The paper reviews the conditions under which phosphate pre- cipitation was observed in the activated sludge process. These include phosphate removal in ...
A Review of Biological Phosphorus Removal in the Activated Sludge Process* JAMES L. BARNARD [P.G.J. MEIRING AND PARTNERS, POBOX 28734, SUNNYSIDE 0132, SOUTH AFRICA]
Reprinted from WATER SA Vol 2, No 3, July 1976
A Review of Biological Phosphorus Removal in the Activated Sludge Process* JAMES L. BARNARD [P.G.J. MEIRING AND PARTNERS, POBOX 28734, SUNNYSIDE 0132, SOUTH AFRICA]
Abstract
Introduction
The paper reviews the conditions under which phosphate precipitation was observed in the activated sludge process. These include phosphate removal in plug-flow systems such as was found in San Antonio, Baltimore and at the Hyperion plant of the City of Los Angeles, in the Pho-strip method and in the Bardenpho method designed originally for denitrification. B·ench scale experiments proved that CO 2 stripping and the resultant raising of the pH alone could not have been responsible for the high removal of phosphates found in the activated sludge systems. Only the incorporation of an anaerobic zone in the activated sludge system could induce the bacteria to remove the phosphates from the liquid phase. The presence of nitrates invariably retarded or completely eliminated the phosphate removal phenomenon, possibly due to the fact that nitrate could serve as an alternative form of electron acceptor in lieu of oxygen - thereby preventing truly anaerobic conditions in any part of the system. For this reason it is essential for the removal of phosphates that either no nitrates be formed in the process or that when formed, nitrates be denitrified in such a way that some part of the system shall be free of both nitrates and oxygen for a sufficient time to promote the release of phosphates in that part of the system. The re-aeration of the mixed liquor will then result in the uptake of the phosphates.
In recent years many reports have been published on the biological removal of phosphates in the activated sludge system. The phenomenon was observed in a waste water treatment plant, San Antonio, Texas, by Vacker et al (1967). Later reports indicated that the removal of phosphates were also observed at plants in Baltimore (Milbury et ai, 1971), Los Angeles (Garber, 1972) and in Tuscon, Arizona (Yall et ai, 1972). Phosphate removal in excess of 90% was also observed in South Africa in a pilot plant specifically designed for the removal of nitrogen without the addition of chemicals (Barnard, 1975), Levin et al (1972) developed a process called the "phostrip" method and insisted that sustained phosphorus removal would not be possible without stripping the accumulated phosphates out of the sludge on a continuous basis. However, this was not deemed necessary at any of the other plants for which a high percentage of phosphorus removal was reported.
The paper proposes an anaerobic basin at the head of those activated sludge plants in which either no nitrification takes place or nitrates are denitrified. The clarifier underflow and the feed can then be mixed in this basin in order to induce the anaerobic conditions required for phosphate removal. This basin is then followed by either an aeration basin in which no nitrification is allowed to take place, or is followed by a series of anoxic and aerobic basins for nitrification and denitrification. In either process, the mixed liquor would need to be aerated to a dissolved oxygen level of between 2 to 4 mg/I before passing it into the final clarifier.
.Presented at the monthly meeting of the Institute of Water Pollution Control (S.A. Branch). Johannesburg. July. 1975.
Two schools of thought developed to explain the phenomenon and hopefully to find a way of designing activated slud plants for the removal of phosphorus on a continuous basis. One school maintained that it was a purely biological phenomenon in that, under certain conditions, bacteria would take up more phosphates than required for cell synthesis, i.e. the so-called "luxury uptake" principle. The other school maintained that it was a purely chemical phenomenon (Menar and Jenkins, 1969) and that the only role played by bacteria was to create the chemical conditions necessary for the precipitation of chemicals. Nobody has as yet proved conclusively that one or the other or even both of these mechanisms come into play although there is strong evidence that the "luxury uptake" phenomenon might be responsible (Yall et ai, 1972).
