SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS
The aim of the article is to study the parameters of wastewater settling in recirculating aquaculture systems (RAS) during the cultivation of African sharptooth catfish. The volumes of freshwater fish farming using such systems are steadily increasing. However, RAS have a disadvantage in that they r...
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Institute of Renewable Energy National Academy of Sciences of Ukraine
2025
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| author | Golub , G. Yaremenko , O. Marus , O. Tsyvenkova , N. Chetveryk , H. |
| author_facet | Golub , G. Yaremenko , O. Marus , O. Tsyvenkova , N. Chetveryk , H. |
| author_institution_txt_mv | [
{
"author": "G. Golub ",
"institution": "National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine"
},
{
"author": "O. Yaremenko ",
"institution": "Іnstitute of Renewable energy, NAS of Ukraine, Kyiv, Ukraine"
},
{
"author": "O. Marus ",
"institution": "National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine"
},
{
"author": "N. Tsyvenkova ",
"institution": "Іnstitute of Renewable energy, NAS of Ukraine, Kyiv, Ukraine; National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine"
},
{
"author": "H. Chetveryk ",
"institution": "Іnstitute of Renewable energy, NAS of Ukraine, Kyiv, Ukraine 1, 3, 4 National University"
}
] |
| author_sort | Golub , G. |
| baseUrl_str | https://ve.org.ua/index.php/journal/oai |
| collection | OJS |
| datestamp_date | 2026-07-18T06:32:21Z |
| description | The aim of the article is to study the parameters of wastewater settling in recirculating aquaculture systems (RAS) during the cultivation of African sharptooth catfish. The volumes of freshwater fish farming using such systems are steadily increasing. However, RAS have a disadvantage in that they require the disposal of contaminated wastewater generated during the washing of the filter meshes of the me-chanical filters through which the recirculated water passes. It is advisable to use wastewater settling tanks in RAS be-cause they are quite effective in removing total suspended solids (TSS) and have a relatively low cost, which leads to their high economic efficiency. The results of studies on the settling of wastewater from mechanical filters using RAS during the cultivation of Clarias gariepinus confirmed the possibility of carrying out this process with high efficiency and using simple equipment in the case of further utilization of sludge and settled wastewater. The TSS content in the settled wastewater after the settling tank was 0.07±0.015 g/l, while the TSS content in the recircu-lating water after the mechanical filter was 0.057±0.023 g/l, which allows the use of the settling tank to replace the mechanical filter for RAS during the cultivation of Clarias gariepinus. Such a replacement will make it possible to remove only the sludge from the RAS, with its amount not exceeding 7 % of the amount of mechanical filter flush water. The expediency of creating a settling system based on two settling tanks was established: the first one for settling the wastewater from mechanical filters and the second one for settling the sludge obtained at the first stage of settling, which will reduce the amount of sludge for further utilization to 3 % of the amount of waste rinse water from mechanical filters. The settled wastewater from the settling tanks can be used in hydroponics systems or for irrigation, and the sludge after two-stage settling can be used for biogas production. |
| doi_str_mv | 10.36296/1819-8058.2025.1(80).148-158 |
| first_indexed | 2025-07-17T11:39:55Z |
| format | Article |
| fulltext |
148
Відновлювана енергетика. №1/2025 | Біоенергетика
UDC 663.142:631.333.92 https://doi.org/10.36296/1819-8058.2025.1(80)148-158
SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE
CULTIVATION OF CLARIAS GARIEPINUS
Received Oct. 31, 2024; accepted Mar. 14, 2025
Available online Apr. 01, 2025
Golub G.1, Yaremenko O.2, Marus O.3,
Tsyvenkova N.4, Chetveryk H.5
Author for correspondence: Golub G.,
e-mail: gagolub@ukr.net
The aim of the article is to study the parameters of
wastewater settling in recirculating aquaculture systems
(RAS) during the cultivation of African sharptooth catfish. The
volumes of freshwater fish farming using such systems are
steadily increasing. However, RAS have a disadvantage in
that they require the disposal of contaminated wastewater
generated during the washing of the filter meshes of the me-
chanical filters through which the recirculated water passes.
