Jaap van Rijn

The potential for integrated biological treatment systems in recirculating fish culture—A review

Intensive aquaculture in recirculating systems is rapidly developing, and with it arises the need for reliable treatment systems. To enable reuse of water in these systems, biological treatment is considered the most economically feasible approach. In this review the advantages and disadvantages of some of the most commonly used biological treatment systems are examined. Using as a comparator the main biological processes in extensive static fish ponds, it is explained how most treatment facilities in recirculating systems achieve only partial water purification as sludge and nitrate are produced. Methods for reducing the accumulation of these materials are discussed. It is concluded that incorporation of such methods would result in more stable water quality conditions within the culture units, and also in a considerable reduction of pollution.

Design and performance of a zero-discharge tilapia recirculating system

A zero-discharge tilapia recirculating system was evaluated during a 331-day grow-out period in which tilapia were raised from fingerling to market size. In addition to conventional water treatment procedures, the system featured an anoxic treatment stage where sludge was biologically digested and nitrate was reduced to nitrogen gas. Total tilapia biomass production over the grow-out period was 81.1 kg m−3 with maximum stocking densities of 61.8 kg m−3. Inorganic nitrogen concentrations were within acceptable levels for tilapia culture. Ammonia production rates corresponded to ammonia removal rates by the nitrifying trickling filter (average ammonia removal: 0.16 g N m−2 per day) while nitrate removal took place in the anoxic treatment stage, consisting of a sedimentation/digestion basin and a fluidized bed reactor. Nitrate removal was most profound in the sedimentation/digestion basin, specifically in those regions close to where the sludge entered the basin. Around 70% of the phosphorus added with the feed were recovered mainly in the anoxic treatment stage of the system.

Aerobic and anaerobic biofiltration in an aquaculture unit - nitrite accumulation as a result of nitrification and denitrification.

Biological oxidation and reduction of inorganic nitrogen were studied in an aquaculture unit equipped with an aerobic trickling filter and two, anaerobic, fluidized bed columns. The rate of ammonia oxidation in the trickling filter was shown to be dependent on the ambient ammonia concentration in the unit, indicating that ammonia oxidation by the nitrifying bacteria was substrate-limited with respect to ammonia. The maximum removal rate of ammonia was 0·43 g NH4N, m−2 day−1. Nitrite removal by the trickling filter took place when ambient ammonia concentrations were lower than 1 mg liter−1 NH4N, while at higher ambient ammonia concentrations nitrite accumulated. The fluidized bed columns, employed in the system, were fed with endogenous organic material from the pond. Under these conditions high specific nitrate removal rates were obtained at relatively short retention times. Nitrate removal, however, fluctuated sharply over a diurnal cycle and nitrite accumulation took place under all running conditions examined. Explanations for the nitrite accumulation observed in both filtration systems are presented and possible improvement of the treatment system is discussed.

Atypical Polyphosphate Accumulation by the Denitrifying Bacterium Paracoccus denitrificans

ABSTRACT Polyphosphate accumulation by Paracoccus denitrificanswas examined under aerobic, anoxic, and anaerobic conditions. Polyphosphate synthesis by this denitrifier took place with either oxygen or nitrate as the electron acceptor and in the presence of an external carbon source. Cells were capable of poly-β-hydroxybutyrate (PHB) synthesis, but no polyphosphate was produced when PHB-rich cells were incubated under anoxic conditions in the absence of an external carbon source. By comparison of these findings to those with polyphosphate-accumulating organisms thought to be responsible for phosphate removal in activated sludge systems, it is concluded thatP. denitrificans is capable of combined phosphate and nitrate removal without the need for alternating anaerobic/aerobic or anaerobic/anoxic switches. Studies on additional denitrifying isolates from a denitrifying fluidized bed reactor suggested that polyphosphate accumulation is widespread among denitrifiers.

