Category / type: Chemical or Biological Nutrient Removal (BNR).
Application: medium, large and municipal wastewater treatment, industrial water process.
Size: from 3 up to 20 m3.
Flow: from 1,0 up to 100 m3 per day.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
Focus: high efficiency treatment.
The most widespread process for phosphorous removal is chemical precipitation. It is typically performed through aluminium and iron coagulants, set before biological treatments; anyhow phosphorus can be precipitated in the biological treatment or as improvement step of the treatment process as well.
Wastewater may contains phosphorous, organic and inorganic. For municipal wastewater, the individual contribution tend to increase in some (residential populated) areas, since phosphorous is one of the main constituent of synthetic detergents. The contribution is significant also because of some industrial sectors as fertilizer and explosive production.
It’s worth to mention that thanks to the late European regulation, the amount of discharged phosphorous is continuously decreasing.
Phosphorus removal (P removal), through special dedicated treatment systems, is often necessary, due to eutrophication problems increasing, no longer exclusively interested on lake or marsh environments.
As the phosphorus mass fraction in volatile sludge is about 2.5% of the VSS concentration in a conventional activated sludge process, the excess sludge discharge will also result in the partial removal of phosphorus from the wastewater. In case of discharge of organic phosphorus with the excess sludge is the only mechanism of phosphorus removal, only under favourable conditions (i.e. a low P/BOD ratio combined with a short sludge age), it is possible to achieve the goal. The system efficiency can reasonably range from 10 up to 30%.
An equivalent inhabitant (or PE) quantity is considered as 1.8 g P/PE per day. Most of the European regulations require to lower the effluent phosphorus concentration to a value less than 1 mg P/l.
In wastewaters with a higher level of nutrients or activated sludge systems operating at a higher sludge age (or extended aeration), an additional methods of phosphorus removal is necessary.
Phosphorus removal from wastewater can be achieved either through chemical removal, advanced biological treatment or a combination of both. The chemical removal involves the addition of calcium (lime), iron (ferric chloride) and aluminium (aluminium polychloride) salts to achieve phosphorus precipitation by various mechanisms.
An alternative process is the biological excess phosphorus removal, in which biomass with an increased phosphorous content develops in the activated sludge system.
The accumulated phosphorus is subsequently removed from the wastewater through discharge with the excess sludge.
If the biological removal capacity is insufficient, supplementary chemical dosing may be applied.
How It Works
Phosporous removal by chemical precipitation has been used for long time. The employed chemicals are most often a compounds of calcium, aluminium and iron. Chemical addition points can be installed prior to a primary settling, during a secondary treatment or as part of a tertiary treatment process.
A major concern with chemical precipitation is the production of additional sludge. This can be important, especially if the method selected is lime application during primary treatment, due to high organic matter concentration in primary inlet flow.
The removal of phosphorus compounds can be also carried by a out simultaneous precipitation method. This process takes place at the same time of insoluble phosphorus precipitation and of biological phosphorus removal (formation of new cells).
Co-precipitation does not require an additional reaction basin, but often requires the dosage of a polyelectrolyte upstream to secondary sedimentation.
Since chemical and biological sludge are mixed, biological available volume decreases.
Use of alum after secondary treatment can produce less sludge, but it requires a post sedimentation chamber or a tertiary filtration step.
This layout is suitable for high purification standards (0.2 up to 0.5 mg p/L) or in wastewater recycling plants where filtration is often required obtaining a stable separated chemical sludge.
The common element in enhanced biological phosphorus removal (EBPR) implementations is the presence of an anaerobic tank (where nitrate and oxygen are absent) prior to the aeration tank. Under these conditions a group of heterotrophic bacteria, called polyphosphate accumulating organisms (PAO) are selectively enriched in the bacterial community within the activated sludge. These bacteria accumulate large quantities of polyphosphate within their cells, reaching a high phosphorus removal.
Sludge retention time (SRT) for bio-P bacteria is 3 days maximum, significantly less then BOD on nitrogen removal (nitrification).
Various system configurations have been developed for bio-P removal, all of which have been extensively applied in practice. The main difference between these configurations is the way in which an anaerobic zone is maintained and how this zone is protected against the introduction of nitrate.
There are two basic different layouts. The main difference is in the way of phosphorus removing: mainstream or sidestream.
The combined use of chemical and biological treatment leads to a good result in effluent quality.
Operation and maintenance
Chemical phosphorous removal produces excess sludge increasing in addition to the biologic one.
Sludge needed volume changes depending on dosing position with advantages and disadvantages for each choice.
Chemical phosphorous removal increases BOD removal performance, because organic matter is captured by reagents (called secondary reactions).
Equipment for chemical phosphorus removal is usually encased in a plastic container in the main control room. The device is connected to the WWTP dispensing hose. The metering pump is set to automatically dose a specified amount of reagent. It always requires power supply.
In the biological removal of phosphorous, the phosphorous in the influent wastewater is incorporated into cell biomass, which is subsequently removed from the process as a result of sludge wasting. The reactor configuration provides the P accumulating organisms (PAO), with a competitive advantage over other bacteria. PAO are encouraged to grow and consume phosphorous. The reactor configuration is comprised of an anaerobic tank and an activated sludge tank.
If the process is not properly managed, phosphorus release into the water or in the supernatant is possible.
Maintenance operations require skilled personnel.
The main problem associated to phosphorous presence in effluent water is the extravagant growth of aquatic life, also called eutrophication. This reduces the quality of the receiving water and the suitability for reuse. For this reason in many countries effluent standards have been implemented with less phosphorus outlet concentration. Normally effluent quality required in phosphorous concentration for public sewer discharging is between 1 e 2 mg/l up to 0.5 for critical conditions (industrial, zootechnical, etc.).
Features and benefits
Thanks to the considerable reduction of phosphorous in cleaning products, the greatest contribution nowadays comes from industry (polyphosphates as corrosion inhibitors or anti-fouling) and livestock farming.
For small population or industrial users (especially if there is no equalization chamber) the large discharge variation can cause serious problems to biological process. Extended aeration and low volumetric organic loading (which is defined as the BOD applied per unit volume of aeration tank, per day) is not always compatible with biologic phosphorous removal.
By contrast, chemical process has less problems with flow variation, but it is normally much expensive (reagents supply).
The most powerful solution for medium and large WWTP is system integration: biological and chemical working together.
For small and micro plats, chemical removal is still the best solution.
Automatic (or remote) controls is needed for systems management and interaction.
- eutrophication reduction;
- stable biological sludge (chemical);
- no additional agents (flocculants) necessary (biological);
- decrease of salts in the effluent of a WWTP (biological);
- lower additional sludge production (biological);
- no additional heavy metals in the sludge (biological);
- for environmentally sensitive areas (ESA).
- higher investment cost;
- high costs of the consumed chemicals (chemical);
- significant increase of the excess sludge production (chemicals);
- a little bit more unstable process (biological);
- higher tendency of bulking sludge in winter time (biological);
- skilled personnel required.