It is advisable to use wastewater settling tanks in RAS be-
cause they are quite effective in removing total suspended solids (TSS) and have a relatively low cost, which leads
to their high economic efficiency. The results of studies on the settling of wastewater from mechanical filters using
RAS during the cultivation of Clarias gariepinus confirmed the possibility of carrying out this process with high
efficiency and using simple equipment in the case of further utilization of sludge and settled wastewater. The TSS
content in the settled wastewater after the settling tank was 0.07±0.015 g/l, while the TSS content in the recircu-
lating water after the mechanical filter was 0.057±0.023 g/l, which allows the use of the settling tank to replace
the mechanical filter for RAS during the cultivation of Clarias gariepinus. Such a replacement will make it possible
to remove only the sludge from the RAS, with its amount not exceeding 7 % of the amount of mechanical filter
flush water. The expediency of creating a settling system based on two settling tanks was established: the first
one for settling the wastewater from mechanical filters and the second one for settling the sludge obtained at the
first stage of settling, which will reduce the amount of sludge for further utilization to 3 % of the amount of waste
rinse water from mechanical filters. The settled wastewater from the settling tanks can be used in hydroponics
systems or for irrigation, and the sludge after two-stage settling can be used for biogas production.
Keywords: recirculating aquaculture systems, settling tank, total suspended solids, sludge, settled wastewater.
Aquaculture is one of the fastest-growing sectors in the
food industry. The increasing demand for fish and seafood
is accompanied by a steady growth in aquaculture produc-
tion. To meet human demand for fish products by 2029, an-
nual production should increase to 102 million tonnes [1].
Aquaculture produces more than half of the world's fish
products. The production of aquaculture products using re-
circulating aquaculture systems (RAS) is becoming increas-
ingly common. RAS are systems where water is reused in
aquaculture production after mechanical and biological
treatment. Such systems have a low environmental impact
and reduce water and energy consumption. The volumes of
freshwater fish (mainly eel and catfish) and trout produced
using such systems are steadily increasing, reaching several
thousand tonnes in Europe, and the use of freshwater
aquaculture is also growing in Asian countries. The use of
RAS is the most promising global trend [2].
According to experts, aquaculture produces much less
greenhouse gas emissions compared to livestock produc-
tion. This means that breeding and consuming protein from
fish can help mitigate the effects of climate change. How-
ever, RAS are not without a drawback, which relates to the
need to dispose of waste-contaminated water generated
during washing of the filter meshes of mechanical filters
through which recirculated water passes [1, 2].
One of the methods of aquaculture wastewater purification
is the use of wetlands. For example, when treating
wastewater from a trout aquaculture farm using wetlands,
it was found that the highest treatment efficiency for re-
moving total suspended solids (TSS) reached 68 % at a
1 Dr. of Tech. Sciences
https://orcid.org/0000-0002-2388-0405
2 Researcher
https://orcid.org/0000-0001-5440-4682
3 Cand. of Tech. Sciences
https://orcid.org/0000-0003-1521-2885
4 Cand. of Tech. Sciences
https://orcid.org/0000-0003-1703-4306
5 Cand. of Tech. Sciences
https://orcid.org/0000-0001-9398-1968
2, 4, 5 Іnstitute of Renewable energy, NAS of
Ukraine, Kyiv, Ukraine
1, 3, 4 National University of Life and Environmental
Sciences of Ukraine, Kyiv, Ukraine
149
Відновлювана енергетика. №1/2025 | Біоенергетика
hydraulic loading rate (HLR) of 13.6 m3/day. Although wet-
lands show some efficiency in the treatment of aquaculture
wastewater under the influence of natural factors, the pol-
lution factors themselves are not eliminated from the eco-
system. [3].
Hydroponic systems are also used to treat aquaculture
wastewater. [4]. Thus, it was found that a hydroponic sys-
tem with barley cultivation allowed to reduce TSS by 52.7
to 60.5% [5], and therefore the authors recommend the use
of additional sedimentation.
The simplest and most cost-effective way to treat aquacul-
ture wastewater is to settle it and then use the sludge and
settled wastewater [6]. The use of sedimentation (the pro-
cess of separation of wastewater into sludge and settled
wastewater due to the settling of sludge particles by gravity
[7]) for sludge separation in RAS is considered inefficient
due to the large volumes of recirculated water, which leads
to a short residence time in the settling tank. However, the
use of settling tanks may be appropriate for settling
wastewater sludge from rotating microsieve mechanical fil-
ters that treat the main stream of recycled water [8]. The
particles retained on the mesh are washed into a collection
chute and discharged from the RAS. The volume of
wastewater from the mechanical strainers is less than 1%
of the recirculation flow, and therefore, sedimentation is
considered to be an effective way to concentrate sludge
[9]. The sedimentation efficiency (settling tank efficiency) is
determined by the following expression [10]:
0
0
C
CC
E −
= , (1)
where: C0 – TSS concentration at the inlet to the settling
tank, %; C — TSS concentration at the outlet of the settling
tank at the current time τ, %.
