About Sub Sand Sea Water Intakes

In 1966 a marine Zoo on the East coast of England needed to draw sea water from a net sandy shore. A tidal recession of up to 400 metres and amplitude of seven metres, with an unstable beach, prevented the use of a conventional open water intake. In addition the inshore water was always brown with silt. A system based on a specially designed intake buried deep in the sand solved all these problems and provided a continuous supply of clean filtered sea water. This aroused such interest that a company was formed to meet the demand for this specialised service in the U.K. and overseas.

[ Advantages | Considerations ]

The advantages of abstracting sea water through the sand include: Freedom from blocking of intake by sea weed and debris. Freedom from suspended silt, even after severe storms. Suspended solids are normally reduced to c. 2 mg/l and 2 micron but many smaller particles, including most of the clay fraction, are also excluded, giving crystal clear water. Exclusion of all stages of all species of plankton, including those such as mussels and sea squirts which are able to colonise pipelines and also the organic particles on which they filter-feed. There is no fouling and no need to provide for routine cleaning. The sub-sand intake pipeline to Skegness Marine Zoo has run for more than 25 years without fouling. Destruction of bacteria and yeasts, often listed as "too numerous td count" in inshore waters, is above 90% and often complete. The beach system is not subject to interruption or damage by oil pollution, and the system pumps without interruption or loss of quality. (This was demonstrated when heavy crude oil from a tanker wrecked off Lowestoft covered much of the beach alongside the Laboratories: top quality water was pumped throughout the incident.) Reduction of heavy metals. Monitoring at Lowestoft (18 tests in 24 days) showed reduction of zinc by above 80% and of copper by above 50%. Little work has so far been done in badly polluted seas but there is evidence that lead and other heavy metals also are usefully reduced.

SWS systems are based on specially designed hardware, resistant to marine conditions, which allow the evacuation of fine material and pollutants during the development which creates a permeable bed holding back progressively fine particles. Single Units provide up to 20 m3/h from favourable terrain while larger volumes are obtained from a complex of Units or wells at c. 10 m centres. Filtration takes place in two more or less separate zones. At the surface particulate matter is filtered out while below this the passage of oxygenated water creates a biologically active layer which consumes bacteria and biodegrades ammoniacal nitrogen etc.

In Great Britain SWS beach intakes provide pre-filtered water for Marine Laboratories, also for many Universities and industrial laboratories to which it is taken in bulk by road tanker. This water is normally fed straight into working circuits. Other systems in Great Britain service fish farms and lobster storage facilities; also marine aquaria, sea life centres etc. Overseas, SWS intakes are also used to fill swimming pools and Oceanaria basins.

Other important applications are for desalination, which requires water free of particles above 5 micron: for cooling water and heat exchange circuits where the water serves as a source of low grade heat for heat pumps: in both cases it is imperative to avoid pipeline fouling.

Factors to be taken into account when considering sub-sand abstraction: Basic site factors:- (i) depth (ii)extent (iii) texture of the bed: i.e. its particular size pattern and stratification, which may vary widely both up and down and along the beach, Considerable modification may be possible, especially of(iii), but these three, separately or together, normally set a limit to the volume that can be drawn from any site. Tidal amplitude and recession may also be limiting factors, especially in north temperate zones, while on beaches with recession of above, say 100 m, the beach may serve as both filter and reservoir. In the Mediterranean, the Middle East, and in the tropics, the rise and fall are seldom excessive. Stability of beach. In areas subject to heavy tidal currents, severe storms, typhoons etc., it is seldom possible to use shallow beaches (i.e. 1 - 2 m depth). Where the sand is deep, screen wells can be installed and the suction line buried deeply for security. Underground fresh water channels may be tapped. Where full salinity water is required for marine aquaria, laboratories etc., such dilution must be avoided. For desalination etc., such a source is an advantage, while for cooling water and general purposes it is immaterial. The possible presence of iron in the bed. Most seas have negligible iron content (0.002 mg/l), but iron may occur in the bed, especially in temperate and northern zones, often in the form of organic pollution for example when algal material is buried by storms or tidal action. Anaerobic decay is so slow that this may remain unchanged for years, but the passage of oxygenated water, preceded by thorough jetting of the bed, quickly breaks down & evacuates the iron. If the presence of iron is noted during feasibility survey its origin should be checked. Sewage. It is generally stated that intakes should not be within 1km of a sewage outfall. The biological activity of a developed sub-sand system makes such a distance completely safe.

Where there is little or no sand it may be possible to excavate by blasting or machine before filling with selected media to form an excellent source.

The above variable factors make it essential to carry out a thorough beach survey for any installation other, perhaps, than a single Unit system. Such a study first determines whether the beach can be used and then allows a design to be worked out for a system providing the required flow. SWS Filtration is happy to carry out feasibility studies to advise on the possibilities for such schemes.

Where beach conditions totally prohibit sub-sand abstraction it may be possible to create an on-shore system based on the same principles.

For further details please contact Richard Cansdale

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