Bioavailability and Characteristics of Dissolved Organic Nutrients in Wastewater Effluents
April Z. Gu
OverviewExcessive phosphorus (P) is considered to be the primary cause for eutrophication in fresh water bodies and, the increasingly stringent discharge limits have pushed wastewater facilities to achieve extremely low effluent total phosphorus (TP) levels (e. g. 10-50 ¼g/L). Pushing the P removal to the level of limit of technology has drawn significant attention from both regulatory agencies and wastewater utilities to the removal and bioavailability of various wastewater derived phosphorus fractions such as non-reactive phosphorus (NRP) and dissolved organic phosphorus (DOP). As more advanced treatment processes have been applied to nearly completely eliminate the soluble reactive P (sRP, i.e. ortho-P), these P fractions contribute an increasingly larger percentage and often become the dominant P fractions in the final effluents.
For utilities, the concern arises from the substantial capital cost that is required to research and implement more advanced treatment technologies beyond the current practices in order to remove these P fractions. On the regulatory side, current watershed protection plans (e.g. TMDLs) use total phosphorus for setting limits without considering the possibility those different P fractions may differ in their bioavailability and therefore their potential to cause eutrophication.
Bioavailable P (BAP) is defined as the sum of immediately available P and the P that can transform into an available form by naturally occurring physical, chemical and biological processes. Although rarely analyzed in routine monitoring, effluents from municipal treatment plants contain various P forms, all of which may have different availabilities to algae and other primary producers. In addition, the bioavailability of different P fraction depends on the different sources of wastewater as well as the method and incubation length of the bioassay. Currently, little information is available about the bioavailability of various P fractions in various wastewater effluents. Particularly, there is limited information on the bioavailability of wastewater derived DOP to eutrophying phytoplankton in receiving waters. Therefore, it's critical to further identify DOP components and its bioavailability.
This study aims to investigate and assess: 1) the bioavailability of various P fractions, particularly dissolved P fractions (DOP), in advanced wastewater effluents; 2) the nature and composition of effluent DOP and its association with effluent dissolved organic nitrogen (DON); 3) the link and correlation of wastewater characteristics and treatment process with bioavailable effluent DOP. Tertiary effluents from the selected representative wastewater treatment plants were collected and subjected to a detailed and comprehensive analysis for wastewater characterization, phosphorus fractions analysis, and bioavailability assays. Standard methods were employed for the phosphorus fractionation and speciation analysis. Waster effluent fingerprinting was performed using fluorescence spectrum and Parafac analysis to reveal the nature and characteristics of the effluent organic nitrogen and phosphorus. The bioavailability of P fractions in effluents were assessed using both chemical enzymatic assays and algal bioassay. For selected effluents, simultaneous DOP and DON bioavailability assay were performed to elucidate the differential availability of waste effluent-derived organic nitrogen and phosphorus and their association with hydrophilic versus hydrophobic fractions.