Waste activated sludge phosphorus and magnesium stripping process and struvite production system

A method of treating a mixture of microorganisms with readily biodegradable carbon compounds (RBCs) in the form of one or more volatile fatty acids (VFAs), by first inducing the mixture microorganisms to release phosphorus and magnesium which is then tapped o as the mixture is thickened, to produce a phosphorus/magnesium-nch liquid and a phosphorus/magnesium-reduced treated mixture This treated mixture is placed in an anaerobic digester where ammonia is formed, but combines very little with phosphorus or magnesium Next the high-ammonia mixture is dewatered to produce an ammonia-rich liquid, which is combined with the phosphorus and magnesium-rich liquid and reacted to form struvite In one preferred embodiment, VFAs are formed in situ via an upstream unified fermentation and thickening (UFAT) process and added to the waste sidestream to strip phosphorus and magnesium found therein In another preferred embodiment a usable struvite product is harvested.

FIELD OF THE INVENTION

The invention relates generally to the field of “waste-activated sludge” (WAS) stripping. More particularly, the invention relates to adding readily biodegradable carbon compounds (RBCs) to biological sludge to reduce downstream struvite build-up in a digester and to produce a usable struvite product therefrom.

BACKGROUND OF THE INVENTION

As part of secondary sewage treatment, primary treated sewage is treated with air or pure oxygen. In what is termed the “activated sludge” process, microorganisms utilize the oxygen to metabolize the incoming waste sewage, forming a mixture of microorganisms and sewage known as “mixed liquor.” This mixture is moved to settling tanks for concentration, thereby forming concentrated activated sludge. A majority of this sludge is returned to the activated sludge process tankage. A separate portion of this sludge, termed waste-activated sludge (WAS), is removed from the activated sludge process and sent to a sludge handling system for further treatment and disposal. In a stable system, the daily WAS is equal to the daily conversion of sewage into microorganisms so no net increase in mixed liquor bio-mass occurs. By manipulating the activated sludge process, phosphorus and magnesium are removed from the liquid stream and concentrated in the mixed liquor. The process is known as Enhanced Biological Phosphorus Removal (EBPR).

Referring toFIG. 1, in one typical scheme10, the WAS is sent to a centrifuge (or other thickening apparatus)14for thickening, the liquids are tapped off and returned to the wastewater plant for treatment, whereas the resultant thickened sludge is sent to an anaerobic digester16with other sludges, where it remains for 15 days or more before being sent to a second centrifuge (or other dewatering apparatus)18for dewatering.

Unfortunately, struvite tends to form in digester16, and other equipment downstream because of the ammonia, magnesium and phosphorus that are present can precipitate as struvite. This struvite is impractical to harvest and also has the deleterious effect of being deposited on surfaces in the reactor16and plugging pipes and equipment leading from the reactor.

A further centrifuge (or other dewatering apparatus)18produces further dewatered sludge20, which is either beneficially reused or disposed of, and liquids22, which are rich in ammonia and phosphorus. It has been learned that prilled struvite can be harvested from liquids22, by a struvite reactor24. This prilled struvite is a marketable product that can be used as a timed release fertilizer, thereby defraying some of the costs of sewage treatment. Unfortunately, the struvite harvest requires the addition of magnesium into the process, which forms a large part of the costs of the process and reduces the profitability.

InA Feasible Approach of Integrating Phosphate Recovery as Struvite at Waste Water Treatment Plants, Proceedings, Institute Of Environmental Engineering, pp. 551-558 (2007), D. Montag, et al. describe a phosphate recovery system that effectively teaches away from the addition of one or more volatile fatty acids (VFAs) for phosphorous removal. They do so by teaching long retention times instead of the addition of external organic or inorganic acids. InThe Modified Renphosystem: A High Biological Nutrient Removal System, Wat. Sci. Tech., Vol. 35, No. 10, pp. 137-146 (1997), J. H. Rensink, et al. describe a so-called modified Renpho system. They fail to teach the addition of VFAs as dosing agents to WAS, fail to teach magnesium as well as phosphate release/removal, fail to teach fermentation, fail to teach pH adjustment prior to mixing w/centrate, and fail to teach the use of a dewaterer in connection with a digester.

Neither of these articles nor any other known prior art publication teaches separation of a VFA-enabled reagent into a phosphorus-rich and magnesium-rich liquid stream to a struvite reactor for pelletized struvite production nor into a phosphorus-poor and magnesium-poor sludge stream to a digester to reduce nuisance struvite build-up therein.

