Treatment of wastewater from slaughterhouse by biodigester and Vetiveria zizanioides L

Wastewater from a slaughterhouse is very harmful to the environment (Polprasert et al 1992). The main contaminants are organic matter, consisting of manure, gut contents, blood, suspended material, urine, grit and colloidal particles. Full scale conventional wastewater system such as anaerobic lagoon, activated sludge and biological nutrient removal technologies have been used to treat wastewater from slaughterhouse but they are expensive. In Vietnam, slaughterhouses apply some types of primary pre-treatment to minimize the pollutant before going out to the environment such as using settling tank in combination with biodigester tank or a fish pond. At farm level, anaerobic treatment with biodigesters is widely employed for treating pig wastewaters containing high concentrations of organic matter. The advantages of the biodigester are widely reported by many authors (Bui Xuan An et al 1997a,b; Sophin and Preston 2001; Duong Nguyen Khang et al 2002; San Thy 2003).

In Vietnam cities, available land area is limited, thus it is very important to design a treatment system for slaughterhouse to be environmentally friendly, efficientt and able to meet the requirements of the environmental regulations. Vetiver grass (Vetiveria zizanioides) has been introduced as a new phyto-technology for various environmental protection applications (Truong 2001). Vetiver grass is a “super absorbent” plant, mainly used for soil conservation, with a strong root system, which can develop to a depth of 5 m, is able to penetrate compacted soil layers and has minimal lateral growth. According to Truong (2000) and Truong and Hart (2001), Vetiver grass has been used for the disposal of leachate and effluent generated from landfill and wastewater treatment plants in Australia, China and Thailand. 

The aim of this study was to evaluate the effects of a wastewater treatment model, to process influent from a slaughter house passing it through a biodigester followed by Vetiveria zizanioides. prior to discharge to the environment. Biochemical parameters were compared against the National Technical Regulations on Industrial Wastewater (QCVN 24:2009/BTNMT). 

The study was conducted at a slaughterhouse in Hau Giang province. The wastewater treatment system included a settling tank with volume of 3m³ connected to a biodigester tank of 30m³ from which it drained directly into a pond of 330m² with Vetiver grass (Vetiveria zizanioides) surrounding the pond (Figure 1). Some 50 pigs were slaughtered daily producing 30m³ of wastewater per day. 

Figure 1. Sample collection points

Monthly sample of the influent and effluent were taken for analysis. Samples were collected at three positions (Figure 1), settling tank (2.00-3.00 am after cleaning the slaughterhouse), biodigester and in the planted Vetiver pond (6:00 to 9:00 am) with four replicates. Collected samples were placed in plastic bottles, previously cleaned by washing in non-ionic detergent, rinsed with tap-water and finally with non-ionic water. 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.

Temperature and pH were determined by using a pH meter (Hanna instrument). Total nitrogen (tot N) was determined by using Kjeldahl digestion. Total phosphorus (tot P) was determined by using a colorimetric ascorbic-molybdate method after digesting with a H₂SO₄/H₂O₂ mixture (Murphy and Riley 1962). Suspended solids (SS) were determined gravimetrically after oven drying at 105°C (APHA 1995). Chemical oxygen demand (COD) was analysed by the potassium permanganate method. Five-day biological oxygen demand (BOD₅) samples were stored at 20°C and analysed by Winker’s method (APHA 1995). Total Coliforms were determined using the Most Probable Number method (APHA 1998). Evaluation of the wastewater quality was based on the National Technical Regulation on Industrial Wastewater (QCVN 24:2009/BTNMT).

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. 

The temperature was lower in Vetiver pond (25oC) than that in settling tank (26.9oC) and biodigester (26.8oC), while the pH of settling tank (6.9) and biodigester (6.3) was slightly lower than that of Vetiver pond (7.4), because of the fermentation into acids and gases of organic matter in anaerobic digestion (Table1; Figure 2). The temperature and pH in a three sample collection points reached the column A of National Technical Regulation on Industrial Wastewater (QCVN 24:2009/BTNMT). 

