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Solid, slightly soluble phosphorus compounds as nutrient source in activated sludge treatment of forest industry wastewaters
~
Pergamon
War. Sci. Tech. Vol. 35, No. 2-3. pp. 131-138,1997. Copyright © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain.
PH: S0273-1223(96)OO924-9
0273-1223/97 $17'00 + 0'00
SOLID, SLIGHTLY SOLUBLE PHOSPHORUS COMPOUNDS AS NUTRIENT SOURCE IN ACTIVATED SLUDGE TREATMENT OF FOREST INDUSTRY WASTEWATERS Pirjo-Riitta Rantala and Hannu Wirola Regional Environmental Agency of Harne, P.O. Box 297. FIN-33JOJ Tampere, Finland
ABSTRACT The aim of the study was to detennine if solid, slightly soluble compounds can be used as nutrient source in activated sludge treatment plants instead of liquid phosphoric acid. Four different solid materials were tested in lab-scale solubility tests to find compounds which are least soluble. Two materials were chosen for further studies: apatite and raw phosphate. The use of apatite and raw phosphate as nutrient source was studied in lab-scale activated sludge reactors along with a control reactor where phosphorus was added in liquid form. The phosphorus dosage, measured as elementary phosphorus, was the same for aU three reactors. The reactors were fed with pre-clarified chemi-thermomechanical pulp miU (CTMP) wastewater. There were no significant differences in the reductions of organic matter between the three reactors. The mean effluent concentration of total phosphorus was 3 mg PII in the control reactor and less than I mg PII in the other two reactors. The soluble phosphorus concentration was more than 2 mg PII in the control reactor and less than 0.5 mg PII in the other two. Apatite was an even better nutrient source than raw phosphate. Further lab-scale tests were conducted using two different grain sizes of apatite. No significant differences were found between the studied grain sizes « 0.074 mm and 0.074 mm-0.125 mm). Apatite was then used in full-scale at a CTMP-mill two different times. The experiments showed that the mean concentrations of phosphorus can be reduced radically by using apatite as a nutrient source instead of liquid phosphorus. Solid phosphorus compounds are a viable alternative to reduce the phosphorus load from forest industry wastewater treatment plants. © 1997 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Activated sludge; apatite; CTMP-wastewaters; nutrients; solid phosphorus; slightly soluble compounds. INTRODUCTION In Finland, the pulp and paper industry has 34 biological wastewater treatment plants. Most of them are activated sludge plants; three are aerated ponds and two are anaerobic plants with aerobic sequence. Five papennills have only chemical purification and three have only mechanical clarification (Metsateollisuus ry., 1995). In activated sludge plants it is often essential to add nutrients (phosphorus, nitrogen) to the treatment process to achieve efficient purification. Since nutrients are added in liquid form, the excess liquid 131
P.-R. RANTALA and H. WIROLA
132
phosphorus and nitrogen that do not separate in secondary clarification escape to the recipient promoting eutrophication. A research project focused on the use of solid phosphorus compounds was started based on the following hypotheses: 1) 2) 3) 4)
slightly soluble solid phosphorus compounds provide nutrition for micro-organisms and keep soluble phosphorus concentration low at the same time solid phosphorus compounds serve as carriers for micro-organisms solid phosphorus compounds separate in secondary sedimentation preventing the escape of phosphorus solid phosphorus compounds are cheap because of their raw material nature (as opposite of industrially manufactured phosphoric acid).
