Tannery effluent treatment using reed bed and nanofiltration technology

The reed bed treatment is low in operation costs because no aeration and mixing are required and is, therefore, considerably cheaper in operation and maintenance compared to conventional treatment. The reed bed plant achieves a significant reduction of COD of up to 80% with COD levels reduced down to 100-250 mg/l in the final effluent. However, the reed bed performance varies in the course of the year and COD concentration discharged tends to increase in autumn and winter. This fact made an upgrade of the current plant with nanofiltration necessary. TCIM now consistently achieve the stringent discharge limit to river of <125 mg/l COD. Reed beds - the low cost tannery effluent treatment Reed bed treatment is based on the principle of aquatic plants, growing in a bed of gravel, which is flooded with effluent (Picture 1). Reed beds have a high degree of physical, chemical and biological complexity. The treatment of effluent is achieved by a combination of the micro-organisms, the physical and chemical properties of the solid media and the reeds. The reeds provide oxygen for the bacteria and the root growth restructures the gravel bed and keeps channels open so that the flow of the effluent remains unrestricted. The microorganisms are utilising the organic compounds present as a source of nutrition. The aerobic and anaerobic conditions in the reed beds results in a high diversity of microorganisms, which improves the biological treatment capacity of the system. The main benefits of reed bed treatment are the low operational costs compared to conventional biological treatment systems [1]. There are no requirements for pumping, as the flow through the system is governed by gravity. Similarly, as the reeds transfer oxygen, there is no requirement for blowers to aerate the system. A further advantage is that reeds do not produce surplus sludge and are free from odour. Reed bed systems have a high diversity of microorganisms, which allows them to adapt to diverse types and varying shock loads of effluents, including highly loaded effluents containing organic compounds, such as chlorinated hydrocarbons, dyes and sulphur containing aromatics and heavy metals and pathogens. Nanofiltration technology: the high tech tertiary treatment The application of recycling normally relies on suitable process technology for water purification. The wide fluctuation in tannery effluent quality coupled with the requirements for process water of reliable quality tend to favour the application of membrane processes. Nowadays, membrane filtration processes inevitably play a key role in modern water recycling since they can produce a water of consistent and reliably high quality. Membranes form a highly selective barrier and are tolerant to shock loads. Therefore, the produced permeate quality varies little with the feed water quality [2]. The main advantage of a membrane based process is that the concentration and separation is achieved without a change of state and without use of chemicals or thermal energy, thus making the process energy efficient and ideally suitable for recycling [3]. Nanofiltration technology is suitable where a final polishing of effluents, without salt retention, is required. The nanofiltration membrane offers a small pore size of 400-600 Dalton, which retains efficiently multivalent ion such as total hardness and certain charged or polar molecules. However, sodium chloride, a mono-valent salt, passes the membrane. The spiral-wound membrane modules are densely packed offering a high membrane surface and, therefore, require only a minimum of space. High reductions of COD, BOD and colour are achieved, due to the fact that the nanofiltration membrane retains organic fractions. The produced permeate is reduced in COD, completely clear and contains minor concentrations of salt. The consistent quality of the NF enables water re-use. Pilot testing A pilot scale nanofiltration plant was tested to assess the suitability for polishing the effluents from the reed beds (Picture 2). The nanofiltration pilot plant consisted of a 100 litre working tank as well as a permeate tank and a cleaning tank. Reed bed effluent was collected in a 1,000 litre container from the final discharge sump. The effluents were then pumped with a submersible pump from the container to the working tank of the nanofiltration plant. A cartridge filter with a pore size of 5 microns was installed in the feed line to protect the membranes from particular matter. During operation the effluent was transferred via a recirculation pump and a high pressure pump to the spiral-wound NF membrane filtration unit. Flow and pressure in the recirculation loop was adjusted by a valve and controlled by pressure gauges and flowmeters. The recirculation pump was operated at 2-3m3/hr in combination with the high pressure pump at up to 12 bars pressure. The NF membrane, BFA-NF 8040, supplied by the company Berghof with a filter-area of 7m2 and a MWCO of 600 Da was operated at a recovery rate of 50 and 80% by adjusting the concentrate drain valve according to the permeate flow achieved. The pilot plant was adjusted to various recovery rates to simulate the increased loading across a series of NF modules that would be expected in a full-scale plant. Salts were retained by the NF membranes, which were concentrated up to four-fold during the filtration. The NF plant was operated for eight hours a day and NF permeate and concentrate was drained continuously. The membranes were flushed with NF permeate prior to plant shut down. Results of the piloting The results of the nanofiltration trial demonstrated a good and stable membrane performance with COD concentrations of around 50 mg/l achieved in the permeate (Picture 3). The permeate flow of average 31 l/m2/hr, at a concentration rate of 70-80%, did not show a major decline in the course of the test period, which indicated that no biofouling or scaling of the nanofiltration membrane had occurred. The selected nanofiltration membrane was suitable and achieved the required treatment performance. However, by implementing nanofiltration a concern