Aquacultural Engineering 53 (2013) 49–56Contents lists available at SciVerse ScienceDirect Aquacultural Engineering j our nal homepage: www. el sevi er . com/ l ocat e/ aqua- onl i ne Waste treatment in recirculating aquaculture systems Jaap van Rijn ∗ Department of Animal Sciences, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel a r t i c l e i n f o Keywords: Recirculating aquaculture systems RAS Waste treatment Waste production Onsite treatment Waste disposal a b s t r a c t Recirculating aquaculture systems (RAS) are operated as outdoor or indoor systems. Due to the intensive mode of fish production in many of these systems, waste treatment within the recirculating loop as well as in the effluents of these systems is of primary concern. In outdoor RAS, such treatment is often achieved within the recirculating loop. In these systems, extractive organisms, such as phototrophic organisms and detritivores, are cultured in relatively large treatment compartments whereby a considerable part of the waste produced by the primary organisms is converted in biomass. In indoor systems, capture of solid waste and conversion of ammonia to nitrate by nitrification are usually the main treatment steps within the recirculating loop. Waste reduction (as opposed to capture and conversion) is accomplished in some freshwater and marine indoor RAS by incorporation of denitrification and sludge digestion. In many RAS, whether operated as indoor or outdoor systems, effluent is treated before final discharge. Such effluent treatment may comprise devices for sludge thickening, sludge digestion as well as those for inorganic phosphate and nitrogen removal. Whereas waste disposed from freshwater RAS may be treated in regional waste treatment facilities or may be used for agricultural purposes in the form of fertilizer or compost, treatment options for waste disposed from marine RAS are more limited. In the present review, estimations of waste production as well as methods for waste reduction in the recirculating loop and effluents of freshwater and marine RAS are presented. Emphasis is placed on those processes leading to waste reduction rather than those used for waste capture and conversion. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Harmful effects attributed to aquaculture practices are of fore- most concern to the industry and are subject to increased public awareness (Sapkota et al., 2008; Subasinghe et al., 2009). Often, these harmful effects are related to the environmental impact of aquaculture activities, among those: (1) destruction of natural sites such as wetlands and mangroves, (2) spread of diseases, (3) decreasedbiodiversity of natural fishpopulations by escape of non- native fish species, and (4) pollution of ground and surface waters by effluent discharge (Boyd, 2003). Recirculating aquaculture systems (RAS), in which water is recirculated between the culture and water treatment stages, provide an answer to some of the above mentioned problems since they enable fish production in relative isolation from the surrounding environment. However, this advantage is not with- out a price as many challenges face the production of fish in these highly contained systems. In this respect, water quality control and waste management are among the most critical of these challenges. ∗ Tel.: +972 8 9489302; fax: +972 8 9489024. E-mail addresses:
[email protected],
[email protected] Careful design and management of RAS are the basis for a suc- cessful waste management with respect to both waste production and treatment. Operation of RAS under well controlled culture conditions contributes significantly to an efficient feed utilization, hence, low waste production. Furthermore, proper incorporation of treatment procedures within the recirculating loop or in the effluent stream may further contribute to a significant reduc- tion in waste production by these systems. In most indoor RAS, the bulk of waste produced by the fish is captured and removed in a concentrated effluent stream that may be treated onsite before final discharge. Such onsite treatment generally involves sludge thickening and flow stabilization but may also be designed to allow bacterial decomposition of solid waste. Outdoor RAS, mostly situated in warmer climates, are often operated with par- tial waste reduction within the recirculation loop. In the latter systems, phototrophic organisms such as plants and algae are often involved in treatment of recirculation as well as of effluent water. This review summarizes some selected issues related to waste management in RAS. Estimations of waste production are pre- sented as well as methods for waste reduction in the recirculating loopandeffluents of freshwater andmarineRAS. Emphasis is placed on those processes leading to waste reduction rather than those used for waste capture and conversion. 