Pollutant removal efficiency from swine biodigester effluents by an integrated treatment system using rice straw, effective microorganism liquid and Vetiver grass (Vetiveria zizanioides)

Han Quang Hanh, Nguyen Van Duy, Nguyen Thi Phuong and Vu Dinh Ton

Faculty of Animal Science, Vietnam National University of Agriculture, Hanoi, Vietnam
hqhanh@vnua.edu.vn

Abstract

The experiment was conducted at a pig farm in Hung Yen province, North Vietnam to identify the efficiency of an integrated wastewater treatment system using rice straw, effective microorganism liquid, and Vetiver grass (Vetiveria zizanioides). Wastewater flowed from pig house down to biodigester (20m³), then settled in the primary tank (8m³with 1kg/m³rice straw added as biofilter) and secondary tank (8m³with 1L/m³of effective microorganism liquid) before turning into a hydroponic pond (200m²) with four Vertiver grass platforms (2m ² per bed). Water samples were taken at day 7, 14, 21, 28, and 35 after treatment at four discharging points to identify the removal efficiency of BOD, COD, total nitrogen, total phosphorus, copper, zinc, total coliforms, and Escherichia coli after each stage. Rice straw samples that sank into settling tanks were also taken to measure nitrogen and phosphorus absorption efficiency. Results showed that integrated treatment system was highly effective in removing organic pollutants and heavy metals from wastewater, while generating gases for farm’s energy consumption, producing composted product, and providing feed to cattle. The concentrations of BOD, COD, copper, zinc, total nitrogen, and total phosphorus decreased gradually from biodigester to primary settling tank, secondary tank and hydroponic pond (BOD concentration reduced by 73.91%, 79% and 88.58% respectively). Rice straw showed a high capacity of nitrogen and phosphorus absorption. The integration between settling tanks added with rice straw and effective microorganism and hydroponic pond with vetiver floating platforms was a highly effective system for swine wastewater treatment, while showing some additional benefits of renewable resources.

Keywords: animal waste, recycling of wastewater, waste treatment

Introduction

The intensification of pig production may cause an extreme environmental and pollution if animal waste is not managed and treated appropriately and effectively. Nowadays, there have various methods of pig waste treatment that were developed and applied to different farm sizes. Some methods are very effective in pollutant removal but unfeasible or costly, thus not be adopted by small or medium-scale farms. Therefore, developing an applicable and cost-effective swine wastewater treatment system is now in need for sustainable development of pig production.

Biodigester is one of the most popular animal waste treatment systems applied by pig farms in many countries around the world and in Vietnam, but its pollutant removal efficiency is strongly affected by many factors. Animal waste treatment by biodigester is quite effective in pollutant removal and to produce biogas for household’s energy consumption. Due to the surplus of animal waste, the effluent from biodigester still contain much pollutants that requires a completed management and treatment system (Vu Dinh Ton et al 2008, Nguyen Thi Hong and Pham Khac Lieu, 2012).

Recently, the approach of using low-cost adsorbents from agricultural by-products and aquatic plant in removing pollutants from wastewater has been emerged as an appropriate method for sustainable animal wastewater management. Rice straw has been studied as an excellent natural sorbent (Dilamian et al 2021, Bhadoria et al 2022). Vetiver grass, which is native to tropical and subtropical areas, has a deep and extensive root system that shows a great potential for pollutant removal from bio-digester effluents. This study aims at identifying the removal efficiency of pollutants from swine biodigester effluents by setting up an integrated treatment system between settling tanks added with rice straw and floating platforms cultured by Vetiver grass.

