Báo cáo khoa học nông nghiệp " Improvement of water quality of outflows from ponds to waterways " pot

Technical Report (CARD 023/06 VIE)

Improvement of water quality of outflows from ponds to waterways


Cao van Phung
1
and Bell R.W.
2

1. Cuu Long Rice Research Institute, O’Mon, Cantho Province, Vietnam. Email:

2. School of Environmental Science, Murdoch University, Murdoch 6150, Australia.


Summary
In the Mekong Delta of Vietnam, water quality is critically important for human
health and wellbeing as well as ecosystem function. Catfish farming in the Mekong
Delta, which is mostly confined to the provinces of An Giang, Cantho, Dop Lap and
Vinh Long, is considered to be a significant source of water pollution in those
provinces. A survey of 240 catfish farmers in An Giang and Cantho provinces, and of
Clarias fish farmers in Cantho Province confirmed that discharge of waste water and
solid waste from these operations is directly to public water sources in canals and
rivers in over 75 % of cases. Water consumers in the areas where catfish ponds are
prevalent had a strong perception that catfish pond discharge was the major cause of
degraded water quality. Water sampling at 2 points of discharge in An Giang over a
5-month period confirmed that discharge causes canal water to exceed discharge
standards for COD, TSS and NH
4
. However, even background levels were generally
above the levels set for household use in Vietnam. Two case studies showed that

passing Clarias fishpond waste water or water laden with catfish waste through rice
fields improved its quality substantially, by decreasing TSS, COD, BOD, N and P.
Coupled with evidence that catfish and Clarias fish pond wastes can be an effective
partial replacement of rice fertiliser requirements, these results indicate that recycling
solid waste and waste water from fishponds could be promoted as an effective
technology for improving quality of public water ways. The recent decline in density
of active catfish ponds in the Mekong Delta, during to poor profitability, may be
having a beneficial effect on water quality.

1
Corresponding author Cuu Long Rice Research Institute, O’Mon district, Cantho city-Vietnam.
Phone No (84) 710861452. Fax: (84) 710861457. Email:
Contents
 Introduction
 Baseline study
 Water quality in canals receiving discharge from fish ponds
 VAC water quality study
 Water quality after passage through padi fields
 Fishpond waste loading in rice padi fields
 A strategic framework for determining application rates for fishpond waste


Introduction
The main aim of the present project was to develop effective methods for decreasing
pollution of water from catfish pond waste discharge. In other reports, the treatment
of solid waste and waste water has been examined (Cao et al. 2010a,b). In the present
report, the emphasis is on quality of water in fishponds and in water affected by the
discharge. The report considers water quality issues associated with catfish as well as
for Clarias fish raised in the VAC system.
In this report, the results of a baseline study were used to demonstrate forms of water

pollution caused by discharge from catfish ponds, from the perspective of farmers,
and also by field sampling at two locations.
Water quality sampling was then used to determine the effectiveness of wastewater
treatment by recycling it through rice padi fields. Finally, there is a discussion of the
principles that are expected to govern future use of waste water treatment through
discharge to rice padi fields.
Baseline study in Cantho and An Giang
A survey was conducted in October-November 2007 in Cantho city and January-
February, 2008 in An Giang province. In total 240 questionnaires completed by
stakeholders (rice and fish farmers were sampled in equal numbers) were collected (2
districts/province) (Table 1, 2). The baseline survey was reported fully in April 2009
(Cao et al. 2009). Key findings of the study are discussed below to highlight the main
water quality concerns, as perceived by the farmers. In addition, the survey reported
on the prevalence of fish and human diseases, which may be indicators of water
quality.
Water quality
Generally half or less of the farmers were satisfied with water quality, and 15-24 %
classified it as bad. Yet, only 5-8 % of fish farmers had their own settling pond and
the vast majority discharged waste to canals and rivers directly without treatment.
Phan et al. (2009) in a survey of 89 catfish farms along the main rivers of the Mekong
Delta found only 3 % of farmers reported having settling ponds. However, 15-24 % of
catfish farmers in the present study used rice padi fields for discharge of waste water.
Waste from fishpond was perceived to be the major source causing pollution in An
Giang while in Can Tho 37 % of respondents considered catfish ponds the major
cause for poor water quality. Part of the reason for the different responses may be that
50 percent of respondents in An Giang used river water for household purposes while
a similar proportion of people in Cantho utilized tubewell water (Cao et al. 2009).
Farmers were also concerned of threats to fish culture from pesticides discharges
from paddy fields.
The type of feed and additives supplied to catfish may have a bearing on water

quality. In Cantho province the majority of catfish farmers supplied pellets to the fish,
while in An Giang, most farmers prepared their own feed (Table 3). Both
supplemented the feed with vitamins, enzymes and minerals. According to De Silva et
al. (2010) farm-made feed increases P discharge but not N discharge relative to
pelleted feed.
Table 1. Farmers’ perception of water quality in surface water close to their residence,
and current waste discharge practices for catfish farmers. Values are percentages for
respondents, where N=60 for each of the 4 groups of respondents.
Items
Cantho An Giang
Fish culture Rice
culture
Fish culture Rice
culture
Water quality (%)
- Good
- Medium
- Bad

30
52
18

55
22
23

51
25
24

46
39
15
Wastewater discharge
- River or canal
- Paddy
- Settling pond

80
15
5

100

68
24
8

100

Table 2: Reasons given for poor quality of water for irrigation and for household use
by catfish and rice farmers. Values are a percentage of respondents where N= 120 in
each province.
Items An Giang (%) Can Tho (%)
Waste from fishpond 91 37
Pesticides from paddy 9 15
Others 0 48