0144-8609/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aquaeng.2012.11.010 50 J. van Rijn / Aquacultural Engineering 53 (2013) 49–56 2. Waste discharge regulation Discharge regulations differ from country to country. Whereas in some jurisdictions effluent standards are provided, in others, restrictions are placed on the amount of feed or water that can be used by individual farms. However, the general tendency in many countries is that, rather than effluent standards, guidelines for best management practices or codes of conduct are provided together with measures to ensure compliance to such guidelines (e.g. Environmental ProtectionAgency, 2004; FoodandAgricultural Organization, 1995). The rational of this approach is based on the fact that universal guidelines as to effluent standards are difficult to formulate due to differences in hydro-geographic, climatic and environmental conditions within countries and regions. One such generic approach is the life cycle assessment (LCA). This method has received increased attention in recent years and has become a recognized instrument in assessing the environmental impact of agricultural as well as other production processes. Recently, it has also been applied for evaluating the environmental impact of sev- eral aquaculture systems, including RAS (Martins et al., 2010). Not only legislative bodies but also producer organizations advocate policies for well monitored production regimes. Product quality, production transparency and the added value of “environmentally friendly” raised products are major incentives for promotion of these policies by such organizations (Boyd, 2003). With respect to RAS, it is to be expected that operators of these generally well-managed systems are able to comply with compulsory monitoring and reporting regimes. The high degree of fish confinement, the year-round production regime, the use of monitoring systems, and the possibility for treatment of the con- centrated waste are all factors contributing to a transparency in reporting on the production process in such systems. 3. Waste production 3.1. Feed conversion in RAS Although liable to imprecision due to large differences in oper- ational parameters, it might be concluded that feed utilization by fish cultured in RAS often compares favorably to that of fish raised in other type of culture systems (Table 1). Production of waste in RAS, like in any other aquaculture system, depends on a number of factors with as most important ones: (a) the type and age of fish, (b) the feed composition, (c) the feeding regime, and (d) the prevalent water quality conditions in the system. In RAS, high feed utilization efficiencies can be attained by controlling some of these factors. For instance, feeding in RAS, whether performed manu- ally or automatically, is well monitored. Hence, lapses of off-feed are easily identified thus minimizing overfeeding and consequent accumulation of uneaten feed in the system. In addition, batch- wise growth of uniform size classes of fish further contributes to an efficient feed utilization in RAS (Karipoglou and Nathanailides, 2009). Another factor contributing to reduced feed wastage in RAS is water quality control. Treatment systems in RAS are designed to control water temperature and critical water quality parameters within an acceptable range hence avoiding inferior water qual- ity conditions and concomitant reduced feed utilization efficiency. Finally, intheserelativelywell monitoredsystems, aquickresponse to changes in water quality conditions may also contribute to an efficient feed utilization (Martins et al., 2010). 