment, to otherwise reduce solids in the process tanks, or to protect added membranes from solids. Another type of product involves a medium to support attached growth, for example a membrane aerated biofilm module, to complement the biological reactor. Another type of product involves membrane filtration to complement secondary clarification. The primary clarifier in the examples described below is optional.
Micro-sieving, side-stream screening, MABR, and membrane filtration are described in other contexts in, for example, U.S. Pat. Nos. 6,942,786; 6,814,868; and, 6,645,374, which are incorporated herein by reference. In this specification they are used, optionally together, to upgrade or expand a CAS plant.
In one example, use of a micro-sieving product involves installing a micro-sieve in parallel with primary treatment to remove suspended solids. In the example of
In another example, a micro-screen or other screen is used to extract solids from mixed liquor or RAS. Removing these solids may supplement primary treatment or provide a substitute for primary treatment if the plant has none. Alternatively or additionally, screening of the mixed liquor or RAS may remove trash and larger particles from the mixed liquor to protect membranes in, or added to, the plant. The screen optionally has pores of about 300 microns or less, or about 200 microns or less, or about 100 microns or less. In a case where only the membrane protection function is required, the screen can optionally have pores up to 1000 microns in size. In the example of
Adding a supported biomass medium augments the biological treatment capacity of a plant 10. In the example of
A membrane filtration product is used to extract treated effluent from the mixed liquor. One example of a membrane filtration product is a ZEEWEED immersed ultrafiltration (UF) or microfiltration (MF) module by GE Water. In the example of
In the example of
Adding filtration membranes to process a minor portion of the mixed liquor 22 does not convert the CAS plant into a membrane bioreactor (MBR). The fraction of the influent flow rate (Q) extracted as permeate 50 is limited to 25% (ore one third of treated effluent 24 discharged from secondary clarifier 16). The MLSS concentration of the mixed liquor 22 is optionally not increased or at least not materially increased. The MLSS concentration of the modified plant is about 2,000 to 4,000 mg/L, which is typical of a CAS, rather than 6,000 to 12,000 mg/L which is typical of an MBR.
Two or all three of the types of products can be combined to improve the use of existing infrastructure. While each existing CAS plant may be limited differently, it may be possible to address each limitation to increase plant throughput by up to 25%. Optionally, the products can be installed without materially interrupting the operation of the CAS plant. Both the MABR and filtration membranes can be deployed as floating cassettes.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.