Advanced Wastewater Treatment for the Separation and Minimisation of Residual Oil at Tata Steel
Authors:   , Lester-Card Edward 
Published: November 2021

At Port Talbot Tata steelworks, an increasingly large amount of oil is being found in effluent streams. This problem can be seen visually as well as in the analytical records kept by the company. Although effluent is treated onsite at the steelworks, oil is leading to a high burden on the wastewater treatment plant in place and is causing excessive volumes of oily sludge. Breaches of oil consent limit (5mg/L) at the Long Sea Outfall (LSO), where the wastewater exits into the Bristol Channel, have also been regularly recorded. To give this problem some weight, 55% of the samples collected from the LSO during daytime operations in 2019 were above the discharge consent limit. An investigation into current steelworks operations related to oily wastewater, revealed that the largest source of waste oil at the steelworks is the Hot Mill, which ejected in 2019 nearly 2,000,000L of oil into its wastewater known as the Dirty Water Return (DWR). Being a large source of oil, wastewater from the Hot Mill is currently treated by synthetic polymer dosing (Nalco 9908) separately from the rest of the other effluents in a dedicated treatment plant managed by Tata’s Energy department. Therefore, the wastewater is recycled to avoid discharges to the LSO. The waste oil from the treatment plant ends up in a sludge, which is collected and used to form pellets for the Blast Furnace. In some cases the oil is recovered by heating to remove the water in the Million Gallon Tank (MGT) and then re-used in the Coke Ovens as bulk density modifier. However, the main sources of waste oil that place a burden on the LSO were found to originate from the Deep Drain, which collects oil from the Hot Mill overflow, Coke Ovens (Sump 6 ), and the Cold Mill, each releasing an estimated daily average of 464, 361 and 195 kg of oil respectively (data from 2019 records). These are only estimated values, which are associated with large standard deviations due to issues around poor precision in flow rate and oil concentration data. The highest average oil concentrations were found in wastewaters from the Continuous Annealing Process Line (CAPL) (158mg/L), Cold Mill (118mg/L) and Sump 6 (115mg/L). Despite, CAPL being the oiliest effluent, it is not such a burden on the LSO since its flowrate is small at only 9m3/hour. The most common technology around the steelworks to tackle the oily wastewater and in particular emulsified oil is the process of coagulation-flocculation. Current research trends highlight a high demand for more novel and environmentally friendly coagulants to replace traditional metal salt and synthetic polymer varieties, which have drawbacks including hazardous non-biodegradable voluminous sludge, sensitive pH and dosage conditions. Natural coagulants however, are non-toxic, biodegradable, and safe to handle. Thus, this project evaluated a natural coagulant/flocculant extracted from the seeds of the plant Moringa Oleifera (MO) for the removal of oil. This is in line with the vision of Tata as a Sustainable and Environmentally responsible steel producer company. To obtain the natural coagulant, the MO seeds were grinded, ethanol extracted, salt (NaCl) extracted, and filtered. The crude salt extracted coagulant is named MOCE in this thesis and was tested using model emulsions and real wastewater collected from Tata steel site at different locations (DWR and CAPL). The MO seeds and obtained coagulant were characterised using different techniques including Fourier Transform Infra Red (FTIR), total organic carbon analysis (TOC), zeta potential, protein content through the Bradford and Lowry assays and protein molecular weights using SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis). The MOCE was further purified with membrane filtration and freeze drying (MP-MOCE-FD) to remove unwanted organics and salt. The coagulation/flocculation experiments were carried out in 6-paddle jar test flocculator. Oil analysis was made using liquid-liquid extraction combined with FTIR analysis in tetrachloroethylene solvent. In addition, the wastewater was characterised using techniques such as turbidity, UV absorbance, pH, zeta sizer, and chemical oxygen demand (COD). The FTIR analysis of MO seeds identified the main functional groups responsible for its coagulating-flocculating capabilities to be 3287-3294cm-1 (O-H, N-H), 1642-1648cm-1 (N-H), 1542-1534cm-1 (C-N, N-H) and 1231cm-1 (C-N). These are all linked to the protein content in the seeds. The average protein concentration for aqueous MO extract (MODI) (i.e. without salt) was 3,000mg/L and for crude salt extract (MOCE) (i.e. with 1M salt) was 8,000 mg/L. Extracting the active agents in the presence of salt (0.6M) increased protein solubility by 74%. In addition, it was found that the salt concentration affected the isoelectric point of the obtained coagulant. For example, the isoelectric points of MODI (i.e. no salt) and MOCE extracted at 1.0 M salt were pH 11-12, and pH≈2, respectively. The coagulant extracts MODI and MOCE (0.2M) both had a natural pH of 6-6.6 with a zeta potential of 4.68mV and -0.62mV respectively. The reduced charge of the MOCE could be due to charge neutralisation effect from the chlorine ion (Cl-). The molecular weights of the extracted proteins ranged from 13.5 to 74.45kDa for MOCE and from 13.59 to 28.74kDa for MP-MOCE-FD. Jar test experiments on model oil-water emulsions evaluated the coagulation-flocculation performance of MOCE, aluminium sulphate (alum) expressed as aluminium (Al), and a synthetic polymer (Nalco 9908); each with optimum coagulant dosages of 50, 5 and 2.5mg/L respectively. It should be noted that the concentration of MOCE is based on protein equivalent as determined by the average Bradford and Lowry assays. The results show that MOCE was effective over a wide range of pH values (pH 3-11) while alum and the polymer required optimum pH values of pH 5-9, and pH 8, respectively. This suggests that MOCE does not require pH adjustment as opposed to alum and the polymer offering an advantage of using MOCE. In addition, MOCE (and the polymer) were found not to affect the final pH of the solution as opposed to alum, which decreased pH by about three pH units. This study has also revealed that the MOCE coagulation mechanism is predominantly by sweep flocculation with some charge neutralisation, while charge neutralisation and sweep flocculation also occur for alum, whereas charge neutralisation with bridging mechanisms take place for the polymer. Analysis of the sludge from model emulsion experiments showed that the polymer-produced-sludge had a near zero volume because of the non-formation of visible flocs, instead a sticky layer attached to the walls of the glass beaker was formed. MOCE produced the greatest amount of sludge (35mL/L) compared to alum (25mL/L). In addition, MOCE sludge had high organic content, with a carbon percentage reaching 60%, indicating that the sludge could be valorised for example by anaerobic digestion to produce biogases. MOCE was also found to lead to rapid oil removal with 95% of the oil being removed within 15 minutes of settling as compared to 71% and 48% for alum and the polymer respectively for the same settling time. However, MOCE can add more organic load to the treated wastewater, which is a limitation commonly observed for natural coagulants. In order to resolve this issue, a purification study using membrane filtration combined with freeze drying was attempted in this study to produce a purified MO proteins containing the bulk fraction of organics that contain the coagulating/flocculating properties. In comparison to MOCE, MP-MOCE-FD reduced the organic load in the treated wastewater by 10% from non-purified MOCE. The molecular weight analysis, thus shows that the majority of the coagulating proteins are ≤28.74kDa. Application of MOCE to treat real wastewaters collected from the Hot Mill wastewater (DWR) and CAPL showed that MOCE rapidly coagulated the site wastewaters, but the final floc size was small and required extended settling times. However, by combining the MOCE as a primary coagulant with the polymer as a flocculent, a synergistic effect was produced, which reduced the settling time to 15 minutes and increased oil removal from solely MOCE by 17% and 77% on the DWR and CAPL, respectively. Optimum coagulant dosages for the combined MOCE/polymer were: (i) for DWR 5mg/L MOCE and 0.5mg/L polymer, and (ii) for CAPL 2mg/L MOCE and 0.1mg/L polymer. Life Cycle Assessment (LCA) and cost analysis were then carried out to evaluate the feasibility of using MOCE-polymer as a coagulant-flocculent for treating the DWR (6,000m3/hour) and CAPL (9m3/hour) wastewaters. The LCA was supported by simulating the process in SuperPro Designer (Intelligen, Inc.), which is a software package widely used in environmental and wastewater simulation studies. The simulation assumed that the MOCE was produced from the seed cake after its MO oil was extracted; the MO oil is the most economical valuable part of the MO seed and is sold approximately at £50/kg. The coagulating proteins are assumed to be extracted onsite using water and salt. Thus, to produce sufficient MOCE (3.751 m3/hour) for DWR and CAPL, a 276 kg/h of raw MO seeds (producing 188kg/hour of seed cake) are required; 99% of the MOCE produced is used in DWR. In addition to MO seeds, the process requires 3.75 m3/hour water and 45 kg/h NaCl salt. The production cost, based on materials, comes at £1.1/m3 MOCE solution. This cost was found to be about three times lower than the polymer (£3.29/m3 polymer solution). These costs were then converted to specific costs for treating the wastewater taking into account of the doses required for treatment. Treating the DWR wastewaters with MOCE-polymer would cost £2.33/ML in comparison to £4.11/ML for polymer (ML: million litre), while treating CAPL would cost £0.60/ML and £4.93/ML for MOCE-polymer and polymer, respectively. The greenhouse gas emissions to produce MOCE were calculated to be approximately 34.33gCO2/kgMOCE, which is in the lower range compared to literature reported values for traditional metal-based coagulants such as ferric and alum (29-295gCO2/kg coagulant). This study shows that MO natural coagulant has great promise in treating oily wastewaters effectively, economically and sustainably.