For water treatment in catfish culture, nearly 50 % of farmers at both sites had used

Vikong, BKC, Bioca, Yulai, Aquapure, Prawbac and some others (Table 4). They
also used antibiotics to control mainly bacterial diseases in the digestive tract of
catfish. Some common antibiotics like Amoxycilline, Cotrim, Penicilline,
Kanamycine, Oxamet, Tetracylline.were used by mixing into feeds in An Giang (96
%) or putting directly into water by 40% of farmers in Cantho. Almost all of farmer
reported using antibotics at recommended dosage.
For control of algae in fishponds, 93 % of farmers in Cantho and about 66 % in An
Giang province applied copper sulphate, BKC, Vikong, Chlorine, juka and even salt
or lime (Table 4). It is worth noting that fishpond solid waste generally contain
significant but not excessive levels of Cu (Table 14). Amongst the treatment
chemicals used, BKC and chlorine were the most common both in An Giang and
Cantho.
Table 3: Characteristics of feed and additives used in catfish farms. Values are a
percentage of respondents where N= 60 in each province.
Items Can Tho An Giang
Type of feed (%)
Pellet (from industry) 55 27
Ball (farm-made produce) 37 58
Mixed (pellet+ ball) 8 15
Use of additive 88 66
Types of additive
Vitamins 64 69
Enzyme 57 74
Minerals 23 36

Table 4: Chemicals used for water treatment in catfish farms. Values are a percentage
of respondents where N= 60 in each province.
Items Cantho An Giang
Bio-products (%) 48 43
Antibiotics (%) 72 54

Mixing to feed 60 96
Mixing with fishpond water 40 4
Water treatment 97 100
BKC 21 28
Vikong 43 5
Copper sulfate 7 8
Chlorine 21 10
Others 8 49

All farmers normally spread lime or salt along the sides and bottom of ponds after
making new ponds or after draining out the water from harvested ponds. Dosages of
lime and salt varied from 300-400 kg/ ha. Pond are kept dried for 3-5 days before re-
filling with water for a new crop of fish. More than 95 % of farmers changed and/or
added water to fish ponds regularly (about 1/3 volume of pond daily) and there was
about 50 % farmers at both sites practiced bottom of fishpond cleaning by pumping
out sludge during the growth of catfish. In addition, all of the farmers pumped sludge
out of their pond after harvesting fish (Table 5). The practice of discharge of solid
waste directly to water sources was practiced by 63-70 % of catfish farmers (Table 6).
Most farmers had a handheld pH meter to monitor water quality (Table 5). Other
ways to detect water quality were by observation of water colour or smelling the
odour of water. Farmers conclude that oxygen is deficient if most of fishes come to
the pond surface during the early morning.
Table 5: Water management in catfish culture. Values are a percentage of respondents
where N= 60 in each province.
Items Cantho An Giang
Renewed fishpond water (%) 100 100
Lime (kg/ha) 425 350
Salt (kg/ha) 325 300
Dried bottom of pond (%) 100 100
Days of drying 3.5 4

Filled up and/or changed water
periodically (%)
98 100
Volume of added or changed water 1/3 1/3
Water quality monitoring

pH 95 90
Color or odour 8 12
NH
3
and Oxygen 62 65

Farmers raising catfish usually clean organic wastes in fishpond to prevent disease
spread to fish (Table 6). Survey data indicated that most of sludge was discharged
directly to water source (63 %), other uses of sludges including filling up low
elevation areas or mulching of fruit trees (37 %). Sludge removal occured 1 or 2 times
(97 %), but more than 2 times was uncommon. Sludge removal was by using bottom
pond pump (86%) or draining out all water in fishpond then removing sludge by hand
(14 %)
Fish disease
As shown above, most of the surveyed farmers in Cantho and An Giang discharged
waste directly into canals and rivers. The discharge of waste from fishponds may be a
significant vector for the spread of infection to neighbouring ponds. In the baseline
survey, there was a high prevalence of fish diseases reported by farmers in both
Cantho and An Giang provinces (Table 7). Over 65-73 % of fishpond farmers
experienced symptoms of haemorhage or swelling head in their fish, especially at
fingerling stage.
In Chau Phu, Phu Tan, Thot Nhot and Cantho districts there is a concentration of
fishponds within a relatively small area, usually in locations with good access to river
or main canal water. All fishponds along a section of the canal or river are obtaining

water for catfish ponds from a common source. Hence disease organisms may spread
readily from one operator to another. The dispersion of disease-causing organisms in
the canals and river has not been investigated in the present study, although the
dispersion plume for TSS in waste water was at least 600 m from the point of
discharge. While the farmers reported that effective control measures for major
diseases were generally available, reducing the spread of the disease through better
water quality may be an outcome from more effective treatment of wastes from fish
ponds.

