Fish waste management by conversion into heterotrophic bacteria biomass

Just as all other types of animal production, aquaculture produces waste. This waste can be managed outside the production system, comparable to terrestrial husbandry systems. However, particularly recirculation aquaculture systems (RAS) are suited to manage waste within the system. In this case, processes have to be selected to convert the waste into a re-usable product. Dissolved and solid waste conversion by heterotrophic bacteria is one of these processes. In the present study, the potential of the latter process was investigated. An operational scheme was followed, which contained five steps: (1) to evaluate nutrient flows in integrated aquaculture systems, (2) to select and to investigate a conversion process, (3) to improve the process and analyze its sensitivity, (4) to evaluate the product suitability, (5) to derive the kinetics, reactor design, and to determine the integration possibilities into RAS.In chapter 2 nutrient flows, conversions and waste management were evaluated, which are taking place in integrated intensive aquaculture systems. In these systems, fish is cultured next to other organisms, which are converting nutrients, which would be otherwise discharged. These conversions were evaluated based on nitrogen (N) and phosphorous (P) balances using a mass balance approach. In the reviewed examples, fish culture alone retained 20-50% feed N and 15-65% feed P. The combination of fish culture with phototrophic conversion increased nutrient retention of feed N by 15-50% and of feed P by up to 53%. If in addition herbivore consumption was included, then the gained nutrient retention decreased by 60-85% feed N and 50-90% feed P. The conversion of nutrients into bacteria and detrivorous worm biomass contributed only to a smaller extent (e.g. 7% feed N and 6% feed P and 0.06% feed N 0.03xl0"3% feed P, respectively). AIl integrated modules had their specific limitations, which were related to uptake kinetics, nutrient preference, unwanted conversion processes and abiotic factors and implications.Chapters 3 to 5 focused on the experimental production of heterotrophic bacteria biomass on carbon (C) supplemented fish waste under different operational conditions. The results covered step two and three in the operational scheme.In chapter 3, the drum filter effluent from a RAS was used as substrate to produce heterotrophic bacteria in suspended growth reactors. Effects of organic C supplementation (0, 0.9, 1.7, 2.5gC/l as sodium acetate) and of hydraulic retention times (HRT: 1 l-lh) on bacteria biomass production and nutrient conversion were investigated. Bacteria production, expressed as VSS (volatile suspended solids) was enhanced by organic C supplementation, resulting in a production of 55-125gVSS/kg fish feed (0.2-0.5gVSS/gC). Maximum observed crude protein production was ~100g protein/kg fish feed. The metabolic maintenance costs were 0.08Cmol/Cmol h"', and the maximum growth rate was 0.25-0.5h"'. Approximately, 90% of the inorganic nitrogen and 80% of ortho-phosphate-phosphorus were converted.The influence of nitrogenous waste on bacteria yields was investigated in chapter 4. RAS effluents are rich in nitrate and low in total ammonia nitrogen (TAN). This might result in 20% lower bacteria yields, because nitrate conversion into bacteria is less energy efficient than TAN conversion. In this chapter, the influence of TAN concentrations (1, 12, 98, 193, 257mgTAN/l) and stable nitrate-N concentrations (174+29mgfl) on bacteria yields and N conversions was investigated in a RAS under practical conditions. The effluent slurry was supplemented with 1.7gC71 sodium acetate, due to C deficiency, and was converted continuously in a suspended bacteria growth reactor (6h HRT). TAN utilization did not result in different yields compared to those for nitrate (0.24-0.32gVSS/gC, p=0.763). However, TAN was preferred compared to nitrate and was converted to nearly 100%, independently of TAN concentrations. TAN and nitrate conversion rates differed significantly for increasing TAN levels (p 50%, 2, 5, and 10 minutes after feeding). It was, therefore, inferred, that all bacteria biomass and commercial feed combinations were basically attractive for the shrimp. This response was not instantaneous. After feeding (2min) more than 80% of the shrimp were present at the feeding places and showed a significant preference for the commercial feed compared to the aerobically produced bacteria slurry. For the other diet combinations no significant differences could be detected for 2min. For 5 and lOmin after feeding, shrimp behavior changed from the commercial feed to the aerobically and anaerobicaliy produced bacteria biomass segments. From this study it was conc! uded that although the commercial diet was preferred above the aerobic slurry, the bacteria slurries had also attracted the shrimps. There was no unambiguous conclusion to be made regarding the preference for aerobic or anaerobic produced slurry. In chapter 8, the design of a suspended bacteria growth reactor integrated in a lOOMT African catfish farm was determined. This study integrated results from the earlier chapters to calculate the bacteria kinetics (yield=0.537gVSS/gC; endogenous decay coefficienl=0.033h^'; maximum specific growth rate=0.217h^ ; half-velocity constant=0.025g/l; and maximum rate of substrate utilization-0.404gC/gVSS*h). As part of the study a model was developed and validated. This model was used to calculate the VSS production and nutrient conversion by heterotrophic bacteria conversion for a lOOMT African catfish farm. The VSS production was 187gVSSAcg feed and the inorganic nutrients (N and P) were removed with an efficiency of 85 and 95% for a C supplementation level of 3.5gC/l (455gC/kg feed). A reactor integrated in a lOOMT farming facility would have a volume of 11m , based on a minimum HRT of6h. The production and potential re-use of heterotrophic bacteria biomass is, therefore, a prospective tool to lower nutrient discharge and to increase nutrient retention and sustainability of RAS in the future.