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Title: Waste treatment in recirculating aquaculture systems
Author: Jaap van Rijn
PII: S0144-8609(12)00094-5DOI: doi:10.1016/j.aquaeng.2012.11.010Reference: AQUE 1670
To appear in: Aquacultural Engineering
Received date: 5-7-2012Accepted date: 19-11-2012
Please cite this article as: van Rijn, J., Waste treatment in recirculating aquaculturesystems, Aquacultural Engineering (2010), doi:10.1016/j.aquaeng.2012.11.010
This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
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Waste treatment in indoor and outdoor RAS is reviewed
Little waste reduction takes place in indoor RAS
Outdoor RAS generally produce less waste than indoor RAS
Many on and off-site methods exist for waste reduction in freshwater RAS effluents
Treatment of effluents from marine RAS is little developed
*Highlights (for review)
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Waste treatment in recirculating aquaculture systems
Jaap van Rijn
The Robert H. Smith Faculty of Agriculture, Food and Environment
The Hebrew University of Jerusalem
P.O. Box 12, Rehovot 76100
Postal address: 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. Phone: +972 8 9489302; Fax: +972 8 9489024; Email:
*Manuscript
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Abstract
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.
Keywords: recirculating aquaculture systems; RAS; waste treatment; waste
production; onsite treatment; waste disposal
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1. Introduction
Harmful effects attributed to aquaculture practices are of foremost 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) decreased biodiversity of natural
fish populations 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 without 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. Careful design and management of RAS are the basis for a
successful 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 reduction 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 partial
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 presented as well as methods for waste
reduction in the recirculating loop and effluents of freshwater and marine RAS.
Emphasis is placed on those processes leading to waste reduction rather than those
used for waste capture and conversion.
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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 Protection Agency, 2004; Food
and Agricultural 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 several 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 concentrated 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 operational parameters, it
might be concluded that feed utilization by fish cultured in RAS often compares
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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 manually 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 Nathanailidis, 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 quality
conditions and concomitant reduced feed utilization efficiency. Finally, in these
relatively well monitored systems, a quick response 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
approach through determining the apparent feed digestibility of fish or is directly
analyzed by quantification of excretion 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 digestibility 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 with the 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 simplest 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,
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analytical 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 environmental impact of aquaculture waste. Except for site
specific instances or in cases of highly concentrated effluents, other potential
environmental 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 concluded from these studies that,
in general, most of the nitrogen waste (60-90%) is in the dissolved form (mainly
ammonia) 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 production 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
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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 compounds by biological
decay. The extent of this decay, mainly due to heterotrophic microorganisms, largely
depends on the specific 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 low as 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., 1997; 1993). Also
Suzuki et al. (2003) found similar low sludge production 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 system and 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 recirculating loop are
mainly found in outdoor, marine and freshwater RAS. Here, nutrients from the culture
water are removed by a combination 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 integrated 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,
microalgae 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
aquaculture systems (Brune et al., 2003), active suspension ponds based on bio-flocs
technology (Avnimelech, 2003; Crab et al., 2007), periphyton systems (Schneider et
al., 2005; Verdegem et al., 2005), and constructed wetlands (Lin et al., 2005; Tilley et
al., 2002; Zachrits et al., 2008; Zhong et al., 2011). In many of these IMTA systems,
production 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
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productivity with concomitant 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 common in marine systems, with solid and partial water discharge.
Most of above systems, in which treatment within the recirculation 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 CO2. 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 discharge for nitrogen and organic solids could
be reduced by 81% and 60%, respectively. An alternative treatment method based on
sludge digestion and bacterial nitrogen removal within the recirculation loop was
described by Tal et al. (2009). In this marine recirculating system, digestion of sludge
within a sludge digestion tank was allowed to proceed at low redox potentials to
produce sulfide which was subsequently used to fuel autotrophic denitrifiers in an
additional reactor. RAS incorporating sludge digestion and denitrification may be
operated with little to no effluent discharge as 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
considerable 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 systems incorporating sludge digestion and
denitrification within the recirculating loop, a considerable part of the dissolved
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orthophosphate was found to be immobilized during the latter treatment stages (see
next section).
