WO2012054812A2

WO2012054812A2 - Method for biological treatment of hydrolyzate from pulp washing by balancing chemical oxygen demand

Info

Publication number: WO2012054812A2
Application number: PCT/US2011/057239
Authority: WO
Inventors: Maureen Barry; David Barzyk; Scott Feil; Namhee Shin; Robert Peretin
Original assignee: Packaging Corporation Of America
Priority date: 2010-10-21
Filing date: 2011-10-21
Publication date: 2012-04-26
Also published as: WO2012054812A3

Abstract

A method to treat hydrolyzate with biological organisms including: discharging hydrolyzate from a pulp washing system; storing the hydrolyzate for first stage of pretreatment; determining a chemical oxygen demand (COD) level of the discharged or stored hydrolyzate; adding a dilution liquid to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to a second stage of pretreatment; determining an amount of the dilution liquid to be added to the hydrolyzate or a ratio of the dilution liquid to the hydrolyzate based on the determined COD level, and transferring the diluted hydrolyzate with the nutrients to a reactor feed tank where it is combined with a recycle stream from a bioreactor having the biological organisms.

Description

METHOD FOR BIOLOGICAL TREATMENT OF HYDROLYZATE FROM PULP WASHING BY BALANCING CHEMICAL OXYGEN DEMAND

CROSS RELATED APPLICATION

[0001] The application claims priority to USSN 61/405,442 filed on October 21, 2010 which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to a method of treatment of hydrolyzate discharged by a pulp washing with biological organisms in a biogas plant and particularly to balancing the chemical oxygen demand (COD) of the discharged hydrolyzate and of diluted hydrolyzate entering the biogas plant.

[0003] In conventional alkali pulping process, Pulp generated from cellulosic material, e.g., wood chips, is washed to remove cooking chemicals from pulp. The wash liquid flows through the pulp and is discharged as washer filtrate from the pulp washers. The washer filtrate is conventionally burned in a recovery boiler to recover the cooking chemicals removed from the pulp by the wash liquid. However, these processes can be operationally expensive due to energy inefficiencies and environmental treatment required.

[0004] A new pulping process using water or other non- alkali-liquor as the cooking liquor is disclosed in U.S. Patent 7,771,565. This pulping process avoids the alkali- chemicals used in conventional pulping processes. Pulp produced using water or other non-alkali cooking liquor is washed to remove hydrolyzate extracted from the cellulosic material in the pulping process.

[0005] It is known to process mill discharge streams containing organic material using anaerobic biological treatments. For example, anaerobic biogas plants have been used to convert the organic matter, e.g., sugars, from pulp mill discharge streams to fuel, such as is disclosed in U.S. Patent Application Publication 2008/0057555.

[0006] There is a need for an enhanced method and apparatus to pretreat hydrolyzate discharged from washing pulp. The method and apparatus would preferably be energy efficient, inexpensive to build and operate, and discharge waste gases that are environmentally friendly and provide useable energy for a mill.

BRIEF DESCRIPTION OF THE INVENTION

[0007] A method and apparatus has been developed to treat hydrolyzate from washing pulp made using water or other non-alkali as cooking liquor. The hydrolyzate is amenable to biological treatment to convert the chemical oxygen demand (COD) of the hydrolyzate to methane gas. For example, anaerobic biological organisms in a biogas plant which consume hydrolyzate organics and generate methane gas . The methane gas provides energy for the mill, such as fuel for boilers driving steam turbines and for gas turbines that generate electricity to power a pulp mill . [0008] Biological agents, e.g., microorganisms, are used in various conventional methods of anaerobicaly converting organic streams to methane gas, including Upflow Anaerobic Sludge Blanket reactors (UASB) and Expanded Granular Sludge Bed (EGSB) reactors. The anaerobic systems of these conventional methods are sensitive to changes in the concentration of COD in the input feed streams, such as the hydrolyzate feed stream. The changes in the concentration of feed stream may require corresponding changes in nutrient dosing of the stream to maintain the health of the biomass in the reactor tanks. In addition, unintentional high loadings to the anaerobic system can result in overwhelming the methanogenic bacteria resulting in inhibitory or toxic conditions in the anaerobic system. To address these issues a new method has been developed for controlling COD concentration to the Reactor Feed Tank.