Conditions under which phosphate removal was observed There is general agreement in the literature that the following conditions are necessary for the removal of phosphates in the activated sludge system (Vacker et ai, Milbury et ai, Garber, Yall et al and Levin et al):
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Figure J Diagram of Bardenpho process for Denitrification and Phosphate removal without addition of chemicals
1. Plug-flow conditions in the aeration basin which should have a length to width ratio of at least 20 to one. 2. Completely mixed conditions - tapered feed or step feed must be avoided. 3. The return sludge and the mixed liquor should be introduced at the inlet end of the aeration basin. 4. The aeration in the basin should not be tapered to such a degree that the dissolved oxygen concentration (DO) is low at the end of the aeration basin. On the contrary, the DO concentration in the mixed liquor passing to the clarifier should be high, preferably between 3 and 4 mg/R.. 5. The degree of nitrification should be minimized. 6. Solids should be rapidly removed from the final clarifier and returned to the aeration basin. If sludge is allowed to go slightly anaerobic, phosphates would be released into solution. 7. Liquid should not be returned to the aerobic plant from a subsequent sludge treatment step that may release the phosphates . In addition to these conditions some researchers found that other conditions should also be satisfied in order to remove high percentages of phosphates from the sewage. Levin et at (1972) insisted that the sludge must in addition pass through a stripping process consisting of a holding tank in the form of a thickener where the underflow from the final clarifiers is allowed to go anaerobic through the endogenous respiration of the sludge. Phosphates are released to the supernatent which is decanted and treated separately with lime to precipitate the phosphates. The underflow, having passed through this anaerobic stage is then fed back to the influent of the plug-flow aeration basin. Levin (1971) maintained that the system will not work without this stripping unit, yet Vacker et at (1967) and Milbury et at (1971) apparently succeeded in reliably removing more than 90% of the influent phosphates in their full-scale plants on a continuous basis.
Garber (1972) found at the Hyperion plant that he required a certain minimum carbon to phosphate ratio in the plant feed in order to sustain the high phosphate removal efficiency of the plant. In a pilot plant the phosphate removal could be correlated with the carbon to phosphate ratio in that the addition of glucose to the influent resulted in the improved precipitation of phosphates. Milbury et at (1971) found that in all full-scale plants in the United States that removed phosphates without major alterations, phosphates were released from the sludge at the influent end of the aeration basin to a degree far in excess of the phosphate concentration in the plant feed. This was then followed by a slow uptake of phosphates along the length of the aeration basin until the phosphate content of the filtrate towards the end of the aeration basin indicated a removal of up to 97% when compared with the plant feed. The author found that removals of as high as 97% could be obtained in a new denitrification process, (Barnard, 1975) termed the "Bardenpho" process. This plant is significantly different from the normal plug-flow plant that was considered to be essential for the removal of phosphates and this led to an investigation of the mechanism for the removal of phosphates in this plant and in other different plants. A flow diagram of this plant is shown in Figure 1. Raw or settled sewage entered the plant in the first completely mixed basin which was stirred but not aerated while mixed liquor from the second aerated basin and the under-flow from the final clarifiers were recycled to this basin. The rate of recycle of the mixed liquor from the second basin exceeded the feed rate by four times. Nitrates which formed in the second aerobic basin were then recycled continuously with the mixed liquor to the first basin and rapidly denitrified. This accounted for the removal of more than 85% of the nitrogen in the feed. The mixed liquor that was displaced from the second basin was passed on to the third basin where it was again stirred in the absence of oxygen. The endogenous respiration of the bacteria reduced the remainder of the nitrates leaving little nitrogen in the effluent. The mixed liquor was then re-aerated before clarification. This process resulted in the continuous removal of over 90% of the nitrogen in the feed.
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containing an average concentration of P equal to 0,7 mgt J! over a period of 43 days, sampling every two hours. During a period of 6 months, this high removal efficiency was observed with interruptions as mentioned before. The study actually concerned the removal of nitrogen and the optimization of the process. It was considered that the retention time in the second anoxic basin was too long and it was reduced from 4 hours to 3 hours based on the feed rate. The efficiency of phosphate removal was immediately lowered to less than 80%. During the period when high removal efficiencies were experienced, it was noticed that the phosphates in the filtrate of the mixed liquor in the four consecutive basins varied considerably. While the phosphate concentration in the plant feed varied between 9 and 12 mgt R. as P, the average values of the phosphate concentrations in the filtrate of the four basins were typically 2,5; 2,5; 30 and 0,3 mgt R. respectively. Due to some release of phosphates in the final clarifier the effiuent P concentration would be approximately 0,6 mg/R.. It was also noticed that good removals of phosphates did not always coincide with a high phosphate concentration in the filtrates of the second anoxic basin, but that the best removals were obtained when phosphates were released in this basin, thereby indicat ing an anaerobic condition.
