SUSTAINABLE FISHERIES FOR ENHANCED WATER RESOURCES IN ARMENIA (SFEWRA) PROJECT

Transcription

1 Identification and suggestion of internationally recognized Best Management Practices of waterefficient fish farming for implementing in targeted fish farms of Ararat Valley SUSTAINABLE FISHERIES FOR ENHANCED WATER RESOURCES IN ARMENIA (SFEWRA) PROJECT USAID PEER Program October,

2 Recirculation of fishery waters in Ararat valley aquaculture facilities 1. General concepts of recirculation The traditional pond and raceway systems of fish production in Ararat valley use large amounts of clean artesian water, exerting pressure on groundwater reservoirs in the region, and thus might face increasingly tight restrictions if environmental legislation and water prices are upgraded. The development of a recirculating aquaculture systems (RASs) (closed or quasi-closed) can significantly reduce the excessive water usage concerns in this area. In recirculating aquaculture productions water is alternately polluted by the crop and cleaned by the filtration systems. As long as the filtration systems adequately remove enough pollution to restore water quality to acceptable levels, the water can be recycled indefinitely with some makeup water to replace water that evaporates or leaks from the system. Thus, the secret to RASs is removal of the waste from the systems and at a cost that allows the operator to make a profit. Aquatic production system wastes generally fall into 2 categories: particulate/solid matter (sludge) and dissolved composites such as nitrogenous compounds (ammonia (NH + 3 ), nitrite (NO - 2 ), nitrate (NO - 3 )), phosphorus (primarily phosphate (PO 3-4 )) and carbon dioxide (CO 2 ). Of these waste metabolites, fish produce roughly mg/l Total Ammonia Nitrogen, mg/l CO 2, and mg/l solids for every 10 mg/l of dissolved oxygen that they consume (Hagopian and Riley, 1998). Stocking density often is a primary determinate in the level of these wastes in RAS and rapid removal of particulate and dissolved wastes from recirculating systems is of upmost importance if recirculation is addressed. There are numerous designs for RAS (Timmons et al., 2002; van Rijn, 1996) which generally work effectively to accomplish the following: 1. Aeration, 2. Removal of particulate matter, 3. Biological filtration to remove ammonia and nitrite by nitrification, 4. Buffering of ph. 2

3 Although not normally employed in commercial aquaculture facilities today, the denitrification process (removal of nitrate) is becoming increasingly important, especially in systems where stocking densities increase and water exchange rates are reduced, resulting in excessive levels of nitrate in the culture system. Currently, to help maintain good water-quality conditions in RAS, aquaponic solutions can be also integrated for nitrate (and in some cases solids) removal from RAS wastewater. Each of these biofiltration units and the potential setups for RAS for Ararat valley fishery productions is discussed. 2. RAS biofiltration units 2.1. Solids removal Uneaten feed, feed fines, fish fecal matter and dead microorganisms are all sources of solids production within recirculating systems (Chen et al., 1997). Solids control is one of the most critical processes that must be managed in recirculating systems, because solids decomposition can degrade water quality and thus directly and indirectly affect fish health (Mirzoyan et al., 2008). The volume of the solid waste produced depends on stocking density and feeding rate and the higher the both, the more efficient biofiltration units are required for particulate matter removal from aquaculture waste. Removal of particulate matter (i.e. removal of large particles (>20 µm) by sedimentation or mechanical filtration and small particles (<20 µm) by either chemical or biological oxidation (Timmons et al., 2002; van Rijn, 1996) is a commonly used practice in RASs worldwide. Stocking densities are typically very low in Ararat valley fisheries and combined with high water exchange rates, this results in very low levels of particulate matter production. Typically solids are washed out with water and are discharged into receiving environment. However, if water recirculation has to be implemented, solids removal becomes crucial. Given higher capital and operational costs of complex mechanical filtration units and relatively low rates of particulate matter in fisheries in Ararat valley, a simple conical clarifier design (Fig.1) is advised for particulate matter removal. Conical clarifiers trap solids in their tanks through a couple of added baffles. The water column along with the solids enters the tank and 3

4 solids drop out or fall to the base of a conical bottom because of the difference in density between particles in the water and the water. The particles are then trapped and later pumped out. A series of baffle obstacles make suspended particles easier to trap as the water flows down, up and out through the tank (Timmons and Ebeling, 2007). Figure 1. Conical clarifier (source: Nitrogen removal In the aquaculture environment, nitrogenous compounds, e.g., ammonia, nitrite and nitrate, are of primary concern as components of waste products generated by rearing fish. There are four primary sources of nitrogenous wastes: urea, uric acid, and amino acid excreted by the fish; organic debris from dead and dying organisms; uneaten feed, and feces; and nitrogen gas from the atmosphere (Timmons and Ebeling, 2007). Ammonia, nitrite, and nitrate are all highly soluble in water and are inhibitory / toxic for fish at different levels (Table 1). 4