The project proceeded from solubility tests of four different solid materials (Wirola et ai., 1991) to laboratory tests (Rantala et ai., 1993) and to full-scale mill experiments. The aim was to find out if the microbes in activated sludge can mobilize slightly soluble, solid phosphorus compounds enough for their growth. Also the capability of these solid materials to attach themselves to flocs was studied. In this paper results from the solubility tests and laboratory experiments as well as preliminary results from full-scale mill tests are presented. MATERIALS AND METHODS The study was started with solubility tests of four different solid materials: apatite [(Ca4P3012-Ca(F,Cl,OH)], raw phosphate [(CalOF2(P04)6-CalO(OHh(P°4)6], triplephosphate and superphosphate. Triplephosphate and superphosphate are made from apatite with addition of sulphuric acid. The materials used in the tests are generally considered fertilizers. Test materials were ground and sifted. The grain sizes studied were: < 0.075 mm, 0.075-0.125 mm and 0.125-0.250 mm. Different amounts (10-70 mgll) of these materials were mixed first with distilled water and then with waters that had different pHs (pH 2, 3, 4, 5, 6 and 7; apatite and raw phosphate only). pH was adjusted with weak HCl- and NaOH-solutions. Total phosphorus was determined from grab samples. Activated sludge tests were made with three lab-scale reactors. The reactors were fed'with pre-clarified chemi-thermomechanical pulp (CTMP) wastewater and were operated under similar conditions in room temperature for about nine months. The average BOD 7 and CODer values of the influent were 1500 mgll and 3300 mgll, respectively (softwood pulp periods). There were four different test periods. The volume of each reactor was 8.8 1 of which the volume of the aeration section was 7.0 1. Table 1 shows the process parameters during the test periods. Table 1. Test periods and different set values of the parameters Parameters
Period I
Period II
Period III
Period IV
Sludge load (kg BOD7/kg MLSS*d) MLSS (gil) Sludge age (d)
0.3 5.2 13
0.3 5.0 14
0.3 4.0 12
0.1 2.6 29
To reactors 1 and 2 phosphorus was added in solid form as apatite and raw phosphate, respectively. To reactor 3 (control) phosphorus was added in liquid form as diammoniumhydrogenphosphate «NH4)2HP04)' The dosage, measured as elementary phosphorus, was the same for all three reactors except during the fourth period. The average nutrient ratio, defined as the ratio of biological oxygen demand to nitrogen and phosphorus, was BOD7 :N:P=100:4:0.4-0.3 in the first and the second period. During the third period the addition of phosphorus was half of the original. During the fourth period the phosphorus addition varied between the reactors: in reactors 1 and 3 the average nutrient ratio was BOD :N:P=100:5:1.1. In the
Treatment of forest industry wastewaters
133
beginning of the fourth period the phosphorus addition to reactor 2 was six times more than in the beginning. The aim was to test the effects of an overdose to the concentration of phosphorus in the effluent. The quality of the sludge was studied using a microscope and diluted sludge volume index (DSVI). After these tests apatite was chosen to further studies. The next step was to determine if there are significant differences with different grain sizes of apatite. The studied grain sizes were: < 0.074 mm and 0.074-0.125 mm. In addition there was also a control reactor with liquid phosphorus addition. Full-scale mill experiments were conducted in two different periods. The first period lasted six months and the second one only two weeks. In both cases the amount of apatite feed to the process was the same when using liquid phosphorus, counted as phosphorus. The grain size of apatite was planned to be between 0.075• 0.125 mm, but the apatite supplier could not provide the desired grain size. Therefore the grain size was between 0.075-0.350 mm and the fraction of particles larger than 0.125 mm was 25-30%. Analyses included BOD? (SFS 3019) chemical oxygen demand (COD Cr) (SFS 5504), suspended solids (SS), volatite suspended solids (VSS), phosphorus (P) (SFS 3026), phosphate (SFS 3025), nitrogen (N) (SFS 5505), mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS). Influent and sludge samples were grab samples (in full-scale experiments the influent samples were 24 hour composite samples). Effluent samples were 24 hour composite samples. Analyses were made according to SFS-standards (SFS Catalogue, 1990). RESULTS AND DISCUSSION Solubility tests The results from the solubility tests showed that the most slightly soluble materials were apatite and raw phosphate. When mixed with distilled water, the amount of total phosphorus in filtered samples after 24 hours dissolution time was 3.3% and 8.8% for apatite and raw phosphate, respectively. This was counted from the theoretical amount of total phosphorus in these materials. The grain size did not much effect the results. For apatite the most soluble grain size was between 0.075• 0.125 mm. The least soluble grain size was < 0.075mm. This is surprising because when the grain size gets smaller the nominal grain surface area gets larger. This is supposed to increase solubility. In this case the explanation may be found in colloidal phenomenona. The solubilities of apatite and raw phosphate started to increase thoroughly when pH was lowered below pH 4 and pH 6, respectively. This indicates that if it is necessary to increase the solubility, it is possible to add acid to the solid phosphorus compound slurry, before it is fed to the biological process. Laboratory tests with activated sludge The results from the laboratory tests showed that there were no significant differences in reductions of organic matter between the three reactors except during the third period (Figs 1 and 2). During the third period there was a process failure in reactors 1 and 2: there were problems with aeration and clogging of the influent tubes. The reduction in the phosphorus addition might also have been a cause for reduced purification efficiency. The BODTreductions in all reactors were on the average of 75-90% and the COD Cr• reductions between 45-80%.
P.-R. RANTALA and H. WIROLA
134
100 90
L-~-----~-~----~--~=--
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Figure I. BOD? reductions in laboratory scale reactors during different test periods. (BOD? was not analyzed during the third period).
