Trace metal elements vaporization and phosphorus recovery during sewage sludge thermochemical treatment - A review

Attempts to obtain clean biochar from hyperaccumulator through pyrolysis: Removal of heavy metals and transformation of phosphorus

The sound disposal of the ensuing heavy metal-rich plants can address the aftermath of phytoremediation. In this study, the first attempt was made to obtain heavy metals-free and phosphorus-rich biochar from phytoremediation residue (PR) by pyrolysis, and the effects of chlorinating agent type, chlorine dosage, and pyrolysis residence time on heavy metal removal, phosphorus (P) transformation, and biochar properties were investigated. The results showed that as chlorine dosage and pyrolysis residence time increased, added polyvinyl chloride (PVC) reduced the concentration of Zn in biochar to one-tenth of that in PR by intensified chlorination, where both Zn concentration (2727.50 mg/kg) and its leaching concentration (29.13 mg/L) met the utilization requirements, in which the acid-base property of biochar plays a key role in heavy metal leaching. Meanwhile, more than 90% of P in PR remained in biochar and the bioavailability of P in biochar enhanced with the decomposition of organic P to inorganic P, where the concentration of plant-availability P (Pnac) expanded from 1878.40 mg/kg in PR to 8454.00 mg/kg in biochar. This study demonstrated that heavy metal hyperaccumulator can be converted into heavy metal-free and phosphorus-rich biochar with promising applications, which provides new perspectives for the treatment of such hazardous wastes.

The high dose of biochar reduces polycyclic aromatic hydrocarbons losses during co-composting of sewage sludge and wheat straw

The aim of the study was to investigate the effect of the biochar (BC) dose on solvent extractable (Ctot) and freely dissolved (Cfree) polycyclic aromatic hydrocarbons (PAHs) content during co-composting. A significantly better reduction of Σ16 Ctot PAHs after 98 days occurred during composting with BC (for 1% of BC - 44% and for 5% of BC - 23%) than in the control (15%). Despite the relatively high reduction of Ctot PAHs in the experiment with 5% BC rate, the content of the PAHs was still the highest compared to other variants. Regarding Cfree PAHs, 5% rate of BC resulted in the best reduction of PAHs, while the 1% BC dose resulted in a lower reduction of Cfree than the control. For 1% BC, PAHs losses was more effective, and sequestration processes played a less significant role than in the experiment with 5% dose of BC. The total and dissolved organic carbon, and ash were predominantly responsible for Ctot and Cfree losses, and additionally pH for Cfree. The results of the experiment indicate that BC performs a crucial role in composting, affecting the Ctot and Cfree PAHs in the compost but the final effect strictly depends on the BC dose.

Phosphate recovery from sludge-incinerated ash by adsorption with hydrotalcite synthesized by metals in the ash

Selective adsorption of phosphorus (P) from the acidic leachate of sludge-incinerated ash (SIA) becomes more attractive due to avoiding removing heavy metals. Especially, layered double hydroxides (LDHs) as an anion adsorbent could be applied into this area owing to their good capacity on P-adsorption and low cost on preparation. Interestingly, SIA contains more aluminum (Al) and iron (Fe) needed to be removed prior to P-recovery, and removed Al and Fe could be utilized to synthesize LDHs, like Mg/Al-LDH and Mg/Fe-LDH. With this study, Mg/Al-LDH-r and Mg/Fe-LDH-r were economically synthesized with Al and Fe removed from SIA, which were similar in their chemical structures to commercial LDHs. The synthesized LDHs had a high P-adsorption capacity, up to 95.0%. The maximal phosphate capacity of the recovered LDHs (Mg/Al-LDH-r and Mg/Fe-LDH-r) was 239.0 and 199.8 mg P/g LDHs, respectively. "NaOH + desalinated brine" as a new desorption solution could achieve a desorption ratio at about 80%, which could reduce the liquid-solid ratio by at least 60%, greatly decreasing the desorption cost. Pot trials demonstrated that the desorbed and precipitated CaP could promote the growth of maize as well as a commercial P-fertilizer. Furthermore, the adsorbed phosphate by LDHs could be directly used as a slow-released P-fertilizer and also improve the pH value of acidic soil, completely deleting the desorption process.

Phosphorus recovery from sewage sludge ash (SSA): An integrated technical, environmental and economic assessment of wet-chemical and thermochemical methods

Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5∼6 €/kg P, while the cost of recovering P by thermochemical methods is about 2∼3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions.

Transformation of Zn and Cr during co-combustion of sewage sludge and coals: influence of coal and steam

Combustion experiments of sewage sludge (SS) blended with low-rank coal were conducted through a drop tube furnace (DTF) to explore the effects of low-rank coal type, blending ratio, and steam on the transformation of Zn and Cr. The results showed that the retention rates of Zn and Cr in ash increased from 24.35% and 71.49% for sludge combustion alone to 53.77% and 117.49%, respectively, for coal blended to SS with a mass ratio of 7:3. The greater the proportion of low-rank coal in the fuel, the greater the residual rate of heavy metals in the ash. Meanwhile, rapid diffusion of vapor occupied adsorption sites on metal mineral surfaces, reducing the retention of Zn and Cr in the co-combustion ash. The leaching toxicity analysis of ash showed that the co-combustion ash of SS with coal was free from leaching toxicity hazards in simulated scenarios. The extraction rate of Zn in co-combustion ash increased from 90.72% with hydrothermal acid leaching to 95.46% with microwave-assisted in 2 mol/L H2SO4 extract. The extraction rate of Cr in hydrothermal acid leaching was 62.80%, which was much higher than that in microwave-assisted extraction (31.76%).

