The present invention relates to systems and processes for biologically treating wastewater such as systems and processes that nitrify and denitrify wastewater.

BACKGROUND OF THE INVENTION

Biological treatment has been used for many years to treat wastewater. In the earlier years of biological treatment, primary treatment was employed to reduce the biological process reactor volume which had the effect of increasing the capacity of the biological process. Some wastewater treatment plants were even built with only primary clarifiers because only BOD removal was required. As biological processes evolved, it became a common practice to use biological processes to remove nitrogen. But in biological processes aimed at removing nitrogen, primary treatment was utilized less. Without primary treatment, it was relatively easy to maintain an appropriate carbon-to-nitrogen ratio for biological denitrification. There is, however, a drawback to biological nitrogen removal processes that do not employ some form of primary treatment. This is because biological nitrogen removal processes require relatively long sludge retention times and this means that without primary treatment the sludge being retained in the biological nitrification/denitrification process includes an excess amount of total suspended solids. This in turn results in an inefficient nitrogen removal process and because of the excess suspended solids this requires that the biological nitrogen removal reactors be made larger.

Even without primary clarifiers, there is often insufficient carbon to support an efficient and effective denitrification process. This is because, without primary clarification, the additional carbon source entering the biological process is mainly in particulate form and is slowly biodegradable (swbCOD) and must be hydrolyzed in order to be useful in a denitrification process. Hydrolysis of swbCOD is a slow process and occurs mainly in the oxic zone of an anoxic-oxic-anoxic nitrification/denitrification process. So most of the swbCOD is oxidized in the oxic zone and essentially wasted while, at the same time, consuming aeration (more energy) when primary clarification is not used. This is why a relatively high influent BOD to TKN ratio is required for efficient denitrification.

Thus, the problem is to provide a biological nitrogen removal process that provides a sufficient carbon-to-nitrogen ratio for efficiently denitrifying the wastewater and at the same time prevents the accumulation of excess suspended solids in the nitrogen removal process. SUMMARY OF THE INVENTION

The present invention addresses these problems by actually producing readily biodegradable chemical oxygen demand (rbCOD) and directing it into the anoxic zone of a denitrification process. By this process, rbCOD can readily be used for denitrification without hydrolysis, and relatively little of the rbCOD is consumed in the oxic zone.

The present invention provides a biological nitrogen removal process that employs primary treatment and which addresses the excess suspended solids problem discussed above. Moreover, the primary treatment employed produces primary sludge which, optionally with other sludges produced in the process, is employed to yield biogas which is ultimately converted to a clean biodegradable carbon source which serves as a supplemental carbon source to support biological denitrification.

The present invention also relates to a nitrification/denitrification process which recovers biogas from the anaerobic digestion of primary sludge and optionally other sludges produced in the overall process and converts the biogas to syngas which is then synthesized to form a carbon-containing liquid stream that is used as a supplemental carbon source for the benefit of denitrifying biomass.

In one embodiment, the biogas is pre-treated to remove contaminants and optionally compressed. Thereafter the biogas is directed into a reformer where the biogas is converted to another gas, syngas, which includes hydrogen and carbon monoxide. In this embodiment, the reformer is an internal combustion engine. The biogas, along with an oxidant, is directed into the internal combustion engine and through a partial oxidation reaction in the engine, the biogas is converted to syngas. Thereafter, the syngas is synthesized to form a liquid stream which includes carbon in the form of readily biodegradable chemical oxygen demand (rbCOD), volatile fatty acids or methanol. This liquid stream containing carbon is routed to the mainstream of the process where the carbon in the liquid stream is used to supplement the carbon in the wastewater stream to improve the efficiency of the biological process.

Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of a biological wastewater treatment process where biogas produced in the course of treating the wastewater is converted to a liquid stream containing carbon which is directed to a mainstream portion of the process.

Figure 1 A is a schematic illustration of a process similar to that shown in Figure 1 except that primary treatment includes the removal of primary solids with a drum filter.

Figure 2 is a schematic illustration of a biological nitrification/denitrification process where biogas produced in the course of anaerobically digesting sludge is converted to syngas which is in turn synthesized to form a liquid stream containing carbon that is directed to the anoxic zones of a nitrification/denitrification system.

