Báo cáo khoa học nông nghiệp " Irrigating rice crops with waste water to reduce environmental pollution from catfish production in the Mekong Delta " doc

Technical Report
Irrigating rice crops with waste water to reduce environmental pollution from
catfish production in the Mekong Delta
Cao van Phung
1
, Nguyen be Phuc
2
, Tran kim Hoang
2
and Bell R.W.
3

1. Cuu Long Rice Research Institute, O’Mon, Cantho Province, Vietnam.
Email:
2. An Giang University, Long Xuyen, An Giang Province, Vietnam
3. School of Environmental Science, Murdoch University, Murdoch 6150,
Australia.

Abstract
Waste from intensive catfish (Pangasianodon hypophthalmus) aquaculture production
has become a pollutant of surface waters in the Mekong Delta, Vietnam. In the
present study, the aim was to treat the wastewater from catfish ponds in the Mekong
Delta by land application to padi fields so that the nutrients could be recovered by rice
crops as a fertilizer substitute. A survey in the dry season 2007 of paired fields in An
Giang Province showed that rice yield in 16 paddies receiving waste from fishpond
was 1 t/ha higher than in another 16 paddies that did not use wastes. In six field
experiments using fishpond waste water for irrigation, decreasing fertiliser N by 33 %
and P and K by 50 % had no effect on rice yields. In other cases, decreasing N by 40
% or P by 50 % did not decrease yield. The variation in nutrient composition in waste
water among sites, and in yield potential and irrigation requirements especially
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1
Corresponding author Cuu Long Rice Research Institute, O’Mon district, Cantho city-Vietnam.
Phone No (84) 710861452. Fax: (84) 710861457. Email:
between wet (lower yield potential and lower irrigation requirement) and dry seasons
may account for the different extent of fertiliser replacement feasible by waste water
without decreasing yield.
Ten -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 may be required for every 1 ha of fishpond. If waste water
application on this cycle causes excess nutrient loading on padi fields, a further
increase in padi land is required to apply this approach as a sustainable strategy for
treating fishpond waste water.

Keywords: catfish, fishpond waste, nutrients, pollution, rice.

Introduction
Catfish culture in the Mekong Delta has been practiced for a long time but this
industry became important for export only after the year 2000 with a subsequent
annual growth rate up to 2008 of about 15-20 % per annum. Total catfish production
in the Mekong delta was 0.68 million tonnes of fillets in 2007 (Phan et al. 2009). Cat
fish production expanded to cover about 6,000 ha of ponds in the Mekong delta,
Vietnam (Bosma et al. 2009). The production of each tonne of catfish consumes 4,023
m
3
of water and releases 47.3 kg of N (Phan et al. 2009). Wastewater discharged
directly from intensive catfish aquaculture production is polluting surface waters in
the Mekong Delta, Vietnam. From the catfish ponds, large quantities of liquid waste
are discharged to waterways without treatment (Phan et al. 2009). It is estimated that
about 2754GL of water was discharged annually back to the surface water system of

rivers and canals of the Mekong Delta from catfish ponds. Consequently, the
pollution of canals or rivers by loading of fishpond waste, rich in nutrients (especially
nitrogen, phosphorus and carbon) has emerged as a major concern for sustainability of
the industry (Phan et al. 2009).
The introduction of the National Environment Law in 2005 which prohibits direct
wastewater discharge of aquaculture water into rivers and canals, is an underlying
driver for this study. Although enforcement and compliance with this regulation
currently appears to be low, the future sustainability of fish pond aquaculture relies
greatly on the ability of farmers to comply with environmental and export regulations.
For this reason cost-effective wastewater treatment strategies need to be developed
and applied by farmers. Presently only 15-24 % of catfish farmers in Cantho and An
Giang provinces currently practice recycling of wastewater from their ponds by
irrigating rice fields (Cao et al. 2009).
Pollution due to fishpond waste is generally attributed to high organic carbon and
nutrients (Pillay, 1992) although high total suspended solids, NH
4
-N and COD may
also downgrade its acceptability for a range of uses. Moreover, this form of discharge
is also contributing to the spread of diseases of catfish since downstream operators are
pumping the infected water from canals into their ponds (Phan et al. 2009). Levels of
fish-disease organisms in the river peak at the beginning of the rainy season in the
Mekong Delta. The quantity of waste produced depends upon the quantity and quality
of feed (Cowey and Cho, 1991). This is associated with lower feed conversion ratios
from manufactured feed pellets than to farm made feed (1.69 vs 2.25; Phan et al.
2009). Hence the latter feed results in greater waste production. The frequency of
water replacement and stocking densities in the fishponds will also affect the quantity
and quality of waste water. However, integration of aquaculture into existing
agricultural systems has been reported to improve productivity and ecological
sustainability of both operations through better management and improved soil
fertility arising from waste recycling (Bartone and Arlosoroff 1987). Moreover,