Prediction of calcium phosphate generation and behaviors of metals during phosphorus recovery using a modified thermodynamic model

In this study, behaviors of metals and their effects on phosphorus recovery by calcium phosphate were investigated by the laboratory and pilot experiments as well as by the modified thermodynamic model. Batch experimental results indicated that the efficiency of phosphorus recovery decreased with the increase in metal content and more than 80% phosphorus can be recovered with a Ca/P molar ratio of 3.0 and a pH of 9.0 for the supernatant of an anaerobic tank in the A/O process with the influent containing a high metal level. The mixture of amorphous calcium phosphate (ACP) and dicalcium phosphate dihydrate (DCPD) was assumed to be the precipitated product with an experimental time of 30 min. A modified thermodynamic model was developed using ACP and DCPD as the precipitated products, and the correction equations were incorporated to simulate the short-term precipitation of calcium phosphate based on the experimental results. From the perspective of maximizing both the efficiency of phosphorus recovery and the quality or purity of the recovered product, the simulation results showed that a pH of 9.0 and a Ca/P molar ratio of 3.0 were the optimized operational condition for phosphorus recovery by calcium phosphate when the influent metal content was at the level of actual municipal sewage.

Hydrothermal carbonization combined with thermochemical treatment of sewage sludge: Effects of MgCl2 on the migration of phosphorus and heavy metal

Phosphorus (P) is a non-regenerative and finite raw material. Due to its decreasing availability, and to protect the environment, recycling methods are needed. With the focus on closing nutrient cycles, sewage sludge (SS) is a potential source for P recovery. The objective of this study was to produce a mineral P-reach fertilizer. For this purpose, the treatment of SS in a multi-stage process, consisting of a hydrothermal carbonization (HTC) and thermochemical post-treatment was examined and compared with a direct thermochemical treatment. The focus was on the transformation of P and the migration of the heavy metals during the processes. In addition, the role of MgCl₂ as an additive was examined. During the HTC, most of the P remained in the HTC-char, so that the P content increased in the HTC-char compared with the SS. The addition of MgCl₂ to the process resulted in lower transportation rates of P in the liquid phase and higher P solubilities in water, citric acid, and alkalic ammonium citrate out of the solid phase. The thermochemical treatment of SS and the HTC-chars further concentrated P in the ash. Retention rates of >97% were achieved, and P₂O₅ contents in the ash were as high as ∼16 wt-%. The presence of the additive resulted in (i) higher retention rates of P in the ashes (ii) higher P-solubility and (iii) higher removal rates of easily volatile heavy metals such as Pb and Zn, and the treatment of HTC-char favored these effects compared with the direct treatment of SS.

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 SO₄^2- 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 SO₄^2-). 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.

Mining phosphorus from waste streams at wastewater treatment plants: a review of enrichment, extraction, and crystallization methods

Two interrelated problems exist: the non-renewability of phosphate rock as a resource and the excess phosphate in the water system lead to eutrophication. Removal and recovery of phosphorus (P) from waste streams at wastewater treatment plants (WWTPs) is one of the promising solutions. This paper reviews strategies for P recovery from waste streams in WWTPs are reviewed, and the main P recovery processes were broken down into three parts: enrichment, extraction, and crystallization. On this basis, the present P recovery technology was summarized and compared. The choice of P recovery technology depends on the process of sewage treatment and sludge treatment. Most P recovery processes can meet the financial requirements since the recent surge in phosphate rock prices. The safety requirements of P recovery products add a high cost to toxic substance removal, so it is necessary to control the discharge of toxic substances such as heavy metals and persistent organic pollutants from the source.

CaO-assisted hydrothermal treatment combined with incineration of sewage sludge: Focusing on phosphorus (P) fractions, P-bioavailability, and heavy metals behaviors

Dewatering of sewage sludge (SS) was the prerequisite for saving its drying energy and sustaining its stable combustion. Hydrothermal treatment (HT) has been a promising technology for improving SS dewaterability with high energy efficiency. However, the knowledge of phosphorus (P) transformation and heavy metals (HMs) behaviors in the combined HT and incineration process was still lack. P fractions, P-bioavailability, HMs speciation, and their environmental risk in the ash samples from this combination process were evaluated and compared with those from the co-incineration of SS and CaO. The combination process was superior to the latter one in the light of P and HMs. CaO preferred to enhance the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP) initially with enriched P and increased P-bioavailability in the resultant ash samples. The combination process further reduced the values of risk assessment code and individual contamination factor with the increment of the stable F4 fraction in HMs. Significant reduction of potential ecological risk was observed with the lowest global risk index of 43.76 in the combination process. Optimum CaO addition of 6% was proposed in terms of P and HMs. The work here can provide theoretical references for the potential utilization of P from SS to mitigate the foreseeable shortage of P rocks.

Recovery of phosphorus from wastewater: A review based on current phosphorous removal technologies

Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.