Recirculating Aquaculture Systems

Transcription

1 Recirculating Aquaculture Systems Introduction, principles and examples (1) Avdelningen för reglerteknik Institutionen för signaler och system Chalmers Aquaculture (2) 1

2 Land Based Recirculating Aquaculture Systems (3) Land Based Recirculating Aquaculture Systems (4) 2

3 Different types (5) Partially recirculating More than 5% water exchange per day Fully recirculating Less than 5% water exchange per day Land based Outdoor Indoor Aquaponics Sea based Hard shell Soft shell Outline (6) Fish basins/tanks Oxygen Water treatment Particulate matter Dissolved organic matter Nitrification Denitrification Photochemical treatment RAS configurations Conclusion 3

4 Fish basins/tanks (7) Circular (or near circular) tanks + Easy collection of feed loss and faeces + Homogenous water quality _ Difficulties catching the fish _ Inefficient use of area Raceways + Easy to catch and sort fish + Efficient use of area + Occular inspection _ Inhomogenous water quality (by def) D ended raceway (Akva group) Shallow basins (e.g. wolffish); Shelves only raceways? Oxygen Oxygen is provided to the rearing tanks by Off equilibrium between ambient air and the concentration in the water causes a flux [g/d] that can be calculated from Water surface area (A) Oxygen concentration (S O ) Mass transfer coefficient (K Ls A), which increases rapidly (from appr. 100 m/d) with wind and water movements. Oxygen saturation concentration (S O,sat ), which is temp. dependent). Incoming water. The required minimum water flow Q needed without additional oxygen can be calculated by a steady state massbalance (in = out) using Maximum respiration rate (r 0 ) [go2/d/kg fish] Stocking density (ρ) [kg/m3] Water volume (V) Aeration. The supplied oxygen can be calculated with another offequilibrium expression where the mass transfer coefficient (k L A) depends on the efficiency of the diffusors and the airflow. go 2 /m 3 Oxygen saturation concentration (8) Temperature ( o C) Super saturation by compressed or liquid oxygen in the incoming water. 4

5 Water treatment (9) Particulate matter Irritation and damage to the gills that increases the risk of bacterial gill disease (also spread by particles) Accumulation of suspended particles leading to anoxic degradation causing muddy taste (e.g. Geosmin) Reduced sight increasing feed loss May reduce the efficiency of water treatment (UV, filter cloughing etc.) Particulate Matter (10) The most common methods: Drum filters Band filters 5

6 Dissolved organic matter (11) Hydrolysis Biodegradable particulate organic matter dissolved organic matter Heterotrophic bacteria use the degradable (readily or more difficult) organic matter as substrate. Often measured as COD (Chemical Oxygen Demand) or TOC (Total Organic Carbon). All matter is not degradable and therefore a better measure is BODk (Biological Oxygen Demand), the amount that has been biologically degraded in k days. What value of k (5 and 7 standard) that is relevant can be related to the average Hydraulic Retention Time (HRT) in the system, which only depends on the water exchange: where Q 0 is the effluent flow from the system and V tot is the total volume of water in the system. 10% water exchange therefore corresponds to 10 days. Dissolved organic matter (12) In an aerob environment heterotrophic bacteria use oxygen as electron acceptor: HC + O 2 CO 2 + H biomass The bacteria can either be suspended in the water, aggregated to granules or in a matrix (biofilm) attached to a surface. Using suspended bacteria (activated sludge) means that they must be collected, normally by sedimentation, and recycled in order to not be flushed out of the system. Using biofilm reactors => No need for settlers and no risk of sludge escape Examples of some biofilm substrate material: Moving bed biofilm Reactor 6

7 Nitrification (13) Ammonia (NH3) is toxic to most fish already at low concentrations. The equilibrium is shifted towards ammonium (NH 4+ ) NH 3 + H 3 O + NH 4+ + H 2 0 As a consequence threshold values depend on ph. Ammonium can be degraded biologically by nitrifying bacteria in two steps. 1. Ammonium oxidization (Nitrosomonas, Nitrosospira, Nitrosolobus,...): NH O 2 NO 2 + H 2 O + 2H + which is acidifying. Generalized, this is compensated for by a reduction in alkalinity (bicarbonate equiv.) 2H + + 2HCO 3 2CO 2 + 2H 2 O 2. Nitrite oxidization (Nitrospira, Nitrobacter,...): NO O 2 NO 3 Nitrifyers are autotrophic and much slower growing than heterotrophs. As a consequence they are outcompeted if the BOD concentration is not very low. Both AOB and NOB are temperature dependent with optimum around o C. Nitrification (14) Nitrification is preferably maintained in biofilmreactors for two reasons: 1. They attach readily to most substrates and usually build up to a firm biofilm that resist shear well. substratum 2. They can cope with heterotrophic competition in a better way than if they were suspended. Note: Also NO 2 is toxic to the fish. It is therefore important that the system is operated and dimensioned such that a sufficiently complete nitrification from NH 4+ to nitrate can take place. 7

