preservation and burial
Preservation of diatoms in sediments ranges from excellent in diatomaceous earth (see Flower, this volume) to poor in some salt lakes (Fritz, this volume), and in extreme cases, no diatoms are present in the sediments. Factors affecting preservation include salinity, temperature, alkalinity and sediment accumulation rate as well as the silica content of the cells and the concentration gradient of dissolved silica between diatomite the sediment and the overlying water. Differential dissolution may occur, causing the composition of the assemblage to be biased towards a more robust form (Ryves et al., 2003). A key question is whether diatoms preserved in sediments truly reflect their community of origin. Burial processes include removal from lake effluents, resuspension and reprocessing of old sediments, losses from grazing, bioturbation and dissolution in sediments. Some studies have shown good agreement between the composition of living communities and the sediment record (eg, Cameron, 1995), while some studies have shown considerable differences between diatoms found in the water column and the sediment record. In Lake Baikal, for example, most diatoms dissolve before reaching the sediment surface (cf. Mackay, this volume).
Granular media filters include filters comprising sand, diatomaceous earth or other granular media, packed beds, layers or surfaces over which water passes or passes. A large number of filtration designs have been developed and put into use - slow sand, 32 fast sand, 33 dual media, 34 deep bed dual media, deep bed single media, multi media, 35 upflow, downflow, dual flow, cross flow, etc. 36 As the fluid passes through the filter, particles larger than the space within the filter matrix are trapped and mainly accumulate on the surface of the filter. 37 These filters retain microorganisms through a combination of physical and chemical processes, including physical strain, sedimentation, and adsorption. Some may also incorporate chemically active antimicrobial or bacteriostatic surfaces or other chemical modifications. Other granular media filters are biologically active because they form a layer of microorganisms and their associated extracellular polymers on the surface or within the granular media matrix. This bioactive layer, called a schmutzdecke in conventional slow-sand filters, retains microorganisms and often leads to their inactivation and biodegradation (WHO, 2011).