A review article [11] concluded that settling tanks are rele-
vant for use in thickening wastewater from mechanical
mesh filters.
A review was conducted in [12], which found that the den-
sity of fish faeces was in the range of 1,050–1,080 kg/m3,
and fish feed was 1,150–1,200 kg/m3. The average density
was 1,190 kg/m3, and according to other data, the average
density range was in the range of 1,050-1,160 kg/m3. The
authors concluded that the low-density difference between
TSS and freshwater (1,000 kg/m3), combined with the wide
size distribution, makes it challenging to separate TSS from
RAS wastewater.
The use of a pilot aquaculture wastewater treatment sys-
tem showed that the efficiency of TSS removal by sedimen-
tation was more than 70%, and due to the subsequent com-
bined action of ozonation and chemical flocculation, 99% of
TSS was removed [13].
Article [14] presents the results of a study of a wastewater
treatment system for trout farming, where a mechanical fil-
ter and a flush wastewater sludge settler were used at the
last stage of treatment. Low TSS removal efficiency by a
mechanical filter was found (at the level of 33-53%).
Coagulation was also studied to ensure the sedimentation
of sludge from aquaculture wastewater [15]. For this pur-
pose, the following coagulants were used: gypsum, alum
with agricultural limestone, ferric chloride, and ferric chlo-
ride with non-ionic polymer (polyacrylamide) in doses that
are commonly used in wastewater treatment [16]. The use
of coagulation for aquaculture wastewater treatment, alt-
hough it shows the effectiveness of pollutant removal, still
requires further research and raises the issue of disposal of
chemical-contaminated sediments generated during the
coagulation process.
Based on the review of scientific publications, the following
conclusions can be drawn: it is advisable to use wastewater
settling tanks in RAS, as they are quite effective in removing
TSS; settling tanks have a relatively low cost, which leads to
their high economic efficiency; settling tanks in RAS
wastewater treatment systems are used at the first stage
of wastewater treatment and do not allow for the complete
utilization of the generated sludge and settled wastewater.
The aim of the article is to study the parameters of
wastewater settling in RAS during the cultivation of African
sharptooth catfish (Clarias gariepinus). To achieve this goal,
the following tasks need to be completed:
1. To study the process of sedimentation of the effluent
from mechanical filters using RSA in the culture of
Clarias gariepinus, with subsequent disposal of the sed-
iment and sludge removed;
2. To determine the content of TSS in the settled effluent
after the settling tank and in the recirculated water after
the mechanical filter with further conclusions on the pos-
sibility of using the settling tank instead of the mechani-
cal filter for RSA in the cultivation of Clarias gariepinus.
3. To determine the average value of the TSS removal ef-
ficiency and to prove the feasibility of creating a settling
system based on two settling tanks: the first one for set-
tling the waste rinsing water of the mechanical filters;
the second one for settling the sludge obtained in the
first stage of settling.
4. To investigate the possibility of using the sludge, after
settling in the settling tank, as an organic fertiliser.
5. To determine the biogas yield from sewage sludge for a
typical RSA system, based on the methodology for cal-
culating the parameters of the process of preparing aq-
uaculture sewage sludge for methane digestion.
Materials and Methods
The objects of study are the sludge from the rinsing water
of the mechanical filters of the RSA, sedimentation tanks,
and biogas plant.
The subject of the study is the sedimentation properties of
the sludge from the rinsing water of the RSA mechanical fil-
ter during the cultivation of Clarias gariepinus.
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Відновлювана енергетика. №1/2025 | Біоенергетика
Hypothesis – achievement of optimal values of RSA sludge
moisture content to ensure its further anaerobic digestion
can be achieved by two-stage sedimentation of mechanical
filter flushing water.
The assumptions and simplifications in this article are due
to the fact that only RSA mechanical filter sludge was used
in the Clarias gariepinus culture. It does not take into ac-
count the different growth stages of the Clarias gariepinus
in each tank and the associated different feeding regimes.
The research was conducted using the mechanical filter
flush water for RAS during the cultivation of Clarias gariepi-
nus. The general scheme of the RAS, which is equipped with
a settling tank for separation of mechanical filter flush wa-
ter into sludge and settled wastewater, is shown in Fig. 1.
At the first stage of the study, the mechanical filter
wastewater was collected in measuring cylinders for set-
tling (Fig. 2). The flush water was sampled before it entered
the settling tank (position 5 in Fig. 1).
At the second stage of the study, the wastewater from the
mechanical filter was settled in a sump (position 6 in Fig. 1).
