Closing the loop on wastewater management31 October 2002
Increasingly stringent environmental legislation and the rising costs associated with the discharge of contaminated water have increased the need for recycling and reuse of process wastewater by the leather industry. Over the past three years, the BLC has been involved in a DTI funded LINK project to investigate the recycling and reuse of end of pipe effluent, after treatment by MBR and membrane technologies. This forms part of a programme of research and development towards achieving a closed loop production. It has been shown that technologies exist that can lead to full water recovery. Recover and recycle Water intensive industries, such as the leather industry, are under increasing pressure to reduce water consumption and increase the quality of the effluent they discharge. Tanneries may use up to 50m³ of water per tonne of raw material in the production of leather. The production of leather also involves the addition of a large number of varied compounds including biocides, surfactants, syntans and dyes. As such, tanneries may produce large quantities of effluent carrying a significant pollution loading in terms of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). A fraction of the total COD is due to the presence of soluble xenobiotic organic compounds, many of which have been found to be refractory to biological treatment. This soluble COD fraction, the 'hard' COD, may ultimately be discharged to surface waters. Membrane technology Membrane technologies for the treatment of wastewater streams have been an area of intense development in recent years. Membrane filtration has been applied to the treatment of wastewaters including the separation of oily wastewaters. Membranes have also been used to 'polish' secondary treated effluents, allowing for the reuse of industrial wastewater as process water, thereby reducing freshwater consumption and discharge charges. Membrane bioreactors (MBR) are considered as an emerging technology of major potential in wastewater treatment. They provide a relatively compact alternative to conventional biological treatment options, producing very little excess sludge and a 'guaranteed' high quality effluent even at high and varying organic loading rates(1, 2, 3). The MBR is based on the combination of an activated sludge process and membrane filtration in one step. The separation of activated sludge and effluent takes place, using membranes, where all suspended material is removed from the water. In comparison to traditional activated sludge methods, the MBR technology has a number of major advantages: * The complete retention of the biomass allows for considerably higher levels of mixed liquor suspended solids (MLSS) in the reactor, with a theoretically infinite sludge age * Significantly lower sludge production than conventional activated sludge systems due to complete retention of the biomass under low food: micro-organisms (F:O) conditions * The space requirement is greatly reduced due to the absence of settlement tanks * Retention of biomass allowing the growth of slow growing micro-organisms, including nitrifying bacteria, facilitating the biodegradation of 'hard COD' compounds and conversion of ammoniacal nitrogen to nitrate * The effluent produced contains no suspended solids or colloidal material The main disadvantages of present MBR technologies are the relatively high capital cost and energy demand arising from the membrane separation process and maintenance of the increased oxygen transfer efficiencies required to maintain the high biomass loading. MBR technology has been applied to the treatment of wastewaters with high loading from industries such as food, paper and textiles. Research The aim of the research carried out by the BLC was chiefly to investigate the technical development and operation of MBR and membrane filtration technologies for recycling and reuse of process water. The trial objectives were defined as follows: * To determine the minimum quality of effluent suitable for reuse in tannery production * To assess the technical feasibility of the MBR system for treatment of different tannery and fellmongery wastewater * To investigate the use of MBR technology for the removal of recalcitrant compounds * Determine optimal process and operational requirements of the MBR system * Removal of remaining inorganic and organic compounds from the MBR permeate to facilitate in recycling and reuse The experiments were carried out using an MBR plant inoculated with a mixture of biological sludges that had previously been exposed to tannery waste for an extended time period. The reactor was run in a semi-continuous format and yielded data on the quality of the permeate in relation to changing hydraulic retention time (HRT), membrane flux rates, and sludge characteristics. The acclimatisation of the inoculated sludge to the incoming feed took approximately two weeks. Once a steady state was achieved, the HRT was changed and the permeate quality over a range of HRT was determined. The COD removal achieved by the reactor, with a 24 hour HRT, is shown in figure 1. Also shown, after an acclimatisation period of 29 days, the COD removal by the reactor was in excess of 90% of the influent concentration. MBR and membrane performance COD removals of 90% have been achieved using MBR over a 24 hour HRT. Reductions in BOD5 and suspended solids of 99.9% and 100%, respectively, were also achieved. However, despite these removal rates, some COD remained in the treated effluent, which may potentially