[0009] Further, the composition and nature of hydrolyzate prevents it from being efficiently converted to methane by the above-identified conventional methods. To address the problems associated with converting hydrolyzate to a biogas, a new pretreatment method has been developed to allow sustainable, efficient conversion of hydrolyzate organics into methane gas .

[0010] Balancing of COD Levels:

[001 1] Unit operations between the pulping and the anaerobic processing, such as brown stock washing of the pulp, affect the concentration of chemical oxygen demand (COD) in the hydrolyzate flowing to the biogas plant. Controlling liquor to wood ratio alone in the pulping process may not be adequate to stabilize hydrolyzate for biological conversion in the biogas plant. The COD level of the hydrolyzate flowing to the biogas plant may be controlled by regulating the amount of dilution liquid added to the hydrolyzate.

[0012] A desired COD level of the combined flow of hydrolyzate and dilution water to the biogas plant is set by adjusting the ratio of dilution water to the hydrolyzate. The COD level of the diluted hydrolyzate is determined based on the desired COD level for the biogas plant and the desired COD level of the pulp discharged from the pulping process.

[0013] The COD level of the pulp from the digester is dependent on the ratio of cooking liquor to wood chips in the digester. Maintaining a relatively high ratio of water or non-alkali cooking liquor should reduce scaling or cladding of the interior metal components of the chip feed system and digester vessels. In addition, a low COD level of the pulp discharged from the washing stages indicates that much of the organic compounds have been filtered from the pulp to the hydrolyzate in the pulp washing stages. A high COD level of the hydrolyzate discharged from the washing stage indicates a high concentration of hydrolyzate organic which can be consumed by the anaerobic biological organisms in the biogas plant. The COD levels in the pulp and hydrolyzate may be inferred or measured using refractometers . [0014] The biogas plant may have an optimal COD level for the hydrolyzate flowing to the plant. This optimal COD level might correspond to the maximum flow of hydrolyzate which could be effectively processed by the biogas plant. The optimal COD level for the biogas plant may not be the only factor determining the desired COD levels for the hydrolyzate discharged from the pulp washers. Other considerations, for example pulp productions and costs, are involved in setting the COD levels including the desired COD level of the wash pulp discharged from the washer and the desired liquor to wood ratio in the chip flowing through the chip feed system and digester. A balance is determined based on each of these factors for the COD level of the hydrolyzate and dilution water flowing to the biogas plant.

[0015] For a high-rate anaerobic biological treatment, the hydrolyzate is initially stored in a first stage pretreatment tank, and then diluted and dosed with nutrients, such as nitrogen and phosphorous, as the hydrolyzate flows to a second stage pretreatment tank that provides diluted hydrolyzate to the reactor feed tank ( s ) .

[0016] The concentration of the COD in the hydrolyzate in the first stage pretreatment tank may be inferred using a refractometer placed in line with the storage tank. The inferred concentration of the COD is applied in a feed forward control loop to determine the ratio of the dilution liquid to hydrolyzate, and the nutrient feeds for the anaerobic bioreactor system. The inferred concentration of the COD may be applied to adjust the ratio of dilution liquid to hydrolyzate and adjust the supply rate of nutrients to achieve predetermined, constant levels COD and nutrients being fed to the bioreactor tanks in the anaerobic bioreactor system.

[0017] PreTreatment Of Hydrolyzate:

[0018] In a conventional high-rate anaerobic method, the substrate, in undiluted form, is stored for a typical period of three ( 3 ) to eight ( 8 ) hours in a preacidification/pretreatment step. Subsequently, the substrate is diluted to an appropriate concentration and nutrients are added as the substrate flows to a reactor feed tank. The reactor feed tank typically combines the diluted hydrolyzate and the recycle stream from the bioreactor .

