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.

NPHarvest process has been optimized for membrane stripping and pretreatment with several different wastewater streams. The results can be found in publications section. Most recent development concerns use of natural coagulants and further optimization for membrane performance.

Considering the pre-treatment unit, the possibility to use natural coagulants has been studied. Wastewater streams considered are reject water from Viikinmäki wastewater treatment plant (already tested with the pilot) and Ämmässuo landfill leachate being it the new wastewater stream to be tested with the pilot. Among the natural coagulants available on the market, tannin-based and starch-based ones were provided to be tested in lab with jar tests. After screening tests starch-based coagulant PrimePHASE 3545 was selected to be used with Viikinmäki reject water, while the tannin-based coagulant HTH was selected to be used to treat landfill leachate. By using Design Expert software, Response Surface Methodology was applied to model the parameters selected to evaluate the effectiveness of the treatment. Optimization of the process based on generated models gave best combination of dose and pH, and the corresponding predicted values for each parameter. Parameters for optimization are all set to minimum concentration, while ammonia nitrogen and TN are set at maximum concentration. Results are as following:

Viikinmäki

pH range: 8-10

Dosage range: 10-20 ml/l (25% diluted solution)

Ämmässuo alkaline pH

pH range: 8-10

Dosage range: 5-10 ml/l

Ämmässuo neutral/acidic pH

pH range: 6-8

Dosage range: 5-20 ml/l