Recirculating aquaculture system RAS

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The recirculating aquaculture system (RAS) is a new type of aquaculture model, through a series of water treatment units, the wastewater produced in the aquaculture pond is treated and recycled again.

The main principle of RAS is to integrate the advanced technologies in the fields of environmental engineering, civil engineering, modern biology, electronic information, and other disciplines, so as to remove the harmful pollutants such as residual bait feces, ammonia nitrogen (tan), nitrate-nitrogen (NO2 — N) and so on, For the purpose of purifying the culture environment, the purified water body is re-input into the culture pool by physical filtration, biological filtration, CO2 removal, disinfection, oxygen enrichment, and temperature regulation.

It can not only solve the problem of the low utilization rate of water resources, but also provide stable, reliable, comfortable, and high-quality living environment for breeding organisms, and provide favorable conditions for high-density breeding.

Outline of recirculating aquaculture system

1. Culture pool: incubation pool, seedling pool, and culture pool.

2. Physical filtration: pre blowdown device; diverting and collecting device.

Sedimentation: sedimentation tank, inclined plate precipitator, vertical flow precipitator, hydro cyclone precipitator.
Sand filter: sand filter, sand filter tank, active sand filter, parabolic screen [sieve bend screen, DSM screen].
Microfiltration machine: all plastic microfiltration machine, spin microfiltration machine, intelligent microfiltration machine, adjustable speed microfiltration machine, micro microfiltration machine, stainless steel microfiltration machine.
Filter: belt filter, bag filter, membrane filter, pressure filter.
Protein separator: external protein separator; internal protein separator; overflow; dissolved gas releaser.
Heavy metal (iron, manganese) removal equipment and its activated carbon linkage process remover.

3. Biofiltration: mobile bed bioreactor: trickle filter; biological turntable: Immersion filter; biological bypass reactor; biological flocculation purifier; integrated Physicochemical / biochemical device. Bamboo ring packing; bamboo ball packing; bamboo piece packing; suspended packing; filter strip packing; polyhedral hollow ball packing; glass ring packing; three-dimensional elastic packing; comet fiber filter material; asymmetric fiber packing.

4. Sterilization: ozone system. Closed UV sterilizer: manual cleaning UV sterilizer, pneumatic cleaning UV sterilizer, mechanical cleaning UV sterilizer, self-cleaning UV sterilizer. Open UV sterilizer; open channel UV sterilizer. Air ultraviolet sterilizer.

5. Oxygenation and pure oxygen oxygenation: low-pressure oxygenator; jet oxygenator; turbulent oxygenator; pressure oxygenator; oxygen cone; gas stone; oxygenator; oxygen recovery device. PSA oxygen generator; liquid oxygen; oxygen source filter.

6. Temperature control system: temperature monitoring; heat source: geothermal; solar energy; electricity; coal, waste heat. Heat exchanger, heater, heat pump; boiler.

7. Monitoring system: pH monitoring; dissolved oxygen monitoring; water level alarm; salinity monitoring; light monitoring. Power distribution system.

8. Baiting system: automatic baiting monitoring system.

9. Electronic measurement: automatic weighing. Automatic sorting. RFID system.

Main treatment unit of Ras in circulating aquaculture system

. Culture pond:

In factory culture, the removal of N and P in water mainly depends on the removal of a large number of feces and residual baits (from the source), followed by biological purification. As the first technology of the water treatment system, the technology of sewage discharge by the way of aquaculture pond is very important. It is the premise of system purification.

Foam and oil on the surface of the fish pond:

The technology of branch drainage in aquaculture is the mainstream technology in foreign fish ponds. It is a combination mode of the bottom row and surface overflow, that is, through the bottom row, effective discharge of precipitated particulate matter is effective, and multiple slots or porous horizontal overflow pipes are arranged on the surface of the water body above the fish pond, so that the oil and foam floating on the water surface can achieve good removal effect, and at the same time play the role of maintaining water level, which has become an alternative technology of the traditional single-channel bottom row mode.

Design of sedimentation tank:

In factory culture, the removal of N and P in water mainly depends on the removal of a large number of feces and residual baits (from the source), followed by biological purification.