[0019] In contrast to the conventional methods, the novel method disclosed herein enhances the preacidification/pretreatment (referred to herein simply as pretreatment ) by increasing the storage time of the substrate (undiluted hydrolyzate) followed by a second stage in which the substrate is diluted with water and receives nutrients . Pretreatment involves the retention of the undiluted hydrolyzate in a first stage pretreatment tank, the downstream addition of nutrients and dilution liquid and the retention of the diluted hydrolyzate with nutrients in a second stage pretreatment tank. Pretreatment preferably occurs before the diluted hydrolyzate is fed to a reactor feed tank which receives a recycled stream from the bioreactor. [0020] The increased storage time of the undiluted hydrolyzate in the first stage pretreatment tank may be as long as, for example, thirty (30) to fifty (50) hours. The diluted hydrolyzate is stored in the second stage pretreatment tank for an exemplary retention time from two (2) to eight (8) hours. The retention time of the diluted hydrolyzate may depend on the measured COD of the undiluted hydrolyzate. From the pretreatment stage, the diluted hydrolyzate flows to a reactor feed tank 54 where the reactor recycle stream 60 is added and fed forward to the bioreactors .

[0021] Hydrolyzate from the pulp wash stage and in the first pretreatment tank may have a COD level ranging from 45,000 to 90,000 ppm. Downstream from the first pretreatment tank, the hydrolyzate is diluted such as with water. The dilution ratio of water to hydrolyzate may be between 1.5 and 3. The diluted hydrolyzate may be retained and agitated in a second stage pretreatment tank or series of tanks for two (2) to eight (8) hours or more. The size and number of second stage pretreatment tanks may be varied to achieve a desired COD concentration and retention time of the diluted hydrolyzate. The hydrolyzate retention time in the second stage pretreatment tank(s) removes oxygen, which is toxic to anaerobic biomass, from the dilution water.

[0022] Nutrients, such as nitrogen and phosphorous, may be added in the first stage pretreatment tank, the second stage pretreatment tank or after second stage pretreatment tank and prior to a reactor feed tank. Feeding nutrients to the second stage pretreatment tank allows more efficient utilization of those nutrients which can lower the desired nutrient to COD ratio needed to maintain the health of the microorganisms in the bioreactor. Additional nutrients may also be added to the reactor feed tank and the bioreactor tanks .

[0023] Pretreatment of hydrolyzate discharged from pulp washing may convert fifteen percent (15%) to eighty percent (80%) of the hydrolyzate to a form readily digestible by methanogenic bacteria in the bioreactor tanks. This conversion reduces the growth of acidogenic bacteria in the anaerobic system of the bioreactor tanks . Balancing the growth rate of the different species of bacteria in the bioreactor tanks allows for sustainable operation of the bioreactor system. Without pretreatment of the hydrolyzate the ratio of methanogenic to acidogenic bacteria decreases and may reduce the capacity of the bioreactor system.

[0024] Phosphorous Control :

[0025] The pretreatment and COD control methods disclosed herein may be embodied to control phosphorus residual in the bioreactors . Phosphorus residuals in the bioreactors can be controlled by adding phosphorus and other nutrients at various locations of the pretreatment stages, increasing the residence period in the pretreatment stages, and controlling COD level in the pretreatment stages .

[0026] Biomass Size Control: [0027] Conditions are controlled in bioreactor to reduce the tendency of biomass to float. Such control includes adjusting amount of nitrogen and other nutrients added to the biomass tanks, and the residence period of the hydrolyzate in the biomass tanks .

[0028] The method and apparatus disclosed herein may be embodied to control, e.g., reduce, granular sizes of the biological agents in biomass tanks including biomass through a static mixer, centrifugal pump or orifice plate to create a rapid pressure drop in piping between biomass tanks .

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIGURE 1 is a schematic diagram of a pulping process having pulp washing stages and a biogas stage.

[0030] FIGURE 2 is a schematic diagram of the biogas stage shown in Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

[0031] FIGURE 1 is a schematic diagram of a pulping process 10 including a chip feed system 12, a digester 14, a wash stage 16 cleansing the pulp generated in the digester, and a biogas plant 18 treating the hydrolyzate 20 discharged from the wash stage. The chip feed system 12 receives wood chips to be converted to pulp. The term wood chips is used herein generically to refer to various types of cellulosic material. [0032] The pulping process 10 may be used to form corrugating materials, such as brown papers and other materials used for packaging. Pulping processes for corrugating materials often do not require bleaching to whiten the pulp. The wash stage may be referred to as a brown stock wash stage.