Figure 2A is a schematic illustration similar to Figure 2 but showing an alternative biological nitrification/denitrification process.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

With further reference to the drawings, the basic process of the present invention entails feeding a wastewater influent stream to a primary treatment unit where the wastewater undergoes primary treatment. Primary treatment results in the production of a primary sludge and a primary effluent. The primary effluent is directed to a biological treatment unit or system. The focus of the biological treatment can vary. For example, it may include a

nitrification/denitrification process, a process aimed at BOD removal or phosphorus removal. These are only examples of biological treatments for wastewater. The biological system or unit produces an effluent that is typically directed to a clarifier, which in turn produces a treated effluent and settled sludge. A portion of the settled sludge is returned to the biological treatment unit as returned activated sludge (RAS). Typically a portion of the return activated sludge is wasted. The primary sludge produced in primary treatment, along with potentially other sludges produced in the total wastewater treatment process, is directed to an anaerobic digester. In the course of anaerobically digesting the sludge in the anaerobic digester, biogas is produced. In the processes disclosed herein, the biogas provides an indirect source for carbon that can be used to support biomass in the mainstream.

The processes described in this application use biogas produced in the anaerobic digester to ultimately produce a liquid stream that includes supplemental carbon that can be directed to the mainstream and used by the biomass performing biological treatment. In the processes disclosed here, the biogas, which is essentially methane and carbon dioxide, is converted to a syngas which includes hydrogen and carbon monoxide. Through a synthesis process, the syngas is converted to a liquid stream that includes carbon. Various specific processes can be utilized to convert the syngas to rbCOD. Particular forms of rbCOD are volatile fatty acids (for example, acetic acid) or methanol.

With reference to Figure 1 , shown therein is a schematic illustration depicting a biological wastewater treatment process. The system and process is indicated generally by the numeral 10. Wastewater influent is directed into a primary clarifier that removes primary solids (block 12). The primary clarifier produces a primary effluent that is directed through line 14 to one or more biological reactors 16. In addition, primary solids removed by the primary clarifier are directed through line 26 to an anaerobic digester 28. Various biological wastewater treatments can be carried out in the biological reactors 16. For example and as discussed below in other embodiments, the biological reactors can perform nitrification and denitrification of the wastewater. In addition, other biological processes, such as BOD removal, phosphorus removal, are typically performed by biomass in biological reactors. Biological reactors 16 produce a treated effluent that is directed to a secondary clarifier or settling tanks 18. Here sludge, including biomass and suspended solids, are settled, collected and returned by a return activated sludge (RAS) line 22 back to the mainstream of the process. The return activated sludge is mixed with primary effluent to form mixed liquor that is directed into the biological reactors 16. Some of the secondary sludge produced by the secondary clarifier or settling tanks 18 can be wasted. As shown in Figure 1 , this waste sludge, referred to as waste activated sludge (WAS), is directed through line 24 where it is ultimately sent to the anaerobic digester 28.

Secondary clarifier or settling tanks 18 produce a clarified effluent that is directed from the secondary clarifier or settling tanks via line 20. Typically treated effluent in line 20 is subjected to further treatment such as filtration and/or disinfection.

The sludge collected and sent to the anaerobic digester 28 is subjected to an anaerobic digestion process. After the sludge has been subjected to anaerobic digestion, it is directed from the anaerobic digester 28 via line 30 to further treatment such as dewatering. In the process of anaerobically digesting the sludge, biogas is produced in the anaerobic digester 28.

While physical and chemical processes are used in wastewater treatment, biological treatment employing microorganisms in activated sludge is a main stay for removing certain nutrients from wastewater. In order to achieve efficient nutrient removal in biological treatment processes, it is important to have available a readily biodegradable carbon source for the microorganisms to use. Microorganisms utilize carbon as an energy source to drive metabolism and for the synthesis of new cellular material. As will be discussed in more detail below, a deficient carbon source in a wastewater stream is especially problematic in biologically denitrification and phosphorus removal processes.

The aim of the processes described herein is to utilize biogas produced in the anaerobic digestion process as an indirect supplemental carbon source. That is, the process of the present invention captures the biogas and converts the biogas to syngas which comprises hydrogen and carbon monoxide. Through a synthesis process, the syngas is converted to a readily biodegradable carbon source, sometimes referred to as rbCOD, which is directed to the mainstream and mixed with the wastewater or mixed liquor therein to supplement the carbon source in the wastewater. In the Figure 1 embodiment, rbCOD is produced and directed to the mainstream where the rbCOD functions as a supplemental carbon source for the biomass. Examples of rbCOD are volatile fatty acids, such as acetic acid as well as methanol.

