METHOD TO ANAEROBICALLY TREAT FLUSH DAIRY MANURE

Abstract

An improved treatment of manure from animal husbandry results from using a hybrid upflow, anaerobic contact reactor that maintains a flocculent bed of solids in combination with a microfiltration/ultrafiltration system and a recycle flow arrangement. This hybrid anaerobic contact reactor arrangement provides a high conversion of COD and volatile suspended solids in the anaerobic contact reactor by maintaining a flocculent region of solids and direct recycle of an aliquot portion of the effluent stream from the anaerobic contact reactor. The direct recycle facilitates the adjustment of the hydraulic retention time versus the solids retention time within the anaerobic contact reactor. The provision of concentrate from the microfiltration/ultrafiltration system increases solids retention time in the anaerobic contact reactor. This arrangement eliminates additional processing steps and equipment typically used in treating flush manure.

Claims

1. A method to reduce the concentration of coarse fibrous solids and/or suspended solids (SS) in flush manure (FM) emanating from animal husbandry operations by using anaerobic conversion wherein: a.) initially treating the FM in a pretreatment zone to remove suspended solids and any coarse fibrous solids to provide an FM liquid containing TSS, VSS, soluble COD (sCOD) and highly degradable particulate COD (pCOD); b.) passing at least a portion of the FM liquid to an anaerobic contact reactor (ACR) having sufficient upward liquid velocity to maintain SS as a flocculent bed in the lower portion of the ACR with the ACR converting the sCOD and pCOD to a gas stream comprising methane and CO2 and an ACR liquid effluent stream having reduced concentrations of sCOD and pCOD relative to the FM liquid; c.) Recovering and cleaning the gas stream from the ACR and upgrading to renewable natural gas (RNG); d.) returning an aliquot portion of the ACR effluent stream directly to the ACR to provide additional liquid to in part maintain the desired upward liquid flow in the ACR such that the flocculant bed is maintained with an increased retention time of solids in the ACR; e.) passing a portion of the ACR effluent stream to a membrane separation unit including at least one of a microfiltration unit and an ultrafiltration membrane unit that separates the ACR effluent stream into a permeate stream comprising primarily water and a concentrate stream having an increased concentration of SS relative to the ACR effluent stream; f.) discharging an aliquot portion of the ACR effluent stream in a manner that bypasses the membrane separation unit to provide an ACR discharge stream for disposal, recovery, and/or further utilization; g.) passing at least a portion of the concentrate stream to the ACR to provide additional liquid to in part maintain the desired upward fluid flow in the ACR such that the flocculant bed is maintained with an increased retention time of solids in the ACR; and, h.) discharging at least a portion of permeate stream to provide a permeate discharge for disposal, recovery and/or further utilization.

2. The method of claim 1 wherein a portion of the concentrate stream is discharged to provide a concentrate discharge for disposal, recovery, or further utilization.

3. The method of claim 2 wherein at least one of the ACR discharge, the permeate discharge and the concentrate discharge passes to a lagoon from which it can provide manure flush water.

4. The method of claim 1 wherein the FM stream contains nitrogenous compounds and at least one of the ACR discharge, the permeate discharge and the concentrate discharge is treated to recover ammonium and/or ammonium compounds.

5. The method of claim 1 wherein the pretreatment zone comprises at least one of an FM separation unit for removing coarse solids and a biological treatment unit.

6. The method of claim 5 wherein the pretreatment zone includes a biological pretreatment unit that converts a portion of the pCOD to sCOD to reduce the TSS/VSS concentrations going the ACR.

7. The method of claim 1 wherein the pretreatment zone passes the FM stream to an FM separation unit that comprises screening and a screened FM stream then passes to a biological treatment unit or wherein the FM stream passes to a biological treatment unit and the biologically treated FM passes to a solids removal unit comprising at least one of centrifugation and membrane filtration.

8. The method of claim 2 wherein at least a portion of the concentrate discharge passes to a solids management zone that produces a converted concentrate discharge suitable for product production or energy recovery.

