Elsevier

Water Research

Volume 43, Issue 9, May 2009, Pages 2363-2372
Water Research

A novel sulfate reduction, autotrophic denitrification, nitrification integrated (SANI) process for saline wastewater treatment

https://doi.org/10.1016/j.watres.2009.02.037Get rights and content

Abstract

This paper reports on a lab-scale evaluation of a novel and integrated biological nitrogen removal process: the sulfate reduction, autotrophic denitrification and nitrification integrated (SANI) process that was recently proposed for saline sewage treatment. The process consisted of an up-flow anaerobic sludge bed (UASB) for sulfate reduction, an anoxic filter for autotrophic denitrification and an aerobic filter for nitrification. The experiments were conducted to evaluate the performance of the lab-scale SANI system with synthetic saline wastewater at various hydraulic retention times, nitrate concentrations, dissolved oxygen levels and recirculation ratios for over 500 days. The system successfully demonstrated 95% chemical oxygen demand (COD) and 74% nitrogen removal efficiency without excess sludge withdrawal throughout the 500 days of operation. The organic removal efficiency was dependent on the hydraulic retention time, up-flow velocity, and mixing conditions in the UASB. Maintaining a sufficient mixing condition in the UASB is important for achieving effective sulfate reduction. For a typical Hong Kong wastewater composition 80% of COD can be removed through sulfate reduction. A minimum sulfide sulfur to nitrate nitrogen ratio of 1.6 in the influent of the anoxic filter is necessary for achieving over 90% nitrate removal through autotrophic denitrifiers which forms the major contribution to the total nitrogen removal in the SANI system. Sulfur balance analyses confirmed that accumulation of elementary sulfur and loss of hydrogen sulfide in the system were negligible.

Introduction

In Hong Kong the capacity of the landfills for sewage sludge will be suppressed in 2015. Sludge incineration seems to be a last remaining option for Hong Kong. However, it would impact the rapidly deteriorating air quality as well as create a problem of locating the incinerator without large public discontent. Reduction in the volume of sewage sludge is a first step in the solution of the sludge problem. Anaerobic digestion of secondary sludge hardly decreases the sludge and construction of more sludge digesters is a problem in Hong Kong because of limited available land. In this respect, reduction of sludge within the treatment works without the need for extra space is ideal for Hong Kong.

Various sludge-minimizing options have been studied, such as disintegration of excess sludge by thermal, ultrasonic and ozone pretreatments (Nickel et al., 1998, Rocher et al., 2001, Saby et al., 2002) or modification of a biological nitrogen removal (BNR) process into an oxic-settling-anaerobic (OSA) process by inserting a sludge holding tank in the sludge retune line between its final clarifier and bioreactor (Saby et al., 2002, Saby et al., 2003, An and Chen, 2008). However, these options lead to either high costs or the need for more space. The best option for sludge minimization is using low sludge production processes. For chemical oxygen demand (COD) conversion of low sludge production can be achieved by using low quality electron acceptors instead of oxygen or nitrate. Combination of denitrification and methanogenesis has been proposed (Akunna et al., 1994, Hendriksen and Ahring, 1996, Del Pozo and Diez, 2003). In such systems, heterotrophic denitrification and methanogenesis both occur in the anaerobic reactor. As the sludge production from heterotrophic denitrification (0.4 g volatile suspended solids (VSS)/g COD) is much higher than that from anaerobic reactions (0.1 g VSS/g COD), diversion of the substrate from methanogenesis towards denitrification increases the overall sludge production (Inamori et al., 1996). In addition, methanogenesis is much slower than denitrification, which requires a very long hydraulic retention time (HRT) to remove COD efficiently.

Besides methanogenesis, sulfate reduction also leads to a low sludge yield (Lens et al., 1995, Lens et al., 1998) because the growth yield of sulfate-reducing bacteria (SRB) is only 0.2 g VSS/g reduced sulfate (Kleerbezem and Mendez, 2002, van den Bosch et al., 2007). The minimal COD requirement in sulfate reduction is 2 g of COD consumed per g of SO42-S reduced. Hong Kong sewage contains 500 mg/L sulfate or 167 mg/L SO42−-S and 400 mg/L COD, indicating a sufficient reduction potential. SRB can out-compete methane producing bacteria (MPB) for organic substrates because of their higher specific growth rate and lower Monod's saturation coefficient than that of MPB (Widdel, 1988). In a closed anaerobic environment, hydrogen sulfide generated from sulfate reduction tends to dissolve in water as pH increases (Harada et al., 1994), thereby generating adequate amounts of dissolved sulfide. It is known that various sulfur sources are able to serve as electron donors for autotrophic denitrification to remove nitrate (Kleerbezem and Mendez, 2002, van den Bosch et al., 2007). Since the growth yield of autotrophic denitrifiers is low, combining sulfate reduction and denitrification based on sulfide leads to low net sludge production.

