PROCESS FOR PREPARING COAGULANTS FOR WATER TREATMENT
The present invention relates to a process for preparing proteins that can act as effective coagulants in the treatment and purification of contaminated water. In particular it relates to a process for extracting coagulant protein derivatives from the seeds of trees within the family Moringaceae and especially those of Moringa oleifera Lam (syns Moringa pterygosperma Gaertn.).
The seeds of Moringa oleifera Lam (hereinafter referred to as Moringa) are utilised primarily to obtain an edible oil, which may be extracted using a mechanical press. The residue from this extraction process is known as presscake. It has been found that the seeds of Moringa contain water soluble, low molecular weight, highly basic proteins that can act as coagulants in contaminated water treatment. The crushed/powdered seed suspensions and presscake, which remains following oil extraction, have been found to be effective coagulants but they suffer from the disadvantage that they result in a large amount of residual insoluble material which requires disposing of.
The object of the present invention is to provide an extraction process which results in a useful yield of coagulation proteins from Moringa seeds. The extraction process should preferably, but not exclusively, be suitable for use in less developed countries thereby providing inexpensive coagulant proteins for water treatment.
According to one aspect of the invention there is provided a process for preparing coagulation proteins from Moringa seeds which comprises the steps of:
1. treating Moringa seed presscake and/or whole seed inclusive of shell to produce an evenly divided granular powder having, for example, a particle size of from 0.5 to 2.5 mm diameter; and
2. adding the granular presscake to hydrochloric acid, preferably a solution of from 0.05 M to 0.2 M hydrochloric acid, and leaving the resultant dispersion to infuse for at least 1 hour and up to 24 hours; and
3. separating the acid protein solution from the insoluble residue of the presscake; and
4. precipitating the proteins from the acid extract using salts, for example, ammonium salts, or alkali, e.g. sodium hydroxide, ammonium hydroxide, or a combination thereof; and
5. separating the protein precipitate to produce a slurry.
The process preferably comprises the additional step of drying the protein slurry to a moisture content of 15% or less, preferably 10% or less, most preferably 5% or less.
The preferred alkali for precipitating the proteins from the acid extract is sodium hydroxide. When sodium hydroxide is used it is preferred to wash the protein precipitate in step (5) of the process to produce a slurry having a lower pH, preferably a pH of less than 8.
According to a second aspect of the invention there is provided a coagulation protein preparation suitable for use in the purification of water when prepared according to the process as described above.
According to yet another aspect there is provided the use of such a coagulation protein precipitation for the treatment and purification of contaminated water.
The extraction process comprises a series of unit operations, each designed to perform an essential step in the extraction of proteins, and their conversion into a state for practical use, from the seeds of the Moringa tree.
The process will be further described with reference to the accompanying figures in which:
Figure 1 is a schematic drawing of the first part of the process; and
Figure 2 is a schematic drawing of the second part of the process.
The unit operations of the process are taken in sequence.
A. Size Reduction
The presscake is received as an unevenly sized, randomly agglomerated granular material.
This operation consists of a feed hopper (1) from which the presscake is fed at a controlled rate into a mill (2) suitable for milling the presscake into a granular powder.
The mill achieves size reduction by its cutting action. Product is fed to the centre of a high speed impellor where centrifugal force moves it to the impellor tips which propel it tangentially against a cylindrical stationary cutting head. The mill is designed for either dry operation or wet operation in which the product is suspended in water or in which water is directed into the mill when necessary to dislodge deposits of the product. On leaving the mill the presscake has been converted to an evenly divided granular powder of a particle size, typically 0.5-2.5 mm diameter, preferably 1-2 mm diameter, and with a minimum of very fine particles which would reduce the efficiency of a later separation operation.
The milled product is collected in a receiving hopper (3) incorporating an exhaust duct for the airflow induced by the mill. The receiving hopper has a device such as a rotary valve (3a) to facilitate discharge of the product into containers or onto a conveyor (4) for transport to the extraction operations.
B. Acid Extraction
This consists of one or more process vessels (5) and a facility (7) for preparing and storing hydrochloric acid.
The process vessel is typically a vertical cylindrical tank with a base sloping to a discharge outlet and equipped with a mixer (6) employing a propeller or dispersion type rotating agitator.
