Gravel trap

In dry climates, a mulch of rocks and gravel will often trap and condense moisture and direct it to the soil surface. 

Gravel and Sand with Portland Cement A.. 44 Trap with Portland Cement A. 

This is the anaerobic equivalent of the trickling filter. It contains support media (rock, gravel, plastic) with a void space of 50% or more. The bulk of anaerobic bacteria grow attached to the filter medium, but some form floes that become trapped inside this medium. Wastewater upflow through the reactor helps retain suspended solids in the column. This system achieves a modest BOD removal, but a higher removal of solids. Approximately 20% of the BOD is converted to CH4. 

One factor in assessing the quality of surface waters is the amount of suspended solids or, "how muddy is it " For example the standard for many rivers and streams is 100 mg/L. If more than this occurs that segment of the stream is declared "impaired." Much of this solid material is colloidal and cannot be filtered under ordinary means. In water treatment plants, aluminum hydroxide is used to surface adsorb or simply trap the solids, and this floe then either settles out or is filtered by a gravel bed. It has been found that cationic surfactants will surface adsorb to most of the solids suspended in water, and this coating will attach to air bubbles passing th ough the system. Some surfactants proven effective are cetyldimethylethylammonium chloride, cetyldimethylbenzylammonium chloride, cetylpyridinium chloride, and ethylhexadecyldimethyl ammonium bromide. These are all antiseptics as well and are used in many mouthwashes. Only a few ppm are necessary (10 to 60) to provide clarification, and the LDjq s are in the range of 500 mg/kg. 

The structures for separation of substances by sedimentation — gravel and sand traps, sedimentation and slot tanks — are simple to construct and undemanding in their operation. 

S. Dubus, J. F. GraveL B. Le Drogoff, P. Norbert, T. Veres and D. Boudreau, PCR-free DNA detection using a magnetic bead-supported polymeric transducer and microeleclromagnetic traps. Anal. Chem., 78, 4457-4464 (2006). 

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