Head of Research & Development Department
SF-Soepenberg GmbH
Germany
j.clemens@soepenberg.com
Christine Oeppert
Wastewater Expert
SF-Soepenberg GmbH
Germany
Wastewater Expert
SF-Soepenberg GmbH
Germany
Sewage sludge in the Baltic Sea catchment
Overall, around 52 million people were connected to tertiary wastewater treatment plants (WWTPs) in the Baltic Sea catchment area in 2020, representing 72% of the total population. These WWTPs largely reduce (70% – 90%) the discharge of nitrogen and phosphorous into the Baltic Sea. While nitrogen is transformed to gaseous N2 to a certain extent during wastewater treatment, all of the phosphorous (P) is “stored” is the sewage sludge. This makes sewage sludge an interesting source as fertilizer in agriculture.
The Baltic Sea countries follow different approaches to use the sludge. The majority of the countries field apply most of their produced sludges in agriculture directly or after composting (e.g. Denmark, Estonia, Lithuania, Sweden). However, in Germany, that produces 60% of the Baltic Sea related Phosphorous load in sewage sludge, most of the sludge is incinerated. There, field application of sewage sludge was more and more reduced because of environmental concerns.
Due to the EU Urban Wastewater Treatment Directive, P recycling from sewage sludge or wastewater will become more important in all EU countries. To recycle at least half of the P from wastewater (or sewage sludge) could be an envisaged aim for all European countries under the umbrella of the EU directive. Similar aims are found in current legislations in Austria, Switzerland and Germany already.
Germany is a current hot spot of P recycling activities
In Germany, starting from the year 2029, wastewater treatment plants are required to recycle phosphorus if the phosphorus content exceeds 2% in the dried sewage sludge. Bigger plants need to recycle P from 2029 on, smaller ones from 2032. It is expected that most of the phosphorous will be recycled in form of fertilizers or prerequisites for fertilizers.
Because of the legal changes in Germany, the country has become a focal point for phosphorous recycling technologies. Technologies are competing but so far none of the technology providers has proven that their technology can operate steadily. Experts assume that most of the sewage sludge will be treated at centralized sites. In this case, mono-incineration of sewage sludge is the first technological step to enable phosphorus recovery. It is followed by physico-chemical processes that extract phosphate from the ash and concentrate it. The end products can be for example phosphoric acid or tricalcium phosphate. However, many established pathways for sewage sludge recovery, such as co-incineration in the cement industry, are lost due to mono-incineration of the sludge. In addition, in less populated areas, long and climate unfriendly transports are required to bring the sludge to the centralized phosphorous recycling facilities.
A closer look to phosphorous recycling at WWTPs
On site, P recycling technologies have not managed to recycle more than 50% of the P input into the WWTP, so far. Why is this so? It is necessary to have a closer and more technical look into the WWTP: Basically, there are two ways to reduce the P from wastewater. The biological uptake of P into the microorganisms at a WWTP requires a special treatment regime inside the plant. The process has its specific challenges and not all plants are able to remove enough P from the wastewater constantly. The second option is the precipitation of P with iron or aluminum salts. It requires the dosing of Fe and Al and it is the predominant technology used in WWTP. Even most of the WWTP that are using the biological P removal technology, dose Fe or Al certain extent to reliably reduce P in the effluent water.
Until now, only biological bound P in the sludge could be remobilized into the water phase and then precipitated in form of e.g. struvite. The key for higher P remobilization from wastewater (or sewage sludge) is to break up iron-phosphate salts again, after these salts were deliberately formed. Besides excessive use of acid or alkaline, reductive processes are able to remobilize P from FeP salts. Such a reductive method for P recovery was developed as an on site technology in Germany.
Process description
In a nutshell, the whole recycling process covers five treatment steps. The activated excess sludge in a WWTP is first reduced by sulfur-containing reducing agents. It is the time limiting step of the process. An average residence time of 10-24 hours is optimal. The next three steps are fast and comprise slight acidification to a pH of 4, a heavy metal removal and a flocculation of the sludge for subsequent sludge dewatering. After sludge dewatering the remobilized phosphate is in the water phase. Now, it can be precipitated in the form of magnesium ammonium phosphate (struvite).
