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A novel treatment process for dairy wastewater with chitosan produced from shrimp-shell waste
e>
Wat. SeL rfCh. Vol. 34. No. II. pp. 33-40. 1996. Copyright 0 1996IAWQ. Publishedby ElsevierScience LId
Pergamon
Pll: S0273-1223(96)OO818-9
Printedin Oreat Britain.All rightsreserved, 0273-1223196 SU 'OO + 0-00
A NOVEL TREATMENT PROCESS FOR DAIRY WASTEWATER WITH CHITOSAN PRODUCED FROM SHRIMP-SHELL WASTE E. Selmer-Olsen*.***. H. C. Ratnaweera** and R. Pehrson* • Deptpanment. ofFoodScience. AgriculturalUniversity ofNorway. P.O. Box 5036. N-J432 As. Norway •• Norwegiallinstitutefor WaterResearch. P.O. Box 173 Kjelsds. N.(J4J I Oslo. Norway ••• Norwegian Dairies. P.O.Box 905J Gnnland;N..()133 Oslo.Norway
ABSTRACf Recovery of proteins and fats from dairy wastewater has two advantages; the recovery process results in a pretreatment of wastewater prior to discharge to municipal sewers; and the recovered sludge can be used as 1I food additive. Carboxy Methyl Cellulose (CMC) is commonly used for the treatment of dairy wastewater after reducing the wastewater to pH 4.2. A novel application of a non-toxic cationic biopolymer - chitosan is evaluated as a substitute for CMC. The results indicate that chitosan can achieve results similar to the CMC process even at pH as high as S.3. Thus, the novel method can save up to SO % of pH-adjusting chemicals requires for both for acidification and neutralisation. The process sludge contains valuable components which have been evaluated and found to be suitable as a food additive. A stable demand for chitosan is also expected to solve the existing shrimp-shell waste disposal problems along the west and north Norwegian coasts. The process is found to be both environmentally and economically attractive for all partners. Copyright ~ 1996 ]AWQ. Published by Elsevier Science Ltd
KEYWORDS Chemical pretreatment; chitosan; dairy wastewater, proteinrecovery.
INTRODUCTION Wastewaters from food processing plants represent a greatly untapped source of recoverable proteins and fats. Given the world's enormous protein shortage these effluents are becoming increasingly important both for humanfood and especially for animal feed. Proteins and fats can be reclaimed from wastewaters by a multitude of physicaVchemical and biological techniques. Reclamation of proteins yields not only economically valuable products. but also the pretreatment of food industry wastewater whichis becoming a common requirement prior to discharge to the municipal sewersystem.
33
E. SELMER-OLSEN et al.
34
Commercial processes for protein recovery with CMC (carboxymethylcellulose) used together with sulphuric acid or formic acid combined with dissolved air flotation. are available. CMC is exclusively used in Norway for this purpose owing to its biologically non-toxic nature. The process requires adjustment to pH 4.2 in order to function with anionic CMC. The Norwegian Dairies was seeking alternative methods for protein and fat recovery which are economically attractive. It was decided to evaluate the efficiency of a process using chitosan as an alternative to the CMC process. Chitosan is a non-toxic polymer made from shrimp shells. As the Norwegian shrimp industry currently has a waste shell disposal problem. the suggested process seemed to be both economically and environmentally interesting for the two industries. This paper presents the results of the above investigation. The goals of the investigation were to establish optimal process conditions and to evaluate recovered sludge as a feed additive. BACKGROUND Chitin is an N-acetyl-2-arninocellulose. or more specifically. 2-acetamido-2-deoxycellulose (Whistler. 1993). It has a molecular weight of 2-3 million daltons. Like cellulose. chitin is insoluble in water but can be converted to water-soluble derivatives. much in the same way cellulose is converted to useful watersoluble derivatives. Chitin forms in the tough. fibrous exoskeletons of insects and in some fungi. However. the present source of chitin is the shells. or skeletal mantles of particularly shrimp. Chitin is the second most abundant biopolymer in nature after cellulose. Removal of N-acetyl groups from chitin to produce chitosan takes place by deacetylation with concentrated alkali. During the deacetylation reaction. some alkaline cleavage of the polysaccharide occurs. resulting in a decrease in viscosity. The deacetylated form. chitosan, consists of two monosaccharides; N-acetyl-D-glucosarnine and D-glucosamine. Chitosan is a soluble biopolymer or polysaccharide at pH values less than 7. but preferably below pH 6. and most often in solutions of organic acids (1.0 %). The soluble chitosans usually contain above 70 % D-glucosamine. Chitosan is well approved for animal feeds where it can be present in amounts up to 0.1 % (Bough et al.