BLC have just completed a landmark project to design and commission the world’s first Membrane Bioreactor (MBR) and Reverse Osmosis (RO) treatment plant, which can process tannery effluent to desalination levels.
The plant, based in Lorca, Spain, treats effluent from a group of 25 tanneries based in the region.
The modern tannery wastewater treatment plant consists of basic chemical/physical treatment with catalytic oxidation of sulfide to sulfate and primary solids separation of the mixed tannery effluent. The subsequent secondary biological treatment removes high COD and BOD and suspended solids.
However, salts such as sulfates and sodium chloride (found in tannery effluent in concentrations of SO4 >3g/l and NaCl >20g/l) cannot be eliminated with conventional treatment techniques and as such contribute significantly to the environmental impact. An unsolved problem up to now had been the persistence of salts in effluent as there has been no economically feasible treatment technique available.
The presence of salts also complicates the overall intentions for recycling and re-use due to concentration effects. The industry has been urged to implement new treatment processes in order to cope with stringent environmental legislation and to reduce organic pollution, in particular the salt impact on surface waters.
BLC has developed a combination of MBR and RO treatment, which enables efficient polishing of tannery effluent and complete elimination of salts. The application of such technologies allows complete water recycling of mixed tannery effluent as process water and water re-use for irrigation purposes.
BLC in co-operation with AGBAR (Aguas de Barcelona) carried out a long-term pilot study, which was funded under a LIFE EC research project at Lorca, Spain, to realise the potential of large scale application of MBR and RO tannery wastewater treatment. Extensive experimental trials were performed over 14 months to determine the operating conditions for the Ultra filtration (UF) membrane filtration in respect of cross flow velocity and transmembranal pressure and to establish ideal hydraulic retention times (HRT) and sludge retention times (SRT) for the bioreactor to achieve optimal MBR process performance.
Simultaneously, the RO pilot plant was tested with MBR permeate at various pressures and recovery rates to obtain the required optimum design parameters. The results provided an outline design for the construction of a full-scale MBR and RO plant with 5,000m3/day treatment capacity for mixed tannery effluent from 25 local tanneries.
The construction of the wastewater treatment plant was completed in June 2003 and is currently operated by Aquagest Levante. The wastewater treatment plant is part of a co-generation concept in which electricity is generated on-site in a gas power plant which also provides heat for the effluent treatment plant (Figure 1).
MBR and RO operation
The mixed effluent are collected and filtered through a rotating screen to remove gross solids, hair and fibres prior to being pumped to the effluent treatment plant.
Initially sand, earth and fats are removed in a trap, following a balancing step in a 4,000m3 holding tank. Pure oxygen is introduced via a venturi aeration system to guarantee sufficient sulfide oxidation and mixing.
The effluent are continuously transferred to a primary treatment plant, where specific coagulation and flocculation systems are applied.
The primary sludge is settled in a lamella separator and dehydrated with centrifuges and further dried in a tunnel sludge drier, which is operated with the off-heat from the gas power station. After primary treatment the effluent are screened to remove residual hair and fibres and 230m3/hr is fed into the bioreactor.
The bioreactor tank is operated at 8,000-10,000mg/l MLSS with a retention time of 19 hrs. Aeration and mixing is provided by a Jetox venturi aeration system to guarantee a dissolved oxygen concentration of not less than 2mg/l DO (Figure 2). Blowers to provide the necessary air to facilitate ‘bio removal’ according to the parameters determined in the pilot trial feed the submerged aeration plant.
The incoming effluent is efficiently mixed within the 4,000m3 bioreactor tank with the MLSS. This mixed liquor is drawn from the tank continuously by a series of pumps, to the UF filtration units, which are operated in a feed and bleed modus (Figure 3). The ultrafiltration membranes were specified to allow for passage of treated permeate, but retaining the solids/biomass.
The plant is designed on a 23 hour/day basis and with sub-units to prudently provide for spare capacity and downtime
due to back washing, without detriment to discharge volume. The plant, therefore, allows for passage of nominally 5,000m3/day of permeate from the bioreactor to the reverse osmosis treatment plant. The residual COD (<400mg/l), BOD (<10mg/l) and ammonia concentration (<35mg/l) of the MBR permeate is significantly reduced by 90-100% and is a considerable improvement on current effluent qualities achieved with conventional treatment.
The MBR was shown to be an excellent pre-treatment prior to Reverse Osmosis technology, due to the high removal efficiency of suspended solids and organic compounds.
The high quality permeate reduces bio-fouling and scaling of the subsequent RO treatment and, therefore, improves the overall RO treatment performance. The UF permeate is collected in a 500m3 holding tank and continuously transferred to three Reverse Osmosis units. The RO plant operates in ‘Christmas tree’ configuration and is operated at a permeate recovery of 75% and produces 158m3 permeate and 52m3 concentrate per hour (Figure 4).
The conductivity of the RO permeate shows a significant reduction of 98% and has a better quality with average 340µS compared to the 770µS of the local tap water. Figure 5 shows the overall plant performance, achieving a reduction of all relevant wastewater parameters including COD, BOD, suspended solids and conductivity to close to zero.
Conclusions
BLC have successfully designed and commissioned an industrial scale MBR for tannery wastewater treatment currently operational for a tannery cluster based in southern Spain.
The plant shows excellent pollutant removal efficiency of up to 92% COD and 99% BOD.
Due to the reduction of suspended solids and organic compounds, MBR treatment can be considered as the preferable technology to prevent bio-fouling of the subsequent RO treatment. This technology shows specific advantages over conventional activated sludge systems or plain membrane filtration, due to the high effluent quality combined with a degradation of organic pollutants.
The surplus sludge generation can be reduced to 8-10% of the organic load, in comparison with conventional systems where approximately 50% surplus sludge is generated. The industrial scale RO treatment showed, at 75% recovery rate, a highly reduced conductivity of the final effluent of 337µS which was of a better quality than local tap water, enabling water re-use for irrigation.
The economic feasibility was calculated for the industrial scale MBR and RO plant, treating wastewater of 25 local tanneries in Lorca, Spain. The calculations are based on the MBR and RO plant with 5,000m3 treatment capacity per day using the system discussed.
Taking into account the overall energy costs for ultra filtration, RO membranes and aeration and capital depreciation, the costs for treating effluent for this plant are less than €1/m3. This is comparable to conventional industrial effluent treatment costs with the advantage of high quality water recovery and re-use.
