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 liquid production resulted from solid-liquid separation to waste water treatment. This treatment method has the advantage of low investment, simple operation, and environmental friendliness. Thereby, treatment capacity for solid-liquid separation is the key factor for determining the treatment capacity of waste liquor.

The problem existing in the traditional method for treating the waste liquor resulted from ethanol production through cassava fermentation, is that the time for solid-liquid separation is too long, thus causing the low treatment capacity of waste liquor, and causing the low load of the equipments used in producing ethanol. Therefore, the production capacity of the equipments for producing ethanol can not meet the requirement.

Summary of the Invention

The object of the present invention is to overcome the long time disadvantages in the available methods for treating the waste liquor resulted from ethanol production through cassava fermentation, and provides a method for treating the waste liquor resulted from ethanol production through cassava fermentation which takes short time for solid-liquid separation.

The present invention provides a method for treating the waste liquor resulted from ethanol production through cassava fermentation, comprising the steps of:

(1) introducing water-soluble ester of alginic acid and/or water-soluble salt of alginic acid into the waste liquor, and subjecting the waste liquor to solid-liquid separation to obtain solid production and liquid production; and (2) subjecting the liquid production obtained in step (1) to the waste water treatment. The inventive method can significantly reduce the time for solid-liquid separation, and enormously increase the treatment capacity of waste liquor in unit time without adding extra equipments, by introducing water-soluble ester of alginic acid and/or water-soluble salt of alginic acid into the waste liquor. Thereby, the inventive method solves the technical difficulty resulting in low load of the equipments used in producing ethanol through cassava fermentation, and enormously increases the production capacity of the equipments for producing ethanol.

Embodiment The present invention provides a method for treating the waste liquor resulted from ethanol production through cassava fermentation, comprising the steps of:

(1) introducing water-soluble ester of alginic acid and/or water-soluble salt of alginic acid into the waste liquor, and subjecting the waste liquor to solid-liquid separation to obtain solid production and liquid production; and (2) subjecting the liquid production obtained in step (1) to the waste water treatment.

According to the present invention, there is no special limitation on adding amount of the water-soluble ester of alginic acid and/or water-soluble salt of alginic acid, as long as it can reduce the time for solid-liquid separation. Preferably, the adding amount of said water-soluble ester of alginic acid and/or water-soluble salt of alginic acid is 0.05-1 kg, relative to 1000 kg of waste liquor; and more preferably, the adding amount of said water-soluble ester of alginic acid and/or water-soluble salt of alginic acid is 0.1-0.5 kg, relative to 1000 kg of waste liquor.

There is no special limitation on the type of said water-soluble salt of alginic acid, as long as it can intensify the impacting among the molecules, and make the small molecules combine into larger flocculate. Preferably, the relative molecular weight of water-soluble salt of alginic acid is 30000-200000. The term "the relative molecular weight" refers to a ratio of average mass of substance molecule or special unit to 1/12 atomic mass of nuclide C. For example, said water-soluble salt of alginic acid can be one or more selected from the group consisting of sodium alginate, potassium alginate, and ammonium alginate. The water-soluble salt of alginic acid which meets the requirement above can be purchased commercially, such as sodium alginate manufactured by Qingdao Nanyang Seaweed Industrial Co., Ltd., potassium alginate manufactured by Qingdao Bright Moon Seaweed Group Co., Ltd., and ammonium alginate manufactured by Handan Marine Medical Seaweed Co., Ltd.. There is no special limitation on the type of said water-soluble ester of alginic acid, as long as it can intensify the impacting among the molecules, and make the small molecules combine into larger flocculate. Preferably, the relative molecular weight of water-soluble ester of alginic acid is 30000-200000. The term "the relative molecular weight" refers to a ratio of average mass of substance molecule or special unit to 1/12 atomic mass of nuclide ¹² C. For example, said water-soluble ester of alginic acid can be one or more selected from the group consisting of propylene glycol alginate, ethylene glycol alginate, and butanediol alginate; preferably, said water-soluble ester of alginic acid is propylene glycol alginate. The water-soluble ester of alginic acid which meets the requirement above can be synthesized through common method known by person skilled in the art, and also can be purchased commercially, such as propylene glycol alginate manufactured by Qingdao Bright Moon Seaweed Group Co., Ltd.. Preferably, both of the water-soluble ester of alginic acid and water-soluble salt of alginic acid can be introduced into the waste liquor before the step of liquid-solid separation. While both of the water-soluble ester of alginic acid and water-soluble salt of alginic acid are introduced into the waste liquor, the time for solid-liquid separation can be further decreased, and the treatment capacity of waste liquor in unit time can be further increased. There is no special limitation on adding amounts of the water-soluble ester of alginic acid and water-soluble salt of alginic acid; preferably, the adding amount of the water-soluble salt of alginic acid is 10-1000 parts by weight, relative to 100 parts by weight of the water-soluble ester of alginic acid . More preferably, the adding amount of the water-soluble salt of alginic acid is 40-300 parts by weight, relative to 100 parts by weight of the water-soluble ester of alginic acid.

