Ardışık kesikli reaktörlerde biyolojik aşırı fosfor giderimi

Abstract

Biyolojik fosfor giderimi prosesi ötrofikasyonun sebep olduğu su kirlenmesinin önlenmesi için üzerinde geniş olarak çalışılan bir konudur. Biyolojik aşırı fosfor giderimi ise son yıllarda önem kazanmaya başlayan ve düşük maliyeti sayesinde kimyasal fosfor giderimine alternatif olabilecek bir arıtma yöntemidir. Bu proses, anaerobik ve aerobik reaktörlerin arka arkaya kullanılmasını gerektirir. Anaerobik bölgeyi takip eden aerobik bölgede bakteriler tarafından normal seviyenin üzerinde fosfor alımı gerçekleşir. Acinetobacter aşın fosfor gideriminde öncelikli mikroorganizmalardan biridir. Bu mikroorganizmalar uçucu yağ asitlerini anaerobik ortamda fosor salımı için kullanırlar. Uçucu yağ asitleri Acinetobacterierin heterotroflarla rekabet edebilmesi için gerekli bir substrattır. Aşın fosfor gideren proseslerde, anaerobik bölgeye giren oksitlenmiş azot denitrifikasyon için substrat tüketir. Ortamda substratın kısıtlı olması fosfor salimim düşürür. Substratın kısıtlı olmadığı sistemlerde hem denitrifikasyon hem de fosfor salımı gerçekleşir. Fosfor depolayabilen denitrifikasyon bakterilerinin anaerobik bölgenin başında oluşacak ön-anoksik bölgede fosfor şahmını düşürdüğünü bildiren çalışmalar yanında anoksik bölgede fosfor saliminin gerçekleşebileceğini söyleyenler de bulunmaktadır. Son yıllarda yapılan deneysel çalışmalar biyolojik fosfor gideren proseslerde oksijen yerine elektron alıcısı olarak nitratın da kullanılabileceğini ortaya koymuşlardır. Bu çalışmada, ardışık kesikli reaktörde, biyolojik aşırı fosfor gideren çamurla yapılan deney sonuçları ile anaerobik bölgede fosfor salımı ve aerobik bölgede fosfor alımı üzerine substrat cinsi, KOİ/TKN oranı, havalanma süresi ve anoksik şartların etkisi ortaya konmuştur. Buna göre, sentetik atıksu ile yürütülen sürekli deneylerde aşın fosfor gideriminin gerçekleşebilmesi için substrat olarak fermantasyon ürünlerinin gerekliliği ve artan KOİ/TKN oranının uygun substratla beslenen sistemde fosfor giderme verimini düşürdüğü ortaya konmuştur. Aşın fosfor gideren sistemden alınan çamurla yürütülen kesikli deneylere göre ise ortamda substratın varlığında nitrat, fosfor salımı ve fosfor alımının beraber gerçekleşmesinden dolayı net fosfor saliminin düşmesine sebep olur. Anaerobik bölgede substrat tükendiğinden ardından gelecek anoksik şartlarda fosfor alımı gerçekleşebilir. Ancak, anoksik şartlarda fosfor alım hızı aerobik şartlara göre daha düşüktür. Yapılan kesikli deneylerde, çamur özelliklerine bağlı olarak, aerobik şartlarda fosfor alım hızının 3.7-6.7 mg P/mg UAKM.saat arasında iken anoksik şartlarda bu oranın 1.7 - 5.5 mg P/mg UAKM.saat arasında değiştiği görülmüştür. Fosfor giderimi yanında nitrat gideriminin istendiği durumlarda aerobik bölgede oluşan nitratın anoksik şartlarda kullanılması yoluna gidilmelidir. Aşın fosfor gideren sistemlerde nitrat korkulması değil kullanılması gereken bir parametre olarak gözönüne alınmalıdır.