Performance of a treatment system for inorganic nitrogen removal in intensive aquaculture systems

A concrete fish culture unit (50 m3) stocked with common carp at an initial density of 20 kg m−3 was operated on a semi-closed mode with a minimum of fresh water addition (3 m3 day−3). Inorganic nitrogen levels in the unit were controlled by a combination of an aerobic, nitrifying trickling filter and an anoxic, denitrifying fluidized bed reactor. Organic debris accumulating in the culture unit was diverted into a sedimentation basin and supernatant from this sedimentation basin, rich in nitrate and dissolved organic matter, was pumped into the fluidized bed reactor. The effect of this treatment system was examined over a 3-month period and was compared to a similar culture unit operated at different dilution rates using clean, unpolluted water. Levels of inorganic nitrogen (ammonia, nitrite and nitrate) in the treated culture unit were well within the acceptable range of concentrations tolerated by the fish. As compared to previous studies, it was found that denitrification in the fluidized bed reactor was highly improved by the incorporation of a sedimentation basin. In addition, considerable degradation of organic matter was found to take place in the sedimentation basin.

The use of hydrous iron(III) oxides for the removal of hydrogen sulphide in aqueous systems

The potential for iron (hydr)oxides to remove dissolved hydrogen sulphide from seawater has been examined under flow-through conditions. Ferrihydrite (a hydrous iron (III) oxide) was stabilised by precipitation onto zeolite pellets, and rates of sulphide removal were determined under laboratory conditions at pH 8.5. Sulphide removal kinetics were dependent on the initial sulphide concentration, substrate mass and flow rate. The experimental data suggest that these parameters can be optimised to result in the rapid and effective removal of hydrogen sulphide. The results from laboratory experiments compared favourably with sulphide removal kinetics determined in a series of experiments performed online in a recirculating mariculture production system. However, the presence in solution of ligands such as phosphate may also significantly affect reaction rates; a 50% reduction in sulphide removal rate for substrate removed from an online system was partly attributed to phosphate adsorption. The formation of a more crystalline, less reactive iron (hydr)oxide in recharged substrate was the likely result of FeS oxidation, which may also have contributed to the observed reduction in sulphide removal rates. Ferrihydrite-coated zeolite would appear to provide an efficient, low-cost method for sulphide removal, which is particularly suited to relatively small-scale aqueous flow-through systems. The reaction of iron (hydr)oxides with dissolved sulphide is also accompanied by a distinct colour change due to the formation of black FeS(s) which, under appropriate conditions, may be used as a rapid indicator of sulphidic conditions.

Biological phosphate removal in a prototype recirculating aquaculture treatment system

Efforts to reduce phosphorus concentrations in aquaculture systems have mainly dealt with improving the bioavailability of phosphorus in fish feed. Once released into the culture water, phosphorus is generally left untreated and discharged with the effluent water. In the present study, results are presented on a prototype recirculating treatment system originally designed for removal of organic matter and inorganic nitrogen. Phosphorus determinations in the various compartments of the treatment system (a digestion basin, a denitrifying fluidized bed reactor and a nitrifying trickling filter) revealed that, after 210 days of operation, more than 90% of the added phosphorus was retained within the organic matter of the trickling filter. By means of batch experiments with bacterial consortia from the reactors and with denitrifying isolates, it was found that denitrifiers were capable of phosphate uptake in excess of their metabolic requirements. The phosphorus content of organic material in the fluidized bed reactor was as high as 11.8% (on a dry-mass basis) while it was much lower in the trickling filter (around 1.9%). Anoxic incubation of the trickling filter material in the presence of an external carbon donor resulted in considerable denitrification activity and phosphate uptake. This finding served as an additional indication for the fact that phosphate removal from the water in the system was mainly mediated by denitrifying organisms. Based on these findings, the feasibility of using denitrification to control phosphate levels in the culture and effluent water of recirculating aquaculture systems is discussed.

