Sand trap
A sand trap is a basin or lake-like widening and deepening of a body of water that is continuously traversed. Sufficiently large solid particles that are heavier than water can settle here and are not transported further. Decisive for this is the reduction of the flow velocity in such basins. This technique is also used in sewage treatment plants .
Sand trap in flowing waters
The task of catching sand and rubble is to hold back the solids carried by the water in order to prevent deposits in underwater areas at risk of silting, culverts and piped sections. A distinction must be made here between a river sand trap and a sand and rubble trap, such as that used in smaller bodies of water in front of pipes and culverts.
Sand trap to protect system parts (pumps, turbines, gate valves, etc.)
In order to protect sensitive system parts against hard suspended matter, the larger suspended matter particles D = 0.2-0.5 mm are separated out in a desander. The principle is based on calming the flow and reducing the flow velocity. This leads to a sinking of the suspended matter. The most common desanding type is the elongated sedimentation tank. The settled material is removed from the desander basin by a flushing device. By opening a gate , the sunken particles are returned from the channel to the underwater area of the river.
Sand traps can be made in the main or shunt.
Main conclusion
Sand traps in the main connection are mostly unsecured widenings of the flow cross-section. They represent a massive migratory barrier for most river life. Although the mode of action is more effective than in a bypass, since the solid matter is constantly precipitated, it represents a migratory barrier for most aquatic animals.
Shunt
In order to enable the river fauna to hike in both directions, a bypass version is necessary. Here the channel is led around a sedimentation basin so that fish and creatures of the macrozoobenthos can pass the sand trap unhindered. The sedimentation basin is only filled during high water. This mode of operation is usually sufficient, as the sand load only increases so much during heavy rain events that the sand trap becomes necessary.
Sand trap in sewage technology
The sand trap of a sewage treatment plant is a settling basin within the mechanical stage with the task of removing settable, mineral impurities from the wastewater, such as sand , stones or glass splinters . These substances would lead to operational disruptions and even damage to the system (wear, blockages, deposits). In addition to the non-ventilated long sand trap , a ventilated long sand trap or roller sand trap , in which grease and oil are separated on the surface at the same time, or a round sand trap is used. Sand traps are usually arranged between the rake or sieve and the primary treatment
The flow speed in the long sand trap, the aeration in the ventilated sand trap or the flow in the round sand trap are designed in such a way that mainly sand and coarse organic material are separated. A horizontal flow velocity of a maximum of 0.2 m / s is aimed for. The air entrainment in ventilated sand traps induces a secondary flow that is independent of flow fluctuations and keeps organic particles in suspension. In modern systems, the sand traps are washed after they have been removed from the sand trap. H. Freed from organic ingredients to enable better drainage and subsequent usability (e.g. in road construction).
The round sand trap is a round basin with a tangential inlet. The resulting circular or vortex flow ensures that the solids move to the center of the bottom of the pool (see also the teacup effect ), which in turn is designed in the form of a funnel. There the deposited substances are removed.