Membrane Technologies and Resource Optimization
Considerations for using membranes in large-scale plants
The role of water recycling and reuse is gaining immense importance. This is primarily due to the increasing disposal cost and the realisation that water is a valuable resource that must be used with great care. Fine chemicals, agro-chemicals and pharmaceutical industries have now products and processes which show a high degree of complexity resulting in effluents streams which are difficult to treat efficiently.
Ciba Expert Services possesses significant know-how in the usage of membranes in large-scale plants. For the last 25 years, many large scale facilities are in operation at various production sites. Major contributions of Membrane Separation Processes can be summarised as follows:
- Production related simplifications
- Environmentally improved processes
- Versatile and new processes
However, all these advantages require adequate membranes providing economical advantage. As processes are being continuously redefined and new processes are being implemented new membranes are needed which can meet the requirements of a new chemical. Hence the goals can be listed as:
- Wastewater reduction (efficient use of water resource)
- Recovery of water from wastewater
Membrane separation processes contributed significantly towards the reduction of wastewater generation. This was done through a combination of in-process optimisation and the treatment of wastewater.
Membrane bioreactor technology
Ciba Expert Services developed a membrane bioreactor technology (MBR) to treat wastewaters from pesticide production as early as 1990.
The combination of biological treatment, ultra-filtration and subsequent activated carbon adsorption was new at the time. The system was able to remove active ingredients from the wastewater effectively and economically with minimal consumption of carbon. However, at the time, bio fouling effects imposed the selection of ceramic membranes, which have proven to be extremely reliable over a long period.
In the meantime the quality and the economy of membranes materials have greatly improved so that membrane systems are gaining acceptance in the chemical-pharmaceutical industry.
Several new chemical wastewater treatment plants using Membrane Bioreactor, designed by Ciba Expert Services, were recently put into operation. Membranes were selected for these plants because of the following advantages:
- Formation of floating sludge and poorly settling solids is a widespread problem in conventional biological treatment plants. It causes elevated suspended solids concentrations at the outlet. In contrast, the effluent from membrane filtration is free from suspended solids and exhibits correspondingly better quality
- Membrane Bioreactor processes are usually operated with a high activated sludge concentration in the range of 10 – 20 grams/litre. Because of this, the volume required for the bioreactor can be significantly reduced in comparison with conventional activated sludge plants. The secondary clarification stage is also not required giving the treatment plant, a much smaller ‘footprint’
- The sludge is completely retained in the bioreactor due to the membranes which leads to a very stable operation and to a high sludge age. Because of this, more slow-growing specialists can accumulate, resulting in improved biodegradation of even the most difficult substances
- If the removal of non-biodegradable substances (active ingredients) is required, a subsequent activated carbon adsorption stage may be necessary. Because of the complete removal of suspended solids and increased bio-removal of soluble organics, blockage and biological fouling of the carbon filter is greatly reduced, leading to much longer service life of the activated carbon
Membrane bioreactor for treating highly contaminated chemical wastewaters
An American polymer production plant owned by a chemical company produces 800m3 per day of wastewater which is heavily loaded with organic contaminants, and has a salt content of up to three percent.
Various processes were pilot tested because of the very demanding wastewater composition. It was found that conventional biological processes could not be used because of poor sludge settling properties.
The legal discharge requirements could only be met consistently using the Membrane Bioreactor process, which was pilot tested on site for several months.
Due to the stable operating conditions, the organic contaminants are degraded effectively even with severe load fluctuations and high salt concentrations. On the basis of the successful pilot test results, the existing treatment plant was replaced by an efficient Membrane Bioreactor plant in the second half of 2005.
Due to complete automation, the plant also proves to be very efficient and reliable in operation, so that the cost of technical service is very low.
Membrane bioreactor for treatment of wastewaters containing active pharmaceutical ingredients
A formulation site of a pharmaceutical company produces about 700m3 per week of wastewater containing active pharmaceutical ingredients (API).
Until recently, the wastewater had been treated in a nearby municipal wastewater treatment plant. However, many active pharmaceutical ingredients are not biodegradable and therefore were not eliminated in the municipal WWTP and were eventually released to the river. This can result in surface water contamination due to bioaccumulation of these substances.
Therefore various alternatives were tested as part of a process evaluation. It was found that treatment of the contaminated wastewater in a combination consisting of a membrane bioreactor followed by activated carbon adsorption presented an attractive solution.
Most of the non-problematic organic substances can be degraded in the membrane bioreactor with low consumption of energy and resources. After the biological treatment, the solids-free permeate contains only traces of non-degradable active substances, which are removed from the wastewater very efficiently with minimal consumption of carbon in the following activated carbon filtration stage. With the new pretreatment plant, which went into operation in the fall of 2005, on average, more than 95% of the pharmaceutical active substances are removed from the wastewater.
The two plants described above went into operation in 2005. Membrane bioreactors with externally mounted and pressure-operated ultra filtration were used in both cases for two key reasons:
- Pressure-operated membrane filtration requires significantly lower membrane areas than immersed membranes. A distinguishing advantage in particular at low wastewater flows is the compact and space-saving construction
- Because of the high velocity through the membrane tubes, pressureoperated systems are fundamentally less susceptible to plugging and can be expanded relatively simply with additional modules if the volumes of wastewater increase in the future. Membrane purification is fully automatic and operates without interruption. Because the process is closed, there are no odor problems
Because of the successful operating experience, this process is now to be studied for use at other sites. One of these plants is being planned for India, where water is a scarce resource, and part of the wastewater treated by Membrane Bioreactor will be desalinated by an additional reverse osmosis system and reused.
