Phosphate precipitation or phosphate removal in wastewater treatment
Glossary of ETL Verfahrenstechnik GmbH
The removal of phosphorus compounds from wastewater, known as phosphate precipitation or phosphate removal, is a key process in wastewater treatment plants. Originally introduced in the 1980s under the term "elimination," this method aims to prevent the eutrophication of water bodies caused by phosphate compounds, which act as fertilizers. The use of phosphate-based detergents until the 1990s significantly increased phosphate levels in municipal wastewater systems.
In modern wastewater treatment plants, phosphates are primarily removed by two methods:
- Chemical phosphate precipitation using precipitating agents
- Biological phosphate removal in wastewater treatment processes
These methods convert dissolved phosphates into solid forms, which are then either reused or disposed of as part of the sewage sludge. Common precipitating agents include ferric chloride, ferrous sulfate, aluminum sulfate, sodium aluminate, and milk of lime. These substances can be added directly to the biological treatment stage, applied in wastewater side streams, or used in pre- or post-precipitation processes. Modern treatment plants can adjust chemical precipitation based on current phosphate concentrations, and many also employ biological phosphate removal to conserve resources.
Simultaneous precipitation, in which the precipitating agent is added before the aeration tank, is a well-established process that combines phosphate removal with biological treatment. This results in increased sludge production but also improves sludge structure and settling behavior, thereby enhancing overall plant performance.
Pre-precipitation, also known as pre-flocculation, focuses on relieving the biological treatment stage by removing phosphates beforehand. This approach is particularly beneficial in older, overloaded plants but increases the consumption of precipitating agents.
Post-precipitation, carried out after secondary clarification, uses trivalent metal salts to precipitate phosphates and achieves excellent effluent phosphate levels, although it also requires significant chemical input.
Biological phosphate removal relies on polyphosphate-accumulating organisms that release phosphates under anaerobic conditions in order to survive. This sensitive process is often combined with chemical methods to maximize efficiency.
Given the limited global phosphate reserves and the essential role of phosphorus in plant growth, efforts are increasing to recover phosphorus from wastewater and sewage sludge to promote sustainability and resource efficiency in wastewater treatment.
Summary:
The precipitation and removal of phosphates in wastewater treatment plants is an essential process for purifying wastewater by eliminating phosphorus compounds to prevent the eutrophication of water bodies. These processes include both chemical methods using precipitating agents such as ferric chloride and aluminum sulfate, as well as biological techniques based on the use of polyphosphate-accumulating organisms.
Modern treatment plants often use a combination of both methods to ensure effective phosphate removal. The goal of the treatment is to convert dissolved phosphates into solid forms that can be reused or safely disposed of. In light of the limited availability of phosphate resources, increasing emphasis is placed on recovering phosphorus from wastewater to promote sustainability and resource conservation in the wastewater industry.
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