The sludge and wastewater were sampled after settling in
the settling tank for a day (Fig. 3). To compare the TSS con-
tent of the wastewater after settling in the settling tank and
the recirculating water after the mechanical filter, samples
of the recirculating water after the mechanical filter were
collected and analyzed.
After settling in the settling tank for a day, the sludge was
further settled in a measuring cylinder and directly in the
settling tank for several days (Fig. 4).
The experiments were conducted on three different days
to neutralize the influence of technological factors and in
triplicate. Based on the data obtained, the standard devia-
tion of the data and the confidence interval were deter-
mined at a 5% level of significance. The amount of sludge in
the cylinders was determined with a change in the settling
time, and the dry matter content in the mechanical filter
wastewater, sludge and settled wastewater in the cylin-
ders, wastewater and sludge after settling in the settling
tank, and recirculation water after the mechanical filter
were determined at the end of the settling process.
Fig. 1. General scheme of the recirculation system of aqua-
culture wastewater purification and sedimentation of
sludge from the mechanical filter wash water: 1 – fish
pool; 2 – mechanical filter; 3 – biological water purification
system; 4 – pipeline for discharge of waste washing water
from the mechanical filter for utilization; 5 – sampling
point for waste wash water; 6 – settling tank; 7 – settled
wastewater pipeline; 8 – sewage sludge discharge pipeline
Fig. 2. General view of the sampling site for the waste wash water (a) and the wash water from the mechanical filter in
the measuring cylinders (b)
151
Відновлювана енергетика. №1/2025 | Біоенергетика
Fig. 3. General view of the sampling site of wastewater after the settling tank (a) and recirculation water after the me-
chanical filter, wastewater and sludge after settling in the settling tank for a day (b)
Fig. 4. General view of the sampling site for sludge after
the settling tank (a) and sludge after settling in the settling
tank for a day (b)
The assessment of agrochemical parameters of sludge and
wastewater after settling in the settling tank was carried
out according to standard methods in a specialized labora-
tory of the National University of Life and Environmental
Sciences of Ukraine.
The dry matter content was determined by filtering the
wastewater and sludge and then drying the residues on the
filters in a drying oven at 105°C.
Results and Discussion
Wastewater settling using measuring cylinders
The study of the settling of the mechanical filter
wastewater in liter measuring cylinders for RAS during the
cultivation of Clarias gariepinus showed that the maximum
volume of sludge is found at 70 ml (Fig. 5).
Fig. 5. The result of settling the wash water of a mechani-
cal filter
It was found that sedimentation occurs in 3-4 minutes, and
then, within 15 minutes, a combination of wastewater pu-
rification (clarification) and sludge compaction under the
influence of the weight of the wastewater occurs. This pro-
cess is completed within 20 minutes (Fig. 6). Thus, it can be
assumed that the level of sedimentation in the mechanical
filter wash water sludge is about 7% of the total amount of
mechanical filter wash water.
152
Відновлювана енергетика. №1/2025 | Біоенергетика
Fig. 6. Dependence of sludge volume on settling time in measuring cylinders
It was also found that the TSS of the mechanical filter wash water at 0.866±0.092 g/L was concentrated in the sludge and
amounted to 0.859±0.091 g/L (Fig. 7). The wastewater sludge contained 0.007±0.002 g/l TSS. The distribution of TSS be-
tween sewage and sludge was 1:120-125. The average TSS removal efficiency in the measuring cylinders was 99.2%.
Fig. 7. Distribution of the DM content of mechanical filter wash water as a function of settling time in measuring cylin-
ders
Settling of wastewater in a settling tank
The study of the settling of mechanical filter wastewater in
the settling tank during the day showed the TSS content in
the settled wastewater after the settling tank at a level of
0.07±0.015 g/l, which is an order of magnitude higher than
when settling in measuring cylinders. Such an excess of TSS
content in the settled wastewater after the settling tank
compared to the settling of mechanical filter wash water in
measuring cylinders is due to the dynamic mode of settling
in the settling tank, which operated in a continuous mode.
The assessment of the TSS content in the recirculating wa-
ter after the mechanical filter showed a value of
0.057±0.023 g/l. The TSS content in the sludge from the set-
tling tank was 32.453±0.663 g/l, which is approximately 30
times higher than when settling in measuring cylinders
since the settling time significantly exceeded the settling
time in measuring cylinders (Fig. 8). This phenomenon is
153
Відновлювана енергетика. №1/2025 | Біоенергетика
due to the compaction of sludge under the weight of the
sludge and wastewater in the settling tank. The average
value of TSS removal efficiency in the settling tank was
91.9%.
