Which parameters are critical for flow-through fish farming technology?
The critical parameters are temperature, dissolved oxygen, pH, ammonia, nitrite, nitrate, stocking density, feeding rate, hydraulics and redundancy of key equipment.
Technology – Direct-flow farms
Technology of direct-flow fish farming systems
Direct flow can be a simple and inexpensive technology - but only with the “right” water. Let's look at how a direct-flow farm is structured, what requirements are placed on the water source, and where this approach is really justified.
Direct-flow workshop with round pools
Short answer
Flow-through fish farming technology explains how to organize fish production with controlled water quality, oxygen, temperature, stocking density, feeding and routine operations. Flow-through technology depends on continuous inflow of good water, so flow rate, temperature, discharge treatment and seasonality are key risks.
What is a straight-through farm
A flow farm is a farm where water is constantly supplied from a source, passes through containers or growing channels and is discharged. Water is used once and is not returned to circulation in significant volumes - unlike RAS.
Analogy from practice
If the direct flow is a waterfall, then the RAS is a fountain that works in a circle.
What is the main point
Main limitation
Direct flow can indeed be a simple technology - but only with a very strict choice of site. If the water source does not provide the required flow rate, temperature and water quality are stable all year round, the direct flow turns into an expensive water intake and treatment facility.
Requirements for the water source
Before designing a forward flow, six basic water parameters and discharge conditions are checked.
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Debit (expense)
Minimum consumption during the driest periods of the year.
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Temperature
Seasonal range and daily variations.
O₂
Oxygen
O₂ at the entrance and its fall in the household - especially in summer.
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Water quality
pH, iron, nitrogen, suspended matter, organic matter, mineralization - by analysis.
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Pollution risks
Upstream discharges, agricultural runoff, floods, blooms, accidents.
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Reset
Where and under what conditions water and sediment are discharged (ecology, permits).
What is desirable to have at the start
- Water analysis for at least 2–4 seasons.
- Actual data on the minimum flow rate.
- Understanding of discharge limitations and settling basin/treatment needs.
5 Key Limitations of Forward Flow
These five factors determine whether a co-flow will be cheap and simple - or turn into a complex engineering project.
01
Amount of water
A practical guideline for the required fresh water consumption per 1 ton of fish in the system:
| Type of fish | With oxygen (pure O₂) | No oxygenation |
|---|---|---|
| Sturgeon / trout / whitefish / tilapia | 20–30 m³/hour for 1 t | 50–80 m³/hour per 1 t |
| African catfish | 3–5 m³/hour for 1 t | — |
Scale: a farm for 50 tons of trout per year with a one-time biomass of ~20 tons will require about 400–600 m³/hour fresh water (with pure O₂ saturation) or 1000–1500 m³/hour (without saturation).
Comparison with wells
A typical well produces 1–10 m³/hour (10 m³/hour is already “very powerful”). With an average of 5 m³/hour, this means dozens of wells for an average project.
02
Temperature
Typical temperatures for optimal growth (guidelines):
- Trout: 15–17 °C
- Sturgeon: 22–24 °C
- Tilapia / African catfish / crayfish / shrimp: 25–28 °C
In wells it is often 8–12 °C: warm-water species will not grow; trout at 8–10 °C grow noticeably slower. In open reservoirs, overheating is possible in summer (critical for trout), in winter - near zero (growth stops).
Example of calculation of heating in direct flow
To heat 500 m³/hour of water from 5 to 15 °C (ΔT = 10 °C) in winter, you need about 5,815 kWh per hour (≈5.8 MW) — approximately 700 m³ of gas per hour, taking into account efficiency. With a gas price of $0.09/m³, this is about $1,600/day only for heating, plus a 6-7 MW boiler and infrastructure. Numbers are approximate; with cooling it is usually even more expensive.
03
Water quality
Not all water is suitable for fish. The well can be hard, with iron; river - with organic matter, parasites and pathogens; excess dissolved gases are possible. The solution is water treatment (mechanical cleaning, UV/ozone, degassing, iron removal, softening). But then the forward flow ceases to be “simple” and turns into a separate water treatment station.