Table 6: Solid waste management on catfish farms. Values are a percentage of
respondents where N= 60 in each province.
Items Cantho An Giang
Removing sludge purpose

Sanitation 51 65
Disease control 46 35
Methods of discharge

Pumping sludge 86 89
Removing by hand 14 11
Location of sludge discharge

Water course 70 63
Filling up low lying surface 30
Mulching of fruit trees 37
Times of sludge discharge

One time 37 35
Two times 60 59
Three times 3 6

Sludge discharge schedule

During raising time (%) 50 50
After harvesting 60 75

Reports on human diseases caused by water pollution:
Investigations on human diseases attributable to water pollution were completed in
OMon and Thot Not districts of Cantho City and in Chau Phu and Phu Tan districts of
An Giang province. According to data collected at the Preventive Medicine Center in
O Mon and Thot Not districts, during 2006 and 2007, prevalence of dengue increased.
This disease re-occurred at the end of 2008 mainly in the south of Vietnam; the
situation was most severe in An Giang and Dong Thap provinces (Department of
Preventive Medicine – MOH Vietnam, 2008). Data in Table 8 also indicated that
diahorrea in 2007 was higher than in 2006.
Dengue, diahorrea and dysentery in Phu Tan were much higher than those in Chau
Phu and they had a tendency to increase (Table 9). These might be related to the
closed dike system at Phu Tan district where water in canals could not be drained. As
reported in the Baseline survey, most of people in An Giang province (57 %) are still
using river or canal waters as the main source for household use.
Table 7: Diseases of catfish, occurrence stage and control measure. Values are a
percentage of respondents where N= 60 in each province.
Items Cantho An Giang
Symptoms

Haemorhage 55 67
Swelling head 10 6
Slimy loss 0.1 4
Occurrence stage

Fingerling up to 1 month 72 88

From 1-3 month old 17 8
More than 3 month old 11 4
Effective Control

Haemorrhage 92 82
Swelling head 77
Slimy loss 100

While not a rigorously collected set of data, the information on human disease trends
in two districts with a high concentration of catfish ponds may be related to the rapid
spread of fishponds in recent years.

Table 8: Disease reports to Preventative Medical Centre in 2006 & 2007 at O Mon
and Thot Not districts
Disease O Mon Thot Not
2006 2007 2006 2007
Dengue 179 237 246 546
Typhoid fever 2 0
Diarrhea 159 220 3210 2822
Dysentery 0 1 138 31

Table 9: Disease reports to Preventative Medical Centre in 2006 & 2007 at Phu Tan
and Chau Phu districts
Disease Phu Tan Chau Phu
2005 2006 2007 2005 2006 2007
Dengue 112 300 495 411 595 438
Typhoid fever 169 153 71
Diarrhea 752 957 1502 189 187 200
Dysentery 202 250 463 21 10

Water quality in canals receiving discharge from fish ponds
Three study areas were selected for water quality sampling: Chau Phu and Phu Tan in
An Giang Province; Thot Not in Cantho district. Study areas have a high
concentration of catfish ponds (Phan et al. 2009). Water quality sampling was also
conducted in Binh Thuy and Phong Dien districts near Cantho city, in areas practicing
VAC.
Water samples at Chau Phu and Phu Tan during the dry and wet season (especially at
flooding time) were collected at the flush gate, 100 metres up-stream and 100, 200,
300, 400 and 500 metres down-stream. These were measured for pH, EC, TSS, COD
and NH
4
-N. Samples of water in the canal were collected at 15-day intervals for 5
months both before discharge and 1-2 hours after discharge had occurred (December
to April).
Data in Figure 1 shows that pH of waters before opening the flushing gate was higher
than after discharge; however, the values were not significantly different (P < 0.05) in
both dry and wet seasons. During flooding time, the difference between these two sets
of data were even smaller than those taken in the dry season because the dilution
factor during the flooding time was much greater (data not shown). Generally pH in
the canal after waste water discharge was within limits accepted for living use
(TCVN-5492-2005).
Electrical conductivity taken after opening the canal gate to release fishpond waste
water were statistically higher than values measured before (Fig. 2). However, as with
pH, EC values during flooding time were not significantly different between times of
water sampling.
Unlike pH but similar to EC, average COD values were very significantly different
before and after opening the flushing gate (Fig. 3). The increase in COD persisted
even during flooding time. On average, the COD levels declined with distance from
the point of discharge. At 500 m from the discharge, they had almost halved but were
still well above the pre-release values.

Figure 1: pH of water in canal before and after opening the flushing gate to discharge
fishpond waste water. Values are means of 40 values sampled at 15-day intervals for
5 months (December to April) in two sites of An Giang Province (Phu Tan and Chau
districts).


Figure 2: Electrical conductivity (EC) of water in canal before and after opening the
flushing gate to discharge fishpond waste water. Values are means of 40 values
sampled at 15-day intervals for 5 months (December to April) in two sites of An
Giang Province (Phu Tan and Chau districts).

Normally COD of liquid waste from fish ponds exceeded values allowable for living
use (below 10 mg/L) except during the flooding time (October-January). Indeed, even
the values in the main canal before discharge of waste water were generally > 10 mg
/L. All values, before and after discharge were in the range accepted for water
discharge from fish ponds (35 mg/L <COD<100 mg/L).

0
50
100
150
200
250

300
350
-100 0 100 200 300 400 500
m
Ec (uS/cm)
Before
After

6.9
7
7.1
7.2
7.3
7.4
7.5
-100 0 100 200 300 400 500
m
pH
Before
After

Figure 3: COD (mg/L) of water in canals before and after opening the flush gate to
discharge fishpond waste water. Values are means of 40 values sampled at 15-day
intervals for 5 months (December to April) in two sites of An Giang Province (Phu
Tan and Chau districts).
Ammonium levels were high at the flushing gate and generally remained higher than
the before-discharge level for up to 500 m down-stream of the gate (Fig. 4). These
concentrations generally were higher than the Vietnamese Standard allowable for
living use (less than 0.5 mg/L) based on TCVN5492-2005 but they were still
acceptable for waste discharge according to announcement No. 2 of the Ministry of

