WO2009140871A1

WO2009140871A1 - A method for treating waste liquid in preparation of ethanol by fermenting cassava

Info

Publication number: WO2009140871A1
Application number: PCT/CN2009/070628
Authority: WO
Inventors: Shuhai Liu; Lianbin Ren; Chengguo Jia; Yongxin Zhang; Qingxun Liu; Guoqing Wu; Xu Wang
Original assignee: Cofco Limited
Priority date: 2008-05-22
Filing date: 2009-03-04
Publication date: 2009-11-26
Also published as: CN100554190C; CN101293716A

Abstract

A method for treating waste liquid in preparation of ethanol by fermenting cassava is provided, which comprises the following steps of: subjecting waste liquid in preparation of ethanol by fermenting cassava to solid-liquid separation to provide filter mass and waste water with chemical oxygen demand (COD) of 30000-35000 mg/L, and introducing waste water obtained by solid-liquid separation into an anaerobic reactor, an aerobic reactor, a deposit tank, an oxidization tank for oxidizing the supernatant and a floatation tank for removing suspended substance from the supernatant in turn.

Description

A method for treating waste liquor resulted from ethanol production through cassava fermentation

Technical Field The present invention relates to a method for treating waste liquor resulted from ethanol production through cassava fermentation.

Technical Background

Cassava is one of the three major root and tuber crops in the world, and is the first choice for bio-ethanol production. By 2005, the planting area of cassava in China has already reached 6,570,000 Chinese acres, and there are more than 200 factories producing starch and ethanol from cassava in China, with annual starch production of

500,000 ton, and cassava-derived ethanol 250,000 ton. As waste liquor resulted from ethanol production through cassava fermentation contains high concentration of organic compounds, and the waste liquor is directly discharged from most ethanol factories without any effective treatment, local and downstream water environment is severely affected.

Presently, treatment methods of waste liquor resulted from ethanol production through cassava fermentation include: 1. incineration method, i.e. incinerating the waste liquor after concentration, which is likely to cause secondary pollution after incineration; 2. marsh gas production through fermentation of the waster liquor, which has the disadvantage of high investment and high requirement for control techniques.

Thus, the available treatment method for the waste liquor resulted from ethanol production through cassava fermentation usually subjects the waste liquor to solid-liquid separation by filtering, then uses the solid residue obtained from solid-liquid separation as feed, and subjects the waste water resulted from solid-liquid separation to waste water treatment. Although this treatment method recovers partial solid residue in the waste liquor, the waste water resulted from solid-liquid separation still has high COD (chemical oxygen demand), usually above 45,000mg/l, which brings heavy load for subsequent water treatment; thus the treated waste water still can not satisfy environment requirement.

Summary of the Invention

The object of the present invention is to overcome the disadvantages in the available methods for treating the waste liquor resulted from ethanol production through cassava fermentation, such as heavy load for waste water treatment and unqualified treated waste water, and provide a method for treating the waste liquor resulted from ethanol production through cassava fermentation, which can significantly lower load of waste water treatment, and provide treated waste water satisfying environment requirement.

The inventors of the present invention have done intensive research on the available methods for treating the waste liquor resulted from ethanol production through cassava fermentation, and have found that the waste water resulted from solid-liquid separation by the available methods has high COD (usually above 45,000mg/l), and brings heavy load for subsequent waste water treatment; and the COD value of the waste water resulted solid-liquid separation is difficult to be further decreased by the available methods, which brings heavy load to environment.

The present invention provides a method for treating the waste liquor resulted from ethanol production through cassava fermentation, comprising the steps of: subjecting the waste liquor resulted from ethanol production through cassava fermentation to solid-liquid separation to give filter cake and waste water having a COD value within 30,000-35,000mg/l; and subjecting the waste water to the waste water treatment. The method according to the present invention lowers the load of subsequent waste water treatment by reducing COD value of the waste water resulted from solid-liquid separation. For example, the COD value of the waste water Al resulted from solid-liquid separation in example 1 is 34,200mg/l, while the COD value of the waste water CAl resulted from solid-liquid separation in comparison example 1 is 50,000mg/l; after waste water treatment under same conditions, the resulted waste water Dl of the example 1 has COD value of 87mg/l while the resulted waste water CDl of the comparison example 1 has COD value as high as 350mg/l. Therefore, the inventive method can lower load for waste water treatment by reducing COD value of the waste water resulted from solid-liquid separation, and the treated waste water can satisfy environment requirement.

