Pretreatment of hydrodesulfurization wastewater
Hydrodesulfurization (HDS) wastewaters are generated from the refining industry during the hydrorefining process for hydrotreating of petroleum products. HDS is an important process considered in the design of most petroleum refineries, not only due to environmental concerns with sulfur compounds but also to improve product quality. The presence of high levels of metals and sulfides in HDS wastewaters makes them more challenging to treat, as they are more toxic and present a higher risk during disposal. This has prompted refineries to consider pretreatment methods to reduce the level of metals and sulfides found in such wastewater before wastewater treatment is performed.
The most common pretreatment techniques used for HDS are oxidation, neutralization, and flocculation. Oxidation techniques such as chemical oxidation or aerobic/anaerobic biodegradation, may be applied to remove sulfides and break down organic compounds. Neutralization techniques, including chemical precipitation and membrane adsorption, are used to reduce acidity and increase pH. Flocculation techniques, such as ion exchange and reverse osmosis, may be applied to remove organic compounds and metals, while filtration techniques may be used to separate suspended solids.
Oxidation techniques have been found to be beneficial in treating HDS wastewater. Chemical oxidation techniques are commonly used and involve adding chemicals such as potassium permanganate and sodium hypochlorite to the wastewater to oxidize sulfur compounds. Chemical oxidation has been reported to reduce levels of total nitrogen, suspended solids, and sulfates. Aerobic biodegradation, particularly activated sludge, has been found to reduce the concentration of oils and fats found in HDS wastewater. Anaerobic biodegradation has also been reported, although this technique is less often applied in the treatment of HDS wastewater.
Neutralization is another commonly used pretreatment technique for HDS wastewater. Chemical precipitation, often involving the addition of lime or other basic compounds, can be used to reduce acidity and increase the pH of HDS wastewater. Membrane adsorption processes may also be applied to remove metals and other contaminants, with reverse osmosis used to remove organic compounds.
Flocculation techniques have been applied to reduce total suspended solids in HDS wastewaters. Ion exchange, using cation exchange resins, has been found to bind and remove manganese, copper, and zinc. The process also removes other metals, reducing their presence in wastewater significantly. Reverse osmosis has been applied to reduce suspended solids by trapping and concentrating them in a semipermeable membrane.
Filtration techniques have been used to remove suspended solids from HDS wastewaters. Sand filters, packed towers, and other filtration media can be used to reduce the presence of suspended solids. The efficiency of the filtration process will depend on the type of filtration media used and the characteristics of the wastewater.
In summary, pretreating HDS wastewater is beneficial for reducing the presence of metals and sulfides. Oxidation techniques, neutralization, flocculation, and filtration are all effective pretreatment techniques. Each method should be considered depending on the type of wastewater and the desired treatment objectives.