100
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,-----------------------------~-------------
90 80
------------------
----------
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,
~
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o I tot
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II sol
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IV tot
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Figure 2. CODer reductions in laboratory scale reactors during different test periods.
55 and V55 concentrations in the effluents varied in every reactor, probably due to the variations in the quality of the incoming water. Values of BOD? and COD Cr and 55 concentrations in the influent were doubled during the aspen wood pulping period (most of the time softwood pulp was used in the mill). The concentration of total phosphorus in the effluent was on the average of 3 mg PII in reactor 3 and less than 1 mg P/l in the other two reactors (Fig. 3). The average concentration of soluble phosphorus was more than 2 mg PII in reactor 3 and less than 0.5 mg P/l in reactors 1 and 2. The concentration of the effluent phosphorus bound with suspended solids was twice as high in the control reactor than in the reactors 1 and 2 despite the fact that the suspended solids concentrations were on the average little higher in reactors 1 and 2 than in reactor 3 (Table 2).
Treatment of forest industry wastewaters
135
Table 2. Phosphorus bound with suspended solids in the effluents Reactor
ugPss/mgSS
REACTOR 1 (apatite) REACTOR 2 (raw phosphate) REACTOR 3 (control)
9 ',--,---------I period
5.0 5.4 10.5
,---_._-------.....,-------, III
II period
8
76
IV period
---+- Reactor III (control) - .;( - Reactor I - . . - Reactor II
'§,5 E
1
Days Figure 3. Total phosphorus concentrations in the effluents.
DSVI was measured during the first and second period. DSVI was under 100 mg/l in every reactor; in reactors 1 and 2 the DSVI was a little lower than in reactor 3. The microscopic examinations showed that there were no significant differences in the quality or quantity of the microbe populations in the sludges of the three reactors. The most common protozoa in the sludge were Oxytrichia, Vorticella, Paramecium and Uronema. The crystals of apatite and raw phosphate were mostly covered with sludge. This could mean that the sludge will separate better in secondary clarification. The same kind of phenomenon was also detected in another research, where paper mill wastewaters were studied. China clay and other fillers used in the process were attached effectively to the biomass and the settling properties improved (Wirola, 1979). The attachment of bacteria to apatite crystals can also present one form of releasing phosphorus for other microbes. Certain bacteria, as in biological phosphorus removal (Acinetobacteria) may be able to attach to apatite crystals and take up more phosphorus than they need from the crystals (luxury uptake or overplus phenomenon). When more micro-organisms attach to the crystal, finally anaerobic conditions are formed at the surface of the crystal and the phosphorus stored by the bacteria will be released (e.g. Jorgensen and Pauli, 1992; Pauli, 1994). The results showed that apatite was even better material to be used as a nutrient source compared to raw phosphate. Grain size tests showed no significant differences between the two grain sizes used (data not shown). Full scale mill experiments The results obtained in the first full-scale mill experiments were similar to the laboratory tests. There were three periods: first period with phosphoric acid, second period with apatite and third with apatite and phosphoric acid (50% + 50% as phosphorus) a total of 6 months in all. As can be seen (Fig. 4) the CODer
P.-R. RANTALA and H. WIROLA
\alue-. varied a little more during the apatite running. This cannot all be explained with the use of apatite. hecause there were several problems at the mill at that time (lack of oxygen. aspen wood period when the BOD 7 values doubled. the sludge removal pump was damaged. etc.). Also at the same time there was a study going on where the purpose was to optimise nutrition addition, both phosphorus and nitrogen (Jansson, 1992). Regardless of these problems the phosphorus concentrations were very low during the apatite period (Fig. 5). 7 000
~rp=los~OrTC--------------------------------
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Days Figure 4. CODer values of the first full scale experiment.
9---
- - - -------+-------1
I p,osp,oric o::::id
8
II q:xtite
III q:xtit
p,osp,oriC
-----1--------------------.--------1--
7
----
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--------f----
-------
- ----
'd --
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I
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--.
- - Ptot influent - .. - Ptot effluent
::::::: 5
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--[}- Psol effluent
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Days Figure 5. Total and soluble phosphorus concentrations in the first full scale experiment. Influent phosphorus concentrations are before phosphorus addition to the wastewater treatment plant.