Fisheries.
In the case of nitrite and nitrate levels taken at different time intervals before and after
opening the flushing gate, they were lower than allowable values for living use even
in the dry season when nitrite and nitrate levels in canal waters were more
concentrated than in the flooding season (data not shown).
On average, TSS values in canal waters after the opening the flushing gate (261± 64
mg/l) were five times higher than those recording before (54 ± 20 mg/l). Figure 5
demonstrated that most of the TSS values collected in canals under the effect of waste
water discharge were 10-13 times higher than TCVN 5942-1995 (column A- TSS less
than 20 mg/l). Even the allowable value (less than 80 mg/L) for waste from fish pond
discharged to water sources under Announcement No 02 of the Ministry of Fishery
was generally exceeded.
Hence these sampling results indicate clearly measurable effects of wastewater on
canal water quality for at least 100 m up-stream and 500 m downstream of the
discharge. The measurable decline in water quality was attributed to elevated COD,
NH
4
and TSS.
These water quality data support the results of the baseline survey of farmers. In
Cantho and An Giang, 7-36 % of farmers assessed water quality to be bad, and of
these 37 to 91 % attributed the poor water quality to catfish pond discharge. A higher
proportion of farmers were troubled by poor water quality in An Giang than in
0
20
40
60
80
0 100 200 300 400 500
Distance (m)
COD

After
Before
Cantho, presumably because a higher proportion of these households still rely on river
or canal water for household use.


Figure 4: Ammonium levels of water in canal before and after opening the flushing
gate to discharge fishpond waste water. Values are means of 40 values sampled at 15-
day intervals for 5 months (December to April) in two sites of An Giang Province
(Phu Tan and Chau districts).


Figure 5: Total suspended solids levels of water in canal before and after opening the
flushing gate to discharge fishpond waste water. Values are means of 40 values
sampled at 15-day intervals for 5 months (December to April) in two sites of An
Giang Province (Phu Tan and Chau districts).
Wet season flooding conditions ameliorated the effects but still did not prevent the
water from exceeding discharge standards for wastewater set by TCVN 5942-1995 or
the allowable value for waste from fish pond discharge under Announcement No 02
of the Ministry of Fishery. If adjacent flushing gates on canals discharging fishpond
waste are less than 500 m apart there will be cumulative effects on water quality.
While Bosma et al. (2009) have argued that the aggregate effect of catfish pond waste
on the quality of water in the main streams of the Mekong River will be insignificant,
0.0000
0.0200
0.0400
0.0600
0.0800
0 100 200 300 400 500
Distance (m)

Ammonia (mg/L)
After
Before
0
50
100
150
200
250
300
-100 0 100 200 300 400 500
Distance (m)
TSS (mg/L)
After
Before
in the tributary canals and in the closed-dyke system, measurable decline in water
quality is evident in the Phu Tan and Chau district sampling. Indeed, after discharge,
water sampled in canals was below the discharge standards for TSS, COD and NH4.
In the present study we did not study the longevity of these peaks in water quality to
ascertain whether they persist for minutes, hours or days. The persistence of the peaks
in water pollution due to fishpond discharge are likely to relate to the level of
pollution in the discharge water, and the volume of discharge in relation to water flow
in the canal. Small canals in the dry season with limited flow for flushing are most
likely to suffer declines in water quality.

Water quality in catfish ponds:
Investigations were conducted from December 2007 to July 2008 at Phu Tan and
Chau Phu districts of An Giang province on water quality in 12 catfish ponds that
used two types of feed, namely manufactured pellets and farm-made feed.
Results showed that pH values were still within the acceptance limit (7.0-8.0) even

though there were differences between ponds using pellet and farm-made feed (data
not shown). All catfish ponds were slightly higher in turbidity than the allowable limit
for fishpond discharge (80 mg/L; data not shown). Ponds at Phu Tan were lower in
TSS than that in Chau Phu because most of ponds in Phu Tan are located near a large
river so they can readily change pond water with better quality water (data not
shown). The TSS and BOD were lower when farmers used pellets than farm-made
feed but not significantly. The COD at both sites was higher than allowable limit for
waste water discharge to surface water which is consistent with the canal discharge
sampling (Fig. 3).
Table 10: Comparison of water qualities in catfish ponds in An Giang province (Phu
Tan and Chau Phu districts) using different feeds. Values are means of 4 ponds for
farm-made feed and 8 for pellets.


TAN
(mg/L)
NO
3
-

(mg/L)

PO
4
3-
(mg/L)

TN
(mg/L)
TP

(mg/L)
BOD
(mg/L)
COD
(mg/L)
TSS
(mg/L)
Pellet 0.145 0.032 0.134 0.662 0.416 13.3 69.4 86.9
Farm-
made 0.138
0.077 0.104 0.569 0.318
18.0
73.3
90.7
LSD
(5%) 0.017
0.012 0.012 0.246 0.093
1.94
14.5
15.3
Ponds using pellets had higher values of total available N and TP than farm-made
feed since the pellets were much more concentrated in nutrients (Table 10). However,
the nitrite and nitrate concentrations were lower in the ponds with pelleted feed
rations.
Total available N, nitrite, nitrate, phosphate, total N and total P increased steadily
with time after stocking the ponds and they were not much different between the two
sites (data not shown).

