The Research Of PH Regulation Methods In Industrialized Recirculating Aquaculture Water

Compared with traditional culture methods, industrialized recirculating aquaculturehas a lot of advantages, such as high density, resource conservation, pollution-free andso on, and this way is also consistent with sustainable development, so it has beenlarge-scale application in recent years. However, water quality control is one key tosuccess in industrialized recirculating aquaculture. There are many water qualityparameters which should be controlled in recirculating aquaculture system, includingtemperature, DO, CO₂, pH, ammonia, etc.. In which, pH can affect other water qualityparameters and biochemical reaction rate, it is a very important parameter. Thus, it is ofgreat significance to research the effective pH regulation methods in industrializedrecirculating aquaculture system.With the culture time increased, pH in aquaculture waters will be declined. Thereare two reasons, one is CO₂gas accumulation produced by fish respiration andnitrification, the other is alkalinity consumption caused by nitrification. Compared withforeign countries, pH regulation methods are relatively backward in domesticindustrialized recirculating aquaculture enterprises, which mainly use on-linemonitoring and artificial way of adding alkaline reagents. And the research ofCO₂removal technology is not thorough and perfect, still in its infancy, manyhigh-density recirculating aquaculture systems don’t set the CO₂removal link. Theresearch purpose of this paper is to develop a pH regulation method with CO₂removaland adding alkaline reagents combined, at the same time, using automatic controltechnology, pH real-time monitoring and control system in recirculating aquaculturewater is designed with a good control effect and a high degree of automation.In this paper, CO₂removal technology has been studied, in order to adjust pHdecline caused by CO₂concentration accumulation. Based on principle of gas exchange,CO₂removal device has been designed for recirculating aquaculture water, and DOE （design of experiment） method has been used to study CO₂removal efficiency.Three-factor two-level orthogonal experiment results show that: G/L （gas to liquid）hasthe most significant effect on CO₂removal efficiency, but hydraulic load （HLR）, inletCO₂concentration and the interaction between factors have no significant effect on CO₂removal efficiency. Therefore, G/L should be adjusted to improve CO₂removalefficiency in the actual operation of CO₂removal device. The testing results of G/Leffect on CO₂removal efficiency show that: when G/L=1⁵, CO₂removal efficiencyincreases rapidly with the increase of G/L; when G/L>8, CO₂removal efficiencyincreases gently with the increase of G/L. Considering both system energy saving andeffective removal of carbon dioxide, G/L=5⁸is considered to be the best region forCO₂removal device running, CO₂removal efficiency=80%⁹2%. Thus, CO₂removaltechnology can remove high CO₂concentration accumulated in the aquaculture watereffectively, at this time, pH decline in the aquaculture water is only concerned withalkalinity consumption.In this paper, the principle of adding alkaline reagents has been studied, inorder to adjust pH decline caused by alkalinity consumption. First, typical pHneutralization curve and mathematical model have been analyzed, the results show thatpH neutralization process possesses significant non-linearity and hysteresis; second, thetheoretical and experimental studies of pH change principle in recirculating aquaculturesystem have been carried out, the results show that nitrification is an acid productionand base consumption process, effect of changes in alkalinity on changes in pH islogarithmic, at the same time, pH change in the recirculating aquaculture system is veryslow, so we have sufficient time to adjust pH in the actual production; Then, bycomparing and analyzing of several properties of alkaline reagents,the alkaline buffer used in this article has been determined, and according to therelationship between the feeding amount and the amount of adding alkaline reagent, thedaily amount of adding alkaline reagent has been determined to adjust pH declinecaused by alkalinity consumption; Finally, the non-linearity and hysteresis of pHadjusted by adding alkaline reagent have been researched, the results show that the dailychange amount of pH in the recirculating aquaculture water is in the linear range ofalkaline buffer titration curve, and its control scope is also in the linear range, so pHreal-time monitoring and control system in recirculating aquaculture water has beenconsidered as a linear system, and recirculating aquaculture system with more than 1time/hour of cycle rate is more uniform mixed, its effect on hysteresis is very little.Based on the above research results and the request of recirculating aquaculture forpH, control indicators and control requirement of pH real-time monitoring and controlsystem in recirculating aquaculture water have been made. Through comparing classicpH control algorithms, incremental PID control algorithm has been chosen for thissystem, a dual pH regulation method with CO₂removal and adding alkaline reagentscombined has been determined, and the overall system control program has been built.Under the overall program guidance, the hardware and software design of pH real-timemonitoring and control system has been done. Siemens S7-200PLC,mcgsTPC embedded integrated touch screen, analog input module, analog outputmodule, MA8-3SMS alarm module, pH sensors and metering pumps have been used tobuild hardware platform. With the use of MCGS configuration software, pH real-timemonitoring and control system in recirculating aquaculture water has been designed,with the functions of control engineering display, communication connections,parameter setting, real-time curve display, historical curve display, historical datastorage, query and export, data acquisition and processing, pH overrun alarm and so on.This control system has many advantages, such as good human interface, goodscalability, simple, flexible, practical, high degree of automation and a certain degree ofinnovation.Finally, pH real-time monitoring and control system has been carried out forfour-month trial running in the freshwater high-density recirculating aquaculture systemof Ministry Fishery Equipment and Key Opening Laboratory. The results show that themonitoring and control system is stable and reliable, and its control performance issignificant, realizing pH constant control and meeting the requirement of recirculatingaquaculture for pH, with a good promotional value and practical value.