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

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 operat

Technical Report (CARD 023/06 VIE) Improvement of water quality of outflows from ponds to waterways

Cao van Phung1 and Bell R.W.2

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

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 NH4 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: caovanphung@hcm.vnn.vn

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

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 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

January-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

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

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

Type of feed (%)

Table 4: Chemicals used for water treatment in catfish farms Values are a percentage

of respondents where N= 60 in each province

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

Water quality monitoring

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

Removing sludge purpose

Filling up low lying surface 30

Times of sludge discharge

Sludge discharge schedule

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

Effective Control

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

Table 9: Disease reports to Preventative Medical Centre in 2006 & 2007 at Phu Tan and Chau Phu districts

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 NH4-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

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)

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,

NH4 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

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,

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)

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-

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/m2) 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 m2 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

Type of fish culture (%)

Average time for fish raising (month) 3.5-5 3.5-5

From fingerling – selling size (months) 7.2 7.5

Table 12: Water quality of Clarias fishponds sampled in Binh Thuy and Phong Dien

districts of Cantho province

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

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)