Precipitation, filtration, and ion exchange are widely used ex situ treatment technologies for inorganics in ground water and are discussed in the following paragraphs. In situ treatment technologies are used less frequently.
The combination of precipitation/flocculation and sedimentation is a well-established technology for the removal of metals from ground water. This technology pumps ground water through extraction wells and then treats it to precipitate lead and other heavy metals. Typical removal of metals employs precipitation with hydroxides, carbonates, or sulfides. Hydroxide precipitation with lime or sodium hydroxide is the most common choice. Generally, the precipitating agent is added to water in a rapid-mixing tank along with flocculating agents such as alum, lime, and/or various iron salts. This mixture then flows to a flocculation chamber that agglomerates particles, which are then separated from the liquid phase in a sedimentation chamber. Other physical processes, such as filtration, may follow.
Metal sulfides exhibit significantly lower solubility than their hydroxide counterparts, achieve more complete precipitation, and provide stability over a broad pH range. At a pH of 4.5, sulfide precipitation can achieve the EPA-recommended standard for potable water. Sulfide precipitation, however, can be considerably more expensive than hydroxide precipitation, as a result of higher chemical costs and increased process complexity; also, there are safety concerns associated with the possibility of H2S emissions. The precipitated metals would be handled in a manner similar to contaminated soils. The supernatant would be discharged to a nearby stream, a POTW, or recharged to upstream of the site aquifer. Selection of the most suitable precipitant or flocculent, optimum pH, rapid mix requirements, and most efficient dosages is determined through laboratory jar test studies.
Filtration isolates solid particles by running a fluid stream through a porous medium. The driving force is either gravity or a pressure differential across the filtration medium. Pressure differentiated filtration techniques include separation by centrifugal force, vacuum, or positive pressure. The chemicals are not destroyed; they are merely concentrated, making reclamation possible. Parallel installation of double filters is recommended so ground water extraction or injection pumps do not have to stop operating when filters backwashed.
Ion exchange is a process whereby the toxic ions are removed from the aqueous phase in an exchange with relatively innocuous ions (e.g., NaCl) held by the ion exchange material. Modern ion exchange resins consist of synthetic organic materials containing ionic functional groups to which exchangeable ions are attached. These synthetic resins are structurally stable and exhibit a high exchange capacity. Other types of ion exchange materials include clays, zeolites, and peat derivatives. They can be tailored to show selectivity towards specific ions. The exchange reaction is reversible and concentration-dependent; the exchange resins are regenerable for reuse. The regeneration step leads to a 2 to10% wastestream that must be treated separately.
All metallic elements present as soluble species, either anionic or cationic, can be removed by ion exchange. A practical influent upper concentration limit for ion exchange is about 2,000 mg/L. A higher concentration results in rapid exhaustion of the resin and inordinately high regeneration costs.