Materials and methods

The present study was conducted at a pilot level at a medium-scale pig farm (with 50 sows) in Hung Yen province, North Vietnam. The liquid wastewater (effluent) from sow house was flowed directly to biodigester (with a volume of 20 cubic meters). The post-biodigester effluents were then flowed into two settling tanks (with a volume of 8 cubic meters per tank), in which the primary settling tank was added with rice straw (1kg/1m³) and the secondary tank was added with liquid of effective microorganism (1 litter/1m³, containing Bacillus subtilis 1 x 10⁹CFU/L, Bacillus amyloliquefaciens 1 x 10⁹ CFU/L, Saccharomyces cerevisiae1 x 10⁸CFU/L). Finally, the effluents were discharged into a hydroponic pond (200 m²) with four vetiver floating beds (total of floating bed area was 8m²). The detail design of the pilot at the model farm was expressed in figure 1.

Figure 1. Design of the pilot of an intergrated physical and biological wastewater treatment at the model pig farm

Water samples were collected at four locations (post biodigester, post primary settling tank, post secondary settling tank, and post hydroponic pond) according to every stage of treatment procedure at a constant day (at day 7,14, 21, 28, and 35 after treatment) according to National Standard Sampling Method (TCVN 5999:1995).

The pH value was measured by pH meter. BOD concentration was identified by winkler titration method in a strong base environment at 20°C for 5 days. COD concentration was tested by photometric detection with The AQUALYTIC® COD VARIO photometer test. Total concentration of heavy mental (zinc and copper) was identified by Atomic Absorption Spectrophotometer (AAS) method. Total phosphorus, amonia and nitrate concentration were measured by molybdate spectrometric method. Total nitrogen was identified by Kjeldahl titration. Detection and enumeration of thermotolerant coliform organisms and presumptive Escherichia coli was measured by Multiple tube (most probable number) method according to National Standard Method (TCVN 6187-2: 1996).

Data was analyzed by the descriptive analysis method. Comparison of mean was conducted by analysis of variance with the General Linear Model using Tukey test at a confidence interval of 5% by Minitab 16.0 software.

Results and discussion

General characteristics and benefits of the integrated treatment system

This system had a high operational efficiency in term of both decontamination and additional benefits such as gas or energy production, compost fertilizer production, and cattle feed supplementation Firstly, the biodigester worked well and generated a high quantity of gases that was used for farm’s energy consumption for lighting and cooking purpose. The gas was measured with an average quantity of 9.1m³ per day in summer and 4.2m³ per day in winter that was enough for daily energy consumption of the farm. Secondly, the rice straw was collected from the primary settling tanks (after about four to five weeks of treatment) and mixed with pigs’ dung for composting. The composted product was then used as organic fertilizer for crop cultivation. Thirdly, the vetiver grass was cut at young stage and used as roughage for cattle. Maffei M (2002) reported that young leaves of vetiver grass have a good nutritional value (7% crude protein) that can be cut and make fodder for animal during dry season. El Shereef A A and Shehata M F (2020) found that vetiver grass has 10% crude protein, 24.6% crude fiber, and 3.2 % lipids that makes it a promising feed source for small ruminants. Therefore, this integrated system is effective for not only water decontamination, but also for energy generation, composted production, and animal feed provision.

Removal efficiency of BOD and COD concentration

Both BOD and COD concentration are the most important indicators of water pollution which indicates the concentration of organic materials contaminated in water samples. The removal efficiency of BOD and COD concentration by an integrated physical and biological treatment system is presented in table 1 and table 2.

The concentration of BOD descreased significantly thoughout the settling tanks and the hydroponic pond. Results in table 1 showed that the BOD concentration in the effluent from post biodigester was much higher than that in the settling tanks and hydroponic pond (p<0.05). The addition of rice straw as a biofilter helped to filter the wastewater and a high amount of solid materials was kept and sank into the settling tanks. Moreover, the vetiver grass cultivated in the floating beds in hydroponic pond helped to absorb organic materials. The BOD concentration was reduced by 73.9%, 79% and 88.6% in the primary settling tank, secondary settling tank, and hydroponic pond, respectively in comparison with that in the post biodigester effluent. The water in hydroponic pond has a lowest concentration of BOD and this was below the limited standard of type B (≤60 mg/l) for wastewater according to the National Technical Regulation on the Effluent of Livestock (QCVN 62:2025/BTNMT). Hegazy et al (2007) reported that using rice straw as filtration of wastewater helped reduce the BOD concentration by 35.5% and 50% in combination with cemen kiln dust. Therefore, in term of BOD concentration, the water from hydroponic pond can be discharged into the environment with low contamination of organic materials.