In operation, the tank is filled with water and hydrochloric acid to provide a solution of a specified strength, from 0.05 M to 0.2 M, typically 0.1 M or 0.37% w/w. The milled presscake is added to this solution in a specified proportion, typically
w/w and thoroughly dispersed before leaving to infuse for at least one hour up to one day. During this operation, protein is leached out of the presscake as an acidic solution.
C. Clarification of Acidic Solution
This operation separates the acidic protein solution from the insoluble residue of the presscake now referred to as spent cake.
The spent cake is a by-product from the main process.
The acid protein solution may be separated from the insoluble residue by settlement, settlement and filtration or centrifugation.
Separation is most effectively achieved by use of a centrifuge (9) but a clarification filter has also been used. The acid protein solution and insoluble residue are pumped using a pump (8) to the centrifuge (9).
The preferred centrifuge is a continuous disk-stack type commonly used in the food and drink industry for separation of suspended solids from liquids and discharges the separated solids as a sludge. An impellor pump may be incorporated into the rotor which discharges the clear protein solution at sufficient pressure to assist transfer by pipeline to a holding vessel (10) where it is referred to as the acidic liquor.
D. Protein Precipitation
This operation recovers the protein from the acidic liquor by converting the liquor to an alkaline solution in which the protein is insoluble. The alkali which may be used to achieve this is sodium hydroxide and a facility (12) is provided for preparation and storage of concentrated solution in water, typically 4% w/v. When required, this concentrated solution is added to the acidic liquor in the proportion typically of 12.5% v/v to achieve a pH of from 10.5 - 11.5.
Precipitation is rapid and the addition of the concentrated alkali may be achieved in a process vessel equipped with a dispersing mixer (similar to (5) plus (6)) or preferably using an in-line mixer (14) through which the acidic liquor is continuously pumped by pump (11). In this system the alkali is continuously dosed (13) into the acid liquor before the mixer at an automatically controlled rate to achieve the specified proportion. The resultant liquor is discharged as an alkaline slurry into one or more holding vessels (15) intended to act as break tanks or buffer capacity for the pump transferring this dilute slurry to the next operation.
Some sedimentation occurs in the holding vessel and advantage may be taken of this to decant a part of the clear liquid away to waste reception.
However, it is significant that the residence time must be limited preferably to less than two hours to avoid degradation of the protein.
Precipitation may alternatively be carried out using an ammonium salt such as ammonium sulphate.
£. Separation of the Protein from the Dilute Slurry
Separation by filtration whilst found applicable is made difficult by the coagulant nature of the suspended protein tending to cause a phenomenon known as blinding of the filter medium. This is to be avoided as it is important to complete the recovery of the protein from the dilute slurry without excessive delay.
The preferred method is to use one or more continuous disk-stack centrifuges designed to separate solids from liquid suspensions and discharge them in the form of a thick slurry, typically having a solids content in the range of 5-25%, preferably 10-20%, most preferably 15-20%.
The dilute slurry is transferred by a pump (16) via a screen (17) for the purpose of removing any relatively gross particles which it is known might cause a blockage in the subsequent process equipment. The dilute slurry is thereby transferred to the centrifuges arranged in the form of a multistage operation.
The first stage centrifuge (18) separates the protein as a thick slurry from the residual clear alkaline liquor which is discharged to waste or other recovery location. In the next stage, the thick slurry is continuously mixed with water or clear liquor from a subsequent stage, and then further separated by centrifuge (19) as a thick slurry but with a part of the original alkaline liquor thus washed out. Known as a wash stage, this is repeated using fresh water (20) to wash out the
alkaline liquor and so reduce the pH of the thick slurry to a preferred level of less than 8. Alternatively, the additional wash stages may be achieved with one centrifuge by using intermediate batch holding vessels. For continuous and efficient production it is preferable to use a series of interlinked centrifuges as described in what is commonly referred to as a countercurrent washing system.
The finally recovered slurry has a solids content suitable for the next operation which, typically requires it in the range of 15-20%. A positive displacement pump (21) transfers the slurry to a holding vessel (22) which acts as a buffer storage necessary to facilitate a controlled unbroken supply to the next operation.
