Subsections

• • Designing a water intake
  • Screening and grit removal
  
  
• Pumps

Construction of water source intake

In the case of underground water source, the water intake catchment or pumping system should be designed to extract as little as possible earth or sand with the water. In the case of water boreholes, the design is left to specialists.

Grit removal may be necessary of wellbore water and is discussed below. In the case of surface water the intake is designed to handle the different types of material which may be present in the water.

Designing a water intake

In lakes with a virtually constant water level, the intake should be set at such a depth that the quantity of suspended solids, colloidal matter, metals and plankton is as low as possible throughout the year. Seasonal changes in temperature can be considerable, and while it may in some areas be desirable to take advantage of increased temperature, one should also in warmer climates take care to avoid excessive changes in temperature that induces thermal stress on the fish. If the lake is deep, effect of light is very small below 30-35 meters and plankton content and temperature changes are minimal. The intake should, however, preferably be 7 meters above bottom to minimize effects of bottom currents and deposited particles.

In shallow lakes it may not be possible to meet these criteria at all. The depth of the water intake may there depend on the type of bottom sediments, risk of high temperature etc. In some cases an intake tower, enabling water to be drawn off at different depths according to season may be useful.

Intake from a stream must be protected from matter such as earth, sand, leaves, reeds, floating objects and films of oil carried along with the current. There is no ideal model and the type may be a side, bottom or siphon intake, depending on the nature of the most common entrained matter, variation in flow and ease of construction.

Screening and grit removal

Screening of the water may be done with a coarse bar screen, with bars 8-10 cm apart, followed by a medium bar screen, with bars 25-40 mm apart. Finer screening (1-5 mm) and grit removal may be necessary or essential, when the water has to be conveyed through a long pipeline or when it has to be pumped. Grit removal is to eliminate gravel, sand or mineral particles larger than 200 microns from the water. It is done to protect pumps or sensitive gear from abrasion and to avoid deposits in channels and piping. A grit removal device or a sand trap can be a simple rectangular channel or a box-like structure, where the current speed is limited, so the grit settles at the bottom. Its dimension is determined by the following equations

$${\frac{{flow \:(m^3/s)}}{{ v_s \:(m/s)}}}$$ A_h(m²) =

$${\frac{{depth \:(m)}}{{ v_s \:(m/s)}}}$$ t_hr(s) =

$${\frac{{length\:(m)}}{{ v_f \:(m)}}}$$ =

where A_h is the horizontal surface area, v_s is the settling speed of the smallest particles to be removed, v_f is the current or speed of flow and t_hr is the hydraulic retention time. The settling rate of 200 micron sand particles is approximately 1.5-2 cm/s (0.015-0.02 m/s) at low current speed. The current speed should be no more 0.25-0.3 m/s for particles down to this size, as else they will not settle out. The depth is suitably 0.5-1 m. As an example, a sand trap for 100 L/s flow should have a surface area of A_h = 0.10 m³/s / 0.015 m/s = 7 m². The minimum cross-sectional area of the trap is determined by the flow rate and the maximum current speed and is A_cs = 0.10 m³/s / 0.25 m/s = 0.4 m². If the depth is 0.5 m, the minimum width would be 0.8 m. Of course a sand trap for this flow may have a larger cross-sectional area, it will slow down the current speed, increasing the effectiveness of the sand trap for small particles.

Pumps

The purpose of pumps is to increase the pressure of water so that it will flow into the desired direction.

Water pumps are available in many types and sizes. In fish farming immersible centrifugal pumps are mainly used as other types are more sensitive to particles in the water. In cases where large volumes with relatively low pressure increase are needed propeller pumps are also used.

Figure: A pump with the centrifugal principle

It is important that the pressure is as high as possible at the sucking side of the pump to avoid air bubbles to get in to the pump which can decrease the flow or even stop it and damage the pump as well.

Generally it is not advisable that the pump is above the water level and has to suck up the water. The risk of false air is to big. A much safer approach is to either immerse the pump or having the water flowing with pressure towards the pump (i.e. pump lower than the surface of water). In the figure here below is some example of pump locations and the design of a pumping station.

Figure: Examples of pump locations and the design of pumping station.

Although pumps are good and handy tools when water is not self-flowing, the possibility of breakdown is always there (all pumps break down in the end). A repair can take a few days and thus a reserve pump has always to be at hand and be able to take over automatically. There is also the risk of power cut, therefore a power station has to be at hand as well. This power station has to take over the power production automatically as well.