Results

The reject waters of digesters in Finland have an interesting marketing potential for NPHarvest-technique. The amount of biogas plants has increased over the last decade: in 2017 total amount of 330 000 tons of sludge or organic waste was digested. Reject water was produced over 5 million cubic meters. Approximately 60 % of the reject water was produced in digesters attached to waste water treatment plants and 30 % in biogas plants. In addition to reject water nitrogen rich liquid wastes are produced also in composting plants. Also, water originating in active landfill sites have high nitrogen content. It is common to circulate water in landfill sites back to the site which decreases the liquid volume but increases concentrations. After the landfill site closes the concentrations decrease however. Furthermore, not depositing organic waste in landfill sites further decreases nitrogen concentration in seeping waters.

Based on an inquiry sent to several wastewater treatment plants and biogas plants it is clear that nitrogen load in reject water and finding a suitable treatment method are the most common factors affecting sludge treatment process in WWTPs because the reject water is usually sent back to the beginning of the process. In biogas plants, the reject water is pre-treated to reduce N, P, NH4, suspended solids and BOD. After that, it is sent to a WWTP to be further treated as wastewater. Based on this is may be concluded that there indeed is a need for a process such as ours.

Optimization tests were run with a lab scale pilot plant. The reject water tested came from HSY (Helsingin seudun ympäristöpalvelut, Environmental services for Helsinki area) Viikinmäki waste water treatment plant and HSY Ämmässuo biogas plant. For this phase, hydraulic retention time, acid circulation rate and concentration as well as membrane thickness were optimized for the reactor. HRT wise the harvesting efficiency increases linearly between 2 and 12 hours. Based on retention time tests a correlation curve with membrane surface area was drawn. By adding membrane surface area, it is possible to decrease retention time without harvesting efficiency going down. Membrane thickness did not have any significant effect on harvesting efficiency. Results regarding the composting waters were similar to digester water results. Sulphuric acid and phosphoric acid work equally well.

The process has been tested in lab scale with urea and reject water. Produced substances are commercial-like white powder. From left to right: commercial ammonium sulfate, ammonium sulfate produced from urea, ammonium phosphate produced from reject water and solid phosphate compound.

Results are as follows:

- The recovery efficiency improved linearly between 2 to 8-hour hydraulic retention time (HRT)

• 60 % recovery efficiency was reached in 8 hours

- Optimal acid circulation rate was determined to be 300 l/h/m2 related to reactor membrane surface area.
- The efficiency was also affected by the pH of the reject water.

• Higher pH -> better efficiency

- Membrane thickness and acid type did not have any significant impact on harvesting efficiency.