The scope of this thesis was to lay the basis for a phosphate recovery technology that can be applied on sewage sludge containing iron phosphate. Such a technology should come with minimal changes to the existing sludge treatment configuration while keeping the use of chemicals or energy as small as possible. The research focused on understanding the exact mechanism for phosphate release from iron in sewage sludge in order to find a method to release phosphate in an elegant way. Phosphate is an essential nutrient for plant growth, but at the same time the resources of phosphate are limited and concentrated in a few countries outside Europe. Recovery of phosphate can secure the access to phosphate for food production and is therefore an important topic.
Iron based phosphate removal is still used by a majority of sewage treatment plants (STPs) but no viable technology is available to recover phosphate from sludge without sludge incineration. The addition of iron is a convenient way for removing phosphate from wastewater, but this is often considered to limit phosphate recovery. Struvite precipitation is currently used to recover phosphate, and this approach has attracted much interest. However, it requires the use of enhanced biological phosphate removal (EBPR). Phosphate removal relying solely on EBPR is not yet widely applied and the recovery potential is low (<50%). Other phosphate recovery methods, including sludge application to agricultural land or recovering phosphate from sludge ash, also have limitations. Energy-producing STPs increasingly rely on phosphate removal using iron, but the problem (as in current processes) is the subsequent recovery of phosphate from the iron. In contrast, phosphate is efficiently mobilized from iron by natural processes in sediments and soils. Iron–phosphate chemistry is diverse, and many parameters influence the binding and release of phosphate, including redox conditions, pH, presence of organic substances, and particle morphology. The current poor understanding of iron and phosphate chemistry in sewage systems is preventing processes being developed to recover phosphate from iron–phosphate rich wastes like municipal wastewater sludge. In the first chapter parameters that affect phosphate recovery were reviewed, and methods are suggested for manipulating iron–phosphate chemistry in wastewater treatment processes to allow phosphate to be recovered.
Iron is omnipresent in STPs. It can be present unintentionally, for e.g. due to groundwater seepage into sewers, or it is intentionally added for odour and corrosion control, phosphate removal or prevention of hydrogen sulphide emissions into the biogas. The strong affinity of iron to phosphate has advantages for efficient removal of phosphate from sewage but it may also reduce recovery efficiencies in struvite precipitation technologies or for some phosphate recovery methods from ash. On the other hand iron may also have positive effects on phosphate recovery. Acid consumption was reported to be lower when leaching phosphate from sewage sludge ash with higher iron content. Also, phosphate recovery efficiencies may be higher if an iron phosphate compound, like vivianite, Fe(II)3(PO4)2x8H2O, could be harvested from sewage sludge. Developers of phosphate recovery technologies should be aware of the potential and obstacles the iron and phosphate chemistry bears.
The mineral vivianite, is already present in digested sewage sludge and can be an alternative phosphate recovery option to current technologies. To evaluate this, surplus and digested sewage sludge was sampled from full-scale STPs and analysed using XRD, (e)SEM-EDX and Mössbauer spectroscopy. Vivianite was observed in all plants where iron was used for phosphate removal. In surplus sludge before the anaerobic digestion ferrous iron dominated the iron pool (≥50%). XRD and Mössbauer spectroscopy showed no clear correlation between vivianite bound phosphate versus the iron content in surplus sludge. In digested sludge, ferrous iron was the dominant iron form (>85%). Phosphate bound in vivianite increased with the iron content of the digested sludge but levelled off at high iron levels. 70-90% of all phosphate was bound in vivianite in the sludge with the highest iron content (molar Fe:P = 2.5). The quantification of vivianite was difficult and bears some uncertainty probably because of the presence of impure vivianite as indicated by SEM-EDX. eSEM-EDX indicates that the vivianite occurs as relatively small (20 -100 µm) but free particles that could potentially be separated from the sludge. We hypothesize that chemical/microbial Fe(III) reduction is relatively quick and triggers vivianite formation in the treatment lines. Once formed, vivianite may endure oxygenated treatment zones due to slow oxidation kinetics and due to oxygen diffusion limitations into sludge flocs.