9. The method of claim 8 wherein the solids management zone screens and/or centrifuges the concentrate discharge stream to produce concentrated solids and then dries the concentrated solids to produce nutrient rich solids suitable for use as fertilizer or a soil amendment.

10. The method of claim 1 wherein no chemicals are added in practicing the method.

11. The method of claim 1 wherein at least a portion of the permeate discharge is stripped using air to remove dissolved CO2 and dissolved ammonia, and to produce a stripped permeate.

12. The method of claim 11 wherein the stripped permeate passes to a gas scrubber using CO2 and produces an ammonia bicarbonate solution.13. The method of claim 1 wherein the AC coupled with the treatment of the concentrate from the membrane separation unit increases the solids retention time in the AC by a sufficient amount to reduce the required size of the membrane separation unit.

13. The method of claim 1 wherein the animal husband operation maintains ruminant animals.

14. The method of claim 1 wherein the treating of step a.) is especially adapted for the removal of coarse fibrous solids.

15. The method of claim 1 wherein the pretreatment zone includes a biological treatment unit that converts a portion of the pCOD to sCOD to reduce the TSS/VSS concentrations going the ACR.

16. A method to reduce the concentration of coarse fibrous solids and/or suspended solids (SS) in FM flush manure (FM) streams emanating from animal husbandry operations by using anaerobic conversion wherein: a.) initially treating the FM stream in a pretreatment zone to remove suspended solids and any coarse fibrous solids to provide an FM liquid containing TSS, VSS, soluble COD (sCOD) and highly degradable particulate COD (pCOD); b.) passing at least a portion of the FM liquid to an anaerobic contact reactor (ACR) having sufficient upward liquid velocity to maintain SS as a flocculent bed in the lower portion of the ACR with the ACR converting the sCOD and pCOD to a gas stream comprising methane and CO2 and an ACR liquid effluent stream having reduced concentrations of sCOD and pCOD relative to the FM liquid; c.) recovering and upgrading the gas stream from the ACR to make RNG; d.) returning an aliquot portion of the ACR effluent stream directly to the ACR to provide additional liquid to in part maintain the desired upward fluid flow in the ACR such that the flocculant bed is maintained with an increased retention time of solids in the ACR; e.) passing a portion of the ACR effluent stream to a membrane separation unit including at least one of a microfiltration membrane unit and an ultrafiltration membrane unit MF/UF and separating the ACR effluent stream into a permeate stream comprising primarily water and a concentrate stream having an increased concentration of SS relative to the ACR effluent stream; f.) discharging an aliquot portion of the ACR effluent stream in a manner that bypasses the membrane separation unit to provide an ACR discharge stream for disposal, recovery, and/or further utilization; g.) passing a portion of the concentrate stream to the ACR to provide additional liquid to in part maintain the desired upward fluid flow in the ACR such that the flocculant bed is maintained with an increased retention time of solids in the ACR; h.) passing at least a portion of the permeate stream to a nitrogen recovery zone to produce an ammonium stream comprising ammonium and/or ammonium bicarbonate and a stream suitable for use as flush water; and, i.) passing a portion of the concentrate stream to a solids management zone and treating the concentrate stream to produce a converted stream suitable for product production or energy recovery.

17. The method of claim 16 wherein the pretreatment zone includes a biological treatment unit that converts a portion of the pCOD to sCOD to reduce the TSS/VSS concentrations going the ACR.

18. The method of claim 16 wherein the animal husband operation maintains ruminant animals and the treating of step a.) is especially adapted for the removal of coarse fibrous solids.

19. The method of claim 16 wherein the AC coupled with the treatment of the concentrate from the membrane separation unit increases the solids retention time in the AC by a sufficient amount to reduce the required size of the membrane separation unit.