VAC water quality study
VAC systems are located mainly on high elevation areas along rivers. These are areas

of silty soils having high infiltration capacity which are suitable for growing
vegetables and fruit crops.
A small survey was carried out on the VAC system in two districts of Cantho city,
namely Binh Thuy and Phong Dien. Orchards or fruit tree plantations were mainly
kept on elevated soil adjacent to ditches or ponds. Longan, milky fruit, durian,
rambutan or oranges/lime are the most common fruit trees in these areas. Duck was
raised in the largest numbers followed by chicken or pigs. Tilapia, silver carp and
Osphonerrus gouramy are mainly raised in ditches between raised beds of orchards.
These species are stocked at low density (<30 fish/m
2
) and they did not cause
noticeable water pollution. Catfish other than Pangasianodon hypophthalmus namely
Clarias botrachus, Clarias gariepinus, Clarias macrocephalus or hybrids of the first
two species were raised in ponds for sale in local markets only.
There was not much variation in size and depth of fish ponds between two locations
(Table 11). Since pond size is not so large, household labour can manage almost all
activities relating to raising fish. Clarias fish reaching 200-250 g each (commercial
size) are sold for meat. Bigger or smaller than commercial size are sold at reduced
price. Most of Clarias fish raising farmers make farm-made feed by purchasing
ingredients from their neighbours at 2,500-3,000VND/kg. Clarias fish could be sold at
the price of 12,000-20,000VND/fish depending on demand of local market.
Cultivation of Clarias sp. (or walking fish) caused higher surface water pollution than
Pangasianodon sp. because waste waters were mainly discharge to small rivers or
canals. The waste water from Clarias sp. ponds are typically black in colour, very
high in turbidity (TSS), BOD and COD, and stale smelling due to anoxia (Table 12).
The most striking effect of liquid waste from Clarias pond is increased death to
naturally-occurring fish, possibly due to very low oxygen levels in the waste water.
Sludge accumulation in Clarias fishponds is not significant because most of solid
wastes remain suspended by the high density of fish present in ponds (about 120-150
fish per m

2
of pond surface). Solid wastes are mixed with liquid and then discharged
to the environment. However, suspended particles will then settle along water ways
and the canals which obstructs navigation.
Table 11: General information on Clarias sp. Culture in two districts of Cantho
Province.
Items Binh Thuy Phong Dien
Type of fish culture (%)

Clarias sp. (%) 86 92
Others 14 8
Average time for fish raising (month) 3.5-5 3.5-5
From seed – fingerling (months) 1.2 1.3
From fingerling – selling size (months) 7.2 7.5
Size of fishpond (m
2
) 500-3000 1000-3000
Depth of fishpond (m)

Fingerling raising 1.2 1.1
Selling size 1.8 1.6
Density of fish/m
2
90-120 70-100
Conversion factor (feed/meat) 3.5 3.6

Table 12: Water quality of Clarias fishponds sampled in Binh Thuy and Phong Dien
districts of Cantho province.
Items Binh Thuy Phong Dien
BOD

5
(20
0
C) (mg/L) 120 104
TSS (mg/L) 312 258
Dissolved oxygen (mg/L) 0.05 0.1
COD (mg/L) 128 117

All of the Clarias fish ponds in the survey reported that they deliver liquid waste
directly to waterways resulting in high pollution in these areas. The conflict between
Clarias fish raising farmers and people living in these areas are quite common so it is
quite difficult to convince them to cooperate in the treatment of fishpond waste.
Although local authorities had issued regulations banning direct discharge waste from
fishpond to waterways this situation seems out of control. Since most of farmers
raising Clarias fish do not have settling ponds or paddy for land application, the water
pollution in these areas is very critical.
Although Clarias fish can survive in polluted water, it is also subjected to some
bacterial diseases but the percentage of loss is much lower than for Pangasianodon
sp. fish. Chemicals and biochemicals used for cultivation of Clarias fish are more or
less similar to those that are used for catfish culture (Table 13).

Table 13: Chemicals used for water treatment and diseases control of Clarias fish
culture. Values are the percentage of farmers using a product.
Items Binh Thuy Phong Dien
Bio-products: biozyme, US-zyme, biomix 56 34
Water treatment (sanitation) 76 82
BKC, 32 24
Vikong 16 22
Copper sulfate, potassium permanganate 39 48
Chlorine, formol 7 110

Soil treatment (zeolite, lime) 62 75

Water quality after passage through padi fields
The effect of passage of fishpond waste water through padi fields on water quality
was assessed at two locations, Chau Phu and Phong Dien. In the former case the water
had first passed through a main canal receiving discharge from a large number of
catfish ponds. It was then directed through rice fields to another main canal. Water
samples were collected along this water course. In the second case, water was
sampled from a Clarias fishpond and then at a series of downstream locations to
determine changes in composition and quality.
In Chau Phu, water samples were collected along the course of water from a main
drain that received fishpond discharge, through small drains adjacent to rice padi
fields, settling ponds and small wetlands to discharge into another main canal (Fig. 6).
Water samples were analysed for ammonium-N, nitrate-N, total N, TSS, COD and
total P (Table 14).
During the passage of the water in Chau Phu from the fishpond-polluted main canal to
the discharge canal, levels of N and P dropped 3- to 4-fold (Table 14). The water
COD also dropped from over 230 mg /L to about 100 mg/L. There was however,
only a small decrease in TSS with the values at VTT10 and VTT11 still being about
100 mg/L and above both the standard for fishpond discharge (80 mg/L) and for
household use (20 mg/L).
In the settling ponds and wetland, the composition of water hardly changed compared
to that in the small drains upstream, indeed ammonium-N and nitrate-N increased.


Fig. 6. Water sampling points in Chau Phu from the main canal (VTT1, VTT2- top
left), small field drains (VTT3,4,5), a settling pond (VTT6), a small wetland
(VTT7,8), a field drain (VTT9) and in the main canal (VTT10,11).

Water from Clarias fishponds at Phong Dien was directed through rice padi fields

before discharge to the canal. Water sampling was conducted at 15 locations (Fig. 7)
from the fishpond to the main canal and analysed for pH, EC, BOD, COD, TSS, Total
N, nitrate-N and Total P (Table 15).
Table 14. Water composition at sampling points shown in Fig 6.

