ePhos – Electrochemical phosphorus recovery

© Fraunhofer IGB

Recovered struvite from wastewater that can be used directly as a slow releasing fertilizer.

The need for phosphorous fertilizers is growing with the increasing global demand for foodstuffs. The ePhos® process developed by Fraunhofer IGB is used for the recovery of phosphate and ammonium from municipal wastewater treatment plants. The substances are precipitated in an electrochemical process as ammonium magnesium phosphate (struvite) which is a high-quality slow-release fertilizer and can be used directly in agriculture as a fertilizer.

The main advantage of the ePhos® process is that no chemicals are added at all. The magnesium required for struvite formation is added by means of a sacrificial anode in an electrolysis cell.

Challenges

One approach to recover phosphorus from wastewater is the crystallization of P, N and magnesium (Mg) as struvite (ammonium-magnesium-phosphate: NH4MgPO4*6 H2O). In state-of-the-art struvite precipitation processes, the limiting reactant magnesium must be added as a solution of MgCl2, Mg(OH)2 or MgO. Furthermore, the pH value has to be raised to a range between 8.5 and 9.5, usually by adding sodium hydroxide. The need for these chemicals, however, increase the costs of the production process.

Our solution

Fraunhofer IGB has developed and patented a novel process to recover ammonium (NH4+) and phosphate (PO43–). In this electrochemical process with a magnesium electrode, NH4+ and PO43– are precipitated as struvite. This new process has the advantage that no chemicals such as MgCl2 or NaOH need to be added for the precipitation process.

Advantages

© Fraunhofer IGB

Design of an installation plan for a wastewater treatment plant with 500,000 PE.

The ePhos process has the advantage that no chemicals such as MgCl2 or NaOH need to be added for the precipitation process. The ePhos® plant concept is based on the series connection of electrolytic cells in parallel. As this is a purely electrochemical process, the cells or cell pathways can be switched on or off by a process control system depending on the demand. This beneficial mode of operation and the efficient, chemical-free operation represent unique selling propositions that insure competitiveness. 

Moreover, the technology has a low energy consumption (50–70 Wh/m³ wastewater). Thus, with this process, a simple, efficient and flexible technology is available for recycling struvite as a high quality fertilizer.

The recovered product struvite can be used directly in agriculture as a high-quality, slow releasing fertilizer. Struvite was tested in plant-pot experiments showing excellent results. Plant yield and plant nutrient-uptake with struvite were up to 4 times higher than with commercially available mineral fertilizer (calcium ammonium nitrate and triple superphosphate). This result demonstrates that struvite is easily plant-available and has a positive effect in plant growth.

Process development

© Fraunhofer IGB

Mobile pilot plant for phosphorus recovery.

Process principle

Water-cleavage takes place as a result of the cathodic reduction. OH­–- ions are formed, while hydrogen (H2) is released. Oxidation takes place on the anode. Magnesium ions are released into the solution and react with the P and N in the water to form struvite. Here, magnesium is the limiting reactant. As a result of the electrolytic water-splitting at the cathode, the pH value of the wastewater increases and remains constant at pH 9. This has the advantage that no base has to be added for the struvite precipitation process.

 

Feasibility study in a pilot plant

In the course of a feasibility study the process was tested using a pilot plant with a flow rate of up to 1 m3/h at a sewage treatment plant with biological phosphorus elimination. The average phosphorus elimination rate from the centrate water of the digested sludge dewatering and the phosphorus conversion to struvite was more than 80 percent.

The phosphorus concentration in the centrate water was reduced by an average of 180 mg/L to 20.8 mg/L. The phosphorus load that no longer has to be treated when the filtrate water is recirculated, decreases by 37 percent; this amounts to 9284 kilograms annually and results in a reduction of sludge production by 7 percent. The design of the process for the client’s plant shows that the electrochemical phosphate precipitation would require approx. 10 tons of magnesium in the form of sacrificial electrodes per year. From this, approx. 73 tons of struvite per year would be obtained which can then be used directly as a fertilizer. The total quantity of chemicals that would have to be used at the treatment plant would decrease by 40 tons or 20 percent per year. 

 

Industrial-scale implementation with flat-panel reactors

With the results from the first pilot tests, in which tubular electrolytic cells were used, the process was further developed with flat-panel reactors for industrial use. 

© Fraunhofer IGB

Demonstration operation: ePhos® reactors of the large-scale plant, flow rate up to 3 m³/h.

Continuous operation at sewage treatment plant    

The continuous operation of a pilot plant was successfully tested under real-life conditions last year on the premises of a municipal wastewater treatment plant, with a flow rate of up to 3 m³/h. An average elimination rate of more than 80 percent was achieved over the one-year operating period. The average energy requirement for the treatment of one cubic meter of filtrate water from dewatering waste sludge was just under 0.5 kWh. The technology is further optimized with the know-how gained from the pilot test phase.

A first industrial-scale implementation is in preparation at a North German wastewater treatment plant with a population equivalent (PE) of 70,000. It is used there to recover phosphate from filtrate water produced in dewatering sludge after digestion.    

Market launch

A license agreements for industrial use has been concluded with the US company OVIVO, an established supplier of water management equipment and systems, for the market in the USA, Canada and Mexico. Biological phosphorus elimination with subsequent stabilization of the biomass in anaerobic stages is being employed to an increasing extent at sewage treatment plants in North America. In this method of operation, phosphorus has to be specifically removed from the sewage treatment process to avoid uncontrolled precipitations in the filtrate water pipes and sludge dewatering equipment, which otherwise would cause enormous damage and operating costs.

Fields of application

The process is also suitable for the food industry and agricultural biogas production, whose wastewater is rich in ammonium and phosphate. In the future, further process modules will be added to the ePhos® process in order to achieve sustained cycle management of the nutrients at sewage treatment plants.