Treating pig waste by fish and water-hyacinth ponds in Vinh Khanh State Farm, An Giang province

An Giang province has an agriculture- based economy. Pig production is considered one of the most important livestock activities in the province, mainly by smallholders. Recently, pig production has rapidly increased to supply the demand for meat. The increased concentration of intensive livestock is associated with the pollution and the pig waste becoming a serious environmental hazard. Livestock waste causes incessant and pervasive damage to waterways, human health and aquatic life. Water bodies become unusable, while in the remote areas people are still using these for their daily activities. According to a study of Bui Thi Le Minh (2006), more than 25% of householders had no waste treatment method. At the state farms, the common method used to treat wastewater, consisting of feces, urine and leftover feeds is to directly drain the wasterwater into a closed fishpond or into canals or rivers (Huynh Tan Tien 2006). A model for the wastewater treatment in Vinh Khanh state farm directs the wastes from the pig farm to a fishpond, followed by a water-hyacinth pond, which is considered a bio-pond. However, there is a lack of data on the wastewater quality in the treatment ponds and in rivers surrounding the pig farm.

This study was undertaken by a team from the faculty of Agriculture and Applied Biology, Cantho University to evaluate a more efficient treatment model and to find out the optimum number of pigs per unit of water surface area to meet the government standard (TCVN 5945 – 2005). 

In total there were 931 pigs consisting of sows and boars (but excluding piglets) (Table 1).

 

Table 1: Number of pigs per unit of water surface area of fish pond*

Samples were taken between 06:00 and 09:00h after cleaning the pens. The representative sites were chosen from 6 blocks of pens (Figure 1) from which wastewater consisting of feces, urine and leftover feed were discharged, piped and drained into the fishpond. For each block of pens, there were 4 selected sample points. The first point was before the water pipe where the water flows into the fish pond. The second point was between the entrance and exit to the fish pond. The third point was in the fishpond and the fourth was in the water hyacinth pond. Samples collected were from four replicates. Water samples in the river surrounding the farm were also taken at three points (up, middle and down stream). The total number of samples was 100. They were held in plastic bottles, previously cleaned by washing in non-ionic detergent, rinsed with tap-water and finally with deionised water. During sampling, the bottles were rinsed with the sample wastewater and then filled to the brim from a depth of 20 to 30 cm below the level of the wastewater from each of the four designated sampling points.

For COD and DO analysis, samples were stored in dark bottles and a few drops of manganese sulfate solution added to fix dissolved oxygen. Bottles were labeled, transported to the laboratory and stored at 4°C prior to analysis

Suspended solids (SS) wére determined gravimetrically after oven drying at 105°C (APHA 1995). Total nitrogen (Ntot) was determined using Kjeldahl digestion. Total phosphorus (Ptot) was determined by using a colorimetric ascorbic-molybdate method after sample digestion with a H₂SO₄/H₂O₂ mixture (Murphy and Riley 1962). Samples for chemical oxygen demand (COD) and five-day biological oxygen demand (BOD₅) were stored at 20°C and analysed by Winker’s method (APHA 1995). DO was also determined using the Winkler method.

Total Coliforms were determined using the Most Probable Number method (APHA 1998). The isolation and identification of parasitic eggs were done by the methods of Fiilleborn (1920) and Benedek (1940). Evaluation of the water quality was based on Industrial Wastewater Discharge Standards (TCVN 5945-1995) according to the Environmental Law (ASEAN-10) (1998)

Data were subjected to analysis of variance (ANOVA) using the General Linear Model (GLM) available in Minitab (2000). The Tukey test in the same software was used to detect significant differences among treatment means. 

