Pretreatment of landfill leachate using deep shaft aeration bioreactor (DSAB) in cold winter season

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Abstract

A pilot-scale deep shaft aeration bioreactor (DSAB) with 110 m in depth and 0.5 m in diameter for the pretreatment of landfill leachate in winter was operated at a daily treatment scale of around 10–20 tons. It was found that the performance of the DSAB mainly depended on the inflow loads and concentrations of pollutants. NH3-N, TN, COD, TOC removals of 66–94%, 41–64%, 67–87%, 55–92% at organic load rate of 1.7–9.4 g COD L−1 day−1 and hydraulic retention time of 1–2 d were obtained using DSAB, respectively, with the lowest ambient temperature of −3 °C. The effluent COD can be reduced to below 1000 mg/L, an acceptable level for advanced treatment using reverse osmosis system, when the influent COD was below 7000 mg/L at 10 t/d. The EEM and GPC analysis implied that the non-biodegradable contaminants such as humic- and fulvic-like DOM dominated in the organic fractions of the effluent, which rendered the biological treatment ineffective. Compared with 20–40% removals obtained using traditional biological processes below 15 °C, DSAB showed a higher treatment efficiency for COD and NH3-N, even though at adverse conditions of poor carbon source, lower C/N ratio and high nitrite concentrations in the leachate of test.

Highlights

  • The deep shaft aeration bioreactor (DSAB) technique is firstly used to pretreat the landfill leachate in cold winter reason.

  • The DSAB showed a high treatment efficiency for the pollutants in landfill leachate below 15 °C, i.e. with a removal of 66–94% NH3-N and 67–87% COD, respectively.

  • The DSAB had a good resistance to shock pollutants load at COD and NH3-N concentrations of 4000–10,000 mg/L and 1500–2250 mg/L, respectively.

  • EEM and GPC analysis was used to explore the role of DSAB in removal of organic matters from landfill leachate.

Introduction

Leachate is a by-product generated in the municipal solid wastes (MSW) landfills and its advanced treatment to meet the environmental constraints is still a challenge in the MSW management field. Many biological and physico-chemical methods have been developed and practiced, such as anaerobic and aerobic treatment, advanced chemical oxidation, absorption, etc. [1], [2], [3], [4], [5]. The discharge standards set for the leachate have been become more stringent and an effluent with chemical oxygen demand (COD) <60–100 mg/L may be required. In this case, a combination of biological and chemical methods should be used.

Biological processes have been widely used in practice due to their cost-effectiveness. However, at relatively low ambient temperatures, the COD values may be over 1000 mg/L when the biological processes are used alone. In the cold winter seasons, the leachate may have to be heated during biological process so that the COD can be reduced to an acceptable level, e.g., below 1000 mg/L, facilitating the following advanced physical process such as reverse osmosis (RO) technique to make the end effluent quality reach the discharge standards.

Deep shaft biological technique was a modification of the conventional aeration sludge process (ASP) without any requirement for external heating system [6]. A deep shaft, normally 50–150 m deep and 5–6 m wide, could ensure higher partial pressure of oxygen at the bottom of the reactor, resulting in high oxygen transfer efficiency (OTE) [7]. The deep shaft aeration process enhanced the OTE up to 60–90% in comparison with the OTE of less than 30% in conventional ASP [8]. Under high hydraulic pressure, the high OTE of the deep shaft aeration process drastically reduced the power consumption. Moreover, this process was well-received by many small land area countries like Japan due to a small space requirement [9].

The deep shaft biological technique has been applied to the treatment of municipal wastewater [10], food processing waste [11], potato processing starch waste and pulp and paper mill effluent [7], brewery waste [12], municipal sludge [13] and sulfite mill evaporator condensate and saponification wastewater from propylene oxide plant [14]. However, there is no information regarding the treatment of landfill leachate using deep shaft bioreactor in literature.

Landfill leachate, a high-strength typical wastewater, may be characterized by four groups of pollutants including dissolved organic matter (CH4, volatile fatty acid, fulvic-like and humic-like compounds), inorganic macrocomponents (Ca, Mg, Na, K, NH4+, Fe, Mn, Cl, SO42− and HCO3), heavy metals (Cd, Cr, Cu, Pb, Ni and Zn), and xenobiotic organic compounds (aromatic hydrocarbons, phenols and chlorinated aliphatics). Moreover, the varied quality and quantity of leachate subjected to seasonal variations, refuse compositions, waste ages and landfilling techniques caused a challenge to meet the increasingly stringent discharge standards in many countries [15], [16].