The COD concentration in the biodigester effluent was very high that may contaminate the environment if organic materials is not removed and treated effectively. Results in table 2 indicate that after 35 days of treatment, the COD concentration of wastewater in the primary settling tank, secondary settling tank and hydroponic pond were decreased by 27.4%, 58.7% và 84.5%, respectively, compared with that in the biodigester effluent sample. Similar with BOD, the COD concentration of the wastewater in the hydroponic pond after 35 days of treatment was below the limited standard for type B (≤ 150mg/l) according to the National Technical Regulation on the Effluent of Livestock (QCVN 62:2025/BTNMT). According to Hegazy et al (2007) rice straw was effective in COD concentration reduction (32.4% and 50% compared with that in effluent). It could conclude that the integration between rice straw as a biofilter, effective microorganism and vetiver grass hydroponic system had a great efficiency in removing organic materials from biodigester effluent.

The results in table 1 and 2 also pointed out that the pollutant removal efficiency from biodigester effluent by rice straw, EM and vetiver grass system increased gradually by the time of treatment. The BOD and COD concentration was very high in post biodigester. When the effluent was flowed daily to the settling tanks and hydroponic pond, the reduction of BOD and COD concentration was much more significant than that in the previous period. However, when the rice straw was decomposted, its absorption and adsorption capacity reached at the maximun level of saturation, it is in need of replacing new rice straw samples to increase the removal efficiency.

The combination of settling tanks added with rice straw as biofilter with EM supplement and the vetiver grass cultivated on floating beds in hydroponic pond showed a significant efficiency of organic material removal. The BOD concentration of wastewater in primary settling tank, secondary settling tank and hydroponic pond at day 28 were decreased by 40.6%, 64.7%, and 87.1% respectively compared with that in the biodigester effluent sample. When supplementing with effective microorganism, the BOD concentration decreased much more significantly, by 73.9%, 79%, and 88.6% respectively at day 35. This decreasing trend was similar with COD concentration. Therefore, the removal of organic material from biodigester effluent was more effectively when integrating the physically settling tanks added with rice straw with the biologically EM supplement and vetiver grass cultivation in hydroponic pond.

Removal efficiency of nitrogen and phosphorus concentration

Nitrogen and phosphorus pollution from pig wastewater is an extreme concern due to the intensification of pig production and the surplus of wastewater. The removal of nitrogen and phosphorus from effluent plays an important role in wastewater treatment. Table 3 shows the removal efficiency of total nitrogen,amonium, nitrate and phosphorus concentration from biodigester effluents by an intergrated treatment method.

Phosphorus is not a poisonous, but a surplus of phosphorus concentration in water can cause the extreme multiply of some types of algae (algae bloom), leading to the over consumption of dissolved oxygen in the water. The removal of phosphorus from effluents is of great importance in wastewater treatment. Results in table 3 indicated that phosphorus concentration in the hydroponic pond was reduced by 54.6% compared with that in the post biodigester effluents. Vetiver grass and microorganisms have consumed a high amount of phosphorus in the water.

The total nitrogen concentration has decreased significantly from post biodigester to hydroponic pond (from 426mg/l to 248mg/l). However, the nitrogen concentration in hydroponic pond was still high and over the standard limit for wastewater (≤ 60 mg/l for type B according to the National Technical Regulation on the Effluent of Livestock (QCVN 62:2025/BTNMT). The concentration of total nitrogen in wastewater post treatment which was still high might be due to the capacity of nitrogen absorption and adsorption of rice straw and vetiver grass has been saturated.

Similarly, the concentration of ammonium and nitrate have decreased significantly in the hydroponic pond compared with that in the previous treatment stages. The ion ammonium is a compound that is ready for plant adsorption. Therefore, the vetiver grass in the hydroponic pond helped reduce the ammonium concentration in the water.