SUMMARY

The present invention may take the form of a method of treating a first mixture of waste solids and microorganisms containing phosphorus and magnesium, by first inducing the mixture microorganisms to release phosphorus and magnesium which is then tapped off as the mixture is thickened, to produce phosphorus and magnesium-rich liquid and phosphorus and magnesium-reduced treated mixture. This treated mixture is placed in an anaerobic digester where ammonia is formed but combines very little with phosphorus or magnesium as these elements have been greatly reduced in concentration. Next the high-ammonia mixture is dewatered, to produce an ammonia-rich liquid, which is combined with the phosphorus and magnesium-rich liquid. In one preferred embodiment a useable struvite product is harvested from this combination.

Additionally, the production of nuisance struvite in the anaerobic digester is greatly reduced, in comparison with prior art waste treatment methods.

Systems and methods of practicing the present invention are shown inFIGS. 2,3, and2A and are described in the accompanying text, which should help to clarify the invention in its various embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 2, in a preferred embodiment of a waste activated sludge (WAS) treatment method110, phosphorus and magnesium are released by microorganism action in an anaerobic reactor112, where the WAS is held, for a minimum time of 0.5 hours. One method for effecting this release is by adding one or more readily biodegradable carbon compounds (RBCs), such as one or more volatile fatty acids (VFAs) to the sludge in the anaerobic reactor, with 3 to 8 grams (and preferably 4-6 grams) of the one or more VFAs added per gram of planned phosphorus release. In another technique, the activated sludge is held for 36 to 96 hours, without the addition of VFAs, for endogenous respiration and fermentation to release phosphorus and magnesium.

The resultant WAS is sent to a thickening device114, such as a centrifuge, thickening belt or rotating screens and the resultant liquids115, having enhanced phosphorus and magnesium levels, are sent to a struvite reactor124, which will be discussed further below. There is only very minimal struvite production in the liquids115, because they have a very low ammonia level. VFAs or other forms of RBCs can be generated by fermentation as in the unified fermentation and thickening (UFAT) process disclosed in U.S. Pat. No. 6,387,264 B1. Other methods of obtaining VFAs, include various fermentation methods, harvesting from various waste products and purchase as industrial chemicals, such as acetic acid.

The thickened WAS with reduced phosphorus and magnesium levels is sent to an anaerobic digester116with other sludges and is typically held there for a minimum of fifteen days, where it further treated by anaerobic bacteria which generate high concentrations of ammonia. The production of struvite in digester116, is however, greatly reduced in comparison with the amount of struvite produced in digester16of the prior art system (which could be identical to digester116) because of the reduction in phosphorus and magnesium in the thickened WAS, both of which are necessary for the formation of struvite. This reduction in struvite formation greatly reduces the formation of struvite deposits in the digester and pipes and equipment downstream from anaerobic digester116.

The treated sludge from digester116is dewatered118, by use of a centrifuge, dewatering belt, screen, plate and frame presses, etc. with the resultant dewatered solids being beneficially reused or disposed. The ammonia-rich liquids122, which are less able to make struvite in the associated pipes and equipment because of the reduced phosphorus and magnesium, are sent to struvite reactor124, where the abundant ammonia combines with the phosphorus and magnesium of the liquids115to form struvite.

Referring toFIG. 3, in a second preferred embodiment, input to the system210is in the form of mixed liquor suspended solids (MLSS)214taken from the anaerobic zone212of the aeration basin, in an enhanced biological phosphorus removal (EBPR) system. The majority of the MLSS progresses to a further portion of the aeration basin218. RBCs are added to the added to the MLSS in a standard EBPR system, thereby causing phosphorus and magnesium to be released from the microorganisms. Other than this difference the processing is largely the same, although some variation is necessary to accommodate the larger flow214into the thickener216, as MLSS is typically three times as dilute as WAS. To handle the dilute flow, thickener216may utilize gravity thickening that is optionally followed by a belt or centrifuge or other thickening device. The anaerobic reactor112and supplemental addition of RBCs, shown inFIG. 2, can be eliminated.

FIG. 2Ashows yet another embodiment of the invented system similar to that ofFIG. 2(and having identical reference designators for identical elements). System220may be seen to include a mainstream flow220aand a sidestream flow220b, as illustrated, respectively above and below the dashed horizontal line. System220is referred to herein as providing for the in situ, i.e. closed or contained, production of usable struvite byproduct (e.g. marketable products such as regularly or irregularly shaped and sized pellets or particles, non-marketable products such as struvite sludge, etc.) from WAS, without external inputs being required to realize the production process.