The second full scale experiment lasted only 15 days. After the first full scale experiment changes were made at the mill. The aspen wood is not used any more, which effects the incoming water. The quality of the water is at the moment more balanced. Also the phosphoric acid addition has been reduced to about half of the dosage it used to be. As a result, the phosphorus load from the mill is nowadays less than half of what it
Treatment of forest industry wastewaters
137
was earlier. Yet, with apatite period lower phosphorus concentrations in the effluent could be achieved (Fig. 6).
apatite period --Ptotinf. - • - Ptot eU. -::«:-Psol info -0--- Psol eU.
--- - -
-
--
--
...---- ---_ .......... ~ 00
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~
0
~
~
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~
~
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~
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~
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~
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Figure 6. Total and soluble phosphorus concentrations of the second full scale experiment. Influent phosphorus concentrations are before phosphorus addition to the wastewater treatment plant.
CONCLUSIONS
According to the research conducted, it was proved that solid slightly soluble compounds can be used as a nutrient source for forest industry wastewaters. Apatite was found to be the least soluble of the tested materials. When properly operated there are not significant differences in the reduction of organic matter between activated sludge processes using solid or liquid phosphorus addition. Yet, the phosphorus load from a treatment process using solid phosphorus addition can be reduced substantially compared to liquid phosphorus addition. When using apatite the grain size did not affect purification results and apatite did not escape from clarification, either. Apatite particles serve as carriers or nuclei for floes. Slight overdose of apatite is not critical, because excess apatite will be removed with surplus sludge. Vice versa, slight overdosing may have a positive effect for the process ensuring sufficient phosphorus release. If needed, phosphoric acid can be dosed together with apatite or apatite can be acidified before use to get more phosphorus dissolved. Because apatite is the raw material for phosphoric acid it should also be more economical than phosphoric acid. REFERENCES Jansson, K. (1992). Reduction of Phosphorus Load from a Forest Industry Wastewater Treatment Plant. Msc. thesis. Abo Akademi University, Turku, Finland (in Swedish). JOrgensen, K. S. and Pauli, A. (1992). Microbial Transformations of Phosphorus and Nitrogen in the Activated Sludge Treatment of Forest Industry Wastewaters. Publication of the National Board of Waters and Environment, SITYKE-programme, 1. Helsinki, Finland. (in Finnish). Metsateollisuus ry. (1995). Environmental Protection in Finnish Forest Industries, Yearbook 1994. Metsateollisuus ry., Helsinki, Finland. (in Finnish). Pauli, A. (1994). The Role of Acinetobacter sp. in Biological Phosphorus Removal from Forest Industry Wastewaters. Publication ofthe Water and Environment Research Institute, 16. National Board of Waters and the Environment, Helsinki, Finland. Rantala, P.-R., Wirola, H. and Isoaho, S. (1993). Slightly Soluble Solid Phosphorus Compounds as Nutrient Source in Biological Treatment of Forest Industry Wastewaters. Lab-scale Tests with Activated Sludge Reactors. Publication of Tampere University of Technology, Institute of Water and Environmental Engineering, B56. Tampere, Finland. (in Finnish). SFS Catalogue (1990). Finnish Standards Association SFS, Gummerus Kirjapaino Oy, Jyviiskylii, Finland.
138
P.-R. RANTALA and H. WIROLA
Wirola. H. (l979). Combined and Separate Treatment of Board Mill Wastewaters and Municipal Wastewaters. Msc thesis Tampere University of Technology, Tampere, Finland. (in Finnish). Wirola, H., Isoaho. S. and Tanhuala, T. (1991). Slightly Soluble Solid Phosphorus Compounds as Nutrient Source in Biologics Treatment of Forest Industry Wastewaters. Part I: Solubility Tests. Publication of Tampere University of Technology Institute of Water and Environmental Engineering, C35, Tampere, Finland. (in Finnish).
Pergamon
War. Sci. Tech. Vol. 35, No. 2-3. pp. 131-138,1997. Copyright © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain.