Fig. 8. Distribution of DM content in recirculating water after a mechanical filter, settled wastewater after a settling
tank, and sludge from a settling tank after a day of settling
The evaluation of agrochemical parameters of sludge and
wastewater after settling in the settling tank for a day
showed the results presented in Table 1.
Table 1. Agrochemical parameters of sludge and wastewater after settling in the settling tank for a day (for initial moisture)
Name of indicators Units of measurement Sludge
Settled
wastewater
Organic matter (С) % 0.04±0.004 ‒
Mass fraction of total nitrogen (N) % 0.25±0.003 0.17±0.02
Mass fraction of ammonia nitrogen (NН4) % 0.11±0.0001 0.06±0.01
Mass fraction of total phosphorus (Р2О5) % 0.06±0.0005 0.001±0.0001
Mass fraction of total potassium (К2О) % 0.02±0.002 0.007±0.001
Mass fraction of total calcium (СаО) % 0.1±0.001 0.005±0.0008
Salt extract рН 5.93±0.05 6.22±0.02
The results of the agrochemical parameters of the sludge
indicate the possibility of using the sludge after settling in
the settling tank as an organic fertilizer for vegetable crops
when diluted with water in a ratio of 1:4.
Sludge settling using measuring cylinders
After settling in the settling tank for a day, the sludge was
further settled in a measuring cylinder. It was found that the
sludge volume continued to decrease for 15 days (Fig. 9).
Fig. 9. General view of the sludge after settling (a) and the dependence of the sludge volume and its estimated moisture
content on the settling time (b)
154
Відновлювана енергетика. №1/2025 | Біоенергетика
During the first day, the sludge level as a percentage of the
current value of the sludge volume to the initial value de-
creased by up to 35-38%. The maximum value of the sludge
level reduction was 45%. Based on the sludge level data,
the dependence of the calculated sludge moisture content
on the settling time was determined. It was found that
when the sludge was settled for 15 days, its estimated
moisture content was about 92.8%.
Settling of sludge in the settling tank
Settling the sludge directly in the settling tank for 15 days
allowed obtaining a differential and integral distribution of
the results of determining the moisture content of the
sludge (Fig. 10).
Fig. 10. Distribution of sludge moisture content after settling in a settling tank for 15 days
It was found that a sludge moisture content of 92% could
be obtained after settling the sludge for 15 days, with a
probability of 69.23%. As for the sludge moisture content
of 93%, it can be obtained after settling the sludge for 15
days with a probability of 92.31%. No sludge sample with
more than 94% moisture content was obtained during the
study. These data indicate the possibility of further anaer-
obic digestion of the resulting sludge to produce biogas and
use it to increase the energy autonomy of an aquafarm.
Table 2. Agrochemical parameters of sludge and wastewater after settling in a settling tank for 15 days (for initial moisture)
Name of indicators
Units of measure-
ment
Sludge Settled wastewater
Organic matter (С) % 1.95±0.05 ‒
Mass fraction of total nitrogen (N) % 0.28±0.01 0.05±0.01
Mass fraction of total phosphorus (Р2О5) % 0.28±0.03 0.02±0.003
Mass fraction of total potassium (К2О) % 0.01±0.002 0.009±0.001
Mass fraction of total calcium (СаО) % 0.26±0.03 0.008±0.001
Salt extract рН 5.55±0.01 6.59±0.05
Engineering methodology for calculating parameters of the
process of preparation of aquaculture wastewater sludge
for biogas fermentation
Studies have shown the feasibility of creating a system for
settling mechanical filter wash water based on two settling
tanks: the first one is for settling mechanical filter wash wa-
ter; the second one is for settling the sediment obtained at
the first settling stage. The sediment obtained at the first
settling stage can be returned to the RSA system in the case
of effective operation of the biological treatment system
for recirculating water. It is obvious that the working vol-
ume of the first settling tank can be determined by the ex-
pression:
11 ·tqV = , (2)
where V is the working volume of the first settling tank, l; t1
is the retention time of the mechanical filter wastewater in
the first settling tank, min; q is the volume of the mechani-
cal filter wastewater, l/min.
The annual yield of sedimentation during the settling of the
wastewater wash water of mechanical filters in the first set-
tling tank will be:
qkQ )1(
1000
365·1440
11 −= , (3)
where Q1 is the volume of sedimentation from the first set-
tling tank, l/min; 1440 min/day is the number of minutes in
one day; 365 days/year is the number of days in one year;
155
Відновлювана енергетика. №1/2025 | Біоенергетика
1000 l/m3 is the number of minutes in one day; k1 is the
sediment yield coefficient from the first settling tank, rel.
units. The annual sediment yield when settling the
wastewater wash water of mechanical filters in the first set-
tling tank will be:
qkQ ·
1000
365·1440
12 = , (4)
where Q2 is the volume of sediment from the first settling
tank, l/min. The working volume of the second settling tank
(for settling the sediment obtained at the first settling
stage) can be determined by the expression:
qktV ···
1000
1440
122 = , (5)
where V2 is the working volume of the second settling tank,
l; t2 is the retention time of the sludge from the first settling
tank in the second settling tank, days.
