Oxic-settling-anoxic (OSA) process combined with 3,3′,4′,5-tetrachlorosalicylanilide (TCS) to reduce excess sludge production in the activated sludge system

Abstract

The potential of oxic-settling-anoxic (OSA) process with addition of 3,3′,4′,5-tetrachlorosalicylanilide (TCS) to reduce excess sludge production was investigated. TCS was dosed into aeration tank with 0.05, 0.10 and 0.15 g every other day in three lab-scale OSA processes, respectively to form the TCS and OSA combined processes. The OSA and TCS combined processes reduced sludge yield by 21–56% under the same sludge retention time (6.75 h) in sludge anoxic holding tank. Substrate removal capability, effluent NH₃-N concentrations and total phosphorus removal rates were not adversely affected by the presence of TCS or insertion of sludge anoxic holding tank, but total nitrogen removal rates only decreased significantly in the system with addition of 0.15 g TCS during the 60-day continuous operation. The settleability of sludge in four systems was qualitatively comparable and not significantly different. Microscopic examination and the banding patterns of DGGE profiles demonstrated that microbial population changed after TCS addition and insertion of anoxic sludge holding tank. The results suggest that TCS and OSA combined process is effective in reducing sludge yield, and process performance as well as sludge settleability are not significantly affected by introduction of the chemical uncoupler. The results imply that reduction of excess sludge production is due to uncoupled metabolism at low TCS dosage, but microbial death at high TCS dosage.

Introduction

Both municipal and industrial wastewater treatment plants worldwide are mainly based on activated sludge process. However, one of the drawbacks of this process is the high excess sludge production. Secondary sewage treatment plants are being built rapidly throughout China in recent years. With the growing applications of activated sludge processes and upgraded wastewater treatment processes, excess sludge production will escalate. In 2005, China produced 11 million tons dewatered sludge cake (water content of 80%), and it was anticipated that the number will rise to 35 million tons in 2015 [1]. The rising cost from treatment and disposal of excess sludge and restrictive legislation have being emphasized the need for reduction of excess sludge production.

To achieve this goal, several methods have been investigated, such as lysis/cryptic growth, uncoupled metabolism, maintenance metabolism, and predation on bacteria [2], [3], [4]. Although the lysis/cryptic growth method may be feasible in achieving effective reduction of excess sludge production in activated sludge processes, the constraints of the two-step reaction, i.e. lysis and oxidation of lysate, may limit the efficiency of excess sludge reduction, and result in additional costs in inducing lysis and oxidizing lysate. Maintenance metabolism can reduce sludge production in aerobic wastewater treatment processes by 44% when biomass concentration was increased from 1.7 to 10.3 g/L by increasing sludge retention time [5]. However, it is impossible to increase the sludge concentration largely in conventional activated sludge processes by sedimentation. Another method of reducing sludge production is to employ higher organisms such as protozoa and metazoa that predate on the bacteria in the activated sludge processes in a two-stage system. Nevertheless, the long hydraulic retention time in the first stage may greatly increase not only the working volume of bioreactor, but also its capital and operation costs. In general, it is difficult to apply such a two-stage process in practice [6]. The uncoupled metabolism can reduce microbial growth yield by increasing the discrepancy in the energy level between catabolism and anabolism and limiting the energy supply to anabolism. Oxidation of the substrate still occurs but the phosphorylation of ADP to ATP is decreased and consequently there is less energy available for formation of new biomass.

The chemiosmotic mechanism of oxidative phosphorylation (by which ATP is produced during catabolism) can be uncoupled by some chemical uncouplers and under some abnormal circumstances, such as oxic and anoxic (or anaerobic) cycling. Uncouplers of oxidative phosphorylation act by short-circuiting the proton gradient that drives production of ATP by adenosine triphosphates in the cell membrane, limiting cells’ ability to capture energy from substrate oxidation, thereby inhibiting cell growth. Therefore, energy is dissipated and less energy is available for the formation of biomass. When aerobic sludge turns into anaerobic circumstance, it exhausts the intracellular stock of ATP (anaerobic starvation). When the sludge returns into aerobic condition again, it re-synthesizes ATP and stocks it first, then grows with surplus ATP. So it is accompanied by reduction in excess sludge production.

The purposeful promotion of uncoupled metabolism can not only achieve effective reduction of excess sludge production, but also do not need large change in configuration of the conventional activated sludge process. The introduction of metabolic uncouplers seems to be feasible provided that low-price and non-toxic uncouplers can be found. Significant sludge yield reduction was observed in organic protonophore-containing cultures [2], [5], [7], [8]. Our previous investigation found that 3,3′,4′,5-tetrachlorosalicylanilide (TCS) can effectively reduce the sludge yield [9]. TCS at 0.4 mg/L was also found to be the threshold triggering sludge reduction, which can be reduced by around 40% by the addition of 0.8–1.0 mg/L TCS [7], [10]. However, it was found that the addition of TCS somewhat influenced sludge settleability [8], [10], [11].

The oxic-settling-anaerobic or anoxic (OSA) activated sludge process is a modified activated sludge process based on sludge oxic and anoxic (or anaerobic) cycling to induce uncoupled metabolism, by inserting a sludge holding tank in the sludge return line between the aeration tank and the secondary clarifier where activated sludge is exposed to an anaerobic or anoxic zone under no food and low oxidation–reduction potential (ORP) conditions periodically [12]. It is believed that the secondary clarifier can minimize both substrate residues in the liquid and food storage in the sludge, which induces sludge starvation under anaerobic or anoxic conditions. Re-circulation of activated sludge among an oxic (aeration tank), starvation (settling tank), and anaerobic or anoxic environment (sludge holding tank), can reduce 30–50% excess sludge [13], [14]. The sludge settleability in OSA process was excellent and total phosphorus (TP) removal efficiency increased by 19% [12], [14].

In order to maximize reduction of excess sludge together with least negative effect on the process performance and sludge characteristics, we proposed a combined process, i.e. OSA with addition of TCS process. Based on our previous work [14], the optimum sludge retention time in sludge holding tank was 6–7 h, so we investigated excess sludge production, substrate removal capability, and sludge settleability of combined process under different TCS dosage and with the same sludge retention time (6.75 h) in the sludge holding tank. TCS, a component in the formulation of soaps, rinses, and shampoos, etc., was selected as an uncoupler in this study because it is non-toxic to Escherichia coli, and was not detected in the effluent in the previous study [10]. It belongs to chemicals termed as protonophores because of their ability of shuttle electrons across the cell membrane. But TCS is a xenobiotic chemical and its use should be great care. The objective of this research was to maximize excess sludge reduction by employing TCS and OSA combined process with addition of TCS as little as possible, and not undermining the treatment performance. It is hoped that the results generated from this study may provide useful information for future investigation on the feasibility of uncoupled metabolism approaches in reduction of excess sludge.