properly managed inputs of waste materials can reduce the need for fertilisers (Falahi-
Ardakani et al. 1987).
Rice uses large volumes of water, especially in the dry season , when the crop is fully
irrigated. Instead of using river or canal water to irrigate rice, wastewater, if available
from adjacent catfish ponds, could supply most of the water requirements for rice
while also providing a significant supply of nutrients. The present study aims at
recycling waste water from fishponds by using it to irrigate rice. The rationale for the
treatment was to use the assimilation capacity of rice to absorb nutrients and the
filtering processes of the rice padi and associated water distribution canals and
sediment traps to improve the quality of water discharged from fishponds before it re-
enters the main canals and rivers. The objective of the present study was to determine
the fertiliser substitution value of the fishpond waste water in order to determine how
to adjust recommended fertiliser rates for rice when using this water to irrigate crops
instead of river water.

Materials and methods
Field experiments on recycling of waste water were carried out on rice crops
commencing with the wet season 2007 and ending with the dry season 2010. Site
locations and soil characterisation are given in Table 1.
A preliminary survey on the beneficial use of fishpond waste water for rice
cultivation on farmers’ fields was carried out in the dry season 2007 at Chau Phu and
Phu Tan districts of An Giang province. In each district, 16 fields were selected
comprising 8 which used waste water from fishponds and the other 8 paired sites
which were protected by levees to prevent inflow of waste water. Rice samples were
harvested from 5 m
2
with 3 replications for yield evaluation.
Experiments on recycling of waste water for rice production were carried out
at two communes of Chau Phu district namely My Phu during wet season 2007 and
dry season 2008 and at Vinh Thanh Trung for dry season 2009 and wet season 2009.

Another 2 experiments were conducted during the dry season 2008 at Phu Tan district
(two sites) of An Giang province. Further wastewater experiments were conducted at
CLRRI in the dry season 2009 and at Phong Dieng in the dry season 2009. At the
latter site, the wastewater was sourced from a Clarias fish pond, while for the
remaining sites the water was from catfish ponds. Nutrient composition of wastewater
at each site is shown in Table 2.
There were 6 treatments for experiments at Chau Phu and Phu Tan using
chemical fertilisers (N-P-K rates in kg/ha given in parentheses) as follows: T1 (90-26-
50); T2 (60-13-25); T3 (30-0-25); T4 (30-26-25); T5 (30-13-50) and T6 (0-13-50).
Experiments in My Phu did not include T5. Inorganic fertilisers treatments for
experiments at Vinh Thanh Trung and CLRRI were adjusted as follows: T1 (100-
17.5-25); T2 (80-14-25); T3 (60-10.5-25); T4 (40-14-25); T5 (40-10.5-25). Finally,
experiments at Phong Dien tested two treatments viz, irrigation with wastewater and
reduced inorganic fertiliser (45-13-30, NPK in kg/ha) vs irrigation with river water
and the recommended fertiliser rate (83-21-17, NPK in kg/ha).
Irrigation with wastewater occurrred 5 times for the wet season and 10 times for dry
season rice crops. The volume of wastewater applied at each irrigation event was
1000 m
3
/ha (i.e. 10 cm depth of water). Nutrients composition of the wastewater at
different sites is presented in Table 2.
All experiment was laid out in randomized complete block design with 4 replications
except Phong Dien which comprised 8 replications, each of 2 treatments. Plot size
was 8 x 7 m and each plot was separated from others by bunds. Soil sampling on each
plot was done before planting and after harvesting every crop. Yield components were
estimated by sampling two 0.5 x 0.5 m quadrats on each plot. Actual yield was
measured by harvesting 5 m
2
per plot. Rice crops were protected against leaf folder,
thrips, brown plant hopper and rice blast by using effective pesticides as required.