Several organics of wastewater and natural waters are important in establishing and controlling water quality. The concentrations of organic substances in water are increased both by the wastewaters, treated or untreated, that are dicharged to it. The elements nitrogen and phosphorus are essential to the growth of protista and plants and as such are known as nutrients or biostimulatants. Because of noxious algal blooms that occur in surface waters, there is presently much interest in controlling amount of phosphorus compounds that enter surface waters in domestic and industrial waste discharges and natural runoff. Municipal wastewaters, for example, may contain from 4 to 15 mg/l phosphorus as P. Since some blue-green bacteria have the ability to fix atmospheric nitrogen gas to support primary production, the eutrophication control strategies are generally based upon the control of phosphorus in the effluent discharges to water bodies. Although the chemical phosphate removal has been generally applied for phosphorus removal, because of high chemical cost, biological excess phosphorus removal has gained importance since 1970's. Biological phosphorus removal techniques are based on the principle that, given the right conditions, certain bacteria are able to remove phosphates from wastewaters. Such systems incorporate an anaerobic phase somewhere in the process in which phosphorus is released from the microorganisms. The uptake of the released phosphorus in the aerobic phase results in much lower phosphorus concentration than the influent so-called luxury phosphorus uptake. In this way, the sludge containing higher phosphorus than the conventional activated sludge systems occurs and this diverts more phosphorus to the waste solids yields high biological phosphorus removal. Acinetobacter which is able to accumulate more phosphate than is required for cell synthesis is mostly reported as BEPR bacteria. Acinetobacter is normally present in activated sludge, but in minority due to the low growth rate. Acinetobacter organisms prefer low fatty acids, especially acetate, as a growth substrate. The phosphorus release in anaerobic zone is typically accomplished by the consumption of organic substrates by facultative anaerobic bacteria. The energy provided by releasing the stored phosphate is used to consume fatty acids and stored as reserve material. During the aerobic phase, the stored substrate products are depleted and soluble phosphorus is taken up with excess amounts stored as polyphosphate, in aerobic zone Acinetobacter metabolizes the stored materials via oxidation and synthesis of new cells. This cycle repeats itself until the point when after an aerobic zone the accumulated polyphosphate in Acinetobacter is discharged with the surplus sludge. It can be concluded that the more low fatty acids there are in the anaerobic zone, the higher the P release, the more growth of Acineobacter and hence the more P removal occurs. xv Although an anaerobic zone is a prepequisite for biological P removal, many parameters can positively or negatively affect the degree of P removal, such as the composition of the wastewater, the oxygen supply, nitrate concentration entering the anaerobic zone. It has been generaly accepted that the phosphorus release only occurs under anaerobic conditions in the absence of both oxygen and nitrate. In wastewater treatment plants combining denitrification with biological phosphorus removal, there is a risk that the denitrification will reduce biological phosphorus removal capacity if some of the organic matter taken up by the denitrification bacteria is not utulized for phosphorus release. It has been reported that the presense of nitrate disturbs the anaerobic period (Hascoet, 1985) and inhibits the biological phosphorus removal process in the anaerbic zone because denitrification consumes low fatty acids (Van Stankerburg et. a/., 1993). However, it is also shown that the magnitude of the detrimental effect of nitrate depends on the substrate existing in the anaerobic zone. The low fatty acid concentration in the anaerobic zone is reported to be a limiting factor. If this concentration is high enough, P-release and denitrification takes place simultaneously (Van Stankerburg et. al., 1993). Despite the negative effect of nitrate in anaerbic zone, its existance in aerobic zone does not affect phosphorus uptake. Since many biological phosphorus removal systems involve nitrification and denitrification, the ability for phosphorus-storing microorganisms to reduce nitrate is an important issue. Since they have the potential to remove a large portion of the influent substrate, denitrification in an anoxic zone following the anaerobic zone should be lower compared to possible rates without the