The sedimentation tank is an important facility for the treatment of solid waste in the water. The most commonly used settling tank is the gravity separation facility, which uses the method of gravity settlement to separate the suspended particles with high density from the natural water. A well-designed sedimentation tank can remove 59% ~ 90% suspended solids.

The primary sedimentation tank and secondary sedimentation tank can be set as required. Generally, the sedimentation tank is built on a high position, which uses the potential difference to supply water automatically. Its structure is mostly made of reinforced concrete. It is equipped with a water inlet pipe, water supply pipe, sewage pipe, and overflow pipe. The drainage slope at the bottom of the tank is 2% – 3% and its volume should be 3-6 times the maximum daily water consumption of the fish factory.

The common inclined plate sedimentation tank is equipped with numerous plastic inclined plates in the cement tank. The water overflows from the bottom up through the inclined plate, and the solid matter in the water settles on the inclined plate. Because the inclined plate is placed in water for a long time, biofilm is formed on its surface. Biofilm can decompose toxic chemicals such as ammonia and nitrogen in water to purify water quality.

The key point of the design is that the settling velocity of the suspended solids should be less than 4 m3 / min, the appropriate velocity is 1 m3 / min, and the flow per unit area is 1.0-2.7 m3 / m2h.

Although natural precipitation has a good effect, due to the low flow rate-limiting the circulation flow, it will reduce the breeding density and efficiency, so it needs to be considered comprehensively.

Impurities such as residual ear feces deposited at the bottom of the aquaculture pond are discharged through the blowdown pipe under the pond bottom and enter the vertical flow precipitator. The water inlet direction of the vertical flow precipitator is circular and radial, so as to settle the impurity particles at its bottom as much as possible, so as to facilitate the discharge from the bottom of the precipitator. The water outlet is on the upper part of the device.

Regularly open the valve at the lower part of the device to discharge the culture dirt deposited at the bottom. The water to remove the culture waste can be input into the microfilter to enter the circulating water treatment system for reuse.

The removal efficiency of vertical flow precipitator for solid particles such as residual bait and feces can reach more than 80%.

2. Solid-liquid separator (residual bait and excrement separator)

In Ras, the wastewater discharged from the aquaculture pond contains a large number of large particles such as residual baits and feces, which need to be removed as much as possible in the early water treatment unit, so as to reduce the organic load of the subsequent water treatment unit. As the first water treatment unit of the whole system, the solid-liquid separator can not only use the centrifugal and gravity action to remove the large particles in the waste and feces of the residual bait, so as to avoid the blockage of the subsequent treatment unit pipeline and the corrosion of the equipment, but also reduce the local head loss of the pipeline and save the energy consumption of the system.

The separate drainage process of the fish pond: the density of industrial fish farming is very high, and the amount of solid waste produced is very large, which is the main source of water pollution. The removal of organic matter and ammonia nitrogen in the water must first rely on the timely removal of a large number of feces and residual bait (from the source), and then rely on biological purification.

Fish excrement and food residue can increase the content of organic matter in aquaculture water, too much organic matter can cause the reproduction of organic matter digestive bacteria, and compete with ammonia nitrogen transformation bacteria in the biofilm for growth space, dissolved oxygen and nutrients. The reproduction rate of organic matter digesting bacteria is much faster than that of ammonia nitrogen transforming bacteria. It is difficult for nitrifying bacteria (autotrophic bacteria) to compete with heterotrophic bacteria, which results in the slow growth of nitrifying bacteria and low ammonia nitrogen removal rate of Biofilter in the culture system.

When the ratio of biochemical oxygen demand (COD) to ammonia nitrogen is greater than 2.7, the removal rate of ammonia nitrogen will decrease by about 70%. If the organic capacity of a biofilter is 2.5gbod/m/day, its nitration rate is only 30%. For example, 5gbod / M / day efficiency is theoretically zero.

In the traditional single-channel bottom discharge mode, there is a certain distance between the discharge port of the aquaculture sewage and the microfilter. When the aquaculture sewage reaches the microfilter, a lot of residual bait and fish body waste is easily decomposed into smaller particles or ammonia nitrogen in the movement of the pipeline and the impact of the microfilter rotating filtration; If these particulate organics pass through the microfiltration machine, it will increase the processing burden of the follow-up process.