It was shown that vivianite can indeed form relatively quickly in activated sludge systems. Kinetics of iron reduction, the microbial community and the mechanism of vivianite formation in activated sludge from two STPs were studied; one STP with a low iron dosing (STP Leeuwarden, EBPR) and the other STP with a high iron dosing (STP Cologne, applying chemical phosphorous removal, CPR) were studied. The sludges were incubated under anaerobic conditions in batch experiments. The iron reduction rate in the CPR sludge (2.99 mg-Fe g VS-1 h-1) was 3 times higher than the rate observed in the EBPR sludge (1.02 mg-Fe g VS-1 h-1). The higher iron reduction rate in the CPR sludge is probably caused by its 3 times higher iron content. The rate constants (k) in both sludges are comparable (0.06 h-1 in EBPR sludge vs 0.05 h-1 in CPR sludge), thus the potential rates in both sludges are similar. For calculating the time it takes to turn over all Fe(III) to Fe(II) in the sludge, the Fe(III) reduction rates at the total ferric iron content of the experiments were used and assumed to be constant over time. Calculations then suggest that all iron in STP Leeuwarden and STP Cologne can be turned over within 15 h and 44 h respectively. Sequencing showed that both of the sludges were dominated by proteobacteria (65 – 89% of all operational taxonomic units, OTUs) and that the dominant class of bacteria were β-proteobacteria (38-63% of all OTUs). The microbial communities in both sludges contained genera that comprise iron oxidizing and iron reducing bacteria. These genera were more abundant in the CPR sludge with a higher iron content. XRD and Mössbauer spectroscopy showed that significant quantities of vivianite were formed in the sludges within 24 h. Our study suggests that iron metabolizing bacteria are more abundant in sludge which is rich in iron and that significant vivianite formation can already take place before the anaerobic digestion process.
Based on the findings, vivianite is the most important phosphate phase provided enough iron is present, vivianite separation from sewage sludge was studied using a tailor made magnetic separator. Vivianite particles are paramagnetic and present as free particles. Magnetism is an elegant technology as it exclusively separates the liberated and paramagnetic vivianite (and perhaps some pyrite or iron carbonates that are present in the sludge). For this purpose a magnetic separator with Jones magnetic plates was designed and tested on two digested sewage sludges with different iron content. Varying feeding rates were used for the separation. A higher phosphate separation efficiency was achieved with sludge that contained more iron (up to 60% of all input phosphate was recovered) compared to the sludge with lower iron contents (up to 40% of all phosphate could be recovered). The iron and phosphate content was double sometimes even three times higher in the separated (magnetic) fraction when compared to the initial sludge solids. The crystalline fraction of the separated material consisted mainly of vivianite (68%) but also quartz was found (32%) as shown by XRD. The separated material had still a relatively high volatile solid content ranging between 30 – 40% of the dry matter. This fraction is related to organic compounds and other compounds that lose weight during heating (such as carbonates or vivianite). Based on these observations a new phosphate recovery technology for vivianite containing sludge was proposed that makes use of relatively cheap magnetic separation equipment from the mining industry. In this process iron is dosed in high quantities during the treatment process. This would result not only in low effluent phosphate concentrations but, additionally, vivianite formation is not limited by iron during the anaerobic digestion and this would probably result in the transformation of all available phosphate to vivianite. Then vivianite can be separated using a magnetic separator. This separation could be combined with a liberation or pre-separation step by using e.g. a hydrocyclone. Once vivianite is separated from sludge it could be directly used, preferably to produce high valuable products, or it could be dissolved to produce fertilizer. Pure vivianite can easily be dissolved at alkaline pH of about 12. At this pH, phosphate goes in solution and iron and most other metals remain in the precipitate. The phosphate solution obtained from the separated vivianite can directly be used for fertilizer production. Iron could be re-used for phosphate elimination in the STP.
