Synergy of phosphate recovery from sludge-incinerated ash and coagulant production by desalinated brine.

Wet-chemical approach is widely applied for phosphate recovery from incinerated ash of waste activated sludge (WAS), along with metals removed/recovered. The high contents of both aluminum (Al) and iron (Fe) in WAS-incinerated ash should be suitable for producing coagulants with some waste anions like Cl- and SO42- With acid (HCl) leaching and metals' removing, approximately 88 wt% of phosphorus (P) in the ash could be recovered as hydroxylapatite (HAP: Ca5(PO4)3OH); Fe3+ in the acidic leachate could be selectively removed/recovered by extraction with an organic solvent of tributyl phosphate (TBP), and thus a FeCl3-based coagulant could be synthesized by stripping the raffinate with the original brine (containing abundant Cl- and SO42-). Furthermore, a liquid poly-aluminum chloride (PAC)-based coagulant could also be synthesized with Al3+ removed from the ash and the brine, which behaved almost the same in the coagulation performance as a commercial coagulant on both phosphate and turbidity removals. Both P-recovery from the ash and coagulant production associated with the brine would enlarge the markets of both 'blue' phosphate and 'green' coagulants.

Effect of surfactants on phosphorus release and acidogenic fermentation of waste activated sludge containing different aluminium phosphate forms.

The effects of different surfactants (rhamnolipid, trehalolipid and citrate) on phosphorus (P) release and acidogenic fermentation of waste activated sludge (WAS) containing different aluminium phosphate forms (AlPO4, Al(PO3)3) were investigated. Results showed that rhamnolipid was the most effective surfactant to release P from aluminum phosphates (AlPs)-rich sludge. Al(PO3)3 was easier to release P than AlPO4 in WAS due to their different crystal structures. Different surfactants promoted the production of different types of protein. The addition of rhamnolipid was conducive to produce propionate from WAS, while trehalolipid and citrate increased the production of n-butyrate and acetate, respectively. Citrobacter played an important role in producing phosphatase continuously for P release with rhamnolipid addition. Predictive functional profiling indicates that rhamnolipid greatly facilitated adenosine triphosphate (ATP)-binding cassette transporter and quorum sensing. These important discoveries help to enrich P recovery paths from sludge produced with Al-based coagulants in wastewater treatment plants.

Effects of ferric-phosphate forms on phosphorus release and the performance of anaerobic fermentation of waste activated sludge.

Five ferric-phosphate (Fe(III)Ps) with amorphous or crystalline structures were added to waste activated sludge (WAS) for anaerobic fermentation, aiming to investigate effects of Fe(III)Ps forms on phosphorus (P) release and the performance of WAS fermentation. The results revealed that the Fe(III) reduction rate of hexagonal-FePO4 was faster than that of monoclinic-FePO4·2H2O, thanks to its lower crystal field stabilization energy. FePO4·nH2O was reduced to vivianite and part of the phosphate was released as orthophosphate (PO4-P). Giniite (Fe5(PO4)4(OH)3·2H2O) as an iron hydroxyphosphate was transformed to ßFe(III)Fe(II)(PO4)O-like compounds without PO4-P release. In addition, Fe(III)Ps had an adverse effect on the anaerobic fermentation of WAS. The specific hydrolysis rate constant and volatile fatty acids (VFAs) yield decreased by 38.4% and 41.9%, respectively, for the sludge sample with amorphous-FePO4·3H2O, which dropped the most. This study provides new insights into various forms of Fe(III)Ps performance during anaerobic fermentation and is beneficial to enhancing P recovery efficiency.

Migration and transformation mechanism of phosphorus in waste activated sludge during anaerobic fermentation and hydrothermal conversion.

This study investigated migration and transformation mechanism of P in waste activated sludge (WAS) during anaerobic fermentation (AF) process and the subsequent hydrothermal conversion (HTC) process. Control of pH during the AF processes was found to be significant, whereby the use of acidic (pH = 5.5) or alkaline conditions (pH = 9.5) facilitated the release of either apatite phosphorus (AP) or non-apatite inorganic phosphorus (NAIP) and organic phosphorus, respectively. At the same pH of 9.5, NaOH promoted the transfer of P into liquid phase, and P in the solid phase was mainly in the form of NAIP. In contrast, Ca(OH)2 enhanced the incorporation of P into the solid products, with the P mainly in the form of AP. The subsequent HTC process promoted the NAIP transferred to AP, and the bioavailability of P in the HTC solid products was decreased. The P K-edge X-ray absorption near edge structure analysis provided detailed information about the phosphates. It demonstrated that the conversion of Ca8H2PO4·6.5H2O to Ca5(PO4)3·OH was facilitated by HTC under the alkaline condition. This study sheds lights on transformation mechanism of P speciations during AF and HTC processes, which would provide fundamental information for effective utilization of P in bio-wastes.

A novel approach of synchronously recovering phosphorus as vivianite and volatile fatty acids during waste activated sludge and food waste co-fermentation: Performance and mechanisms.

This research proposed an innovative approach to synchronously enhance the recovery of phosphorus (P) as vivianite and volatile fatty acids (VFAs) during waste activated sludge (WAS) and food waste (FW) co-fermentation. A high performance was achieved under 30% FW addition and pH uncontrolled, which gained 83.09% of TP recovery as high-purity vivianite (93.90%), together with efficient VFAs production (7671 mg COD/L). The FW supplement could enhance VFAs production and subsequently lower pH to contribute to the release of Fe2+ and PO43-. Also, it could dampen disrupting effects of strong acidic pH on microbial cells (lowering LDH release). Moreover, the flexible pH variation caused by biological acidification could maintain relatively higher microbial activities (increasing enzymes' activities), which was advantageous to the biological effects involved in Fe2+ and PO43 release and VFAs generation. Therefore, this research provide a promising and economic alternative to dispose of WAS and FW simultaneously for valuable resource recovery.