20. A method to reduce the concentration of coarse fibrous solids and/or suspended solids (SS) in flush manure (FM) from animal husbandry operations by using anaerobic conversion wherein: a.) initially treating the FM stream in a pretreatment zone to remove suspended solids and any coarse fibrous solids to provide a FM liquid containing TSS, VSS, soluble COD (sCOD) and highly degradable particulate COD (pCOD); b.) passing at least a portion of the FM liquid to an anaerobic contact reactor (ACR) having sufficient upward liquid velocity to maintain SS as a flocculent bed in the lower portion of the ACR with the ACR converting the sCOD and pCOD to a gas stream comprising methane and CO2 and an ACR liquid effluent stream having reduced concentrations of sCOD and pCOD relative to the FM liquid; c.) recovering an upgrading the gas stream from the ACR to RNG; d.) returning an aliquot portion of the ACR effluent stream directly to the ACR to provide additional liquid to in part maintain the desired upward fluid flow in the ACR such that the flocculant bed is maintained with an increased retention time of solids in the ACR; e.) passing a portion of the ACR effluent stream to a nitrogen recovery zone to produce an ammonium stream comprising ammonium and/or ammonium bicarbonate and a recovery zone effluent stream comprising SS including VSS; f.) passing a portion of the recovery zone effluent stream to a membrane separation unit including at least one of a microfiltration unit and an ultrafiltration membrane unit and separating the recovery zone effluent stream into a permeate stream comprising primarily water and a concentrate stream having an increased concentration of SS relative to the ACR effluent stream; g.) discharging an aliquot portion of the ACR effluent stream in a manner that bypasses the membrane separation unit to provide an ACR discharge stream for disposal, recovery, and/or further utilization; h.) passing at least a portion of the permeate stream to the ACR to provide additional liquid to in part maintain the desired upward fluid flow in the ACR such that the flocculant bed is maintained with an increased retention time of solids in the ACR; and, i.) discharging at least a portion of permeate stream to provide a permeate discharge for disposal, recovery and/or further utilization.

21. The method of claim 20 wherein at least a portion of the concentrate stream passes to a solids management zone and treats the concentrate stream to produce a converted stream suitable for product production or energy recovery.

22. The method of claim 21 wherein the solids management zone screens and/or centrifuges the concentrate stream and then dries the resulting concentrated solids to produce nutrient rich solids suitable for fertilization.

23. The method of claim 20 wherein the animal husband operation maintains ruminant animals and the treating of step a.) is especially adapted for the removal of coarse fibrous solids.

24. The method of claim 20 wherein the pretreatment zone includes a biological treatment unit that provides hydrolysis and converts a portion of the pCOD to sCOD to reduce the TSS/VSS concentrations going the ACR.

25. The method of claim 20 wherein the AC coupled with the treatment of the concentrate from the membrane separation unit increases the solids retention time in the AC by a sufficient amount to reduce the required size of the membrane separation unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 is a flow diagram for the practice of the invention showing an ACR, a MF/UF unit, and a solids management section arranged in accordance with this invention.

[0040] FIG. 2 is a flow diagram for the practice of the invention showing an ACR, a MF/UF, and a solids management section arranged in accordance with this invention that includes nitrogen recovery, H2S removal and biogas upgrading.

[0041] FIG. 3 is a flow diagram for the practice of the invention showing a variation in the arrangement of the processing unit shown in FIG. 2

DETAILED DESCRIPTION

[0042] Any advanced FM manure treatment method requires the basic steps of separating fiber from the FM; treating the resulting FM liquid to breakdown suspended solids; and separating solids from the liquid effluent of the FM treatment. Advanced FM manure treatment methods use a wide variety of equipment and equipment arrangements to carry out these basic steps. In addition to these basic steps most FM treatment method involve additional treatment steps and ancillary equipment to tailor the method to specific characteristics of the FM and the desired outputs.