5 Table 1. Water quality parameters in terms of nitrogenous waste for warm and cool water species (according to Timmons and Ebeling, 2007) Parameter Warm water species Cool water species Temperature ( C) Oxygen (mg/l) Carbon dioxide (mg/l) Total suspended solids (mg/l) <20 <10 Total ammonia nitrogen (mg/l) <3 <1 Nitrite nitrogen (mg/l) <1 <0.1 Nitrate nitrogen (mg/l) or higher Ammonia, nitrite and nitrate typically are dissolved compounds and will require biofilter units to be removed. The process of ammonia removal by a biological filter is called nitrification, and consists of the successive oxidation of ammonia to nitrite and finally to nitrate. The reverse process is called denitrification and is an anaerobic process where nitrate is converted to nitrogen gas (Timmons and Ebeling, 2007). Nitrification Ammonia is produced as the major end product of the protein catabolism and is excreted by fish as unionized ammonia across their gills. In addition, uneaten feed and other organic material are quickly broken down by bacteria into ammonia through the process called mineralization. Rule of thumb: about 3% of the daily feed ends up as ammonia-nitrogen in the water. Ammonia is highly toxic for fish. In general, warm-water fish are more tolerant to ammonia toxicity than cold-water fish, and freshwater fish are more tolerant than saltwater fish (Timmons and Ebeling, 2007). Rule of thumb: for commercial production, un-ionized ammonia concentrations should be held below 0.05 mg/l and Total Ammonia Nitrogen concentrations below 1.0 mg/l for long-term exposure. 5

6 In RAS biofilter units, ammonia should be oxidized to decrease its levels and convert into lower toxicity nitrate. Nitrification (e.g. ammonia oxidation to nitrate) is a 2 step process. First, ammonia is oxidized to nitrite by several bacterial species, the most commonly known genus of which is Nitrosomonas. Nitrite is then further oxidized by several groups of bacteria, the most widely known of which is Nitrobacter, to nitrate. The two steps in the reaction are normally carried out sequentially (Timmons and Ebeling, 2007). Rule of thumb: one gram of ammonia yields 4.42 grams of nitrate and 5.93 grams of carbon dioxide, plus a small amount of cell mass. It consumes 4.57 grams of oxygen and 7.14 grams of alkalinity during nitrification. The rate at which the conversion of ammonia to nitrate takes place is dependent on nitrogen concentration in the water, the oxygen concentration, alkalinity, temperature, and other variables. This dictates that aeration to increase oxygen concentrations and adjustment of alkalinity in nitrification biofilters to increase the nitrification efficiency are of utmost importance. Aeration is achieved by either passive or active methods and is an important parameter for biofilter unit design choice. Rule of thumb: alkalinity should be maintained at 50 to 100 mg/l CaCO 3 and for every kg of feed fed, approximately 0.25 kg of sodium bicarbonate is needed to replace the lost alkalinity consumed during nitrification. There is considerable debate as to the most appropriate biological filter technology for RAS applications. Each of the known biofilter units have their advantages and disadvantages, and therefore their best application areas. Requirements for a relatively small footprint, inexpensive media use, no water pressure or maintenance to operate, and no solids capture/ fouling lead to the large scale commercial recirculating systems choosing granular filters, distinguished by the strategy used to provide oxygen and the techniques used to handle excess biofilm growth. Among all possible types of nitrification reactors the commonly used in RASs are: submerged biofilters, trickling biofilters, rotating biological contactors, floating bead biofilters, dynamic bead biofilters, and fluidized-bed biofilters (Timmons and Ebeling, 2007). 6