Returning to Figure 1 , biogas produced by the anaerobic digester 28 is directed via line 32 to a biogas pre-treatment unit 34. Biogas pre-treatment unit 34 removes impurities from the biogas including hydrogen sulfide gas, carbon dioxide and water vapor. After pre-treatment, the biogas may be compressed. After pre-treatment, the biogas is directed to a conversion unit 36. Here the biogas is converted to syngas. Various systems and processes can be used to convert the biogas to syngas. One approach is by an internal combustion engine having one or more cylinders, reciprocating pistons in the cylinders, an intake manifold, and an exhaust manifold. The biogas and an oxidant, such as air or oxygen, are directed into the engine. The partial combustion of the biogas and oxidant in the engine results in the release of heat and the formation of syngas, a mixture of hydrogen and carbon monoxide. Syngas produced by the engine may be compressed and sent to a chemical reactor, or in the case of one embodiment, an rbCOD synthesis reactor 40 where the syngas is converted to a liquid which contains readily biodegradable carbon. As discussed below, the syngas, through a synthesis process, can be converted to a liquid that contains rbCOD volatile fatty acids or acetic acid as well as methanol. Moreover, the combustion of the biogas in the internal combustion engine produces power that can be harnessed and utilized in and around the wastewater treatment facility.

As shown in Figure 1 , the rbCOD resulting from the rbCOD synthesis reactor 40 is directed through line 42 to the biological reactors 16 where the rbCOD functions to supplement the carbon in the primary effluent being directed to the biological reactors 16. Biological reactors 16 can perform various biological wastewater treatment processes. As discussed below, one particular biological process entails nitrification/denitrification. In that case, it may be preferable to convert the syngas to methanol and use the methanol as a supplemental carbon source for denitrification. Here, the methanol would be directed to one or more anoxic zones that perform denitrification. Another example of biological wastewater treatment is phosphorus removal. Phosphorus removal processes can stand alone or can be used with other mainstream biological wastewater treatment processes. In any event, biological phosphorus removal is usually facilitated by one or more reactors operated under anaerobic conditions. Here, if there is a carbon deficiency in the wastewater influent, it is preferable to supplement with volatile fatty acids.

There are other ways to convert biogas to syngas other than employing an internal combustion engine reformer. For example, rbCOD including volatile fatty acids or methanol can be produced from the biomass through a thermochemical process known as gasification.

Biogas is subjected to elevated temperatures and pressures to form syngas. The syngas can then be treated to remove impurities, such as tars and methane, and to adjust the hydrogen-to- carbon monoxide ratio. The syngas can then be reacted over a catalyst and at elevated temperatures and pressure to form rbCOD.

Figure 1A depicts a biological wastewater process that is similar in many aspects to that just described and shown in Figure 1 . The main difference is that the primary treatment process shown in Figure 1 has been replaced with a primary treatment process that entails a drum filter 54. In the case of the Figure 1A embodiment, wastewater is directed through a line to a coagulation tank 50. Upstream of the coagulation tank 50, a coagulant is added to the wastewater. The effluent from the coagulation tank is directed to a flocculation tank 52. Between the coagulation tank 50 and the flocculation tank 52, a flocculant polymer is added. The effluent from the flocculation tank 52 is directed to a drum or rotary disc filter 54 which filters the primary solids from the influent wastewater stream and produces a primary affluent that is directed to the biological treatment reactors 16. Primary solids captured by the drum or rotary disc filter 54 are directed through line 26 to the anaerobic digester 28. As discussed above, biogas produced by the anaerobic digester 28 is converted to syngas which in turn is converted to rbCOD or a liquid that contains biodegradable carbon.

Turning to Figure 2, a biological nitrification/denitrification process is shown. In some cases, the wastewater stream or primary effluent is carbon deficient. Hence, there is insufficient carbon in the wastewater to completely denitrify the wastewater. As discussed in the Figures 1 and 1 A embodiments, the biogas produced by an associated anaerobic digester 28 can be employed to ultimately produce rbCODmethanol which can be used to augment the carbon present in the influent to the nitrification/denitrification process. More particularly, the biogas is converted, in one embodiment by an engine reformer, to syngas and the syngas is converted to methanolrbCOD.

In Figure 2, a wastewater influent stream is subjected to primary treatment upstream of the nitrification/denitrification unit. The influent wastewater stream is mixed with a coagulant and a flocculent polymer and thereafter filtered by a microscreen filter 49. Microscreen filter 49 produces a primary effluent and primary sludge. The resulting primary sludge is directed from the microscreen filter 49 via line 54 to a primary sludge thickener 56 that thickens the primary sludge. Once the primary sludge is thickened, it is directed to the downstream anaerobic digester 28.

