New evidence for rejuvenation of phosphorus retention capacity in EAF steel slag

Phosphorus removal by steel slag from tile drainage water: Lab and field evaluations

Basic oxygen furnace (BOF) and blast furnace (BF) steel slags are well suited for phosphorous (P) removal from nonpoint sources such as agricultural runoff. However, the reported mechanism(s) of removal varies from study to study which complicates implementation for unique environmental conditions that may interfere with the removal mechanism(s). This work compared laboratory column experiments and field filter experiments to provide insights on the influence of relevant field conditions (water alkalinity, slag grain size distribution, BF:BOF slag ratio, and water stagnation) on P removal by BF and BOF steel slag mixtures. Alkalinity was the most influential variable in lab-scale slag columns that received 250 mg/L alkalinity water and achieved complete P removal throughout the 3-h experiment, while identical columns receiving 500 mg/L alkalinity water averaged 52% P removal and only 14% removal after 2.5 h. Batch regeneration and adsorption experiments were conducted on the exhumed BOF/BF slag mixture from the field filter to evaluate strategies for increasing field P removal capacity. The adsorption capacity of steel slags was effectively regenerated by 0.01 M Al₂(SO₄)₃, which allowed for an additional 34% P removal in batch adsorption tests. The acid neutralization capacity of slag samples was effectively regenerated by 1 M NaOH, which allowed previously expended slag to reach a pH of 9.7 even in high alkalinity test water. The results presented here show that BF slag and Al₂(SO₄)₃ regeneration of BF slag is best suited for high alkalinity influent conditions and removes P through adsorption while BOF slag and NaOH regeneration perform best under low alkalinity conditions and removes P through mineral precipitation.

An Inventory of Good Management Practices for Nutrient Reduction, Recycling and Recovery from Agricultural Runoff in Europe’s Northern Periphery and Arctic Region

The excess loading of nutrients generated by agricultural activities is a leading cause of water quality impairment across the globe. Various management practices have been developed and widely implemented as conservation management strategies to combat water pollution originating from agricultural activities. In the last ten years, there has also been a widespread recognition of the need for nutrient harvesting from wastewaters and resource recovery. In Europe’s Northern Periphery and Arctic (NPA) areas, the expertise in water and runoff management is sporadic and needs to be improved. Therefore, the objective of this research was to perform a comprehensive review of the state of the art of Good Agricultural Practices (GAPs) for the NPA region. A set of questionnaires was distributed to project partners combined with a comprehensive literature review of GAPs focusing on those relevant and/or implemented in the NPA region. Twenty-four GAPs were included in the inventory. This review reveals that there is a large level of uncertainty, inconsistency, and a gap in the knowledge regarding the effectiveness of GAPs in nutrient reduction (NRE), their potential for nutrient recycling and recovery (NRR), and their operation and maintenance requirements (OMR) and costs. Although the contribution of GAPs to water quality improvement could not be quantified, this inventory provides a comprehensive and first-of-its-kind guide on available measures and practices to assist regional and local authorities and communities in the NAP region. A recommendation for incorporating and retrofitting phosphorus retaining media (PRMs) in some of the GAPs, and/or the implementation of passive filtration systems and trenches filled with PRMs to intercept surface and subsurface farm flows, would result in the enhancement of both NRE and NRR.

Materials in constructed wetlands for wastewater remediation: A review

The wastewater treatment industry is constantly evolving to abate emerging contaminants and to meet stringent legislative requirements. The existing technologies need to be modified, or new innovative treatment techniques need to be developed to ensure environmental protection and secure sustainability in the future. Emphasis is mainly on nutrient recovery, energy-efficient systems, zero waste generation, and environmentally friendly techniques. Constructed wetlands (CWs) have evolved as natural, eco-friendly, economical, and low-maintenance alternatives for wastewater remediation. These wetlands employ several materials as adsorbents for the treatment, commonly known as media/substrate. This review paper presents an assessment of various materials that can be used as substrates in CWs for the efficient removal of organic and non-biodegradable pollutants in different types of wastewaters. The effect of pH, mineral composition, specific surface area, and porosity of various natural materials and agricultural and industrial wastes used as media in CWs for wastewater remediation was discussed. The study showed that different substrates like alum sludge, limestone, coal slags, rice husk, and sand had removal efficiency for chemical oxygen demand (COD): 71.8%-82%, total phosphorous (TP): 77%-80%, and total nitrogen (TN): 52%-82% for different types of wastewaters. It also highlights the challenges related to the long-term sustainability of these materials. PRACTITIONER POINTS: Physicochemical characteristics influence the removal efficiency of the materials Life of media is also important along with removal efficiency and cost The sustainability of materials is very crucial for the overall performance of the system.

