Đề tài Towards Zero Discharge of Wastewater from Floating Raceway Production Ponds (Milestone No 5)

Ministry of Agriculture & Rural Development PROGRESS REPORT Intensive in-pond floating raceway production of marine finfish (CARD VIE 062/04) MILESTONE REPORT NO.5 Development of a zero-discharged system Report Author: Michael Burke, Tung Hoang & Daniel Willet December 2007 1 AUSTRALIA COMPONENT 2 Towards Zero Discharge of Wastewater from Floating Raceway Production Ponds (Milestone No. 5) D.J. Willett1, C. Morrison1, M.J. Burke1, L. Dutney1, and T. Hoang2 1Department of Primary Industries and Fisheries, Bribie Island Aquaculture Research Centre, Bribie Island, Queensland, Australia. 2Nha Trang University, International Centre for Research and Training, NHATRANG City, Vietnam Correspondence: Daniel Willett, Bribie Island Aquaculture Research Centre, PO Box 2066 Bribie Island, Queensland, 4507 Australia. daniel.willett@dpi.qld.gov.au EXECUTIVE SUMMARY A major problem with intensified pond-based aquaculture production systems has been managing water quality and discharge quotas due to the accumulation of waste nutrients. This is exacerbated in the current CARD project which demonstrated the very high production capability of in-pond raceways in excess of 35 ton/ha of combined mulloway and whiting. While the current operation managed water quality through exchanging water (approximately 10% per day on average – see MS No.4), it is recognised that with water conservation issues and environmental nutrient discharge impacts, flushing pond water to waste is a less desirable solution. One of the original goals of this project was to investigate strategies that limited water discharge to show that raceway production of fish could be sustainable. This report summarises details of water remediation strategies investigated to progress towards zero water discharge. Waste sumps were installed into the raceways as a proposed means for collecting and concentrating uneaten feed and faeces, thereby reducing nutrients entering the ponds. A trial tested the effectiveness of these solids traps by comparing Total Solids, TN and TP collected in the sump with those flowing out of the raceway through the end screens. Results showed that the waste sumps are generally not effective at concentrating solids for periodic removal. This was primarily due to flow dynamics within the raceways causing eddies to form that keep solids from going down into the collector. In addition, fish within the raceways continually stir up and resuspend particulate waste, allowing it to be expelled into the pond. However, the sumps may be useful as a discharge point in a remediation system which recirculates pond water via an external treatment pond. 3 An original objective of the project was to investigate the culture of the red marine macrophyte Harpoon Weed (Asparagopsis armata) as a nutrient sink. While much previous research at BIARC has looked to develop seaweed biofilters for pond-based aquaculture, the culture of A. armata was novel and offered advantages over commonly used green seaweed species, according to new literature. Several attempts to collect seed stock and culture the specific tetrasporophyte phase of this species however proved problematic and the seaweed failed to thrive and eventually died. Specific factors responsible are discussed. Concurrent research at BIARC is developing technologies that overcome many of the common impediments to seaweed culture and these are discussed in light of future work evaluating A. armata as a biofilter. Recent international research has demonstrated the successful use of bacterial-based processes (termed Bio-floc treatment) for water quality management in pond-based aquaculture. The concept involves manipulating substrate Carbon:Nitrogen ratios to promote heterotrophic nutrient assimilation. A series of experiments were conducted to determine whether bio-floc treatment may be incorporated effectively as part of the raceway production system, specifically as an external component of a recirculating system. The trial defined a Carbon dose rate that achieved almost complete elimination of toxic N species (TAN and NOx) from raceway effluent within 12 hours and prolonged the period prior to remineralisation. A successful shift from a phytoplankton-dominated waste stream to a bio-floc community was also achieved by applying this carbon dose in a replicated continuous-flow treatment system. The bio-floc community was characterised by lower, stable pH (8.0-8.2) and DO (6.9-8.8) levels, increased biomass and a decreased proportion of phytoplankton present. This demonstrated that effluent treated in an external bio-floc pond would be suitable for recirculation, and a schematic of a proposed integrated production system is presented. Of the wastewater remediation strategies investigated in this project, it is evident