Rtsults oj Phosphatt removal in tht Bardtnpho Proem
Bench-scale investigations of the mechanism of phosphate removal During experiments on the removal of nitrogen, it was found th'at the phosphates were also removed in this system provided that the mixed liquor was re-aerated after passing through the second anoxic* basin to leave a residual DO of3 to 4 mgt J! and provided that the sludge was not allowed to go anaerobic in the final clarifier. There was a definite correlation between the nitrates in the effiuent and the phosphates. Even though it happened that the effiuent had a high phosphate content and a low nitrate content, the reverse was never true. The correlation between the nitrates and the phosphates in the effiuent could best be illustrated by the results of a bench-scale unit that treated a particularly strong domestic waste. These results are shown in Figure 2. During the time of the experiment, the nitrogen content was at times exceptionally high due to high peaks of ammonia arriving at the plant at the time of sampling. In addition, the bench unit was too small for the nitrogen load and this resulted in incomplete denitrification. The correlation between the nitrates and the phosphates can clearly be discerned. Pilot plant studies of the denitrification system at first resulted in erratic removals of phosphates. However the lack of phosphate removal could almost invariably be traced to incomplete removal of the nitrates. This sometimes resulted from interruptions of the feed or problems with the oxygen control system which resulted in over-aeration and incomplete denitrification. Incomplete nitrification did not affect the plant's ability to remove phosphates. At one stage, the ammonia nitrogen in the effiuent reached 10 mgt J! without any visible effect on the removal of phosphates which was 93% at the time. It was possible to operate the pilot plant to produce an effiuent
-In this paper the term "anoxic:" refers to the c:ondition where dissolved oxygen is replac:ed by nitrates as electron ac:ceptor, while "anaerobic:" refers to the absenc:e of both nitrates and dissolved oxygen.
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Bench-scale experiments were then run to investigate the mechanism of phosphate removal. Since it was established that phosphates could be removed from the particular sewage under certain conditions, the phosphate removal capacity of the normal plug-flow system was investigated. A 30 litre plexi-glass laboratory unit was divided into 9 compartments in series and a clarifier added as shown in Figure 3. The raw feed, consisting of settled sewage, was fed to the first compartment and the underflow from the clarifier was also returned to this compartment. High rates of aeration were maintained in all the compartments. Nitrification was virtually complete. Effiuent phosphate concentrations are shown in Figure 4. Phosphate removal efficiency was of the order of50%. Menar and Jenkins (1969) concluded that the only function of aeration in the plug-flow unit as related to the removal of phosphorous, was that of stripping the CO 2 and lowering the hydrogen ion concentration of the mixed liquor to a point where chemical precipitation of the phosphates take place. This could also be achieved by the addition of an alkaline solution. A lime slurry was added under carefully controlled conditions to the same unit described above and shown in Figure 3, to maintain the pH value of the mixed liquor at a constant value of8,5 ± 0,1. All the compartments were well-aerated. The improvement in the removal of phosphates was insignificant as can be seen in Figure 4. The effect of the presence of nitrates as such on phosphate removal was investigated by adding methanol to the seventh and eighth compartment and stirring the mixed liquor under anoxic conditions. The final basin was again well-aerated. Even though the effiuent phosphate concentrations were lower than in the previous two experiments, as can be seen in Figure 4, they still never approached the low concentrations achieved in the Bardenpho Unit.
FEED
AERATED COMPARTMENTS
1
2
3
4
5 6
7
8
9
Figure 3 Bench scale Plant Jor the removal oj Phosphates
In none of these experiments was it possible to achieve removal rates of the order of 90% which would have resulted in effluent P values of less than 1 mg/ R.. The unit was then converted to operate according to the denitrification principle previously described but with sufficiently long retention times in the first and second anoxic basins to ensure that no nitrates ould be present in the efRuent and furthermore to maintain a ...ondition of anaerobiasis in the second anoxic basin. The retention time in the first anoxic basin was 4 hours and that in the second anoxic or rather "anaerobic" basin was 6 hours, based on the feed rate to the unit. The total retention time was 24 hours while the solids retention time was 20 days. The results of this experiment are shown in Figure 5.
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Some problems were experienced as indicated on the graph in Figure 5. Sludge tended to settle in the second anoxic basin. It was difficult to reproduce the conditions that would prevail in a large scale plant on so small a scale since small variations in the pressure of the central compressed air system would interfere with the control of the aeration in the aerobic compartments. This led to over-aeration at times and interference with maintaining proper anaerobic conditions in the second anaerobic basin. In spite of this it was possible to obtain high removals of phosphates especially towards the end of the experiment. The effluent P concentration reached as low as 0,3 mg/R. and was below 1 mg/ R. for much of the time.
Figure 5 Percentage phosphate removal in bench scale Bardtnpho process
These experiments were not of long duration and they were performed more in the way of probing for possible clues to the necessary conditions for phosphate precipitation or removal in the activated sludge system. It is difficult to draw absolute conclusions from laboratory experiments unless precautions are taken to ensure that conditions would be similar to those that prevail in large scale plants. However, it was felt that the experiments clearly indicated that plug-flow conditions with full nitrification or the maintenance of the pH at 8,5 through the addition of lime did not result in low concentrations of P in the effluent while it was possible by creating anaerobic conditions in the system.
