Division 8.6 Techniques to reduce emissions to water
United Kingdom
| Technique | Description |
|---|---|
| Adsorption on activated carbon | The retention of soluble pollutants on the surface of solid, highly porous particles (the adsorbent). Activated carbon is typically used for the adsorption of organic compounds and mercury. |
| Aerobic biological treatment | The biological oxidation of dissolved organic pollutants with oxygen using the metabolism of microorganisms. In the presence of dissolved oxygen — injected as air or pure oxygen — the organic components are mineralised into carbon dioxide and water or are transformed into other metabolites and biomass. Under certain conditions, aerobic nitrification also takes place whereby microorganisms oxidise ammonium (NH4 +) to the intermediate nitrite (NO2 –), which is then further oxidised to nitrate (NO3 –). |
| Anoxic/anaerobic biological treatment | The biological reduction of pollutants using the metabolism of microorganisms (e.g. nitrate (NO3 –) is reduced to elemental gaseous nitrogen, oxidised species of mercury are reduced to elemental mercury). The anoxic/anaerobic treatment of waste water from the use of wet abatement systems is typically carried out in fixed-film bioreactors using activated carbon as a carrier. The anoxic/anaerobic biological treatment for the removal of mercury is applied in combination with other techniques. |
| Coagulation and flocculation | Coagulation and flocculation are used to separate suspended solids from waste water and are often carried out in successive steps. Coagulation is carried out by adding coagulants with charges opposite to those of the suspended solids. Flocculation is carried out by adding polymers, so that collisions of microfloc particles cause them to bond thereby producing larger flocs. |
| Crystallisation | The removal of ionic pollutants from waste water by crystallising them on a seed material such as sand or minerals, in a fluidised bed process |
| Filtration | The separation of solids from waste water by passing it through a porous medium. It includes different types of techniques, e.g. sand filtration, microfiltration and ultrafiltration. |
| Flotation | The separation of solid or liquid particles from waste water by attaching them to fine gas bubbles, usually air. The buoyant particles accumulate at the water surface and are collected with skimmers. |
| Ion exchange | The retention of ionic pollutants from waste water and their replacement by more acceptable ions using an ion exchange resin. The pollutants are temporarily retained and afterwards released into a regeneration or backwashing liquid. |
| Neutralisation | The adjustment of the pH of the waste water to the neutral pH level (approximately 7) by adding chemicals. Sodium hydroxide (NaOH) or calcium hydroxide (Ca(OH)2) is generally used to increase the pH whereas sulphuric acid (H2SO4), hydrochloric acid (HCl) or carbon dioxide (CO2) is generally used to decrease the pH. The precipitation of some pollutants may occur during neutralisation. |
| Oil-water separation | The removal of free oil from waste water by gravity separation using devices such as the American Petroleum Institute separator, a corrugated plate interceptor, or a parallel plate interceptor. Oil-water separation is normally followed by flotation, supported by coagulation/flocculation. In some cases, emulsion breaking may be needed prior to oil-water separation. |
| Oxidation | The conversion of pollutants by chemical oxidising agents to similar compounds that are less hazardous and/or easier to abate. In the case of waste water from the use of wet abatement systems, air may be used to oxidise sulphite (SO3 2–) to sulphate (SO4 2–). |
| Precipitation | The conversion of dissolved pollutants into insoluble compounds by adding chemical precipitants. The solid precipitates formed are subsequently separated by sedimentation, flotation or filtration. Typical chemicals used for metal precipitation are lime, dolomite, sodium hydroxide, sodium carbonate, sodium sulphide and organosulphides. Calcium salts (other than lime) are used to precipitate sulphate or fluoride. |
| Sedimentation | The separation of suspended solids by gravitational settling. |
| Stripping | The removal of purgeable pollutants (e.g. ammonia) from waste water by contact with a high flow of a gas current in order to transfer them to the gas phase. The pollutants are removed from the stripping gas in a downstream treatment and may potentially be reused. |