Routes from Sewage Sludge to Phosphate Supply
The EU imports an estimated 975,000 tons of phosphate fertilizers annually. This non-renewable resource is mined in only a few locations, primarily the U.S., China, Morocco and Western Sahara. Evaluation of the supply chain reveals that only 20 percent of mined phosphorus is actually consumed, with the balance lost in the process. Recovery from secondary sources is increasingly being recognized as being part of a more sustainable wastewater treatment process.
By Divya Inna
The EU imports an estimated 975,000 tons of phosphate fertilizers annually. This non-renewable resource is mined in only a few locations, primarily the U.S., China, Morocco and Western Sahara.
Evaluation of the supply chain reveals that only 20 percent of mined phosphorus is actually consumed, with the balance lost in the process. Recovery of phosphorus from secondary sources such as municipal wastewater is increasingly being recognized as being a part of a more sustainable wastewater treatment process.
Routes for Recovery
Most phosphorus entering a wastewater treatment plant (WWTP) ends up in the sludge, leaving three principle routes for recovery:
- Application of sludge to land
- Recovery of phosphorus as struvite or magnesium ammonium phosphate
- Recovery of phosphorus from sewage sludge ash
Phosphorus can be recovered in the form of struvite crystals or magnesium ammonium phosphate (MAP) through crystallization from the aqueous sludge phase either prior to or after sludge dewatering.
Crystallization of struvite directly from digested sludge improves sludge dewatering. For instance, installation of the AirPrex® system between the digester and dewatering unit in Waßmannsdorf (Berliner Wasserbetriebe) and Neuwerk (Niersverband) has led to substantial savings in operational costs for sludge handling.
Struvite recovery became a commercial technology in 2014, seven years after being adopted by innovators. It is also taking off across various industrial applications like pharmaceuticals and food processing. An increasing trend in this area is led by companies like NuReSys and Paques, who have installed struvite recovery systems particularly in food processing facilities across Europe (see Table 1).
Based on BlueTech’s technology adoption curve, phosphorus recovery technologies are currently entering the ‘Early Adopter’ section of the technology market adoption curve (see Table 2). We anticipate that over the next five years those companies that have established reference facilities and process know-how will do well and will benefit from faster rates of market adoption.
BlueTech’s research interviews and patent analysis show that it is generally believed that enhanced biological phosphorus removal (EBPR) followed by struvite crystallization is the safest and most favorable option. Biological phosphorus treatment typically removes 80 percent of the incoming phosphorus, which ultimately ends up in the sludge and can be recovered if directly applied to land.
However, if application onto land is restricted and if, as in conventional EBPR, sidestream sludge treatment is applied, in particular sludge stabilization, then roughly 50 percent of phosphorus is re-released from the sludge matrix into the liquid phase. This means roughly 45 percent of the phosphorus is recovered from the total incoming load.
Bearing in mind that EBPR is in limited use, if roughly 10 percent of the total mass of phosphorus goes through EBPR treatment plants, then struvite can only recover roughly 5 percent of the total phosphorus load going through WWTPs, offering very little recovery potential.
Chemical Precipitation + Incineration
Chemical precipitation followed by sludge incineration and raw material capture from ash provides the best opportunity for maximum phosphorus recovery. The challenge with this approach is the separation of remobilized heavy metals from phosphorus and the disposal of the waste contaminated acid.
There is no technology demonstrated at industrial scale that can do this, but BlueTech Research believes that many are under development or still being constructed, such as EcoPhos or RecoPhos thermal.
If countries like Sweden, that have regulations stipulating the need to recover 60 percent or more phosphorus from WWTPs, enforce land application, this creates a huge market potential for thermal technologies like AshDec, Mephrec and LeachPhos. However these technologies need high capital investment on mono-incinerators.
Thermal Treatment with Mono-Incinerators
A third route is recovery from incinerated ash of sludge solids through thermal treatment using mono-incinerators. Sludge incineration is the controlled combustion of waste at high temperatures in a furnace.
The process destroys all pathogens and organic pollutants and the resulting ash contains the highest concentrate of phosphorus from the waste stream. However this waste stream also contains heavy metals that are not degraded in the incineration process and are present in concentrations higher than their allowed limits, restricting their use in agriculture. Treatment is therefore necessary and a crucial part is often the separation of the nutrient from the pollutants.
Given the lack of sufficient ash treatment facilities together with the high costs of investment and operation required for a mono-incinerator, co-incineration of dewatered sewage sludge in existing coal power plants or cement works offers an economic alternative. However the dilution of phosphorus with other substances restricts its recovery and therefore a 50 percent reduction of phosphorus from sludge prior to co-incineration is usually a prerequisite.
Based on future uncertainties associated with the availability of reserves, the market and policy, there is a need to further investigate and develop newer technologies that address factors like ease of technology implementation and scale of operational benefits. The recovery of phosphorus in a suitable form and extent of environmental and societal impacts will be critical.
BlueTech has identified a number of upcoming technologies that aim to maximize phosphorus (P) recovery. One such process is the adapted A-B process developed at Wetsus in the Netherlands that combines chemical dosing of iron for phosphorus and chemical oxygen demand (COD) removal in stage A followed by low temperature Annamox at stage B. The settled sludge is digested to produce biogas and P is recovered through the re-release of iron salts from sludge to produce struvite or apatite for the fertilizer industry.
Although the dosing of iron is effective in lowering phosphorus solubility, forming insoluble iron precipitates in water, what is still missing is a method to effectively recover phosphorus from sludge rich in iron phosphate (FeP). Wetsus researchers believe that P is easily mobilized from FeP in nature, especially in aquatic ecosystems, and are currently developing a biomimetic process that could provide an attractive alternative to the expensive acid leaching process.
About the Author: Divya Inna is a water technology market analyst for BlueTech Research, a consultancy that provides water technology market expertise and strategic advice on technology development and commercialization to start-up firms, the investment community and larger water corporations. Learn more at www.bluetechresearch.com.