Studies have established that the retention time of the
wastewater wash water of mechanical filters in the first set-
tling tank to achieve the maximum level of sedimentation
should be at least t1=15 minutes, the sediment yield coeffi-
cient from the first settling tank k1=0,07 rel. units, the re-
tention time of the sediment from the first settling tank in
the second settling tank to achieve the maximum level of
sedimentation should be at least t2=15 days, and the sedi-
ment yield coefficient from the second settling tank k2=0,45
rel. units.
The annual yield of sediment when settling the sediment
obtained at the first stage of settling in the second settling
tank will be:
123 ·)1(
1000
365·1440
kqkQ −= , (6)
where Q3 is the volume of sediment from the second set-
tling tank, l/min; k2 is the coefficient of sediment yield from
the second settling tank, rel. units.
The annual sediment yield when settling the sediment ob-
tained at the first stage of settling in the second settling
tank will be:
qkkQ ··
1000
365·1440
214 = , (7)
where Q4 is the volume of sludge from the second settling
tank, l/min.
The working volume of the biogas reactor (for the fermen-
tation of sludge obtained at the second settling stage) can
be determined by the expression:
3213
1000
1440
qtkkV = (8)
where V3 is the working volume of the third settling tank, l,
л; t3 is the retention time of the sludge in the biogas reac-
tor, days.
The annual biomethane yield can be determined by the ex-
pression:
3··365
4
VVCH = , (9)
where VCH4 is the annual yield of biomethane, m3/year; is
the specific yield of biomethane in the biogas reactor, m3
CH4/m3 CH4 per day.
The annual electricity production based on the obtained bi-
omethane can be determined by the expression:
−=
100
1
100
··278.0
44
CCEE
CHCHEL
kk
QVW , (10)
where WEL is the annual electricity production, kWh/year;
0.278 kWh/MJ is the conversion factor of MJ to kWh; QCH4
is the calorific value of biomethane, MJ/m3 CH4; kE – the ef-
ficiency of electricity production by a cogeneration (gas in-
ternal combustion engine with an electric generator) plant,
%; kCCE – the coefficient of electricity consumption for the
internal needs of a biogas plant, %.
An example of calculating the volume of settling tanks,
sludge yield and indicators of the biotechnological process
is given in Fig. 11.
The calculation shown in this figure demonstrates that with
the volume of wastewater wash water of mechanical filters
Q1=3 l/min and in the case of returning the sediment from
both settling tanks to the RAS system, in the case of effi-
cient operation of the biological treatment system of recir-
culated water, the amount of sediment that requires fur-
ther disposal is 49,7 m3/year. This is about 3% of the total
volume of wastewater wash water of mechanical filters.
The sediment from settling tanks can be used in hydropon-
ics systems for growing vegetable crops or for irrigation
when growing field crops, and the sediment after two-stage
settling can be used for the production of biomethane and
electricity based on it. Thus, the feasibility of creating a set-
tling system based on two settling tanks has been estab-
lished: the first one is for settling wastewater wash water
of mechanical filters; second - for settling the sediment ob-
tained at the first stage of settling, which will allow reduc-
ing the amount of sediment for further disposal to 3% of
the amount of wastewater flushing water from mechanical
filters.
The results of the study of settling the wastewater from
mechanical filters using RAS during the cultivation of Clarias
gariepinus confirmed the possibility of carrying out this pro-
cess with high efficiency and using simple equipment. It
should also be noted that the efficiency of wastewater set-
tling can be ensured, provided that the sludge and settled
wastewater are further utilized. This review of scientific
publications also focuses on the efficiency of settling the
wastewater from mechanical filters during the cultivation
of tilapia, shrimps, rainbow trout, and the use of other RAS.
The use of mechanical filters for settling wastewater and
the resulting sludge using measuring cylinders and experi-
mental settling tanks showed a significant difference in the
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Відновлювана енергетика. №1/2025 | Біоенергетика
results obtained. This is due to the dynamic mode of sedi-
mentation in the settling tank, which operated in a contin-
uous mode, as well as the compaction of the sludge under
the influence of the weight of the sludge and wastewater
in the settling tank. This should be taken into account when
using different types of decanters for settling studies, as the
results obtained will be only preliminary in comparison with
the use of settling tanks.