[0043] Initially raw FM recovered by the flushing of husbandry animal stalls, housing, and areas of confinement contain a high fiber loading. Such fiber typically comprises undigested and/or partially digested animal feed along with straw, sawdust, or other bedding that becomes mixed with the manure. Removal of fiber is essential to the processing of the remaining FM primarily because of the lignin and lignin-carbohydrate complexes present in grasses which are resistant to breakdown comprise a large portion of the undigested animal feed along with saw dust that may be present. Recovered fiber can also have value in a wide range of other uses from fuel product, animal bedding and even construction materials.

[0044] This invention starts with fiber treatment that will at minimum separate, from the raw FM, the larger fibrous suspended solids and other suspended solids that will not contribute significantly to the amount of methane produced by this method. Equipment for basic fiber removal is well known and will comprise dewatering and compression larger solids. Centrifuges, screw presses, high-capacity drum screens coupled with screw or roller presses can capture most of the fibrous manure particles. This type of equipment will deliver an FM liquid essentially devoid of fibers and having a lower concentration of TSS.

[0045] The fiber treatment may include additional screening of TSS and/or biological pretreatment. Biological pretreatment enhances the amount of hydrolysis of the smaller particulate COD and acidification to organic acids. Preferred arrangements will include both screening and biological treatment in either orderscreening for TSS removal followed by biological pretreatment or biological pretreatment followed by TSS removal using unit processes such as centrifuging or membrane filtration.

[0046] The FM liquid essentially free of large solid matter passes to an anaerobic treatment step to breakdown TSSNSS contained therein. The anaerobic treatment step of this invention uses an anaerobic contact process in the form of an anaerobic contact reactor (ACR). The ACR is an upflow hybrid anaerobic contact reactor that retains a flocculent (i.e. non-granular) sludge with high levels of organic and suspended solids. The ACR of this invention operates with much longer SRT than HRT. The ACR comprises a vertically extended tank or vessel that operates at a liquid upflow velocity that results in retention of the TSS after fiber removal as a blanket with higher TSS at the vessel bottom and gradually diminishing TSS concentration at higher elevations in the vessel. This further enhances the overall SRT for the solids ensuring the highest COD conversion efficiency.

[0047] An ACR effluent is withdrawn from the ACR vessel, a portion of which directly contributes to the maintaining the desired SRT. More specifically the ACR achieves longer SRT in a simple and readily controllable manner by recycling an aliquot portion of the ACR effluent directly back into the ACR in combination with the MF/UF that allows the longer retention time of the solids. The effluent contains TSS/VSS that the direct recycle returns to the ACR. As hereinafter described the ACR can also receive a portion of a concentrate stream containing TSS/VSS recovered from the ACR effluent and/or a portion of a purified water stream derived from the ACR effluent.

[0048] Returning a portion of the concentrate stream or purified water stream allows greater control of the TSS loading in the ACR and the ACR's HRT and SRT. For example, recycling a portion of concentrate or purified water can enable further control the overall particle density of the liquid that gets recycled to ACR. Thus, the direct recycle of the ACR effluent combined with the selective return of concentrate or purified water provide, in addition to HRT and SRT adjustment, great flexibility in maintaining desired mixing; liquid flow-distribution; agitation within the ACR; and internal recycling of particle sludge.

[0049] After the direct recycle of a portion of the ACR effluent, the remaining ACR effluent passes at least in part to a solids removal/solids concentration step that uses an MF/UF unit. The MF/UF unit separates the ACR effluent into a concentrate stream and a permeate stream. The concentrate stream contains a high density of particles in the liquid. The permeate stream contains no more than trace amounts of pCOD.

[0050] The MF/UF unit will typically have a microfiltration or ultrafiltration section. A typical microfiltration membrane with a pore size ranges from 0.5 microns or above. The pore size of ultrafiltration membrane typically range between 0.05 and 0.2 microns.