Organic carbon is determined by wet digestion; analysis of nutrients (N, P, K, Ca,
Mg, Fe, Cu, Zn, Mn) followed standard methods for soil (Page et al. 1982), plant and
water analysis (Chapman and Pratt, 1961).
Statistical analysis was completed with IRRISTAT software version 5.1 by applying a
balanced one-way ANOVA.
Nutrient (N, P) balances were calculated following the approach of Dobermann and
Fairhurst (2000) to estimate total input and output. Values reported by Dobermann
and Fairhurst (2000) were replaced where possible with locally relevant values.
Nutrient budgets for N and P under the double rice system were calculated for the
scenario where 2/3 of the straw is removed, which is current practice, and for 100 %
retention of straw as an indication of the consequences of different straw management
strategies.

Results
Preliminary investigation of farmers’ use of waste water
The preliminary study showed that rice yields in farmers’ fields using wastewater
from fishponds for irrigation were higher than in paddies using an equivalent volume
and time of application of river water for irrigation. Yield difference between the two
methods was about 1 t/ha (Table 3). This indicates that wastewater, applied at
appropriate rates, can help to further increase rice yield.
Analysis of soil samples at harvest time showed that total nitrogen, phosphorus and
potassium in paddies with wastewater application were significantly higher than plots
without wastewater application but organic carbon was lower (Table 4). Wastewater
was rich in nitrogen, phosphorus, and potassium (Table 2) which is likely why soils
receiving it had higher nutrient contents. By contrast, the high bacterial loading in
waste water may accelerate decomposition of organic matter leaving lower organic C
levels but higher mineralized nitrogen levels.
The survey also recognized that farmers usually added zeolite, lime and dolomite
while cleaning fishponds after harvesting (Bosma et al. 2009). This may be the reason
for higher contents of calcium and magnesium in paddies receiving wastewater.

Besides that, iron and manganese were also significantly higher in wastewater-treated
fields (Table 4).
Recycling of wastewater for rice cultivation at Chau Phu
At Chau Phu, rice yields of all treatments were in the range 3.9- 4.2 t/ha in the wet
season 2007 and not statistically different. However, in the dry season 2008 rice
yields of T1 and T2 were significantly higher than the other treatments (T3, T4 and
T5) with no added P or N or only 33 % of the recommended N (Table 5). This
suggests that irrigation by wastewater from fishponds can save 1/3 of recommended
N, P and K. The lower yields in T3 were attributed to the acidity of soils in which
phosphorus supply is a key factor for crop growth (Cong et al. 1995). Besides that,
nitrogen in T3, T4 and T5 was only 33 % of the recommended rate and not sufficient
to achieve potential yields for the dry season. Rice yield in the wet season is usually
lower than in dry season in the Cuu Long Delta due to lower solar radiation (Hung et
al., 1995)
Analysis of soil, straw and grain samples at harvesting time showed no significant
difference among treatments in concentrations of N, P and K (data not shown).
Recycling of wastewater for rice cultivation at Phu Tan
At Phu Tan site 1 in the dry season, rice yields in T1 and T2 were close to 7 t/ha and
not significantly different (Table 5). This suggests that irrigation by wastewater from
fishponds can save 1/3 of recommended N and ½ of the recommended P and K.
Further decrease in N fertiliser resulted in reducing yield. Omission of P produced the
lowest yield at Phu Tan 1, because low P on these soils limits N use efficiency (Cong
et al., 1995). At Phu Tan 2, the maximum yield was only 5.7 t/ha, but T2 again
produced the same yield as the recommended fertiliser rate. Further decreases in N
and P decreased yield significantly.
Macro and secondary nutrient uptake at the Phu Tan sites (Table 6) showed that plots
with high yield were also high in nutrient uptake (kg/ha) in straw and grain apart
from P in straw of Phu Tan 1. In the experiment at Phu Tan 2, nutrient uptake in grain
followed the same trend as in the experiment Phu Tan 1 but K and Ca uptake in straw
were not statistically different among treatments (Tables 6).

