Treating pig wastewater by applying a model of fishpond or biodigester tank followed by a fishpond in Phuoc Tho state farm

Luu Huu Manh, Nguyen Nhut Xuan Dung, Bui Thi Le Minh and Mai Thuy Duong

College of Agriculture and Applied Biology, Cantho University, Vietnam
lhmanh@ctu.edu.vn

Abstract

Two methods of  treating the wastewater from pig farms (by draining directly into a fish pond or through a biodigester and subsequently a fish pond) were evaluated with two levels of pigs/surface water area. Measurements were made of temperature, pH, dissolved oxygen (DO), chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), suspended solids (SS), total nitrogen, total phosphorus, total Coliforms and parasitie eggs. 

The model of biodigester followed by the fish pond was the more effective and the quality of the wastewater in the fish pond met the government standards (type A Vietnam Standard: TCVN 5945 – 2005). The standards were not met by treating the wastewater only with the only fish pond.

Key words: BO, Coliforms, DO,  parasite eggs, pH, pig manure, Vietnamese Standard

Introduction

Pig production is one of the most important livestock sections in agriculture in the Mekong delta.  Intensive pig farms and small scale farms have grown remarkably; in most cases,  pig waste is directly drained to the water surfaces, resulting in severe environmental pollution. Realizing that problem, some waste bio-treatment methods have been applied to reduce environmental pollution by using fish ponds or fish ponds followed by water hyacinth ponds (Luu Huu Manh et al 2009).

The aim of this study was to evaluate the effects of two wastewater treatment models:  fish pond alone or biodigester followed by fish pond; and numbers of pigs per unit of surface water in the fish pond. Biochemical parameters were compared against the Vietnamese government standard (TCVN 5945 – 2005).

Materials and Methods

The study was conducted with two models in the Phuoc Tho state farm.

Model 1: Treating pig waste by using fish pond

Model 1 consisted of two fish ponds with water surface of 600 and 1400m², pig numbers were 2.35 and 9.12 heads/100m² water surface (Table 1).

Figure 1. Sample collection points (1and 2) in model 1.

  Model 2: Treating pig waste by biodigester tank in combination with fish pond

In the second model, pig waste was treated in a biodigester tank before going to the fish pond; this model consisted of two different treatments, pig numbers per 100m² water surface area were 9.12 and 5.4 heads (Table 2).

Figure 2. Sample collection points (3, 4 and 5) in model of biodigester in combination with fish pond

Sample collection

Samples were collected from 06:00 to 09:00 in the morning after cleaning the pig pens. The washings from the pen, consisting of faeces, urine and leftover feed, were discharged, piped and drained into a fishpond ora biodigester tank. There were 5 selected sample points (Figures 1 and 2). The first and third points were at the end of the pipe where the flow of wastewater went into the fishpond and biodigester tank.  The second and fifth points were in the fishponds and the fourth was in the biodigester tank. Samples collected included three replications. Total number of the samples was 78. They were held in plastic bottles, previously cleaned by washing in non-ionic detergent, rinsed with tap-water and finally with non-ionic water.  During sampling, the bottles were rinsed with the sample wastewater and then filled from a depth of 20 to 30cm below the level of the wastewater. To determine total Coliforms, samples were stored in sterile bottle. Bottles were labeled, transported to the laboratory and stored at 4^°C prior to analysis.

Analytical methods

Suspended solids (SS) were 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). For chemical oxygen demand (COD) and five-day biological oxygen demand (BOD5), samples were stored at 20°C and analysed by Winker’s method (APHA 1995). DO was also determined using the Winkler method. Temperature and pH were as determined using a pH meter (Hanna instrument).

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 Vietnamese Standard (TCVN 5945-2005) according to the Environmental Law (ASEAN-10) (1998).

 Statistical analysis

Data were subjected to analysis of variance (ANOVA) using the General Linear Model (GLM) available in Minitab 13.2. The Tukey test in the same software was used to detect significant differences among treatment means. 

Results and Discussion

Effect of treating pig waste by fish pond

The temperature in the wastewater collected in the pig pen (Table 3) was lower (28.3°C) than that of the fishpond (29.9°C). This increased temperature in the fishpond could be from absorbing the sunlight and from the oxidation of organic matter. The pH of the fishponds (6.9 and 6.7) was reduced as compared to the sample collected from the pig pen (7.4). The reduction of pH was perhaps cause by carbon dioxide from respiratory processes of aquatic animals and oxidation of growing organic matter occurring in the fish pond. However, these parameters were less than type A of Vietnamese Standard TCVN 5945-2005.