3.2. Quantifying of waste production Waste production in aquaculture systems is quantified either by the nutritional approachthroughdetermining the apparent feed digestibilityof fishor is directlyanalyzedbyquantificationof excre- tion products in the culture water (Cho et al., 1991). Calculated values are often derived from feed trials under well-controlled experimental conditions and not always reflect the feed digestibil- ity of the fish under more realistic culture conditions. In addition, due to partial breakdown of the waste to gaseous forms within the culture system, not all of the generated fish waste is discharged withthe effluent water. Despite these shortcomings, the nutritional approach is often preferred over the alternative method in which waste is directly quantified in the culture system. Quantification of waste production by means of this latter method, even in the sim- plest of experimental systems, is complicated due to the difficulty in fitting a sampling regime to accurately estimate the fluctuating waste production by fish. Furthermore, factors such as the cleaning regime of the culture system, the frequency and duration of water replacement in the culture systems as well as analytical errors in quantifying the waste products (e.g. sample preservation, analyti- cal inaccuracies) contribute to the inaccuracy of the latter method (Roque d’Orbcastel et al., 2008). Organic matter, nitrogen and phosphorus utilization by the fish are main indicators for the efficiency of feed utilization. Often these same parameters are also used to quantify the environmen- tal impact of aquaculture waste. Except for site specific instances or in cases of highly concentrated effluents, other potential envi- ronmental harmful ingredients of aquaculture waste, such as other inorganic compounds, metals, drugs and pathogens, are monitored to a lesser extent. Clearly, production of organic matter, nitrogen and phosphorus is directly linked to the food conversion ratio and differs with different diets, temperatures, fish species, fish sizes and culture systems (Table 2). By means of direct quantification, the partitioning of nitrogen and phosphorus in solid and dissolved waste has been studied for most of the commercially produced fish species (e.g. Azevedo et al., 2011; Lupatsch and Kissil, 1998; Piedrahita, 2003; Roque d’Orbcastel et al., 2008). Despite the large variability among fish species and culture methods, it can be con- cluded from these studies that, in general, most of the nitrogen waste (60–90%) is inthe dissolvedform(mainlyammonia) whereas for phosphorus, a larger proportion is excreted within the fecal waste (25–85%). In intensive production systems such as flow-through systems and cages, waste production based on the nutritional approach (digestibility) might provide a fairly accurate estimate for the waste that is discharged since in these systems most of the fish waste is flushed out by water exchange. However, in RAS with a high degree of recirculation, some of the waste is either passively or actively digested (Chen et al., 1993; van Rijn et al., 2006) and waste pro- duction in these systems is lower than what would be predicted by the nutritional approach. Due to differences in configurations and management of RAS, losses of nitrogen and carbon within the system differ widely among the different RAS (Chen et al., 1997; Piedrahita, 2003). A true quantification of the waste production in these systems is therefore only possible by direct measurements of waste in the effluent stream. 4. Onsite waste treatment 4.1. Reduction of waste within the RAS In most indoor RAS, ammonia removal and solids capture are the primary treatment processes within the recirculation loop. Although intended to collect or convert fish waste, these online treatment processes might lead to a considerable waste reduction through production of mainly gaseous carbon and nitrogen com- pounds by biological decay. The extent of this decay, mainly due to heterotrophic microorganisms, largely depends on the specific J. van Rijn / Aquacultural Engineering 53 (2013) 49–56 51 Table 1 Feed conversion ratios in different types of culture systems. Species Flowthrough RAS Earthen pond Cage Reference Rainbowtrout (Oncorhynchus mykiss) 0.8–1.2 0.8–1.1 – 1.1–1.3 Bureau et al. (2003), Roque d’Orbcastel et al. (2009a,b,c) Barramundi (Lates calcarifer) – 0.8–1.1 1.5–2.2 1.6–2.0 FAO (2012), Peet (2006), Schipp et al. (2007) Tilapia (Oreochromis spp.) – 1.0–2.2 0.8–3.5 >1.5 El-Sayed (2006), Leenhouwers et al. (2007), Little et al. (2008), Martins et al. (2009), Perschbacher (2007), Shnel et al. (2002) Gilthead seabream(Sparus aurata) – 0.9–1.9 – 1.4–2.2 Cromey and White (2004), Zohar et al. (2005) Cobia (Rachycentron canadum) – 1.0 1.5 1.5–2.0 Benetti et al. (2008), Kaiser and Holt (2005) system configuration. In particular, the water and solid retention time of the