Fig. 11. Scheme and indicators of the biotechnological process of processing wastewater wash water of mechanical fil-
ters
The TSS content in the settled wastewater after the settling
tank was found to be 0.07±0.015 g/l, while the TSS content
in the recirculating water after the mechanical filter was
0.057±0.023 g/l. Thus, the wastewater settling after the
settling tank corresponds with a slight excess to the recir-
culation water after the mechanical filter. This situation al-
lows the settling tank to be used to replace the mechanical
filter for RAS during the cultivation of Clarias gariepinus. Re-
placing the mechanical filter with a settling tank will make
it possible to remove only sludge from the RAS, the amount
of which, based on the analysis of settling in measuring cyl-
inders, will be no more than 7% of the amount of waste
wash water from mechanical filters. Given that the TSS con-
tent in the sludge obtained in the settling tank was
32.453±0.663 g/l, which is approximately 30 times higher
than when settling in measuring cylinders, the amount of
sludge that will need to be disposed of will be drastically
reduced. This will significantly reduce the environmental
impact. However, on the other hand, replacing the me-
chanical filter with a settling tank is only possible for RAS
with small volumes of recycled water. For comparison, sim-
ilar results were obtained by the authors of [11, 16].
Studies have shown the feasibility of creating a settling sys-
tem based on two settling tanks: the first one for settling
the waste wash water from mechanical filters; the second
one - for settling the sludge obtained at the first stage of
sedimentation. The sludge obtained at the first stage of
sedimentation can be returned to the RAS system in case of
efficient operation of the biological purification system of
recycled water.
The introduction of anaerobic digestion for biogas produc-
tion is often hindered by the lack of sufficient energy and
economic efficiency [17]. I have shown in my previous study
[18] that in the vast majority of cases, this is due to insuffi-
cient biomass preparation for anaerobic digestion. It is
known that the biogas produced is only sufficient to main-
tain the required anaerobic digestion temperature when
the biomass moisture content is 96%, and the optimum bi-
omass moisture content is 90-92%. Studies have shown
that it is possible to achieve a given concentration of TSS in
the biomass for further anaerobic digestion by using two
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Відновлювана енергетика. №1/2025 | Біоенергетика
settling tanks, one for settling the mechanical filter rinse
water and the other for settling the resulting sludge from
the first settling stage. Such a two-stage settling of the me-
chanical filter flush water in the RSA during the cultivation
of Clarias gariepinus made it possible to obtain a sludge
moisture content of 92% for 15 days with a probability of
69.23%, which is the level of the sludge entering the anaer-
obic digestion process. As for the sludge moisture content
of 93%, it can be achieved with a probability of 92.31%.
Thus, the moisture content of the biomass is acceptable for
starting the anaerobic digestion process in the RSA.
Conclusions
1. The results of studies on the settling of wastewater
from mechanical filters using RAS during the cultivation
of sharptooth catfish confirmed the possibility of carry-
ing out this process with high efficiency and using sim-
ple equipment in case of further utilization of sludge
and settled wastewater.
2. The TSS content in the settled wastewater after the set-
tling tank was 0.07±0.015 g/l, while the TSS content in
the recirculating water after the mechanical filter was
0.057±0.023 g/l, which allows the use of the settling
tank to replace the mechanical filter for RAS during the
cultivation of sharptooth catfish. Such a replacement
will make it possible to remove only the sludge from the
settling tank from the RAS, the amount of which, based
on the analysis of settling in measuring cylinders, will be
no more than 7% of the amount of mechanical filter
wash water.
3. It was found that the average value of TSS removal effi-
ciency in the measuring cylinders was 99.2%, and in the
settling tank - 91.9%. This indicates the feasibility of cre-
ating a settling system based on two settling tanks: the
first one for settling the waste wash water of mechani-
cal filters; the second one - for sedimentation of sludge
obtained at the first stage of sedimentation, which will
significantly reduce the amount of sludge for further
utilization by anaerobic digestion to produce biogas.
4. It was found that the sludge after settling in the lagoon
can be used as an organic fertiliser for vegetable crops
when diluted with water in a ratio of 1:4.
5. The biogas yield for a typical RSA system was deter-
mined on the basis of the methodology for calculating
the parameters of the process of preparing aquaculture
wastewater sludge for methane digestion, with a me-
thane yield of 1 m3 per 1 m3 of wastewater.