• 1976; Whistler. 1991). It is recommended for recovering proteinaceous materials. for clarifying e.g. beer. wine. fruit juices. and for recovery of by-products from the food industry. Modem processing of shrimps. crabs and lobsters result in the availability of substantial quantities of waste materials. These wastes consist mainly of compounds that can be processed to yield chitin. The world market for chitin was is in 1993 estimated to be 1000 to 2000 tons. The traditional addition of an anionic polymer to dairy wastewater after acidification and adding of polymer causes cloud particles to aggregate to larger units which sediment and are easily removed by flotation. filtration or sedimentation. The cloud particles are essentially composed of protein carrying an outward positive charge which is coated with negatively charged polymer molecules. This is the situation when an acidification takes place and the pH is < isoelectric point (pI) for the protein. On the other hand if the pH is > pI for the protein the polymer has to have a positive charge to give the same result. Chitosan is an example of a cationic polysaccharide and theoretically it should be an efficient coagulant. The most widely researched application for the biopolymer chitosan is perhaps as a coagulant of suspended matter in food processing wastes. Chitosan has been used to treat the waste effluents of a wide number of food process applications including; egg breaking. vegetable. shrimp. cheese. meat. beer and apple juice processing. In these operations. chitosan was demonstrated to be a very good coagulating agent. The minimum reduction of suspended solids reported was 70 % (Enriquez and Flick. 1989). Dairy wastewater has a considerably varying water quality during any day. The variation of CODICOD (filtered/total che~cal oxygen demand) ratio is signific~t and ~sually controls the protein removal efficiency by chemIcal methods. Rusten et al. (1993) studied chemical pretreatment of dairy wastewater. They used dissolved air flotation for solids separation at pH 4.2. About 60 % removal of total COD could
A novel treatment process for dairy wastewater
35
be achieved at a COD"CODt ratio of 0.6. Ferric chloride removed 2-3 % more COD than sulphuric acid combined with CMC. and 4-6 % more COD than lactic acid combined with CMC.
EXPERIMENTAL DESIGN The experiments at laboratory scale were carried out at a dairy in Oslo. The dairy produces consumption milk. including yoghurt, sour cream. juice and butteroil. The wastewater characteristics varied on a day to day basis, and the samples used in this investigation had water qualities as given in Table 1. Table 1. Composition of untreated dairy wastewater Samples' Parameters
Turbidity (NTU) Total COD (mgll) Filtered COD (mg/l)" Total P (mgll) Total N (mgll) Suspended Solids (mg/l)" Oil and Grease (mgll)
Range
360- 910 1160 - 2690 810 - 1860 11.5 - 27.4 38 - 53 290- 500 300-920
• Samples were taken from a stirred collection tank of 1/3 of the daily volume. •• Glass fibre filters with an average pore size of 1 J.1IIl were used for determinati of CODr and SS. Four anionic polymers (Na-CMC. Na-alginate (Protanal XL). Na-k-Carragenan, dextran (neutral» and four chitosan samples (cationic polymers with different molecular weight) were tested in combination with pH adjustment with lactic acid or hydrochloric acid. The results have been corrected for the added chemical oxygen demand from lactic acid. The experiments with ferric chloride and poly-aluminium chloride were also conducted for comparison. Only the data for CMC and the chitosan sample (Seacure 443). shown in Table 2. will be discussed in detail later in this paper. The remaining data are presented elsewhere (SelmerOlsen, in preparation). Chitosan was 81 % deacetylated and was delivered from Pronova Biopolymer a.s.• Norway. The experiments were conducted using a semi-automated jar-test apparatus type Flocculator 90 from Kemira Chemicals. Details of the experimental set-up are presented elsewhere (Ratnaweera, 1991). After adjusting pH to the required levels. coagulant was added under rapid mixing of 400 rpm for 1 min. Where applicable polymers were added after this under the same mixing conditions. A slow mixing of 30 rpm for 10 min and a sedimentation of 30 min then followed prior to sampling. RESULTS COD removal from dairy wastewater is very dependent on the pH. since the suspension's colloidal stability depends on the pH. The efficiency of Seacure 443 was studied at various pH values for COD removal. The results are presented in Fig. 1. The results indicate the increasing importance of the coagulant dosage with the increase of process pH. At dosages over 15 mg/l, similar COD removal was observed at pH < 5.25. while dosages of 5-10 mgll worked similarly only at pH < 4.75.