In China, the combined processes of membrane reactor (MBR) and RO system were widely used to meet the new discharge standard (GB16889-2008) of landfill leachate in recent years. However, most of the MBRs should be operated at ambient temperature to obtain a relatively high removal of pollutants [17], [18], [19]. Furthermore, RO system has become a predominant leachate advanced treatment process to reach the China Discharge Standard of COD <100 mg/L. However, the RO system may be seriously deteriorated and even disrupted and a huge quantity of concentrated liquid may be generated, when the influent COD is higher than 1000 mg/L. Few studies on leachate pretreatment at low temperature (below 15 °C) can be traced in the literature. Generally, lower temperature has a critical effect on the growth of bacteria, which resulted in a poor removal of NH3-N and COD [20], [21]. Sundaresan and Philip found that only 40% removal of COD was obtained at 10 °C using the submerged bed reactor and the nitrification efficiency reduced drastically with the decrease of temperature [22]. Uemura and Harada also observed that 58% of the entrapped particulate organic matter was liquefied at 15–18 °C but only 33% at 13 °C which means the hydrolysis rate of the entrapped organics was significantly affected by the temperature [23].

Hence, the extra warming-up devices were needed to provide an ambient temperature for the effective removals of COD and NH3-N in large-scale MBRs during the cold winter season so that the effluent COD can be below 1000 mg/L to meet the requirement of the coming RO system. The operating cost of the process of “anaerobic/aerobic + MBR” was usually overspent due to the membrane fouling and the variable quality of leachate, which always resulted in economically overburden for the leachate treatment plant, especially at low temperature.

In this paper, a pilot-scale deep shaft aeration bioreactor (DSAB) was constructed and operated on site at Shanghai Laogang Landfill, the largest landfill in Asia, for the pretreatment of the landfill leachate at cold winter with a lowest temperature of −3 °C. The aim of this project is to find a solution for the preliminary treatment to reduce COD to below 1000 mg/L, an acceptable level for the advanced treatment using RO system. The parameters of COD, total organic carbon (TOC), ammonia nitrogen (NH3-N), total nitrogen (TN), nitrite (NO2) and nitrate (NO3) before and after treatment using the bioreactor were analyzed to explore the operation performances of the DSAB. The DOM in leachate was characterized using the gel permeation chromatography (GPC) and the excitation-emission matrix fluorescence spectroscopy (EEM) to explore the role of DSAB in removal of organic matters from landfill leachate.

Section snippets

Sampling sites

Shanghai Laogang Refuse Landfill is the largest municipal landfill in China, and was constructed in 1985 along the shore of the East China Sea and started operation at the end of 1989. At present, Shanghai Laogang Refuse Landfill receives nearly 10,000 tons municipal solid wastes every day and the leachate produced is about 3000 m3/d.

Scheme of the DSAB

The landfill leachate distribution process is schematized in Fig. 1. In the pilot-scale project, the DSAB was built with the concentric circular structures and

Characteristics of the leachate of test

The leachate used in this work was categorized into aged leachate and fresh leachate. The aged leachate was obtained from the collecting tank of the first-stage aged refuse biofilter (ARB) consisting of 8–10 years old aged refuse. The three-stage ARB has been conducted with a daily treatment scale of 400 m3 leachate in Shanghai Refuse Landfill and operated for over 7 years in our previous research work [25]. The aged leachate was used for the start-up of the DSAB and the low values of COD and NH3

Conclusions

The DSAB had a good resistance to shock pollutant impact in spite of the dramatic fluctuation of the influent COD concentrations from 4000 mg/L to 10,000 mg/L and could remove organic pollutants and nitrogen matters in the leachate effectively in cold winter. Under a high OLR (1.7–9.4 gCOD L−1 day−1) and a relatively low HRT (1–2 d) conditions, the DSAB can still stably remove 67–87% COD, 55–92% TOC, 66–94% NH3-N and 41–64%, respectively, even though at low carbon source, low C/N ratio and high

Acknowledgements

This work was supported by the Science and Technology Commission of Shanghai Municipality (Nos. 10DZ1200109; 11DZ0510200), and the Science and Technology Commission of Suzhou Municipality (No. SS201106).

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