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|
| id | veorgua-article-518 |
| institution | Vidnovluvana energetika |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2026-07-19T01:15:27Z |
| publishDate | 2025 |
| publisher | Institute of Renewable Energy National Academy of Sciences of Ukraine |
| record_format | ojs |
| resource_txt_mv | veorgua/9a/bca9e1c5d880892c39a2b2a3cf2dfa9a.pdf |
| spelling | veorgua-article-5182026-07-18T06:32:21Z SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS Golub , G. Yaremenko , O. Marus , O. Tsyvenkova , N. Chetveryk , H. recirculating aquaculture systems, settling tank, total suspended solids, sludge, settled wastewater. The aim of the article is to study the parameters of wastewater settling in recirculating aquaculture systems (RAS) during the cultivation of African sharptooth catfish. The volumes of freshwater fish farming using such systems are steadily increasing. However, RAS have a disadvantage in that they require the disposal of contaminated wastewater generated during the washing of the filter meshes of the me-chanical filters through which the recirculated water passes. It is advisable to use wastewater settling tanks in RAS be-cause they are quite effective in removing total suspended solids (TSS) and have a relatively low cost, which leads to their high economic efficiency. The results of studies on the settling of wastewater from mechanical filters using RAS during the cultivation of Clarias gariepinus confirmed the possibility of carrying out this process with high efficiency and using simple equipment in the case of further utilization of sludge and settled wastewater. The TSS content in the settled wastewater after the settling tank was 0.07±0.015 g/l, while the TSS content in the recircu-lating water after the mechanical filter was 0.057±0.023 g/l, which allows the use of the settling tank to replace the mechanical filter for RAS during the cultivation of Clarias gariepinus. Such a replacement will make it possible to remove only the sludge from the RAS, with its amount not exceeding 7 % of the amount of mechanical filter flush water. The expediency of creating a settling system based on two settling tanks was established: the first one for settling the wastewater from mechanical filters and the second one for settling the sludge obtained at the first stage of settling, which will reduce the amount of sludge for further utilization to 3 % of the amount of waste rinse water from mechanical filters. The settled wastewater from the settling tanks can be used in hydroponics systems or for irrigation, and the sludge after two-stage settling can be used for biogas production. Institute of Renewable Energy National Academy of Sciences of Ukraine 2025-04-01 Article Article application/pdf https://ve.org.ua/index.php/journal/article/view/518 10.36296/1819-8058.2025.1(80).148-158 Vidnovluvana energetika ; No. 1(80) (2025): Scientific and applied Journal renewable energy ; 148-158 Возобновляемая энергетика; ##issue.no## 1(80) (2025): Scientific and applied Journal renewable energy ; 148-158 Відновлювана енергетика; № 1(80) (2025): Науково-прикладний журнал Відновлювана енергетика; 148-158 2664-8172 1819-8058 10.36296/1819-8058.2025.1(80) en https://ve.org.ua/index.php/journal/article/view/518/425 Copyright (c) 2025 G. Golub , O. Yaremenko , O. Marus , N. Tsyvenkova , H. Chetveryk https://creativecommons.org/licenses/by-nc-nd/4.0 |
| spellingShingle | recirculating aquaculture systems settling tank total suspended solids sludge settled wastewater. Golub , G. Yaremenko , O. Marus , O. Tsyvenkova , N. Chetveryk , H. SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS |
| title | SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS |
| title_full | SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS |
| title_fullStr | SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS |
| title_full_unstemmed | SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS |
| title_short | SEDIMENTATION PROPERTIES OF AQUACULTURE WASTEWATER SLUDGE DURING THE CULTIVATION OF CLARIAS GARIEPINUS |
| title_sort | sedimentation properties of aquaculture wastewater sludge during the cultivation of clarias gariepinus |
| topic | recirculating aquaculture systems settling tank total suspended solids sludge settled wastewater. |
| topic_facet | recirculating aquaculture systems settling tank total suspended solids sludge settled wastewater. |
| url | https://ve.org.ua/index.php/journal/article/view/518 |
| work_keys_str_mv | AT golubg sedimentationpropertiesofaquaculturewastewatersludgeduringthecultivationofclariasgariepinus AT yaremenkoo sedimentationpropertiesofaquaculturewastewatersludgeduringthecultivationofclariasgariepinus AT maruso sedimentationpropertiesofaquaculturewastewatersludgeduringthecultivationofclariasgariepinus AT tsyvenkovan sedimentationpropertiesofaquaculturewastewatersludgeduringthecultivationofclariasgariepinus AT chetverykh sedimentationpropertiesofaquaculturewastewatersludgeduringthecultivationofclariasgariepinus |