system as well as methods used for water treatment within the recirculating loop are major factors underlying such heterotrophic bacterial activity. Sludge recoveries as lowas 14% of the added feed, much lower than the calculated sludge production (38–46%), were reported for recirculating systems not equipped with dedicated treatment steps for sludge digestion (Chen et al., 1993, 1997). Also Suzuki et al. (2003) found similar lowsludge pro- duction values of 18% of the added feed in a RAS not equipped with dedicated treatment for sludge removal. Not only organic carbon but also nitrogen is lost from RAS. The loss of nitrogen is mainly due to denitrification in oxygen depleted zones in the systemand may account for as much as 21% of the nitrogen loss in some RAS (reviewed by van Rijn et al., 2006). Dedicated processes for waste reduction within the recirculat- ing loop are mainly found in outdoor, marine and freshwater RAS. Here, nutrients from the culture water are removed by a combi- nation of assimilatory and dissimilatory processes, mediated by phototrophic and heterotrophic organisms. In this modern form of polyculture, production of fed species (e.g. fish, shrimps) is inte- grated with that of extractive species. In most of these so called integrated multi-trophic aquaculture systems (IMTA), extractive species comprise phototrophic organisms such as plants, microal- gae and macroalgae but in some, also other organisms such as filter feeders, detritivores and heterotrophic bacteria are produced. Examples of IMTA systems are integrated marine systems (Neori et al., 2004), high rate algal ponds (Metaxa et al., 2006; Pagand et al., 2000), aquaponic systems (Racocy, 2007), partitioned aqua- culture systems (Brune et al., 2003), active suspension ponds based on bio-flocs technology (Avnimelech, 2006; Crab et al., 2007), peri- phyton systems (Schneider et al., 2005; Verdegemet al., 2005), and constructed wetlands (Lin et al., 2005; Tilley et al., 2002; Zachritz et al., 2008; Zhong et al., 2011). Inmany of these IMTAsystems, pro- duction of the primary aquatic species is combined with growth of other economical valuable crops such as plants, filter feeding fish and detritivores (e.g. clams and oysters). They provide, therefore, an elegant solution for increasing system productivity with con- comitant reduction of waste output (Nobre et al., 2010). Depending on the particular design and operating conditions, these IMTA systems are operated without effluent discharge (e.g. partitioned aquaculture systems, active suspension ponds), with discharge of solids (e.g. aquaponic systems, high rate algal ponds), or, as com- mon in marine systems, with solid and partial water discharge. Most of the above systems, in which treatment within the recir- culation loop partially depends on phototrophic organisms, are outdoor systems operated with relatively large treatment areas under favorable climatic conditions. Hence, these latter systems are more site-dependent than the more compact, indoor RAS systems. Some indoor RAS, where ammonia is nitrified to nitrate, employ special reactors to induce bacterial reduction of nitrate to nitrogen gas under anoxic conditions. Most of these reactors are supplied with external carbon sources to fuel heterotrophic denitrification. Others are designed to allow denitrification on internal carbon sources which are produced in the RAS (van Rijn et al., 2006). In the latter case, bacterial fermentation processes play an important role in supplying carbon compounds for denitrification whereby most of the organic carbon is eventually oxidized to CO 2 . Therefore, not only nitrogen but also organic carbon is removed by means of this treatment combination (Eding et al., 2003; van Rijn et al., 1995). Eding et al. (2009) calculated that by incorporating waste digestion and nitrate removal within the recirculating stream, waste dis- charge for nitrogen and organic solids could be reduced by 81% and Table 2 Waste production of different fish species as determined by the nutritional approach. Fish species Total solids Total N Total P Reference (kg per ton fish production) Rainbowtrout (Oncorhynchus mykiss) 148–338 41–71 7.5–15.2 Azevedo et al. (2011), Bureau et al. (2003), Roque d’Orbcastel et al. (2008) Brown trout a (Salmo trutta) 438 (589) 49.2 (45.8) 6.2 (10.5) Cho et al. (1994) Lake trout a (Salvelinus namaycush) 564. (562) 65.3 (59) 6.8 (6.8) Cho et al. (1994) Barramundi (Lates calcarifer) 29.0–302.3 21.8–101.7 4.2–15.4 Bermudes et al. (2010) Gilthead seabream(Sparus aurata) 447.5 102.9 17.8 Lupatsch and Kissil (1998) Tilapia (Oreochromis spp.) 520–650 72.4 23–29 Beveridge (1984), Beveridge and Phillips (1993) Tilapia (O. niloticus) 192–268.8 48–72.7 0.6–8.9 Schneider et al. (2004) Atlantic salmon (Salmo salar) 224 32 1.1 Reid (2007) a Numbers in parenthesis represent values that were obtained by direct quantification of the waste in the culture water. 