Table 1: Effect of wastewater treatment on biochemical parameters and compared against the National Technical Regulation on Industrial Wastewater QCVN 24:2009/BTNMT

 

Figure 2. Effect of wastewater treatment model by biodigester and vetiver grass on BOD₅, COD, SS and total nitrogen compared against the Technical Regulation on Industrial Wastewater QCVN 24:2009/BTNMT

  

The rates of removal of BOD₅ and COD from the settling tank through the biodigester were 11.3% and 13.9%. The BOD₅ and COD concentrations of the effluent from the biodigester exceeded the QCVN 24:2009/BTNMT. After the vertivia pond, the concentrations of BOD₅ and COD were reduced by 59.8% and 60%, respectively (P<0.01). These parameters were close to the QCVN 24:2009/BTNMT standard. In principle, COD reflects the amount of organic matter oxygenated by chemical factors and BOD₅ is the amount of organic matter oxygenated by biological factors. Bwire et al (2011) reported that Vetiver grass tolerated leachate with high loading of COD up 14,000 mg/liter. Vetiver is very effective in removing COD. According to Tran Van Nhan et al (2002) wastewater is considered well treated by biologically effectively methods if the ratio of BOD₅ to COD is equal or more than 0.05. In this study, the ratio of BOD₅ to COD was 0.56 and thus should be subjected tofurther application of biological treatment methods.

The suspended solids (SS) was high in the settling tank (306 mg/l) and was reduced slightly in the biodigester (247 mg/l), but still exceeded the QCVN 24:2009/BTNMT standard), while in the vertivia pond, SS were reduced to 82.4 mg/l). The removal of SS from biodigester to the Vetiver pond was 73%. The SS is an important factor that affects development of aquatic organisms. The high concentration of SS was also caused by high fecal Coliform counts in the slaughterhouse as mentioned below.

Nitrogen and phosphorus are necessary for aquatic organisms, but their high concetration causeseutrophication and this can be toxic for aquatic animals (Meybeck et al 1996). From the settling tank through biodigester, total nitrogen (tot N) was reduced 42.3% and by 77.2% at the Vertivia pond. Total phosphorus (tot P) was reduced by 11 and 46% at these points. However, in the Vetiver pond, the wastewater still contained a high level of tot P. This could be explained that the removal of P and N by Vetiver grass increases with time, while the Vetiver pond continued to receive waster water from the slaughterhouse with very high amount of nitrogen and phosphorus from animal waste products. Vetiver grass has been shown to have a very high capacity of absorbing nitrogen (up to 10,000 kg N/ha/year) and phosphorus (up to 1,000 kg/ha/year); the capacity to use phosphorus was found to exceed that of other tropical and subtropical grasses (Wagner et al 2003).

Total Coliform bateria in the settling tank and biodigester were very high (370x10⁶, 315x10⁶ MPN/100ml, respectively) as compared to that in the Vertivia pond (634,500 MPN/100ml). The removal efficiency for total coliforms was 100%, but their amount was still very high (634,500 MPN/100ml) and far exceeded the QCVN 24:2009/BTNMT standard. The high level of Coliforms in the Vetiver pond reflected the high load received from the biodigester. These levels contrast with the report of Truong and Hart (2001) in which fecal coliforms declined with Vetiver treatment by 95% (from 500 to 23 MPN/100 ml). As far as we are aware our results are the first to demonstrate the use of Vetiver for treating wastewater from a slaughterhouse. It is possible that the area dedicated to the Vetiver was too small (330m²) to permit greater reduction in the Coliform load. This aspect requires further research. The results of this study nevertheless are in agreement with the report of Boonsong and Chansiri (2008) that Vetiver has a good potential to treat wastewaters. 

The model of a biodigester tank followed by Vetiveria zizanioides L. was suitable for treating wastewater from a slaughterhouse, reaching the standards of the Technical Regulation on Industrial Wastewater (QCVN 24:2009/BTNMT), in all criteria other than tot P and total Coliforms. 

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