Removal of ammonia from aqueous solution for swine wastewater with swine manure compost-based char

Although for total of ammonia, ammonium compounds, and nitrate and nitrite compounds, 100 mg-N L⁻¹ is decided by the Ministry of the Environment Japan as the National Effluent Standards, the temporary standard of 700 mg-N L⁻¹ is applied to a swine wastewater treatment plant and it would be 100 mg-N L⁻¹ in the future. Therefore, nitrogen removal from swine wastewater is an issue of capital important.

Swine wastewater treatment process is composed of solid removal (primary treatment) and activated sludge treatment (secondary treatment). Denitrification is an effective way to remove nitrogen from swine wastewater and more than BOD/N ratio of 3 in influent is necessary to achieve high denitrification rate (Osada et al. 1991). Waki et al. (2010) reported that average BOD/N ratio in primary treatment effluent was 2.7, which led insufficient nitrogen removal in secondary treatment. Since ammonium accounted for 80% in total nitrogen in primary treatment effluent, it was important to remove ammonia from primary treatment effluent before secondary treatment in order to improve BOD/N ratio.

To remove ammonium from solution, air stripping, wet-oxidation, membrane process, and precipitation of magnesium ammonium phosphate have been conducted (Boopathy et al. 2013). They required complicated configuration and had difficulties in maintenance due to the scale formation during precipitation and stripping (Benefield & Judkins 1982; Melcer & Nutt 1988; Hung et al. 2003; Bodalo et al. 2005). Adsorption process was compatible with these processes in terms of removal efficiency and cost. Coconut shell (Boopathy et al. 2013), cacao shell, corncob (Hale et al. 2013), and rice husk (Zhu et al. 2012) have been considered as a source of biochar for ammonium removal, and adsorption isotherm and adsorption kinetics were studied. However, the considered pH for adsorption was initial solution condition, not equivalent condition. Research under high ammonium concentration considering ammonium concentration of primary treatment effluent, about 1,000 mg-N L⁻¹ (Waki et al. 2010), was little (Boopathy et al. 2013).

On the other hand, compost production from livestock manure is increasing, because the Act on the Appropriate Treatment and Promotion of Utilization of Livestock Manure forces to forward appropriate livestock manure treatment such as composting. Therefore, compost would excess in some region in Japan, and development of usage of compost except direct fertilizer was expected. Biochar was considered as one of other usage of compost.

In this study, biochar produced from swine manure compost was used to evaluate the effect of pH, temperature, size of biochar on ammonium adsorption property considering wine wastewater treatment.

The compost used for pyrolysis was obtained from a swine farm in Miyazaki prefecture, where swine manure was mixed with sawdust, agitated with a rotary fermenter for about 2 weeks, and then stacked for about 3 weeks. The characteristics of the compost were as follows: water content, 27%; ash content, 33% on a dry basis; and combustible content, 67% on a dry basis. Swine manure compost-based biochar was produced in a pyrolysis unit (30 kg-compost h⁻¹) at 450 °C for 1 hour, and sieved into less than 1 mm and 1–2 mm of particle size (denote as C1 and C2, respectively). Surface area of biochar was determined by sorption of N₂ at 77 K using an automated surface area analyser. Cation exchange capacity of biochar was determined by according to Schollenberger & Simon (1945).

Batch adsorption experiment was carried out in 100 mL plastic bottles containing 5 g of biochar and a 100 mL of ammonium solution with NH₄Cl. The ammonium solution was also prepared with HCl to prepare the desired pH of solution after shaking. The bottles were shaken at 140 rpm for 6 hours in a water bath at 20 or 40 °C. After shaking, pH of solution was measured and filtrate was obtained with 0.45 µm membrane filter. Ammonium concentration was determined by indophenol blue absorptiometry. Potassium concentration was determined with ICP-AES.

Adsorption kinetics experiments were conducted by preparing 5 plastic bottles of 100 mL containing 5 g of biochar and a 100 mL of ammonium solution (1,000 mg-N L⁻¹) with HCl as same as the batch adsorption experiment. The bottles were shaken at 140 rpm at 20 °C. Each bottle was taken after 10, 20, 30, 40, 60 minute.

Table 1 shows the characteristics of biochar. Specific surface and CEC of C2 was about two times larger than those of C1.

Langmuir model

Freundlich model

where q_max is the adsorption capacity of biochar (mg g⁻¹), K is the Langmuir adsorption constant (mg L⁻¹), and K_F and n are the Freundlich adsorption constant.

Figure 1 shows an example of adsorption isotherms fitted with the Langmuir model, and Table 2 shows the correlation coefficient (R²) in all condition. Except the condition of 40 °C and pH 9, R² of the Langmuir model were higher than the Freundlich model. It can be concluded that the Langmuir model was suitable for describing the adsorption equilibrium of ammonium by biochar from swine manure compost. Table 3 shows the removal ratio of ammonium at an initial concentration of 1,000 mg-N L⁻¹. The average removal ratio was 19%, which was not so high. It indicates that the roll of adsorption by the biochar was not to remove ammonium completely, but to reduce the nitrogen load for the secondary treatment.

Figure 2 shows the effect of temperature, pH, and size of char on q_max. q_max was increased with increase in pH up to pH 8 and kept constant over pH 8. The reason of increase of q_max with increase in pH would be considered that at alkali pH, the active sites of the biochar became negatively charged, which enhanced the binding of the ammonium ion onto the biochar (Boopathy et al. 2013). It was considered that q_max was saturated over pH 8.

Higher temperature tended to lead higher q_max, which suggests that the adsorption reaction of the biochar would be exothermic (Kučić et al. 2103). Boopathy et al. (2013) reported that higher temperature led the lower q_max. Therefore, the effect of temperature on q_max would depend on the raw material of biochar.

The difference in particle size affected little q_max even though the CEC of C2 was about twice lager than C1. The reason could be considered that the biochar contained water soluble potassium of 98 mmol (100 g)⁻¹, which exceeded the CEC of biochar and potassium ion occupied a large amount of adsorption site of biochar.

The possibilities for ammonium removal from aqueous solution considering swine wastewater onto biochar produced from swine manure compost were studied. The low removal rate of ammonium at an initial concentration of 1,000 mg-N L⁻¹ considering primary treatment effluent indicated that the roll of adsorption by the biochar was not to remove ammonium completely, but to reduce the nitrogen load for the secondary treatment. The Langmuir model was demonstrated to provide the best fit for the adsorption of ammonium on the biochar. Higher temperature and pH promoted the adsorption capacity although the effect of particle size of the biochar was little. The kinetic studies suggested that the adsorption of ammonium on the biochar was described by the pseudo-first order kinetic model and the rate constant was affected by pH.

Financial support was provided by the Scientific Technique Research Promotion Program for Agriculture, Forestry, Fisheries and Food Industry, Project ‘Building a Business Model of Utilizing Resources in Swine Manure’.