PH: S0273-1223(96)OO924-9
0273-1223/97 $17'00 + 0'00
SOLID, SLIGHTLY SOLUBLE PHOSPHORUS COMPOUNDS AS NUTRIENT SOURCE IN ACTIVATED SLUDGE TREATMENT OF FOREST INDUSTRY WASTEWATERS Pirjo-Riitta Rantala and Hannu Wirola Regional Environmental Agency of Harne, P.O. Box 297. FIN-33JOJ Tampere, Finland
ABSTRACT The aim of the study was to detennine if solid, slightly soluble compounds can be used as nutrient source in activated sludge treatment plants instead of liquid phosphoric acid. Four different solid materials were tested in lab-scale solubility tests to find compounds which are least soluble. Two materials were chosen for further studies: apatite and raw phosphate. The use of apatite and raw phosphate as nutrient source was studied in lab-scale activated sludge reactors along with a control reactor where phosphorus was added in liquid form. The phosphorus dosage, measured as elementary phosphorus, was the same for aU three reactors. The reactors were fed with pre-clarified chemi-thermomechanical pulp miU (CTMP) wastewater. There were no significant differences in the reductions of organic matter between the three reactors. The mean effluent concentration of total phosphorus was 3 mg PII in the control reactor and less than I mg PII in the other two reactors. The soluble phosphorus concentration was more than 2 mg PII in the control reactor and less than 0.5 mg PII in the other two. Apatite was an even better nutrient source than raw phosphate. Further lab-scale tests were conducted using two different grain sizes of apatite. No significant differences were found between the studied grain sizes « 0.074 mm and 0.074 mm-0.125 mm). Apatite was then used in full-scale at a CTMP-mill two different times. The experiments showed that the mean concentrations of phosphorus can be reduced radically by using apatite as a nutrient source instead of liquid phosphorus. Solid phosphorus compounds are a viable alternative to reduce the phosphorus load from forest industry wastewater treatment plants. © 1997 IAWQ. Published by Elsevier Science Ltd.
KEYWORDS Activated sludge; apatite; CTMP-wastewaters; nutrients; solid phosphorus; slightly soluble compounds. INTRODUCTION In Finland, the pulp and paper industry has 34 biological wastewater treatment plants. Most of them are activated sludge plants; three are aerated ponds and two are anaerobic plants with aerobic sequence. Five papennills have only chemical purification and three have only mechanical clarification (Metsateollisuus ry., 1995). In activated sludge plants it is often essential to add nutrients (phosphorus, nitrogen) to the treatment process to achieve efficient purification. Since nutrients are added in liquid form, the excess liquid 131
P.-R. RANTALA and H. WIROLA
132
phosphorus and nitrogen that do not separate in secondary clarification escape to the recipient promoting eutrophication. A research project focused on the use of solid phosphorus compounds was started based on the following hypotheses: 1) 2) 3) 4)
slightly soluble solid phosphorus compounds provide nutrition for micro-organisms and keep soluble phosphorus concentration low at the same time solid phosphorus compounds serve as carriers for micro-organisms solid phosphorus compounds separate in secondary sedimentation preventing the escape of phosphorus solid phosphorus compounds are cheap because of their raw material nature (as opposite of industrially manufactured phosphoric acid).
The project proceeded from solubility tests of four different solid materials (Wirola et ai., 1991) to laboratory tests (Rantala et ai., 1993) and to full-scale mill experiments. The aim was to find out if the microbes in activated sludge can mobilize slightly soluble, solid phosphorus compounds enough for their growth. Also the capability of these solid materials to attach themselves to flocs was studied. In this paper results from the solubility tests and laboratory experiments as well as preliminary results from full-scale mill tests are presented. MATERIALS AND METHODS The study was started with solubility tests of four different solid materials: apatite [(Ca4P3012-Ca(F,Cl,OH)], raw phosphate [(CalOF2(P04)6-CalO(OHh(P°4)6], triplephosphate and superphosphate. Triplephosphate and superphosphate are made from apatite with addition of sulphuric acid. The materials used in the tests are generally considered fertilizers. Test materials were ground and sifted. The grain sizes studied were: < 0.075 mm, 0.075-0.125 mm and 0.125-0.250 mm. Different amounts (10-70 mgll) of these materials were mixed first with distilled water and then with waters that had different pHs (pH 2, 3, 4, 5, 6 and 7; apatite and raw phosphate only). pH was adjusted with weak HCl- and NaOH-solutions. Total phosphorus was determined from grab samples. Activated sludge tests were made with three lab-scale reactors. The reactors were fed'with pre-clarified chemi-thermomechanical pulp (CTMP) wastewater and were operated under similar conditions in room temperature for about nine months. The average BOD 7 and CODer values of the influent were 1500 mgll and 3300 mgll, respectively (softwood pulp periods). There were four different test periods. The volume of each reactor was 8.8 1 of which the volume of the aeration section was 7.0 1. Table 1 shows the process parameters during the test periods. Table 1. Test periods and different set values of the parameters Parameters
