Chemical precipitation and ammonia air stripping as effective pre-treatment methods before membrane filtration of co-digestion effluents

Abstract

The aim of the research presented was to develop an effective treatment method of highly polluted post-digestion effluents, to a degree allowing the treated waters to be directly released into a natural reservoir. The post-digestion effluents characterized by varied ammonia–nitrogen content underwent high-pressure membrane filtration (nanofiltration [NF], reverse osmosis [RO]) and chemical precipitation (struvite)/ammonia stripping as pre-treatment steps. The application of struvite precipitation and ammonia stripping turned out to be an effective method of binding nitrogen and phosphorus compounds from post-digestion liquors. Waters met all discharge limits after struvite precipitation at stoichiometric ratio of reagents, ammonia stripping of 2–8 h (depending on the initial NH4+ concentration) and RO as a post-treatment. Introducing air stripping between struvite precipitation and membrane filtration allowed to reduce the amount of reagents used during struvite precipitation in order to remove the ammonia–nitrogen below discharge limit. The waters purified with NF as a post-treatment did not meet the discharge limits; however, their reuse as technological waters should be taken into consideration.