Variations in retention efficiency of bivalves to different concentrations and organic content of suspended particles

Retention efficiencies (RE) of scallop (Chlamys farreri),oyster (Crassostrea gigas) and mussel (Mytilus edulis) in a flow-through system were measured to understand the short-term response to various particle and organic matter concentrations.By comparing the RE of C.farreri with that of C.gigas and M.edulis,we gained further knowledge on the feeding physiological characteristics of C.farreri and ascertained the possible cause of high summer mortalities of this species.The experimental feeding conditions included natural differences in the abundance and composition of suspended seston,as well as conditions in which seston abundance and composition were manipulated by adding natural silt or cultured microalgae.The results show that in natural sea water,the minimum particle size for maximal retention in M.edulis,C.gigas and C.farreri was approximately 4,6,and 8 μm,respectively;the RE of 2-μm (equivalent spherical diameter) particles was 17%,19%,and 8%,respectively;and the relative standardized RE was 58%,49%,and 18%,respectively.In C.gigas and M.edulis,the minimal particle size for maximal retention did not change with food quality (organic content).C.farreri was more sensitive to fluctuations in particle concentration and organic content.With particle concentration increase,the minimal particle size for maximal retention in scallop shifted to large particles and the RE for 2-μm particle decreased from 8% in natural seawater to 1.6%-6% in silt-enriched groups.With organic content increase,the minimal particle size for maximal retention shifted from 8 to 5 μm in natural seawater.Variation in RE of C.farreri with food conditions and the relationship between lower RE and smaller particle size may hamper C.farreri from food taking due to the decrease in the size distribution of phytoplankton in Sungo Bay.

In Situ Filtration Rates of Blue Mussels(Mytilus edulis)Measured by an Open-Top Chamber Method

Blue mussels, Mytilus edulis , form dense beds of both commercial and ecological importance, and many attempts have been made to determine their filtration rate. The total time in which mussels actually utilise their filtration capacity in nature varies greatly, making in situ methods for filtration rate measurements relevant. Further, it is being debated to what extend filtration rates measured in the laboratory using cultivated algal cells may apply for mussels in nature. In the present study, we have used an open-top chamber setup in order to allow repeated in situ filtration rate measurements of M. edulis using ambient natural phytoplankton and free-living bacteria. We found that the in situ measured filtration rates are comparable to filtration rates obtained in laboratory studies using different methods and controlled diets of cultivated algal cells. Further, we found that the retention efficiency of free-living bacteria was between 22.2% and 29.9%, in good agreement with values from laboratory studies. Our findings support the assumption that mussels in nature tend to use their filtration capacity when the phytoplankton concentration is above a certain lower trigger level.

Settling basin design in a constructed wetland using TSS removal efficiency and hydraulic retention time

Using total suspended solid （TSS） removal efficiency and hydraulic retention time （HRT） as design parameters a design guideline of a settling basin in a constructed wetland （CW） was suggested; as well as management of sediment and particle in the settling basin. The CW was desiEned to treat the piggery wastewater effluent from a wastewater treatment plant during dry days and stonnwater runoff from the surrounding paved area during wet days. The first settling basin （FSB） in the CVV was theoretically designed with a total storage volume （TSV） of 453 ms and HRT of 5.5 hr. The amount of sediment and particles settled at the FSB was high due to the sedimentation and interception of plants in the CVV. Dredging of sediments was performed when the retention rate at the FSB decreased to approximately 80%. Findings showed that the mean flow rate was 21.8 m3/hr less than the designed flow rate of 82.8 m3/hr indicating that the FSB was oversize and operated with longer HRT （20.7 hr） compared to the design HRT. An empirical model to estimate the length of the settling basin in the CW was developed as a function of HRT and desired TSS removal efficiency. Using the minimum tolerable TSS removal efficiency of 30%, the length of the FSB was estimated to be 31.2 m with 11.8 hr HRT.

Three-dimensional computational fluid dynamics （volume of fluid） modelling coupled with a stochastic discrete phase model for the performance analysis of an invert trap experimentally validated using field sewer solids

Invert traps are used to trap sewer solids flowing into a sewer drainage system, The performance of the invert trap in an open rectangular channel was experimentally and numerically analysed using field sewer solids collected from a sewer drain. Experiments showed that the free water surface rises over the central opening （slot） of the invert trap, which reduces the velocity near the slot and allows more sediment to be trapped in comparison with the case for the fixed-lid model （assuming closed conduit flow with a shear-free top wall） used by earlier investigators. This phenomenon cannot be modelled using a closed conduit model as no extra space is provided for the fluctuation of the water surface, whereas this space is provided in the volume of fluid （VOF） model in the form of air space in ANSYS Fluent 14.0 software. Additionally, the zero atmospheric pressure at the free water surface cannot be modelled in a fixed-lid model. In the present study, experimental trap efflciencies of the invert trap using field sewer solids were fairly validated using a three-dimensional computational fluid dynamics model （VOF model） coupled with a stochastic discrete phase model. The flow field （i.e., velocities） predicted by the VOF model were compared with experimental velocities obtained employing particle image velocimetry. The water surface profile above the invert trap predicted by the VOF model was found to be in good agreement with the experimentally measured profile. The present study thus showed that the VOF model can be used with the stochastic discrete phase model to well predict the performance of invert traps.