Period I
Period II
Period III
Period IV
Sludge load (kg BOD7/kg MLSS*d) MLSS (gil) Sludge age (d)
0.3 5.2 13
0.3 5.0 14
0.3 4.0 12
0.1 2.6 29
To reactors 1 and 2 phosphorus was added in solid form as apatite and raw phosphate, respectively. To reactor 3 (control) phosphorus was added in liquid form as diammoniumhydrogenphosphate «NH4)2HP04)' The dosage, measured as elementary phosphorus, was the same for all three reactors except during the fourth period. The average nutrient ratio, defined as the ratio of biological oxygen demand to nitrogen and phosphorus, was BOD7 :N:P=100:4:0.4-0.3 in the first and the second period. During the third period the addition of phosphorus was half of the original. During the fourth period the phosphorus addition varied between the reactors: in reactors 1 and 3 the average nutrient ratio was BOD :N:P=100:5:1.1. In the
Treatment of forest industry wastewaters
133
beginning of the fourth period the phosphorus addition to reactor 2 was six times more than in the beginning. The aim was to test the effects of an overdose to the concentration of phosphorus in the effluent. The quality of the sludge was studied using a microscope and diluted sludge volume index (DSVI). After these tests apatite was chosen to further studies. The next step was to determine if there are significant differences with different grain sizes of apatite. The studied grain sizes were: < 0.074 mm and 0.074-0.125 mm. In addition there was also a control reactor with liquid phosphorus addition. Full-scale mill experiments were conducted in two different periods. The first period lasted six months and the second one only two weeks. In both cases the amount of apatite feed to the process was the same when using liquid phosphorus, counted as phosphorus. The grain size of apatite was planned to be between 0.075• 0.125 mm, but the apatite supplier could not provide the desired grain size. Therefore the grain size was between 0.075-0.350 mm and the fraction of particles larger than 0.125 mm was 25-30%. Analyses included BOD? (SFS 3019) chemical oxygen demand (COD Cr) (SFS 5504), suspended solids (SS), volatite suspended solids (VSS), phosphorus (P) (SFS 3026), phosphate (SFS 3025), nitrogen (N) (SFS 5505), mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS). Influent and sludge samples were grab samples (in full-scale experiments the influent samples were 24 hour composite samples). Effluent samples were 24 hour composite samples. Analyses were made according to SFS-standards (SFS Catalogue, 1990). RESULTS AND DISCUSSION Solubility tests The results from the solubility tests showed that the most slightly soluble materials were apatite and raw phosphate. When mixed with distilled water, the amount of total phosphorus in filtered samples after 24 hours dissolution time was 3.3% and 8.8% for apatite and raw phosphate, respectively. This was counted from the theoretical amount of total phosphorus in these materials. The grain size did not much effect the results. For apatite the most soluble grain size was between 0.075• 0.125 mm. The least soluble grain size was < 0.075mm. This is surprising because when the grain size gets smaller the nominal grain surface area gets larger. This is supposed to increase solubility. In this case the explanation may be found in colloidal phenomenona. The solubilities of apatite and raw phosphate started to increase thoroughly when pH was lowered below pH 4 and pH 6, respectively. This indicates that if it is necessary to increase the solubility, it is possible to add acid to the solid phosphorus compound slurry, before it is fed to the biological process. Laboratory tests with activated sludge The results from the laboratory tests showed that there were no significant differences in reductions of organic matter between the three reactors except during the third period (Figs 1 and 2). During the third period there was a process failure in reactors 1 and 2: there were problems with aeration and clogging of the influent tubes. The reduction in the phosphorus addition might also have been a cause for reduced purification efficiency. The BODTreductions in all reactors were on the average of 75-90% and the COD Cr• reductions between 45-80%.
P.-R. RANTALA and H. WIROLA
134
100 90
L-~-----~-~----~--~=--
--
-
._-~--~------.--------_.-._----
------
o Reactor 1 (apatite) El Reactor 2 (raw phosphate) o Reactor 3 (control)
- --""":0--
80
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o -t---C:',J;;;;;L-4-.J.:,;,I:=-'-4--+L-~--'-t---+----t~----"--t-'--L....JI tot
IV tot
II sol
I sol Periods I - IV
IV sol
total and soluble BOD7
Figure I. BOD? reductions in laboratory scale reactors during different test periods. (BOD? was not analyzed during the third period).
100
..-.
,-----------------------------~-------------
90 80
------------------
----------
---
o Reactor 1 (apatite) Reactor 2 (raw phosphate) o Reactor 3 (control)
,
~
- - - - - - - - - - - - 1 .•
70 - - - - >
- -----
60
z-'
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20
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o I tot
I sol
II tot
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II sol
III tot
III sol
IV tot
IV sol
total and soluble CODer
Figure 2. CODer reductions in laboratory scale reactors during different test periods.