NH
4

(mg/L)
NO
3

(mg/L)
TSS
(mg/L)
COD
(mg O
2
/l)
TP
(mg/L)
TN
(mg/L)
VTT1 0.22 0.16 142 248 1.16 1.34
VTT2 0.16 0.54 128 232 1.14 1.28
VTT3 0.15 0.44 122 214 1.08 1.12
VTT4 0.17 0.64 118 220 1.02 0.94
VTT5 0.22 0.84 116 180 1.06 0.86
VTT6 0.26 1.24 115 152 0.84 0.78
VTT7 0.16 0.33 122 124 0.56 0.64

VTT8 0.22 0.35 114 128 0.62 0.82
VTT9 0.06 0.08 112 82 0.34 0.28
VTT10 0.06 0.08 98 98 0.28 0.31
VTT11 0.09 0.05 104 64 0.32 0.34

Sites 13,14,15
Fig 7. Schematic representation of sampling locations: Clarias fishpond (Grey
rectangle- Sites 1,2,3); rice padi fields (green rectangle- Sites 4,5,6); canal (open oval
shapes- Sites 7,8,9 (upstream), Sites 10,11,12 (downstream)); discharge to main canal
(blue circle- Sites 13,14,15).
The Clarias fishpond water was highly turbid, excessively high in BOD and contained
high concentrations of total N, nitrate-N and total P (Table 15). The water after
passing through the padi field had decreased 15-fold in TSS and both the TSS and the
BOD had dropped to acceptable levels for discharge. Total N and nitrate-N had both
dropped 3 to 4-fold, while total P only dropped by 25 %. The composition of water
draining from the padi field was remarkably similar to that in the nearest canal, in
both upstream and downstream directions, as well as at the discharge to the nearest
main canal.

Table 15. Water quality in Clarias fishponds in Phong Dien (Sites 1-3), after passing
through padi fields (Sites 4-6) and in downstream canals (Sites 7-15). Values are
means of 3 sampling points at each location. See Fig. 7 for a schematic of sampling
locations.
Site pH
EC
(mS/cm)
TSS
(mg/L)
COD
(mg

O
2
/L)
TN
(mg/L)
NO
3

(mg/L)
TP
(mg/L)
BOD
(mg
O
2
/L)
1,2,3 6.64 0.22 618 35 17.0 16.3 1.2 119
4,5,6 6.86 0.14 43 19 4.8 3.7 0.9 70
7,8,9 6.83 0.14 35 19 4.1 3.5 0.9 69
10,11,12 6.81 0.14 32 16 3.9 3.3 1.1 68
13,14,15 6.76 0.14 42 13 3.5 3.1 1.0 70

Fishpond Waste Loading in Rice Padi Fields
The above two case studies suggest that passage of catfish and Clarias fish waste
water through padi fields can improve water quality through filtration of sediment and
assimilation of nutrients within drains and in the padi fields. In another reports, we
have shown that the solid waste from catfish ponds and liquid waste from both catfish
and Clarias fish can be used to replace a significant proportion of the fertilizer
requirement of rice (Cao et al. 2010a,b). For the widespread use of this practice to
treat waste and improve the quality of water in canals and rivers, a number of factors

need to be considered. The loading of waste to land depends on optimising four
criteria: hydraulic loading; solids loading; chemical reactivity of the waste, and;
nutrient assimilation capacity of the system. The optimisation of these factors in
relation to fishpond waste application to rice padi fields in the Mekong Delta is
discussed below.
Fishponds generate wastewater continuously throughout the year, while the solid
waste from the bottom of the fishponds is removed infrequently, usually at harvest of
the catfish after 6 months (Table 6). Application of both types of waste is considered
below. About half of the fishponds surveyed in Cantho and An Giang, periodically
flushed out solid waste during the raising of catfish (Cao et al. 2009; Phan et al. 2009)
which probably increases the nutrients, COD, BOD and TSS levels in the waste water
discharge. The concentration of nutrients and levels of COD and BOD are much
higher in solid waste than wastewater, but the large volume of wastewater means that
total nutrient loading may be as high. With very frequenct water replacement, the
composition of waste water may not be too different from that of canal water (Bosma
et al. 2009).
In the survey of Cantho and An Giang, fishpond operators replaced 33 % of the
fishpond water daily to avoid build up of high levels of nutrients and organic matter,
and to minimise levels of infection of fish by disease organisms (Cao et al. 2009). In
another study, the range was reported to be 20-40 % replacement daily (Phan et al.
2009), although the frequency is also reported to vary with the growth cycle of the
catfish. During the early growth of catfish, less frequent changes occur and at close to
maturity more frequent changes of water occur. The ponds with more frequent
changes probably also accumulate solid waste slowly. By contrast, when fishpond
operators change water less frequently, it will result in a wastewater with higher
nutrient and carbon loading, and a greater accumulation of solid waste at harvesting.
The stocking rate of fish and age of the fish (both of which affect the amount of feed
supplied per day) will also affect the composition of the wastewater. Use of bought
pelleted feed vs farmer-produced feed may also affect wastewater and solid waste
accumulation based on differences in feeding rates, the composition of the feed, and

