Developing a low-cost recirculating aquaculture system using locally available media as biofilters for removing ammonia and nitrite
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Date
2022
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Sokoine University of Agriculture
Abstract
Aquaculture is the fastest growing food producing sector in the world and remains a
vibrant and important sector for production of high-quality protein food. Success for the
aquaculture industry is attributed to growing demand for healthy, tasty and affordable
food as well as the sharp decline in wild fish supply due to increasing human population
and over exploitation of natural water bodies. To increase production from aquaculture
and reduce the widening gap between fish demand and supply, technologies that increase
production efficiency and intensity as well as use less water and are environmentally
friendly need to be promoted. One of the methods for intensive aquaculture production
system is recirculating aquaculture system. Recirculating Aquaculture System (RAS) is a
technology designed for holding and growing a wide variety of aquatic species in defined
production unit which recycles water by passing it through mechanical and biological
filters to remove suspended and dissolved wastes, respectively. This technology (RAS) is
widely used in developed countries and the sector is growing tremendously. Yet,
developing countries’ aquaculture production rarely uses RAS for intensive fish farming.
This discrepancy is partially driven by the high costs of the RAS unit and the associated
media such plastic beads and Kaldnes (KMT) media, commonly used as biomedia in
moving bed bioreactor (MBBR).
Therefore, a study was conducted to develop a low-cost RAS and identify cheap and
locally available media that can be used as biofilters in RAS. The use of cheap and locally
available media will reduce costs and make RAS affordable to small-scale fish farmers.
Development of low-cost RAS unit and availability of cheap, locally available biomedia
that can be used in biofilters will encourage local small-scale fish farmers in Tanzania to
embark on intensive fish farming using RAS technology. In this study four experiments
were conducted. The first experiment was conducted to establish the suitable water flow velocity to be
applied in the development of an ideal RAS for use in developing countries. Three
freshwater pilot scale RAS stocked with rainbow trout and fixed bed biofilters were used.
Removal of total ammonia-nitrogen (TAN) and nitrite-nitrogen were assessed at four
different water velocities in the biofilters (i.e. 1.4, 5.4, 10.8 and 16.2 m/h) under identical
conditions (temperature, dissolved oxygen, pH, alkalinity). Results indicated low TAN
and nitrite removal rates at water velocities below 10.8 m/h. Five-fold elevated nitrite
levels were found in the RAS when biofilters were operated at 1.4 m/h compared to the
other velocities, substantiating the significant effect of water velocity on biofilter
performance. The best water flow velocity determined in this pilot scale RAS was applied
in the second trial.
The second experiment involved developing an ideal low-cost RAS to be used for
experiments and production of fish in developing countries. In this study, two pilot RAS
units, each with capacity of 900 L of water in circulation were developed and ran for 10
weeks. Synthetic ammonia and nitrite were added in the first four weeks to trigger the
development of nitrifying bacteria, after which 20 kg bulk weight of Nile tilapia was
stocked in each RAS unit. The average water quality parameters throughout the
experimental period were 179.09 ± 85.6 mg CaCO 3 /L, 6.18 ± 0.8 mg/L, 7.59 ± 0.4, 24.69
± 1.1 °C, 197.23 ± 92.2 mg/L and 0.20 ± 0.1 ppt for alkalinity, dissolved oxygen, pH,
temperature, total dissolved solids and salinity, respectively. The stocked fish biomass
increased by 9 kg in each tank for a period of six weeks. Volumetric TAN and nitrite
conversion rate exponentially increased from week two and became stable after the 6 th
week with an average concentration of 450 g/m 3 /d and 100 g/m 3 /d, respectively. The
developed simple low-cost RAS showed performance that is similar to other commercial
RASs and, therefore, it is ideal for teaching, research and fish production purposes in
developing countries. The third experiment was done to test locally available materials as biomedia in RAS. In
this experiment, six biological filters, of which five media were made from locally
available materials (dry cattle horns, dry local ceramic, dry activated charcoal, dry
bamboo sticks and dry coconut shells) and one commercial plastic media were evaluated
in duplicate in a 1 m 3 tank under pilot scale. Volumetric TAN and Nitrite removal were
assessed. The highest VTR recorded in this study was 598.65 ± 15.8 g TAN/m 3 /d from
coconut shells while the lowest was 343.45 ± 8.93 g TAN/m 3 /d from horns. Biofilters
containing plastic recorded the highest VNR (704.24 ± 50.30 g NO 2 -N/m 3 /d) while the
horns biofilters recorded the lowest (457.38 ± 46.09 g NO 2 -N/m 3 /d). This study, therefore,
revealed that coconut shells can be used as biomedia in place of plastic materials in
recirculation aquaculture system biofiltration.
The fourth experiment was done to further evaluate the ability of coconut shells in
comparison with commercially available biomedia (Foam, Leca and plastic beads). Water
quality parameters were monitored and the performance of different biofilters were
assessed in terms of VTR, VNR and bacterial activity by the use of hydrogen peroxide
(H 2 O 2 ) degradation method. Nitrification kinetics for volumetric TAN conversion rates
were 4.6 ± 0.3, 3.8 ± 0.3, 3.3 ± 0.3 and 1.7 ± 0.2 g/m 3 /d for biofilters containing foam,
coconut shells, leca and Plastic beads, respectively. The calculated first order rate
constant k 1a (1’ order) for volumetric TAN conversion rates were 0.05, 0.04, 0.04 and 0.01
m/d for biofilters containing foam, coconut shells, leca and plastic, respectively. On the
other hand, the 0’ order nitrification kinetics for VNR were 5.1 ± 0.8, 4.5 ± 0.6, 3.7 ± 0.2
and 1.4 ± 0.2 g/m 3 /d for biofilters containing foam, coconut shells, leca and Plastic beads,
respectively. This study found that foam is the best biomedia for RAS, followed by
coconut shells and leca. High porosity state of foam makes it easy to clog and, therefore,
cannot maintain its nitrification performance for a long time. Therefore, this study
recommends the use of coconut shells and leca for biofiltration in RAS. This study concludes that coconut shell is as good as plastic in serving as a biomedia in
RAS. Therefore, it is recommended that coconut shell be used as biomedia in RAS in
developing countries. Further research is recommended to determine appropriate sizes of
this biomedia during application and its durability during operation.
Description
Thesis
Keywords
Low-cost recirculating, Aquaculture system, Media, Biofilters, Ammonia, Nitrite