Recycling of wastewater for rice cultivation at CLRRI.
At CLRRI, rice yields of all treatment were not different (Table 7). Even though T1,
with the highest applied inorganic N, P and K fertiliser rates (100-18-25 kg/ha), had
the highest yield, due to high variability among the plots, the effect was not
significant. Nitrogen contents in grain at harvesting time of treatments T1, T2 and T4
were among the highest and they were statistically different to others (Table 8). This
suggests that differences in N on plots irrigated with waste water from fishpond were
not a decisive factor for rice growing on acid soil where P is deficient. Low N content
in T3 plot might have resulted from low P application in this treatment as compared to
T4. As regard to P content in grain of this experiment, treatment T1 had lower P
concentration than others which may result from a dilution effect because this
treatment had the highest yield.
Recycling of wastewater for rice cultivation at Chau Phu in 2009
At the two Chau Phu sites in the wet and dry seasons of 2009, respectively, there were
no significant yield différences among treatments. Even when 40 % of the
recommended N and 67 % of the recommended P were applied as fertiliser, no
decrease in yield was obtained in the rice irrigated with waste water (Table 7). At
Chau Phu, the crops were a healthy green colour during growth with 40 kg N/ha.
This suggests moderate but not excessive levels of N in waste water at Chau Phu,
perhaps because there is greater use of settling ponds here.

Recycling of wastewater for rice cultivation at Phong Dien
Total fertiliser application in treatment T1 (irrigated with waste water from fishpond)
at planting was about 1/3 of treatment T2 (which used only river water). Later on
fertilisers used in treatment T1 was based on plant diagnosis for N (leaf colour), P
(tillering capacity) and K (leaf turgour). Overall, T1 had about 45 % less N and 40 %
less P application as fertiliser, but a 76 % increase in K in order to minimise insect
damage.
There was no difference in yields between two treatments over two succeeding crops
in 2009 (Table 15). This demonstrated that 40-45 % decrease in N and P fertilisers for

rice irrigated with waste water from Clarias fishpond did not induce any nutrient
disorders (Tables 16,17).
Nutrient budgets
Nutrients budgets of 4 sites were presented in Tables 19 and 20. If rice straw was
removed, all treatments at Phu Tan and Phong Dien had negative N balances
regardless of differences in N dosages of treatments. But experiments in Chau Phu
and CLRRI showed that only treatment T5 & T6, with 30 kg N/ha applied as fertiliser
had negative N balances. When straw was recycled in situ, N balance was positive for
most cases except treatment T5 in Chau Phu, T6 in Phu Tan and T1 in Phong Dien
which all had a small N deficit.
Phosphorus was in surplus even when straw was completely removed at all 4 sites
except treatment T1 at Phong Dien. However, P was in surplus to the extent of 18-70
kg P/ha in the waste water irrigated rice crops if rice straw was retained except if no P
fertiliser at all was applied (Table 20).
Changes in soil properties
Soil properties of 4 sites were presented in Table 21. There was not much change in
nutrient availability for the double rice system (CLRRI) because in this system rice
straw was mostly retained. In Chau Phu and Phu Tan, where a closed dyke was built
to protect rice crops in the wet season, rice straw is usually burnt to facilitate rapid
land preparation. Soil data of Chau Phu and Phu Tan showed reduction of organic
carbon, total N and available N, P and K but not in available P at Phu Tan. Soil
analysis at Phong Dien after the 2
nd
rice crop did not show much variation of soil
parameters except available N was sharply reduced, which is consistent with the
negative N balances.
Effect of wastewater irrigation on crop profit
In the farmer survey in An Giang, rice crops irrigated with wastewater had 1 t/ha
extra yield. The extra income from 1 t of rice is 4 millions VND. The other benefit
from using wastewater to irrigate crops is the cash saving from reduced fertiliser

rates. In the scenario where fertiliser was reduced to 33 % of N, 50 % for P and for K,
the saving in fertiliser costs was equivalent to 1,161 million VND /ha. If full straw
removal is practiced, reducing fertiliser requirements to 50 % of recommended N, 33
% of P and 0 % of K, the saving in fertiliser costs decline to 0.658 million VND /ha
(Calculations based on: paddy price= 4,000 VND/kg, urea = 6,000 VND/kg, super
phosphate (16% P2O5)=3,600 VND/kg and KCl (60% K2O)= 9,000 VND/ha).
Discussion
Rice fertiliser rates and yield
Two rice crops per year are generally grown in the Mekong Delta and fertiliser is
applied by farmers for both crops. The CLRRI recommendations indicate that N
levels should be reduced in the wet season when yield potential is lower (Hung et al.
1996). The total nutrient supply to 2 crops, following the CLRRI recommendation,
would be 140 kg of N, 100 kg of P and 100 kg of K per hectare. With straw removal
these rates maintain close to neutral N balance, a positive P balance of about 16 kg
P/ha/crop and a modest K balance of 5 kg/ha/crop (Cao et al. 2010). However, it is
common for farmers to exceed these recommendations in their N fertiliser
application. The cost of these fertilisers has risen substantially in recent years
following international price trends. Hence any technology that reduces fertiliser input
costs while maintaining or increasing yield would be more profitable for rice farmers.
A conservative conclusion from the experiments conducted in the present study is that
use of wastewater for irrigation in the wet season and dry season could save 33 % of
the N, and 50 % of both P and K fertiliser recommended for rice by CLRRI. While
the wet season crop requires only half as much waste water for irrigation as the dry
season crop, due to the smaller water deficit from rainfall, the lower nutrient input is
offset by lower yield. Hence a similar reduction in fertiliser rate was applicable in
both wet and dry seasons.
With the exception of the preliminary yield assessment of farmers’ rice crops irrigated
with waste water in 2007, the effect of the waste water irrigation was to reduce
fertiliser inputs without affecting yield. Hence the main economic benefit of using the
waste water would be to decrease fertiliser costs, rather than to achieve increased