04
Stability of water supply
Even if the flow seems “unlimited” in the spring, in the summer the source may lose a significant part of the flow or dry up. For direct flow, this is a direct risk of stopping production - in contrast to RAS, where replenishment volumes are many times lower.
05
Effluent discharge
Draining 400–600 m³ of water every hour is a separate task. City sewer and "ravine" are usually not suitable. A realistic scenario is discharge into an open reservoir near the site, with compliance and often with sludge settling and treatment.
Where co-current really works
Direct flow is justified where nature itself provides the required flow, temperature and quality of water.
Trout
The bank of a mountain river (clean, cool in summer, does not dry out) or northern reservoirs, where in summer the water does not warm up above ~20 °C. The downside is the winter growth pause.
sturgeon
Rivers and lakes are easier to grow, but growth slows or stops in winter. The best options are power plant cooling ducts.
Catfish and tilapia
Almost only the cooling channels of power plants provide stable warm water all year round.
Growing technology
The basic logic of direct flow is linear: water passes through the farm once.
Water intake → if necessary, preparation and protection (grids, filtration, aeration / oxygenation, UV, iron removal) → distribution to pools and channels.
Growing: feeding, temperature and O₂ control, container maintenance, sediment removal.
Water collection after the tanks → discharge to the discharge → if necessary, sedimentation and purification of waste water.
Sorting and replanting → pre-sale holding → shipment.
Key control points in operation
- O₂ at the entrance and after the last section (especially in summer).
- Water temperature and its peaks/troughs.
- Uniform distribution of flow across lines and basins.
- Sediment, siltation and sanitation.
- Reset and meet environmental requirements.
Truss composition and equipment
Typical structure of direct-flow facilities and set of equipment. The list depends on the water and type of fish.
Blocks and modules
- Water intake (gravity / pump) + flow metering unit
- Input water treatment: grilles, filters, aeration / oxygen, UV / ozone, iron removal
- Water distribution through pools and canals (hydraulics, valves, overflows)
- Pools and canals growing
- Collection node and water drainage
- Reset/Clear: settling tanks, sludge collection
- Pre-sale preparation and shipment
- Feed warehouse, sanitary areas, workshop
An oxygen cone with a pump is a unit for saturating water with oxygen in a forward flow.
Equipment
- Pumps (if there is no gravity flow), fittings, flow meters
- Mechanical cleaning at the inlet: gratings, mesh, filters
- Aeration and oxygen: aerators, injectors, saturation, oxygen station
- UV disinfection (by risk)
- Removal of sediment, flushing and draining
- Autofeeding and feed warehouse
- Parameter control: temperature, O₂, pH + alarm
- Inventory for sorting, catching and packaging
Design Features
- Worst case scenario project: minimum flow rate + maximum temperature + peak biomass.
- Hydraulics: uniform distribution of water and the same conditions across all growing lines.
- Oxygen: calculation of O₂ drop by farm and section; if necessary, supplement with oxygen.
- Sanitation and sediment: the design must allow washing and removal of bottom sediments.
- Reset: Sludge and cleanup are started immediately, otherwise the project will run into regulations.
- Source risks: pollution, floods, blooms - protection and emergency scenarios.
Project implementation
Technology design — our profile (service “Design of fish farms”).
Concrete pouring and construction — hydraulic engineering and general construction works.
Supply of equipment.
Installation technology: pumps, hydraulics, water treatment, oxygen / aeration, instrumentation.
Launch and stocking.
Let us help you design a once-through farm
From water source assessment to working documentation and equipment selection.
Direct flow farm projects
Examples of implemented direct-flow farms - from an incubation and fry workshop to a large complex on the shore of a reservoir.
Moscow region
Direct-flow incubation and fry complex for growing sturgeon
Mangystau region, Kazakhstan
Direct-flow complex for growing sturgeon for caviar
FAQ
Frequently asked questions
Clear answers to the questions that usually appear before a RAS fish farm project starts.
Can standard equipment be used without calculation?
It is risky. Equipment must match the species, biomass, feed load, water quality and building layout. A generic set can lead to weak filtration, unstable oxygen or difficult maintenance.
How is the technology selected for a project?
The choice depends on fish species, target capacity, water source, climate, available building, energy costs, staff skills, market format and the required level of automation.
Calculate a direct-flow farm to suit your conditions
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