E. SELMER-OLSEN ~I al.
36
Table 2. Characteristics of polymers compared Polymer
Charge
Viscosity (ml'a.s)"
Stock solution (%)
Seacure 443
cationic anionic
1340 1000-2800
0.5 0.5
CMC
• Viscosity is given for 1 % solution (v/v) at 25°C. Seacure443 COD/COOl =0.62
I---;=====~~~~~~~~-I
..... 60
c 50 ~
E
40
~ 30
-e-pH 4.5 -e-pH 4.75 ___ pH 5.0
Cl
8
20
~
10
-*-pH 5.25 0+...-----=F--------1f-----f----+----f-----i o 5 10 15 20 25 30 Polymer dosage (mgll)
Figure I. Removal of totalCODas a function of polymer dosageand pH for Seacure443.
It was then interesting to investigate the coagulation behaviour of commonly used chemicals over a broad
pH range. The results are given in Fig. 2. The results clearly indicate that the coagulation efficiency of all chemicals, except for Seacure 443, dramatically decreases when pH increases above 4.5.
=
COD/COOl 0.62, polymer dosage 15 mgll 60
~
~E
50 40
l!! 30 0 o 20
-e-CMC -+- Seacure 443 ___ Protanal XL -*-Dextran --0- Seacure 443+Protanal XL
Cl
~
10 0 3,5
3,75
4
4,25
4,5
4,75
5
5,25
pH Figure2. Removal of totalCODas a function of pH for differentchemicals tested.
5,5
A novel treatment process for dairy wastewater
37
The dairy wastewater usually has a high initial pH (even> 9.0) and a high buffer capacity. When a treatment process should be conducted at pH values as low as 4.3, the consumption of pH-adjusting chemicals dramatically increases together with their cost. To elaborate this relationship, we have conducted a series of experiments starting from pH 7.5, and the selected results are summarised in Figure 3. The results indicate a dramatic increase of acid consumption for pH reduction after pH = 5.0, confirming a high buffer capacity there. The total amount of acid required to reduce pH from 7.5 to 4.5 was similar to the amount required to reduce the pH further by 0.5 units.
=
~ c
.
.:.
6 ~
iilc
10,------------------------, 9 c=J acidconsumption per 0.5 pH unit 8 7 --cummulative acid 6 consumption from 5 pH=7,5
8 'tl
~
4
3
2 1 0..J.--.C==:L-+--1---'+-J----'L...f-J'---4--~-.y.....L..--...I.+....L-_...L.j
6,56,0
7,06,5
7,57,0
6,05,5
5,55,0
5,04,5
4,54,0
Change of pH Figure3. Acidconsumption for pH reduction from7.S to 4.0 foreachO.S pH unit
The treatment of dairy wastewater does not only conclude in COD removal, but also in the removal of particles, proteins and phosphates. We have studied the treatment efficiencies for these parameters over a broad pH range for Seacure 443 and the selected results are given in Fig. 4. It is clear from the results that Seacure 443 is able to give stable and good treatment efficiencies over a broad pH range up to pH = 5.3.
Seacure 443 COO,lCOOt = 0.7, polymer dosage 15 mgll
~ ~ c Gl
'0
IS Gl
~E
Gl
c::
100 90 80 70 60 50 40 30 20 10
•
•
•
:
•
t::::
:
--------. S;; -.- TUrbidity -.- Tot.-P -.-COOt -M- Tot.-N
0
4,7
4,8
4,9
5
5,1
5,2
5,3
pH Figure4. Removal efficiencies as a function of pH for Seacure 443.
5,4
E. SELMER-OLSEN et al.
The sludge was collected from a series of experiments conducted in full-scale experiments carried out at another dairy (Frya Dairy), which also produces consumption milk. The chemical analyses indicate that the sludge is equally good as the sludge from the CMC-process, which is currently used as an additive for pig food.
