| Recirculating aquaculture system(RAS)is an advanced mode of facility aquaculture.In China,RAS has been developing rapidly in recent years.It plays a significant leading role in promoting the upgrading and modernization of fishery.The core of RAS is the purification of water quality.Removing solid wastes,including uneaten feed and feces,is the crucial water treatment process for maintaining the stable operation.At present,the basic characteristics and removal mechanisms of suspended solids are insufficient known,and the effects of different solids removal technologies on removal efficiency and particle size distribution(PSD)are unclear as well.The removal efficiency of particles is thus not high,especially the fine particles.It has been a key factor for restricting the fast development and the aquaculture benefit improvement in RAS.This thesis aims at removing particles(i.e.suspended solids)in marine RAS.The relevant optimization studies on application technologies were conducted from the perspective of practical production.The contents and results are summarized as follows:1.In an experimental RAS for grouper farming,the original water was taken from the outlet of a drum filter.It was carried out for the batch trials of venturi jet-based foam fractionation to explore the removal effects of particles.The results showed that the venturi jet-based foam fractionator had a good effect for removing particles of size≤90μm,especially for particles of size≤10 and 50-90μm.And on this basis,the removal effect of different mesh sizes(120,90,60 and 40μm)on solids microscreen filtration combined with foam fractionation was studied.The results showed that the mean filtration and total removal efficiency increased with the decrease of mesh size.However,the breakup of relatively large particles caused by microscreen filtration also increased simultaneously.Although the best total removal efficiency was 32.33%,obtained with mesh size 40μm.It probably causes adverse effects due to the serious particle breakup and the more produced fine particles(size<20μm).Therefore,mesh size 60μm is better in terms of solids removal efficiency and fine particles management.2.Five ozone dosages(0.5,1.0,2.0,2.5 and 3.5 mg/L)and five contact time(1,2,3,4and 5 min)were adopted for the ozonation treatment of recirculating seawater taken from Pacific white shrimp farming respectively.To study the changes of PSD,it was real time measured.The results showed that the flocculation and breakup of particles were demonstrated during the ozonation treatment.Due to the relatively high total residual oxidants(TRO)concentration at the beginning of contact time,the particle flocculation increased.With the increase of contact time,the TRO concentration decreased,and the particle flocculation could decrease too,while the particle breakup could be obvious gradually.By using a fractal characteristic value(D_f)of cumulative particle size-volume distribution,the flocculation process could be divided into three stages of preparation(D_f<1.22),growth(D_f>1.22)and steady(stable or decreased D_f).Besides,the flocculation efficiency varied parabolically with contact time.On experimental conditions,the best flocculation efficiency was 43%at ozone dosage of 3.5 mg/L with 5 min contact time.A higher ozone dosage resulted in a shorter preparation stage,along with a longer growth stage(namely lasting a longer flocculation effect)and a relatively higher extremum of parabolic flocculation efficiency.However,the increase of extremum would reach a peak,and probably be accompanied with a more breakup of particles and a more produced ammonia.It would increase the load of water treatment.Thus,a relatively high ozone dosage may not be suitable for promoting solids removal in marine RAS.3.To study the promoting effect of ozonation treatment on solids microscreen filtration combined with foam fractionation,three ozone dosages(0.8,1.6 and 2.4 mg/L)of two ozonation positions(before microscreen filtration and before foam fractionation)were adopted for the treatment of recirculating seawater taken from Pacific white shrimp farming respectively.The results showed that ozonation treatment obviously improved the foam fractionation efficiency.The increased total removal efficiency was 13.5%compared to the free of ozonation,obtained with ozonation of 0.8 mg/L before filtration,and it produced relatively less ammonia.If ozonation is added to foam fractionation,1.6 mg/L is preferred for avoiding a large amount of particles breakup.As a whole,the high TRO concentration is not suitable for releasing to microscreen filtration and foam fractionation treatment.4.In order to improve the foam fractionation efficiency of fine particles,the bubble generating of aeration and venturi jet were studied.The results showed that the increase the water salinity,the decrease the Sauter mean diameter(SMD)and the increase the gas holdup.The decrease the air inflow,the decrease the SMD,as well as the gas holdup.In general,the SMD of aeration was larger than that of venturi jet.Based on the above results and combining the relevant flotation theories,an integrated foam fractionation of the synergy between aeration and venturi jet was therefore proposed.The gas holdup of integrated foam fractionation could reach to 0.100(aeration and venturi jet alone was 0.031 and 0.074,respectively),and the removal efficiency of fine particles could reach up to 55.84%under experimental conditions(aeration and venturi jet alone was 19.06%and 39.67%,respectively).The aeration-based foam fractionation had a good removal effect of fine particles that were<50μm.The venturi jet-based foam fractionation probably led to the breakup of relatively large particles,and the increasement of fine particles that were 2-30μm.The integrated foam fractionation could save energy and generally reduce the amounts of fine particles that were 2-15μm produced by venturi jet. |
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