[0033] The chip feed system 12 may include a chip bin

22 or other device providing chips to a feed screw 24, high pressure transfer device (HPT) or other device which pressurizes the chips and conveys the chips to the digester 14. In addition, the feed screw 24, HPT or other device receives steam 26 and a white cooking liquor 28 which is added to pressurize, heat, and provide cooking liquid for the pulping process. The cooking liquid may be water or other non-alkali liquid which promotes the delignification of the chips in the digester 14.

[0034] The proportion of cooking liquor added to the wood chips determines a liquor to wood ratio of the chip fed to the digester. Especially where the cooking liquor is water or other non-alkali liquid, the concentration of sugars in the cooking liquor can cause scaling and damage the cladding of the metal components in the digester. This scaling, e.g., calcium oxide scaling, may damage the rotating and moving components in the digester, such as the screws which move the chips through the digester vessels 28. Increasing the proportion of cooking liquor, e.g., increasing the liquor to wood ratio, tends to minimize scaling and damage to cladding. However, excessive liquor may not be suitable for the defibrator 30 and the blow tower 32. [0035] The chips and cooking liquid flow from the feed screw 24 to the digester 14, which may be a conventional pulp digesting system, such as a cascade of cylindrical digesting vessels 28 having internal screw conveyors. The digester is arranged, e.g., with respect to the screw speed and flow path through the digester, to provide a desired retention period, e.g., ten minutes, for the chips .

[0036] The digester 14 may include a defibrator which is a thermo-mechanical pulping refiner 30 that acts on the chips flowing from digesting vessels to further delignify the chips and form pulp.

[0037] The chips flow from the digester 14 to a blow tower 32. The pulp, at atmospheric pressure, flows from the blow tower to one or more conventional pulp washers in the wash stages 16. Washed pulp 34 is discharged from the washing stages and is used to form corrugating paper, and other products .

[0038] Chemical Oxygen Demand (COD) sensors 36 measure the COD level of the pulp and of the hydrolyzate. The sensor may be a refractometer which provides data from which COD concentration levels may be inferred. A refractometer may also be used to infer the COD level of the hydrolyzate discharged from the wash stages.

[0039] Wash liquid 34, such as a mixture of clean water and hydrolyzate discharged from a downstream washer, is sprayed on the pulp mats moving through each of the washers. The pulp is washed as the wash liquid flows through the pulp and is drained from the pulp as hydrolyzate .

[0040] The hydrolyzate 20 discharged from the wash stage flows to a pretreatment stage 21 for the biogas plant 18. The pretreatment stage may include a first stage pretreatment tank 37 and a second stage pretreatment tank 48, such as an equalization/pretreatment tank. The hydrolyzate 20 flows from the pulp wash system to the first stage pretreatment tank 37. The hydrolyzate may be retained in the first stage pretreatment tank 37 in an undiluted form. From the first stage pretreatment tank, the hydrolyzate is diluted with a dilution liquid 39, e.g., water. Nutrients 56 and 58 may be added to the hydrolyzate flowing to the second stage pretreatment tank 48.

[0041] The dilution liquid may be obtained from a secondary waste water treatment facility 41 for the pulp mill. The secondary waste water treatment facility 41 may also provide clean water 38 for the washing stages 16.

[0042] The ratio of hydrolyzate and dilution water in the second pretreatment tank may be set to achieve a desired COD level of the diluted hydrolyzate flowing to the reactor feed tank 54. Increasing the portion of dilution water reduces the COD level of the liquid flowing to the biogas plant. The desired COD level may be determined based on the maximum amount of hydrolyzate which can be reasonably handled by the biogas plant to generate biogas . [0043] RefTactometer measurements may be used to infer COD level of the hydrolyzate discharged from the washing stages 16 , the unwashed pulp entering the wash stages and the washed pulp 34 leaving the wash stages. Further, the COD level of the unwashed pulp is dependent on the liquor to chip ratio in the chip flow from the chip feed system 12 to the digester 14 . In view of this dependence, the setting of the liquor to chip ratio to reduce cladding damage and scaling may be done in conjunction with the setting of desired COD levels for the unwashed and washed pulp as well as the desired COD level of the combined hydrolyzate and dilution flow to the biogas plant.