DISCUSSION Dairy wastewater is reported to have a pI around 4.2 (Selmer-Olsen, in preparation). Consequently, the colloidal destabilisation occurs at this pH, and therefore the anionic polymers must be functioning only as flocculation-aids here. It is earlier reported that under special circumstances the coagulation may occur at slightly non-zero zeta-potentials (Ratnaweera, 1991). The high particle content in the colloidal system of dairy wastewater together with sufficient amount of flocculation-aids can therefore result in coagulation. This explains the results for anionic polymers observed in Fig. 2. In practice, however, the CMC dosages used are lower and the process is extremely sensitive to the pH (Selmer-Olsen, in preparation). Thus it seems that one may increase the process stability by increasing the CMC-like polymer dosages while operating at slightly higher pH than pI. However, one must be careful not to restabilise the system by addition of too much anionic polymers. Cationic polymers, on the other hand, function both as coagulants and flocculants. This is evident from Fig. I, where the relationship between the dosage and efficiency seems to have a stoichiometric trend. Although one could have used higher dosages of cationic polymers to obtain coagulation even at higher pH values than 5.3, this was not considered owing to economic reasons in practical applications. Considering these data. we can construct the application diagram given in Fig. 5 for treatment of diary wastewater with chitosan. Regarding the total treatment efficiencies, it is important to note the advantages of a chemical treatment. Our results have indicated COD I removals up to 61 % depending on their CODtlCODt ratio. A further reduction is not possible only by chemical methods. However, this reduces the load to a consecutive biological plant while reclaiming the valuable materials as a food additive. The observed phosphate results were between 5261 %, which is an excellent result for an organic polymer. The inorganic coagulants can remove almost all phosphates by forming phosphate precipitates, but they result in enormous amounts of non-edible chemical sludges. For these reasons the use of cationic polymers seems to be well justified. 25
og;
20
.Sal Ol
15
'0
c:
10
III
9
:.c U
5 • 0 4,25
4,5
4,75
5
5,25
pH Figure 5. Application diagram for treatment of dairy wastewater with chitosan COD~CODI = O.5-c).7. for 50.5R
A novel treatment process for dairy wastewaler
39
The treated wastewater often has discharge criteria including pH neutralisation. In practice, the Norwegian Dairies using the CMC method for wastewater treatment first reduce pH down to 4.2 by sulphuric acid and then increase it again to neutral levels using caustic soda. As shown in Fig. 3, the reduction of pH from 4.5 to 4.0 requires an amount of acid equal to that required for the reduction from 7.5 to 4.5. The same is true when upwards adjusting. This indicates that if one can conduct the process at pH > 4.5, in practice it is possible to save 50% of both acid and NaOH. Consideringthese factors and taking it into the current product costs we have compared the alternative treatment costs in Fig. 6. Thus it is possible to adopt a chitosanbased treatment process with a considerableeconomic and environmental profit at prices where chitosan is even 15%higher than the CMC.
s
25%
:E
20%
E
e
15%
';c
~~ u:t::
10%
U
Chitosan price 8s%of CMC-price
U
:t::.c
~u cQ) s: ~
~
e 0-
-0-115% -x-100% --11-90 % --+-75 % __ 60%
5% 0% -5% -10% pH Figure6. Treatment cost comparison withchitosan andCMC.
CONCLUSIONS Chitosan, a biological cationic polymer, can treat dairy wastewaterat pH values up to 5.25 by coagulation. Treatment efficiencies vary with the quality of wastewater, but the results with chitosan indicate a nearly 60% removal of phosphatesand COOl and over 90% removal of particles. Chitosan can efficiently function at pH ranges even as high as 5.25, while other commercial polymers functioned only at pH below 4.5. The full-scale references indicate that the CMC process functions only at pH <4.3. Since chitosan works also in higher pH ranges, the process can save almost 50% of pH-adjusting acid and base consumption. Preliminary investigationson the applicability of chitosan-sludgeas a food additive for pigs have confirmed the possibility. Chitosan can be made from shrimp shells, which may be the solution for waste disposal problems along the west and north Norwegiancoasts. The chitosan process will be considerablycheaper in practice compared with the CMC process, even if the chitosan prices may be 15% higher.
40
E. SELMER-QLSEN et al.
ACKNOWLEDGEMENT This paper is based on a research project funded by the Norwegian State Pollution Authority. Norwegian Institute for Water Research, The Agricultural University of Norway and Norwegian Dairies. The authors would like to thank the sponsors for funding and for the Norwegian Dairies for granting publication.