anaerobic zone. The phosphorus accumulating bacteria can also take up phosphate under anoxic conditions as nitrate can serve as oxidant which has been demonstrated by Hascoet etal. (1985) and Gerber etal. (1987). However, many investigators have now observed phosphorus uptake in anoxic zones concurrent with nitrate reduction, but it is not known if the denitrification rate is equivalent to that possible if no anaerobic zone and substrate storage exist. It is generaly belived that biological oxication reactions are faster using readily available substrates than intracellular products. Gerber et al. (1987) compares the rate of which phosphate is taken up under aerobic and anoxic conditions and finds that under anoxic conditions the rate is considerably lower than is the case under aerobic conditions. The phosphorus uptake was slower under anoxic conditions, because only part of the phosphorus accumulating bacteria take up phosphate under anoxic conditions, whereas all the phosphorus accumulating bacteria take up phosphate under aerobic conditions (Henze,1993). If the amount of organic matter is limiting to the biological phosphorus removal, anoxic conditions will in that case be regards phosphate removal. XVI But, organics such as acetate and propionate induce phosphate release from activated sludge with a history of excess phosphate accumulation not only under anaerobic conditions but also in anoxic and aerobic environments. This contrasts sharply with a compaund such as glucose, which does not give rise to phosphate release unless strict anaerobiosis prevails. The release occurs immediatelly upon exposure to acetate or propionate irrespective of whether the sludge is partially or fully reconstituted with respect to accumulated phosphate (Gerber, 1987). The above reported background suggest that better knowledge has to be gained about the effect of nitrate on BEPR. In this context, this study describes a lab-scale experimentattion carried out to stidy the effect of nitrate under both anaerobic and aerobic conditions of phosphorus removal in a sequencing batch reactor together with different organics substrates fed. Experimental Study An anaerobic-aerobic SBR was operated over 280 days in the experimental work. The SBR had a working volume of 8.8 I. and it was operated in a cycle of 8 hours. The synthetic wastewater fed to the system consisted of different combinations of TSB, sodium acetate and glucose. Additional nutrients and minerals, including inorganic phosphorus and nitrogen were also added to the feed solution. The feed was prepared daily by using tap water. Experimental work can be evaluated in two ways, namely continuous SBR tests and batch tests; Continuous SBR Test Phosphorus removing sludge was taken from anaerobic-aerobic SBR that was feed with the feed solution containing 80% TSB and 20% acetate. The phosphorus content of the sludge was 15% on VSS and 12% on SS basis. The synthetic watewater was used during the experimental study. The feed solution contained 50% TSB and 50% glucose for SET 1-SET 3 and 80% TSB and 20% acetate for SET 4-SET 6. Ratio of influent COD/TKN was changed in each set. Operation conditions of these sets are shown in Fig. 1. Average results of the experimental study for each set are also given in Table 1. Figure 2 shows daily variations in phosphorus parameter throughout a cycle during SET 1 to SET 6. I_0 l! £ |_3 |4 [5 [6 hours ? F, M, A, S, PI, (a) |F| M | A | M | A | S | PI | (b) F: Fill only M: Mixed A: Aerated S: Settle PI: Decant and Idle (a) From SET 1 to SET 5 (b) SET 6 Figure 1. Operational conditions of experimental sets. XVII Although an anaerobic zone is a prepequisite for biological P removal, many parameters can positively or negatively affect the degree of P removal, such as the composition of the wastewater, the oxygen supply, nitrate concentration entering the anaerobic zone. It has been generaly accepted that the phosphorus release only occurs under anaerobic conditions in the absence of both oxygen and nitrate. In wastewater treatment plants combining denitrification with biological phosphorus removal, there is a risk that the denitrification will reduce biological phosphorus removal capacity if some of the organic matter taken up by the denitrification bacteria is not utulized for phosphorus release. It has been reported that the presense of nitrate disturbs the anaerobic period (Hascoet, 1985) and inhibits the biological phosphorus removal process in the anaerbic zone because denitrification consumes low fatty acids (Van Stankerburg et. a/., 1993). However, it is also shown that the magnitude of the detrimental effect of nitrate depends on the substrate existing in the anaerobic zone. The low fatty acid concentration in the anaerobic zone