[0051] Whether it uses ultrafiltration or microfiltration the MF/UF section will usually include multiple modules. Available modules configurations and module materials are well known to the skilled in the art. Typical module configurations include flat sheet, tubular, and hollow fiber. Any of the well-known classes of membrane material will give suitable performance. Well know classes of membrane materials include ceramic membranes, stainless steel, polymeric membranes, and composite membranes. Commonly used materials include cellulose acetate, polyvinyl chloride, polysulfones, polycarbonates, and polyacrylonitriles. Purity requirements and pressure drop considerations will determine the number of modules, their sizing, and their operating conditions. Ceramic or stainless-steel membranes are preferred

[0052] Because the MF/UF permeate contains at most trace TSS and essentially consists of water it can be used directly or from a storage lagoon for land use by a conventional irrigation system, such as a center pivot system, with no plugging issues. When sent to lagoons, the purity of the permeate water eliminates the need for dragging the effluent lagoons to remove settled solids. The no TSS condition of the permeate makes additional downstream processing easier.

[0053] As previously described, a portion of the concentrate TSS of the MF/UF unit typically returns to the anaerobic contact reactor with one or more stream derived from the concentrate stream. This TSS return further increases the solids retention time, thereby improving the conversion of the VSS to methane.

[0054] The method of this invention can include additional processing steps and equipment. Specifically, VSS retained in the concentrate may be wasted or otherwise further processed in a concentrated form. Options for further processing these wasted solids can involve further concentration and drying using any number of well-known approaches to both unit operation. Preferably, these unit operations are done without chemical addition so the material does not lose its organic designation and associated higher value.

DETAILED DESCRIPTION OF THE DRAWINGS

[0055] A more complete description of the invention is given in conjunction with the following detailed description of the figures. A network of flow meters, sensors, pumps, compressors, controls, and control loops ensure smooth operation from feed to output and every step in between. For clarity the figures omit such additional process equipment that is well known to those skilled in the art and readily incorporated without explanation.

[0056] FIG. 1

[0057] FIG. 1 shows a basic arrangement for the practice of this invention with an ACR 10, a MF/UF unit 20, and a solids management unit 30. FIG. 1 also shows the necessary and optional lines that interconnect these units in a manner to practice this invention.

[0058] A feed stream 12 supplies an FM liquid to ACR 10 as the basic feed to the process. A recycle stream 14 supplies at minimum a direct recycle of an aliquot portion of ACR effluent stream 22 via a direct recycle stream 16. An optional concentrate recycle stream 18 passes a portion of concentrate from the MF/UF unit. FIG. 1 shows an arrangement for all of streams 12, 14, 16, and 18 to flow into ACR 10 as a combined input stream 12. Together streams 12, 16, and optionally 18 produce a flocculent bed in the ACR having the desired upflow velocity and disengagement of the gas stream produced therein.

[0059] Different arrangements for practicing the method of this invention may introduce liquid and particles via streams 12, 14, 16, and 18 into ACR 10 in a variety of ways. Combined input stream 12 may enter the ACR through a plurality of inlet nozzles. Similarly, any streams 12, 14, 16, and 18 may be combined with to produce a combined input stream that enters ACR 10 through one of more inlets. Any such stream or combined stream may enter ACR 10 through multiple inlet nozzles spaced about an ACR vessel (not shown) to distribute the input streams over circumference and/or different elevations of an ACR vessel. In a similar manner effluent stream 22 may exit the ACR vessel through multiple liquid effluent nozzle distributed circumferentially and/or at different height with all such nozzle preferably located above any input nozzles. Preferably any input streams will enter ACR 10 at a lower vessel elevation and any liquid effluent from ACR 10 will exit above the top region of the flocculent bed and all of the entry locations of feed and recycle components into ACR 10.

[0060] The breakdown of VSS produces an ACR gas stream 24 that comprises mainly methane and lesser amounts of sulfur compound. ACR gas stream 24 exits ACR 10 at an upper location of the ACR vessel. Gas stream 24 typically includes dissolved CO2 and ammonia and exits from an upper portion of the ACR vessel as off a gas stream. As shown in FIGS. 2 and 3, gas stream 24 will in almost all gases undergo further processing for recovery of a purified biogas/green methane, and sulfur compounds along with a rejection of CO2.