REFERENCES Bough. W.A., Landes, D.R.• Miller. I. , Young. C.T. and McWorther. T.R. (1976). Utilizat ion of Chitosan for Recovery of Coagulated By-Products from Food Processing Wastes and Treatment Systems. In: Proceedings of the Sixth National Symposium on Food Processing Wastes. Wisconsin. USA. 1-18. Enriquez. L.a. and Flick. a.l. (1989). Marine Colloids . In: Developments in FoodScience19 - FoodEmulsifiers. Charalambous. a. and Doxastakis, a. (Ed .), Elsevier Science Publishing Company Inc. New York. 322-326. Halliday. PJ . and Beszedits, S. (1984). Proteins from food processing wastewaters. Engineering Digest. 30 (10), 24-29. Ratnaweera, H.C. (1991). Influence of the Degree of Coagulant Prepolymerization on Wastewater Coagulation Mechanisms. Doctoral thesis. Norweg ian Institute of Technology. University of Trondheim, 165. Rusten, B.• Lundar, A.• Eide, O. and idegaard •. H. (1992) . Chemical Pretreatment of Dairy Wastewater. In: Wat. Sci Tech. 28 (2)
67-76 Selmer-Olsen. E. (in preparation (1996». Doctoral thesis. Agricultural University of Norway. Whistler. R.L. (993). Chitin. In: Industrial Gums Polysaccharides and Their Derivatives. Whistler. R.L. and BeMiller, I.N (ED). Academic Press. INC. London. 601~ . »
Wat. SeL rfCh. Vol. 34. No. II. pp. 33-40. 1996. Copyright 0 1996IAWQ. Publishedby ElsevierScience LId
Pergamon
Pll: S0273-1223(96)OO818-9
Printedin Oreat Britain.All rightsreserved, 0273-1223196 SU 'OO + 0-00
A NOVEL TREATMENT PROCESS FOR DAIRY WASTEWATER WITH CHITOSAN PRODUCED FROM SHRIMP-SHELL WASTE E. Selmer-Olsen*.***. H. C. Ratnaweera** and R. Pehrson* • Deptpanment. ofFoodScience. AgriculturalUniversity ofNorway. P.O. Box 5036. N-J432 As. Norway •• Norwegiallinstitutefor WaterResearch. P.O. Box 173 Kjelsds. N.(J4J I Oslo. Norway ••• Norwegian Dairies. P.O.Box 905J Gnnland;N..()133 Oslo.Norway
ABSTRACf Recovery of proteins and fats from dairy wastewater has two advantages; the recovery process results in a pretreatment of wastewater prior to discharge to municipal sewers; and the recovered sludge can be used as 1I food additive. Carboxy Methyl Cellulose (CMC) is commonly used for the treatment of dairy wastewater after reducing the wastewater to pH 4.2. A novel application of a non-toxic cationic biopolymer - chitosan is evaluated as a substitute for CMC. The results indicate that chitosan can achieve results similar to the CMC process even at pH as high as S.3. Thus, the novel method can save up to SO % of pH-adjusting chemicals requires for both for acidification and neutralisation. The process sludge contains valuable components which have been evaluated and found to be suitable as a food additive. A stable demand for chitosan is also expected to solve the existing shrimp-shell waste disposal problems along the west and north Norwegian coasts. The process is found to be both environmentally and economically attractive for all partners. Copyright ~ 1996 ]AWQ. Published by Elsevier Science Ltd
KEYWORDS Chemical pretreatment; chitosan; dairy wastewater, proteinrecovery.
INTRODUCTION Wastewaters from food processing plants represent a greatly untapped source of recoverable proteins and fats. Given the world's enormous protein shortage these effluents are becoming increasingly important both for humanfood and especially for animal feed. Proteins and fats can be reclaimed from wastewaters by a multitude of physicaVchemical and biological techniques. Reclamation of proteins yields not only economically valuable products. but also the pretreatment of food industry wastewater whichis becoming a common requirement prior to discharge to the municipal sewersystem.
33
E. SELMER-OLSEN et al.
34
Commercial processes for protein recovery with CMC (carboxymethylcellulose) used together with sulphuric acid or formic acid combined with dissolved air flotation. are available. CMC is exclusively used in Norway for this purpose owing to its biologically non-toxic nature. The process requires adjustment to pH 4.2 in order to function with anionic CMC. The Norwegian Dairies was seeking alternative methods for protein and fat recovery which are economically attractive. It was decided to evaluate the efficiency of a process using chitosan as an alternative to the CMC process. Chitosan is a non-toxic polymer made from shrimp shells. As the Norwegian shrimp industry currently has a waste shell disposal problem. the suggested process seemed to be both economically and environmentally interesting for the two industries. This paper presents the results of the above investigation. The goals of the investigation were to establish optimal process conditions and to evaluate recovered sludge as a feed additive. BACKGROUND Chitin is an N-acetyl-2-arninocellulose. or more specifically. 2-acetamido-2-deoxycellulose (Whistler. 1993). It has a molecular weight of 2-3 million daltons. Like cellulose. chitin is insoluble in water but can be converted to water-soluble derivatives. much in the same way cellulose is converted to useful watersoluble derivatives. Chitin forms in the tough. fibrous exoskeletons of insects and in some fungi. However. the present source of chitin is the shells. or skeletal mantles of particularly shrimp. Chitin is the second most abundant biopolymer in nature after cellulose. Removal of N-acetyl groups from chitin to produce chitosan takes place by deacetylation with concentrated alkali. During the deacetylation reaction. some alkaline cleavage of the polysaccharide occurs. resulting in a decrease in viscosity. The deacetylated form. chitosan, consists of two monosaccharides; N-acetyl-D-glucosarnine and D-glucosamine. Chitosan is a soluble biopolymer or polysaccharide at pH values less than 7. but preferably below pH 6. and most often in solutions of organic acids (1.0 %). The soluble chitosans usually contain above 70 % D-glucosamine. Chitosan is well approved for animal feeds where it can be present in amounts up to 0.1 % (Bough et al.