limit its application in leather processing. To supplement this investigation, tight nanofiltration (TNF) membranes were applied to further polish the MBR-treated effluent and thus facilitate the use of the effluent in tannery processing. The TNF membrane successfully separated the organic and the majority of the inorganic fractions from the effluent, producing a high quality permeate (0-20mg/l COD). The organic component of the permeate was concentrated into 10% of the initial permeate volume, leaving 90% of the treated effluent for recycling and reuse in processing. The TNF membrane also removed 99% of the salts in the permeate, including chlorides. The results are shown in table 1. This is a significant finding as, currently, salt removal from effluent is only thought to be achievable with reverse osmosis (RO) membrane technology. RO has a higher capital cost than TNF and carries a high power requirement. TNF membrane technology uses considerably less power and is potentially a more economic means by which treated effluent could be recycled and reused. This application also raises the possibility of infinite water recycling. Biodegradation of recalcitrant compounds The majority of tannery effluents will contain a number of organic compounds that are resistant to biodegradation in conventional biological treatment plants. These compounds include biocides, surfactants, syntans and dyes. Recent studies have shown that polar, recalcitrant organic compounds, such as naphthalene sulfonate-formaldehyde condensates (SNFC), used in syntans and benzothiazoles, used as fungicides, will also persist in the permeate from MBR4. These recalcitrant compounds may, therefore, be present in the concentrate from the TNF treatment of MBR permeate and will require disposal. The use of selected micro-organisms for the degradation of one group of recalcitrant compounds, syntans based on polymers of SNFC, has been investigated at the University of Kent at Canterbury. SNFC are polar and highly soluble in water. They also display very low biodegradability with typical BOD:COD ratios of less than 0.1. The polymeric nature of the syntans is thought to prevent their uptake and degradation by bacteria. However, research at the University of Kent found that syntans may be depolymerised by certain fungi to monomers of naphthalene sulfonate. These monomers were then entirely degraded by NSA-degrading bacteria. A combined treatment with both fungi and bacteria was found to degrade 65% (+-5) of a proprietary SNFC-based syntan. If the organic content of the concentrate could be fully elucidated and appropriate micro-organisms identified, it may then be possible to apply selected consortia of micro-organisms to degrade the organic content of the concentrate from combined MBR-TNF effluent treatment and achieve 100% recycling of the process effluents in the tannery. Future developments The reuse and recycling project forms part of a programme of research which incorporates not only the treatment of existing effluents but aims to generate impacts with lower environmental impacts. This, combined with MBR technology, could ultimately lead to closing the loop for water use in the tannery. Reduced impact processing is an objective of the EU-funded RESTORM project. Additionally, in a bid to recover byproducts from treatment, the RESTORM project will address a further problem associated with beamhouse effluent, that being the removal of sulfur containing waste. The project will exploit novel anaerobic reactor technology to treat the sulfur components within the waste streams to reduce significantly the environmental impact of effluent and waste sludge. The aim of this task is to investigate the integration of a novel anaerobic reactor design to treat waste concentrates collected from the previous part stream treatment by membrane filtration. The highly organic loaded wastewater will be treated in an anaerobic reactor (UASB) where removed COD is transferred to biogas: a mass balance of the conversion of waste protein into energy will be constructed. The generated biogas will be combusted and the exhaust CO2 gas will be polished in a gas scrubber to remove impurities like residual H2S and NOx to enable further re-use for de-liming. In the process of de-liming pH has to be reduced from 13 to 8.5, which is currently achieved by addition of ammonia sulfate. The resultant CO2 can be collected in a gas tent and re-used in compressed form for pH reduction in the deliming process (replacing addition of ammonia sulfate), enabling complete gas recycling. The CO2 collected from biogas combustion will be introduced directly into the drum, therefore replacing the addition of chemicals. To achieve this task, compressed CO2 will be introduced via the drum axle and bubbled through a porous diffuser to achieve good gas transfer to the liquid. The RESTORM project will also look closely at the management of wastewater from individual waste streams. This will facilitate a scientific study on each of the process stages prior to combined effluent treatment and eventual recommendation to the industry on the most effective and efficient closed loop system for wastewater management. A further objective of the RESTORM project is to develop and demonstrate a combined membrane bioreactor (MBR) system for the enhanced and cost-effective treatment of anaerobic pre-treated effluents from the leather industries. RESTORM will primarily focus on improving the economics of MBR technologies and tailor the process to meet the needs of the leather and related industries.