Nanofiltration in wastewater treatment for a chemical plant
Membrane technology is only a separation method for TOC/COD from the effluent stream. It does not eliminate them. As a consequence, implementing a membrane unit alone for wastewater treatment is not enough. The membrane technology can only do an important job of concentrating the undesirable organic compounds. Volumetrically larger permeate is relatively easy to dispose.
Hence, two goals are to be simultaneously achieved. First, the separation of bio non-degradable species. The second to reduce their volume to an absolute minimum in the Nano Filtration Concentrate. Treatment of the later is a separate but equally important issue.
For smooth operation of the Nano Filtration unit it was necessary to define the goals. Depending upon the concentrate treatment method, the goals defined for wastewaters and their mixtures during the lab and pilot tests were as follows:
- TOC elimination between permeate and feed > 70%
- A volumetric reduction of feed to at least a factor of 10
- To increase TOC concentration in Nano filtration concentrate to such an extent that, after mixing wastewaters and concentrates, a TOC concentration in the WAO feed into WAO would be within the defined range
- Permeate performance for a long term operation should at least be high enough to prevent the feed flow to the unit from dropping below 8 m3/h
- To prevent the negative effects of salts on membrane performance (such as sulphate)
- To prevent the negative effects caused by mixing different wastewaters
At the onset, the nano filtration plant concept was developed as a three stage continuous unit. The third stage was left open as a possible future expansion. Hence two stage plants were designed and realised which consisted of five main groups:
- Pre-treatment with pH regulation and continuous separation of solids
- Permeate tanks and online analytical tools to monitor the permeate quality
- The first stage, known as the feed stage, contains a buffer vessel, a high-pressure pump and five independent membrane units, each as a racks with an individual area of 120 m2 (total area therefore of 600 m2). The membrane units are connected sequentially
- The second stage, known as the concentrate stage, contains a buffer tank, a high-pressure pump and four independent membrane units, each as a rack with an area of 60 m2 (total area therefore of 240 m2) and a circulation pump. The membrane units are connected sequentially
- Cleaning group, including cleaning and washing tank, a highpressure pump and piping. Such a construction enables the on-line cleaning or washing of each membrane unit independent of any other units. Thus any membrane unit can be coupled to the cleaning or washing operation even during production
For this plant, tubular membranes were used. The reason for using tubular membranes, which are more expensive than spiral wound type, was that they can work even with a high solid content. Over the years, the primary function of TOC separation was achieved as planned. The overall elimination of non bio degradable TOC lies between 70-85%.
Water optimisation
In a process that has been in operation for the last 15 years, a chemical product is washed (diafiltration) for salt and finally concentrated. The process was designed using a commercially available Nanofiltration membrane in spiral wound format. The membrane (polyamide) worked well.
However about 4 years ago, a new membrane was introduced in the plant which improved the performance in terms of reduction of wastewater generation (from 186 to 166 mc/day and the membrane life (from 9 to 30 months). The reason for these improvements is based on the fact that the new nano filtration membrane innovatively capitalises on surface charge rather than ‘one for all’ structure of the old membrane. Hence it offers less hydraulic resistance yet provides adequate product rejection (retention). The extra-ordinary surface charge also provides for an enhanced salt passage (Donnan exclusion) – thereby reducing the diafiltration water requirement.
Neutral surface membrane and water recycling
In many wastewater facilities, the use of flocculation agents and subsequent filtration is a common process. Normally cationic flocculants are used. Subsequent to flocculation and filtration, these wastewaters do not contain large amount of salts or organic load (typically <0.5% Salt and < 1000 mg/l COD). These waste waters are thus an ideal candidate for recycling of purified water for reuse.
Two strategies were developed which have been thoroughly studied:
- Advanced Oxidation Process to reduce the COD below 50 mg/l and subsequent Reverse Osmosis/Nano Filtration process
- Filtration and RO/NF process
Several projects are being investigated at the moment that stand a very good chance of success. In many of these investigations, it was found that the neutral surface charge on either Nano Filtration or Reverse Osmosis membrane is an important contribution towards long term trouble free operation. This is due to the fact that no adsorption of cationic species on membrane surface takes place – especially the cationic flocculation agents. Initial estimates indicate that for a 10 m3/h water recycling facility, the cost of recycled water can be estimated at €1-2 per m3.
Considering the fact that the recycled water is practically de-ionized water, the strategy seems to be highly attractive. In the total cost calculation, the reduction of wastewater treatment cost should also be considered – making this approach a very viable proposition.
Conclusion
In conclusion, it can be said that the proper use of membrane separation technology offers interesting possibilities to solve wastewater problems. These possibilities can often only be exploited in combination with other unit operations. To achieve an economically attractive result, wastewaters need to be looked at in their totality including the chemistry, the business environment and the goals.
To save water, judicial use of highly innovative membranes can provide marginal benefits for an individual process. However, in totality, many such individual savings result in substantial savings.
Hence systematic implementation of water saving strategies, especially those which give additional economic benefits, should be rigorously followed up and realised.
Membrane technology also offers unique possibilities to recover usable water from effluents. Such processes require adequate pretreatment steps and proper use of membranes. The surface chemistry of the membrane should also be included when designing such processes.
