Phosphate Recovery From Sewage Sludge Containing Iron Phosphate
Year: 2018
Type: dissertation
Abstract: 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 ma
Author: Wilfert, P.K.
Language: English
Subfield: Waste Management and Disposal
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2789415802