52 J. van Rijn / Aquacultural Engineering 53 (2013) 49–56 Table 3 Some characteristics of outdoor and indoor RAS with treatment components within the recirculating loop. Organismcultured Type of treatment Maximum biomass (kg) Treatment volume and area Reference Total Per kg of cultured biomass Outdoor RAS Sea bass (Dicentrachus labrax) High rate algal pond a 320 14.0m 3 26.0m 2 0.044m 3 0.081m 2 Metaxa et al. (2006) Gilthead seabream(Sparus aurata) High rate algal pond a 520 12.0m 3 43.7m 2 0.023m 3 0.084m 2 Schuenhoff et al. (2003) Tilapia (Oreochromis. mossambicus ×O. aureus) Wetland b 1230 50.0m 3 55.0m 2 0.041m 3 0.045m 2 Zachritz et al. (2008) Shrimps (Litopenaeus vannamei) Wetland b 924 21.0m 3 32.0m 2 0.023m 3 0.035m 2 Lin et al. (2005) Tilapia (O. niloticus) Aquaponics b 2184 80.0m 3 232.0m 2 0.037m 3 0.106m 2 Rakocy et al. (2004) Indoor RAS Tilapia (O. niloticus ×O. aureus) Denitrification/sludge digestion c 4800 40.0m 3 23.0m 2 0.008m 3 0.005m 2 Shnel et al. (2002) Gilthead seabream(Sparus aurata) Denitrification/sludge digestion c 106 1.55m 3 2.75m 2 0.015m 3 0.026m 2 Gelfand et al. (2003) Gilthead seabream(Sparus aurata) Denitrification/anammox/sludge digestion c 1752 14.4m 3 11.1m 2 0.008m 3 0.006m 2 Tal et al. (2009) a Treatment systemwas equipped with additional solids removal and nitrification units. b Treatment systemwas equipped with additional clarifier for solids removal. c Treatment systemwas equipped with additional nitrification unit. 60%, respectively. Analternativetreatment methodbasedonsludge digestion and bacterial nitrogen removal within the recirculation loop was described by Tal et al. (2009). In this marine recircu- lating system, digestion of sludge within a sludge digestion tank was allowed to proceed at lowredox potentials to produce sulfide which was subsequently used to fuel autotrophic denitrifiers in an additional reactor. RAS incorporating sludge digestion and denitri- ficationmay beoperatedwithlittletonoeffluent dischargeas much of the waste is converted to gases. They are, furthermore, operated with relatively small treatment volumes and areas as compared to outdoor RAS (Table 3). Whereas in outdoor RAS, a consider- able part of the released phosphorus is assimilated by extractive organisms, in indoor RAS, phosphorus is not removed within the system and is discharged in the effluent stream. However, in sys- tems incorporating sludge digestion and denitrification within the recirculating loop, a considerable part of the dissolved orthophos- phate was found to be immobilized during the latter treatment stages (see next section). Additional water treatment in the formof disinfection through ozonation and UV irradiation of culture and discharge water are used in many indoor RAS operated today (Goncalves and Gagnon, 2011; Summerfelt et al., 2009). Furthermore, adsorption methods for removal of therapeutants have also been used in such systems (Aitcheson et al., 2000). These compact, indoor systems potentially lend themselves for use of recently developed water treatment technology such as electrochemical and bio-electrochemical meth- ods for removal of organic matter and inorganic nitrogen (Mook et al., 2012; Virdis et al., 2008). 4.2. Onsite treatment of the effluent stream 4.2.1. Sludge thickening Usually, RAS effluents are characterized by a low solid content (<2%) and fluctuate in volume as a result of specific feeding and cleaning regimes. As direct disposal of these effluents is costly, solids thickening and stabilization of the effluent flow is often required before final disposal. Thickening of the sludge through settling of solids in basins or ponds (Bergheimet al., 1993), through solids capture by means of geotextile bags (Schwartz et al., 2004, 2005) or, more recently, by means of belt filters (Timmons and Ebeling, 2007) and membrane reactors (Sharrer et al., 2007) are applied in RAS. The various methods are often used in combination with coagulation/flocculation processes to allow a more complete removal of suspended solids as well as phosphorus fromthe efflu- ent water (Danaher et al., 2011b; Ebeling et al., 2003, 2006; Sharrer et al., 2009). In combination with dewatering, the various methods used for sludge thickening may produce sludge witha solid content of between 5 and 22% (Sharrer et al., 2009). 