As will be understood, primary sludge contained in a primary clarifier222is processed through an UFAT224in accordance with U.S. Pat. No. 6,387,264 or an equivalent process that includes a fermenter226and a thickener228or combined fermenter/thickener that collectively process primary sludge into VFAs and a thickened sludge. The VFAs from UFAT224are inputted to an EBPR aeration basin112aand to a separator/thickener114including, for example, an anaerobic release tank114aand a second thickener such as a thickening centrifuge114b. (Anaerobic reactor112ofFIG. 2in this alternative embodiment of the invention thus takes the form of an EBPR aeration basin112aand a secondary clarifier112b, as illustrated inFIG. 2A.)

The thickened sludge is fed to a digester116, as shown inFIG. 2A. Either downstream from digester116at the struvite reactor (as described above by reference toFIG. 2) or upstream from digester116, the pH of the P-rich and Mg-rich liquids is adjusted by a pH adjuster117(shown in dashed outline since it is optionally located in this upstream location instead of in struvite reactor124). Those of skill in the art will appreciate that, despite adjusting the pH of the P-rich and Mg-rich liquids before they reach the struvite reactor nevertheless struvite does not form in the upstream pipe because no ammonia is present). Moreover, peak concentrations of the fluids in the downstream struvite reactor are reduced. In this way, optionally upstream-located pH adjuster117produces a pH-adjusted phosphorus-rich and magnesium-rich liquids stream115′ as an input to struvite reactor124. (Thus, upstream pH adjuster117provides a system220topology that is more cost-effective and straightforward than the system110topology described above by reference toFIG. 2wherein pH adjustment is performed in the struvite reactor.)

A separator/thickener114acts to separate the WAS input from a secondary clarifier112bdownstream from EBPR aeration basin112aand the VFA input from UFAT224into two distinct output streams. A first relatively phosphorus-rich and magnesium-rich (P- & Mg-Rich) liquids stream115(or, preferably, pH-adjusted liquids stream115′) is fed into struvite reactor124, as described above. A second relatively phosphorus-reduced and magnesium-reduced (P- & Mg-Poor) mixture230is fed into a digester116followed by a dewaterer or dewatering centrifuge118to produce an ammonia-rich liquids stream122that is also fed to struvite reactor124. Phosphorus-reduced (P-reduced) biosolids are produced as another byproduct of the dewatering step. By separating the WAS into two separate, differentially concentrated streams containing phosphorus and magnesium, downstream so-called nuisance struvite production within digester116is minimized while concurrent struvite production within struvite reactor124is maximized.

Those of skill in the art will appreciate that further downstream treatment232within mainstream process220acan be accomplished via precipitation, filtration, and disinfection (e.g. chlorination followed by de-chlorination) of the output of secondary clarifier112b(some of which is returned to the input of EBPR aeration basin112a, and some of which is detoured to the sidestream process220b, as illustrated). Thus, the output of downstream treatment232is suitable for return to a river or other body of water is the mainstream output of the invented process and system while usable and potentially sellable struvite product, e.g. pelletized fertilizer, is the sidestream output of the invented process and system. Those of skill also will appreciate that other output of struvite reactor124can be recycled as shown to the plant influent stream in what may be thought of as a substantially “closed-loop” system220.

For the embodiments, the struvite reactor can take any form that permits the combination of the phosphorus and magnesium with the ammonium, to form struvite, including a simple settling tank, where spontaneously precipitated struvite would form and settle for reuse as a raw material, a usable product such as fertilizer, or a waste product. In one preferred embodiment prilled struvite is formed by a method disclosed in International Publication Number WO 2005/077834 A1.

In a first preferred variant of either the MLSS or the WAS embodiment the diversion of magnesium from the anaerobic digester and the resulting reduced nuisance struvite formation protects process equipment and reduces operational costs. In a second preferred variant, magnesium is added to capture additional phosphorus, thereby causing the system to produce additional struvite and a waste stream with less phosphorus and ammonia to be recycled back to the wastewater plant for re-treatment. In addition, phosphorus and magnesium can be added to increase struvite production and reduce the amount of ammonia sent back for re-treatment.

The above description is intended to provide an example of one method and system that falls within the scope of the invention. Skilled persons will recognize that other methods and systems will also fall within the scope of the invention.

It will be understood that the present invention is not limited to the method or detail of construction, fabrication, material, application or use described and illustrated herein. Indeed, any suitable variation of fabrication, use, or application is contemplated as an alternative embodiment, and thus is within the spirit and scope of the invention.

It is further intended that any other embodiments of the present invention that result from any changes in application or method of use or operation, configuration, method of manufacture, shape, size, or material, which are not specified within the detailed written description or illustrations contained herein yet would be understood by one skilled in the art, are within the scope of the present invention.

Accordingly, while the present invention has been shown and described with reference to the foregoing embodiments of the invented process and system, it will be apparent to those skilled in the art that other changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.