Returning to the primary effluent produced by the microscreen filter 49, it is directed through line 51 to a nitrification/denitrification system or unit. The nitrification/denitrification system includes multiple treatment zones. Typically, a nitrification/denitrification system will include an oxic or aerobic zone and one or more anoxic zones. As seen in Figure 2, the nitrification/denitrification system of this embodiment includes two anoxic (AX) or denitrification zones 60 and 62. Between the two denitrification zones 60 and 62 is an aerobic zone 64 that functions to nitrify the wastewater or mixed liquor passing therethrough. A recycle line 66 extends from a downstream portion of the aerobic zone 64 to a point just upstream of the first anoxic zone 60. A biological nitrification/denitrification process is a two-step process. In the first step, ammonia-nitrogen is converted to nitrate and nitrite. In the second step,

microorganisms oxidize carbon compounds - using nitrate as an electron accepter - which ultimately converts the nitrates to nitrogen gas.

Treated wastewater from the nitrification/denitrification system is directed to a clarifier

70. Clarifier 70 in conventional fashion produces a clarified effluent and sludge. Some of the sludge produced by the clarifier 70 is directed through return activated sludge line 72 to the mainstream where it is mixed with the primary affluent to form mixed liquor. Another portion of the sludge produced by the clarifier 70 is termed "waste activated sludge" and this is directed through line 74 to a waste activated sludge thickener 76 where the waste activated sludge is thickened. Once thickened, the waste activated sludge is directed through line 78 to the anaerobic digester 28. The drawings depict settling tanks 18 and clarifier 70. It is understood and appreciated by those skilled in the art that various types of solids-liquid separators could be employed for the settling tanks 18 or the clarifier 70.

Clarified effluent is directed to tertiary filters 82. From the tertiary filters, the wastewater is directed to a disinfection unit 84 which produces a treated effluent. It is appreciated that various types of tertiary filters and disinfection units can be employed downstream of the nitrification/denitrification system. Moreover, other forms of treatment can be incorporated downstream of where nitrification and denitrification take place.

As discussed before, the nitrification/denitrification process shown in Figure 2 can be used in nitrification/denitrification processes where the primary effluent is deficient in carbon. A deficiency in carbon means that there is insufficient carbon available to the biomass to completely denitrify the wastewater (or otherwise to completely biologically treat the

wastewater) passing through the nitrification/denitrification system. One way of assessing the carbon content in a wastewater stream is to measure carbonaceous biological oxygen demand (cBOD). This is indirectly a measure of the organic carbon content in a wastewater stream. For purposes of biological treatment processes, some (but not all) wastewater streams are deemed deficient in carbon when the cBOD to total kjeldahl nitrogen (TKN) ratio is less than 4.

Biogas produced by the anaerobic digester 28 is directed to a biogas holding tank 71 . From the biogas holding tank 71 , the biogas is directed along with an oxidant, such as air or oxygen, into an internal combustion engine reformer 92 as discussed in the case of the Figure 1 embodiment above. Engine reformer 92 converts the biogas to syngas, hydrogen and carbon monoxide. The syngas is then directed to a syngas reactor 94 where through a synthesizing process the syngas is converted to a carbon-containing liquid. By exercising control over the reactions that take place in the syngas reactor 94, various carbon compounds can be produced in the liquid produced by the syngas reactor 94. In the case of this embodiment, the aim is simply to convert the syngas to methanol. The methanol is directed through line 96 to the anoxic zones 60 and 62 of the nitrification/denitrification system. The methanol supplements the carbon supplied via the primary effluent and increases the carbon concentration in the influent to the nitrification/denitrification unit sufficient to enable the biomass to efficiently denitrify the wastewater. By directing the methanol into the denitrifying zones 60 and 62, the ratio of cBOD to TKN can be increased to 4 or more.

Figure 2A shows another embodiment of the present invention. This embodiment is similar to the embodiment shown in Figure 2 and discussed above, except that in this embodiment a tertiary denitrification unit 81 is disposed downstream of the BNR reactors. As illustrated in Figure 2A, the rbCOD or methanol can also be directed into the tertiary denitrification unit 81.

From the foregoing specification, it is appreciated that effective and efficient biological denitrification can be achieved even where the influent to the denitrification process is deficient in carbon by converting biogas produced in an associated anaerobic digestion process to a carbon containing liquid (methanol) that is used to supplement the biodegradable carbon in the influent stream to the nitrification/denitrification system. In a biological wastewater treatment process that employs the biomass to remove phosphorus from the wastewater, the syngas can be converted to volatile fatty acids which are utilized to supplement the carbon source in the mainstream of the process for efficient phosphorus removal.

The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.