Postulation of the necessary conditions for the removal of phosphates
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O=27~3~1~~~8~~12--~16--~20--~24--~2-8~2--~6--1~O--I~4--1~8--2~2-J novl!lllber 1973
december 1973
Figure 4 Results of tests with Plug-jiow mode, lime addition and Methanol addition
Taking into account the findings of research workers all over the world, including those of the author, there seems to emerge one necessary pre-condition to the removal of phosphates in the activated sludge system. This condition is that the mixed liquor or the return sludge from the clarifier underflow must go through a period of anaerobiasis during which phosphates would be released to the liquid, followed by a period of aeration during which the phosphates would be either precipitated or absorbed by the bacterial mass. The presence of nitrates would tend to raise the oxidation-reduction potential above the minimum value that would be required to ensure that anaerobic conditions are created. The published findings ofresearch workers could be tested against this hypothesis.
The findings by Vacker et at (1967), Milbury et at (1971) and Garber (1972) could be explained as follows. In all reported cases, the plug-flow mode was used. The returned activated sludge was mixed with the incoming raw or settled sewage at one end of the elongated aeration basin and passed through the length of the unit while the aeration was increased to maintain a high DO towards the outlet end. In spite of efforts to introduce sufficient air to the influent end of the aeration basin, the oxygen demand in this section of the plant was so high that reducing conditions developed. This would explain the release of phosphates in this section reported by Milbury et at to have been observed at all plants in the United States that successfully removed phosphates. In laboratory-scale plants the introduction of sufficient oxygen in the initial section of the aeration basin would not present any problems and phosphorous removal is difficult to simulate. The full-scale plants were purposefully operated to minimize nitrification and where it did take place, it could not have presented a serious problem since the return sludge was usually of the order of 25% of the feed. Under these conditions only a low degree of nitrification is possible. Any nitrates that formed would be denitrified immediately and since the total load of nitrogen would not be high due to the small quantity of sludge returned, these nitrates would not have a severe impact on the anaerobic conditions in the zone at the inlet to the aeration basin. When the sewage arrives at the works in a reduced or slightly septic state, the nitrates that might be returned with the underflow of the clarifier would react immediately with the reduced substances in the sewage and would have little effect on the degree of anaerobiasis in the inlet zone. This could explain the erratic removal of phosphates in some activated sludge systems. This degree of anaerobiasis of the incoming sewage or the deficit of oxygen would differ from plant to plant or in the same plant from day to day or from season to season. Obviously, preaeration of the sewage in the grit chambers or pre-aeration of the return activated sludge would have the same effect on the anaerobic zone as a lack of reduced substances in the sewage. Conversely, cutting back the aeration in the inlet zone of the activated sludge system and increasing the aeration near the outlet zone would have a beneficial effect on the removal of phosphates in the system if the hypothesis being postulated holds true. Garber (1972) observed that a high carbon to phosphorous ratio in the feed correlated well with good phosphorous removal in the plant. Increasing the C:P ratio in the influent to the plant would also have the effect of increasing the oxygen demand in the inlet zone and promoting an anaerobic zone in the inlet zone of the plug-flow plant unless the oxygen supply is increased. Garber proved that the ability ofthe plant to remove phosphates was related to the sewage and not to sludge itself and that by increasing the COD of the influent he could enhance the plant's ability to remove phosphates. Menar and Jenkins (1969) tried to simulate the conditions that prevailed at the San Antonio plant, but operated the pilot plant such that a high rate of nitrification was achieved. It is probable that the aeration of the inlet basin was sufficient to prevent the formation of an anaerobic zone. This could result from the fact that whereas it is quite simple to introduce sufficient air into a small plant, the largescale plants usually have fixed aeration devices which would preclude a significant increase in the aeration rate. Also, to maintain nitrification in the plant, a high rate of returned sludge is required which would return a larger proportion of nitrates to the influent 140
Water SA Vol. 2 No. 3 July 1976