4.2.2. Sludge digestion In addition to methods for sludge thickening, methods for enhancing biological degradation of sludge are also used in the treatment of RAS effluents. Waste stabilization ponds such as aero- bic and anaerobic lagoons might be used for this purpose as well as sludge digesters (Chen et al., 1997). In the various ponds/reactors used for sludge digestion, sludge residence time (sludge age) is a major factor dictating the extent of sludge degradation. Apart from the length of time during which the sludge is exposed to micro- bial decay, the residence time also influences the type of electron acceptors that are involved in sludge degradation. At relatively low retention times (e.g. settling basins), oxygen will serve as the major electron acceptor while at higher retention times (e.g. anaer- obic lagoons), due to oxygen depletion, other electron acceptors such as nitrate, sulfate (in marine systems) and carbon dioxide will be respired. Fast decay of sludge in the presence of oxygen also coincides with fast growth in heterotrophic biomass of the microorganisms involved in the sludge decay. Aerobic degrada- tion constants of “fresh” sludge were found to range from 0.07 to 0.40day −1 (Boyd, 1973; Chen et al., 1997). In settling basins oper- ated at relatively long retention times, such rapid breakdown of sludge and concomitant production of gases might cause poor sett- ling sludge properties (Timmons and Ebeling, 2007). In reactors operated at longer retention times in which, besides oxygen, addi- tional electron acceptors are respired, decay of sludge proceeds at lower rates than under aerobic conditions and produces less het- erotrophic bacterial biomass. Sludge decay constants ranged from 0.024 to 0.006day −1 in a reactor operated with a high sludge age with nitrate as the main electron acceptor (van Rijn et al., 1995). Despite this apparently slowdecay, this type of reactor, whenprop- erly sized, can be operated for prolonged periods of time without sludge wastage and, as discussed in the previous section, may be used as an on-line treatment stage within the treatment loop. Sludge degradation of 30–40% was reported for denitrifying reac- tors fedwithmarineRASeffluents andoperatedat shorter retention times of up to 11 days (Klas et al., 2006). Laboratory-scale sequencing batch reactors, operated under aerobic and anoxic conditions, for removal of organic matter and nitrogen from concentrated sludge from a shrimp facility were operated by Boopathy et al. (2007) and Fontenot et al. (2007). They showed that at a hydraulic retention time of 8 days, a 74% reduc- tion in organic matter and a total reduction of nitrogen could be achieved with this kind a treatment scheme. J. van Rijn / Aquacultural Engineering 53 (2013) 49–56 53 Fully anaerobic, methanogenic digestion of aquaculture sludge has been reported by several authors (reviewed by Mirzoyan et al., 2010). Although operational conditions differ considerably among the fewstudies conducted, it can be concluded that a considerable degradationandstabilizationof aquaculturesludgecanbeachieved through methanogenic digestion. Issues such as inhibition of the methanogenic activity by unionized ammonia concentrations due to low C/N ratios of the sludge, optimal dry weight content of the sludge, and optimal hydraulic retention times of the methanogenic reactors, still require further investigation prior to the full scale use of these systems. 4.2.3. Inorganic nutrient transformations Concentrations of inorganic nutrients in the supernatant of set- tlers and digesters are dictated by the balance between chemical, physical and biological processes responsible for their release from or removal by the sludge layer of the settler/digester. Sludge resi- dence time has a major influence on these processes. With respect to nitrogen, ammonia concentrations are often found to increase due to ammonification of nitrogenous organic matter (e.g. Conroy and Couturier, 2010; Stewart et al., 2006). Various processes may counteract this ammonia accumulation. Ammonia assimilation is particularly evident in reactors operated at high redox potentials due