Embodiment

The present invention provides a method for treating the waste liquor resulted from ethanol production through cassava fermentation, comprising the steps of: subjecting the waste liquor resulted from ethanol production through cassava fermentation to solid-liquid separation to give filter cake and waste water having a COD value within 30,000-35,000mg/l; and subjecting the waste water to the waste water treatment.

According to the present invention, there's no special limitation on the solid-liquid separation methods, for example, the method may comprise filtering and squeezing the waster liquor.

The present inventors have done intensive research on the available solid-liquid separation methods of the waste liquor resulted from ethanol production through cassava fermentation, and have found that the available methods usually adopt common filter press for filtering the waste liquor. During filtering, the waste liquor enters the space between the common filter plates of the filter press, solid particles are trapped in space between the filter plates by the filter medium of the filter plate, while liquid passes through the filter medium and is discharged via outlet; with ongoing of the filtering process, thickness of the filter cake increases gradually, and resistance to the dehydrating multiples, which leads to dramatic decrease of treatment capacity; moreover, due to limitation of inherent properties of the common filter plate, filter pressure can not be further increased (usually below IMPa), and thus solid content of the obtained filter cake can not be further increased, which leads to high COD value of the waste water resulted from solid-liquid separation, and heavy load to environment.

In the present invention, the waste liquor resulted from ethanol production through cassava fermentation is subjected to filtering and squeezing, COD value of the waste water resulted from solid-liquid separation can be significantly reduced, and subsequent waste water treatment load is lowered. Preferably, membrane filter press is adopted for filtering and squeezing. The membrane filter press has not only filtering function same as that of common filter press, but also squeezing function for further dehydrating filter cake. In the membrane filter press, common filter plate and membrane filter plate are alternately arranged, and the middle core plate and two side membranes of the membrane filter plate are tightly pressed to form two sealed sandwich layers. The membrane filter plate has the same filtering function as common filter plate, but additionally has squeezing function, i.e. after filtering is finished, squeezing medium is injected into the sealed sandwich layer via the inlet hole of the core plate, and the filtered filter cake can be swollen and squeezed for further increasing solid content by the elastic action of the membrane; therefore COD value of the waste water resulted from solid- liquid separation can be reduced.

The numbers of common filter plates and the membrane filter plates in the membrane filter can be adjusted according to the concentration of the waste liquor, and preferably, the number of the common filter plate is 50-60. The number of the membrane filter plates is preferably 50-60. The common filter plates and the membrane filter plates are alternately arranged at number ratio of 1:1. The membrane filter press satisfying the aforementioned requirements can be commercially purchased, such as X_M ^AZG600/1500 X 2000U^Bκ-type membrane filter press manufactured by Jingjin Filter Press Group.

The inventors of the present invention have found the membrane filter press can not only significantly reduce COD value of the waste water resulted from solid-liquid separation, but also dramatically decrease impurity content in water removed therefrom, so as to alleviate impurity-induced wearing to equipment and lower maintenance cost.

In the present invention, the waste liquor resulted from ethanol production through cassava fermentation refers to residual solid-liquid mixture resulted from ethanol distillation during ethanol production through cassava fermentation. In the present invention, there is no special limitation on filtering conditions, preferably, the filtering conditions comprise filtering pressure of 0.4-1MPa, and filtering time of 2-8hr; more preferably, filtering pressure of 0.5-0.8MPa, and filtering time of 4-6hr.