zone. Even without the return of nitrates the oxygen input in the first unit could have been sufficient to satisfy the oxygen demand which would infer that the degree of nitrification has no effect on the ability of the plant to remove phosphates. Many reported attempts to remove phosphates in plug-flow systems met with no success or with erratic results. The considerations mentioned above might explain much of the problems that were experienced. The "Pho-strip" method reported by Levin et at (1972) and Levin (1971) also consists of a plug-flow unit with the exception that the underflow from the clarifier was first thickened and the bacterial mass allowed to become anaerobic owing to the endogenous respiration of the bacteria. The underflow from the thickener was passed to the inlet zone of the plug-flow aeration basin where the mixed liquor was aerated in the normal way. In the thickener some phosphates were released to the liquid which was then decanted and treated with lime to precipitate the phosphates. Sludge was wasted in the normal way or from the anaerobic thickener. Levin insisted that the system will not work without adding the stripping process to the plug-flow aeration basin (Levin, 1971). However, reports by Vacker et at (1967), Milbury et at (1971), Garber (1972) and that of author showed that phosphate removal took place without stripping process . The essential part of the stripping process is the anaerobic stage and the stripping operation was not essential but perhaps helpful. Without stripping the only outlet for the sludge would be through the waste sludge, the phosphate concentration of which should rise to some 10 percent to account for the phosphates in the influent. Vacker et at (1967) reported phosphate concentrations in the sludge of as high as 7% while others maintained that the maximum concentration that would be reached would be of the order of3%. It is clear that in order to obtain high phosphate concentrations in the sludge one should first create the conditions under which this would be possible. The maximum concentration when using a simple plug-flow aeration system without the necessary anaerobic conditions, would therefore be low. The value obtained by Vacker et at (1967) accounted for the loss of phosphates in their plant. Similarly, the lower value of about 4% obtained by Levin et at (1972) could be explained by the stripping of the phosphates from the sludge. It is evident that the presence of nitrates would not undul' upset the system of Levin et at (1972), provided that t anaerobic basin or thickener be large enough both to completely denitrify the nitrates and create the necessary anaerobic conditions. Again, since the sludge return in high rate plants is a relatively small ratio of the influent flow rate, the nitrates returned with a sludge stream would be a small fraction of the total nitrates in the effiuent. Levin has also allowed for the denitrification of some nitrates by the incorporation of the Wuhrmann (1964) procedure for denitrification and by reaerating the mixed liquor before clarification. The Wuhrann procedure for denitrification, consists of interposing a stirred basin between the aeration basin and the final clarifier for denitrification by endogenous respiration of bacteria, of nitrates that may be formed. This system of denitrification had been proven by a number of researchers to be ineffective (Barth et at 1968). In high rate activated sludge plants not designed for nitrification, some nitrates may form during the warmer months. The mixed liquor of such a high rate process would have a considerably higher respiration rate than that of a plant designed for nitrification all the year round. Also, the denitrification rates in summer would be much higher than those
during the colder months (Dawson and Murphy, 1972, Barnard, 1975). Whereas it would then be possible to reduce the nitrates by endogenous respiration alone during the summer provided that the nitrogen content of the waste is low - the amount of nitrates removed in this way during the winter would be negligible. We thus have the position that the system of Levin would not be unduly upset by the presence of nitrates since these would only occur during the warm season when the denitrification rate would also be high and nitrates would therefore be removed rapidly and not be formed at all during the colder period. For much the same reasons, it is also unlikely that nitrates would interfere with the performance of other plants for which phosphate removals were reported.
bacterial mass in plants having a long solids retention time (SRT) is slow and large basins are required for the reaction to take place. In the Bardenpho process the bulk of the nitrates formed in the nitrification zone is denitrified by recycling of the mixed liquor to the influent end of the aeration tank where the mixed liquor is deprived of oxygen. The rate of denitrification is then much faster than that due to endogenous respiration. It is therefore proposed to use the same method for lowering the redox potential in the sludge to create the anaerobic conditions that would be necessary for the removal of phosphates - this process is hereafter termed the "Phoredox" process. When a plant was designed for nitrification, the nitrates would be removed by internal recycling of the mixed liquor.