is reported to be a limiting factor. If this concentration is high enough, P-release and denitrification takes place simultaneously (Van Stankerburg et. al., 1993). Despite the negative effect of nitrate in anaerbic zone, its existance in aerobic zone does not affect phosphorus uptake. Since many biological phosphorus removal systems involve nitrification and denitrification, the ability for phosphorus-storing microorganisms to reduce nitrate is an important issue. Since they have the potential to remove a large portion of the influent substrate, denitrification in an anoxic zone following the anaerobic zone should be lower compared to possible rates without the anaerobic zone. The phosphorus accumulating bacteria can also take up phosphate under anoxic conditions as nitrate can serve as oxidant which has been demonstrated by Hascoet etal. (1985) and Gerber etal. (1987). However, many investigators have now observed phosphorus uptake in anoxic zones concurrent with nitrate reduction, but it is not known if the denitrification rate is equivalent to that possible if no anaerobic zone and substrate storage exist. It is generaly belived that biological oxication reactions are faster using readily available substrates than intracellular products. Gerber et al. (1987) compares the rate of which phosphate is taken up under aerobic and anoxic conditions and finds that under anoxic conditions the rate is considerably lower than is the case under aerobic conditions. The phosphorus uptake was slower under anoxic conditions, because only part of the phosphorus accumulating bacteria take up phosphate under anoxic conditions, whereas all the phosphorus accumulating bacteria take up phosphate under aerobic conditions (Henze,1993). If the amount of organic matter is limiting to the biological phosphorus removal, anoxic conditions will in that case be regards phosphate removal. XVI Table 1 Average Results (SET 1-SET 6) 90 120 1EO Time (minutes) 180 210 240 270 Figure 2 Variation of phosphorus in cycles XVIII Although an anaerobic zone is a prepequisite for biological P removal, many parameters can positively or negatively affect the degree of P removal, such as the composition of the wastewater, the oxygen supply, nitrate concentration entering the anaerobic zone. It has been generaly accepted that the phosphorus release only occurs under anaerobic conditions in the absence of both oxygen and nitrate. In wastewater treatment plants combining denitrification with biological phosphorus removal, there is a risk that the denitrification will reduce biological phosphorus removal capacity if some of the organic matter taken up by the denitrification bacteria is not utulized for phosphorus release. It has been reported that the presense of nitrate disturbs the anaerobic period (Hascoet, 1985) and inhibits the biological phosphorus removal process in the anaerbic zone because denitrification consumes low fatty acids (Van Stankerburg et. a/., 1993). However, it is also shown that the magnitude of the detrimental effect of nitrate depends on the substrate existing in the anaerobic zone. The low fatty acid concentration in the anaerobic zone is reported to be a limiting factor. If this concentration is high enough, P-release and denitrification takes place simultaneously (Van Stankerburg et. al., 1993). Despite the negative effect of nitrate in anaerbic zone, its existance in aerobic zone does not affect phosphorus uptake. Since many biological phosphorus removal systems involve nitrification and denitrification, the ability for phosphorus-storing microorganisms to reduce nitrate is an important issue. Since they have the potential to remove a large portion of the influent substrate, denitrification in an anoxic zone following the anaerobic zone should be lower compared to possible rates without the anaerobic zone. The phosphorus accumulating bacteria can also take up phosphate under anoxic conditions as nitrate can serve as oxidant which has been demonstrated by Hascoet etal. (1985) and Gerber etal. (1987). However, many investigators have now observed phosphorus uptake in anoxic zones concurrent with nitrate reduction, but it is not known if the denitrification rate is equivalent to that possible if no anaerobic zone and substrate storage exist. It is generaly belived that biological oxication reactions are faster using readily available substrates than intracellular products. Gerber et al. (1987) compares the rate of which phosphate is taken up under aerobic and anoxic conditions and finds that under anoxic conditions the rate is considerably lower than is the case under aerobic conditions. The phosphorus uptake was slower under anoxic conditions, because only part of the phosphorus accumulating bacteria take up phosphate