Abstract:
An apparatus for recovering nutrients from digestate comprises one or more solid-liquid separation steps, and dryer, an ammonia stripping device, and an evaporator. In a process, digestate is separated into a solids portion and a liquid portion. Ammonia is stripped from the liquid portion and converted into an ammonium solution or salt which may be used as, or blended with, a fertilizer product. At least part of the remaining liquid portion is concentrated to produce brine. The brine is mixed with the solids portion. The mixture is dried and may be used as, or blended with, a fertilizer product.

Description:
FIELD 
     This specification relates to anaerobic digestion, to the recovery of nutrients from waste biomass, and to a fertilizer product. 
     BACKGROUND 
     The following discussion is not an admission that anything discussed below is common general knowledge or citable as prior art. 
     Various organic waste products contain nutrients that make the waste potentially valuable as fertilizer. For example, some animal manures and organic sludges or slurries could be applied directly to land. However, due for example to the large quantities of material involved relative to the nutrient content, and potential problems with odors, this practice is limited to selected appropriate operations located near the source of the waste. The manure, sludge or slurry might be treated to remove large fibers, physically dewatered, partially dried thermally, extruded into a solid fertilizer product and then further thermally dried. However such a product would not be stable and would tend to decompose or attract mold during storage because of its high biodegradable organic matter content. Alternatively, manures, sludges or slurries could be digested in an anaerobic digester to produce a biogas. The digested sludge could then be applied to the land as a fertilizer. While the biogas produced is useful as a fuel, use of the digester sludge as a fertilizer is still limited to selected appropriate operations near the source of the waste. 
     In an activated sludge wastewater treatment plant, ammonia is removed from the wastewater at least in jurisdictions with relevant discharge regulations. In these plants, waste activated sludge may be sent to an anaerobic digester. Sludge from the digester, comprising digestate, is typically de-watered before it is disposed or treated further. The liquid stream from the de-watering device, which may be called reject water, centrate or filtrate, is often returned to the main activated sludge process. This centrate contains ammonia, and there have been some attempts to remove ammonia from the centrate before it is sent back to the main process. A paper by Tim Constantine, presented at the 2006 WEFTEC conference and entitled “North American Experience with Centrate Treatment Technologies for Ammonia and Nitrogen Removal”, provides a summary of ammonia removal technologies that have been used in North American facilities. 
     US Patent Application Publication Number 2007/0297953 to Kemp et al. describes a system in which ammonia is removed from water in a vacuum assisted flash stripping tower. The water is treated before stripping to remove solids and multivalent cations and increase its pH. 
     U.S. Pat. No. 7,416,644 to Bonde describes a fermenter with a side stream ammonia stripping step. Ammonia is stripped from fermented biomass in a shunt. Effluent from the fermenter passes through the shunt while water vapor is injected into the shunt. 
     INTRODUCTION TO THE INVENTION 
     The following paragraphs are intended to introduce the reader to the more detailed description to follow, and not to limit or define any claimed invention. 
     In some applications of anaerobic digestion, the feedstock is rich in nutrients such as Nitrogen (N), Phosphorus (P) and Potassium (K). Nutrients within the feedstock are maintained through the digestion process. The nutrients are concentrated on a dry mass basis in the digestate because a portion of the volatile solids in the feedstock is consumed by anaerobic bacteria during the digestion process and converted into biogas. However, a portion of the nutrients in the digestate is in the form of minerals or salts. For example, nitrogen is mostly contained as organic nitrogen in the feedstock but a large fraction of the nitrogen in the digestate is in the form of ammonia. 
     This specification describes a process and apparatus for recapturing one, or preferably more, nutrients contained in the feedstock in a form that is usable, for example as fertilizer. The nutrients are preferably concentrated in the recaptured form. 