yield. When the catfish farmer also produces rice, as is common in An Giang
Province (Cao et al. 2009), the logistics for the transfer of wastewater to the padi field
and for the capture of the reduced fertiliser costs within the family business enterprise
seems quite obvious and simple. By contrast, in many parts of the Mekong Delta the
fish farmers run a specialised business with no rice production (Cao et al. 2009). Here
the catfish farmers need to negotiate with rice farmers for disposal of their waste
water. If water discharge regulations were enforced, catfish farmers would have a
strong incentive to negotiate such arrangements. In the absence of regulatory
inducements, the onus seems to be on convincing rice farmers of the value of the
waste water resource, so that they seek arrangements with catfish farmers for access
to waste water. A close working relationship between catfish farm operators and rice
farmers would be necessary so that the timing of wastewater release can be
coordinated with irrigation schedules for rice. Catfish farmers may seek to recover
some of their costs by a payment for the use of the waste water but would obviously
need to set the price at less than the cost of the equivalent fertiliser saved by rice
farmers.
Wastewater from fishponds supplies comparatively large amounts of N, P and K
when used to irrigate rice crops. At the dry season irrigation rate, equivalent to 1000
mm depth of waste water (10 lots of 100 mm depth of water), the amounts of N more
than doubled those in recommended N fertiliser while the P additions in wastewater
more quadrupled the P fertiliser addition. Clearly, the use of wastewater for irrigation
affords the opportunity for farmers to greatly reduce their fertiliser costs. Indeed,
unless farmers reduce N fertiliser rate, they risk decreasing rice yield when applying
wastewater due to increased risk of crop lodging. Such experiences have been
reported to discourage farmers in the past from irrigating with wastewater. On the
other hand, some An Giang farmers had, through trial and error, reduced their N rate
to 20 kg N/ha, about 20 % of the recommended N rate. When observed, all Phu Tan
plots were very dark green suggesting excessive N supply, even when only 40 kg
N/ha was applied (40 % of the recommended rate). Only 3 irrigations with waste
water had occurred. The farmer has over the last 5 years of using the wastewater to

irrigate his rice chosen 20 kg N/ha as the optimal N rate to apply to avoid excessive N
or risk crop lodging. Hence the present recommendations are conservative and
especially with greater straw retention could be further reduced, as some farmer
practice suggests.
Nutrient balance
The N additions in 1500 mm of wastewater/yr supplied an additional 50 % to that
recommended for rice, suggesting that use of wastewater without adjusting fertiliser
rates will cause large N surpluses in rice fields (Table 15). The main consequence of
excess N is lodging of rice crops which may decrease yield. Decreasing N fertiliser
additions to 33 % of the recommended rate and relying on wastewater irrigation for
the remaining N requirements would still generally supply more total N than is
required by rice. Straw removal would diminish the surplus of N carried forward to
the following crop, but in the case of 33 % of the recommended N fertiliser rate
would result in a deficit of N.
In order to integrate wastewater irrigation with fertiliser practices, simple tools are
required by farmers. The composition of wastewater may vary from place to place
and time to time as shown in the present study (Table 2). Hence, it is important that
farmers are tooled with means to make decisions on how to adjust fertiliser
application to supply the crop demand without applying excess or too little. For N,
the leaf colour chart is already in use by rice farmers to manage N fertiliser
requirements. The leaf colour chart could be used to determine whether
supplementary N fertiliser was required while irrigating with wastewater. On the
other hand if the leaf colour chart indicates that a crop already contains adequate N,
wastewater irrigation should be deferred in favour of river water irrigation.
The P additions in wastewater were greatly in excess of rice crop requirements and
more than double the fertiliser addition rate. The fate of the surplus P depends on the
P sorption capacity of the soils. On acid sulfate soils with very high P sorption, most
of the P that is not removed in harvested rice grain would be adsorbed by soils. Indeed
in a related study testing the use of fishpond solid waste as a fertiliser substitute for
rice, the increse in extractable soil P from positive P balance was equivalent to only 2-