In these types of plant the recycle rate of the clarifier underflow would normally exceed the influent flow rate but this return sludge would then contain a low concentration of nitrates due to the internal denitrification of the nitrates. The reThe foregoing discussion leads to the inevitable conclusion that turn sludge is then mixed with the raw or settled sewage and the activated sludge returned from the clarifier or the mixed allowed to become anaerobic. Phosphates would then be reliquor must pass through an anaerobic phase where the oxygen leased in this basin and the mixed liquor could then be passed demand exceeds the supply of both oxygen or nitrates at some on to the standard Bardenpho plant for the reduction of the tage except the final stage before clarification at which point it nitrates. The last basin of this plant is aerated and this serves to 'S hould be aerated . In this anaerobic zone or stage, a certain precipitate the phosphates and to maintain the contents of the degree of anaerobiasis or a certain minimum level of the clarifiers aerobic to avoid the redissolution of phosphates. It is oxidation-reduction potential must be reached. At this level of immediately apparent that phosphate removal could be the oxidation-reduction potential phosphates will be released to applied to any process that would achieve a high degree of dethe liquid in the form of dissolved ortho-phosphates and nitrification by means of creating anoxic zones within the aerawhereas it would be difficult to measure the oxidation reduc- tion basin. tion potential, it would be a simple matter of control to measure The application of the "Phoredox" process to the Bardenthe release of phosphates in the anaerobic zone as a means of ensuring that the necessary conditions for the removal of phos- pho process is shown in Figure 6. In this way high rates of phates would prevail. The redox potential should not however nitrogen and phosphate removal could be obtained in a onebe lowered to the point where sulphates would be reduced and sludge activated sludge unit. The addition of the second anoxic odour problems result. basin in the Bardenpho system is specifically for obtaining high nitrogen removal efficiencies. When it is desirable to remove a It was observed that phosphates were not always released in large proportion of the nitrogen but not necessarily more than the anaerobic zone, which leads to the speculation that the de- 90%, the second anoxic basin could be omitted. The nitrates gree of anaerobiasis required is such that phosphates need not being returned to the anaerobic basin would be low and could be released but that the redox potential at the level at which be compensated for by slightly enlarging this basin. In this inphosphates are released should be close to the minimum value stance one would have a sequence of basins being anaerobic, anoxic and aerobic. of the redox potential necessary to ensure good P removal.
Introducing the "Phoredox" method for the removal of phosphates
(
In the process of Levin, both the nitrate reduction and the creation of the anaerobic conditions in the sludge is achieved by endogenous respiration . The endogenous respiration rate of the
If the removal of nitrogen is not important, the aerobic basin could be designed and controlled to minimize nitrification. The return sludge could then still be mixed with the incoming sew-
mixed liquor recycle
FEED
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obic
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aerobic
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aerobic
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sludge recycle Figure 6 Application
of Phoredox to the Bardenpho Process W.t~r SA Vol. 2 No.3 lulv 1976
141
at
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Figure 7 Application
Feed
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Figure 8 Application
of Phoredox to a pure oxygen plant
NITRIFIC AND
DEN ITR I FICATION
EFFLUENT
Figure 9 Application of Phoredox to the Carousel System
age to create anaerobic conditions. This is illustrated in Figure 7. The means of aeration is not important and pure oxygen could be used for this purpose. It might however be desirable to have the last stage as an ordinary aeration stage for stripping excess CO 2 from the mixed liquor before clarification. This is illustrated in Figure 8.
The process could also be applied to denitrification systems such as the Orbal, the Pasveer or the Carousel plant. In each instance it would be necessary to precede the plant with an anaerobic basin and to add a reaeration basin before the clarifier, since these plants operate at low oxygen levels. This application is shown in Figure 9.
2. In plug-flow plants this condition could be created inThe idea of introducing an anaerobic basin ahead of the anoxic basin was conceived and first introduced when the au- voluntarily at the inlet to the aeration basin by overloading this thor was involved in the final stages of the design ofa treatment section of the basin and by the physical inabili ty of the aeration plant for the town of Meyerton in the Transvaal. This plant system to introduce sufficient oxygen into the mixed liquor. In required a design for greater than 90% nitrogen removal and such plants the degree of backmixing is low and the oxygen phosphate removal of the same order. This plant is now under demand along the first few metres of the aeration basin would construction and should produce full-scale results in the first be unusually high, resulting in an oxygen deficit. If the sewage half of next year. The concept was also tried at the Alexandra is strong or septic, the oxygen demand will be higher and the plant in Johannesburg. Nicholls (1975) describes full-scale ex- tendency to remove phosphates would be increased provided periments for phosphorus removal at the Alexandra plant that other