According to the present invention, there is no special limitation on squeezing conditions, preferably, the squeezing conditions comprise squeezing pressure of 10-25MPa, and squeezing time of 0.2-2hr; more preferably, squeezing pressure of 15-20MPa, and squeezing time of 0.5-lhr.

In the present invention, the squeezing pressure can be realized by filling squeezing medium into the membrane filter press, and the said squeezing medium can be various common squeezing medium for membrane filter press, such as compressed air and/or water.

According to the present invention, there is no special limitation on waste water treatment method, for example, waste water treatment can be carried out by the method disclosed in CN1202032C. The method may comprise introducing the waste water resulted from solid-liquid separation into an anaerobic reactor and an aerobic reactor, allowing to stand, oxidizing the supernatant, and removing suspended substance from the supernatant. In the present invention, the anaerobic reactor can be reactors well known to those skilled in the art. The anaerobic reactor is loaded with anaerobic granular sludge containing anaerobic microbes. The species of the anaerobic microbes are well known to those skilled in the art, such as Bacteriodes succinogenes, Butyribibrio fibrisolve, Ruminococcus flavfaciens, and Ruminococcus albus etc. When waste water passes through the anaerobic reactor, organic substances therein are decomposed. The types of the anaerobic granular sludge are well known to those skilled in the art, which can be commercially purchased, such as anaerobic granular sludge from Paques Environmental Technology ( Shanghai ) Co., Ltd. The residence time of the waste water in the anaerobic reactor can be selected according to types of the waste water, preferably, 25-35hr.

The reaction conditions in the anaerobic reactor can be varied within wide range, for example, the reaction conditions in the anaerobic reactor include volume load of 20-25kgCOD/m³.d, temperature of 30-40 °C, and pH of 6.5-7.5. The aerobic reactor is well known to those skilled in the art, preferably the waste water is introduced into A/O reactor (anoxic/aerobic tank) for aerobic treatment, and the A/O reactor is loaded with aerobic granular sludge therein; when the waste water flows in the A/O reactor, the aerobic microbes decompose organic substances in the water and convert them to their nutrients. The types of the aerobic granular sludge are well known to those skilled in the art, which can be commercially purchased, such as aerobic granular sludge produced by Beijing Fengzelvyuan Environment Technology Co., Ltd. The residence time of the waste water in the aerobic reactor can be selected according to types of the waste water, preferably, 40-60hr.

The reaction conditions of the aerobic reactor can be varied within wide range, for example, the reaction conditions in the anaerobic reactor include volume load of 0.1-0.2kgCOD/m³.d, temperature of 20-40 °C , pH of 7-8, and dissolved oxygen content of l-3mg/l, wherein the dissolved oxygen content refers to oxygen content in water when the waste water is subjected to aeration.

According to the present invention, the waste water having passed through the anaerobic reactor and the aerobic reactor is allowed to stand, the supernatant is oxidized, and the suspended substance in the supernatant is removed.

The aforementioned method for oxidizing the supernatant is well known to those skilled in the art, for example oxidant is adopted to oxidize the supernatant, and the types of the oxidant are well known to those skilled in the art, such as one or more selected from sodium hypochlorite, calcium hypochlorite, ferric chloride, and sodium ferrate (VI); and the oxidation treatment time may be 0.5-1.5hr.

According to the present invention, suspended substance in the supernatant can be removed by various known methods, for example, the supernatant can be introduced to floatation tank for floatation treatment, the flocculants adopted by the floatation tank are well known to those skilled in the art, such as one or more selected from aluminum-based flocculant, iron-based flocculant, and composite flocculant. The flocculants are commercially available, such as polyferric sulfate (PFS), polyferric chloride (PFC), polymerized ferric sulfate chloride (PFCS), and polyferric aluminum chloride from Huizhou 3R Environmental Chemical Co., Ltd. The surface loading rate of the floatation tank may be 36-72 m³/m².h, and the residence time of the supernatant in the floatation tank may be 3-5min. The present invention will be further described through the following examples.