under anoxic conditions, whereas all the phosphorus accumulating bacteria take up phosphate under aerobic conditions (Henze,1993). If the amount of organic matter is limiting to the biological phosphorus removal, anoxic conditions will in that case be regards phosphate removal. XVI Although an anaerobic zone is a prepequisite for biological P removal, many parameters can positively or negatively affect the degree of P removal, such as the composition of the wastewater, the oxygen supply, nitrate concentration entering the anaerobic zone. It has been generaly accepted that the phosphorus release only occurs under anaerobic conditions in the absence of both oxygen and nitrate. In wastewater treatment plants combining denitrification with biological phosphorus removal, there is a risk that the denitrification will reduce biological phosphorus removal capacity if some of the organic matter taken up by the denitrification bacteria is not utulized for phosphorus release. It has been reported that the presense of nitrate disturbs the anaerobic period (Hascoet, 1985) and inhibits the biological phosphorus removal process in the anaerbic zone because denitrification consumes low fatty acids (Van Stankerburg et. a/., 1993). However, it is also shown that the magnitude of the detrimental effect of nitrate depends on the substrate existing in the anaerobic zone. The low fatty acid concentration in the anaerobic zone is reported to be a limiting factor. If this concentration is high enough, P-release and denitrification takes place simultaneously (Van Stankerburg et. al., 1993). Despite the negative effect of nitrate in anaerbic zone, its existance in aerobic zone does not affect phosphorus uptake. Since many biological phosphorus removal systems involve nitrification and denitrification, the ability for phosphorus-storing microorganisms to reduce nitrate is an important issue. Since they have the potential to remove a large portion of the influent substrate, denitrification in an anoxic zone following the anaerobic zone should be lower compared to possible rates without the anaerobic zone. The phosphorus accumulating bacteria can also take up phosphate under anoxic conditions as nitrate can serve as oxidant which has been demonstrated by Hascoet etal. (1985) and Gerber etal. (1987). However, many investigators have now observed phosphorus uptake in anoxic zones concurrent with nitrate reduction, but it is not known if the denitrification rate is equivalent to that possible if no anaerobic zone and substrate storage exist. It is generaly belived that biological oxication reactions are faster using readily available substrates than intracellular products. Gerber et al. (1987) compares the rate of which phosphate is taken up under aerobic and anoxic conditions and finds that under anoxic conditions the rate is considerably lower than is the case under aerobic conditions. The phosphorus uptake was slower under anoxic conditions, because only part of the phosphorus accumulating bacteria take up phosphate under anoxic conditions, whereas all the phosphorus accumulating bacteria take up phosphate under aerobic conditions (Henze,1993). If the amount of organic matter is limiting to the biological phosphorus removal, anoxic conditions will in that case be regards phosphate removal. XVI the batch tests with nitrate, the sludge was fully loaded with nitrate, and the nitrate was always present in the bulk liquid. The acetate consumption rate increased due to nitrate addition because of denitrification, phosphorus released as long as acetate was present. Nitrat was added to batch after a period of anaerobic phosphorus release. However, nitrate did not block phosphorus release, since acetate uptake increased because of high nitrate concentration, released phoshorus/utilized acetate ratio decreased. Comparison of biological P removal under aerobic and anoxic conditions The batch tests with the drawn from the end of anaerobic period of continious SBR were also conducted to compare the aerobic and anoxic phosphorus uptake rates. In one of the batch, nitrate was added to the sludge and other batch having the same sludge was aerated. This batch tests were carried out several times by using the sludge having different characteristics during the whole experimental study. The resuls of anoxic and aerobic phosphorus uptake rates observed during these batch tests are given in Table 2. The phosphate concentration as a function of time during the batch test (No. 3) is also seen in Fig. 4. It shows that phosphate accumulation can occur under both aerobic and anoxic conditions. Table 2 Phosphorus Uptake Rates Under Anoxic and Aerobic Conditions 120 150 180 Time (minutes) 210 2-40 270 300 Figure 4 Effect of aerobic and anoxic conditions