MODELLING OF MAGNESIUM OXIDE APPLICATION FOR STRUVITE PRECIPITATION

Transcription

1 MDELLING F MAGNESIUM XIDE APPLIATIN FR STRUVITE PREIPITATIN J. Mzierski * nd S. Połwski ** * Polish Acdemy of Science, Institute of Environmentl Engineering, ul. M. Skłodowskiej-urie, -89 Zbrze, Polnd (E-mil: mzier@iis.zbrze.l) ** University of Bielsko-Bił, Institute of Environmentl Protection nd Engineering, ul. Willow, -09 Bielsko-Bił, Polnd (E-mil: solwski@th.bielsko.l) ABSTRAT The kinetics of struvite reciittion in digestion sludge suerntnt ws described. The effect of ertion nd the mgnesium comound solubility ws investigted. The kinetic eqution ws roosed nd constnt rte vlues were clculted. The removing from digestion sludge suerntnt nd solubility of Mg comound hve min effect on kinetics rocess. EYWRDS struvite, rection kinetics, reciittion. INTRDUTIN It is well known tht ctivted sludge bcteri ccumulte hoshte ions in erobic conditions. In the sludge nerobic digestion rocess reviously formed oly-hoshte hydrolyze to hoshte, nd the high mount of mmonium nd crbon dioxide were formed too s finl roduct. The extremely high mmoni nd hoshte concentrtions re resent in digested sludge suerntnt. In the tmosheric ressure the crbon dioxide solubility decrese nd in the resence of smll mounts mgnesium ions cused the struvite crystls re formed: Mg N P MgN P 6 () Struvite formtion highly deend on substrtes concentrtions nd vlue. In digested sludge suerntnt the following ions equilibriums cn be written s follow: - crbon dioxide equilibrium: k =.7 () = () = ()

2 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion - mmoni equilibrium: N N b = (5) - hoshte equilibrium: P P = (6) P P = (7) P P = (8) n the bsis bove equilibrium the concentrtions of wek cid nd bses for the row digested sludge suerntnt my be clculted from the equtions: = (9) = (0) = () P P = () P P = () P P = () P P P = (5) Digested sludge suerntnt vlue is controlled by the crbonceous system, nd its decresing is necessry to hoshte P - ion concentrtion incresing. It my be done by the suerntnt ertion or by bse ddition. The mgnesium ions deficit cn be eliminte by soluble or insoluble mgnesium comounds ddition. The effect of the lkliztion method, the kind nd dose of mgnesium comound nd the stirring intensity on the struvite reciittion kinetics ws the im of our investigtion. METDS The investigtion on struvite reciittion were crried out in the 000 ml rectors. The mgnesium oxide or mgnesium chloride solution were used. s the source of mgnesium ions. The rection mixture ws stirred nd erted or 0, M sodium hydroxide solution ws dded to decrese vlue.

3 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion The vlue nd mmoni concentrtion were mesured electrochemiclly by glss nd ion selective electrode. The hoshte concentrtion ws determined colorimetriclly by vndomolybdo hoshoric cid method. The kinetics of minerl reciittion for mny soluble slts tye ws described by outsoukos s follow: dm = ks v m v v / m v ( M A ) M A v ( ) / v v v dt A where: M m, A -, M m, A - re the concentrtions of crystl lttice ions in the solution nd t equilibrium resectively k reciittion rte constnt s fctor roortionl to the totl number of growth sites on the dded seed mteril n fctor determined exerimentlly In our exeriments following simlifying ssumtions were done: - the mmoni to hoshte molr rtio is high nd the mmoni concentrtion hs no effect to reciittion kinetics - similrly the mgnesium comounds were dded in excess nd the effect of mgnesium concentrtion cn be negligible. Finlly the following kinetic eqution describing hoshte concentrtion is follow: n (6) dc dt ( c ) ' = k c (7) where: c- hoshte concentrtion c - equivlent hoshte concentrtion The constnt rte k my be determined from the following integrted form: c 0 c ' ln = k t c c (8) The hoshte concentrtion cn be clculted on the bsis of eqution: k ( c c ) e ' t c = 0 c (9) RESULTS AND DISUSSIN In the first exeriment series stirring nd ertion ws used to reelling crbon dioxide nd to incresing. The result showed ertion nd stirring intensity incresing hd the ositive effect on hoshte reciittion. It ws observed both Mg or Mgl ws used s mgnesium ions source, lthough liction of soluble mgnesium comound were fvorble.

4 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure. The hoshte reciittion kinetics in stirred smle (00 min - ), Mg dose 00 mg/l. Figure b. The hoshte reciittion kinetics in stirred smle (00 min - ), Mg dose 00 mg/l.

5 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure. The hoshte reciittion kinetics in erted smle, Mg dose 86 mg/l. Figure b. The hoshte reciittion kinetics in erted smle, Mg dose 86 mg/l. 5

6 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure. The hoshte reciittion kinetics in stirred smle (00 min - ), Mgl dose 6 mg/l. Figure b. The hoshte reciittion kinetics in stirred smle (00 min - ), Mgl dose 6 mg/l. 6

7 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure. The hoshte reciittion kinetics in erted smle, Mgl dose 5 mg/l. Figure b. The hoshte reciittion kinetics in erted smle, Mgl dose 5 mg/l. In the gently erted smle the vlue were 9.5 nd bove units, the mixing of the smle cused incresing only to 9. units. 7

8 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion 9,8 9,6 9, 9, 9 8,8 8,6 8, 8, time h Figure 5. vlue in stirred nd erted smles. stired I stired II erted I erted II The effect of ertion cn be negligible, when the sodium hydroxide solution were used to correction. The finl vlues were 9,5 units nd bove. In these cses the effect of the good solubility mgnesium comound ws clerly observed. The some highest rection rte were obtined when mgnesium chloride were used. Figure 6. The hoshte reciittion kinetics in lklized nd stirred smle (00 min - ), Mg dose mg/l. 8

9 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure 6b. The hoshte reciittion kinetics in lklized nd stirred smle (00 min - ), Mg dose mg/l. Figure 7. The hoshte reciittion kinetics in lklized nd erted smle (00 min - ), Mg dose 99.7 mg/l. 9

10 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure 7b. The hoshte reciittion kinetics in lklized nd erted smle (00 min - ), Mg dose 99.7 mg/l. Figure 8. The hoshte reciittion kinetics in lklized nd stirred smle (00 min - ), Mgl dose 60. mg/l. 0

11 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure 8b. The hoshte reciittion kinetics in lklized nd stirred smle (00 min - ), Mgl dose 60. mg/l. Figure 9. The hoshte reciittion kinetics in lklized nd erted smle, Mgl dose. mg/l.

12 Mzierski & Połwski, Modeling of mgnesium oxide liction for struvite reciittion Figure 9b. The hoshte reciittion kinetics in lklized nd erted smle, Mgl dose. mg/l. REFERENES outsoukos P., Amjd Z., Tomson M.B. nd Nncolls G.. (980). rystlliztion of clcium hoshtes: A constnt comosition study. J. Am. hem. Soc., 7,