Example 1

5 Tons of waste liquor with water content of 95wt% resulted from ethanol production through cassava fermentation is charged into membrane filter press (Jingjin filter press group, X_M ^AZG600/1500 X 2000U^B _K) for filtering, wherein the membrane filter press has 50 common filter plates, and 50 membrane filter plates therein, and the common filter plates and the membrane filter plates are alternately arranged. Filter cake is obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time 2 hr. After filtering process, air is charged as squeezing medium between the membrane filter plates for performing squeezing on the filter cake at squeezing pressure of lOMPa for 2hr to obtain waste water Al resulted from solid- liquid separation.

The COD value of the waste water Al is determined by Chinese National Standard GB 11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

The method for determining impurity content in the waste water Al comprises stirring the waste water Al well to form suspension, taking l,000g of the suspension, allowing it to stand for 24hr, removing the supernatant, and weighing the precipitate to obtain impurity content in the waste water Al resulted from solid-liquid separation. And the result is shown in Table 1.

The obtained waste water Al is subjected to waste water treatment by the anaerobic/aerobic waste water treatment system from Guangxi COFCO Bio-energy Co. Ltd. The waste water treatment system mainly comprises pre-acidifying tank, anaerobic tank, A/O system, settling tank, oxidation reaction tank, floatation tank, and sludge concentrating tank. Waste water distribution pipeline is arranged below the anaerobic reactor, and provided with anaerobic granular sludge (Paques Environmental Technology (Shanghai) Co., Ltd.); the aeration tank is provided with aeration pipe at lower part thereof, and loaded with aerobic granular sludge (Beijing Fengzelvyuan Environment Technology Co. Ltd.); the oxidation reaction tank adopts sodium hypochlorite for oxidation; the floatation tank adopts polyaluminum chloride (Huizhou 3R Environmental Chemical Co., Ltd.) and anionic PAM (polyacrylamide) to carry out decoloration for the waste water; and the sludge concentrating tank is used for concentrating the sludge. The residence time of the waste water in the anaerobic reactor is 25hr, and the reaction conditions in the anaerobic reactor include volume load of 21.5kgCOD/m³.d, temperature of 35⁰C, and pH of 6.8. The waste water discharged from the anaerobic reactor is introduced to the A/O system for aerobic treatment; the residence time of the waste water in the anaerobic tank is 50hr; and the reaction conditions in the A/O system include volume load of 0.1kgCOD/m³.d, temperature of 20⁰C, pH of 7, and dissolved oxygen content of lmg/1. The water treated by the aeration tank is introduced to the settling tank for settling, and the obtained supernatant enters into the oxidation tank for oxidation for 0.5hr, and then enters into the floatation system for treatment before discharge. The surface loading rate of the floatation tank is 36m³/m².h. The residence time of the supernatant in the floatation tank is 3min.

The COD value of the treated waste water Dl is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

Comparison example 1 The waste liquor resulted from ethanol production through cassava fermentation is filtered by the same method in the example 1 to obtain reference waste water CAl, except that the membrane filter plate is replaced by the common filter plate. The filtering conditions include filtering pressure of 0.8MPa and filtering time of 2hr.

The COD value of the waste water CAl is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

The method for determining impurity content in the waste water CAl comprises stirring the waste water CAl well to form suspension, taking 1,00Og of the suspension, allowing it to stand for 24hr, removing the supernatant, and weighing the precipitate to obtain impurity content in the waste water CAl resulted from solid- liquid separation. And the result is shown in Table 1.

The reference waste water CAl is subjected to waste water treatment by the same method in the example 1 to obtain reference treated waste water CDl. The COD value of the treated waste water CDl is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

Example 2

5 Tons of waste liquor with water content of 95wt% resulted from ethanol production through cassava fermentation is charged into membrane filter press (Jingjin filter press group, X_M ^AZG600/1500 X 2000U^B _K) for filtering, wherein the membrane filter press has 60 common filter plates, and 60 membrane filter plates therein, and the common filter plates and the membrane filter plates are alternately arranged. Filter cake is obtained at the filtering conditions of filtering pressure 0.4MPa and filtering time 8 hr. After filtering process, air is charged as squeezing medium between the membrane filter plates for performing squeezing on the filter cake at squeezing pressure of 25MPa for 0.2hr to obtain waste water A2.