7 Figure 2. Trickling filter bioreactor In Ararat valley fishery productions, the choice of biofiltration units is based on more strict evaluation. Since water recirculation is not required by governmental regulations, the most attractive biofilter units should be easy to operate and maintain, require lower capital and operational costs, and have small footprints combined with high nitrification efficiency, and most importantly, might be possible to construct locally. For these reasons, plastic media based attached growth bioreactors, such as Trickling filter and Moving Bed bioreactors are advised. Trickling filters: Trickling filters (Fig. 2) consist of a fixed media bed through which a prefiltered wastewater trickles down across the height of the filter. The wastewater flows downwards over a thin aerobic biofilm and dissolved substrates diffuse into the biofilm where they are consumed by the nitrifying bacteria. As it trickles over the media, the water is continuously oxygenated and carbon dioxide is removed by the ventilated air. Trickling filters have been widely used in aquaculture, because they are easy to construct and operate, are selfaerating and very effective at off gassing carbon dioxide, and have a moderate capital cost. In municipal waste water treatment systems, trickling filters were traditionally constructed of rocks, but today most filters use plastic media, because of its low weight, high specific surface area ( m 2 /m 3 ) and high void ratio (>90%) (Timmons and Ebeling, 2007). Moving bed bioreactor: The moving bed bioreactor (MBBR) (Fig. 3) was developed in Norway in the early 1980 s to reduce nitrogen discharge into the North Sea. The MBBR is an attached growth biological treatment process based on a continuously operating, non-clogging biofilm reactor with low head loss, a high specific biofilm surface area, and no requirement for backwashing. The reactor can be operated under either aerobic conditions for nitrification or anoxic conditions for denitrification. For nitrification, the media is maintained 7

8 Figure 3. Moving bed bioreactor (left) and packing material (right) in constant circulation via a course air bubble aeration system creating aerobic conditions and for denitrification via a submerged mixer for anoxic conditions. Media usually occupies up to 70% of the reactor volume, in that at is higher percentage fill reduces mixing efficiency. The media is kept within the reactor volume by an outlet sieve or screen, which may be vertically mounted, rectangular mesh sieves, or cylindrical bar sieves, vertically or horizontally mounded. The media made of high density polyethylene (density 0.95 g/cm3) and shaped as a small cylinder with a cross on the inside of the cylinder and fins on the outside (Timmons and Ebeling, 2007). Agitation within the reactor maintains the media in constant motion creating a scrubbing effect that prevents clogging and sloughs off excess biomass. Since MBBR s are an attached growth process, treatment capacity is a function of the specific surface area of the media. A significant advantage is its small footprint and low maintenance in comparison to the operational and maintenance issues associated with trickling filters and rotating biological contactors. Denitrification Nitrite is an intermediate product in the process of nitrification of ammonia to nitrate. Although it is usually converted to nitrate as quickly as it is produced, lack of biological oxidation of the nitrite will result in elevated nitrite levels that can be toxic to the fish. Nitrite is constantly 8

9 produced as the intermediary step between ammonia and nitrate. High levels of nitrite are also indicative of biofilter impending failure and should always be addressed (Timmons and Ebeling, 2007). Nitrate is the end product of nitrification and is the least toxic of the nitrogen compounds. In recirculation systems, nitrate levels are usually controlled by daily water exchanges. In systems with low water exchange or high hydraulic retention times, denitrification has become increasingly important. Historically, nitrate has not been of major concern in RAS due to its low toxicity to freshwater organisms (Lee et al., 2002). However, with the high degree of water reuse that is inherent in aquaculture systems, nitrate reduction becomes more important, since accumulations as high as 100-1,000 mg/l nitrate nitrogen are not uncommon. Biological denitrification is the microbial reduction of nitrate or nitrite to nitrogen gas by heterotrophic and autotrophic facultative aerobic bacteria and some fungi widely found in the environment. Under anoxic conditions, nitrate and nitrite replace oxygen as the electron acceptor for the oxidation of a wide variety of organic or inorganic electron donors. The term anoxic defines the conditions optimal for denitrification: low oxygen and high nitrate (Timmons and Ebeling, 2007). In aquaculture facilities, the wastewater generally contains high levels of nitrate-nitrogen and nitrite-nitrogen, especially after nitrification, but little or no electron donors. In order to achieve anaerobic conditions that would stimulate denitrification, the filtration systems require the addition of organic compounds to promote oxygen consumption during degradation by heterotrophic bacteria (and induce anaerobic pockets), as well as to serve as electron donors to support biological nitrate reduction in denitrifying biofilters (Timmons and Ebeling, 2007). It should be noted, that as for nitrification, many different kind of reactor designs are available for denitrification and often the same type of reactor can be used for both systems, taking into account that denitrification is an anaerobic process and access of oxygen to the biofilter should be restricted (Timmons and Ebeling, 2007). An effective denitrification reactor can be designed by considering the following parameters: a) Nitrate-nitrogen production in the system; b) The maximum allowable nitrate concentrations in 9