[0044] Biogas 40 , e.g., methane gas, generated by the consumption of hydrolyzate organic may be used to fuel a power plant 42 . The power plant may include a boiler which generates steam to drive steam turbines coupled to electrical generators or a gas turbine. Fueling a boiler with the biogas may be suitable when a constant flow rate of the biogas is unavailable. A boiler may be operated with a varying flow rate of the biogas to provide steam for the power plant. If a constant flow rate of biogas is available, the biogas may fuel industrial gas turbine (s) which drive electrical generators . A combination of gas turbines and a boiler may be used to consume the biogas and provide power and steam for the pulp mill.

[0045] FIGURE 2 is a schematic diagram of an exemplary biogas plant 18 . There is a continuous flow of hydrolyzate through the biogas plant 18 . The biogas plant produces methane gas and discharges a liquid effluent 44 after the hydrolyzate has been treated in one or more of the biological reactor vessels 46. The biogas plant may use conventional apparatus and methods common to anaerobic digestion, such as using microorganisms to consume the hydrolyzate organics in the absence of oxygen. However the pretreatraent stage 21 is unconventional, especially with respect to the second stage pretreatment tank 48 between a first stage pretreatment tank 37 and a bioreactor feed tank 54.

[0046] The hydrolyzate 20 is retained, in undiluted form, in the first stage pretreatment tank for a period of 30 to 50 hours. The retention period in the first stage pretreatment tank may be shorter, such as the 3 to 8 hours common to a pretreatment storage tank used in conventional biogas plants. An extended retention period in the first stage pretreatment tank may be useful in processing hydrolyzate obtained from pulp washing.

[0047] The hydrolyzate flowing from the first stage pretreatment tank 37 is mixed with a dilution liquid 39 and nutrients 56, 58 before entering or in a second stage pretreatment tank 48. The dilution liquid 39 may be heated to control the temperature of the diluted hydrolyzate flowing through the biogas plant. The diluted hydrolyzate may be at atmospheric pressure.

[0048] The residence time of the diluted hydrolyzate in the second stage pretreatment tank 48 may be relatively long, such as two (2) to eight (8) hours. Further, the diluted hydrolyzate may be agitated in the second stage pretreatment tank. The retention and agitation of the diluted hydrolyzate in the second stage pretreatment tank also allows for the removal of oxygen from the dilution water, as well as converting the hydrolyzate to a form digestible by the methanogenic bacteria. Oxygen is toxic to anaerobic biomass in the rapid mix tank 54 and biological reactor tanks 46. Removal of oxygen from the dilution water assists in maintaining a healthy population of anaerobic microorganisms in the rapid mix and bioreactor tanks 46, 54.

[0049] Hydrolyzate from a pulp washer may have a COD level ranging from 45,000 to 90,000 ppm. As the hydrolyzate flows from the first stage pretreatment tank to the second stage pretreatment tank, the hydrolyzate is diluted with a dilution liquid to reduce the COD level . The ratio of dilution liquid to hydrolyzate may be in a range from 1.5 to 3. This ratio may be determined based on the COD level of the undiluted hydrolyzate discharged from pulp washing or in the first stage pretreatment tank. For example, dilution liquid may be added to reduce chemical oxygen demand (COD) level to a range 15,000 (ppm) to 25,000 ppm in the diluted hydrolyzate.

[0050] A COD sensing device 49, e.g., a refractometer , provides data from which may be inferred the COD level of the hydrolyzate in the first stage pretreatment tank 38. The COD sensing device may be a refractometer in a conduit leading to or from the first stage pretreatment tank .

[0051] The COD level as inferred or measured of the hydrolyzate in the first stage pretreatment tank is used as a feed forward control of the amount of dilution water added to the hydrolyzate or the ratio of dilution water to hydrolyzate at or before the second stage pretreatment tank 48. In addition, the inferred or measured COD level may be used to regulate the amount and types of nutrients added to the hydrolyzate stream entering or in the second stage pretreatment tank. Using the inferred or measured COD level in the first stage pretreatment tank, the COD level may be controlled to maintain a constant level of COD and constant concentrations of nutrients in the hydrolyzate being fed to the biological reactor tanks 46, 54.