The COD value of the waste water A2 is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

The method for determining impurity content in the waste water A2 comprises stirring the waste water A2 well to form suspension, taking l,000g of the suspension, allowing it to stand for 24hr, removing the supernatant, and weighing the precipitate to obtain impurity content in the waste water A2 resulted from solid-liquid separation. And the result is shown in Table 1.

The obtained waste water A2 is subjected to waste water treatment by the anaerobic/aerobic waste water treatment system from Guangxi COFCO Bio-energy Co. Ltd. The waste water treatment system mainly comprises pre- acidifying tank, anaerobic tank, A/O system, settling tank, oxidation reaction tank, floatation tank, and sludge concentrating tank. Waster water distribution pipeline is arranged below the anaerobic reactor, and provided with anaerobic granular sludge (Paques Environmental Technology (Shanghai) Co., Ltd.); the aeration tank is provided with aeration pipe at lower part thereof, and loaded with aerobic granular sludge (Beijing Fengzelvyuan Environment Technology Co. Ltd.); the oxidation reaction tank adopts sodium hypochlorite for oxidation; the floatation tank adopts polyaluminum chloride (Huizhou 3R Environmental Chemical Co., Ltd.) and anionic PAM^" (polyacrylamide) to carry out decoloration for the waste water; and the sludge concentrating tank is used for concentrating the sludge.

The residence time of the waste water in the anaerobic reactor is 35hr, and the reaction conditions in the anaerobic reactor include volume load of 21.5kgCOD/m³.d, temperature of 38⁰C, and pH of 7.2. The waste water discharged from the anaerobic reactor is introduced to the A/O system for aerobic treatment; the residence time of the waste water in the anaerobic tank is 55hr; and the reaction conditions in the A/O system include volume load of 0.2kgCOD/m³.d, temperature of 40⁰C, pH of 8, and dissolved oxygen content of 3mg/l. The water treated by the aeration tank is introduced to the settling tank for settling, and the obtained supernatant enters into the oxidation tank for oxidation for 1.5hr, and then enters into the floatation system for treatment before discharge. The surface loading rate of the floatation tank is

65m³/m².h. The residence time of the supernatant in the floatation tank is 5min. The treated waste water D2 is obtained.

The COD value of the waste water D2 is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

Example 3

5 Tons of waste liquor with water content of 95wt% resulted from ethanol production through cassava fermentation is charged into membrane filter press (Jingjin filter press group, X_M ^AZG600/1500 X 2000U^B _K) for filtering, wherein the membrane filter press has 55 common filter plates, and 55 membrane filter plates therein, and the common filter plates and the membrane filter plates are alternately arranged. Filter cake is obtained at the filtering conditions of filtering pressure 0.5MPa and filtering time 6 hr. After filtering process, air is charged as squeezing medium between the membrane filter plates for performing squeezing on the filter cake at squeezing pressure 20MPa for 0.5hr to obtain waste water A3. The COD value of the waste water A3 is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

The method for determining impurity content in the waste water A3 comprises stirring the waste water A3 well to form suspension, taking l,000g of the suspension, allowing it to stand for 24hr, removing the supernatant, and weighing the precipitate to obtain impurity content in the waste water A3 resulted from solid-liquid separation. And the result is shown in Table 1.

The obtained waste water A3 is subjected to waste water treatment by the anaerobic/aerobic waste water treatment system from Guangxi COFCO Bio-energy Co. Ltd. The waste water treatment system mainly comprises pre- acidifying tank, anaerobic tank, A/O system, settling tank, oxidation reaction tank, floatation tank, and sludge concentrating tank. Waster water distribution pipeline is arranged below the anaerobic reactor, and provided with anaerobic granular sludge (Paques Environmental Technology (Shanghai) Co., Ltd.); the aeration tank is provided with aeration pipe at lower part thereof, and loaded with aerobic granular sludge (Beijing Fengzelvyuan Environment Technology Co. Ltd.); the oxidation reaction tank adopts sodium hypochlorite for oxidation; the floatation tank adopts polyaluminum chloride (Huizhou 3R Environmental Chemical Co., Ltd.) and anionic PAM^" (polyacrylamide) to carry out decoloration for the waste water; and the sludge concentrating tank is used for concentrating the sludge.