3 % of the P surplus (Cao et al. 2010). Hence, on acid sulfate soils in the Mekong
Delta, the capacity for soils to sorb the surplus P seems adequate at present to prevent
the release of P. On alluvial soils, the capacity for positive P balances to result in
marked increases in soil P are much greater. However, continued application of such
large P surpluses would eventually exceed the capacity of the soil to adsorb P and at
this point soils would begin to release P into surface water creating a risk of
eutrophication. Straw removal was not sufficient to significantly decrease the surplus
of P. Clarias fishpond wastewater which contained much lower P did not produce
significant P surpluses.
Relative to crop requirements and fertiliser additions, wastewater contained relatively
modest amounts of K. Nevertheless, addition of wastewater in addition to
recommended fertiliser would result in a K surplus after every crop except if straw
was completely removed.
At CLRRI, when using wastewater to fully irrigate rice fields, it is estimated from the
nutrient budget calculations that fertiliser rates should be adjusted from the
recommended rate to 20 % of N, 0 % of P and 0 % of K if straw is retained. If straw is
removed, then fertiliser should be applied at 50 % of recommended N, 0 % of P and
100 % of K. However, when no P was applied in several experiments, rice yield was
strongly depressed. This suggests that the supply of P in the wastewater was
insufficient or possibly the timing was not optimal for rice. Adequate P is required by
rice at sowing or transplanting for rot growth and tillering. It appears that waste water
does not supply sufficient for early growth and some P fertiliser is required at
planting.
The above calculations were based on wastewater quality at CLRRI. Variable quality
of wastewater will affect the calculated balances and hence the adjustments needed
for fertiliser rates. For N, adjustments can be made using the leaf colour chart, but all
the decisions about fertiliser P have to be made at sowing. While topdressing of K is
worthwhile for crops, there are no tools apart from leaf analysis that could be used in
the season to adjust K fertiliser rates. Leaf analysis is available but not readily
practiced by farmers or extension agents, and relatively costly.

Land requirements for wastewater irrigation
The calculations above were based on 500 mm of irrigation water to wet season crops
and 1000 mm to dry season crops. However, the required volumes of water may be
much less. For example, Hoa et al. (2006) measured irrigation water applied to rice
crops at CLRRI and determined it was only 400 mm. 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 estimate of 800-1000 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 and a 9.2 cm depth of wastewater application to a rice field, 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)
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 (Cao et al. 2009; Phan et al. 2009) 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 may be required for
every 1 ha of fishpond. If waste water application on this cycle causes excess nutrient
loading on padi fields, 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 would be required for every 1 ha of fish pond. Since about 5000 ha of land
is occupied by fishponds, > 350,000 ha of rice land may be required to fully treat
wastewater in the Mekong Delta.

Long term effects on soils
The long term effects of wastewater application on soil properties are unclear.
Clogging of soil pores can be a consequence of using wastewater containing
suspended particulates. However, in padi rice, slowing of percolation rates by pore
clogging is not likely to harm crop growth. If wastewater was used to irrigate crops
like maize or vegetables as discussed below, pore clogging leading to sluggish
drainage and anaerobic soil conditions may be detrimental to crop growth.
Extra organic matter inputs may cause lower redox potential in soils, with the
potential to increase Fe toxicity in some rice paddy soils of the Mekong Delta
(Dobermann and Fairhurst 2000). The continuous loading of dissolved organic matter
may also lead to changes in nutrient forms and availability. Given the number of
uncertainties, monitoring of soil conditions under long term application of wastewater
would seem to be advisable.
Management of waste water
While application of wastewater to rice through irrigation is effective as a fertiliser
substitute, the capacity of rice to absorb nutrients is limited compared to some other
crops. Maize when managed to achieve yield of 10 t/ha has a high N, P and K demand
(Dierolf et al. 2000) and hence may be an alternative crop to irrigate with wastewater,
especially in An Phu district of An Giang province where fishponds occur and maize
is an established crop. Similarly, vegetables have a high nutrient demand and could be
used for wastewater irrigation, although the area of land requiring irrigation is small
compared to rice and may not make a significant contribution to recycling waste
water.
Alternative treatments need to be considered for the management of wastewater.
During the wet season, there needs to be a means of treating wastewater other than
irrigating rice fields because water levels in the fields will already be high due to
heavy rainfall. Settling ponds, with or without aquatic plants and fish species other
than catfish, could be used as holding ponds for wastewater before discharge into
canals and the river. Further research is needed to determine whether settling ponds
would be sufficient treatment to reduce water quality to levels acceptable under the

