Environ. Sci. Technol. 2005, 39, 8505-8511
Desalination of Mixed Tannery Effluent with Membrane Bioreactor and Reverse Osmosis Treatment W . G . S C H O L Z , * , † P . R O U G EÄ , ‡ A. BO Ä DALO,§ AND U. LEITZ| W2O Environment, Queen’s Park, 58 Cecil Road, NN2 6PQ, Northampton, United Kingdom, AGBAR, Aguas de Barcelona, Divisio´n Agua y Sanamiento, Paseig de Sant Joan 39-43, 8009 Barcelona, Spain, Departamiento de Ingenierı´a Quı´mica, Universidad de Murcia, 30007 Espinardo, Murcia, Spain, and Berghof Filtrations und Anlagentechnik, Harretstrasse 1, 72800 Eningen, Germany
A limiting factor for the reuse and recycling of treated tannery wastewater for irrigation and other uses is the high salt content, which persists even after conventional treatment. Reverse osmosis (RO) membrane treatment has been shown to significantly reduce the salt contents of tannery effluents. However, the high organic content of tannery effluent leads to rapid scaling and biofouling of RO membranes with a consequent reduction in flux rates and performance. Membrane bioreactors (MBR) have been shown to be highly effective in the removal of organic pollutants and suspended solids from tannery effluent. This research investigated the use of a combined MBR and RO treatment process to treat tannery effluents to an acceptable level for irrigation purposes. The MBR was operated at 17-20 h retention time, at a F/M ratio of 0.52 kg COD‚kg SS-1‚day-1 and a volumetric loading rate of 3.28 kg COD‚m-3‚day-1. This treatment reduced the COD, BOD, and ammonia concentrations of the effluent by 90-100%. The MBR was shown to be an excellent pretreatment prior to RO technology, due to the high removal efficiency of organic compounds and suspended solids, with average concentrations of 344 mg‚L-1 COD and 20 mg‚L-1 BOD achieved in the permeate. RO treatment reduced the salt content of the MBR permeate by up to 97.1%. The results of the research demonstrated that the MBR system developed was appropriate for the treatment of tannery effluents and, in combination with the RO treatment, reduced the salt content to acceptable levels for irrigation. The MBR pretreatment reduced bio-fouling and scaling of subsequent RO treatment and improved the overall performance of the RO unit. It is believed that this is the first investigation of a combined MBR and RO treatment for tannery effluents. This research provided data for an outline design of a full-scale MBR and RO plant with a treatment capacity of 5000 m3 per day for mixed tannery effluents.
* Corresponding author phone and fax: e-mail:
[email protected]. † W O Environment. 2 ‡ AGBAR, Aguas de Barcelona. § Universidad de Murcia. | Berghof Filtrations und Anlagentechnik. 10.1021/es050330p CCC: $30.25 Published on Web 09/27/2005
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2005 American Chemical Society
Introduction Tanning is a water intensive process and, as a consequence, wastewater disposal is one of the major concerns in tanneries. The characteristics of untreated tannery wastewater are a high chemical and biochemical oxygen demand and high salt content. Tannery effluents will, therefore, require extensive treatment prior to recycling and reuse. A prerequisite for the use of tannery effluent for irrigation of agricultural land is the removal of salt. Significant reductions in the salt loading of tannery effluents were achieved by the use of reverse osmosis (RO) membrane filtration (1). However, fouling of the membranes, due to the organic content of the effluent, resulted in marked reductions in the flux rates, even after pretreatment of the effluent with ultrafiltration membranes. To reduce fouling and scaling of RO membranes several pretreatment technologies were tested by Suthanthararajan et al. (2) using a multistage pressure sand filter, a photochemical oxidizer with UV light, a softener, an activated carbon filter, a polypropylene cartridge, and nanofiltration. The photochemical oxidizer and the activated carbon filter removed 5-20% of the COD present and suspended solids were completely removed by the sand filter and cartridge filter. However, frequent fouling of the nanofilter was observed. Therefore, in order for RO treatment to be a viable option for the treatment of tannery effluents, an effective biological pretreatment to reduce or remove the organic content would be required. Membrane bioreactors (MBRs) are an emerging technology of major potential in wastewater treatment. MBRs provide a relatively compact alternative to conventional biological treatment options, producing a high-quality effluent even at high and varying organic loading rates (3, 4). The process relies on membrane filtration to effectively retain all the biomass in the bioreactor. The membrane is used as an alternative to gravity separation as applied in clarifiers