According to the present invention, various common methods known by the person skilled in the art can be used to perform the solid-liquid separation, such as filtration. The filtration can be performed by filter press, such as chamber filter press manufactured by Jingjin Filter Press Group. There is no special limitation on filtering conditions; preferably, the filtering conditions comprise filtering pressure of 0.4-1 MPa, and filtering time of 1-3 hr.

However, the present inventors have found that while the filtration is performed by chamber filter press, 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, and while liquid passes through the filter medium and is discharged via outlet; with ongoing of the filtering process, thickness of the solid production 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), thereby, water content of the obtained solid production can not be further decreased, thus causing the high chemical oxygen demand(COD) value of liquid production resulted from solid-liquid separation, and increasing the waste water treatment load.

Therefore, the solid-liquid separation is preferably performed by filtration and squeeze. The inventive method can significantly decrease the COD value of the liquid production resulted from solid-liquid separation, and enormously alleviate waste water treatment load. Preferably, various equipments can be used to filter and squeeze the waste liquor, and membrane filter press is preferably adopted. 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 liquid production 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 ^A ZG600/1500X 2000U ^B κ-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 liquid production 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 and squeezing conditions, preferably, the filter conditions and squeezing conditions make the solid production have water content less than 60wt%; more preferably, the filtering conditions comprise filtering pressure of 0.4-1 MPa, and filtering time of 1-2 hr; and the squeezing conditions comprise squeezing pressure of 10-25 MPa, and squeezing time of 0.2-1 hr.

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 liquid production 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 liquid production 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 liquid production in the anaerobic reactor can be selected according to types of the liquid production, 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/m ³ .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 liquid production 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 liquid production 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 liquid production in the aerobic reactor can be selected according to types of the liquid production, 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/m ³ .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 liquid production is subjected to aeration.

According to the present invention, the liquid production 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 m ³ /m ² .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

10 kg of propylene glycol alginate (Qingdao Bright Moon Seaweed Group Co., Ltd., relative molecular weight of 40000) is introduced into the waste liquor(100 tons) with water content of 95wt% resulted from ethanol production through cassava fermentation, and then, the waste liquor is charged into chamber filter press (Jingjin Filter Press Group) for filtering, wherein the chamber filter press has 100 chamber filter plates (Jingjin Filter Press Group). Solid production having water content of 75wt% and liquid production Al are obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time 2 hr. The time of solid-liquid separation and the treatment capacity (single set of equipment) of waste liquor in unit time are shown in table 1.

Comparison example 1

The waste liquor resulted from ethanol production through cassava fermentation is filtered by the same method as that in the example 1, except that the propylene glycol alginate is not introduced into the waste liquor. Solid production having water content of 75wt% and the comparison liquid production CAl are obtained, and the time of solid-liquid separation and the treatment capacity(single set of equipment) of waste liquor in unit time are shown in table 1.

Example 2

30 kg of sodium alginate (Qingdao Nanyang Seaweed Industrial Co., Ltd., relative molecular weight of 80000) is introduced into the waste liquor (100 tons) with water content of 95wt% resulted from ethanol production through cassava fermentation, and then, the waste liquor is charged into chamber filter press (Jingjin Filter Press Group) for filtering, wherein the chamber filter press has 100 common chamber filter plates (Jingjin Filter Press Group). Solid production having water content of 75wt% and liquid production A2 are obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time 1.8 hr. The time of solid- liquid separation and the treatment capacity (single set of equipment) of waste liquor in unit time are shown in table 1.

Example 3

15 kg of potassium alginate (Qingdao Bright Moon Seaweed Group Co., Ltd., relative molecular weight of 150000) and 15 kg of propylene glycol alginate (Qingdao Bright Moon Seaweed Group Co., Ltd., relative molecular weight of 100000) are introduced into the waste liquor(100 tons) with water content of 95wt% resulted from ethanol production through cassava fermentation, and then, the waste liquor is charged into chamber filter press (Jingjin Filter Press Group) for filtering, wherein the chamber filter press has 100 common chamber filter plates (Jingjin Filter Press Group). Solid production having water content of 75wt% and liquid production A3 are obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time 1.4 hr. The time of solid-liquid separation and the treatment capacity (single set of equipment) of waste liquor in unit time are shown in table 1.

Table 1

It can be observed from the table 1 that the inventive method reduced the time for removing water from the waste liquor significantly, and increased the treatment capacity of waste liquor in unit time enormously without adding extra equipments, thus solving the technical difficulty resulting in low load of the equipments used in producing ethanol through cassava fermentation, and increasing the production capacity of the equipments for producing ethanol.

Example 4

10 kg of ammonium alginate (Handan Marine Medicical Seaweed Co., Ltd., relative molecular weight of 100000) and 10 kg of propylene glycol alginate (Qingdao Bright Moon Seaweed Group Co., Ltd., relative molecular weight of 50000) are introduced into the waste liquor(100 tons) with water content of 95wt% resulted from ethanol production through cassava fermentation, and then, the waste liquor is charged into chamber filter press (Jingjin Filter Press Group) for filtering, wherein the chamber filter press has 100 common chamber filter plates (Jingjin Filter Press Group). Solid production and liquid production A4 are obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time 1.8 hr. The COD value of the liquid production A4 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 2.

