Impact of increased instream heterogeneity by deflectors on the removal of hydrogen sulfide of regulated urban waterways-A laboratory study.

Laboratory experiments were conducted to test the hypothesis that increase in physical heterogeneity by deflectors would improve the water quality of urban regulated (straight and prismatic) waterways. Deflectors changed the near-uniform flow to a rapidly varied flow, as such the depth, velocity, and Froude number (Fr) variations were four, 10, and 14 times more than the without deflector scenario, respectively. Removal of hydrogen sulfide (H2 S), the main focus of the study, was significantly high when deflectors were placed in the laboratory urban waterway. Introduction of a sediment bed further improved H2 S removal; however, in this case turbidity and color were significantly high too. These observations endorse the fact that attenuation induced by deflectors and assimilation promoted by the sediment bed aids the H2 S removal. These facts were further strengthened by the significant strong negative correlations H2 S made with DO and pH for all experiments. Further studies are recommended for different deflector orientations and modified sediment beds (e.g., mixture of sediment and gravel), identification of localized water quality hot spots to capture spatial variation of water quality, and impact of increased heterogeneity on flood safety. PRACTITIONER POINTS: Increase in physical heterogeneity (by deflectors) on water quality improvement was studied in a laboratory set up. Deflectors changed the near-uniform flow to rapidly varied flow with several mesoscale physical habitats. Hydrogen sulfide (H2 S) removal was greatest with deflectors and the sediment bed, but was characterized by high turbidity. H2 S removal showed strong negative correlations with dissolved oxygen and pH.

In-stream Physical Heterogeneity, Rainfall Aided Flushing, and Discharge on Stream Water Quality.

Implications of instream physical heterogeneity, rainfall-aided flushing, and stream discharge on water quality control have been investigated in a headwater stream of a climatic region that has contrasting dry and wet seasons. Dry (low flow) season's physical heterogeneity showed a positive correlation with good water quality. However, in the wet season, physical heterogeneity showed minor or no significance on water quality variations. Furthermore, physical heterogeneity appeared to be more complementary with good water quality subsequent to rainfall events. In many cases stream discharge was a reason for poor water quality. For the dry season, graywater inputs to the stream could be held responsible. In the wet season, it was probably the result of catchment level disturbances (e.g., regulation of ephemeral freshwater paths). Overall, this study revealed the importance of catchment-based approaches on water quality improvement in tandem with in-stream approaches framed on a temporal scale.

Phytoremediation of heavy metals by calcifying macro-algae (Nitella pseudoflabellata): implications of redox insensitive end products.

To evaluate the phytoremediation of heavy metals in water and understand the biochemistry of end products of calcifying macro algae (charophytes), an 84-wk laboratory experiment was conducted. Eighteen microcosms were maintained with and without plants. These were given different heavy metal treatments: no heavy metals, 0.2mgL(-1) Cr(6+) and 0.01mgL(-1) Cd. Accumulation observed to be 0.06% Cr by dry weight and for Cd it was 0.02%. The bioconcentration factors were 3000 and 25000 for Cr and Cd, respectively. Ratios of heavy metal accumulation in alkaline (i.e., calcified areas) to acidic areas of plants were 6 to 4 (for Cr) and 1 to 1 (for Cd). This elucidated an association between heavy metal accumulation and calcification. This was validated by sequential extraction of sediments. It was shown that in microcosms with plants, the heavy metals were mainly in redox insensitive and less bioavailable carbonate bound form (39-47%). This was followed by organic-bound form (23-34%). Carbonate bound end products will ensure long term storage of heavy metals and after plant senescence these will not re-enter the water column.

Phycoremediation of Chromium (VI) by Nitella and impact of calcium encrustation.

This article discusses the applicability of the Charophyte, Nitella pseudoflabellata in the remediation of Cr (VI) contaminated waters at different calcifying potentials. Its growth was found to be positively correlated with Ca in water (CaW), but marginally significant in the presence of Cr (VI) in water (CrW). High CaW resulted in calcite encrustation on the plant cell wall. CaW was found to be aiding Cr (VI) fixation in the long run, as this correlated positively with both CaW and CrW. However, Ca interfered with passive Cr (VI) accumulation in live plant matter at low CrW concentrations (1mg/g Cr dry weight of plant. Cr (VI) concentrations greater than 0.4 mg/L were too toxic, showing maximum quantum efficiency of PSII photochemistry (F(v)/F(m)) values<0.63. The opposite was noticed (F(v)/F(m)>0.76) when Cr (VI) was less than 0.2mg/L. Elongation curve patterns based on shoot lengths showed similar scenarios. In all cases high CaW units with calcite encrustation found to be least affected by Cr (VI) toxicity. Optimum remediation was obtained using a combination of high Ca and Cr (VI) in the case of passive (short-term) operation and low Ca and Cr (VI) for active (long-term) operation. Under the passive scenario, plants accumulated above 1.2mg/g Cr dry weight whereas in the active case, accumulation was 0.8 mg/g Cr dry weight. We conclude that Nitella-mediated Cr (VI) remediation is a promising technique within the range and conditions investigated.