10 the culture water; c) The volume of reactor required to match the nitrate load; d) Retention time of the reactor; e) The type of electron donor (organic or inorganic) and its stoichiometric requirements in the denitrification process; f) Oxygen concentrations in the reactor (Timmons and Ebeling, 2007). While one of the most commonly used reactors for denitrification in RAS units are MBBRs (see above), the use of alcohols, volatile fatty acids and sugars often lead to bacterial blooms, toxic by-product production, among other problems (Singer et al., 2008) and result in elevated system costs, and require sophisticated process controls and continuous monitoring (Singer et al., 2008). Since nitrate concentrations in Ararat valley fisheries should not reach high levels because of relatively low stocking densities and higher water exchange rates, and additionally, since freshwater fish are more tolerant to nitrate, a cheap, low cost and maintenance systems should be implemented in this area fօr removal of nitrate from RAS effluents. A system based on an insoluble solid carbon source would therefore be a good solution to these problems. Packed cotton filled reactor, similar to one used by Singer et al. (2008) (Fig. 4) is advised to address nitrate pollution from RAS. To reduce the overall compressibility of the cotton wool plastic beads (identical to those used in the aerobic biofilter) as spacers can be used: arranging the biofilter in a horizontal flow regime will increase the active and prevent the clogging and channeling (Singer). The specific denitrification rate for this type of reactor was estimated to be 3.35 mg/l/hr nitrate nitrogen (Singer et al., 2008) and scaling up the reactor will be sufficient to remove all the nitrate produced in Ararat valley fisheries. 10

11 Figure 4. Two stage nitrogen removal setup for RAS. C represents packed denitrification bioreactor, using carbon as electron donor (Figure is adapted from Singer et al., 2008) Additionally, cotton wool, being completely insoluble in water, can serve both as a carbon source and as a biomass growth bed. Its low cost, availability, and low toxicity are added advantages. In addition, it can act as a physical barrier that traps particles, some of which can serve as an additional carbon source for denitrification (Singer et al., 2008). 3. Aquaponics: integrated fish and plant culture Aquaponics, the combined culture of fish and plants in recirculating systems, has become increasingly popular. Aquaponic systems are RASs that incorporate the production of plants without soil. The metabolic byproducts in RAS need not be wasted if they are channeled into secondary crops that have economic value or in some way benefit the primary fish production system (Timmons and Ebeling, 2007). Plants grow rapidly in response to dissolved nutrients that are excreted directly by fish or generated from the microbial breakdown of fish wastes. In recirculating systems with very little daily water exchange (less than 5%), dissolved nutrients accumulate and approach concentrations that are found in hydroponic nutrient solutions. In aquaponic systems, the plants recover a substantial percentage of these nutrients, thereby reducing the need to discharge water to the environment and therefore extending water use, i.e., by removing dissolved nutrients through plant uptake, the water exchange rate can be reduced. Minimizing water exchange reduces operating costs of aquaponic systems in arid climates and heated greenhouses where water or heated water represents a significant expense. Lennard (2006) demonstrated that nitrate accumulation in culture waters was reduced by up to 97% in the Aquaponic system when compared with the fish-only system. 11

12 Figure 5. Floating raft aquaponic system (source: In Ararat valley fish production systems, although current practices should lead to only moderate concentrations of nutrients in waste water due to low stocking densities and high water exchange rates, the implementation of reactor type-biofiltration and water recirculation will certainly increase the concentration of these nutrients, thus making aquaponics an attractive solution for water reuse. Profitability is always a major concern when considering a RAS. RASs are expensive to construct and operate, and profitability often depends on serving niche markets for live fish. A secondary plant crop, which receives most of its required nutrients at no additional cost, improves system profit potential. The daily feeding of the fish provides a steady supply of nutrients to plants, which reduces or eliminates the need to discharge and replace depleted nutrient solutions or adjust nutrient solutions as is required in hydroponics. The carbon dioxide vented from fish culture water can increase plant yields in enclosed environments. The plants purify the culture water and can, in a properly sized and designed facility, eliminate the need for separate and expensive biofilters, a major capital expense and a minor operational expense. The profitability of recirculating systems can thus be improved substantially with aquaponics, if there is a good market for the vegetable crop. 12