[0061] Upon exiting ACR 10 effluent stream 22 divides into at least a MF/UF unit feed stream 28; a waste stream 26 and the direct recycle stream 16. Recycle stream 16 will typically directly return a liquid volume of from 1.5 to 1.9 times the total volume of the liquid entering the ACR waste stream 26 may go to a lagoon for use as flush water and/or NH3 removal. The portion of ACR effluent 22 by streams 26 and 16 reduces the separation load on the MF/UF unit and can lead to a reduction in the required size of membrane separation unit. In addition, selective passing of a portion of the permeate stream provides more means for controlling SRT in ACR 10 as previously described.

[0062] MF/UF unit separates TSS/VSS from the ACR effluent using any of the membranes or combination of membranes in the manner previously described to produce a concentrate stream 32 and a permeate stream 34. Permeate stream 34 contains essentially no pCOD and comprises mainly treated water and will typically contain soluble ammonia compounds. Stream 34 may pass directly to a holding lagoon and/or find use as flush water. In addition to or alternatively all or a portion of permeate stream may undergo ammonia recovery and subsequent use in solid product or liquid product production.

[0063] Concentrate stream 32 has a variety of uses. At least a portion of concentrate stream 32 usually returns to ACR 10 and may enter the ACR vessel at any point and by direct flow or in combination with any other stream entering ACR 10 in any of the ways previously described. FIG. 2 shows an arrangement where a portion of concentrate stream 32 flows into ACR 10 via streams 18, 14, and 12. The volume of concentrate stream entering ACR 10 will usually range from 0.3 to 0.6 times the total volume of liquid entering ACR 10. At least another portion of stream 32 leaves the core arrangement of the invention as a concentrate waste stream 36.

[0064] Waste stream 36 may undergo additional treatment/processing to manage or utilize the solids contained therein. FIG. 1 shows waste stream 36 passing into a solids management unit 30. Unit 30 has an outlet 38 that can provide a variety of outputs including solid products, recovered liquid and/or energy. In most cases unit 30 will provide ammonia, ammonia derivatives and/or energy production (not shown).

[0065] FIG. 2

[0066] FIG. 2 expands the processing steps of those shown in FIG. 1 by adding a nitrogen recovery section 40, an H2S removal section 50 and a biogas upgrading section 60. All the description of FIG. 1 applies to the method represented by FIG. 2 and like numbering between FIGS. 1 and 2 describe the same elements.

[0067] In this arrangement permeate stream 34 passes to nitrogen recovery section 40 that further process at least a portion of the permeate. Nitrogen recovery section 40 may integrate numerous different process steps (not individually shown) in the processing of permeate stream 34. At minimum the nitrogen recovery stream provides a purified water stream 44 that is deficient in ammonia and a stream 42 that contains ammonia removed from permeate by nitrogen recovery section 40. Thus, nitrogen recovery section 40 can provide ammonia via recovery stream 42.

[0068] In one case, recovery section 40 may strip permeate using air (not shown). Air stripping reduces both the dissolved CO2 and dissolved ammonia in the permeate. In this case recovery stream 42 recovers nitrogen/nitrogen compounds for utilization as products.

[0069] In another case, recovery section 40 includes an air/gas scrubber (not shown). In this case the stripped air/gas scrubber and recovery stream 42 will deliver a solution of ammonia and/or ammonium bicarbonate (AB). Cooling of an AB laden stream that exits the stripping gas scrubber to a temperature that promotes the precipitation of some of the dissolved AB can provide a slurry containing precipitated and/or dissolved AB. Precipitation of AB can be controlled by manipulating the pH of the scrubbing solution and the temperature of an AB laden stream stripped by the stripping gas. The AB laden stream exiting the stripping gas scrubber may also contain ammonium carbonate AC.

[0070] When ammonium bicarbonate is recovered it may be converted for solid storage and product supply to put it in a form that makes it readily available for use on farm fields when desired. It is also possible to further process it to a granular product if desired.

