Electrodialytic Separation of Phosphorus and Heavy Metals from Two Types of Sewage Sludge Ash

Extraction of phosphorus from sewage sludge ash by electrodialysis combined with wet-chemical extraction

Phosphorus (P) recovery from sewage sludge ash (SSA) is considered to be an effective method for P recovery. In this work, P extraction and the removal of heavy metals were realized by electrodialysis. Low-cost, easily available, and environmentally friendly plant extracts were applied as suspension to reduce the inevitable secondary pollution. And the feasibility of using plant extracts was analysed by comparing with using deionized water (DI) and oxalic acid (OA) solution. When SSA was suspended in different solutions (DI, OA, and three plant extracts - Hovenia acerba (HA), Saponin (SA) and Portulaca oleracea (PO)), the effects of reaction time and plant extract concentration on P extraction and heavy metal separation of SSA under ED treatment were compared. After the process of electrodialysis, compared to other experimental groups, electrodialysis with plant extracts obtained more P released from SSA, but less P migrated to the anode chamber. However, when SSA was suspended in PO at a concentration of 80 g/L, the proportion of P transferred from SSA to the anode chamber can still reached 37.86%. In addition, the use of plant extracts as suspension had a positive effect on the removal of heavy metals, but its effect was lower than that of the oxalic acid-treated experimental group. The results indicated that the use of plant extracts for wet-chemical extraction combined with electrodialysis promoted the removal of heavy metals and the extraction of P from SSA, which is a feasible option.

Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus

Phosphorus is a nutrient that is essential to nature and human life and has attracted attention because of its very limited reserves. Dwindling phosphorus reserves and soaring prices have made the recovery of phosphorus from waste biosolids even more urgent. Waste activated sludge, as the final destination of most of the phosphorus in human domestic and industrial water, has been considered as a reliable source of phosphorus recovery. The thermal treatment method of sewage sludge is currently a relatively environmentally friendly disposal method, which mainly includes incineration, pyrolysis and hydrothermal carbonization. This paper reviews the methods for the recovery of different forms of phosphorus (wet chemical, thermochemical and electrodialysis) from solid products obtained from different sludge thermal treatment methods (incinerated sewage sludge ash, pyrolysis of sewage sludge char and hydrochar) and the bioavailability of the recovered phosphorus products. Incineration of sewage sludge is currently the most established and effective method for recovering phosphorus from the thermal treatment products of sewage sludge. One of the wet chemical methods has been applied on a commercial scale and is expected to be further developed for future industrial applications. Pyrolysis and hydrothermal carbonation still have many research gaps in this field. Based on their principles and laboratory performance, both of them have the potential to recover phosphorus and should be further explored.

Removal of Metal Ions in Phosphoric Acid by Electro-Electrodialysis with Cross-Linked Anion-Exchange Membranes

There are numerous metallic impurities in wet phosphoric acid, which causes striking negative effects on industrial phosphoric acid production. In this study, the purification behavior of metallic impurities (Fe, Mg, Ca) from a wet phosphoric acid solution employing the electro-electrodialysis (EED) technology was investigated. The cross-linked polysulfone anion-exchange membranes (AEMs) for EED were prepared using N,N,N',N'-tetramethyl-1,6-hexanediamine (TMHDA) to achieve simultaneous cross-linking and quaternization without any cross-linkers or catalysts. The performance of the resulting membranes can be determined using quaternization reagents. When the molar ratio of trimethylamine/TMHDA/chloromethylated polysulfone is 3:1:1, the cross-linked membrane CQAPSU-3-1 exhibits lower water swelling and membrane area resistance than the non-cross-linked membrane. The low membrane area resistance of CQAPSU-3-1 with long alkyl chains is obtained due to the hydrophilic-hydrophobic microphase separation structure formed by TMHDA. EED experiments with different initial phosphoric acid concentrations of 0.52 and 1.07 M were conducted to evaluate the phosphoric acid purification of different AEMs. The results show that the EED experiments were more suitable for the purification of wet phosphoric acid solution at low concentrations. It was found that the phosphoric acid concentration in the anode compartment could be increased from 0.52 to 1.04 M. Through optimization, with an initial acid concentration of 0.52 M, CQAPSU-3-1 exhibits an enhanced metallic impurity removal ratio of higher than 72.0%, the current efficiency of more than 90%, and energy consumption of 0.48 kWh/kg. Therefore, CQAPSU-3-1 exhibits much higher purification efficiency than other membranes at a low initial phosphoric acid concentration, suggesting its potential in phosphoric acid purification application.