that bio- floc treatment was the most promising technology to progress towards zero water discharge. INTRODUCTION A major goal of this CARD project was to develop a pond-based fish production system that is both sustainable and profitable, designed to increase production and improve stock 4 management efficiencies and ultimately make better use of existing unprofitable aquaculture pond infrastructure in Australia and Vietnam. The development of low-cost in- pond Floating Raceways (FRs) in this project has demonstrated an innovative approach to larval rearing, juvenile nursery and fish growout. As reported in Milestone No.4, the FR system within a pond a Bribie Island Aquaculture Research Centre demonstrated production capability in excess of 35 ton/ha of combined mulloway and whiting. An inherent problem of any pond-based production system is the accumulation of residual organic matter (uneaten feed, faeces) and toxic inorganic nitrogen (specifically ammonia). Even the best practices cannot avoid this since it has been shown that fish and shrimp only assimilate on average about 25% of ingested food – the rest being excreted into the water column predominately as ammonia (Boyd & Tucker 1998; (Funge-Smith and Briggs 1998; Hargreaves 1998). This feeds phytoplankton blooms which are at best only a partial nutrient sink in ponds stocked at densities above 5 ton/ha (Avnimelech 2003; Brune et al. 2003). Dense phytoplankton blooms can cause lethal DO and pH fluctuations and their overgrowth can lead to bloom crashes and subsequent release of ammonia (Krom et al. 1989; Boyd 1995; Boyd 2002; Ebeling et al. 2006). Water exchange is usually required to alleviate this problem and maintain suitable pond water quality; however with water conservation issues and environmental nutrient discharge impacts, flushing pond water to waste is becoming a less desirable solution. Clearly, production of fish in the order of 35 ton/ha as demonstrated in this project cannot be maintained without a means to remediate or exchange water. The current project managed water quality using secchi depth as gauge of appropriate conditions and by exchanging water (approximately 10% per day on average – see MS No.4). One of the original goals of this project was to investigate strategies that limited water discharge. A number of strategies were proposed, including the culture of Harpoon Weed (Asparagopsis armata) as a nutrient sink; partitioning ponds to into ‘fish culture’ and ‘remediation’ zones; and manipulating Carbon:Nitrogen ratios to promote bacterial nutrient processing. This report will summarise details of water remediation strategies investigated, with particular emphasis on partitioned bacterial nutrient processing as it became evident that this was the most promising technology to progress towards zero water discharge. Strategy 1: Raceway sump to trap solids 5 Background: Reducing direct nutrient input into production ponds reduces pressure on biological remediation processes. The regular removal of uneaten feed and faeces directly from raceways before it is allowed to enter the pond will prevent further nutrient release and mineralisation from this waste source over the production period. The amounts of these settleable solids within floating raceways will vary depending on feeding rates and efficiencies. In turn, the ability to harvest these solids depends on flow dynamics within the raceways and the design of the solids trap. A preliminary experiment was designed to gauge the effectiveness of a solids trap built into the raceways as a means for reducing nutrients entering the ponds. Methods: Plastic stormwater drain sumps were inserted into the tail end floor of each raceway as a solids trap (Fig 1). These sumps were connected via a flexible hose to a pump on a timer which periodically (twice daily) pumped collected waste to a holding tank for evaluation of nutrient content. On monthly occasions between February and October 2006, water leaving the raceways through the end screen was also sampled and nutrient data was compared with that from the sump waste to determine differences. Water quality analyses evaluated Total Solids (TS), Total Nitrogen (TN) and Total Phosphorous (TP), and were determined using validated laboratory protocols based on standard methods (American Public Health Association 1989) and nutrient analysis equipment at BIARC. 