Recycled Steel Slag as a Porous Adsorbent to Filter Phosphorus-Rich Water with 8 Filtration Circles

Steel slag is a secondary product from steelmaking process through alkaline oxygen furnace or electric arc furnace (EAF). The disposal of steel slag has become a thorny environmental protection issue, and it is mainly used as unbound aggregates, e.g., as a secondary component of asphalt concrete used for road paving. In this study, the characteristics of compacted porous steel slag disc (SSD) and its application in phosphorous (P)-rich water filtration are discussed. The SSD with an optimal porosity of 10 wt% and annealing temperature of 900 °C, denoted as SSD-P (10, 900) meets a compressive strength required by ASTM C159-06, which has the capability of much higher than 90% P removal (with the effluent standard < 4 mg P/L) within 3 h, even after eight filtration times. No harmful substances from SSD have been detected in the filtered water, which complies with the effluent standard ISO 14001. The reaction mechanism for P-rich water filtration is mediated by water, followed by two reaction steps-CaO in SSD hydrolyzed from the matrix of SSD to Ca2+ and reacting with PO43-. However, the microenvironment of water is influenced by the pH value of the P-rich water at different filtration times and the kind of P-rich water with different free positive ion that interferes the reactions of the release of Ca2+. This study demonstrates the application of circular economy in reducing steel slag deposits, filtering P-rich water, and collecting Ca3(PO4)2 precipitate into fertilizers.

Optimal regulation of N/P in horizontal sub-surface flow constructed wetland through quantitative phosphorus removal by steel slag fed

High concentration of nitrogen and phosphorus and imbalance of N/P can lead to the formation of water and the malignant proliferation of toxic microalgae. This study put forward the advanced nutrient removal with the regulation of effluent N/P as the core in order to restrain the eutrophication and growth of poisonous algae. According to the preliminary study and review, the optimal N/P for non-toxic green algae was 50:1. The horizontal sub-surface flow constructed wetland was filled with steel slag and ceramsite to achieve the regulation of effluent N/P. The results showed that steel slag had the stable P removal capacity when treating synthetic solution with low P concentration and the average removal rate for 1.5, 1.0, and 0.5 mg/L synthetic P solution was 2.98 ± 0.20 mg kg^-1/h, 2.26 ± 0.15 mg kg^-1/h, and 1.11 ± 0.10 mg kg^-1/h, respectively. Combined with P removal rate and P removal task, the filling amount of steel slag along the SSFCW (sub-surface flow constructed wetland) was 3.22 kg, 4.24 kg, and 4.31 kg. In order to ensure the stability of dephosphorization of steel slag, the regeneration of P removal capacity was investigated by switching operation of two parallel SSFCW in 20 days for cycle. The N removal was limited for the deficiency of carbon source (COD (chemical oxygen demand)/TN = 3-4), and was stable at 18.5-31.9% which was less affected by temperature. Therefore, by controlling the process of quantitative P removal of steel slag, the effluent N/P in SSFCW can be stable at 40-60:1 in the whole year, so as to inhibit the malignant proliferation of toxic algae.

Mechanisms of Phosphorus Removal by Recycled Crushed Concrete

Due to urbanisation, there are large amounts of waste concrete, particularly in rapidly industrialising countries. Currently, demolished concrete is mainly recycled as aggregate for reconstruction. This study has shown that larger sizes (2-5 mm) of recycled concrete aggregate (RCA) removed more than 90% of P from effluent when at pH 5. Analysis of the data, using equilibrium models, indicated a best fit with the Langmuir which predicated an adsorption capacity of 6.88 mg/g. Kinetic analysis indicated the equilibrium adsorption time was 12 h, with pseudo second-order as the best fit. The thermal dynamic tests showed that the adsorption was spontaneous and, together with the evidence from the sequential extraction and desorption experiments, indicated the initial mechanism was physical attraction to the surface followed by chemical reactions which prevented re-release. These results suggested that RCA could be used for both wastewater treatment and P recovery.