Enhanced volatile fatty acids production from waste activated sludge with synchronous phosphorus fixation and pathogens inactivation by calcium hypochlorite stimulation.

An efficient approach for synchronous volatile fatty acids (VFAs) promotion, phosphorus fixation and pathogens inactivation during waste activated sludge (WAS) anaerobic fermentation was achieved with optimal calcium hypochlorite (Ca(ClO)2) stimulation. The maximal VFAs were 3.6 folds of control in reactors with 0.01 g Ca(ClO)2/g TSS addition. The low dosage of Ca(ClO)2 enhanced WAS solubilization and hydrolysis by disrupting the extracellular polymeric substance (EPS) effectively. Sufficient substrates for fermentative bacteria were thereby provided with the maintenance of acceptable microbial activity and viability. However, high dosage of Ca(ClO)2 deteriorated the performance of anaerobic fermentation due to its strong oxidative ability, resulting in cell lysis greatly. Moreover, the largely released phosphorus during WAS fermentation was effectively precipitated and removed by the combination of Ca2+ at 0.01 g Ca(ClO)2/g TSS dosage. In addition, Ca(ClO)2 had distinguished effects on pathogens inactivation. The simultaneous phosphorus fixation and pathogens reduction during VFAs production increased the utilization value of fermentation liquid and benefitted the further disposal of fermented sludge.

Effect of complexing agents on phosphorus release from chemical-enhanced phosphorus removal sludge during anaerobic fermentation.

Phosphorus (P) release from sludge containing phosphate precipitates (FePs or AlPs) as well as the anaerobic performance with the addition of complexing agents (citric, tartaric and EDTA) during ambient anaerobic fermentation process were investigated. Results showed that citrate addition was the most effective method to enhance P release from inorganic phosphate by chelation and promote volatile fatty acids (VFAs) production simultaneously during anaerobic fermentation. Equimolar citrate addition with chemical precipitates was the optimal dosage. Microbial analysis revealed that EDTA has the strongest inhibitory effect on microbial activity and community structure, while citrate was more effective in enhancing important acidifying microorganisms than tartrate and EDTA. Therefore, citrate addition can be regarded as an alternative and promising method to recover P and carbon source from sludge containing chemical precipitates. These important discoveries will help to enrich P recovery path from sludge produced in the chemical-enhanced P removal treatment processes.

Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation.

This study presented an innovative method for phosphorus (P) recovery as vivianite from waste activated sludge (WAS) via optimizing iron dosing and pH value during anaerobic fermentation (AF). The optimal conditions for vivianite formation were in the pH range of 6.0-9.0 with initial PO43- >5 mg/L and Fe/P molar ratio of 1.5. Notably, FeCl3 showed advantages over ZVI for the simultaneous release of Fe2+ and PO43- during WAS fermentation, especially in acidic conditions. The FeCl3 dosing at pH 3.0 could contribute to 78.81% Fe2+ release and 85.69% of total PO43- release from WAS. They were ultimately recovered in the form of high-purity vivianite (93.67%). Clostridiaceae (40.25%) was the predominant bacteria in FeCl3-pH3 reactors, which played key roles in inducing dissimilatory iron reduction for Fe2+ formation. Therefore, P recovery as vivianite from WAS fermentation might be a promising and highly valuable approach to relieve the P crisis.

Enhancement of the sludge disintegration and nutrients release by a treatment with potassium ferrate combined with an ultrasonic process.

Sludge disintegration by ultrasound is a promising sludge treatment method. In order to enhance the efficiency of the sludge reduction and hydrolysis, potassium ferrate (K2FeO4) (PF) was used. A novel method was developed to improve the sludge disintegration-sludge pretreatment by using PF in combination with an ultrasonic treatment (PF + ULT). After a short-term PF + ULT treatment, 17.23% of the volatile suspended solids (VSS) were reduced after a 900-min reaction time, which is 61.3% higher than the VSS reduction for the raw sludge. The supernatant soluble chemical oxygen demand (SCOD), total nitrogen (TN), volatile fatty acids (VFAs), soluble protein and polysaccharides increased by 522.5%, 1029.4%, 878.4%, 2996.6% and 801.9%, respectively. The constituent parts of the dissolved organic matter of the sludge products were released efficiently, which demonstrated the positive effect caused by the PF + ULT. The enhanced sludge disintegration process further alleviates environmental risk and offers a more efficient and convenient method for utilizing sludge.

Microwave-assisted digestion and NaOH treatment of waste-activated sludge to recover phosphorus by crystallizing struvite.

A number of studies of waste-activated sludge (WAS) pretreatments, aimed at releasing phosphorus (P) from WAS and increasing the amount of P that can be recovered, have been performed. Here, a microwave-assisted digestion and NaOH treatment (MWs & NaOH) coupled crystallizing struvite, to promote the solubilization, transformation, and recovery of P from WAS, is proposed. Microwaves (MWs) can cause cavities to form in WAS, weakening the bonds between extracellular polymeric substances and the solid phase. Irradiating with MWs significantly increased the efficiency at which P was dissolved (i.e. transferred from the solid to the liquid phase) and the efficiency at which organic P was hydrolyzed and transformed into inorganic P when the NaOH treatment was performed. The P solubilization and transformation characteristic achieved in different treatments was examined by scanning electron microscopy and three-dimensional excitation emission matrix analysis. The MWs & NaOH method released 34.20-43.73% of total P from WAS, and 23.48-32.07% of the total P was recovered by crystallizing struvite at pH 9.5 and Mg:P ratio of 1.5:1. It would cost about USD 85-103 per ton of dry WAS to treat WAS using the MWs & NaOH method.