Trophic status and lake depth play important roles in determining the nutrient-chlorophyll a relationship: Evidence from thousands of lakes globally.

A fundamental problem in lake eutrophication management is that the nutrient-chlorophyll a (Chl a) relationship shows high variability due to diverse influences of for example lake depth, lake trophic status, and latitude. To accommodate the variability induced by spatial heterogeneity, a reliable and general insight into the nutrient-Chl a relationship may be achieved by applying probabilistic methods to analyze data compiled across a broad spatial scale. Here, the roles of two critical factors determining the nutrient-Chl a relationship, lake depth and trophic status, were explored by applying Bayesian networks (BNs) and a Bayesian hierarchical linear regression model (BHM) to a compiled global dataset from 2849 lakes and 25083 observations. We categorized the lakes into three groups (shallow, transitional, and deep) according to mean and maximum depth relative to mixing depth. We found that despite a stronger effect of total phosphorus (TP) and total nitrogen (TN) on Chl a when combined, TP played a dominant role in determining Chl a, regardless of lake depth. However, when the lake was hypereutrophic and/or TP was >40 µg/L, TN had a greater impact on Chl a, especially in shallow lakes. The response curve of Chl a to TP and TN varied with lake depth, with deep lakes having the lowest yield Chl a per unit of nutrient, followed by transitional lakes, while shallow lakes had the highest ratio. Moreover, we found a decrease of TN/TP with increasing Chl a concentrations and lake depth (represented as mixing depth/mean depth). Our established BHM may help estimating lake type and/or lake-specific acceptable TN and TP concentrations that comply with target Chl a concentrations with higher certainty than can be obtained when bulking all lake types.

Sulfate radical oxidation combined with iron flocculation for upgrading biological effluent of coking wastewater.

Because the components of the coking wastewater was biologically toxic and hence inhibit the actions of microorganisms in conventional biological treatment processes,the biological effluent of coking wastewater (BECW) still remains much recalcitrant pollutants. In the current work, we set out to explore the feasibility of using a proposed advanced oxidation method, involving the persulfate-activated zero-valent iron system (PS/ZVI), to realize a deep treatment of BECW. The efficiency levels at which sulfate radical oxidation combined with iron flocculation removed pollutants, specifically TOC, phenolic compounds (PCs), cyanide, and suspended solids (SSs), as well as removing colour were investigated in batch tests. Increasing the persulfate concentration generally resulted in improved pollutant removal, with maximum removal efficiency levels of 58.5%, 68.4%, 61% 99.9% and 91.04% for TOC, PCs, SS, cyanide and colour, respectively. Note that the coexisting inorganic ions CO3 2- and HCO3 - were strong competitors of the radical consumption of TOC, but this interference was eliminated by adjusting the pH to 4.5. Also, flocculation of the generated Fe3+ ions from the radical reaction significantly enhanced SS removal. GC-MS analysis showed that the compositional diversity of the BECW decreased after oxidation. Meanwhile its biodegradability increased, indicating less bio-toxicity reaching the natural water body. This study suggests that the PS/ZVI system may be an alternative safer and more efficient method than Fenton's method for carrying out an advanced treatment of coking wastewater.