13 Figure 6. Nutrient film aquaponic system (source: Any plant commonly grown in hydroponic systems will adapt to aquaponics including the most common types. However, matching the right plant to the right hydroponic system is an important decision. To maximize yields, plants that grow best in drier, well drained soils do best in media aquaponic systems while plants that prefer well-watered, continuously moist soil (most leafy salad crops and herbs) adapt well to either water culture or media systems. The most common hydroponic systems (Tyson et al., 2011) currently being used in aquaponics are the floating raft, nutrient film technique (NFT), and the bench bed systems. Floating raft systems (Fig. 5) are ideally suited for quick-turnaround lettuce crops. Many commercially available NFT systems (Fig. 6) can handle only small-rooted crops like lettuce because of limited trough volume. Large-rooted vegetables such as tomato, cucumber, pepper, and mint can be grown with NFT provided the trough s root-zone space is large enough to accommodate the plant roots and allow water to continue flowing down the trough. Properly designed and operated media (perlite, vermiculite, peat, coconut coir, pine bark, pebbles, and combinations) systems (Fig. 7) such as the bench bed have the broadest crop choices because they can accommodate water-loving plants or plants that need well-drained soils. Oftentimes media-filled plastic pots are placed in the bench bed to facilitate crop cycles and make the beds easier to clean and maintain. 13

14 Figure 7. Bench bed aquaponic system (source: System sizing recommendations depend on a number of variables. RAS recommendations suggest that 5% 10% of fish tank water volume be discharged daily and replaced with fresh water to help keep the tank water clean (Timmons et al., 2002). If the hydroponic subsystem is designed with enough plants to use up this discharge water through plant uptake and system evaporation (evapotranspiration), then the resulting aquaponic system can be maintained at near zero water discharges for both production systems. For this to occur creating optimum system sustainability with minimal water discharges to the environment the hydroponic subsystem should be much larger in surface area than the aquaculture subsystem. Given that plants are often present at different stages of development from seedling to mature, in order to maintain consistent year-round aquaponic system water use, an average use of 1 L per plant per day, depending on the average overall plant size, could be planned for hydroponic subsystems that use larger plants. To help maintain good water-quality conditions, additional filtration of RAS tank-water solids usually is required and a separate biofilter for the nitrifying bacteria is recommended, especially with high densities of fish. Plants do have a biofilter role by removing ammonium from system water. However, research suggests that nitrification is more important than plant uptake for biofiltration of the toxic ammonia (Tyson et al., 2011), and only nitrification can change ammonia to nitrate, the latter being the most common mineral nutrient required by plants. Out of all 3 systems, the floating raft and NFT also require solid removal biofiltration prior to the water use for the plants. Therefore, the optimal choice of the aquaponic system in Ararat valley 14

15 fisheries might be bench bed systems that can simultaneously remove both solids and nitrate, if labor is not a limiting factor. However, the use of the other types of aquaponic systems can be advantages in separate operations, dependent on the farmers choice of fish stock density and crops grown. 4. References Hagopian, D. S., Riley, J. G. (1998). A closer look at the bacteriology of nitrification. Aquacultural engineering, 18(4), Timmons, M.B.; Ebeling, J.M.; Wheaton, F.W.; Summerfelt, S.T.; Vinvi, B.J Recirculating Aquaculture Systems. Northeastern Regional Aquaculture Center; Cayuga Aqua Ventures; Ithaca; NY; USA; 769 pp. van Rijn, J The potential for integrated biological treatment systems in recirculating fish culture-a review. Aquaculture 139; Chen, S.; Coffin, D.E.; Malone, R.F Sludge production and management for recirculating aquacultural systems. Journal of the World Aquaculture Society 28; Mirzoyan, N., Parnes, S., Singer, A., Tal, Y., Sowers, K. and Gross, A., Quality of brackish aquaculture sludge and its suitability for anaerobic digestion and methane production in an upflow anaerobic sludge blanket (UASB) reactor. Aquaculture, 279(1), pp Timmons, M.B.; Ebeling, J.M.; Recirculating Aquaculture. Northeastern Regional Aquaculture Center; Itaca; NY ; USA; 975 pp. Lee, H.W., Lee, S.Y., Lee, J.W., Park, J.B., Choi, E.S. and Park, Y.K., Molecular characterization of microbial community in nitrate-removing activated sludge. FEMS microbiology ecology, 41(2), pp Singer, A., Parnes, S., Gross, A., Sagi, A. and Brenner, A., A novel approach to denitrification processes in a zero-discharge recirculating system for small-scale urban aquaculture. Aquacultural engineering, 39(2), pp Lennard, W.A. and Leonard, B.V., A comparison of three different hydroponic subsystems (gravel bed, floating and nutrient film technique) in an Aquaponic test system. Aquaculture International, 14(6), pp Tyson, Richard V., Danielle D. Treadwell, and Eric H. Simonne. "Opportunities and challenges to sustainability in aquaponic systems."horttechnology 21, no. 1 (2011):