2005 National Environment legislation.
Even if no fertiliser P was added, the amounts of P in wastewater applied at 500 mm
to the wet season and 1000 mm to the dry season would results in large P surpluses. It
is possible to reduce the rate of wastewater added per field, which would reduce
nutrient surpluses of N and K as well as P. However, this strategy is only feasible if
sufficient rice land is available in proximity to the fishpond and arrangements
between the owner of the fishpond and the rice fields are in place.
Other strategies for removing P from wastewater may be needed such as passing the
wastewater through a P-sorbant filter before release into paddy fields. The acid sulfate
soils of the Mekong Delta have high P sorption capacity and may be useful candidate
materials for developing filtration systems. The recovery of struvite (ammonium
magnesium phosphate crystals) is a feasible technology for various types of waste
including animal and human waste (Cordell et al. 2009). As the planet moves towards
the point at which annual global P production from rock phosphate reaches a peak
(Peak P), there is increased interest in technologies that capture P in concentrated
form so that it can be re-used in place of fertiliser from mineral rock phosphate
(Cordell et al. 2009).

Conclusions
While in one survey in An Giang higher yields were obtained in fields irrigated with
wastewater, in most cases the savings to farmers from reduced fertiliser inputs would
be the main incentive for use of wastewater. The use of fishpond wastewater to
irrigate rice can consistently save 1/3 or more of N fertiliser, and up to ½ of the P and
K currently applied to crops as inorganic fertiliser. Savings could be valued at about
1.16 million VND /ha. No phytotoxicity to rice plants was observed from application
of waste water from fishponds to paddies. However, if rice is irrigated with
wastewater without any reduction in fertiliser addition, lodging of rice crops is likely,
and depressed grain yield may be an adverse consequence. While nutrient budgets
indicate that use of fishpond wastewater can save 50 or more % of nitrogen, and up to
100 % of the phosphorus and potassium currently applied to crops as inorganic

fertiliser, field trials showed that yields declined if no fertiliser P was applied and
variations in waste water composition meant that such savings were not always
assured. The removal or retention of rice straw will greatly alter the N and K balances
in fields irrigated with wastewater, but regardless P balances were positive.
Recycling of waste from fishponds for rice cultivation has the potential to alleviate
water pollution by reducing the quantity of nutrients discharged directly to water
sources. Ten -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 may be required for every 1 ha of fishpond.
If waste water application on this cycle causes excess nutrient loading on padi fields,
a further increase in padi land is required to apply this approach as a sustainable
strategy for treating fishpond waste water. To cover these contingencies, 60-80 ha or
more of land may be required for every 1 ha of fish pond.

Continued monitoring of fields under treatment with fishpond waste is necessary to
determine longer term effects on nutrient budgets, soil quality, rice yields and water
quality.
Acknowledgements
This research was financially supported by CARD project VIE/023/06. The assistance
of staff in the Soil Science Department and a student of An Giang University to carry
out this study are greatly appreciated. Thanks also to Cuu Long Rice Research
Institute and the Ministry of Agriculture & Rural Development, Vietnam for the
facilities and services granted to complete this investigation.
References
Bartone, C.R., Arlosoroff, S., 1987. Irrigation reuse of pond effluents in developing
countries. Wat. Sci. Tech. 19, 289-297.
Bosma, R.H., Hanh, C.T.T., Potting, J., 2009. Environmental Impact Assessment of
the Pangasius Sector in the Mekong Delta. Wageningen University. 50 pp
Bush, S. R., Khiem, Nguyen Tri and Sinh Le Xuan 2009. Governing the

