Szennyvízkezelés során képződő, technológiai szempontból káros ásványkiválások feltérképezése és javaslattétel ezek hasznosítására

Abstract

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.