Dissolved oxygen (DO) in the pig pen wastewater was not observed, while it was 1.43 and 3.14 mg/litre for the two different pig densities in the fishpond. The DO concentration in fishpond increased because of the diffusing of oxygen from the air by activities of the Tilapia and common carps, which prefer floating feeds and flocks of fish often swim on surface of the water.

In the pig pen, five-day biochemical oxygen demand (BOD₅) for treatments FP-H and FP-L were very high (957and 877mg/litre, respectively); both were significantly reduced (72.5 and 72.7 mg/litre) in the fishpond, but this parameter exceeded type A of Vietnam Standard TCVN 5942-2005.

Chemical oxygen demand (COD) and suspended solids (SS) were reduced significantly by around 94% after wastewater went to the fishpond. Nevertheless, the values exceeded the type A (TCVN 5942-2005). This could be the result of the  organic matter in the waste waste going into the fish pond being used as a feed source for the fish. In addition, organic matter deposited on the bottom of the fish pond would be disintegrated by microorganisms.

Total nitrogen (N_tot) was reduced more than 84% through the fishpond and was below type A of Vietnamese Standard TCVN 5942-2005.  Similarly, total phosphorus (P_tot) in models 1 and 2 was also reduced rapidly from 330 and 296 mg/litre to 41.9 mg/litre (approximate 87% and 91%, respectively). However, P_tot exceeded the type A of Vietnam Standard TCVN 5942-2005. If fish pond wastewater, which contains a high level of total phosphorus, is discharged directly into rivers it would cause eutrophication and be toxic  for aquatic animals.

Parasitic eggs were not found in the fish pond; this indicated that the fish pond was effective in controlling this component in the pig faeces.

Total Coliforms in fishpond were high (1.1x10⁶ MPN/100ml) and exceeded type A of Vietnamese Standard TCVN 5942-2005. The presence of Coliform bacteria indicates that the water has been contaminated with human or animal feces and is a potential hazard for the community.  

Effect of treating pig waste by biodigester in combination with fish pond

The temperature in the biodigester tank was higher than in the pig pen and fish pond and probably caused by oxidation of organic matter. The temperature was similar to the optimum temperature for the anaerobic disintegration process of 30-35oC recommended by Bitton (1999). This parameter in the fishpond was less than type A of Vietnamese Standard TCVN 5945-2005. The pH in the biodigester was slightly reduced because of the acids and gases resulting from anaerobic digestion organic matter. This parameter was within the range of the type A Vietnamese Standard TCVN 5942-2005. DO was not found in the wastewater of the pig pen and biodigester tank. However, in the fish pond, it was 3.43 mg/litre in treatment BGFP-H and was 3.48 mg/l in treat BGFP-L. The increase of DO could be from the photosynthesis process of aquatic plants and diffusion  of oxygen from the air.

Generally, BOD5, COD and SS were reduced up to 72%, 77% and 74%, respectively through the biodigester tank, but exceeded the type A standard (TCVN 1945-2005) , while in the  fishpond, these parameters were reduced more than 97% and were less than the type A of Vietnamese Standard TCVN 5942-2005. In principle, COD reflects the amount of organic matter oxygenated by chemical factors and BOD5 the amount of organic matter oxygenated by biological factors. wastewater could be treated by biological effectively methods if the ratio of BOD5/COD is equal or more than 0.05 (Tran Van Nhan et al 2002). In this study, the ratio of BOD5/COD was 0.55 and 0.57 in the two different treatments and thus could be subjected successfully  to application of biological treatment methods.

Total nitrogen in the biodigester tank varied between 11.3 and 23.7mg/litre in treatments BGFP-H and BGFP-L. In the fish pond, it was reduced by 89.5% (treatment BGFP-H) and 92.0% (treatment BGFP-L) and was within the type A standard (TCVN 1945-2005). Total phosphorus in the wastewater from the pig pen was reduced through the fish pond (by 98.5 and 98.6% in treatments BGFP-H and BGFP-L, respectively). However, it exceeded the standard type A (TCVN 1945-2005). 

Parasite eggs were not found in the biodigester tank nor in the fish pond. Total Coliforms in the fish pond were 2.4x103 and 2.6x103 MPN/100ml in the two treatments and exceeded the standard type A (TCVN 1945-2005).  

Conclusions  

• The pig ratios per water surface in this study (from 5.4 to 9.12 pigs/100m² water surface) did not affect temperature, pH value and biochemical parameters in both fish pond alone or biodigester and fish pond models. BOD5, COD and SS of the model using only the fish pond to treat pig wastewater exceeded the type A Vietnamese Standard TCVN 5942-2005 and would not be allowed to discharge into rivers.    

• Biochemical parameters of the model of treating pig waste by biodigester in combination with fish pond were less than the type A TCVN 5942-2005 (excluding Ptot). This is the best biological  method to treat pig wastewater.

References 

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