6 Figure 1. Design and configuration of the solids trap inserted within the nursery raceways. A plastic grate cover (not shown) prevented fish from entering the sump. Results & Discussion: Nutrient analyses showed some small differences in concentration between water pumped from the sump and water leaving the raceways through the end screen (Table 1.) The greatest difference was with TS, where the sump captured on average 16% more solids than water discharged from the pond. Differences in TN and TP between sump and raceway screen were smaller but still showed a marginally greater average nutrient removal via the sump. This data cannot be statistically validated however because monthly data from the raceway was from a single water sample (due to budgetary constraints) whereby no measure of error rate can be determined. Regardless, the sump was designed to trap and concentrate solids into a thick sludge that could be periodically removed from the pond. It was clear that only a slightly more concentrated effluent was captured by the sumps and their role in preventing nutrients entering the pond from the raceways was limited. This suggests that the waste sumps are not effective at collecting solids for periodic removal. However, they may be useful as a discharge point in a remediation system which recirculates pond water via an external treatment pond. It is an advantage, in this instance, to discharge the most concentrated effluent as possible into the 7 treatment pond, and this was employed in subsequent bio-floc remediation trials (see below). Similar waste removal systems were employed by Koo et al. (1995) in in-pond raceways developed for channel catfish, and likewise their waste removal system showed poor performance. The primary problem was due to inefficient settling of waste in the solids collectors. A known difficulty with raceways is that when solids reach the end of the tank, the hydraulic forces do not efficiently concentrate the solids around the drain. Water reflected off the end wall generates turbulence, causing eddies to form that may keep solids from going down into the collector (Van Wyk, 1999). In addition, fish within the raceways continually stir up and resuspend particulate waste, allowing it to be expelled into the pond. Table 1. Differences in water collected from the solids trap and water leaving the raceway through the end screen, over seven months (n=7). Constituent Mean concentration in water expelled from raceway (mg/L) Mean concentration in water from sump (mg/L) Total Solids 15.4 18.35 Total Nitrogen 2.07 2.33 Total Phosphorous 0.78 0.83 Strategy 2: Evaluation of Harpoon Weed Summary: The concept of using seaweeds as biofilters for removing waste nutrients from fish and shrimp aquaculture operation is well known, with a seminal review by Neori et al (2004) describing the state of the art of this technology. Presently, the most commonly proposed and researched biofilters are green seaweeds from the genus Ulva and the red seaweed Gracilaria. Yet, in practice most seaweed-based remediation systems have proven not to be economically viable, mainly due to the low value of the produced seaweed and the high labour and area requirements for its cultivation. Other physical impediments to the culture of seaweeds in effluent from aquaculture ponds include their susceptibility to epiphytism (Friedlander et al., 1987), infestation by grazers such as amphipods, and 8 competition for available nutrients with phytoplankton (Palmer 2005). These difficulties are compounded by the accumulation of effluent particulate matter on the seaweed’s surfaces. The result therefore in practice, is that growth rate of the seaweeds (and their corresponding value as a nutrient sink) is very often limited and nutrient removal efficiencies are below optimum rates achieved in scaled trials under more favourable conditions (Palmer 2005; previous BIARC research). The present CARD project proposed to investigate the performance of the red seaweed Asparagopsis armata (also known as Harpoon Weed) as a sink for waste nutrients generated in raceway production system. This species was selected on the basis of new work by Schuenhoff & Mata (2004) which suggested that it had considerably greater market value than other seaweeds due to its high concentration of halogenated organic metabolites. Once extracted, these halogenated compounds are used for antifouling and in the cosmetic industry as fungicides. Schuenhoff & Mata (2004) suggest that these compounds are also responsible for limiting epibiota and epiphytes in culture – an advantage over other cultured seaweeds. In addition, its reported removal rate of ammonia is superior to that of Ulva species and it is also a native species to Australia (Fig 2). Figure 2. Harpoon weed (Asparagopsis armata) growing on rocks in Moreton Bay, S.E. Qld. Photo by Marine Botany Group, University of Qld (2003) 9 A proposal was drafted to collect harpoon weed from Moreton Bay as a seed stock to trial its growth rate and nutrient uptake under effluent conditions generated in the raceway pond at BIARC. In particular, it is the tetrasporophyte phase of the plant that is reported useful for biofiltration. Several collecting expeditions were mounted in conjunction with marine botanists from the University of Qld. Only a small amount of harpoon weed in its tetrasporophyte phase was