to a relative large increase in bacterial biomass while nitri- fication of ammonia may also take place in aerobic parts of the reactors (Cytryn et al., 2005; Klas et al., 2006). Not only under aerobic conditions but also under anaerobic conditions ammonia removal might take place. Under such conditions, nitrate, often present in the RAS effluent stream, will not only be denitrified to elemental nitrogen at appropriate hydraulic retention times, but may indirectly, through reduction to nitrite, serve as an electron acceptor for anammox bacteria whereby both ammonia and nitrite areconvertedtoelemental nitrogengas (Lahavet al., 2009; Tal et al., 2003). In addition to ammonia release, hydrolysis of sludge in thick- ening reactors or digesters leads to a release of orthophosphate. In their study on hydrolysis of aquaculture sludge under static con- ditions, Conroy and Couturier (2010) showed that orthophosphate release from the sludge was strongly correlated to the solubility of calcium orthophosphates at low pH values. The same authors did not observed orthophosphate release at pH values above 7.0. A decrease of orthophosphate in the water column of reactors used for digestion of aquaculture sludge has been observed in many studies (Barak et al., 2003; Barak and van Rijn, 2000a; Klas et al., 2006; Neori et al., 2007; Sharrer et al., 2007; Tal et al., 2009). In addition to chemical precipitation with mainly calcium and iron ions, biologically mediated phosphate seques- tration may be of importance during digestion of aquaculture sludge. In nitrate-rich digestion basins of freshwater and marine RAS it was found that denitrifiers accumulated orthophosphate as intracellular polyphosphate in excess of metabolic require- ments (Barak et al., 2003; Barak and van Rijn, 2000a). In these RAS, sludge from areas of intensive denitrification was found to contain up to 19% phosphorus on a dry weight basis while deni- trifiers isolated from these systems were found to contain up to 9% phosphorus on a dry cell weight basis (Barak and van Rijn, 2000b). Release of reduced inorganic sulfur compounds during sludge thickening/digestion may pose a potential problemwith respect to effluent discharge. This is especially true for marine RAS in which, under anaerobic conditions, sulfide may be produced as a result of organic matter mineralization and sulfate reduction (Cytryn et al., 2003; Schwermer et al., 2010; Sher et al., 2008). In these marine systems it was found that the presence of nitrate during sludge digestion prevents sulfide formation by exclusion of bacterial sul- fate reduction (Schwermer et al., 2010) as well as by promoting the growth of sulfide oxidizing, autotrophic denitrifiers (Sher et al., 2008; Tal et al., 2009). Depending on the accumulation of dissolved organic matter and nutrients in sludge thickening reactors or sludge digesters, further onsite treatment of the supernatant from these reactors may be warranted before final disposal. Brazil and Summerfelt (2006) examined the effect of aerobic treatment of the super- natant overflowing an aquaculture sludge thickening tank. They showed that in aerobic reactors operated at hydraulic retention time of up to 6 days, an 87% reduction of organic matter and total ammonia nitrogen and a 65% reduction in orthophosphate could be achieved. In addition, outdoor treatment systems, simi- lar to those used within the recirculation loop (e.g. wetlands, high rate algal ponds) may also be used for treatment of effluent water before final discharge or may serve both as an online and efflu- ent treatment stage. Largely depending on the size of such systems relative to the waste load, these systems may be fed organic-rich water directly released from the RAS or with supernatant from the sludge thickening stage (Cohen and Neori, 1991; Metaxa et al., 2006; Neori et al., 1991; Pagand et al., 2000; Sindilariu et al., 2009). 