Active filtration of phosphorus in Ca-rich hydrated oil shale ash: precipitation mechanisms and recovery
Date
2012-05-16
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Abstract
Fosfor (P) on üks tähtsamaid biokeemilise tsükli toitaineid, mistõttu juba natuke kõrgenenud fosfori kontsentratsioonid võivad vallandada veekogude eutrofeerumise. Fosforiühendid on põllumajanduses väetistena intensiivses kasutuses. Fosforit kasutatakse ka tööstuses ja olmevaldkonnas mitmesugustes detergentides. Seetõttu on põllumajandusmaastikelt äravool ja eriti majapidamiste heitvesi rikastatud fosfaatidega. Siit tuleneb vajadus puhastada inimtegevusega seotud heitveed liigsest fosforist. Tehismärgalade kasutamine heitvee puhastuseks on viimastel kümnenditel kiiresti laienenud tänu nende töökindlusele, suhteliselt odavale ehitusmaksumusele ja lihtsale kasutamisele ning hooldusele. Tehismärgalade efektiivsust saasteainete eemaldamiseks peetakse üldiselt heaks, kuid keskkonnanormatiividega sätestatud piirnormid saastekomponentidele ja üldisele vee kvaliteedile on muutumas järjest rangemaks ning seega kasvab nõudlus tõhusate, kuid ühtlasi ka odavate filtermaterjalide järgi. Lisaks vajab lahendust ka selliste filtersüsteemide sidumisvõime ammendumisel tekkinud toitainetest küllastunud filtermaterjali taaskasutus.
Eesti põlevkivi baasil töötavates elektrijaamades tekib 5 kuni 7 Mt põlevkivituhka aastas, millel peaaegu täielikult puudub taaskasutus, ning kokku on põlevkivituhaplatoodele ladestatud umbes 300 Mt hüdratiseerunud põlevkivituhka. Põlevkivitööstuse tahkete jäätmete taaskasutus on tänaseni olnud äärmiselt piiratud ulatudes kuni 5%-ni tekkiva tuha kogumahust. Varasemad uuringud on näidanud kaltsiumirikkast hüdratiseeritud põlevkivituhast valmistatud filtersüsteemide häid omadusi P eemaldamiseks reoveest tehismärgalasüsteemides. Need katsed on näidanud, et P eemaldamise efektiivsus põlevkivituha katsetes küünib kuni 99,9%, kusjuures materjali maksimaalne sidumisvõime ulatub kuni 65 mg P g-1. Käesolev doktoritöö uurib esiteks fosfori ärastusmehhanisme ja neid kontrollivaid keemilis-mineraloogilisi protsesse põlevkivituhasette filtermaterjalides. Teiseks selgitab uurimistöö küllastunud filtermaterjalide potentsiaalset taaskasutuse võimalust täiendava fosforväetisena põllumajanduses.
Põlevkivituha kui odava filtermaterjali märkimisväärselt kõrget P–sidumisvõimet põhjustavad ja mõjutavad selle materjali koostis ja füüsikalis-keemilised omadused: kaltsiumifaaside lahustuvustasakaalust tingitud poorivee kõrge pH, erineva lahustuvusega mineraalsed Ca-ja Al-ühendid ja poorsusomadused. Siiski on ilmne, et kuigi eksperimendid näitavad P eemaldamisvõimet kuni 91% varieeruvatel fosfori koormustel kuni 8-15 mg P L-1 ja viibeajaga 12 tundi, siis nõnda üledimensioneeritud voolusüsteemides kaotatakse kiiresti tuhafiltermateriali sidumisvõime tänu filtermaterjali pindade nn. keemilisele ummistumisele läbi sekundaarse kaltsiumkarbonaatide sadestumise. Lisaks näitavad uuringutulemused, et hüdratiseeritud põlevkvituha filtrite sidumisvõime sõltub oluliselt P kontsentratsioonist lahuses ning madalatel sisendkoormustel võib tuhasette sidumisvõime olla ebapiisav rangeimate piirväärtuste tagamiseks. Küllastatud materjaliga läbiviidud taaskasutus katsed kinnitavad, et kuigi P on põlevkivituhasettes seotud raskesti lahustuva Ca-fosfaadi vormina, on fosforiga küllastatud põlevkivituhasete siiski sobilik kasutamaks aeglase toimega väetisena näiteks karjamaadel ja metsades.