The method for determining impurity content in liquid production A4 comprises stirring the liquid production A4 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 liquid production A4 resulted from solid-liquid separation. And the result is shown in Table 2.

The obtained liquid production A4 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 3 R Environmental Chemical Co., Ltd.) and anionic PAM (polyacrylamide) to carry out decoloration for the liquid production; and the sludge concentrating tank is used for concentrating the sludge.

The residence time of the liquid production in the anaerobic reactor is 25hr, and the reaction conditions in the anaerobic reactor include volume load of 21.5kgCOD/m ³ .d, temperature of 35 ⁰ C, and pH of 6.8. The liquid production discharged from the anaerobic reactor is introduced to the A/O system for aerobic treatment; the residence time of the liquid production in the anaerobic tank is 50hr; and the reaction conditions in the A/O system include volume load of 0.1kgCOD/m ³ .d, temperature of 20 ⁰ C, 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 36m /m .h. The residence time of the supernatant in the floatation tank is 3min. The COD value of the treated liquid production Dl is determined by GB11914-89 Water quality-Determination of the chemical oxygen demand -Dichromate method. And the result is shown in Table 2.

Example 5 The waste liquor resulted from ethanol production through cassava fermentation is filtered by the same method as that in the example 1, except that the solid- liquid separation is performed by using membrane filter press (Jingjin Filter Press Group, X _M ^A ZG600/1500 X 2000U ^B κ), wherein the membrane filter press has 50 common filter plates (Jingjin Filter Press Group, chamber filter plate), and 50 membrane filter plates (Jingjin Filter Press Group, 1250 Type) therein, and the common filter plates and the membrane filter plates are alternately arranged. Solid production is obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time lhr. After filtering process, air is charged as squeezing medium between the membrane filter plates for performing squeezing on the solid production at squeezing pressure lOMPa for 0.8hr to obtain liquid production A5.

The COD value of the liquid production A5 is determined by 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 liquid production A5 comprises stirring the liquid production A5 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 liquid production A5 resulted from solid-liquid separation. And the result is shown in Table 1.

The obtained liquid production A5 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 liquid production; and the sludge concentrating tank is used for concentrating the sludge.

The residence time of the liquid production in the anaerobic reactor is 35hr, and the reaction conditions in the anaerobic reactor include volume load of 21.5kgCOD/m .d, temperature of 38 °C, and pH of 7.2. The liquid production discharged from the anaerobic reactor is introduced to the A/O system for aerobic treatment; the residence time of the liquid production in the anaerobic tank is 55hr; and the reaction conditions in the A/O system include volume load of 0.2kgCOD/m .d, temperature of 40 °C, 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 65m ³ /m ² .h. The residence time of the supernatant in the floatation tank is 5min. The treated liquid production D2 is obtained.

The COD value of the liquid production D2 is determined by GB 11914-89 Water quality -Determination of the chemical oxygen demand -Dichromate method. And the result is shown in Table 2. Example 6

The waste liquor resulted from ethanol production through cassava fermentation is filtered by the same method as that in the example 5, except that the solid production and liquid production are obtained at the filtering conditions of filtering pressure 0.8MPa and filtering time lhr. After filtering process, air is charged as squeezing medium between the membrane filter plates for performing squeezing on the solid production at squeezing pressure lOMPa for 0.5hr to obtain solid production A6.

The COD value of the liquid production A6 is determined by GB 11914-89 Water quality -Determination of the chemical oxygen demand -Dichromate method. And the result is shown in Table 2. The method for determining impurity content in the liquid production A6 comprises stirring the liquid production A6 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 liquid production A6 resulted from solid-liquid separation. And the result is shown in Table 2. The obtained liquid production A6 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 liquid production; and the sludge concentrating tank is used for concentrating the sludge.

The residence time of the liquid production in the anaerobic reactor is 30hr, and the reaction conditions in the anaerobic reactor include volume load of 21.5kgCOD/m .d, temperature of 36°C, and pH of 7.0. The liquid production discharged from the anaerobic reactor is introduced to the A/O system for aerobic treatment; the residence time of the liquid production in the anaerobic tank is 45hr; and the reaction conditions in the A/O system include volume load of 0.12kgCOD/m ³ .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 50m ³ /m ² .h. The residence time of the supernatant in the floatation tank is 5min. The treated liquid production D3 is obtained. The COD value of the liquid production D3 is determined by GB11914-89 Water quality -Determination of the chemical oxygen demand -Dichromate method. And the result is shown in Table 2. Table 2

It can be observed from the Table 2 that the COD value of the liquid production A5 resulted from solid-liquid separation in example 5 is 34,100mg/l, while the COD value of the liquid production A4 resulted from solid-liquid separation in example 4 is as high as 50,000mg/l; after waste water treatment under same conditions, the resulted liquid production D2 of the example 5 has COD value of 87mg/l while the resulted liquid production Dl of the example 4 has COD value as high as 350mg/l. Therefore, the inventive method can alleviate waste water treatment load by reducing COD value of the liquid production resulted from solid-liquid separation, and the treated liquid production can satisfy environment requirement.