the rate of conversion of feed to fish biomass. According to Bosma et al. (2009) the
average feed conversion ratio (FCR) among 89 catfish farms in the Mekong Delta was
1.86, while Phan et al. (2009) report that the average was 1.69 for pellet-fed fish and
2.25 for farm-made feed. The range of FCR appears to be from about 1.45 to 3.
1. Hydraulic loading
Rice is grown as a fully irrigated crop in the dry season in the Mekong delta and as a
partially irrigated crop in the early wet season. During the main rainy season,
September- November, there is generally limited rice grown in the Mekong Delta
because of high water levels. The amount of flood water and rainwater and their
distribution during the year will be the main determinants of the hydraulic loading
possible on rice padi fields. Variation in temperature has limited effect on evapo-
transpiration (ET) rate in the Mekong Delta. Cloudiness in the main wet season may
limit ET (Table 16).
With fishpond solid waste, the hydraulic loading per unit area is small compared to
the liquid waste and hence is not considered further in this section.
Flooding of soils to prepare rice fields for land preparation requires a lot of water
(300- 700 mm according to Greenland 1997). Moreover maintaining padi fields in a
submerged condition consumes substantial water during crop growth. Depending on
ET, percolation rate and the amount of throughflow of water in fields, 1020-2860 mm
is required for a 100 day transplanted crop (Greenland 1997). Flooded rice uses 4000
kL/ t of grain produced (Greenland 1997). Hence a 6 t crop of rice in the dry season
requires 24,000 kL of irrigation water (2400 mm depth of water), while a 3 t crop in
the wet season requires only 12,000 kL (2000 mm), of which 75-100 % is supplied by
rainwater.
A typical irrigation schedule for padi rice is as follows (Table 16). This probably
represents the upper limit for hydraulic loading of fishpond wastewater. It illustrates
the seasonal variation in the amounts of wastewater that can be applied. Hence in the
main rainy season, alternative treatments such as a settling pond may be needed to
deal with waste water. In the wet season, larger areas of rice fields are needed to cater
for the water from a single pond. The timing of rainfall also has a bearing on the

timing of wastewater application to padi fields. Antecedent rainfall may block the use
of wastewater in a particular field for several days. Hence, it is important for the
flexibility of operations of fishponds to arrange surplus capacity of padi fields or
settling ponds on which to apply wastewater, to avoid the situation of having no
storage available for wastewater disposal when discharge is required from the
fishpond.
From a case study at Chau Phu, An Giang Province, it was reported that growing 3
rice crops per year required 12-13 irrigations, each supplying about 75 mm of water.
Hence in this location, 900-1000 mm of irrigation water was required per year. Many
locations only grow 2 rice crops per year, but in Chau Phu where 3 crops are grown
the main wet season crop uses no irrigation water. Hence, the above estimate in Table
18 of 835 mm annual irrigation requirement seems reasonable for either 3 crops or 2
crops, although perhaps a conservative estimate.
Large volumes of water need disposal from fishponds. Assuming an average depth of
3 m for fishponds (Phan et al. 2009) and a 9.2 cm depth of each waste water
application, 1 ha of fishpond requires about 33 ha of padi fields for each time the
pond is fully emptied and re-filled. This assumes rice uses 4.6 mm per day
(evaporation and transpiration- see Table 18) so that re-application of 9.2 mm
wastewater to a field needs to occur every 2 days. In practice farmers apply 50-100
mm of irrigation water every 10 days in the dry season and every 18-22 days in the
early wet season. The replacement of 33 % of pond water every day is a common
practice so full replacement notionally takes a 3-day cycle. Hence 10-20 ha of padi
land would be required to use the wastewater produced in the dry season from 1 ha of
fishponds assuming that only wastewater is used for irrigation. In the early wet
season, rainfall would prevent wastewater irrigation on some days, so that 20-40 ha of
padi land would be required for every 1 ha of fishpond.
Table 16. Example of monthly irrigation water required for rice fields in the Mekong
Delta and rainfall (averages for Long Xuyen and Cantho), and potential evaporation
(PE) from Ho Chi Minh City. Note that rice also receive water from groundwater
during daily high tides.

Month Rainfall
(R)
PE Irrigation
(R – PE)*

(mm) L/ha (mm) (mm) L/ha
Jan 15 150,000 160 145 1,450,000
Feb 0 0 175 175 1,750,000
Mar 15 150,000 215 200 2,000,000
Apr 70 700,000 200 130 1,300,000
May 190 1,900,000 150 - -
Jun 170 1,700,000 100 - -
Jul 210 2,100,000 110 - -
Aug 205 2,050,000 130 - -
Sep 255 2,550,000 100 - -
Oct 270 2,700,000 100 - -
Nov 155 1,550,000 110 - -
Dec 55 550,000 130 85 850,000
Annual 1610 16,100,000 1690 835 8,350,000
* calculation does not allow for percolation losses

If waste water application on this cycle causes excess nutrient loading on padi fields
(see below), a further increase in padi land is required to apply this approach as a
sustainable strategy for treating fishpond waste water. Possibly 60-80 ha or more of
land would be required for every 1 ha of fish pond. Since about 6000 ha of land is
occupied by fishponds, > 360,000 ha of land may be required to treat wastewater.
Assuming a fish harvest of 300 t/ha/yr from fishponds, and a feed conversion of 1
tonne of fish biomass for every 1.86 t of feed, 1 ha of fishpond generates about 260 t
of organic waste. Assuming an application rate of 3 t/ha, this requires about 90 ha of
padi land per 1 ha of fishpond. This represents a minimum ratio since greater than 90