Phosphorus is an important nutrient in the biochemical cycle. However, already slightly elevated concentrations of phosphorus (>10 µg L–1) can trigger eutrophication. Phosphorus, mainly in the form of soluble phosphate, is intensively used for agricultural and domestic purposes (fertilizers, detergents etc.) and the run-off from agricultural landscapes and especially the domestic wastewaters are enriched in respect to phosphates. Consequently the effluents from human activities must be treated to remove the surplus P, in order to maintain the good health of natural systems. The use of constructed wetlands has extended rapidly as an alternative choice for waste water purification due to their robustness, relatively low cost, easy operation and maintenance. Although the constructed wetlands have proven to be effective for nitrification and have enhanced the removal of total nitrogen, then the capacity of phosphorus removal is not sufficient. Therefore, there is an increasing demand for efficient, but cheap filter materials for phosphorus removal in constructed wetland systems. Also the nutrient-loaded filter material can be used in agriculture as phosphorous fertilizer afterwards. Ca-rich oil shale ash is derived from burning of low-caloric value solid fuel oil shale (kukersite) in the Estonian electric power plants. Oil shale combustion in electric power plants results in generation of large amounts of oil shale ash waste. The amount of ash deposited in waste plateaus ranges between 5 and 7 Mt each year and the total ash waste volume is close to 300 Mt and less than 5% of ash is recycled. Earlier studies have shown that calcareous oil shale ash have efficient phosphorus retention capacities. These experiments have shown the P removal effectiveness for fly ash and hydrated ash in the batch experiment to be up to 99.9%, with maximum binding capacity as high as 65 mg P g–1. The objectives of this thesis are, to reveal the principal chemical–mineralogical processes controlling the P binding in hydrated oil shale ash, to test the binding capacity dynamics and mechanisms on oil shale ash sediment substrates at varying phosphorus loadings and to assess the potential remobilization and reuse of the P from filter materials. The remarkably high P-binding capacity of the oil shale ash filer material is influenced by the complex physical-chemical properties of the material, especially its high content of different Ca- and Al-compounds, porosity and high pH. The P-removal capacity of the Ca-rich hydrated ash-sediment is not influenced by presence of known inhibitor ions (chlorine, magnesium, sulphate etc.), and the filter material retains high removal rate for prolonged periods. However, our results show that the P-removal efficiency using active filtration, which means precipitation of discrete Ca-phosphate phases, depends significantly on the P-concentration in the inflow. The precipitation mechanism is effective only at P-concentrations exceeding 0.5 mg L–1 and at lower concentrations the P-removal is probably governed only by adsorption. Mineralogical and microscopical observations suggest that at longer retention times, supersaturation in respect to Ca-carbonate in filter material containing several soluble Ca-rich mineral phases is attained, which leads to chemical clogging of the ash particles by carbonate percipitates. Phosphorus in alkaline oil shale ash sediment filter material is immobilised in the form of low soluble Ca3(PO4)2 type phase, but the material can be still regarded as a slow-release fertilizer.
Phosphorus is an important nutrient in the biochemical cycle. However, already slightly elevated concentrations of phosphorus (>10 µg L–1) can trigger eutrophication. Phosphorus, mainly in the form of soluble phosphate, is intensively used for agricultural and domestic purposes (fertilizers, detergents etc.) and the run-off from agricultural landscapes and especially the domestic wastewaters are enriched in respect to phosphates. Consequently the effluents from human activities must be treated to remove the surplus P, in order to maintain the good health of natural systems. The use of constructed wetlands has extended rapidly as an alternative choice for waste water purification due to their robustness, relatively low cost, easy operation and maintenance. Although the constructed wetlands have proven to be effective for nitrification and have enhanced the removal of total nitrogen, then the capacity of phosphorus removal is not sufficient. Therefore, there is an increasing demand for efficient, but cheap filter materials for phosphorus removal in constructed wetland systems. Also the nutrient-loaded filter material can be used in agriculture as phosphorous fertilizer afterwards. Ca-rich oil shale ash is derived from burning of low-caloric value solid fuel oil shale (kukersite) in the Estonian electric power plants. Oil shale combustion in electric power plants results in generation of large amounts of oil shale ash waste. The amount of ash deposited in waste plateaus ranges between 5 and 7 Mt each year and the total ash waste volume is close to 300 Mt and less than 5% of ash is recycled. Earlier studies have shown that calcareous oil shale ash have efficient phosphorus retention capacities. These experiments have shown the P removal effectiveness for fly ash and hydrated ash in the batch experiment to be up to 99.9%, with maximum binding capacity as high as 65 mg P g–1. The objectives of this thesis are, to reveal the principal chemical–mineralogical processes controlling the P binding in hydrated oil shale ash, to test the binding capacity dynamics and mechanisms on oil shale ash sediment substrates at varying phosphorus loadings and to assess the potential remobilization and reuse of the P from filter materials. The remarkably high P-binding capacity of the oil shale ash filer material is influenced by the complex physical-chemical properties of the material, especially its high content of different Ca- and Al-compounds, porosity and high pH. The P-removal capacity of the Ca-rich hydrated ash-sediment is not influenced by presence of known inhibitor ions (chlorine, magnesium, sulphate etc.), and the filter material retains high removal rate for prolonged periods. However, our results show that the P-removal efficiency using active filtration, which means precipitation of discrete Ca-phosphate phases, depends significantly on the P-concentration in the inflow. The precipitation mechanism is effective only at P-concentrations exceeding 0.5 mg L–1 and at lower concentrations the P-removal is probably governed only by adsorption. Mineralogical and microscopical observations suggest that at longer retention times, supersaturation in respect to Ca-carbonate in filter material containing several soluble Ca-rich mineral phases is attained, which leads to chemical clogging of the ash particles by carbonate percipitates. Phosphorus in alkaline oil shale ash sediment filter material is immobilised in the form of low soluble Ca3(PO4)2 type phase, but the material can be still regarded as a slow-release fertilizer.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
heitvesi, vee filtreerimine, aktiivfiltrid, põlevkivituhk, fosfor, waste water, water filtering, active filters, oil shale ash, phosphorus