5. Waste disposal As apparent fromthe previous sections, the nature and quantity of waste disposed from RAS depend largely on the onsite treat- ment facilities used. While several alternatives are available for treatment of waste fromfreshwater RAS, waste treatment of waste frommarine facilities is restricted to fewer methods. Liquid as well as solid waste from freshwater RAS can be treated in centralized facilities such as publicly owned treatment works (POWT) used for treatment of other livestock waste as well as domestic and industrial waste. Where land availability and cost is less of a con- straint, these centralized facilities may be based on treatment by means of stabilizationponds andwetlands. Alternatively, wastewa- ter treatment facilities, primarily used for treatment of domestic and industrial waste, with primary, secondary and tertiary treat- ment steps, may also be used to treat RAS effluent. However, treating aquaculture sludge in these latter systems seems wasteful as concentrations of toxic and other health threatening compo- nents in aquaculture sludge are lowas compared to those in sludge from domestic and industrial origin. As such, the use of aquacul- ture sludge as a fertilizer by direct land application(Bergheimet al., 1993; Yeo et al., 2004) or its use for compost production (Adler and Sikora, 2004; Danaher et al., 2011a) appears to be more sustainable alternatives. Composting might require adjustment of the C/Nratio and a decrease of the water content of the sludge by the addition of a carbonaceous bulking agent in order to provide optimal aerobic decomposition conditions (Adler and Sikora, 2004). Like the sludge alsothe liquidfractionfromRAS effluents may be usedfor irrigation of agricultural crops. Whereas compost production is site indepen- dent, the use of solid as well as liquid waste for fertilizer purposes depends on location. The absence of a properly scaled application in the vicinity of the RAS may prohibit this latter form of disposal (Yeo et al., 2004). As most marine RAS are situated in close vicinity to the sea, waste discharge into the sea is still the most common practice. While in marine RAS with online waste treatment such practice results in little environmental impact such impacts may be profound when waste is discharged fromRAS with little post treat- ment. In the latter case, the quantity of waste produced is not much different fromcage aquaculture. In coastal areas, constructed wetlands seemtobe a promising methodfor the treatment of aqua- culture waste (Gregory et al., 2010; Su et al., 2011). Where, due to site restrictions, discharge to external facilities is not possible, 54 J. van Rijn / Aquacultural Engineering 53 (2013) 49–56 on-site treatment systems can be used by means of which excess nitrogen and carbon are converted into gases (see Section 4.1). 6. Conclusions Water treatment technology has seen a dynamic development inrecent years withnewtreatment methods rapidlyemerging. Also in the field of RAS, a choice can be made frommany different treat- ment methods. The choice of a suitable treatment methoddepends, in addition to a proper cost/benefit analyses, largely on factors, directly or indirectly, relatedtothe locationof the recirculating sys- tem. Climatic conditions, water availability, discharge regulations, and land availability are suchlocation-dependent factors whichare major determinants for the type of treatment methods to be used. These factors, together with the market value of the cultured orga- nisms, may justify the use of sophisticated treatment methods in some cases while in others, optimal economical benefit is accom- plished with relatively simple water treatment techniques at the expense of water savings and production intensity. In most outdoor RAS, waste reduction is generally achieved within the recirculating loop by an integrative approach in which organic carbon and inorganic nutrients are assimilated by pho- totrophic and heterotrophic organisms. Due to site and climatic restrictions, indoor RAS are usually operated according to different treatment protocols in which emphasis is placed on solid capture and ammonia transformation to nitrate within the recirculation loop with optional onsite treatment of the concentrated effluent before discharge. It is expected that with increased fish demand as well as increased public awareness related to issues such as overfishing, water savings, pollution, animal welfare and ethics of animal hus- bandry, research on RAS as well as their commercial exploitation will show a steady growth in the near future. The development of cost efficient and sustainable waste treatment methods will be an important aspect contributing to the wider use of these systems. References Adler, P.R., Sikora, R.J., 2004. Composting fish manure fromaquaculture operations. Biocycle 45, 62–66. Aitcheson, S.J., Arnet, J., Murray, K.R., Zhang, J., 2000. Removal of aquaculture ther- apeutants by carbon adsorption: 1. 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