55 and V55 concentrations in the effluents varied in every reactor, probably due to the variations in the quality of the incoming water. Values of BOD? and COD Cr and 55 concentrations in the influent were doubled during the aspen wood pulping period (most of the time softwood pulp was used in the mill). The concentration of total phosphorus in the effluent was on the average of 3 mg PII in reactor 3 and less than 1 mg P/l in the other two reactors (Fig. 3). The average concentration of soluble phosphorus was more than 2 mg PII in reactor 3 and less than 0.5 mg P/l in reactors 1 and 2. The concentration of the effluent phosphorus bound with suspended solids was twice as high in the control reactor than in the reactors 1 and 2 despite the fact that the suspended solids concentrations were on the average little higher in reactors 1 and 2 than in reactor 3 (Table 2).
Treatment of forest industry wastewaters
135
Table 2. Phosphorus bound with suspended solids in the effluents Reactor
ugPss/mgSS
REACTOR 1 (apatite) REACTOR 2 (raw phosphate) REACTOR 3 (control)
9 ',--,---------I period
5.0 5.4 10.5
,---_._-------.....,-------, III
II period
8
76
IV period
---+- Reactor III (control) - .;( - Reactor I - . . - Reactor II
'§,5 E
1
Days Figure 3. Total phosphorus concentrations in the effluents.
DSVI was measured during the first and second period. DSVI was under 100 mg/l in every reactor; in reactors 1 and 2 the DSVI was a little lower than in reactor 3. The microscopic examinations showed that there were no significant differences in the quality or quantity of the microbe populations in the sludges of the three reactors. The most common protozoa in the sludge were Oxytrichia, Vorticella, Paramecium and Uronema. The crystals of apatite and raw phosphate were mostly covered with sludge. This could mean that the sludge will separate better in secondary clarification. The same kind of phenomenon was also detected in another research, where paper mill wastewaters were studied. China clay and other fillers used in the process were attached effectively to the biomass and the settling properties improved (Wirola, 1979). The attachment of bacteria to apatite crystals can also present one form of releasing phosphorus for other microbes. Certain bacteria, as in biological phosphorus removal (Acinetobacteria) may be able to attach to apatite crystals and take up more phosphorus than they need from the crystals (luxury uptake or overplus phenomenon). When more micro-organisms attach to the crystal, finally anaerobic conditions are formed at the surface of the crystal and the phosphorus stored by the bacteria will be released (e.g. Jorgensen and Pauli, 1992; Pauli, 1994). The results showed that apatite was even better material to be used as a nutrient source compared to raw phosphate. Grain size tests showed no significant differences between the two grain sizes used (data not shown). Full scale mill experiments The results obtained in the first full-scale mill experiments were similar to the laboratory tests. There were three periods: first period with phosphoric acid, second period with apatite and third with apatite and phosphoric acid (50% + 50% as phosphorus) a total of 6 months in all. As can be seen (Fig. 4) the CODer
P.-R. RANTALA and H. WIROLA
\alue-. varied a little more during the apatite running. This cannot all be explained with the use of apatite. hecause there were several problems at the mill at that time (lack of oxygen. aspen wood period when the BOD 7 values doubled. the sludge removal pump was damaged. etc.). Also at the same time there was a study going on where the purpose was to optimise nutrition addition, both phosphorus and nitrogen (Jansson, 1992). Regardless of these problems the phosphorus concentrations were very low during the apatite period (Fig. 5). 7 000
~rp=los~OrTC--------------------------------
crid 6 0 0 0 - - ------
+---- ---.--
--
5000 --
;;:::: 4 000
/I q:dite -- - - - - --1- ---- ----
---
--
--
--
-----
----
-Influmt o Efflumt
--
-----
~
OJ
E
- e- -- - '---
3 000 2000
-
1---
----
--
I---
-
1 000
~~
o
0
00-
r---
~
N
~
C"')
Lei
0 .q
00-
.q
~- ~- ~-
~ ~,
~
~~
r---
~
C'0
N
N
~! N
N
-0
~
---
N N
N
N
N
N
co
C'0
.q
C'0
ci
~ C"')
N
~
.q
.q 0
r---
N
oq
LO
cO
Days Figure 4. CODer values of the first full scale experiment.
9---
- - - -------+-------1
I p,osp,oric o::::id
8
II q:xtite
III q:xtit
p,osp,oriC
-----1--------------------.--------1--
7
----
6
--------f----
-------
- ----
'd --
I
I
I -
--.