[0052] The hydrolyzate is believed to undergo preacidification in the first and second stage pretreatment tanks. In these tanks, 15% (fifteen percent) to 80% (eighty percent) of the COD in the hydrolyzate converts to a form readily digestible by methanogenic bacteria and, thus, requires less growth of acidogenic bacteria in the anaerobic system in the biological reactor tanks 46.

[0053] Converting the COD to a form digestible by the methanogenic bacteria in the first and second stage pretreatment tanks assists in balancing the growth rates of the different species of bacteria in the bio reactor tanks. A balance of growth rates of the different bacteria species assists in sustaining the microorganisms in the anaerobic system of the biological reactor tanks. Without the balance of growth rates provided by the second pretreatment tank 48, the ratio of methanogenic to acidogenic bacteria can decrease which degrades the capacity of the biogas system. Feeding nutrients to the diluted hydrolyzate flowing to or in the second stage pretreatment tank allows for efficient utilization of the nutrients . Because the nutrients are efficiently used, a relatively low nutrient to COD ratio may be used to maintain the health of the anaerobic microorganisms in the bioreactors .

[0054] The nutrients may be added to the diluted hydrolyzate in the conduit before the second stage pretreatment tank, in the second state pretreatment tank, in the conduit between the second stage pretreatment tank and to the rapid mixing tank. The nutrients may include nitrogen and phosphorus, such as provided by urea 56 and phosphoric acid 50. The nutrients provide feed material for the biological microorganisms in the bioreactors 46. The amount of nutrients added to the diluted hydrolyzate may be dependent on the COD level in the first stage pretreatment tank.

[0055] In addition, nitrogen and phosphorus may be added in the upstream portion of the biogas plant, e.g., in the first stage or second stage pretreatment tanks 37, 38, reactor feed tanks 54 and conduits between these pretreatment stage and the reactor feed tanks allows the nitrogen and phosphorous to mix with the diluted hydrolyzate before entering the bioreactors 49 and increases the retention period of the nitrogen and phosphorus with the hydrolyzate. The added nitrogen and phosphorous assist in maintaining the health of the microorganisms in the bioreactor tanks . [0056] The amount of phosphorus to be added may be determined by monitoring the residual phosphorus in the effluent 44 discharged from the bioreactors 46. For example, a residual phosphorus level of 3 to 5 parts per million (ppm) in the effluent may be desired and a level of at least 1 ppm may be acceptable.

[0057] From the second stage reactor tank 48, the diluted hydrolyzate is pumped 52 to a reactor feed tank 54, such as a rapid mix tank. The diluted hydrolyzate flows from the reactor feed tank to the biological reactor tanks 46 that are arranged in parallel with respect to the flow of the diluted hydrolyzate. The residence period in the reactor feed tank may be a few minutes, such as three to five minutes.

[0058] The bioreactor tanks 46 each contain a large population of microorganisms, such as methanogenic and acidogenic bacteria. Anaerobic biomass that is nutrient deficient tends to lose its capacity to generate methane. Controlling the COD by adjusting the ratio of dilution water added to the hydrolyzate based on the feed forward control of the inferred COD level assists in ensuring the health of the biomass and conversion of COD to methane gas. Similarly, using the inferred COD to determine the amount of nutrients to add and adding nutrients early in the biogas system, e.g., before the second stage pretreatment tank, assists in avoiding overdosing nutrients to the hydrolyzate. Similarly, controlling the COD concentration in the diluted hydrolyzate, prevents overfeeding the anaerobic system with hydrolyzate and thereby prevents inhibitory or toxic conditions in the reactors 46 , 54 .

[0059] As they consume the hydrolyzate organics, the microorganisms generate methane gas 64 that flows through conduits to gas treatment devices 66 , such as a sediment trap, scrubber, holding tank, compressor and moisture separator. The treated gas flows to the power plant 42 to be burned in, for example, a boiler to generate steam for a turbine or a combustor of a gas turbine. The turbines drive electrical generators which produce power to operate the pulp mill.

[0060] Boilers can produce useable steam regardless of the flow of methane gas from the biogas plant. Variations in the amount of steam produced by the- boilers can be relatively easily compensated for at the power plant 42 . In contrast, industrial gas turbines tend to operate at steady state conditions and are not tolerant of substantial variations in the gas flow of fuel. Boilers are particularly suitable for the power plant 42 if the biogas plant does not provide a steady flow of methane gas. If the biogas produces a steady flow of methane gas, gas turbines may be applied to burn the gas and generate electrical power.