Phosphorus removal in a vertical flow constructed wetland using dolomite powder and chippings as filter media

Different kinds of natural and artificial filter media are able to retain phosphorus in the constructed wetlands. Due to the fact that the constructed wetland needs huge amounts of the filter media, it is very important to find locally available material which distinguishes itself by its ability to retain phosphorus. The materials found in Lithuania were considered and dolomite was chosen. Two dolomite fractions, dolomite powder (1–2 mm) and dolomite chippings (2–5 mm), and sand media were used in the laboratory- scale installed for the comparative experiments. The laboratory-scale with dolomite as the filter media was on average by 21% more efficient in total phosphorus removal in comparison with the sand media. Based on the laboratory research pilot–scale vertical flow constructed wetland of 160 m2 was installed and planted with reed Phragmites australis. The dolomite chippings as filter media were chosen in order to avoid the danger of the clogging of constructed wetland. Efficiency of total phosphorus removal in the pilot-scale vertical flow constructed wetland was on average 95.7%, phosphates removal – 94.8% within one year.

Modeling phosphate transport and removal in a compact bed filled with a mineral-based sorbent for domestic wastewater treatment

Phosphorus filter units containing mineral-based sorbents with a high phosphate (PO4) binding capacity have been shown to be appropriate for removing PO4 in the treatment of domestic wastewater in on-site facilities. However, a better understanding of their PO4 removal mechanisms, and reactions that could lead to the formation of PO4 compounds, is required to evaluate the potential utility of candidate sorbents. Models based on data obtained from laboratory-scale experiments with columns of selected materials can be valuable for acquiring such understanding. Thus, in this study the transport and removal of PO4 in experiments with a laboratory-scale column filled with a commercial silicate-based sorbent were modeled, using the hydro-geochemical transport code PHREEQC. The resulting models, that incorporated the dissolution of calcite, kinetic constrains for the dissolution of calcium oxide (CaO) and wollastonite (CaSiO3), and the precipitation of amorphous tricalcium phosphate, Ca3(PO4)2, successfully simulated the removal of PO4 observed in the experiments.

Trapping phosphorus in runoff with a phosphorus removal structure

Reduction of phosphorus (P) inputs to surface waters may decrease eutrophication. Some researchers have proposed filtering dissolved P in runoff with P-sorptive byproducts in structures placed in hydrologically active areas with high soil P concentrations. The objectives of this study were to construct and monitor a P removal structure in a suburban watershed and test the ability of empirically developed flow-through equations to predict structure performance. Steel slag was used as the P sorption material in the P removal structure. Water samples were collected before and after the structure using automatic samples and analyzed for total dissolved P. During the first 5 mo of structure operation, 25% of all dissolved P was removed from rainfall and irrigation events. Phosphorus was removed more efficiently during low flow rate irrigation events with a high retention time than during high flow rate rainfall events with a low retention time. The six largest flow events occurred during storm flow and accounted for 75% of the P entering the structure and 54% of the P removed by the structure. Flow-through equations developed for predicting structure performance produced reasonable estimates of structure "lifetime" (16.8 mo). However, the equations overpredicted cumulative P removal. This was likely due to differences in pH, total Ca and Fe, and alkalinity between the slag used in the structure and the slag used for model development. This suggests the need for an overall model that can predict structure performance based on individual material properties.

Chemical techniques for pretreating and regenerating active slag filters for improved phosphorus removal

Active slag filters are an emerging technology for removing phosphorus (P) from wastewaters. Recent research revealed that adsorption onto Fe oxides/oxyhydroxides at near-neutral pH and oxidizing Eh is the key mechanism of P removal by melter slag filters. Currently, filter lifespan is limited by available adsorption sites. This study examined whether the performance and longevity of active filters could be improved via chemical treatment to create additional reactive sites as well as regenerate exhausted ones. Fresh original melter slag as well as slag from an exhausted full-scale filter was tested. Chemical reagents that could manipulate the pH/Eh of the slag granule surfaces and potentially activate them for further P removal were used, namely hydrochloric acid (HCI), sodium hydroxide (NaOH) and sodium dithionite (Na2S2O4). Waste stabilization pond effluent was then applied to the treated slag to assess the effectiveness of the treatments at improving P removal. Fresh slag treated with Na2S204 and HCl, respectively, retained 1.9 and 1.4 times more P from the effluent than the untreated fresh slag. These reagents were even more effective at regenerating the exhausted slag, increasing total retained P by a factor of 13 and six, respectively, compared with untreated slag. Sodium hydroxide was ineffective at increasing P removal. The higher P retention by the 'treated exhausted slag' compared with the 'treated fresh media' indicates that adsorption sites on melter slag filters become increasingly reactive with time. This research is the first study to provide evidence that P retention by active slag filters can be increased by both (1) chemical pre treatment and (2) chemical post-treatment once their P removal is exhausted, thereby potentially transforming them from a single use system to a more viable, reusable treatment technology.