ha would be advisable to minimise the risks of nutrient build up as discussed above
and to allow for the unavailability of some fields at the time when the solid waste was
available.
2. Solid loading
The application of solids can cause "clogging" of soil pores at the soil surface, due to
the organic and inorganic particles applied. Clogging interferes with the movement of
gases and liquids into the soil matrix. In fishpond liquid waste, the level of suspended
solids is 200 mg/L (Fig. 5). Farmers at Chau Phu reported that irrigation with waste
water caused a slowing of infiltration rates. For rice, exact levelling was also required
to prevent waste water accumulating in low patches, where excessive N loading
caused a tendency to lodging of rice. For vegetables, slower infiltration rates were
observed when irrigating with waste water.
Solid waste contains much higher levels of solids. However, the padi rice system is
not a percolation based system of waste treatment, hence the blocking of surface
pores may not be a significant concern except before sowing of rice seeds on the
prepared seedbed. The need for precise field leveling would also apply with the use of
fishpond solid waste.
3. Chemical reactivity of the waste
Organic materials applied to land will produce a biochemical oxygen demand (BOD)
and a chemical oxygen demand (COD). Waste application rates should be designed so
that COD or BOD do not exceed the gas exchange capacity of the soil, otherwise
decomposition of the waste will be slowed by lack of oxygen, and anoxic conditions
will develop. Anoxia in the root zone is less of a concern for rice than for other crops.
A wide variation in BOD has been detected in different wastes (McGowan et al. 2007;
Table 17). The maximum recommended BOD for direct application is 100,000 mg/L
while maximum COD is 250,000 mg/L (Yadav et al. 2002). In fishpond waste water
in Vietnam, levels of BOD and COD were 35 and 100 mg/L, hence well below limits
for soil application.
Wastewater from a number of locations in An Giang and Cantho contained 0.5-5 mg
NH

4
/L, 0.4-12 mg NO
3
/L, 5.4-17 mg N/L and 1.2-8.5 mg P/L (Table 18). The Clarias
fishpond water was notably high in N but lower in P. In a selection of outflow
samples from 9 ponds in the Mekong Delta, Bosma et al. (2009) reported N and P
concentrations within the above ranges. The wide ranges in N (3- to 4-fold) and P (7-
fold) concentrations have significant implications for determining the fertilizer
replacement value of the waste water. However, the variables that probably explain
much of the overall variation, such as feed type, water replacement frequency,
stocking rate of fish, are known at each fishpond and hence each waste water source
can probably be categorized as low, medium or high in the nutrient range. Fertiliser
replacement value of the waste water from each pond can be predicted from the likely
nutrient concentration in the water. For example, water from a pond near CLRRI
varied considerably over a 4 month period of use for irrigation, and the later decrease
in N and P concentrations was explained by the cessation of pellet feeding by the
farmer when catfish market prices dropped (Table 19).

Table 17. Typical BOD levels in liquid wastes. Source: McGowan et al. (2007)
Material / Substrate BOD (mg /L)
Dairy wastewater 500 - 2 000
Silage effluent 30 000 - 80 000
Pig slurry 20 000 - 30 000
Cattle slurry 10 000 - 20 000
Liquid effluent from slurry stores 1 000 - 2 000
Liquid sewage sludge 10 000 - 20 000

Solid waste from fishponds in An Giang in 2007 contained variable C, N, P, K and Ca
(Table 20). Levels of P were relatively high, as were K levels. Zn levels were
elevated, but not excessively high for land application. However, currently many

fishponds are pumping mineral sediment from the bottom of ponds to increase their
depth. The composition at some sites in Table 20 may not reflect that obtained in the
long term when ponds achieve their planned depth. The composition of swine lagoon
solids reported by Duffera et al. (1999) suggests that the catfish pond solid waste is
relatively low in P and to a lesser extent in N.
Table 12: Nutrient composition in wastewater at experimental sites reported by Cao et
al. (2010) and Bosma et al. (2009). Except for the Phong Dien sites, all are water from
catfish ponds. Data reported by Bosma et al. (2009) was for 3 sites of undisclosed
location on the Mekong Delta.
Location pH EC
(µS/cm)
NH
4
-N
(mg/L)
NO
3
-N
(mg/L)
Total N
(mg/L)
Total P
(mg/L)
Total K
(mg/L)
CLRRI 7.18 373 0.46 2.72 5.39 2.53 4.18
Chau Phu 7.13 234 3.4 0.42 5.40 8.46 4.47
Phu Tan 7.32 243 4.84 0.79 7.66 6.44 5.12
Phong Dien 6.65 217 2.18 12.3 16.7 1.21 6.02
Mekong

Delta
- - - - 14.8 3.2 -

Table 13: Composition of wastewater used in four irrigation events for dry season rice
at CLRRI.
No. pH
EC
(mS/cm)
COD
(mg/l)
PO
4
3-
(mg/L)
NH
4
+
(mg/L)
NO
3
-
(mg/L)
1 7.2 0.38 67.4 3.54 0.12 4.39
2 7.2 0.39 16.8 3.45 0.11 4.33
3 7.1 0.35 75.8 1.50 0.81 1.13
4 7.1 0.37 75.8 1.45 0.79 1.12

Table 14. Composition of solid waste from fishponds in Phu Than and Chau Phu, An
Giang, Vietnam. Composition of swine lagoon solids (Duffera et al. 1999) also
reported for comparison.

Solid waste An
Giang 2007 mean of
12 farms
Solid waste An
Giang 2007
Swine lagoon
solids
C 5±0.3 % 2.5-2.7 % 13.7±0.4 %
N 0.32±0.03 % 0.83 % 1.72±0.04 %
C:N 15.6 3 8.0
P 0.28±0.08 0.61 % 2.49±0.11 %
K 1.1±0.03 3.5 % 0.67±0.03 %
Ca 0.013±0.006 % 7.8-8.8 %
Cu 35±1 32 mg/kg
Zn 111±4 109 mg/kg
Fe 3.9 ±0.3 % 3 %
Mn 0.05±0.01 % 331 mg/kg
pH 6.4-6.5
EC 1.9-2.1 mS/cm