In another study it was tested whether sulphide can help to release and recover phosphate from sewage sludge. A series of batch experiments were conducted on different synthetic iron phosphates: Fe(III)P purchased from Sigma, Fe(III)P synthesized in the lab and vivianite. Sulphide was added to these different iron phosphates in a molar Fe:S ratio of 1 to evaluate the total phosphate release and the kinetics of phosphate release into solution. Phosphate release was usually completed within 1 hour. The maximum phosphate release was 92%, 60% and 76% from vivianite, Sigma Fe(III)P and Fe(III)P synthesized in the lab, respectively. However, rebinding of the released phosphate by Fe(II), only in the experiment with Fe(III)P that was synthesized in the lab, reduced the net phosphate release to about 56%. Sulphide induced phosphate release from vivianite is more efficient because sulphide reacts directly with Fe(II) to form FeSx and releases phosphate. No additional sulphide is needed for reducing Fe(III) to Fe(II). At the same time Fe(II) in vivianite is probably more efficient, or as efficient, as Fe(III) in retaining phosphate. Phosphate release from Fe(III)P was, at its maximum (before re-sorption/re-precipitation of the phosphate to other compounds in the sludge) higher than stoichiometry would suggest. Probably because sulphide was acting as a reducing agent, without significant formation of FeSx. FeSx formation requires a larger sulphide input. The high efficiency (moles P released / moles S input) of sulphide acting as a reducing agent to release phosphate was confirmed in additional experiments where sulphide was slowly added to Fe(III)P. Moreover, sulphide addition experiments showed that up to 30% of all phosphate could be released from digested sewage sludge. The highest phosphate release was achieved in experiments with the highest iron content. The total phosphate release from digested sludge was not as high as expected, earlier measurements using XRD and Mössbauer spectroscopy, that were used to quantify iron bound phosphate in the digested sludges, suggested that more phosphate should be iron bound and hence sulphide extractable. The dewaterability (determined using capillary suction test) in digested sludge (0.13 ±0.015 g2(s2 m4)-1) dropped significantly after sulphide was added (0.06 ±0.004 g2(s2 m4)-1). This strongly suggests that sulphide addition to sewage sludge will result in higher sludge disposal costs. Only insignificant phosphate release (1.5%) was observed from sewage sludge ash in response to sulphide addition. Overall, sulphide showed to be a useful tool to release phosphate bound to iron from sewage sludge for its subsequent recovery. Drawbacks are the deterioration of the sludge dewaterability and a net phosphate release that is lower than expected.
In a side project of this thesis biogenic iron oxides (BioFeO) formed by Leptothrix sp. and Gallionella sp. were compared with chemically formed iron oxides (ChFeO) for their suitability to remove and recover phosphate from solutions. The ChFeO used for comparison included a commercial iron based adsorbent (GEH®) and chemical precipitates. Despite contrary observations in earlier studies, our batch experiments showed that BioFeO do not have superior phosphate adsorption capacities compared to ChFeO. However, it seems multiple mechanisms are involved in phosphate removal by BioFeO which make their overall phosphate removal capacity higher than that of ChFeO. The overall phosphate removal capacity of Leptothrix sp. was 26.3 mg P/g dry matter (d.m.), of which less than 6.4 mg P/g d.m. was attributed to adsorption. The main removal is likely due to formation of organic iron phosphate complexes (19.6 mg P/g d.m.). Gallionella sp. had an overall phosphate removal capacity of 39.6 mg P/g d.m. Significant amounts of phosphate were apparently incorporated into the Gallionella sp. stalks during their growth (31.0 mg P/g d.m.) and only one fourth of the total phosphate removal can be related to adsorption (8.6 mg P/g d.m.). Their overall ability to immobilize large quantities of phosphate from solutions indicates that BioFeO could play an important role in environmental and engineered systems for removal of contaminants such as phosphate or arsenic.
This thesis showed that the iron phosphate chemistry in STPs has been neglected in the past and that more research is necessary to understand the complex interactions between iron and phosphate. This knowledge would help to improve the use of iron in STPs for phosphate removal further and pave the way for new phosphate recovery technologies from iron rich sewage sludge. Within the framework of this research the mineral vivianite was identified as a main iron phosphate phase in sewage sludge. Phosphate recovery technologies via vivianite might lead to a significantly higher recovery efficiency compared to routes relying on struvite. Magnetic separation of vivianite from sewage sludge was achieved using equipment from the mining industry. This process will be tested on pilot scale next. Future research related to vivianite based phosphate recovery has to focus on (I) understanding the formation of vivianite in STPs, (II) improving the separation efficiency of vivianite from sewage sludge using equipment that is tailor made for the type of vivianite which is contained in the sludge (density, magnetic susceptibility etc.) or by manipulating the formation of vivianite (by e.g. increasing its particle size) and (III) evaluating the purity of vivianite in sewage sludge to determine its economic value.