Additional water treatment in the form of 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 methods 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 (Bergheim et al., 1993), through solids capture by means of
geotextile bags (Schwartz et al., 2005; 2004) 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 from the effluent water (Danaher et al., 2011b; Ebeling et
al., 2006; Ebeling et al., 2003; Sharrer et al., 2009). In combination with dewatering,
the various methods used for sludge thickening may produce sludge with a solid
content of between 5 - 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 treatment of RAS effluents. Waste stabilization
ponds such as aerobic 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
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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 microbial 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. anaerobic 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 degradation
constants of "fresh" sludge were found to range from 0.07-0.40 day-1
(Boyd, 1973;
Chen et al., 1997). In settling basins operated at relatively long retention times, such
rapid breakdown of sludge and concomitant production of gases might cause poor
settling sludge properties (Timmons and Ebeling, 2007). In reactors operated at longer
retention times in which, besides oxygen, additional electron acceptors are respired,
decay of sludge proceeds at lower rates than under aerobic conditions and produces
less heterotrophic bacterial biomass. Sludge decay constants ranged from 0.024-0.006
day-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 slow decay, this type of
reactor, when properly 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 reactors fed with marine RAS effluents and operated at 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% reduction in organic
matter and a total reduction of nitrogen could be achieved with this kind a treatment
scheme.
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 few studies conducted, it can be concluded
that a considerable degradation and stabilization of aquaculture sludge can be
achieved through methanogenic digestion. Issues such as inhibition of the
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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 settlers 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 residence 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, 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 nitrification 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
are converted to elemental nitrogen gas (Lahav et al., 2009; Tal et al., 2003).
In addition to ammonia release, hydrolysis of sludge in thickening reactors or
digesters leads to a release of orthophosphate. In their study on hydrolysis of
aquaculture sludge under static conditions, 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 and van Rijn, 2003; Barak et al., 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
sequestration may be of importance during digestion of aquaculture sludge. In nitrate-
rich digestion basins of freshwater and marine RAS it was found that denitrifiers
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accumulated orthophosphate as intracellular polyphosphate in excess of metabolic
requirements (Barak and van Rijn, 2003; 2000a). In these RAS, sludge from areas of
intensive denitrification was found to contain up to 19% phosphorus on a dry weight
basis while denitrifiers 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 problem with 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 sulfate 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 supernatant
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, similar 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 effluent 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 from the previous sections, the nature and quantity of waste disposed
from RAS depends largely on the onsite treatment facilities used. While several
alternatives are available for treatment of waste from freshwater RAS, waste
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treatment of waste from marine 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
constraint, these centralized facilities may be based on treatment by means of
stabilization ponds and wetlands. Alternatively, wastewater treatment facilities,
primarily used for treatment of domestic and industrial waste, with primary,
secondary and tertiary treatment 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 components in aquaculture sludge
are low as compared to those in sludge from domestic and industrial origin. As such,
the use of aquaculture sludge as a fertilizer by direct land application (Bergheim et al.,
1993; Yeo et al., 2004) or its use for compost production (Adler and Sikora, 2004;
Danaher et al., 2011a) appear to be more sustainable alternatives. Composting might
require adjustment of the C/N ratio and a decrease of the water content of the sludge
by addition of a carbonaceous bulking agent in order to provide optimal aerobic
decomposition conditions (Adler and Sikora, 2004). Like the sludge also the liquid
fraction from RAS effluents may be used for irrigation of agricultural crops. Whereas
compost production is site independent, 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 from RAS with little post treatment. In the latter
case, the quantity of waste produced is not much different from cage aquaculture. In
coastal areas, constructed wetlands seem to be a promising method for treatment of
aquaculture waste (Gregory et al., 2010; Su et al., 2011). Where, due to site
restrictions, discharge to external facilities is not possible, 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
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Water treatment technology has seen a dynamic development in recent years
with new treatment methods rapidly emerging. Also in the field of RAS, a choice can
be made from many different treatment methods. The choice of a suitable treatment
method depends, in addition to a proper cost/benefit analyses, largely on factors,
directly or indirectly, related to the location of the recirculating system. Climatic
conditions, water availability, discharge regulations, and land availability are such
location-dependent factors which are major determinants for the type of treatment
methods to be used. These factors, together with the market value of the cultured
organisms, may justify the use of sophisticated treatment methods in some cases while
in others, optimal economical benefit is accomplished 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 phototrophic 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 husbandry, 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.