Based on the above rationale, we have recently developed the sulfate reduction, autotrophic denitrification and nitrification integrated (SANI) process for low-cost reduction of excess sludge (Lau et al., 2006, Tsang et al., in press). This new BNR process significantly reduces excess sludge production because the three major microbial populations in the process: SRB, autotrophic denitrifiers and nitrifiers all have low growth yields. In this process, most of the COD is oxidized to CO2 in sulfate reduction by SRB. By considering the full process design, we estimate that the total cost reduction would be more than 50% for a 10,000 m3/day sewage treatment works (unpublished data). Fig. 1 shows a schematic diagram of the experimental setup of the SANI system. It consists of (1) an anaerobic zone to remove COD by SRB; (2) a subsequent anoxic zone for autotrophic denitrification of nitrate with dissolved sulfide generated from sulfate reduction integrated with (3) an aerobic zone to nitrify ammonia and recirculate nitrate to the anoxic zone for the denitrification. It should be noted that phosphorus removal is not mandatory in Hong Kong since all sewage treatment effluents are discharged to the sea. The major challenges we face in the SANI process are the (1) effectiveness of sulfate reduction, (2) effectiveness of the autotrophic denitrification using dissolved sulfide; (3) performance of the process with respect to COD and total nitrogen (TN) removal and excess sludge production; (4) impact of the recirculation flow on autotrophic denitrification; and (5) accumulation of sulfur in the system. A comprehensive study was conducted to deal with these challenges. It was divided into three parts: (1) a lab-scale demonstration of the process with synthetic saline wastewater; (2) development of a steady-state model for process evaluation and (3) a pilot-trial of the process with real saline sewage. Parts (1) and (2) were completed in the last five years, while Part (3) is currently conducted. This paper reports on the laboratory study. The objective of this study was to develop a lab-scale setup of the SANI process to examine the effectiveness of sulfate reduction and autotrophic denitrification and to test the performance of the entire integrated process in terms of COD and nitrogen removal and sludge reduction.

Section snippets

Experimental setup

Fig. 1 shows the lab-scale SANI system. An up-flow anaerobic sludge bed (UASB) was used as the sulfate reduction reactor. It was made with 100 mm in diameter and 400 mm in height and covered on both ends with plastic plates held together by stainless steel fasteners. Rubber O-rings were used to seal the contacts between the plates and the cylinder to make the cylinder complete airtight. The reactor had a liquid volume of 33.2 L plus a headspace of 0.5 L. An anoxic filter had a diameter of 100 mm and

Performance of UASB

The main performance results of the UASB reactor in these four stages are reported in Fig. 2. The UASB was operated for more than 600 days with average organic and sulfate influent concentrations of 265 mg COD/L and 166 mg SO42−-S/L. In Stage 1, the UASB was operated at an HRT of 8 h. After a 60-day start-up period, the UASB achieved 80% COD removal. The up-flow velocity in the reactor was 0.2–0.3 m/h and formation of sulfidogenic granules was observed in the bed. In this anaerobic reactor, 45% of

Competition of SRB and MPB in UASB for COD removal

The average COD-to-sulfate sulfur ratio in this study was 2.4. This suggested that about 83% COD reduction (as calculated from the theoretical value (2) to this measurement (2.4), i.e. 2/2.4 = 0.83) came from SRB and the remaining COD was obviously removed through methanogenesis since MPB and SRB share many ecological and physiological similarities. Three general relationships between MPB and SRB are identified: (1) co-existence through using separate substrates; (2) a synergistic relationship in

Conclusions

The lab-scale SANI process was successfully established to demonstrate the good potential for COD and nitrogen removal from saline wastewater with low sludge production. Experimental work lasted for 500 days to investigate the performance of the UASB, the anoxic filter and the entire SANI process on a laboratory scale under long-term stable process conditions. The main conclusions of this study are as follows.

  • (1)

    The lab-scale SANI system successfully demonstrated high COD and nitrogen removal

Acknowledgements

The authors gratefully acknowledge the financial support from the Research Grants Council of the Hong Kong Special Administrative Region (HKUST6136/04E) and also sincerely appreciate Professor G. A. Ekama for his scientific and technical comments for this paper.

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