Phosphorous recovery from sewage sludge using calcium silicate hydrates

Phosphorous is an essential limiting nutrient for which there is no substitute. Its efficient recovery from sewage treatment plants is important to mitigate both dependence on limited reserves of exploitable phosphate rock and eutrophication of surface waters. Here, we evaluate the use of calcium silicate hydrates (CSH) to recover phosphorous eluted from sewage sludge. Phosphorous elution experiments were conducted with acid and base leaching solutions. The phosphorous recovery efficiency with CSH was compared to that with other calcium compounds, and the final product was analyzed to assess its potential value as fertilizer. Dried sewage sludge from the West Lake Ecological Water Resource Center, South Korea, having 123 g-P kg^-1, was used for these tests. About 55% of the phosphorus in the sludge was released with an elution solution of 0.1 M H₂SO₄. A dose of 15 g L^-1 of CSH recovered 89.6% of the eluted phosphorous without the need for additional pre-treatment, and the resulting calcium phosphate product (in brushite form, based on XRD analysis) exhibited superior settleability than that resulting from Ca(OH)₂- and CaCl₂-induced precipitation. XRD peaks of the calcium sulfate hydrate (in gypsum form) and residual CSH were also observed. The final product contained a relatively high content of the total P₂O₅ eluted in a 2% citric acid solution (43.1%), which suggests that it might be readily used to fertilize crops.

Heavy-metal extraction from sewage sludge using phosphorous-based salts: optimization process with Na2H2P2O7

Land application is one of the important disposal alternatives for sewage sludge, but availability of potential toxic metals often restricts its uses. Three phosphorous-based salts (Na₂H₂P₂O₇, K₄P₂O₇, KH₂PO₄) were studied as potential metal extractants. The conclusions of the research were that greater extractive efficiency is achieved through a 30-min process of vertical shaking with disodium diacid pyrophosphate - Na₂H₂P₂O₇ - at a concentration of 0.2 M at pH 2. Alternatively, the optimized process with oscillating shaking equipment would require 60 min. In both cases the average of set of extracted metals is around 50%. A second extraction process with potassium pyrophosphate - K₄P₂O₇ at pH 6 achieved the reduction of further total amounts of metal, upper 65% with respect to the initial content. In this way the sludge could be used in land applications, with restrictions on each soil, according to the limit values specified in the future regulations.

Comparison of phosphorus recovery from incineration and gasification sewage sludge ash

Incineration of sewage sludge is a common practice in many western countries. Gasification is an attractive option because of its high energy efficiency and flexibility in the usage of the produced gas. However, they both unavoidably produce sewage sludge ashes, a material that is rich in phosphorus, but which is commonly landfilled or used in construction materials. With current uncertainty in phosphate rock supply, phosphorus recovery from sewage sludge ashes has become interesting. In the present work, ashes from incineration and gasification of the same sewage sludge were compared in terms of phosphorus extractability using electrodialytic (ED) methods. The results show that comparable recovery rates of phosphorus were achieved with a single ED step for incineration ashes and a sequential combination of two ED steps for gasification ashes, which was due to a higher influence of iron and/or aluminium in phosphorus solubility for the latter. A product with lower level of metallic impurities and comparable to wet process phosphoric acid was eventually obtained from gasification ashes. Thus, gasification becomes an interesting alternative to incineration also in terms of phosphorus separation.

Sequential electrodialytic recovery of phosphorus from low-temperature gasification ashes of chemically precipitated sewage sludge

Phosphorus recycling from secondary materials like sewage sludge ashes offers an alternative to mining of phosphates from primary resources and a mean to counteract the current phosphorous rock depletion concern. A separation of P from the bulk ash is normally required, due to its low plant availability and the presence of heavy metals. Previously, more than 80% of P was recovered from incineration sewage sludge ashes using a two-compartment electrodialytic cell. In contrast, the recovery was below 30% for ashes from low-temperature gasification using the same setup. The low recovery was due to a high presence of Al- and Fe(III)-P bindings. In the present study, an electrodialytic process combining sequentially a pair of two-compartment cells allowed a recovery of up to 70% of phosphorus from these ashes. The use of a second cell, where the ash was suspended in an alkaline solution, allowed the P solubilisation from aluminium and ferric phosphates. In addition, P was separated from most metals as they became insoluble under the prevailing chemical environment. The obtained ratio of Al, Fe, Mg and most heavy metals to P was comparable to wet process phosphoric acid. Therefore, this sequential process was found to be suitable to recycle P and potentially use it in the production of common fertilizers like diammonium phosphate.