or sedimentation tanks, where a small amount of biomass is discharged continuously with the effluent. As a consequence, the MBR process is operated at much higher mixed liquor concentrations, up to 30-50 g‚L-1 mixed liquor suspended solids (MLSS), than conventional biological treatment (5). Previous research has shown that MBR technology provides a practical option for the treatment of difficult effluents from the leather industry, enabling water reuse. Pilot scale MBR technology was evaluated for the treatment of effluents of ovine and bovine tanneries (6). Highly polluted soaking liquors were treated at a retention time of 2 days in a MBR with cross-flow ultrafiltration, achieving reductions in COD (90-96%), BOD (95-100%), and suspended solids (100%). A major benefit of MBR, resulting from increased sludge retention times and operating temperatures, is the reduction of surplus sludge generation (7). A relatively low cost MBR, developed specifically for the treatment of tannery effluent, was reported to effectively treat difficult tannery effluent streams and enable water reuse in the leather manufacturing process (6). The MBR process is well suited for tannery effluents which generally require long retention times for the effective biological treatment of the less degradable organic pollutants present. MBR would, therefore, be expected to provide an adequate pretreatment to prevent fouling of RO membranes by residual organics, during the polishing of tannery effluents. The aim of this research was to evaluate a novel combined treatment process for the reuse of tannery effluents for irrigation, in a continuous on-site pilot trial. Previous attempts VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic illustration of membrane bioreactor plant. to apply RO membrane treatment at the site had failed due to rapid fouling of the membranes. A MBR system was tested as a pretreatment to prevent biofouling of RO membranes during the subsequent elimination of the salt content of the permeate. Extended on-site trials of MBR were carried out using ultrafiltration (UF) cross-flow membranes and two RO membrane types were evaluated for removal of salt from the MBR permeate. It is believed that this is the first investigation of a combined MBR and RO treatment for tannery effluents. The results obtained from the research with the pilot system were used to assess the technical and economical feasibility of a large scale MBR and RO plant for the treatment of tannery effluent.
Materials and Methods Membrane Bioreactor and Cross-Flow Ultrafiltration Unit. The membrane bioreactor pilot (Figures 1 and 2) consisted of a 3-m3 aerated bioreactor tank connected via a centrifugal pump to a P31C-37100 cross-flow ultrafiltration membrane filtration unit (Berghof, Eningen Germany). The permeate tank (0.5 m3) also served as a cleaning tank and was connected to the membrane filtration system by two three-way valves. The necessary cleaning intervals were determined during the trial. Flow and pressure of the membrane pump were adjusted via a bypass and controlled by pressure gauges and flow meters. The installed pump was a vertical, nonselfpriming, multistage, in-line centrifugal pump, and supplied 56 m3‚hr-1 at 4 bar. The pump was equipped with a 11 kW/21.5 a/2990 rpm motor. The level in the bioreactor was automatically controlled, to prevent dry running or overflow of the tank, by regulating the feed or by re-directing the permeate back into the bioreactor tank. The plant consumed approximately 10 kWh electricity at 380 V, 60 Hz, 3 phase. The bioreactor was inoculated with activated sludge from a local municipal plant and supplemented with adapted biomass from a pilot-scale MBR treating tannery effluents 8506
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FIGURE 2. Membrane bioreactor pilot plant used in the on-site trials. (BLC, Northampton, UK). In the course of the trials, pH values were adjusted to 7-7.5 and the temperature was kept in the range of 25-37° C to maintain nitrification. Two PE cross-flow ultrafiltration modules were installed in series with a total filter area of 6 m2, a tube diameter of 12 mm, and a MWCO of 100 kDa, and were operated at 4.5 m‚sec-1 and 3 bar transmembranal pressure (Table 1). The membranes were operated in the cross-flow filtration mode,
TABLE 1. Technical Specifications of the Ultrafiltration and Reverse Osmosis Modules Used in the Described Trials ultrafiltration
reverse osmosis (Toray)
reverse osmosis (Osmonics)
configuration
31 parallel tubes
membrane polymer
PES
membrane surface area nominal cutoff operating pressure TMP max. operating temperature pH operating range axial flow rate, m3/h permeate flux L/m2‚h
3 m2 100 kDa 3 45° C 2-12 42-56 (4-5 m‚sec-1) 130.5
spiral-wound; 31 mil spacer aromatic polyamide composite (2-layer) 7 m2