[0061 ] Effluent fluids discharged from the bio reactor tanks 46 flow to a stand pipe 68 and are pumped to the secondary treatment facility 41 . The amount of phosphorus retained in the fluid discharged from the reactor tanks 46 may be measured by a lab test at the standpipe 68 or in the conduits immediately upstream or downstream of the standpipe ,

[0062] The amount of residual phosphorus in the discharged effluent fluid 44 discharged by the bioreactor tanks 46 may be used to determine the amount of phosphorus to be added to the diluted hydrolyzate at the second stage pretreatment tank or rapid mix tank. A residual amount of phosphorus of 3ppm to 5ppm, or even as little as lppm, may indicate that a sufficient amount of phosphorus is in the biological reactor tanks 46. Phosphorus assists in maintaining the health of the microorganisms in the tanks 46. If the residual amount of phosphorus is below a predetermined threshold, e.g., lppm, 3ppm or 5ppm, the amount of phosphorus 50 added at the second stage pretreatment tank 48 may be increased. Excessive residual phosphorous increases the difficulty to control phosphorous discharge from the secondary treatment facility to the fresh water body.

[0063] Sludge from the bioreactors 46 and the reactor feed tank 54 may be periodically discharged to a sludge treatment device 70.

[0064] The biomass of microorganisms in the reactor tanks 46 should have a relatively small granular size to improve settling of the biomass in the lower regions of the tank and to increase the surface area of the biomass exposed to the hydrolyzate. The biomass tends to grow to rather large hollow granules that float. Floating granules of biomass are not desired because they have limited surface area exposed to the hydrolyzate, tend to trap methane gas and do not efficiently release methane gas .

[0065] The granular size of the biomass may be measured by inspecting a sample of the biomass taken with a conventional test device, such as a Clark Classifier used to measure pulp quality.

[0066] To reduce hollow granules in the reactor tanks 46, the biomass may be periodically circulated from tank to tank 46, 54. The conduits through which the biomass is circulated may include devices that breakup large granules. These devices may include an orifice plate which causes a rapid pressure drop in the biomass flow, static mixer, and centrifugal pump, all of which mix and agitate the biomass. Further, adding phosphorus and nutrients early in the flow path of the biogas plant may assist in reducing the tendency of the biomass to grow in size .

[0067] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims .

Claims

We claim:

1. A method to treat hydrolyzate with biological organisms comprising: discharging hydrolyzate from a pulp washing system; storing the hydrolyzate for a first stage of pretreatment ; determining a chemical oxygen demand (COD) level of the discharged or stored hydrolyzate; adding a dilution liquid to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to a second stage of pretreatment or in the second stage of pretreatment; determining an amount of the dilution liquid to be added to the hydrolyzate or a ratio of the dilution liquid to the hydrolyzate based on the determined COD level, and transferring the diluted^' hydrolyzate with the nutrients from the second stage of pretreatment to a reactor feed tank where it is combined with a recycle stream from a bioreactor having the biological organisms.

2. The method of claim 1 further comprising transferring the diluted hydrolyzate from the reactor feed tank to a bioreactor tank having the biological organisms, and generating a combustible gas as the diluted hydrolyzate is consumed by the biological organisms in the bioreactor tank.

3. The method of claim 1 or 2 further comprising adding nutrients to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to the second stage of pretreatment or in the second stage of pretreatment .

4. The method of any of claims 1 to 3 wherein the step of storing the hydrolyzate for first stage of pretreatment includes storing the hydroylzate in a first stage pretreatment tank.

5. The method of any of claims 1 to 4 wherein the second stage of pretreatment includes storing the diluted hydroylzate in a second stage pretreatment tank.

6. The method of any of claims 1 to 5 wherein the ratio of dilution liquid added to the hydrolyzate is in a range of 1.5 to 3 of dilution liquid to hydrolyzate.

7. The method of any of claims 1 to 6 wherein the hydrolyzate discharged from the pulp washing system has a chemical oxygen demand (COD) level of 45,000 ppm to 90, 000 ppm.