conditions are also favourable. which consisted of an extended aeration unit designed for treating raw sewage. It was found that by switching off the aerators 3. The presence of nitrates would have a deleterious effect on close to the inlet end of the aeration basin, mixed liquor was the anaerobic conditions in this first zone. In a plant not recycled to the inlet end where the raw sewage was mixed with designed for nitrification, the sludge return rate would be the nitrate containing mixed liquor. A high degree of denitri- low and the nitrates that may form in the plant would also fication was observed resulting in an overall nitrogen removal be low. The underflow from the clarifiers would then be more which at times exceeded 90%. Nicholls then isolated one of the active and contain less nitrates while the return flow rate would four clarifiers and pumped the waste sludge into this unit over- be only a fraction of the feed rate. Under these circumstances, night. When the peak flow arrived at the plant the next morn- the nitrates would not have a serious effect on the ability of the ing, he returned the anaerobic sludge to the inlet. Phosphate plant to remove phosphates. In nitrifying plants the recycle determinations on filtrates of the mixed liquor revealed that the rate would be higher and would contain a high concentration of phosphates were absorbed into the sludge during the passage of nitrates. It is likely that a loss of nitrogen would be experienced the mixed liquor through the aeration basin. The clarifiers were in this type of plant but due to the nitrates in the mixed liquor .: trthe scraper type and the sludge remains in these units for an in the inlet zone, the redox potential would not be lowered to excessive time. The dissolved phosphates in the filtrate of the the level that is required for phosphate removal. mixed liquor dropped to 0,3 mg/Il. as P but some orthophosphates were redissolved in the clarifiers. The stored 4. When it is desired to nitrify the ammonia in the feed or to anaerobic sludge was soon exhausted and the phosphates in remove the nitrogen from the feed, it would be necessary to solution started to increase. The retention time in the aeration nitrify and denitrify the ammonia nitrogen in the one-sludge basin was about 27 hours while that of the clarifiers was ap- system and return the nitrate-free sludge to the inlet of the aeraproximately four hours. tion basin. The sludge of an activated sludge unit designed for full nitrification and denitrification is usually much less active While confirmation of the experimental results is awaited than that of the high rate system and it would then be necessary from full-scale operation, it was felt that we could no longer to cut back the oxygen input in the inlet zone of the aeration delay the incorporation of the anaerobic basins in the new basin or add an anaerobic basin before the inlet to the plant plants being designed. No evidence contradicting the findings and allow the mixed liquor to become anaerobic through the could be uncovered and the alternative of using chemicals for action of the incoming sewage. This is the principle of the the removal of phosphates is so costly that the minimal cost of "Phoredox" method for the removal of phosphates and nitroproviding some extra basin capacity is marginal, compared to gen from sewage. the cost of chemicals which could double the cost oftreatment. 5. The Phoredox method could also be used with plants that When removing phosphates by biological means, the sludge can reduce most of the nitrates through internal recycling of the would have to be treated in such a way that the phosphates mixed liquor. When using this method with systems such as the would not be released to the liquid returned to the treatment Carousel, the Pasveer Ditch or the Orbal system, it would be / rocess or the liquid should be treated for precipitation of the desirable to add not only an anaerobic basin before the inlet to phosphates. In the latter instance the volume would be small the plant, but also a re-aeration zone to maintain the DO ofthe compared to that of the waste flow and the addition of lime to mixed liquor, being passed on to the clarifiers, above 3 mg/Il.. raise the pH of the liquid would not be costly. This step was found to be essential in all biological systems that removed phosphates.
Summary and Conclusions I. The investigation of a number of plants that achieved high phosphate removals led to the conclusion that the only common feature of all the plants that could be responsible for the removal of the phosphates was the intentional or unintentional creation of an anaerobic zone in the plant as opposed to an anoxic zone. The sludge or the mixed liquor passing through this anaerobic zone would then release phosphates to solution in the form of ortho-phosphates. The mixed liquor must be aerated for a period of at least one hour before passing it to the final clarifier. The continual change in the redox potential seems to be a necessary pre-condition for the removal of phosphates in the activated sludge system.
6. The "Pho-strip" method proposed by Levin et at (1972) depends on the endogenous respiration both for the removal of the nitrates and for creating the anaerobic condition necessary for phosphate removal. This type of plant would obviously not be able to effect a high rate of nitrogen removal and phosphate removal. If full nitrification is desired, the SRT must be long and the endogenous respiration rate would be low. The anaerobic retention time would then have to be so long that nitrification would be affected deleteriously. Since nitrogen could only be removed by denitrification, it is essential to have full nitrification and denitrification. Levin also stated that the phosphate removal would not be possible without the stripping of the phosphates even though he only removed 25% of the phosphates through the stripping and recovery with chemicals and 75% by the waste sludge. Water SA Vol. 2 No.3 July 1976