     An apparatus for recovering nutrients from digestate comprises one or more solid-liquid separation steps, and dryer, an ammonia stripping device, and an evaporator. In a process, digestate is separated into a solids portion and a liquid portion. Ammonia is stripped from the liquid portion and converted into an ammonium solution or salt which may be used as, or blended with, a fertilizer product. At least part of the remaining liquid portion is concentrated to produce brine. The brine is mixed with the solids portion. The mixture is dried and may be used as, or blended with, a fertilizer product. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a process flow diagram of a system for treating a feedstock with nutrient recovery. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a system  10  for treating a feedstock  12 . The feedstock  12  is typically a waste biomass. The feedstock  12  is treated first in an anaerobic digester  14 . The digester  14  converts the feedstock  12  into biogas  16  and digestate  18 . Biogas  16  may be used, typically after upgrading, in the system  10  or after being transferred out of the system  10 . Examples of suitable feedstock  12  that result in high nutrient content digestate  18  include animal manure, post-consumer food waste, pre consumer food processing waste, biofuels processing by products, agricultural waste, and municipal wastewater sludge, among others. International Publication WO 2012/109737 is incorporated by reference. 
     The digester  14  can have one or more mixed closed tanks. The digester  14  is preferably operated, for example by using a high solids feedstock or recuperative thickening or both, such that the digestate  18  has a high total solids (TS) concentration, for example between 3 or 5 wt % and 9 wt %. 
     The nutrient recovery apparatus to be described below could be located separately from the anaerobic digester  14 . However, when the nutrient recovery apparatus is co-located with the anaerobic digester  14 , the need to move digestate  18  is reduced, the biogas or waste heat from power generation can be used in the nutrient recovery process, and a liquid stream may be advantageously returned to anaerobic digester  14 . 
     The digestate  18  passes through at least one, but preferably two, solids separation stages. In the system  10 , a first solid-liquid separation device  20  removes fibrous materials from the digestate  18 . For example, the first solid-liquid separation device  20  may be a filter screw press, a screen or roller press, or another similar device. The first solid-liquid separation device  20  may have openings larger than 400 microns but smaller than 1000 microns. Filtrate  22  from the first solid-liquid separation device  20 , alternatively called a reject, filtrate, centrate or pressate, may contain about 2 to 3 wt % total suspended solids (TSS). The first solid-liquid separation device  20  may optionally be omitted if there is not a significant concentration of fiberous solids in the digestate  18 . 
     The filtrate  22  goes to a second solid-liquid separation device  24 . The second solid-liquid separation device  24  may be, for example, a centrifuge, flocculating screw press, or similar device. One or more polymeric or mineral coagulants or flocculants may be added to the second solid-liquid separation device  24 . Second filtrate  26  exiting the second solid-liquid separation device  24  preferably contains 0.2 wt % TSS or less. A cake  28  produced by the second solid-liquid separation device  24  is combined with fibrous solids  30  removed in the first solid-liquid separation device  20 . The combined solids  28 ,  30  are rich in phosphorus and organic nitrogen, whereas the second filtrate  26  contains nitrogen (as ammonium) and potassium, both in solution. 
     Ammonia nitrogen is removed from the second filtrate  26  and concentrated to produce a nitrogen rich fertilizer product. The relative presence of ammonia (NH 3  gas) and ammonium (NH 4 + ion in solution) in the liquid of the digestate  18  is a function of pH and temperature. A larger fraction is present as unionized ammonia (NH 3  gas) with increased temperature and with increased pH. 
     The second filtrate  26  passes through an ammonia stripper  34 . In one example, the second filtrate  26  is heated, for example to 60 to 80 degree C., and flows through one or more tanks or zones, for example 3 to 5 heat jacketed reactors, in series. Sub-surface low pressure air is bubbled into the heated second filtrate  26 . In cold climates the subsurface air can be heated prior to entering the stripper  34 . The air bubbles and elevated temperature first strips CO 2  from the second filtrate  26  resulting in a pH increase. The pH increase and elevated temperature in turn result in ionized ammonium in solution shifting to unionized ammonia gas. The elevated temperature also reduces ammonia solubility. Continued air bubbling then strips ammonia gas out of solution and into a mixed gas stream  36  comprising ammonia gas, air and evaporated moisture. 
