The treatment of wastewater (and sewage) spreads across three main stages: primary treatment, secondary treatment and finally the tertiary treatment. Tertiary treatment process is used to improve the quality of the effluent which has resulted from primary and secondary treatment processes. After the tertiary treatment, the water can be safely discharged into the environment (water bodies like rivers and lakes etc.) And used for agricultural, industrial and, in many cases, domestic purposes.
As the wastewater reaches the tertiary treatment stage, it still has residual suspended matter and fine particulates. Further, it has a relatively high level of nutrients such as nitrogen and phosphorus and has microbes and odor in it. During tertiary treatment process, different methods are used to remove all these contaminants and properties from wastewater.
The first stage of tertiary treatment is filtration which helps primarily to remove residual suspended matter in wastewater. Sand filtration is the usual method that is used for this purpose. In some cases, residual toxins may be present in wastewater and to filter them out, activated carbon is used to adsorb the toxins and remove them from wastewater.
After filtration, some fine particulate matter may still remain in the wastewater. To treat the wastewater further, it is transferred to lagoons with and filter feeders such as Daphne. In the lagoon’s aerobic and biologically enriched environment, further removal of fine particulate matter takes place. The wastewater may still have high nutrient concentration. Nitrogen and phosphorus are usually found in high concentration in wastewater and this, if untreated and released into the natural water environment, can cause excessive growth, consequent death (due to nutrient and space limitation) and eventual decomposition of algae. As the dead algae due to bacterial action, the amount of oxygen becomes much less than is necessary for other aquatic life to survive. Therefore, reduction of nitrogen and phosphorus concentrations is critical before wastewater release into natural water bodies. Both nitrogen and phosphorus can be removed using biological processes involving different bacteria. Nitrogen in the form of ammonia is first oxidized to form nitrates and then nitrates to nitrogen gas which is released to the environment. On the other hand, phosphorus can be removed biologically as well by chemical precipitation with salts of iron, aluminium, or lime. accumulating bacteria can accumulate high percentage of phosphorus and the biomass thus generated can further be of good value as fertilizer. Similarly, phosphate-rich sludge which is produced as a result of precipitation during chemical treatment for removal of phosphorus has good value as fertilizer.
The final steps in tertiary wastewater treatment, before release into the environment, are removing any undesirable microbes through the process of disinfection and also removal of odors from wastewater. The clear and less cloudy nature of wastewater at this stage is critical for efficiency and effectiveness of the disinfection process. Several disinfection agents can be used depending on wastewater condition (pH, clarity etc.) And among them chlorine, ozone, ultraviolet (UV) light are most common. These have their advantages and disadvantages. Chlorine is a time-tested effective disinfection agent. However, long term use and unregulated dosage can lead to generation of chlorinated-organic compounds from chlorination of residual organic matter in water. Such compounds may be carcinogenic in nature. Therefore, a de-chlorination step is important if the water is to be discharged into larger aquatic bodies. Ultraviolet light treatment for disinfection does not leave any residues or compounds behind in the water but for it to be effective, the wastewater must be clear. Proper lamp and equipment maintenance is necessary for long term usage of UV light. Ozone is also a very effective disinfection agent and does not need any storage place as it can be generated on site using the necessary equipment for passing oxygen through high voltage current.
Odor in wastewater is mainly a result of release of gases, such as hydrogen sulfide, from anaerobic processes in wastewater. Carbon reactors, regulated amounts of chlorine, hydrogen peroxide, calcium nitrate etc. Can be effectively used to manage hydrogen sulfide levels and thus reduce odors from wastewater.
Eventually, the wastewater after the tertiary treatment is fitting, after necessary tests, to be released into the environment for further re-use.