Removal of toxic compounds particularly heavy metals from wastewater discharge is critical not only to prevent the material from finding its way to the human population food chain but also to protect the environment. There are different treatment methods available and depending on the type of contaminant, different removal application with varying processes can be applied.
Usually selections of treatment methods are based on the characteristics of the wastewater and content of the toxic compounds. Three of the most basic application include carbon – activated sludge adsorption, use of chemicals and air stripping for volatile organic compounds (VOC). There are even specific methods on removal of dissolved inorganic compound in wastewater through use of different unit operations.
Carbon – Activated Sludge Adsorption
Carbon is usually effective to remove both organic and inorganic compounds especially suspended solids in wastewater effluent. It is usually applied in secondary treatment stage because if it is fixed at the influent side, high concentration of solids will cause severe blockage and render the system ineffective. Granular media filters are usually used in conjunction with activated carbon contactors to remove waste compounds and thus lead to reduction in the COD level. There are several types of contactors like the upflow and downflow columns and there is even the fixed bed or expanded types. Nowadays, there is even a proprietary process which combines use of activated carbon treatment with activated sludge process. The idea of combining both methods is to allow the carbon absorption to work simultaneously together with the biological oxidation process taking place together in the aeration pond.
Chemical Oxidation and Coagulation
Chemical oxidation is effective towards reduction of organic residual compound and removal of ammonia in wastewater. Chlorine is always the automatic choice of oxidant but because it form THMs, alternatives such as ozone disinfection is being considered. Another form of processes involving use of chemical can be performed using coagulation and sedimentation principle targeted specially for heavy metals and PCBs removal. Usually precipitation of phosphorus and other inorganic ions in wastewater can be effective by addition of coagulants such as alum and polymers.
Reverse Osmosis and Ultrafiltration
Reverse Osmosis uses semi-permeable membranes such as cellulose acetate and nylon to achieve separation of dissolved salts from the wastewater by applying pressure greater than the osmotic pressure caused by the dissolved salts using high pressure pumps. Pretreatment steps of effluent water before going for reverse osmosis process is necessary and usually this is made possible by using Ultrafiltration to first remove large molecules and colloidal materials so that this will help to protect the membranes. The spiral wound configuration of the membranes support structure proves to be the best and most effective in use when it comes to municipal wastewater reclamation.