8 Denitrification (15) In absense of oxygen, the heterotrophic bacteria can use nitrate, which is the reduced to elementary nitrogen gas: 4NO 3 + 4H + + 5CH 2 O 2CO 2 + 2N 2 + 7H 2 O The denitrification is the main reason why most RAS have a water exchange exceeding 10%. Assume no denitrification and Threshold NO3 concentration is SNO3 = 250 gn/m3 2% kg feed/kg fish/d 40% protein and 16% N in protein 25 kg fish/m3 Ammonium, nitrite and organically bound nitrogen concentrations are small. Added N is then: 25*0.02*0.40*0.16 = 32 gn/m3/d A nitrogen massbalance for the system is (neglecting the N in the fish) Pathogens (16) Chemicals have to be used with precausion. In particular because Nitrifiers are sensitive. The chemicals will be recirculated and potentially remain in the system for a long time. Alternatives are UV treatment Ozon treatment Catalytic oxidization 8

9 The Nitrogen Treatment Problem (17) Required treatment: Organics (BOD) Tolerant Ammonium (NH 4+ ) Not tolerant Nitrate (NO 3 ) Tolerant Nitrite (NO 2 ) Not tolerant Biological WWT processes: A General Problem (18) Required treatment: Organics (BOD) Tolerant Ammonium (NH 4+ ) Not tolerant Nitrate (NO 3 ) Tolerant Nitrite (NO 2 ) Toxic! Biological WWT processes: Requires BOD = 0 9

10 Huge number of suggested RAS! (19) Landbased RAS typical for partial recirculation without denitrification (AquaOptima) Huge number of suggested RAS! (20) Landbased RAS typical for partial recirculation without denitrification (AquaOptima) 10

11 (21) With post denitrification and hydrolysis Example: Langsand Laks 1000 tons/year (22) With biofilters in a separate loop (Aqua Group): 11

12 With biofilters in a separate loop (Aqua Group): (23) Aquaculture Developments + Low flow through BF possible => complete nitrification can be guaranteed + Not required to remove all BOD in the main flow => reduced oxygen consumption + Particles from BF are caught in the PF before the fish tanks No denitrification Increased flow through PF, UV and degass => increased energy use Possibility of nitrification in degass => risk of elevated NO2 concentrations (24) University of Maryland: Very high degree of recirculation Surprising massbalances explained by combined denitrification and ANAMMOX bacteria 12

13 (25) With Annamox bacteria: High temperatures and moderate salt (<1.6 g/l) Greenfish AB (Kungälv) (26) + Pre denitrifying solution (WWTP) + Nitrification in separate loop => complete nitrification can be guaranteed + Bypass from P => both BOD and NH4 can be controlled to be close to threshold 13

14 A Simulator in Matlab / Simulink (27) 1 (28) Feed time (h) Death Growth: TGC + FCR TGC + DE + DEN... Rate Rate =1 T 1 =1 T 2 =2 0 6 time (h) T 1 =0 T 2 =2 =2 0 6 time (h) 14

15 (29) A Very Dynamic Process! (30) 15

16 Conclusion (31) A RAS configuration that is optimal, in general, has not been found. The best configuration of the water treatment will depend on Species Feed Size Temperature Aeration Local factors, such as water availability effluent restrictions available heat sources price of energy available carbon source etc. Current & Future Research (32) Freeware solution LibRAS Investigate Pros & Cons of different configurations (robustness, resource consumption etc.) Investigate reasons and appropriate actions to different incidents. Modelling and validation for different species Start up Energy consumption Easy to use version for educational purposes 16

17 Nomaculture (33) Spotted wolffish (Fläckig havskatt) European lobster (Hummer) 17