Sludge Trap – Sand Trap
Category / type: physical pre-treatment, e runoff water, stormwater, sand and sludge removal.
Application: service areas, parking lot, gas stations, car wash.
Flow: 1, 2, 5 ,up to 50 l/s.
Normative references: EN 858-2, national an local regulation.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
Upgradable: high sludge level sensor, rain sensor, flow meter.
Sludge/sand trap, also used as grid separator (as it makes a roughing treatment on the incoming material), is a physical pre-treatment and works by gravity effect for sand and gravel removal in wastewater.
Usually small and medium stormwater run-off treatment plants are provided with a sedimentation chamber as sludge strap.
In large and municipal plants, where wastewater are mixed, sand trap reduce also a small quantity of biological material that cause the the decreasing of the biological reactor.
In small and domestic wastewater plants physical pre-treatment coincides with primary treatment. Sand or sludge trap is a continuous flow rate pre-treatment for wastewater or rainwater; it is a settler working by gravity.
Sand and gravel settle to the bottom of the tank due to the decrease of water velocity.
There are many kind of separator: channel sedimentation types allow sedimentation in laminar conditions, wile in aerated types turbulence prevent the organic matter settlement (lighter than sand), that make the sediment putrescible. Biodegradable substances pass downstream towards the biological treatment.
Aerated sand trap is also used as pre-aeration for septic wastewater incoming flow (long sewer), although it allow only a small reduction of the organic load (negligible in design phase).
In large and municipal plants sand and oil removal take place in the same sector. This process requires pumps or air-lifts and mechanical equipment.
Vortex grit chamber is the most innovative sand trap: wastewater enters tangentially and creates a vortex current. Under the influence of centrifugal force and gravity, grit and other solids quickly collect to the centre of the grit chamber
It requires lower areas and low residence times but it can not be associated with oil removal.
The most common prefabricated sand trap is the gravity sedimentation basin, used for rain water or industrial wastewater treatment.
How It Works
Grit removal treatments are normally designed to remove solid particels from 0.20 mm to greater, with a density of about 2600 g/l and terminal velocity of about 2 cm/s (according to Stokes’ law).
There are many different types of grit removal systems, including aerated grit chambers, vortex-type (paddle or jetinduced vortex) grit removal systems, detritus tanks (short-term sedimentation basins), horizontal flow grit chambers (velocity-controlled channel), and hydrocyclones (cyclonic inertial separation). Various factors must be considered to select a grit removal process, including the quantity and characteristics of grit, potential adverse effects on downstream processes, head loss requirements, space requirements, removal efficiency, organic content, and cost. The type of grit removal system chosen for a specific facility should best balances these different parameters.
Hydraulics retention time is usually about 4 minutes on the peak flow rate and from 10 to 15 minutes on calculation flow rate.
Flow and free surface ratio design value is about of 50 m per hour.
Free surface is an important parameter that influences treatment efficiency (as grease trap).
For simple and prefabricated products a simplified approach is to consider a separation time from 5 to 25 minutes.
Simple sludge/sand trap is not a domestic wastewater treatment but it’s only suitable for stormwater or industrial waste as car-washing.
Low flow rates and specific baffle can justify a simplified approach.
Moreover for the main uses of sand traps, prefabricated standard EN 858-2 describes how to locate the nominal dimensions and information on the plant configuration according to use.
During system design attention should be payed to manholes accessibility, also with mechanical equipment (7.3 – EN 124:1994). It is recommended to ensure vehicle access (vacuum truck) due to sand and gravel high specific weight.
Light liquid separators are regulated by the European standard EN 858-1 for nominal size definition and for hydraulics and statics tests to obtaining the CE marking.
Declaration of performance is required for separators, but it is not required for the sand traps.
EN 858-2 on table 5 defines method and parameters for sand trap volume calculation depending on nominal size (NS) and origin of sludge and density factor for the relevant light liquid fd (220.127.116.11 – Appendix A). It also defines minimum volume.
Appendix B defines where separator is needed and which is the proper configuration.
Note that each configuration in table B.2 includes the sludge trap.
Operation and maintenance
Sand or sludge trap works by gravity and does not need a start up procedure; it must be filled with clean water up to the normal operating level before working.
Sediments level should be checked periodically and cleaned once filled the accumulation volume. Exceeding volume causes the hydraulics residence time reduction and worsen the treatment efficiency.
Maintenance and cleaning operation must be done by licensed firms, whose follow the procedures for the correct disposal of hazardous materials.
All scum, sand and gravel must be removed before cleaning the tank. After cleaning operation, the tank is filled again with clean water up to working level.
Manhole diameter must be large enough to allow access for deep cleaning (as EN 476), to avoid the fine sands and sludge compaction problem.
Sludge trap, or settler, is required by EN 858-2 European standard, that describes nominal size calculation methods and specific plant configurations.
It generally works upstream of a light liquid separator. For oil removal system Declaration of Performance (DoP) is required, based on harmonized standard according to Regulation (EU) No 305/2011.
Stormwater or industrial wastewater treatment is subject to prior authorisation by competent authorities.
Some local or national regulations require the use of separators and sand trap for constant flow rate or first rain water treatment.
Features and benefits
Sand or sludge trap is a stormwater or industrial wastewater settler. Upstream to oil and light liquid separator. It is often required by regulation and for preventing drainage network clogging problems.
Grit trap is a continuous flow rate and industrial wastewater plant section.
Sand trap is often use with an oil and light liquid separator to remove oil traces and hydrocarbons from wastewater coming from car-wash, discharging into public sewer.
- simply treatment;
- no energy consumption and supply;
- small required area (underground installation);
- low operating costs;
- long useful life.
- low efficiency (further treatment may be required);
- periodic purges, depending on use, by a licensed contractor (vacuum truck).