Physical processes for particle matter removal from wastewater effluents: A comparative study

摘要

Digestate and manure has been used for biogas production and as fertilizers for a very long time. Both applications result with the same remaining products; valuable and economically viable solid content and digestate water. This water contains certain amounts of nitrogen and phosphorus. After removing bulk solid content, the remaining water is economically unusable. Nitrogen and phosphorus content limits its spread on the fields, therefore, requiring expensive storage or transfer costs.The aim of this thesis is to compare different physical methods available for removing or reducing total solids in digestate by using physical processes. The goal is to have clean enough water within legal limits that can be discharged or sent to a final process, such as reverse osmosis.The current situation is, without having a pre-process reverse osmosis is not a viable option due to membrane clogging and high energy consumption. To achieve the goal, the following technologies were tested: martensitic particle bed and sedimentation with chitosan as coagulant. These were compared with the following state of the art technologies: dissolve air floatation, reverse osmosis, dewatering, wetland, mechanical centrifuge, sand filtration, chemical precipitation, and sedimentation. Another goal was to evaluate whether the methods utilized are energy and total cost efficient, and environmentally friendly.Martensitic particles were used for these reasons. They are cheap, recoverable by magnetic separation and recyclable materials. Chitosan was also chosen for its environmentally friendly properties.Results were organized in tables according to total solids removal percentage, and different particle sizes and flow rates. Initially, the total solids ratios were between 1.25-1.33 percentages. With the smallest particle size and 10 ml/min flow, it was possible to achieve a final total solids ratio of 0.57 percent, which corresponds to the removal of little more than half of the total solids from the digestate. Chitosan caused a gel-like flocculant. This flocculant alone caused around 11 percent removal. This was a relatively low level of removal, although gel-like formation could increase agglomeration to a level at which any other method could be added to system and it would be possible to achieve much higher removal rates. By combining martensitic particle bed with chitosan treatment in particular, it can be possible to have a very low energy consuming and environmentally friendly method that is alternative any state of the art methods, with much lower costs and better sustainability.