Valorisation of ferric sewage sludge ashes: Potential as a phosphorus source

Sewage sludge ashes (SSA), although a waste, contain elements with socio-economic and environmental potential that can be recovered. This is the case of phosphorus (P). SSA from two Danish incinerators were collected during two years and characterized. The sampling was done immediately after incineration (fresh SSA) or from an outdoor deposit (deposited SSA). Although morphology and mineral composition were similar, physico-chemical and metal concentration differences were found between incinerator plants and sampling periods. No differences were observed between deposited and fresh SSA, except for the parameters directly influenced by disposal conditions (e.g. moisture content). All the SSAs had high concentrations of P (up to 16wt%), but they all exceeded Danish EPA Cd and Ni thresholds for direct application at agricultural soil. Fresh and deposited SSA were acid washed aiming P extraction, achieving 50gP/kg (approx. 37% of total P), but metals were also co-extracted to the liquid phase. To avoid and/or minimize the metals pollution of the extracted P, selective P recovery from the SSA was tested, using the electrodialytic (ED) process. ED laboratory cells, with 3 compartments (3c) and 2 compartments (2c), and two acid concentrations (H2SO4, 0.08M and 0.19M) were used for 7days. The most concentrated acid solution increased P solubilization. The 2c-cell combined with the higher acid concentration resulted in higher P recoveries, 125g of P/kg of SSA in the anolyte. The obtained results showed that the ED process is a valuable tool for the SSA valorisation as it promotes simultaneous P recovery and metals extraction from the SSA.

Phosphorous recovery from sewage sludge ash suspended in water in a two-compartment electrodialytic cell

Phosphorus (P) is indispensable for all forms of life on Earth and as P is a finite resource, it is highly important to increase recovery of P from secondary resources. This investigation is focused on P recovery from sewage sludge ash (SSA) by a two-compartment electrodialytic separation (EDS) technique. Two SSAs are included in the investigation and they contained slightly less P than phosphate rock used in commercial fertilizer production and more heavy metals. The two-compartment electrodialytic technique enabled simultaneous recovery of P and separation of heavy metals. During EDS the SSA was suspended in water in the anolyte, which was separated from the catholyte by a cation exchange membrane. Electrolysis at the anode acidified the SSA suspension, and hereby P, Cu, Pb, Cd and Zn were extracted. The heavy metal ions electromigrated into the catholyte and were thus separated from the filtrate with P. More than 95% P was extracted from both SSAs. The charge transfer to obtain this varied when treating the two SSAs, and for one ash it was about 30% higher than for the other as a result of a higher buffering capacity against acidification. The repeatability of EDS results between experiments with the same SSA and the same experimental conditions was good, which shows that the process is easy to control at the studied laboratory conditions. About 80% P and 10% of the heavy metals remained in the filtrate from the anolyte after treatment of both SSAs. The heavy metal content relative to P in the filtrate by far meet the limiting values for use of industrial wastes as fertilizers, thus the filtrate is ready for direct processing into P-fertilizer.

Comparison of two different electrodialytic cells for separation of phosphorus and heavy metals from sewage sludge ash

With decreasing availability of phosphorus from primary resources its recovery from waste streams becomes increasingly more important. Sewage sludge ash is rich in phosphorus, but the direct use as fertilizer is limited because of inorganic contaminants such as heavy metals and strong bonding of phosphorous in the ash. Electrodialysis (ED) can be used to recover phosphorus and simultaneously remove heavy metals. The present work is an experimental screening of different options for ED in relation to experimental setup and combination with acid addition. Experiments for stirred ash suspensions utilizing a three compartment cell setup where the anode, cathode and stirred suspension are separated by ion exchange membranes are reported. Simplifying this experimental setup by removing the anion exchange membrane brings the anode in direct contact with the stirred ash suspension. Through this adjustment, half-reactions at the anode contribute to the acidity of the stirred suspension resulting in increased dissolution of both phosphorus and heavy metals (Cd, Cu, Cr, Pb, Zn, Ni) and better separation of most heavy metals from the stirred ash suspension. When the ash is suspended in an acidic solution, these effects increase significantly in early stages of the experiments. The combination of ED in a two compartment setup and initial acidification of the stirred suspension is most effective in dissolving of phosphorus and separation of heavy metals. In this setup, up to 96% of the phosphorus in the ash was dissolved after 7 d. Using the three compartment setup and initially suspending the ash in distilled water, resulted in 53% dissolution of the total recovered phosphorus after 7 d.