Therefore, as the first technology of the water treatment system in the whole aquaculture system, the separate sewage treatment technology of fish pond is very important, which is the premise of realizing system purification and has a function of four or two thousand kilograms. It can improve the capacity of water treatment and reduce the investment of system equipment.

The sub drainage process is to combine the bottom drainage with the surface overflow process, effectively and timely discharge the precipitated particles, avoid the particles from entering the microfilter, and causing secondary crushing, which can reduce the organic load and stabilize the water quality.

It can reduce the nutrition source of bacteria and other microorganisms, effectively prevent bacteria from multiplying in large quantities, and prevent the consequences such as the decrease of dissolved oxygen in the water and the exceeding of COD.

The circulating water treatment and intake system are composed of a water tank or a porous horizontal overflow pipe arranged on the water surface above the fish pond, which can make the oil sludge and foam floating on the water surface achieve good collection and removal effect, and also play the role of maintaining the water level.

Reducing the organic load as much as possible in the biofilter will contribute to the nitrification of ammonia. The basic method is to reduce the capacity of the biofilter and remove some solid substances (the main source of organic matter).

It is more effective and economical to discharge solid particles in time by using a fish pond shunt system than by increasing the capacity of the biofilter.

Otherwise, the capacity of biofilter will be increased inappropriately because of the same nitrification.

The separate sewage treatment process greatly reduces the load and energy consumption of the water treatment process of the industrial circulating aquaculture system.

Vortex separation: the design of vortex separation is adopted in the culture pool. The water inlet pipe on the wall of the culture pool improves the vortex rotation speed of the water body in the pool, accelerates the speed of solid particle sedimentation, and shortens the sedimentation time.
It is convenient for the collection and discharge of solid waste; it is necessary to turn on the switch at the bottom of the device manually to discharge the settled culture waste.
The settling velocity of the solid particles produced by different fish feed is different. The fine and lose particles can only settle at the rate of 0.01 cm / s, which makes the solid particles can not be effectively concentrated at the bottom of the pool.

Using the water inlet pipe on the wall of the pond to improve the eddy current rotation of the water body in the pond, accelerate the settling speed of solid particles up to 2-5cm / s, shorten the settling time, make the thick and undisturbed feces settle well, so as to solve the problem of solid particles deposition in the pond.
90% of the feces and 98% of the nonedible feed and other solid particles are concentrated in the bottom drain outlet instead of circulating the water treatment system.

3. Microfilter (or sieved bend screen, DSM screen)

Its main function is to intercept and remove the small particles which cannot be removed by the solid-liquid separator by using the mechanical filtration function of the microporous screen, so as to further reduce the organic load of the flow bed biofilter in the subsequent treatment unit.

Sieve bend screen and microfilter:

Sieve bend screen: the sieve bend screen comes from the separation device of ore screening. In the treatment of aquaculture water, the fixed screen surface of the arc-shaped screen seam arranged perpendicular to the direction of inflow water flow is used to realize the solid-liquid separation of the water body.

The treatment capacity of sieve bend screen: the most commonly used screen gap is 0.25 m m, which can effectively remove about 80% of solid particles with particle size greater than 70 μ M.

It is not possible for the sieve bend screen to replace the drum type microfilter:

Compared with drum type microfilter, the sieve bend screen has the biggest advantage of energy-saving without the additional mechanical power. In fact, the arc screen is through the loss of potential energy to achieve water solid-liquid separation, and can not effectively reduce energy consumption.

However, the arc screen has no advantage in the efficiency of removing suspended solids and energy consumption ratio. Especially when a sieve bend screen is used, it is even necessary to brush the screen surface once an hour when the breeding load is high.

At present, the problem of automatic cleaning of the curved-screen surface cannot be solved effectively at home and abroad. This problem can be solved, but aquaculture farmers can not afford the high cost of an arc screen backwash system. Moreover, the stainless steel screen surface made in China has the problem that the material is not resistant to seawater corrosion.

