process control

The need for Process Control in next-generation Aquaponic Systems

In chemical engineering, processes are procedures encompassing mechanical, chemical, physical, electrical and biological steps to create a product from one or more substances. Decades of research and development have led to detailed knowledge of processes that shape our world today: from sawing off a piece of metal, to creating carbon nanotubes, everything that is industrially produced today comes from sets of very specific and controlled steps (i.e. process control (PC)).

Aquaponics -being an emerging field that deals with live organisms that can often be poorly understood-, can benefit from systematic engineering approaches found in process and chemical engineering disciplines. These approaches generally entitle breaking out a large process (a full aquaponic system in this case) into smaller sub-processes that can be studied both in isolation and in interaction with others. The following table is a simple example of such a breakdown:

Process Inputs Outputs PC (abiotic) PC (biotic)
Fish rearing vessels Clean process water; Biomass; Feed; Air (design option) Polluted process water; Biomass; Biosolids Polluted process water; Biomass; Biosolids; Hydraulic retention time (HRT); Water velocities at inlets; Water velocities at outlets; Hydrodynamic regime; Water quality Fish stocking density; Fish behaviour
Water transport Process water (both clean and polluted); Biosolids Process water (both clean and polluted); Biosolids Flow rate; Water velocities; Pressure Biofouling growth
Solids separation Polluted process water; Power (design-dependent); Backwash water (design dependent) Clean process water; Sludge Flow rates; Water velocity (in some cases); Rinsing/purging/backwash frequency (design-dependent); Total suspended solids/turbidity n/a
Total Ammonia Nitrogen (TAN) removal Polluted process water; Air (depending on design) Clean process water; Biosolids HRT; Water velocities; Reynolds number (degree of turbulence); Air flow (design-dependent); Rinsing/purging/backwashing (design dependent) TAN removal rates; TAN concentration; NO2 concentration; NO3 concentration; pH
Gas exchange Clean process water; Air; Gases Clean process water; dissolved gases Flow rate; Water velocities; HRT; Gas to liquid ratios; DO concentration; CO2 concentration; DN concentration n/a

By “peeling off” a complex system into smaller, simpler layers, it becomes easier for the operator to diagnose and control the system, one step at a time.

The most obvious advantage of process control is the capacity to adjust the system’s parameters in a more flexible way: nitrate concentrations can be adjusted according to specific plant’s requirements. Fish feeding can be controlled independently from plants and sludge can be converted to fertilizer, bio-gas or dissolved nutrients. Proper process control also has the advantage to reduce the knowhow burden on the operator. This allows people with less technical training to undertake simpler tasks, which in conjunction keep the whole system running.

Aquaponic systems are currently evolving from simple single-loop systems to more complex configurations with one or two mainstream processes (i.e. a fish loop and a plant loop) and many sidestream processes (denitrification, sludge digestion, foam fractionation, CO2 degassing). Thus, it will become increasingly necessary to keep a sound hierarchy, control procedures and adequate instrumentation for every process found in the system. While the operational parameters of some processes found in aquaponics are well-known (e.g. TAN removal and gas exchange), some others, such as specific plant uptake of nutrients, nutrient accumulation dynamics, and microbial mineralization will require further research.

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