F. Drying
If the process is conveniently located, the protein in the state of a thick slurry is available for immediate use (as a water treatment aid). However, it must be assumed that in the form of a large scale efficient factory, it will be remote from the various points of use of the protein. For this major purpose, the state of the protein is changed to that of a dry powder in which it is resistant to degradation, and is convenient for storage, transport and use.
This operation includes a system for drying the thick slurry to a moisture content preferably 15% or less, and producing the protein in a finely divided form for convenience of use.
Freeze drying is a method which dries the protein to a friable solid which breaks up to a fine granular state. The preferred method is spray-drying which dries the protein without unwanted degradation and simultaneously converts it to a fine powder.
The spray dryer (24) consists of a chamber into which the slurry is fed at a controlled rate by a positive displacement variable capacity pump (23). The slurry is dispersed in the chamber in the form of very small droplets by means of an atomiser nozzle or atomiser rotating disk. A large volume of air is simultaneously introduced which has been heated to a temperature, typically of 200°C. The method achieves a rapid rate of evaporation of the water component of the slurry without raising the product temperature to a damaging level and for this protein the discharge temperature is limited to 100°C or less, preferably a range of 80°-90°, most preferably a range of 84-87°C.
The physical properties of the dry protein powder are such that it is attracted to and clings to the inside walls of the spray dryer chamber and to the exit ducts and receiver cyclone chamber (25). The spray dryer is equipped with devices to dislodge the powder and to ensure that it remains suspended in the flowing air.
The final product is discharged at two available locations (26) from the spray drying system via a rotary valve (26a) or similar method, from which it is filled directly into containers for transport or into a conveyor system for transfer to a filling process.
The invention will be further described with reference to the following examples :-
EXAMPLE 1
Preparation of Coagulation protein from Morinea
Milled Moringa seed presscake (65kg) was added to an extract solution (2600 1) consisting of 0.1 M hydrochloric acid in water. The resultant dispersion was stirred for a minimum of 1 hour prior to the residual solids being removed by centrifugation. This resulted in the recovery of 2200 1 of supernatant containing
solubilised protein. To the supernatant has added a 4% w/v sodium hydroxide solution (275 1, 12.5%v/v). The alkali mixture was then centrifuged to remove the precipitated protein. The resultant slurry was then spray dried giving 3kg of dry protein product suitable for use for water treatment.
EXAMPLE 2
Comparison of Water Treatment Using Moringa Proteins
Materials and Methods
1. Protein Preparations
Two preparations of Moringa seed proteins were made, Moringa 1 and Moringa 2. Moringa 1 was an extract produced using 100 mM HC1 as extractant and 1 M NaOH as precipitant. Moringa 2 was an extract prepared according to the invention using ammonium sulphate precipitation.
2. Test Water
A test raw water was prepared utilising kaolin clay in deionised water with an ionic background provided by sodium bicarbonate. Stock kaolin suspensions for dilution in the test water were prepared using the following procedure: 200g samples of Kaolin (BDH, Grade light, particle size 0.4 - lμm) were mixed to a paste and gradually diluted with deionised water, containing 450mg/l sodium bicarbonate, to a final volume of 1 litre. Samples were then rapid mixed for 2 hours at 300 rpm, allowed to settle for 1 hour following which the top 800 ml was decanted. This was then made up to 1 litre again with deionised water/sodium bicarbonate and mixed for a further 2 hours at 300 rpm. Following settlement for 1 hour the top 800ml was decanted and used as a stock solution for dilution to the required turbidity. An initial turbidity of 300+ 10 NTU was selected as the experimental standard.
3. Test Procedure
A standard jar test procedure was used for all testing - viz fast mix at 300rpm for 2 minutes following coagulant addition; slow mix at 30rpm for 15 minutes; 1 hour quiescent settling. Turbidity samples were taken from 2 cm below the surface of the treated water following the settling period. pH of the treated water was taken when considered necessary.
Results
Figure 3 demonstrates comparative performance between Moringa 1 and 2 using the standard test water and test procedure described above. Both preparations showed good turbidity removal at comparatively low doses. For this test water Moringa 1 provided marginally better turbidity removal than Moringa 2, however, the optimum dose was lower for Moringa 2, viz 4mg/l as compared to 6mg/l.
Figure 4 demonstrates comparative performance on a natural raw water obtained from the Ruvu river in Tanzania. A similar trend to that observed in Figure 1 was achieved.