The residence time of the waste water in the anaerobic reactor is 30hr, and the reaction conditions in the anaerobic reactor include volume load of 21.5kgCOD/m³.d, temperature of 36°C, and pH of 7.0. The waste water discharged from the anaerobic reactor is introduced to the A/O system for aerobic treatment; the residence time of the waste water in the anaerobic tank is 45hr; and the reaction conditions in the A/O system include volume load of 0.12kgCOD/m³.d, temperature of 30°C, pH of 7.5, and dissolved oxygen content of 2mg/l. The water treated by the aeration tank is introduced to the settling tank for settling, and the obtained supernatant enters into the oxidation tank for oxidation for lhr, and then enters into the floatation system for treatment before discharge. The surface loading rate of the floatation tank is 50m³/m².h. The residence time of the supernatant in the floatation tank is 5min. The treated waste water D3 is obtained.

The COD value of the waste water D3 is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand-Dichromate method. And the result is shown in Table 1.

Table 1

It can be observed from the Table 1 that the COD value of the waste water Al resulted from solid-liquid separation in example 1 is 34,200mg/l, while the COD value of the waste water CAl resulted from solid-liquid separation in comparison example 1 is as high as 50,000mg/l; after waste water treatment under same conditions, the resulted waste water Dl of the example 1 has COD value of 87mg/l while the resulted waste water CDl of the comparison example 1 has COD value as high as 350mg/l. Therefore, the inventive method can alleviate waste water treatment load by reducing COD value of the waste water resulted from solid-liquid separation, and the treated waste water can satisfy environment requirement.

Claims

1. A method for treating the waste liquor resulted from ethanol production through cassava fermentation, comprising the steps of: subjecting the waste liquor resulted from ethanol production through cassava fermentation to solid-liquid separation to give filter cake and waste water having a COD value within 30,000-35,000 mg/1; and subjecting the waste water to waste water treatment.

2. The method according to claim 1, wherein the process of the solid-liquid separation comprises filtering and squeezing the waste liquor.

3. The method according to claim 2, wherein the filtering and squeezing is carried out in a membrane filter press.

4. The method according to claim 3, wherein the filtering conditions comprise filtering pressure of 0.4-1 MPa, and filtering time of 2-8 hr.

5. The method according to claim 4, wherein the filtering conditions comprise filtering pressure of 0.5-0.8 MPa, and filtering time of 4-6 hr.

6. The method according to claim 3, wherein the squeezing conditions comprise squeezing pressure of 10-25 MPa, and squeezing time of 0.2-2 hr.

7. The method according to claim 6, wherein the squeezing conditions comprise squeezing pressure of 15-20 MPa, and squeezing time of 0.5-1 hr.

8. The method according to claim 1, wherein the process of the waste water treatment comprises: introducing waste water resulted from solid-liquid separation into an anaerobic reactor and an aerobic reactor; allowing the waste water discharged from the anaerobic reactor and aerobic reactor to stand to obtain supernatant; oxidizing the supernatant; and removing the suspended substance in the supernatant.

9. The method according to claim 8, wherein the residence time of the waste water in the anaerobic reactor is 25-35 hr, and the reaction conditions in the anaerobic reactor include volume load of 20-25 kgCOD/m³.d, temperature of 30-40 °C, and pH of 6.5-7.5.

10. The method according to claim 8, wherein the residence time of the waste water in the aerobic reactor is 40-60 hr, and the reaction conditions in the aerobic reactor include volume load of 0.1-0.2 kgCOD/m³.d, temperature of 20-40 °C, and pH of 7-8.