8. The method of any of claims 1 to 7 wherein the pulp washing system discharges pulp produced from chips digested with water or other non-alkali cooking liquor, and the digesting of the chips avoids use of alkali cooking liquor.

9. The method of claim 8 wherein the washed pulp is used to form a corrugating medium.

10. The method of any of claims 1 to 9 wherein the method is a continuous method in which the hydroylzate continuously flows from the pulp washing system, through the first stage of pretreatment and second stage pretreatment and to the reactor feed tank.

11. The method of any of claims 1 to 10 further comprising adding nutrients to the diluted hydrolyzate.

12. The method of claim 11 wherein the nutrients include nitrogen and phosphorus .

13. The method of any of claims 1 to 12 wherein the biological organisms include methanogenic bacteria and acidogenic bacteria, and the method further comprises balancing growth rates of the methanogenic bacteria and of the acidogenic bacteria by pretreating the hydrolyzate in the first and second stages of pretreatment.

14. The method of any of claims 1 to 13 wherein the biological organisms are contained in an anaerobic environment in the biological reactor tank.

15. The method of any of claims 1 to 12 wherein the determined COD level is measured with a refractometer .

16. The method of any of claims 1 to 13 wherein the determined COD level is the sole factor used to determine the amount or the ratio for adding the dilution liquid.

17. A method to pretreat hydrolyzate with biological organisms comprising: storing the hydrolyzate for an extended period for a first stage of pretreatment; determining a chemical oxygen demand (COD) level of the stored hydrolyzate; adding a dilution liquid and nutrients to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to a second stage of pretreatment, wherein the dilution liquid is added in an amount or at a ratio based on the determined COD level of the stored hydrolyzate, and transferring the diluted hydrolyzate with the nutrients to a biological reactor feed tank which combines the diluted hydrolyzate and nutrients with a recycle stream from a bioreactor having the biological organisms .

18. The method of claim 17 further comprising transferring the diluted hydrolyzate from the reactor feed tank to a bioreactor tank having the biological organisms, and generating a combustible gas as the diluted hydrolyzate is consumed by the biological organisms in the bioreactor tank.

19. The method of claim 17 or 18 further comprising adding nutrients to the hydrolyzate as the hydrolyzate is transferred from the first stage of pretreatment to the second stage of pretreatment or in the second stage of pretreatment .

20. The method of any of claims 17 to 19 wherein the step of storing the hydrolyzate for first stage of pretreatment includes storing the hydrolyzate in a first stage pretreatment tank.

21. The method of any of claims 17 to 20 wherein the second stage of pretreatment includes storing the diluted hydrolyzate in a second stage pretreatment tank.

22. The method of any of claims 17 to 21 wherein the ratio of dilution liquid added to the hydrolyzate is in a range of 1.5 to 3 of dilution liquid to hydrolyzate.

23. The method of any of claims 17 to 22 wherein the hydrolyzate discharged from the pulp washing system has a chemical oxygen demand (COD) level of 45,000 ppm to 90,000 ppm .

24. The method of any of claims 17 to 23 wherein the pulp washing system discharges pulp produced from chips digested with water or other non-alkali cooking liquor, and the digesting of the chips avoids use of alkali cooking liquor.

25. The method of claim 24 wherein the washed pulp is used to form a corrugating medium.

26. The method of any of claims 17 to 25 wherein the method is a continuous method in which the hydrolyzate continuously flows from the pulp washing system, through the first stage of pretreatment and second stage pretreatment and to the reactor feed tank.

27. The method of any of claims 17 to 26 further comprising adding nutrients to the diluted hydrolyzate.

28. The method of claim 27 wherein the nutrients include nitrogen and phosphorus .

29. The method of any of claims 17 to 28 wherein the biological organisms include methanogenic bacteria and acidogenic bacteria, and the method further comprises balancing growth rates of the methanogenic bacteria and of the acidogenic bacteria by pretreating the hydrolyzate in the first and second stages of pretreatment.

30. The method of any of claims 17 to 29 wherein the biological organisms are contained in an anaerobic environment in the biological reactor tank.

31. The method of any of claims 17 to 30 wherein the determined COD level is measured with a refractometer .

32. The method of any of claims 17 to 31 wherein the determined COD level is the sole factor used to determine the amount or the ratio for adding the dilution liquid .