Consequences of ozonation for the limited coagulation of non-proteinaceous AOM and formation of aldehydes as ozonation by-products
Abstract
Limited knowledge of how ozonation affects algal organic matter (AOM) character and removal hinders assessing the suitability of ozonation for treating AOM-laden waters. This study examines pre-ozonation-coagulation and coagulation-post-ozonation of AOM, specifically its non-proteinaceous fraction (NP).
O-3 doses of 0.05-4.0 mg per 5 mg dissolved organic carbon were applied at pH 5.5, 7.8, and 9.0; coagulation was performed in pH range 5.0-9.0. Coagulation alone exhibited maximum removal of 15%, and pre-ozonation before coagulation had different impacts depending on both the O-3 dose and ozonation pH.
In case of pH 5.5 and 7.8, the lowest and highest O-3 doses improved the removal (by max. 7%); by contrast, the medium O-3 dose deteriorated coagulation. In case of pH 9.0, at which different oxidation pathways predominate, the low and medium O-3 doses had negligible effects, while the highest O-3 dose improved NP removal (by max. 11%).
Noteworthy is that despite these trends appeared throughout the whole coagulation pH range, NP removal depended also on coagulation pH. Post-ozonation after coagulation contributed to NP removal (additional 5-6%) at the highest O-3 dose.
In general, positive effects of ozonation were ascribed to charge alterations and partial mineralization of NP, adverse effects to changes in molecular weights. Additionally, aldehydes (up to 34.2 mu g mg(-1) DOC; predominantly formaldehyde and acetaldehyde) were formed whenever ozonation was involved, more with increasing O-3 dose, and regardless the ozonation pH.
The results emphasize the importance of optimizing ozonation so as to reach the benefits and avoid the undesirable effects on water quality.