It is a well-known but not widely produced slow release fertilizer. It is listed in the EU Fertilizer Products Regulation as material that falls under the Component Material Category No. 12.
The remobilization grade depends on factors such as reduction time, wastewater temperature and dosage amount of the reducing agent. The P recovery process was tested continuously with the sludge of 5,000 person equivalents on a WWTP in Germany for three months. P recovery was maximum 70%.
This technology may overcome the current limitations of effective P recycling on WWTPs and may be the base for decentralized P recycling. However, as all other technologies, it must still prove its full scale continuous operation.
In Germany, starting from the year 2029, wastewater treatment plants are required to recycle phosphorus if the phosphorus content exceeds 2% in the dried sewage sludge. Bigger plants need to recycle P from 2029 on, smaller ones from 2032. It is expected that most of the phosphorous will be recycled in form of fertilizers or prerequisites for fertilizers.
Because of the legal changes in Germany, the country has become a focal point for phosphorous recycling technologies. Technologies are competing but so far none of the technology providers has proven that their technology can operate steadily. Experts assume that most of the sewage sludge will be treated at centralized sites. In this case, mono-incineration of sewage sludge is the first technological step to enable phosphorus recovery. It is followed by physico-chemical processes that extract phosphate from the ash and concentrate it. The end products can be for example phosphoric acid or tricalcium phosphate. However, many established pathways for sewage sludge recovery, such as co-incineration in the cement industry, are lost due to mono-incineration of the sludge. In addition, in less populated areas, long and climate unfriendly transports are required to bring the sludge to the centralized phosphorous recycling facilities.
A closer look to phosphorous recycling at WWTPs
On site, P recycling technologies have not managed to recycle more than 50% of the P input into the WWTP, so far. Why is this so? It is necessary to have a closer and more technical look into the WWTP: Basically, there are two ways to reduce the P from wastewater. The biological uptake of P into the microorganisms at a WWTP requires a special treatment regime inside the plant. The process has its specific challenges and not all plants are able to remove enough P from the wastewater constantly. The second option is the precipitation of P with iron or aluminum salts. It requires the dosing of Fe and Al and it is the predominant technology used in WWTP. Even most of the WWTP that are using the biological P removal technology, dose Fe or Al certain extent to reliably reduce P in the effluent water.
Until now, only biological bound P in the sludge could be remobilized into the water phase and then precipitated in form of e.g. struvite. The key for higher P remobilization from wastewater (or sewage sludge) is to break up iron-phosphate salts again, after these salts were deliberately formed. Besides excessive use of acid or alkaline, reductive processes are able to remobilize P from FeP salts. Such a reductive method for P recovery was developed as an on site technology in Germany.
Process description
In a nutshell, the whole recycling process covers five treatment steps. The activated excess sludge in a WWTP is first reduced by sulfur-containing reducing agents. It is the time limiting step of the process. An average residence time of 10-24 hours is optimal. The next three steps are fast and comprise slight acidification to a pH of 4, a heavy metal removal and a flocculation of the sludge for subsequent sludge dewatering. After sludge dewatering the remobilized phosphate is in the water phase. Now, it can be precipitated in the form of magnesium ammonium phosphate (struvite).
It is a well-known but not widely produced slow release fertilizer. It is listed in the EU Fertilizer Products Regulation as material that falls under the Component Material Category No. 12.
The remobilization grade depends on factors such as reduction time, wastewater temperature and dosage amount of the reducing agent. The P recovery process was tested continuously with the sludge of 5,000 person equivalents on a WWTP in Germany for three months. P recovery was maximum 70%.
This technology may overcome the current limitations of effective P recycling on WWTPs and may be the base for decentralized P recycling. However, as all other technologies, it must still prove its full scale continuous operation.