Evaluating the efficiency and temporal variation of pilot-scale constructed wetlands and steel slag phosphorus removing filters for treating dairy wastewater

The performance and temporal variation of three hybrid and three integrated, saturated flow, pilot-scale constructed wetlands (CWs) were tested for treating dairy farm effluent. The three hybrid systems each consisted of two CWs in-series, with horizontal and vertical flow. Integrated systems consisted of a CW (horizontal and vertical flow) followed by a steel slag filter for removing phosphorus. Time series temporal semivariogram analyses of measured water parameters illustrated different treatment efficiencies existed over the course of one season. As a result, data were then divided into separate time period groups and CW systems were compared using ANOVA for parameter measurements within each distinct time period group. Both hybrid and integrated CWs were efficient in removing organics; however, hybrid systems had significantly higher performance (p<0.05) during peak vegetation growth. Compared to hybrid CWs, integrated CWs achieved significantly higher DRP reduction (p<0.05) throughout the period of investigation and higher ammonia reduction (p<0.05) in integrated CWs was observed in late summer. Geochemical modeling demonstrates hydroxyapatite and vivianite minerals forming on steel slag likely control the fate of phosphate ions given the reducing conditions prevalent in the system. The model also demonstrates how the wastewater:slag ratio can be adjusted to maximize phosphorus removal while staying at a near-neutral pH.

Investigations on phosphorus recovery and reuse as soil amendment from electric arc furnace slag filters

Electric arc furnace (EAF) steel slag has been identified as an effective filter material for the removal of phosphorus (P) from both point and non-point sources. To determine the feasibility of land-applying P saturated EAF steel slag this study was undertaken to investigate (i) saturated EAF steel slag material's potential as a P fertilizer or soil amendment and (ii) P desorption and metals leachate from saturated EAF steel slag material to surface runoff. Medicago sativa (alfalfa) was planted in a nutrient depleted washed sand media. Phosphorus was added either as saturated EAF steel slag or as a standard commercial phosphate fertilizer in order to assess the plant availability of the P from saturated EAF steel slag. Four different P application levels were tested: a low (20 lbs acre furrow slice(-1) (5.5 g P m(-3))) two medium (40 and 60 lbs. acre f.s.(-1) (11 and 16.5 g P m(-3))) and a high (120 lbs. acre f.s.(-1) (33 g P m(-3))). The above-ground biomass of half of the plants was harvested after 5 weeks and the second half at 10 weeks. All treatments regardless of the P source used showed high rates of germination. At the first harvest period (5 weeks) significantly higher above-ground biomass (p < 0.01) was seen at the 3 highest P amendment rates in treatments with triple super phosphate fertilizer (TSP) than with EAF steel slag. However, by the second harvest (10 weeks) only the highest amendment rate of TSP showed a significantly higher amount of biomass (p < 0.01), suggesting that EAF steel slag might be an effective slow release P source. In a second experiment, a rain simulator was used to assess desorption of DRP, TP and metals from a saturated and semi-saturated EAF steel slag. The results revealed that the total amounts of DRP and TP released to surface runoff from EAF steel slag were negligible when compared to the total quantities of P retained by this material. Overall the results from this study demonstrated that once the EAF steel slag filter reaches its saturation point, the material could be re-used as soil amendment for the slow release of bioavailable P with minimal risk for loss of P to surface runoff, bringing further benefits to the environment.

Active filtration of phosphorus on Ca-rich hydrated oil shale ash: does longer retention time improve the process?

The purpose of this research was to investigate the phosphorus binding capacity of active filtration through hydrated Ca-rich oil shale ash sediment in a laboratory-scale experiment at different retention times. Three column experiments were established with hydraulic loading rates of 12 L d(-1), 18 L d(-1), and 24 L d(-1), with residence times of 12, 8, and 6 h, respectively. The results showed good removal efficiency (up to 91% at loading of 1.66 gP m(-2) d(-1)) in the experiment with the longest retention time. The overall performance of the filters did not, however, improve with increasing retention time, and the decrease in binding capacity was 41% at longer retention times. Chemical clogging by carbonate precipitates probably reduces the availability of Ca from the dissolution of unstable ash sediment mineral phases, and the phosphate removal decreases rapidly from 91% to 49% at loading of 1.66 gP m(-2) d(-1).