is that salts or organics concentrate up in the overall effluent treatment system. Therefore, a second task of the initial pilot scale investigation was to assess how the concentrate can be treated efficiently in the existing plant to avoid organics and sulfate levels increasing. Concentrate treatment Coagulation and flocculation tests were carried out to investigate whether the concentrate from the nanofiltration (NF) can be efficiently treated with the current primary treatment. Three samples, balanced effluent, NF concentrate and a (1:3) mix of concentrate with balanced effluent were tested according to the standard primary procedures of TCIM. Jartesting was carried out with addition of ferricchloride (250 ppm) and aluminium sulfate (720 ppm) at high mixing velocity over five minutes and an anionic polymer at 2ppm under reduced mixing. The samples were allowed to settle for ten minutes; after that samples were taken and analysed for COD. The mix of concentrate and balanced effluent gave a low concentration of 356 mg/l COD, compared to the balanced sample, which gave a concentration of 454 mg/l. The results showed that the NF concentrate can be coagulated by means of the current primary treatment and that no major increase of COD can be expected in the loop of the overall system. Implementation of a treatment and water recycling system using nanofiltration Based on the result of the pilot plant investigation, an industrial scale membrane plant was designed. The nanofiltration plant with 7m3/hr treatment capacity was installed in November 2006 and is now treating effluents from the reed beds. A part stream of 20-30% of the overall effluent bypasses the plant and is mixed at the final discharge with permeate to achieve the required limits of <125mg/l COD. The NF plant is fed with a submersible feed pump, which is installed in the holding tank. The feed pump with 12 m3 capacity redirects parts of the feed to the plant, whereas as small amount of 2-3 m3 is bypassed to the discharge sump. The amount bypassed can be adjusted via a valve and flow meter to the required volume to achieve a mix of a concentration <125 mg/l discharged. The nanofiltration plant consists of a pre-pressure pump that transfers the effluents via a cartridge filter to the high pressure pump. The NF plant has two streets of two pressure modules containing seven membranes, each assembled in series, and is operated at a pressure between 10-12 bars. The permeate is collected in a 4m3 holding tank with an overflow to the discharge sump. The plant is fully automatic and shuts down at low level or with changes of the operational pressure. Then the membranes are automatically flushed out with permeate. The plant produces 6m3/hr of permeate and 2m3/hr of concentrate. The concentrate is transferred back to the balancing stage and then to the primary treatment and reed beds. The performance of NF membranes tends to decrease during operation as a layer of inorganic deposits builds-up on the membrane surface and on the feed spacer, which reduces flux and permeate quality with a concurrent increase in differential pressure. Blockage of the membranes may lead to precipitation of salts on the membrane surface and in the feed inlets [4]. A specific cleaning regime was developed to prevent the formation of a layer and consequently scaling and fouling using antiscalant, and alkaline and acidic membrane cleaners [5]. The feed is dosed with phosphonates containing antiscalant at 16g/m3 to reduce scaling. The implementation of this combination of reed bed and NF membrane technology has shown to be significantly cheaper compared to conventional activated sludge treatment combined with tertiary polishing, which is nowadays commonly applied effluent treatment technology [6]. The consistent effluent quality of <40 mg/l COD with no suspended solids is excellent compared to conventional plants, where a discharge concentration of COD 250 mg/l to surface water has to be achieved [7]. This combination of low cost reed bed treatment with membrane technology enables to comply consistently with discharge limits to river and enables water recycling. Economics The costs of implementation of the reed beds and nanofiltration plant are documented in Table 2. The costs are based on the reed bed installation and the nanofiltration plant with a daily treatment capacity of 150 m3 of primary treated tannery effluents. The seven reed beds have a total size of 450 m2 and a daily throughput of 150 m3. The Nanofiltration plant, containing 14 membranes of 14 m2, has a capacity of 144 m3 per day. The overall energy consumption of the reed bed and nanofiltration plant is approximately 1.75 kW/m3 permeate produced. Table 3 shows the investment and operation costs for mixed effluent treatment using reed bed and subsequent nanofiltration treatment are based on a treatment capacity of 150 m3 per day. The cost calculations are based on 250 working days per year. The reed bed and NF plant capital is calculated on the current membrane and equipment prices including costs for installation. The energy costs are based on local prices, antiscalant, filter cartridges and cleaning costs are calculated on the current expenses. Conclusions The nanofiltration plant has been in successful operation for six months. The results are good in respect of flow rate and permeate quality. Recycling the highly loaded concentrate back to the primary treatment has caused no negative concentration effects such as accumulation of salts or hard COD in the overall ETP. In the second project phase TCIM are planning to implement water recycling. The flow balance of the overall system will be assessed in respect of concentration effects and to calculate the recyclable permeate volume. The recycled permeate will be transferred from the permeate holding tank via an ion exchanger to the water holding tank of the tannery. This combination of reed bed and membrane technologies offers a route of treatment and recovery of high quality water at low costs. Where appropriate, the quality of the treated effluent can be carefully managed to maximise the discharge limits in a manner that is both legal and cost-effective.