Rice straw played a role as a biofilter to keep the solid parties in the wastewater then helps reduce the nitrogen concentration. In order to identify the capacity of nitrogen removal capacity, the rice straw samples that sank into the settling tank were collected for nitrogen and phosphorus concentration measurement. Results were presented in table 4.

Rice straw showed a good capacity in nitrogen and phosphorus removal from wastewater. The total nitrogen concentration in treated rice straw was twice times higher than that in the dry rice straw. Similarly, rice straw was highly effective in phosphorus removal from wastewater. Rice straw has a large surface area and thus plays as a biofilter. A high amount of solid parties in wastewater was kept in the settling tank and removed from wastewater. Rice straw was reported by many authors that is a low cost and effective adsorbent for nitrogen and phosphorus removal (Nouran 2012, Hegazy et al 2007).

Removal efficiency of copper and zinc concentration

Heavy mental contamination is one of the most concerns due to their toxic effects since they cause severe health problems to animals and human beings. Copper, zinc are the two main heavy metals that get the most concerns according to the World Health Organization (WHO) and they are commonly present in animal wastewater. Table 5 showed the removal efficiency of copper and zinc concentration from wastewater.

The copper and zinc concentration decreased significantly throughout the treatment system, especially after settling tanks. The rice straw added to the settling tank and the vetiver grass cultivated in the hydroponic pond helped to adsorb the heavy mental. For copper, its concentration in the primary settling tank with rice straw addition reduced by 28.89% in comparison with that in the biodigester effluents. The copper concentration in the hydroponic pond was much lower (reduced by 81.1%) than that in the effluents. The zinc concentration in the primary settling tank and hydroponic pond decreased by 38.6% and 76% compared with that in the biodigester effluent sample. It can be seen that settling with rice straw addition and vetiver grass are very effective in copper and zinc removal from biodigester effluents.

Rice straw has binding sites that make it capable to remove metals from aqueous solutions (El-Sayed et al 2010). Rice straw is rich in lignin and cellulose (cellulose: 32-47%, hemicellulose: 19-27%, and lignin: 5-24%) and other active compounds (alcohols, aldehydes, ketones, carboxylic, phenolic) that provide the ions for the reaction with heavy mental in the solution (Pagnanelli et al 2003, Demirbas 2008). Achanai et al (2012) also reported that rice straw was highly effective in copper absorbtion (74.7mg copper per 1 g of dry matter of rice straw).

Removal efficiency of total coliform and Escherichia coli

Total number of total coliform and Escherichia coli were the two important indicators for assessing the bacteria contamination of wastewater. Table 6 presented the change in total coliform and Escherichia coli number throughout the treatment system.

The number of total coliform and Escherichia coli in the primary settling tank were higher than those in the post biodigester wastewater. In the primary settling tank, main solid parties were kept inside the rice straw and these stimulate the multiply of coliform and Escherichia coli, causing the increase in number of these bacteria in the wastewater. Therefore, the effective microorganism liquid was supplemented into the secondary settling tank helped to control the number of coliform bacteria. The number of total coliform and Escherichia coli in secondary settling tank decreased by 34.3% and 42.4% compared with those in the post biodigester effluents. Similarly, the number of coliform and Escherichia coli in the hydroponic pond decreased significantly (by 79.4% and 87% compared with those in the post biodigester effluent) and had a low fecal contamination on the environment.

Conclusions

• The integrated physical and biological wastewater treatment system between biodigester, settling tanks added with rice straw and effective microorganism liquid and the hydroponic pond cultivated with vetiver grass on floating beds showed a high efficiency in removing the pollutants from swine wastewater, while generating gases for energy consumption, producing composted product, and providing feed to cattle.

• Rice straw, effective microorganism liquid and vetiver grass were significantly effective in removing organic materials, nitrogen, phosphorus, heavy mental, total coliforms and E. coli from biodigester effluents and applicable for swine wastewater treatment.

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