Összefoglalás.
A kibányászható foszfátércek fogyóban vannak. A nemzetközi kutatások alapján a
felmerülő foszfáthiány enyhítésére megoldást jelenthet a szennyvíztelepeken
spontán képződő, és ott üzemeltetési problémát is okozó foszfátásványok
hasznosítása. A vizsgált szennyvíztisztítóban két foszfátásvány, a struvit
([NH4]Mg[PO4]·6H2Orombos) és a
vivianit
(Fe3(PO4)2·8H2Omonoklin)
okoz problémát. A technológiai folyamatban azonosítottuk azt az egyik pontot,
ahol ma – szándékolatlanul – szubmikrométeres vivianitkristályok nukleációja
történhet. E ponton beavatkozva, vagy e pont után egy új műtárggyal tudatosan
erősítve a kristályképződést mind a nyersanyag-leválasztás, mind az üzemeltetési
probléma csökkentése lehetővé válhat.
Summary.
Based on the experience of the past decades, the 21th century is challenged with
several environmental problems that call for a solution at a global level. One
such problem of the foreseeable future is, according to scientific forecasts,
the shortage in phosphate rocks. Phosphate minerals forming spontaneously in
sewage plants and causing operational difficulties at the same time, with a
proper technological design can alleviate the emerging problem of phosphate
shortage.
We examined the phosphate mineral precipitation processes in a large-capacity
sewage treatment plant in Hungary. Both the precipitated phases and phosphorus
content characteristics of the sewage sludge were studied.
X-ray powder diffraction (XPD) was used to identify the minerals. The total
phosphorus content (expressed as phosphate concentration) and the dissolved
orthophosphate content of the sewage sludge samples were measured by molybdenate
spectrophotometric method.
Our findings point to two main phosphate minerals: struvite
(Mg(NH4)[PO4] ·
6H2Oorthorhombic) and vivianite
(Fe3[PO4]2 ·
8H2Omonoclinic) formed as technologically harmful
precipitates at the sewage plant (Figure 1). The two minerals
occur downstream of the digester, at separate, well-defined points of the
technological line (Figure 2). Both crystalline compounds are
potentially suitable for the alleviation of the impending global phosphate
shortage.
We determined the total P content (expressed as phosphate concentration) of
sewage sludge samples, along with the quantitative distribution of the dissolved
(liquid; orthophosphate) and solid (organic phosphate, polyphosphate, phosphate
minerals) fractions of the sludge prior and after the anaerobic digester tanks
(Figure 3). The total P content (expressed as phosphate
concentration) – in full agreement with the expectations – has practically not
changed during digestion (Figure 3; columns # K1 MW vs. 5 MW).
Concerning the P forms present in the sludge we expected an increase of
dissolved orthophosphate at the expense of bonded phosphate after the digestion
(Figure 3; column # „elméleti”); however the actual
orthophosphate content dropped by 80% in the sample after the digester
(Figure 3; columns # K1 vs. 5). The misfit between the
stable total P content and the decreasing amount of both the dissolved
(ortho)phosphate and solid polyphosphate in the digester clearly indicates the
formation of submicroscopic vivianite, confirming from the P speciation side the
findings of Wilfert et al. (2018). That process is triggered by
the addition of FeCl3 into the digester. The more controlled
FeCl3 treatment and/or a new technological step (mineral
separator tank) included right after the digester may help the separation of up
to 50% or more of phosphorous from the sludge in the form of vivianite. By that
step the spontaneous and harmful mineral formation, currently visible on the
technological equipment following the digestion, could also be reduced
significantly.
