Introduction
AquaBioTech (ABT) Group in the SEA2LAND project is responsible for providing marine aquaculture sludge for the nutrient extraction methods employed by UVIC (Universitat Central de Catalunya). Based on the circular economy model, SEA2LAND promotes the production of fertilisers in the EU from raw materials. This solution is expected to reduce the soil nutrient imbalance in Europe. However, there are still technological difficulties surrounding the concentration of sludge, , therefore there is a need to modify and change the existing concept in order to search for other ways of concentrating more sludge and perhaps of better quality (nutrient concentration).
It is well known that ozone causes clumping of solids (micro-flocculation), which facilitates their removal by foam-fractionation, filtration, and sedimentation. It was thought that ozone treatment could be effective after drum-filters or even before the drum-filter. In this case, the backwash of drum-filters could provide not just normal sludge but also more flocculated solids (treatments with ozone lead to flocculation). This will result in more sludge removed (parts filtered by the drum and solid clumps) and hopefully could solve the problem identified in the project.
Wastewater was treated with ozone on many occasions at ABT with different protocols of dosing. However, there were not any positive results in the flocculation of solids. Effective doses of ozone for such treatments were found to be above 2mg/L in literature (Bogner et al 2018; Ji et al 2019). However, when the trials were carried out, these levels could not be reached. The saturation level was reached early before ozone reached 1mg/L. Hence, after the saturation point (the highest observed was 680µg/L in 30 minutes post ozone application), ozone started to spread in the atmosphere, becoming dangerous for humans. If that was the case on an experimental level, on a bigger scale these levels of ozone could be lethal for humans. Therefore, it was decided that other alternatives should be trialled.
Specific, water-soluble polymers are widely used in wastewater treatment to remove suspended solids and/or contaminants from the water, and therefore can be found in municipal, industrial and stormwater treatment systems. The liquid/solid separation, that can take hours or days when left to gravity alone, can typically be achieved in minutes or seconds with properly prepared, activated, and applied polymers. A coagulant added to wastewater creates a coagulation process that neutralizes the particles’ negative charge. Once neutralized, the particles can come together to form larger particles called micro-flocs, creating a larger particle with a higher mass-to-drag ration and hence speeding up the natural process of sedimentation.
Research shows most polymers used in wastewater come from the backwash of drum-filters. It is believed that perhaps the application of polymers in the wastewater before entering the drum-filter could flocculate solid particles, increase their size, and therefore increase the efficiency of the drum-filter in terms of solid removal which will be increased furthermore with the application of filter-bags to the wastewater leaving the filter after the backwash. In 2009, Sharrer et al. tried and successfully implemented the use of flocculants after the drum-filter and before geotextile bags. In 2005, Ebeling et al. showed 99% removal of Total Suspended Solids using commercially available polymers from various companies.
After being taken off-line and allowed to dewater and dry, a sludge cake is sufficiently dewatered, i.e., to approximately 20% solids dry weight (Sharrer et al. 2009). At the same time, this removal led to 92-95% reduction of reactive phosphorus (orthophosphate).
Materials and methods
The wastewater from two species will be used in the trials: Atlantic salmon (Salmo salar) and European sea bass (Dicentrarchus labrax). The trials will take place in the facilities of AquaBioTech Group and use a recirculating aquaculture system.
As wastewater is leaving the drum-filter with the use of back-wash pump, it will be collected in a tank (125L). The sludge tank will be connected, through a pipe, with the filter bag (secured in a frame). The filter bag will provide wastewater for each sample to a graduated cylinder, with the initial temperature recorded. To ensure the validity of the results, all water samples will come from the same cycle of backwashing every time.
Collected sludge will be treated with the solutions of flocculants, provided by DERYPOL S.A., in a stirrer/flocculation tester. The flocs will be allowed to settle for 15 minutes. For all screening and flocculation tests, turbidity and reactive phosphorus will be measured. Turbidity will be used as an indicator of suspended solids and reactive phosphorus for phosphorus content. Another important factor that is useful in wastewater treatment is the speed of settling of the flocculated particles, these analyses will also be carried out.
Apart from the quality of the filtered water either with pre-treatment using flocculant or not, an assessment will be carried out for the retained (inside the filter bags) solids composition and define what is their nutrient content. Solids will be collected after settling, without and with filter bags. The samples will be sent to an external lab, where the analysis for the content of these solids will be identified. Any positive results could be further exploited, by upscaling into commercial systems.
Acknowledgement
The SEA2LAND project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement N° 101000402.
References
Bögner, D., Schmachtl, F., Mayr, B., Franz, C.P., Strieben, S., Jaehne, G., Lorkowski, K. & Slater, M.J. 2018. Sludge Pre-Treatment through Ozone Application: Alternative Sludge Reuse Possibilities for Recirculating Aquaculture System Optimization. Ozone: Science & Engineering, DOI: 10.1080/01919512.2018.151076.
Ebeling, J.M., Rishel, K.L. & Sibrell, P.L. 2005. Screening and evaluation of polymers as flocculation aids for the treatment of aquacultural effluents. Aquacultural Engineering, 33, 235-249.
Ji, M., Wu, K., Li, J., Ye, Z., Li, H. & Zhu, S. 2019. The Effect of Ozonation on Particle Size Distribution for Recirculating Aquacultural Seawater: Analysis of Particle Flocculation and Breakup. Ozone: Science & Engineering, DOI: 10.1080/01919512.2019.1644993.
Sharrer, M.J., Rishel, K. & Summerfelt, S. 2009. Evaluation of geotextile filtration applying coagulant and flocculant amendments for aquaculture biosolids dewatering and phosphorus removal. Aquacultural Engineering, 40, 1-10.