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7. The mechanism of phosphate removal seems to be one of luxury uptake. It was not possible to obtain high removals with the available sewage when the pH of the mixed liquor was maintained at pH 8,5 by the addition of lime in a highly aerated plug-flow unit. Switching back to the pure biological system with an anaerobic zone immediately showed phosphate removals of as high as 97%. 8. It is undoubtedly true that the biological system is not fully controllable. However, where the biological system lacks in unpredictability, say by the action of certain toxic elements, a pure chemical process again lacks by mechanical breakdowns due to the complexity ofthe system. In addition, the possibility of a biological system to remove more than 90% of the phosphates has tremendous economic importance. The addition of the small anaerobic basin of about one hour's retention based on the average flow rate to the plant would result in a modest increase in the cost of the plant. The cost of the aeration basin usually constitutes about 15 to 20% of the total plant cost. The addition of the anaerobic basin would add another one percent to this cost. The cost of treatment would be increased by about 0,03 c/m 3 • Alternatively, the cost of chemicals added to the activated sludge system would amount to about 1,3 c/m 3 at present prices. 9. The sludge handling must be such that the sludge remains aerobic all the time. This might require that the waste sludge be thickened by flotation and dewatered. The thickened sludge would then contain an abundance of air bubbles and could be stored for a short period. The requirements for nitrification, denitrification and phosphate removal are such that the SRT of the activated sludge unit would be high. The system should then be designed on the extended aeration principle and the sludge would require no further treatment. The dewatered sludge would contain a high percentage of phosphates, thereby increasing its value as fertilizer. The phosphates would also be readily available to plants, in contrast to the phosphates accumulated in most chemical sludges. 10. The actual retention time in the first anaerobic basin might prove to be a critical factor in the removal process. For this reason, the plant at Meyerton was provided with a recycle to the anaerobic basin. Again, the cost of providing this additional recycling of mixed liquor is minimal and could be provided at the construction stage of the plant. On the other hand, this could be added at a later stage at little cost. II. The removal of phosphates by biological means usually results in an all or nothing removal efficiency. When the plant
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is functioning well, the degree of removal is high. When something goes wrong, the removal efficiency will drop drastically. The biological procedure could therefore be complemented by the addition of chemicals such as Alum. The effluent of the plant could be continuously monitored by an auto-analyzer which could activate a feed pump for feeding Alum whenever the phosphates in the effluent exceed a certain value. The cost of chemicals is so exorbitant that this would result in a great saving while minimizing the possibility of phosphates passing through. 12. An alternative solution might be the addition of Alum in the clarifier effluent with no further settlement of the floc. It has been proven that the AIP0 4 precipitate, once formed, will not redissolve, even under the adverse conditions of an anaerobic digester. Discharge of the floc into a lake could therefore do no harm and with the low phosphate content of the effluent from the biological process, one could possibly reduce the concentration of the soluble phosphates being discharged to the lake to very low levels.
REFERENCES BARNARD, J.L. (1975). Nutrient removal in biological systems. Joumal Inst. Water Pollution ContTol, 74, (2). BARTH, E.F., BRENNER, R.C. and LEWIS, R.F. Chemical-biological control of nitrogen and phosphorus in wastewater effluent, JOUT. Wat. Poll. ContT. Ftd., 40 (12). DAWSON, R.N. and MURPHY, K.L. (1972). Factors affecting biological denitrification of wastewater. 6th International WattT Pollution Rtstarch ConfiTmu, Jtrosalem, 1972. GARBER, W.F. (1972). Phosphorus removal by chemical and biological mechanisms. Applications oj Ntw Conupts oj Physical Chemical Wastt WaItT Trtatmmt, Vanderbilt University Conference Sept. 1972, Pergamon Press. LEVIN, G. V. (1972). Phosphorous removal by Luxury Uptake-Communication. Journal Water Poll. ContTol Ftd., 43, (9), 1972. LEVIN, G. V., TOPOL, G.]., TARNAY, A.G., and SAMWORTH, R.B. (1972). Pilot plant tests on phosphate removal process. Journal Water Poll. ContTol FtdtTation,44, (10). MENAR, A.B. and JENKINS, D. (1969). The fate of phosphorus in waste treatment processes: The enhanced removal of phosphate by activated sludge. PTOC. 24th Ind. Wastt Trtatmtnt Conftrtnct, Purdue University, Lafayette, Indiana, May, 1969. MILBURY, W.F., McCAULY, D., and HAWTHORNE, C.H. (1971). Operation of conventional activated sludge for maximum phosphorus removal. JOUT. Wat. Poll. ContT. Ftd., 43, (9). NICHOLLS, H. A. (1975). Full-scale experimentation on the newJohannesburg extended aeration plants. WaitT SA, I, (3), 121. VACKER, D., CONNELL, C.H. and WELLS, W.N. (1967). Phosphate removal through municipal wastewater treatment at San Antonio, Texas. JOUT. Wat. Poll. ContT. Ftd., 39, (5). WUHRMANN, K. (1964). Nitrogen removal in sewage treatment processes. Ver. into Vmin. Limnol., 15, 580. YALL, I., BOUGHTON, B.H., ROINESTAD, S.A. and SINCLAIR, N.A. (1972). Logical removal of phosphates. ApplicatIOns of Ntw Conupts of Physical Chemical Wastt WattT Trtatmmt, Vanderbilt Univ. Conference Sept. 1972, Pergamon Press.