• 1976; Whistler. 1991). It is recommended for recovering proteinaceous materials. for clarifying e.g. beer. wine. fruit juices. and for recovery of by-products from the food industry. Modem processing of shrimps. crabs and lobsters result in the availability of substantial quantities of waste materials. These wastes consist mainly of compounds that can be processed to yield chitin. The world market for chitin was is in 1993 estimated to be 1000 to 2000 tons. The traditional addition of an anionic polymer to dairy wastewater after acidification and adding of polymer causes cloud particles to aggregate to larger units which sediment and are easily removed by flotation. filtration or sedimentation. The cloud particles are essentially composed of protein carrying an outward positive charge which is coated with negatively charged polymer molecules. This is the situation when an acidification takes place and the pH is < isoelectric point (pI) for the protein. On the other hand if the pH is > pI for the protein the polymer has to have a positive charge to give the same result. Chitosan is an example of a cationic polysaccharide and theoretically it should be an efficient coagulant. The most widely researched application for the biopolymer chitosan is perhaps as a coagulant of suspended matter in food processing wastes. Chitosan has been used to treat the waste effluents of a wide number of food process applications including; egg breaking. vegetable. shrimp. cheese. meat. beer and apple juice processing. In these operations. chitosan was demonstrated to be a very good coagulating agent. The minimum reduction of suspended solids reported was 70 % (Enriquez and Flick. 1989). Dairy wastewater has a considerably varying water quality during any day. The variation of CODICOD (filtered/total che~cal oxygen demand) ratio is signific~t and ~sually controls the protein removal efficiency by chemIcal methods. Rusten et al. (1993) studied chemical pretreatment of dairy wastewater. They used dissolved air flotation for solids separation at pH 4.2. About 60 % removal of total COD could
A novel treatment process for dairy wastewater
35
be achieved at a COD"CODt ratio of 0.6. Ferric chloride removed 2-3 % more COD than sulphuric acid combined with CMC. and 4-6 % more COD than lactic acid combined with CMC.
EXPERIMENTAL DESIGN The experiments at laboratory scale were carried out at a dairy in Oslo. The dairy produces consumption milk. including yoghurt, sour cream. juice and butteroil. The wastewater characteristics varied on a day to day basis, and the samples used in this investigation had water qualities as given in Table 1. Table 1. Composition of untreated dairy wastewater Samples' Parameters
Turbidity (NTU) Total COD (mgll) Filtered COD (mg/l)" Total P (mgll) Total N (mgll) Suspended Solids (mg/l)" Oil and Grease (mgll)
Range
360- 910 1160 - 2690 810 - 1860 11.5 - 27.4 38 - 53 290- 500 300-920
• Samples were taken from a stirred collection tank of 1/3 of the daily volume. •• Glass fibre filters with an average pore size of 1 J.1IIl were used for determinati of CODr and SS. Four anionic polymers (Na-CMC. Na-alginate (Protanal XL). Na-k-Carragenan, dextran (neutral» and four chitosan samples (cationic polymers with different molecular weight) were tested in combination with pH adjustment with lactic acid or hydrochloric acid. The results have been corrected for the added chemical oxygen demand from lactic acid. The experiments with ferric chloride and poly-aluminium chloride were also conducted for comparison. Only the data for CMC and the chitosan sample (Seacure 443). shown in Table 2. will be discussed in detail later in this paper. The remaining data are presented elsewhere (SelmerOlsen, in preparation). Chitosan was 81 % deacetylated and was delivered from Pronova Biopolymer a.s.• Norway. The experiments were conducted using a semi-automated jar-test apparatus type Flocculator 90 from Kemira Chemicals. Details of the experimental set-up are presented elsewhere (Ratnaweera, 1991). After adjusting pH to the required levels. coagulant was added under rapid mixing of 400 rpm for 1 min. Where applicable polymers were added after this under the same mixing conditions. A slow mixing of 30 rpm for 10 min and a sedimentation of 30 min then followed prior to sampling. RESULTS COD removal from dairy wastewater is very dependent on the pH. since the suspension's colloidal stability depends on the pH. The efficiency of Seacure 443 was studied at various pH values for COD removal. The results are presented in Fig. 1. The results indicate the increasing importance of the coagulant dosage with the increase of process pH. At dosages over 15 mg/l, similar COD removal was observed at pH < 5.25. while dosages of 5-10 mgll worked similarly only at pH < 4.75.
E. SELMER-OLSEN ~I al.
36
Table 2. Characteristics of polymers compared Polymer
Charge
Viscosity (ml'a.s)"
Stock solution (%)
Seacure 443
cationic anionic
1340 1000-2800
0.5 0.5
CMC
• Viscosity is given for 1 % solution (v/v) at 25°C. Seacure443 COD/COOl =0.62
I---;=====~~~~~~~~-I
..... 60
c 50 ~
E
40
~ 30
-e-pH 4.5 -e-pH 4.75 ___ pH 5.0
Cl
8
20
~
10
-*-pH 5.25 0+...-----=F--------1f-----f----+----f-----i o 5 10 15 20 25 30 Polymer dosage (mgll)
Figure I. Removal of totalCODas a function of polymer dosageand pH for Seacure443.