At the first sampling point, dissolved oxygen (DO) was zero due to the anaerobic conditions (Table 2). Among the collection points, the third site had highest DO values because the fish pond has a large open water surface and so atmospheric oxygen could diffuse into the fish pond. In addition, photosynthesis of algae provides oxygen for the fish pond and normally fish require at least 4-5 mg/litre of DO to thrive (Corson 1990). This indicates that water quality in the fish pond of the present study was acceptable. For aquatic plant systems, healthy water should contain at least 5.2 mg/litre DO (Ademoroti 1996). There was a low concentration of DO in the water hyacinth pond as compared to the fish pond because the water surface was covered by the developing water hyacinth plants. In the river, DO was less than the type A of TCVN 5942-1995 concentration of 6 mg/litre.

Biological oxygen demand (BOD₅) is a common parameter used in water quality evaluation. BOD₅ concentration at the first collection points was very high and decreased rapidly with the lowest level in the fish pond. Levels in the the water hyacinth pond and in the river were within the allowable limits (40 mg/litre for FEPA; 20 mg/litre FAO; 30 mg/litre type A of TCVN 5945-2005) for water used for agricultural purposes. The fish pond and water hyacinth pond reduced the BOD₅ in wastewater by approximately 90 and 98%, respectively. However, according to WHO standard (2004), the permissible limit of BOD₅ for drinking water is 0.0 mg/litre. This implies that there is a risk in drinking the river water without treatment.

High COD concentration indicates the presence of chemical oxidants in the water. High COD causes soil fixation, resulting in lower availability of nutrients for plants (Chukwu 2005). The COD at the different sites followed a similar pattern to the BOD₅. The COD in the fish pond, water hyacinth pond and river were under FEAP’s maximum limit of 80mg/litre and minimum limit of 50 mg/litre (type A of TCVN 5945-2005). COD concentrations in the fish and water hyacinth ponds were reduced by approximately 94 and 97%. In the river, COD was within the type A of TCVN 5942-1995 limit of 10 mg/litre.

The suspended solids (SS) followed a similar pattern to the BOD₅. The values in the fish and water hyacinth ponds and in the river were within standards of FEFA and TCVN 5945-2005 for slurry discharge to surface water. The fish pond reduced approximately 91% of the SS from the pig farm. In the river, SS was within the type A of TCVN 5942-1995 of 20 mg/litre. 

Table 2: Effect of the fish pond and water hyacinth pond on reducing dissolved oxygen, chemical oxygen demand, biochemical oxygen demand and suspended solids

Total nitrogen level was within the TCVN 5945-2005 standard of 23.9-41 mg/litre at the first sample collecting site (Table 3), then rapidly decreased at the second, third and fourth sites and river. These values are low as compared to the type A of TCVN 5945-2005 standard of 15 mg/litre for wastewater. If there is a high concentration of nitrogen in pig waste water discharged into a water body, some species of bacteria would oxidize ammonia to nitrate, which promotes plant and algae development, leading to water eutrophication.

Phosphorus levels revealed a reduction of 91 to 97% from the initial pig waste water to the fish pond and water hyacinth pond. The concentration of P in the water hyacinth pond was within the type A of TCVN 5945-2005 standard of 4 mg/litre. Nitrogen and phosphorus from manure are major pollutant sources and, at low levels, they cause an excess of algae. At high levels, phosphorus is toxic to fish. Thus, the fish pond significantly reduced the amount of nitrogen and phosphorus and the hyacinth pond takes up almost all the remaining phosphorus before discharge into the river, which prevents the eutrophication of these water bodies. 

Table 3: Effect of the fish pond and water hyacinth pond on total nitrogen and phosphorus contents

There was a significant reduction of Coliform bacteria in the fish pond as compared with the wastewater before entering the pond (Table 4). The population of Coliform in the fish pond was within the type B of TCVN 5945-2005 standard of 5 x10³ MPN/100ml. Coliforms were almost completely eliminated in the fish pond, where the large surface area was subjected to UV irradiation; circulation by fish and the wind reinforced this effect. UV irradiation is harmful to bacteria, but is a necessary factor in the development of aquatic plants. The fishpond was high in dissolved oxygen (Table 2), which is a potential stimulant for algae growth, some strains of which contribute to increasing pH of the water, which is lethal to bacteria.