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Zohar, Y., Tal, Y., Schreier, H., Steven, C., Stubblefield, J., Place, A.R., 2005.
commercially feasible urban recirculated aquaculture: addressing the marine
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Urban Aquaculture. CABI Publishing, Wallingford, UK, pp. 159-171.
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Table 1. Feed conversion ratios in different types of culture systems
Species Flow
through
RAS Earthen
Pond
Cage Reference
Rainbow trout
(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 (2008); Peet (2006);
Schipp et al. (2007)
Tilapia
(Oreochromis
spp.)
- 1.0-2.2 0.8-3.5 >1.5 El Sayed (2006);
Leenhouwer et al., (2007);
Little et al. (2008); Martins
et al., (2009); Perschbacher
(2007); Schnell et al.
(2003)
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)
Table 1
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Table 2. Waste production of different fish species as determined by the nutritional approach
* numbers in parenthesis represent values that were obtained by direct quantification of the
waste in the culture water
Fish species Total solids Total N Total P Reference
(kg per ton fish production)
Rainbow trout
(Oncorhynchus
mykiss)
148-338 41-71 7.5-15.2 Azvedo et al. (2011); Bureau et
al. (2003); Roque d'Orbcastel
et al. (2008)
Brown trout*
(Salmo trutta)
438 (589) 49.2 (45.8) 6.2 (10.5) Cho et al. (1994)
Lake trout*
(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 Lupatch 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)
Table 2
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Table 3. Some characteristics of outdoor and indoor RAS with treatment components within the
recirculating loop
1 Treatment system was equipped with additional solids removal and nitrification units.
2 Treatment system was equipped with additional clarifier for solids removal.
3 Treatment system was equipped with additional nitrification unit.
Organism
cultured
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
pond1
320
14.0m3
26.0m2
0.044m3
0.081m2
Metaxa et al.
(2006)
Gilthead
seabream
(Sparus aurata)
High rate algal
pond1
520 12.0m3
43.7m2
0.023m3
0.084m2
Schuenhoff et
al. (2003)
Tilapia
(Oreochromis.
mossambicus x
O. aureus)
wetland2 1230 50.0 m
3
55.0 m2
0.041m3
0.045m2
Zachritz et al.
(2008)
Shrimps
(Litopenaeus
vannamei)
wetland2 924 21.0m
3
32.0m2
0.023m3
0.035m2
Lin et al.
(2005)
Tilapia
(O. niloticus)
aquaponics2
2184 80.0m3
232.0m2
0.037m3
0.106m2
Racocy et al.
(2004)
Indoor RAS
Tilapia
(O. niloticus x
O. aureus)
denitrification/
sludge digestion3
4800
40.0 m3
23.0 m2
0.008 m3
0.005 m2
Shnel et al.
(2002)
Gilthead
seabream
(Sparus aurata)
denitrification/
sludge digestion3
106 1.55m3
2.75m2
0.015 m3
0.026 m2
Gelfand et al.
(2003)
Gilthead
seabream
(Sparus aurata)
denitrification/
anammox/sludge
digestion3
1752 14.4m3
11.1m2
0.008 m3
0.006 m2
Tal et al.
(2009)
Table 3