It was then interesting to investigate the coagulation behaviour of commonly used chemicals over a broad
pH range. The results are given in Fig. 2. The results clearly indicate that the coagulation efficiency of all chemicals, except for Seacure 443, dramatically decreases when pH increases above 4.5.
=
COD/COOl 0.62, polymer dosage 15 mgll 60
~
~E
50 40
l!! 30 0 o 20
-e-CMC -+- Seacure 443 ___ Protanal XL -*-Dextran --0- Seacure 443+Protanal XL
Cl
~
10 0 3,5
3,75
4
4,25
4,5
4,75
5
5,25
pH Figure2. Removal of totalCODas a function of pH for differentchemicals tested.
5,5
A novel treatment process for dairy wastewater
37
The dairy wastewater usually has a high initial pH (even> 9.0) and a high buffer capacity. When a treatment process should be conducted at pH values as low as 4.3, the consumption of pH-adjusting chemicals dramatically increases together with their cost. To elaborate this relationship, we have conducted a series of experiments starting from pH 7.5, and the selected results are summarised in Figure 3. The results indicate a dramatic increase of acid consumption for pH reduction after pH = 5.0, confirming a high buffer capacity there. The total amount of acid required to reduce pH from 7.5 to 4.5 was similar to the amount required to reduce the pH further by 0.5 units.
=
~ c
.
.:.
6 ~
iilc
10,------------------------, 9 c=J acidconsumption per 0.5 pH unit 8 7 --cummulative acid 6 consumption from 5 pH=7,5
8 'tl
~
4
3
2 1 0..J.--.C==:L-+--1---'+-J----'L...f-J'---4--~-.y.....L..--...I.+....L-_...L.j
6,56,0
7,06,5
7,57,0
6,05,5
5,55,0
5,04,5
4,54,0
Change of pH Figure3. Acidconsumption for pH reduction from7.S to 4.0 foreachO.S pH unit
The treatment of dairy wastewater does not only conclude in COD removal, but also in the removal of particles, proteins and phosphates. We have studied the treatment efficiencies for these parameters over a broad pH range for Seacure 443 and the selected results are given in Fig. 4. It is clear from the results that Seacure 443 is able to give stable and good treatment efficiencies over a broad pH range up to pH = 5.3.
Seacure 443 COO,lCOOt = 0.7, polymer dosage 15 mgll
~ ~ c Gl
'0
IS Gl
~E
Gl
c::
100 90 80 70 60 50 40 30 20 10
•
•
•
:
•
t::::
:
--------. S;; -.- TUrbidity -.- Tot.-P -.-COOt -M- Tot.-N
0
4,7
4,8
4,9
5
5,1
5,2
5,3
pH Figure4. Removal efficiencies as a function of pH for Seacure 443.
5,4
E. SELMER-OLSEN et al.
The sludge was collected from a series of experiments conducted in full-scale experiments carried out at another dairy (Frya Dairy), which also produces consumption milk. The chemical analyses indicate that the sludge is equally good as the sludge from the CMC-process, which is currently used as an additive for pig food.