- - Ptot influent - .. - Ptot effluent
::::::: 5
- x - Psol influent
E 4
--[}- Psol effluent
Ol
3 2
N~o-H-/---\l-/---"'.~ -/----"-~--_/_--"""',£_
,"<---I ·_-------~I
0
,.... OJ OJ
c:i ~
c:i ,.... (Y)
,.... ,....
Lri ,....
,....
cD
C\J OJ OJ
,....
C\J
Lri
N C\J
N
~
C\J
(Y)
~
C".i ,.... ,....
C".i
r--:
C\J
~
cD
C\J
(j)
OJ
~
0)
LO ,....
C\J
Days Figure 5. Total and soluble phosphorus concentrations in the first full scale experiment. Influent phosphorus concentrations are before phosphorus addition to the wastewater treatment plant.
The second full scale experiment lasted only 15 days. After the first full scale experiment changes were made at the mill. The aspen wood is not used any more, which effects the incoming water. The quality of the water is at the moment more balanced. Also the phosphoric acid addition has been reduced to about half of the dosage it used to be. As a result, the phosphorus load from the mill is nowadays less than half of what it
Treatment of forest industry wastewaters
137
was earlier. Yet, with apatite period lower phosphorus concentrations in the effluent could be achieved (Fig. 6).
apatite period --Ptotinf. - • - Ptot eU. -::«:-Psol info -0--- Psol eU.
--- - -
-
--
--
...---- ---_ .......... ~ 00
N
~
0
~
~
N
~
~
~
0
~ 00
~
0
~
~
~
~
~
N
~
Days
Figure 6. Total and soluble phosphorus concentrations of the second full scale experiment. Influent phosphorus concentrations are before phosphorus addition to the wastewater treatment plant.
CONCLUSIONS
According to the research conducted, it was proved that solid slightly soluble compounds can be used as a nutrient source for forest industry wastewaters. Apatite was found to be the least soluble of the tested materials. When properly operated there are not significant differences in the reduction of organic matter between activated sludge processes using solid or liquid phosphorus addition. Yet, the phosphorus load from a treatment process using solid phosphorus addition can be reduced substantially compared to liquid phosphorus addition. When using apatite the grain size did not affect purification results and apatite did not escape from clarification, either. Apatite particles serve as carriers or nuclei for floes. Slight overdose of apatite is not critical, because excess apatite will be removed with surplus sludge. Vice versa, slight overdosing may have a positive effect for the process ensuring sufficient phosphorus release. If needed, phosphoric acid can be dosed together with apatite or apatite can be acidified before use to get more phosphorus dissolved. Because apatite is the raw material for phosphoric acid it should also be more economical than phosphoric acid. REFERENCES Jansson, K. (1992). Reduction of Phosphorus Load from a Forest Industry Wastewater Treatment Plant. Msc. thesis. Abo Akademi University, Turku, Finland (in Swedish). JOrgensen, K. S. and Pauli, A. (1992). Microbial Transformations of Phosphorus and Nitrogen in the Activated Sludge Treatment of Forest Industry Wastewaters. Publication of the National Board of Waters and Environment, SITYKE-programme, 1. Helsinki, Finland. (in Finnish). Metsateollisuus ry. (1995). Environmental Protection in Finnish Forest Industries, Yearbook 1994. Metsateollisuus ry., Helsinki, Finland. (in Finnish). Pauli, A. (1994). The Role of Acinetobacter sp. in Biological Phosphorus Removal from Forest Industry Wastewaters. Publication ofthe Water and Environment Research Institute, 16. National Board of Waters and the Environment, Helsinki, Finland. Rantala, P.-R., Wirola, H. and Isoaho, S. (1993). Slightly Soluble Solid Phosphorus Compounds as Nutrient Source in Biological Treatment of Forest Industry Wastewaters. Lab-scale Tests with Activated Sludge Reactors. Publication of Tampere University of Technology, Institute of Water and Environmental Engineering, B56. Tampere, Finland. (in Finnish). SFS Catalogue (1990). Finnish Standards Association SFS, Gummerus Kirjapaino Oy, Jyviiskylii, Finland.
138
P.-R. RANTALA and H. WIROLA
Wirola. H. (l979). Combined and Separate Treatment of Board Mill Wastewaters and Municipal Wastewaters. Msc thesis Tampere University of Technology, Tampere, Finland. (in Finnish). Wirola, H., Isoaho. S. and Tanhuala, T. (1991). Slightly Soluble Solid Phosphorus Compounds as Nutrient Source in Biologics Treatment of Forest Industry Wastewaters. Part I: Solubility Tests. Publication of Tampere University of Technology Institute of Water and Environmental Engineering, C35, Tampere, Finland. (in Finnish).