Identification of Bacteria Potentially Responsible for Oxic and Anoxic Sulfide Oxidation in Biofilters of a Recirculating Mariculture System

ABSTRACT Bacteria presumably involved in oxygen- or nitrate-dependent sulfide oxidation in the biofilters of a recirculating marine aquaculture system were identified using a new application of reverse transcription-PCR denaturing gradient gel electrophoresis (DGGE) analysis termed differential-transcription (DT)-DGGE. Biofilter samples were incubated in various concentrations of sulfide or thiosulfate (0 to 5 mM) with either oxygen or nitrate as the sole electron acceptor. Before and after short-term incubations (10 to 20 h), total DNA and RNA were extracted, and a 550-bp fragment of the 16S rRNA genes was PCR amplified either directly or after reverse transcription. DGGE analysis of DNA showed no significant change of the original microbial consortia upon incubation. In contrast, DGGE of cDNA revealed several phylotypes whose relative band intensities markedly increased or decreased in response to certain incubation conditions, indicating enhanced or suppressed rRNA transcription and thus implying metabolic activity under these conditions. Specifically, species of the gammaproteobacterial genus Thiomicrospira and phylotypes related to symbiotic sulfide oxidizers could be linked to oxygen-dependent sulfide oxidation, while members of the Rhodobacteraceae (genera Roseobacter, Rhodobacter, and Rhodobium) were putatively active in anoxic, nitrate-dependent sulfide oxidation. For all these organisms, the physiology of their closest cultured relatives matches their DT-DGGE-inferred function. In addition, higher band intensities following exposure to 5 mM sulfide and nitrate were observed for Thauera-, Hydrogenophaga-, and Dethiosulfovibrio-like phylotypes. For these genera, nitrate-dependent sulfide oxidation has not been documented previously and therefore DT-DGGE might indicate a higher relative tolerance to high sulfide concentrations than that of other community members. We anticipate that DT-DGGE will be of general use in tracing functionally equivalent yet phylogenetically diverse microbial populations in nature.

Phosphorus removal in a marine prototype, recirculating aquaculture system

Abstract Phosphorus dynamics were examined in a prototype, zero-discharge, marine-recirculating system. Operation of the system without discharge of water and sludge was enabled by recirculation of effluent water through two separate treatment loops. Surface water from the fish basin was pumped over a trickling filter in one loop, while bottom-water was recirculated through a sedimentation basin followed by a fluidized bed reactor in the other treatment loop. Ammonia oxidation to nitrate in the trickling filter and organic matter digestion together with nitrate reduction in the sedimentation basin and fluidized bed reactor were the main biological features of this treatment system. Orthophosphate concentrations did not exceed 15 mg PO 4 –P/l in the culture water during more than 1 year of system operation. Much of the phosphorus was retained within the sedimentation basin and fluidized bed reactor. In these treatment stages, the phosphorus content of organic matter was as high as 17.5% and 19%, respectively. High concentrations of total phosphorus and low concentrations of soluble orthophosphate were measured in the initial stages of sedimentation under oxic and anoxic conditions, suggesting that most of the phosphorus was associated with organic matter. Depletion of oxygen and nitrate in the sludge layers of the sedimentation basin coincided with sulfate reduction to sulfide and a release of soluble orthophosphate. The observed phosphorus dynamics in this marine system supported findings from previous studies in which it was demonstrated that denitrifiers underlie phosphorus immobilization under these conditions.

Anaerobic treatment of intensive fish culture effluents: volatile fatty acid mediated denitrification

Abstract Denitrifying activity was examined in a laboratory-scale fluidized bed reactor coupled to a digestion basin in which fish feed was anaerobically degraded. Under these conditions volatile fatty acids (VFA; acetate, propionate and butyrate), released during anaerobic feed degradation, were found to fuel the denitrifying activity in the fluidized bed reactor. VFA fatty acid-dependent denitrification was confirmed by laboratory incubation of bacterial consortia, derived from the laboratory-scale fluidized bed reactor and a pilot-plant fluidized bed reactor. Kinetic parameters of denitrification were determined as well as the stoichiometry of VFA uptake and nitrate reduction by these denitrifying consortia. It was concluded that in closed fish culture systems, where nitrate accumulates as a result of nitrification, complete nitrate removal is possible by a combination of denitrification and anaerobic degradation of endogenous organic matter.