Therefore, it is difficult to obtain a satisfactory effect of aquaculture water treatment with a sieve bend screen instead of a drum microfilter. Arc screen has the advantages of simple structure and low cost, but it can only be used in low-level water treatment system reluctantly.

Relationship between mesh number and TSS removal rate: the mesh number (aperture) of the filter is the main working part of the microfilter, which directly affects the removal efficiency of total suspended particles (TSS), backwash frequency, water consumption and electricity consumption of the microfilter.

The larger the mesh number of the filter screen, the smaller the aperture, the more solids are intercepted, but the higher the backwash frequency is. When the mesh number of filter screen increases from 150 mesh to 200 mesh, the removal rate increases rapidly with the increase of mesh number. When the mesh number of the filter screen reaches 200 mesh, the removal rate will not increase significantly again. According to the relationship between removal effect, water consumption, and energy consumption, 200 mesh filter has the best technical and economic effect.

The relationship between mesh number of filter screen and power consumption and water consumption: the power consumption of microfilter is composed of two parts, one is to drive the drum to rotate, the other is the power consumed by the backwash water pump. The energy consumption of the rotating drum is basically stable in the operation of the microfilter. With the increase of mesh number of the filter screen, the frequency of backwashing will also increase, and the power consumption will also increase with the increase of backwashing times.
Water consumption is also an important index to evaluate the performance of microfilter, which is directly proportional to the number of backwashing. With the increase of mesh number of the filter screen, the frequency of backwashing will also increase, and the water consumption will also increase. When the mesh number of filter screen is more than 200 mesh, the water consumption, and power consumption increase rapidly.

Water drive is the first choice: the power consumption of microfilter drive accounts for 8L% to 96% of the power consumption of equipment operation. At present, the worm gear reducer is widely used as the main means of deceleration at home and abroad. Although it has the advantages of a large reduction ratio and small size, it has the disadvantages of low transmission efficiency and short service life. The transmission device should be water driven as the first choice.

Middle support shaft support: as a working part, the drum support mode has a considerable impact on energy consumption. The traditional support with double supporting wheels has high requirements for machining and installation accuracy, so it is difficult to meet the requirements of satisfactory use in microfilter. Using the central shaft to support the drum can significantly reduce the manufacturing and installation accuracy, and make the drum run more smoothly. By adopting water driving and middle supporting shaft, the power consumption can be reduced by 40%, which has a good energy-saving effect.

Rotating speed: the rotating speed of the microfilter is 1-3r / min. too fast rotating speed may break large particles into small particles and pass through the filter screen to reduce the filtering effect. The microfilter with adjustable speed is preferred.

One type of backwash nozzle: the advantages and disadvantages of technical parameters such as the pressure of the backwash nozzle play an important role in reducing energy consumption and water consumption. The backwash intensity of the one type backwash nozzle is higher than that of the traditional cone type backwash nozzle, saving water and electricity by 20-30%.

The replacement cost of filter screen: during the long-term operation of the microfilter, the viscous substances in the aquaculture water will gradually attach to the filter screen, resulting in the smaller mesh diameter of the filter screen and affecting the filtering capacity. Therefore, the cost of replacing the filter screen and the convenience of replacing the filter screen are important aspects to evaluate the comprehensive performance of the microfilter.

Microfiltration machine: it is the most effective and considered the only ideal choice to use microfiltration machine to remove the suspended solids in aquaculture water. (of course, the microfilter also has the function of intercepting solid particles, but this can only be selected as a secondary function of the microfilter. )

Microfiltration is the continuous separation of suspended solids through the filter screen on the drum of the microfiltration machine. Generally, the mesh of the filter set in the microfilter is larger than 50 μ M. With the rotation of the drum, the water flows through the net, and the solids are stuck on the net. When the water flows out of the net, the backwash nozzle is used to wash. The washed sludge is collected in the funnel-shaped container and then transported to the sludge treatment tank. This part of the water loss is compensated by new water, which accounts for about 1% of the total amount.

The solids and related wastes removed are as follows:

Suspended solids 80-95%
Nitrogen 15-25%
Phosphorus 45-55%
Organic matter (biological oxygen consumption) 55-65%

In the above data, the nitrogen removal rate is very low because it is mostly in the form of soluble ammonia.