WATER & EFFLUENT TREATMENT
Dam good river management
By Xavier Mear , Product Manager , Lhoist UK
“ Water is not a commercial product but a heritage to be protected .”
Increasing levels of European legislation , led by the Water Framework Directive , is putting great pressure on companies involved with wastewater treatment .
Nutrient removal and reduction technologies will continue to be at the centre of efforts on water protection . The presence of nutrients , especially nitrogen and phosphorous in wastewater effluents and their impacts on natural water bodies , continue to be a major concern for companies involved with wastewater treatment .
Phosphates , like nitrates , are responsible for eutrophication , causing a deterioration of an aquatic ecosystem by the proliferation of certain plants , including algae . Industrial emissions , but also urban waste streams , contain these pollutants .
Eutrophication – keeping the industry awake at night
The presence of phosphorous in rivers and lakes is responsible for eutrophication , resulting in the uncontrolled proliferation of algae and excessive consumption of dissolved oxygen in the body of the water . It is thereby less available for other living species , and in particular fish , making it necessary to restrict phosphorous discharges into natural water courses .
Separating phosphate from aqueous solutions exist and the physicochemical processes utilised are based on precipitation phenomena by use of salts of calcium , iron or aluminium , or adsorption phenomena . Phosphate removal may also be achieved by biological processes .
Strategies
The removal of Phosphorous can take place successfully through a process called enhanced biological phosphorous removal . Specific bacteria are selectively enriched and accumulate large quantities of phosphorous within their cells ( up to 20 % of the mass ). When the biomass enriched in these bacteria is separated from the treated water , these biosolids have a high fertiliser value .
A well-buffered system maintains the pH where biological activity is optimised and allows for a more efficient use of oxygen , leading to reduced energy costs . Treating wastewaters using lime-based reagents increases the wastewaters ’ buffering capacity and helps the phosphorous removal step .
Phosphate precipitation
Chemical precipitation is used to remove inorganic forms of phosphate by the addition of a coagulant and a mixing of wastewater and coagulant . The multivalent ions most commonly used are calcium , aluminium and iron .
Calcium
Calcium is usually added to the process as hydrated lime Ca ( OH ) 2 . It is reported that addition of lime makes it possible to remove 85-90 % of the inorganic orthophosphates present in wastewater .
The formation of calcium phosphate salts in aqueous solutions takes place following the development of supersaturation , with excess calcium ions reacting with the phosphate to precipitate hydroxyapatite .
Aluminium and Iron
Alum is widely used precipitating phosphates , as are ferric chloride and sulfate . For both reagents however , the resulting AlPO4 precipitates need to be removed by flocculation via the excess addition of a metal hydroxide , and typically lime is added to enhance this treatment step
For more information on Neutralac please contact Xavier Mear , Product manager at Lhoist UK on 07968 755590 or Xavier . mear @ lhoist . com
34 NOVEMBER 2020 twitter : @ surfaceworldmag