[0071] As an alternative to air stripping, recovery section 40 may steam strip the permeate using direct steam injection into a stripper (not shown) or by use of mechanical vapor recompression (not shown). A portion of this stripped permeate now depleted in ammonia may return to the ACR 10 (not shown) thereby reducing the ammonia level in the ACR vessel; this ensures ammonia inhibition of the anaerobic process does not occur. In addition or alternatively, the steam stripped ammonium can be recovered as an aqueous ammonia solution.

[0072] ACR 10 also produces biogas stream gas stream 24 comprising a methane rich stream gas usually referred to as biogas. A typical biogas may include 50 to 80 vol % of methane, 20 to 35 vol % of carbon dioxide, 100 to 5,000 ppm of hydrogen sulfide. Extracting useful products from the biogas requires, at minimum, initial H2S removal and, for further upgrading, removal of additional impurities. Preferably all of the treatment/removal steps for purification of the biogas are carried out without the addition of inorganic chemicals.

[0073] Accordingly, gas stream 24 passes first to H2S removal section 50. Those skilled in the art know a wide range of method to treat/remove H2S from gas streams. Different treatment/removal systems will provide varying benefits depending on characteristics of the biogas. Various adsorbent media can remove both H2S and other sulfurous contaminants such as mercaptans. Additional H2S removal methods include feeding biogas to biological treatment systems that can oxidize the sulfide to produced elemental sulfur and/or sulfate streams and cleaned biogas. Membranes and amine treatment are known to remove CO2 and H2S from such biogas streams.

[0074] H2S removal section 50, except when using adsorptive media, produces at least two output streams. Sulfurous compounds removed from the biogas exit H2S removal section 50 in sulfurous stream 52. Stream 52 will, in most systems, comprise elemental S and/or SO4. H2S removal section 50 also produces a purified biogas stream 54 containing H2S in the range of 50 to 100 ppmv, and principally methane and CO2. This H2S depleted biogas can be recovered by line 56 for energy generation on site.

[0075] Stream 54 passes remaining purified biogas into biogas upgrade section 60. Upgrade section 60 produces an RNG stream 62 by removing other contaminants from purified biogas 54, such as residual CO2, that would disqualify stream 62 from having a designation of RNG. The removed CO2 and other trace gases (such as residual H2S, N2 etc.) removed via line 64 are vented or treated.

[0076] FIG. 3 shows another variation in the arrangement of the invention wherein nitrogen recovery section 40 is upstream of the MF/UF unit. In this variation line 28 carries the remaining ACR effluent downstream of any liquid withdrawn by waste stream 26 and direct recycle stream 16. Remaining ACR effluent flows into nitrogen recovery section 40 for removal/recovery of nitrogenous compounds/product and production of a nitrogen recovery stream 42 as described in conjunction with FIG. 2.

[0077] A nitrogen discharge stream 48 flows to MF/UF unit-20. The MF/UF unit 20 operates in the manner previously described to provide concentrate stream 32 and permeate stream 34. Concentrate stream 32 may pass to a solids management zone or other uses as previously described. A portion of concentrate stream 32 may also return to ACR 10 (not shown) in the manner previously described. At least a portion of permeate stream 34 is recovered for any further processing and the uses as previously described. A water recycle stream 46 or a portion of discharge stream 48 may flow directly to the ACR 10 (not shown) for addition to the ACR vessel in any of the ways as previously described.

[0078] The arrangement of FIG. 3 shows the option of passing a portion of permeate stream 34 to ACR 10 as purified water recycle stream 46. The recycling of purified water allows the adjustment of the liquid flow and the particle density in ACR 10.

[0079] As described, the present invention provides numerous advantages, some of which have been described above and others which are inherent in the invention. Also, modifications may be proposed without departing from the teachings herein. Accordingly, the scope of the invention is only to be limited as necessitated by the accompanying claims.