located. It was transferred to a production unit at BIARC and supplied with pond effluent in order to cultivate larger quantities for use in a replicated bioremediation trial. Unfortunately, the harpoon weed failed to thrive and eventually died preventing the trial being conducted. It is uncertain whether seasonal or effluent-specific factors were responsible. Given the previous considerable work conducted at BIARC evaluating seaweed biofilters and the difficulty in locating, collecting and culturing this specific macrophyte, plans for further trials were terminated for the current project. Future work in evaluating this species as a biofilter, however, is planned as part of ongoing BIARC wastewater remediation studies. Based on current research at BIARC on seaweed biofilters, to effectively incorporate seaweeds into a bioremediation system for pond-based aquaculture it appears that pre- treatment of the effluent would be necessary so that competing plankton levels, fouling organisms and suspended materials are reduced, and so that nutrients are converted into forms available for direct plant uptake. Current work at BIARC, outside of the CARD project, is assessing the role of polychaete-aided sand filtration as one such pre-treatment option (Palmer 2007). Strategy 3: Bacterial nutrient processing Background: There is now recognition that promoting a swing from autotrophic (phytoplankton-based) to heterotrophic (bacterial-based) processing of residual pond nutrients has many advantages for water remediation. Sewage effluent treatment has long employed bacterial digestion of organic matter in activated sludge systems (Arundel 1995) and more recent studies have shown that suspended growth systems, where heterotrophic- dominated processes regulate water quality, have great application for limited-water- exchange shrimp and tilapia production (Avnimelech 1999; Burford, et al. 2003; Erler et al. 2005). In aquaculture, these heterotrophic-dominated growth systems are generally termed Bio-floc systems. 10 The challenge is to determine the best configuration for incorporating biofloc treatment as part of the raceway production system. Two approaches are possible: in-pond biofloc treatment or external biofloc treatment as part of a recirculating system. Most studies on using bio-floc water remediation for aquaculture have advocated floc formation within the culture pond as a supplementary source of dietary protein (Avnimelech 1999; McIntosh et al. 2001; Erler et al. 2005) in addition to controlling water quality. While increased feed utilisation is ideal, the excessive turbidity and high oxygen demand created by bio-flocs may have a negative effect on fish cultured within floating raceways. The high DO demands of the floc colony in addition to those of the cultured species means that cultured stock are even more vulnerable in the event of any aeration failure, especially in intensive production systems such as floating raceways. High suspended solids levels can foul the gills of cultured animals and lead to bacterial, protozoan and fungal infections (Boyd 1994). In addition, not all cultured species will access or target the additional protein source provided by the bacterial flocs – especially higher order species (non filter feeders). Alternatively, establishing a bio-floc zone as a component of a treatment system external to the culture pond (i.e. post-production) is a new approach for this technology and may be more suited to FR production for the reasons detailed above. Waste nutrients potentially could be captured within bio-flocs, which in turn are periodically harvested from the water in isolation from the cultured stock. Significantly cleaner supernatant could then be returned to the culture pond. While sedimentation ponds are routinely used in Australia to treat post-production wastewater, local studies have shown they are generally ineffective at reducing Total Nitrogen, mostly due to remineralisation and inadvertent discharge of the dominating phytoplankton (Preston et al. 2000; Palmer 2005). Directly harvesting phytoplankton is difficult and generally cost prohibitive to farmers, so a need exists for a new approach to enhance the performance of post-production treatment ponds. For a Bio-floc Pond (BFP) to effectively operate as a post-production wastewater remediation system there must be mechanisms for converting phytoplankton-dominated wastewater into a bio-floc community which packages nutrients into the more harvestable ‘floc’ form. A key mechanism for promoting heterotrophic assimilation of waste nutrients is through the manipulation of substrate carbon:nitrogen (C:N) balance. Heterotrophic 11 bacteria utilise organic carbon as an energy source, which is required in conjunction with nitrogen to synthesize protein for new cell material (Avnimelech 1999). For the bacteria to metabolise available nitrogen