Controlling biofouling of pond aerators on marine prawn farms

1. Controlling biofouling of pond aerators on marine prawn farms SeafoodCRC project 2011/ 734 David Mann Dept Agriculture Fisheries and Forestry

2. Biofouling of aeration equipment is a big problemWhat can be done about it? • Problem has been around since aerators put in ponds • Issue worsening • high and rising electricity cost • high cost of labour • difficulty in getting ‘cheap’ labour • efficiency pressures

3. Aerator Biofouling project plan • What control options can be used? • What impact is it having on the industry?

4. Project plan • What control options can be used? • Identify practical solutions or best options in use around the world not just pond aquaculture - examine all marine fouling control approaches to find any appropriate • Test potential options testing in production ponds on a multiple farms • What impact is it having on the industry?

5. Project plan • What control options can be used? • Identify practical options • Test potential options • What impact is it having on the industry? • Estimate cost to farms labour, accelerated wear&tear, electricity, pond productivity • Aerator performance oxygen transfer per hour and per kWh, electricity consumption

6. Project plan • What control options can be used? • Identify practical options • Test potential options • What impact is it having on the industry? • Estimate cost to farms • Aerator performance Completed Planning Questionnaire Some results

7. 1a. Identify practical control options.Review • Undertook a comprehensive review of biofouling controls used in marine industries • Various options outlined – pros and cons • APFA evaluation guidelines • Commercial readiness – ie not developing a new product • No APVMA issues – not restricted • Cost effective – cost of implementation significantly less than current de-fouling approach

8. Review summary Surface coatings • Predominantly biocidal antifouling paints used • marine shipping and structures • off-shore cages – copper • Natural ‘environmentally friendly’ biocides • very few products commercially available • require APVMA approval for pond use • Fouling release coatings • physical properties inhibit attachment • inert coating - wax or silicon based • most logical choice • do not prevent biofouling • couple of products available for testing • performance variable (anti-attachment, longevity) Selectope – medetomidine • anti-barnacle settlement ingredient for paints

9. Review summary Non-surface coating options • Farm management • regular sun drying pdwls • high level water filtration/treatment • reduce No. paddlewheels • Aerator design • construction materials (F-R plastics) • reduce surface area • eg long arm paddlewheels, simpler surfaces • sacrificial covering on float • Lesser known approaches • ultrasound • electric field • biocontrols Paddlewheels most problematic • replace with alternatives • dual roles of circulation and aeration

10. Untreated Treated AFwax test Thailand 2012 1b. Test potential options On-farm testing – Phase 2 of project (2nd season) • Control methods selected by the industry • Yet to be finalised • fouling release coatings fit selection criteria • two products available • silicon and wax based coatings • Assess products/options on • total cost • practicality • durability • effectiveness

11. 2a. Cost to farms - Questionnaire results summary - thanks to responding farms Aeration • Industry-wide aerator fleet • paddlewheels 66.4% : prop. aspirators 33.6% • Aerators are 2hp (1.5kW) motor rating • At peak pond load pond aerator ratio • 70% pdwl : 30% aspirator • Aerator numbers • phased in through crop 4 -10 /ha • average over entire crop – 7.4 /ha • 43,550kWh /ha /crop

12. Questionnaire results summary Defouling • Most farms defoul aerators in the pond • 2/3 farms use high pressure water jet • & some scraping • Labour - 18.4h /ha for defouling • Defouling during crop • Paddlewheel 1.7x (up to 3.4x) • Aspirator 0.3x (up to 0.8x)

13. Questionnaire results summary Fouling organisms • Barnacles and algae ~equal problem (rating 4 & 4.1 /5) • Below and above water surfaces ~equal problem (rating 4.3 & 4.1 /5) • 3 pdwl zones (paddle blades, below & above surfaces) • all received a max. problem rating Biofouling cost per ha Defouling labour $405 ($22/h) Increased electricity $305 (↑ 5%) Increased maintenance labour $248 (↑ 25%) Increased maintenance parts $94 (↑ 25%) TOTAL cost of biofouling $1050

14. 2b. Impact on aerator performance – tank testing Heavily fouled with algae and barnacles Futi, 2hp paddlewheel Total wt of fouling = 21kg SOTR – standard oxygen transfer rate SAE – standard aeration efficiency

15. Impact on aerator performance – tank testing Barnacle and algae fouling • original fouled condition • only paddles defouled • fully cleaned condition Chenta 2hp paddlewheel Heavily fouled with barnacles and algae Total weight of fouling = 35.1kg (out of the water)

16. FRONT BACK Impact on aerator performance – tank testing Filamentous algae on paddles only Paddle and upper surface algae can be controlled by regular daytime ‘OFF’ periods eg 1 or 2 paddlewheels off per pond /day Futi brand 2hp paddlewheel Fouled paddles placed on a clean paddlewheel Paddles with a combined weight of algae = 10.5kg.

17. Comparison of splash from algae fouled and clean paddles Same paddlewheel with and without filamentous algae fouling on the paddles • Note backsplash area difference in droplet density • Indicates possibly more water droplets when filamentous algae on blades

18. Impact on aerator performance Weight loading • Up to 40kg possible • Upper surface algae worst • full wet weight + holds water • Algae in the water has no weight • Barnacle weight - in air 1kg : in seawater ~375g Chenta 17 fouled Chenta 17 cleaned

19. Measuring aerator performance • 40,000L volume • 3x larger than min rec’d (0.0375kW/m3) • For comparisons • calc. oxygen transfer for standard conditions • 0 ppt, 20°C, 101.3 kPa • measurements in freshwater • std for aerator testing • oxygen transfer different for seawater • Oxygen transfer rate higher in seawater • smaller water droplets • smaller air bubbles with longer retention time • Test oxygen transfer rates • always different to aquaculture ponds Aerator test tank at BIRC • 6.5m diam x 1.5m (1.2m water depth)

20. Measuring aerator performance Oxygen transfer values SOTR (kg O2 / h), SAE (kg O2 / kWh) based on oxygen recovery from near zero ppm

21. Aerator Biofouling project – what’s up next • More tank testing – determine impact under various biofouled conditions • More electrical monitoring on farms – accurate assessment of aerator operating conditions • Feedback from APFA on biofouling control options to test this season • Seafood CRC agree to progress project into Phase 2 • Implement biofouling control tests on several farms • collaborators required

22. Environmental influences on aerator performance Salinity Effect on SAE of a paddlewheel and aspirator. Data from Vinatea and Carvalho (2007). Dissolved oxygen Effect on AE of a paddlewheel and aspirator. Data from Ruttanagosrigit, et al. (1991).