The population of Coliforms was high in the water hyacinth pond. This could be explained by the fact that the water hyacinth pond was deep (2.5m) and the abundance of plants that covered the water surface prevented the penetration of the sun rays to the bottom of the pond. Also the retention of the wastewater and decomposing of dead roots in the water hyacinth pond allowed bacteria to work on the organic matter in the sewage. In the water hyacinth pond, the population of Coliforms was not within the type B of TCVN 5945-2005 of 5 x10³ MPN/100ml.

The presence of Coliform bacteria indicates that the water source has been contaminated with feces of humans or animals. Fecal Coliforms are not usually pathogenic and live in the intestine of humans and animals. Some strains of the Coliform Escherichia coli can cause intestinal illness. When a water source is contaminated with fecal material, some pathogenic diseases, such as typhoid, viral and bacterial gastroenteritis can exist. Coliform bacteria is an indicator that a waterway possibly contains other pathogenic organisms which could be a potential health risk. In the river, the population of Coliforms was not within the type A of TCVN 5942-1995 of 5 x10³ MPN/100ml.

Parasitological determination showed that no parasitic eggs were found in the fish and water hyacinth ponds. The efficiency of removal by the fish was 100%. 

Table 4: Changes in microorganism population in water surface before and after the fish pond and water hyacinth pond

The trends in Figures 1 to 4 show that increasing the water surface area per pig significantly contributed to an improvement of water quality in the fish ponds. 

Figure 1. Relationship between fish pond area per pig and DO of the water in the fish pond

 

Figure 2. Relationship between fish pond area per pig and BOD₅ of the water in the fish pond

 

Figure 3. Relationship between fish pond area per pig and P content of the water in the fish pond

 

Figure 4. Relationship between fish pond area per pig and N content of the water in the fish pond

Ademoroti C M A 1996 Standard Method for Water and Effluents Analysis. Foludex Press Ltd, Ibadan pp.22-112.

APHA 1995 Standard for the Examination of Water and Wastewater, American Public Health association, 18th Edition, p3-5.

APHA 1998 Standard methods for the examination of water and wastewater. 19th

Bui Thi Le Minh 2006 Danh gia muc do o nhiem moi truong nuoc mat va hieu qua cua mot so mo hinh xu ly nuoc thai chan nuoi heo o nong ho. Master Thesis, Can tho Univeristy.

Chukwu O 2005 Development of predictive models for evaluating environmental impact of the food processing industry: Case studies of Nasco Foods Nigeria Limited and Cadbury Nigeria PLc. Unpiublished Ph.D Thesis, DEA, FUT, Minna Niger State, Nigeria.

Corson WH 1990 Global Ecology Handbook: What You can do about the Environment crisis. 2nd Edn. Beacon Press, Boston, USA, pp.245.Edition. American Public health Association, Washington, DC pp 45-60.

FEPA 1990 Federal Environmental Protection Agency 1990 Guidlines and Standards for Environmental Control in Nigeria, Abuja, Nigeria, 33-63

Fiilleborn F 1920 Ueber die Anpassung der Nematoden an den ... Evolution of Parasites. Third Symposium of the British Society of Parasitology.

Huynh Tan Tien 2006 Tinh hinh o nhiem moi truong nuoc tu chat thai chan nuoi o mot so trai chan nuoi heo tai TP Can tho. Master Thesis, Can tho University.

Murphy J and Riley J P 1962 A modified single solution method for determination of phosphate in natural waters. Analytical Chemistry, 27: 970-976.

Minitab 2000 Minitab user's guide. Data analysis and quality tools. Release 13.1 for windows. Minitab Inc., Pennsylvania, USA.

Sarin R, Ramteke D S and Moghe U A. Manual on water and waste water analysis, pp.71-73.

TCVN 5945 1995 Wastewater, Industrial Discharge Standards Asia-Pacific Centre for Environmental Law.

World Health Organization 2004 “Guidelines for drinking water quality: Recommendations”. WHO Publications, Geneva, 2004; 1: 30