environmental and social dimensions of Pangasius production in Vietnam: A Review.
Aquaculture Economics & Management 13, 271 – 293
Cao Van Phung and Bell, R.W. (2009). Report on Baseline Surveys in Cantho and An
Giang Provinces. Project Progress Report CARD Project VIE/06/023. Cuu Long Rice
Research Institute, O Mon.
Cao, van Phung, Nguyen, be Phuc, Tran, kim Hoang

and Bell, R.W., 2010. Nutrient
recovery by rice crops as a treatment for aquaculture solid waste: crop yield, nutrient
status and nutrient budgets. Technical Report CARD Project VIE/06/023. Cuu Long
Rice Research Institute, O Mon.
Chapman, H.D., Pratt, P. F., 1961. Methods of analysis for soil, plant and water.
Division of Agricultural Sciences, University of California, Riverside.
Cho, C.Y., Hynes, J.D., Wood, K.R., Yoshida, H.K., 1991. Quantification of fish
culture wastes by biological and chemical (limnological) methods, in: Cowey C.B.
and Cho C.Y. (Eds), Nutritional Strategies and Aquaculture Waste. University of
Guelph, Canada.
Cong, P.T., Sat, C.D, Castillo, E.G., Singh, U., 1995. Effect of phosphorus and
growing season on rice growth and nutrient accumulation on acid sulfate soils, in:
Vietnam and IRRI: A Partnership in Rice Research. International Rice Research
Institute, Manila 1099, Philippines. pp. 123-135.
Cordell, D., Drangert, J-O., White, S., 2009. The story of phosphorus: Global food
security and food for thought. Global Environmental Change 19, 292–305.
Falahi-Ardakani, A., Bouwkamp, J.C., Gouin, F.R., Chaney, R.L., 1987. Growth
response and mineral uptake of vegetable transplants grown in a composted sewage
sludge amended medium. J. Environment Horticulture 5, 559-602.
Guong, V.T, Lap, T.T, Hoa, N.M., Castillo, E.G., Padilla, J.L., Singh, U., 1995.
Nitrogen use efficiency in direct seeded rice in the Mekong River Delta: varietal and
phosphorus response, in: Vietnam and IRRI: A Partnership in Rice Research.
International Rice Research Institute, Manila, Philippines. pp. 150-159.

Hoa, N.M. 2006. Additional sources of K for soils in the Cuu Long delta under
intensive cropping. Viet. J. Soil. Sc. No. 24, pp 62-65.
Hung, N.N, Singh, U., Xuan, V.T, Buresh, R.J., Padilla, J.L., Lap, T.T., Nga, T.T.,
1995. Improving nitrogen use efficiency of direct seeded rice on alluvial soils of the
Mekong River Delta, in: Vietnam and IRRI: A Partnership in Rice Research.
International Rice Research Institute, Manila, Philippines. pp. 138-149.
Page, A.L., Miller, R.H., Keeney, D.R., (eds) 1982. Methods of Soil Analysis.
Number 9 (part 2). Soil Sci. Soc. Amer. Madison, Wisconsin USA.
Phan, Lam T., Bui, Tam M., Nguyen, Thuy T.T., Gooley, Geoff J., Ingram, Brett A.,
Nguyen, Hao V., Nguyen, Phuong T. and De Silva, Sena S. 2009. Current status of
farming practices of striped catfish, Pangasianodon hypophthalmus in the Mekong
Delta, Vietnam. Aquaculture 296, 227-236
Pillay, T.V.R. 1992. Aquaculture and Environment. Blackwell Scientific Publ.,
Cambridge,


Table 1: Soil characterization of experiments at CLRRI and on farmers’ fields in An
Giang and Cantho provinces.
Location
Soil name
(FAO/UNESCO)
pH
(1:5
H
2
0)
Org. C
%
Total (%)
N P K

CLRRI,
Cantho
Eutric Gleysol 4.8-5.2 2.29
0.268 0.021 0.92
Chau Phu, An
Giang
Umbric Fluvisol 5.6-6.2 0.8-1.1 0.161 0.047 1.56
Phu Tan, An
Giang
Thionic Fluvisol 4.9-5.5 0.9-1.3 0.198 0.035 1.37
Phong Dien,
Cantho
Gleyi-Eutric
Fluvisols
5.3-5.5 2.9-3.2 0.210 0.048 1.55

Table 2: Nutrient composition in wastewater at experimental sites.
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