     The mixed gas stream  36  flows to a gas scrubbing unit  38  in which ammonia is scrubbed from the mixed gas stream  36 . The gas scrubbing unit  38  uses sulfuric acid  40  as an absorption liquid. The sulfuric acid  40  reacts with the ammonia gas and forms ammonium sulfate  42 . Ammonium sulfate  42  is a useable nutrient product in liquid form at 28 to 40 wt % concentration. Alternatively, ammonium sulfate  42  can be further concentrated or dried to a crystal or salt form. Ammonium sulfate  42  can be used as a fertilizer product alone or in combination with other products. 
     In one example, the gas scrubbing unit  38  uses a counter flow column configuration with air circulating from the bottom up through a packed bed with plastic media to enhance gas/liquid mass transfer surface area. An acid shower with excess sulfuric acid  40  flows from the top down and reacts with the ammonia gas in the air stream to form ammonium sulfate  42 . Ammonium sulfate  42  is stored in a sump at the bottom of the scrubber column. Ammonium sulfate  42  is pumped for recirculation and sulfuric acid  40  is added. Sulfuric acid addition is controlled automatically based on a pH set point. 
     Part of the stripped filtrate  44  leaving the ammonia stripper  34  can be used as dilution water  46 , optionally after being de-aerated. Dilution water  46  is mixed with the feedstock  12 . In the mesophilic and thermophilic range of digesters, operating at 35 to 55 degrees Celsius and at a pH of between 7.5 and 8.2, most of the reduced nitrogen exists as ammonium ions. Total ammonia concentrations are typically not allowed to exceed about 5000 ppm in mesophilic digesters and about 3000 ppm in thermopohilic digesters since the unionized ammonia fraction is toxic to methanogenic organisms. Therefore digesters for manures with high solids and high nitrogen content, such as digesters for poultry manure, are typically diluted. 
     Phosphorus and potassium that was not retained in the combined solids  28 ,  30  remains in stripped filtrate  44  and can be partially returned to the digester  14  in the dilution water  46 . However, not all of the stripped effluent  44  is required for dilution water  46 . Phosphorus and potassium is recovering from the excess filtrate  48  by blending the excess filtrate with the combined solids  28 ,  30 . 
     Excess filtrate  48  preferably passes through a concentrator  50  before being mixed with the combined solids  28 ,  30 . The concentrator  50  reduces the amount of water added to the combined solids  28 ,  30  while retaining phosphorus and potassium. Concentrated filtrate  52  leaving the concentrator  50  may be concentrated 6 or 7 times or more. The concentrated filtrate  52  may have a solids content of 20 wt % or more. The concentrator  50  may be, for example, one or more thermal evaporators. Thermal energy for evaporation can be supplied in a variety of ways such as burning natural gas. The concentrator  50  is preferably a multiple effect evaporator such that waste heat can be used to provide some of the thermal energy for drying. Waste heat may be taken from a combined heat and power unit burning the biogas  16 , or exhaust from a dryer  54  used to dry the combined solids  28 ,  30  and concentrated filtrate  52  mixture. The concentrator  50  also produces an effluent  56 , for example by condensing steam produced by evaporating the excess filtrate  48 . Effluent  56  may be discharged or used in another device or process, optionally after further treatment steps. 
     The combined solids  28 ,  30  and concentrated filtrate  52  are sent through a drier  54  to remove moisture, preferably to a TS concentration of 90 wt % or more. Drier  54  is preferably an indirect drier. The drier  54  can be, for example, a hollow screw type dryer with steam or hot oil circulation, a disc type dryer or a press type dryer, etc. The drier  54  may use, for example, biogas, natural gas or electricity as an energy source to evaporate water from the combined solids  28 ,  30  and concentrated filtrate  52 . The drier  54  may operate at a temperature of 100 degrees C. or more. 
     The dried product  58  is rich in phosphorus and potassium as well as non-volatile carbon. The dried product  58  can be used as a fertilizer along, or in a mixture with another fertilizer product.