DISCUSSION Dairy wastewater is reported to have a pI around 4.2 (Selmer-Olsen, in preparation). Consequently, the colloidal destabilisation occurs at this pH, and therefore the anionic polymers must be functioning only as flocculation-aids here. It is earlier reported that under special circumstances the coagulation may occur at slightly non-zero zeta-potentials (Ratnaweera, 1991). The high particle content in the colloidal system of dairy wastewater together with sufficient amount of flocculation-aids can therefore result in coagulation. This explains the results for anionic polymers observed in Fig. 2. In practice, however, the CMC dosages used are lower and the process is extremely sensitive to the pH (Selmer-Olsen, in preparation). Thus it seems that one may increase the process stability by increasing the CMC-like polymer dosages while operating at slightly higher pH than pI. However, one must be careful not to restabilise the system by addition of too much anionic polymers. Cationic polymers, on the other hand, function both as coagulants and flocculants. This is evident from Fig. I, where the relationship between the dosage and efficiency seems to have a stoichiometric trend. Although one could have used higher dosages of cationic polymers to obtain coagulation even at higher pH values than 5.3, this was not considered owing to economic reasons in practical applications. Considering these data. we can construct the application diagram given in Fig. 5 for treatment of diary wastewater with chitosan. Regarding the total treatment efficiencies, it is important to note the advantages of a chemical treatment. Our results have indicated COD I removals up to 61 % depending on their CODtlCODt ratio. A further reduction is not possible only by chemical methods. However, this reduces the load to a consecutive biological plant while reclaiming the valuable materials as a food additive. The observed phosphate results were between 5261 %, which is an excellent result for an organic polymer. The inorganic coagulants can remove almost all phosphates by forming phosphate precipitates, but they result in enormous amounts of non-edible chemical sludges. For these reasons the use of cationic polymers seems to be well justified. 25
og;
20
.Sal Ol
15
'0
c:
10
III
9
:.c U
5 • 0 4,25
4,5
4,75
5
5,25
pH Figure 5. Application diagram for treatment of dairy wastewater with chitosan COD~CODI = O.5-c).7. for 50.5R
A novel treatment process for dairy wastewaler
39
The treated wastewater often has discharge criteria including pH neutralisation. In practice, the Norwegian Dairies using the CMC method for wastewater treatment first reduce pH down to 4.2 by sulphuric acid and then increase it again to neutral levels using caustic soda. As shown in Fig. 3, the reduction of pH from 4.5 to 4.0 requires an amount of acid equal to that required for the reduction from 7.5 to 4.5. The same is true when upwards adjusting. This indicates that if one can conduct the process at pH > 4.5, in practice it is possible to save 50% of both acid and NaOH. Consideringthese factors and taking it into the current product costs we have compared the alternative treatment costs in Fig. 6. Thus it is possible to adopt a chitosanbased treatment process with a considerableeconomic and environmental profit at prices where chitosan is even 15%higher than the CMC.
s
25%
:E
20%
E
e
15%
';c
~~ u:t::
10%
U
Chitosan price 8s%of CMC-price
U
:t::.c
~u cQ) s: ~
~
e 0-
-0-115% -x-100% --11-90 % --+-75 % __ 60%
5% 0% -5% -10% pH Figure6. Treatment cost comparison withchitosan andCMC.
CONCLUSIONS Chitosan, a biological cationic polymer, can treat dairy wastewaterat pH values up to 5.25 by coagulation. Treatment efficiencies vary with the quality of wastewater, but the results with chitosan indicate a nearly 60% removal of phosphatesand COOl and over 90% removal of particles. Chitosan can efficiently function at pH ranges even as high as 5.25, while other commercial polymers functioned only at pH below 4.5. The full-scale references indicate that the CMC process functions only at pH <4.3. Since chitosan works also in higher pH ranges, the process can save almost 50% of pH-adjusting acid and base consumption. Preliminary investigationson the applicability of chitosan-sludgeas a food additive for pigs have confirmed the possibility. Chitosan can be made from shrimp shells, which may be the solution for waste disposal problems along the west and north Norwegiancoasts. The chitosan process will be considerablycheaper in practice compared with the CMC process, even if the chitosan prices may be 15% higher.
40
E. SELMER-QLSEN et al.
ACKNOWLEDGEMENT This paper is based on a research project funded by the Norwegian State Pollution Authority. Norwegian Institute for Water Research, The Agricultural University of Norway and Norwegian Dairies. The authors would like to thank the sponsors for funding and for the Norwegian Dairies for granting publication.
REFERENCES Bough. W.A., Landes, D.R.• Miller. I. , Young. C.T. and McWorther. T.R. (1976). Utilizat ion of Chitosan for Recovery of Coagulated By-Products from Food Processing Wastes and Treatment Systems. In: Proceedings of the Sixth National Symposium on Food Processing Wastes. Wisconsin. USA. 1-18. Enriquez. L.a. and Flick. a.l. (1989). Marine Colloids . In: Developments in FoodScience19 - FoodEmulsifiers. Charalambous. a. and Doxastakis, a. (Ed .), Elsevier Science Publishing Company Inc. New York. 322-326. Halliday. PJ . and Beszedits, S. (1984). Proteins from food processing wastewaters. Engineering Digest. 30 (10), 24-29. Ratnaweera, H.C. (1991). Influence of the Degree of Coagulant Prepolymerization on Wastewater Coagulation Mechanisms. Doctoral thesis. Norweg ian Institute of Technology. University of Trondheim, 165. Rusten, B.• Lundar, A.• Eide, O. and idegaard •. H. (1992) . Chemical Pretreatment of Dairy Wastewater. In: Wat. Sci Tech. 28 (2)
67-76 Selmer-Olsen. E. (in preparation (1996». Doctoral thesis. Agricultural University of Norway. Whistler. R.L. (993). Chitin. In: Industrial Gums Polysaccharides and Their Derivatives. Whistler. R.L. and BeMiller, I.N (ED). Academic Press. INC. London. 601~ . »