Membranes for wastewater recyling
Simon Judd and Bruce Jefferson of Cranfield University look at the advantages of wastewater reuse over discharge, improved by advances in membrane technology.
The motivations for recycling of wastewater are manifold. Most often-stated are those pertaining to increasing pressures on water resources. Reuse of wastewater conserves the supply of freshwater, and this presents clear advantages with respect to environmental protection. More pragmatically, wastewater reuse may result directly from legislation, which can constrain the discharge of polluted water by making this option onerous or else forbid such discharges altogether, or it may simply be favoured economically regardless of regulatory stipulations.
There is hardly any industry that does not use large volumes of water (Table one), much of it taken from public water supplies and thus treated to potable quality standards. This means that it is often of better quality with respect to microbial purity but nonetheless may require further purification to reduce the mineral and organic materials content according to the specific duty to which it is to be put. Since the price of supply and discharge has also risen substantially in most countries over the past decade-or-so, it is clearly of economic benefit to acquire a source of water which is commensurate with the quality demanded for its end use.
A whole industry has built up around the above key concept. Pinch analysis has conventionally been employed to maximise heat transfer efficiency within an industrial process. Water pinch is simply pinch analysis based on water quality and volumes, i.e. based solely on mass rather than heat. As such, it is simply an extension of a simple water audit. Volumes
of water used are extremely dependent upon the policy of the company or the individual factory regarding water management, and in particular their housekeeping. Housekeeping relates to the way individual unit operations within the factory are conducted so as, in this instance, to limit the freshwater demand. Actual examples of demand management, reuse and recycling (Table 2) demonstrate the degree of sophistication of the solution to achieve a significant cost benefit depends upon the existing state of water management.
The application of recycling, that is the reuse of water for the same or similar duty, normally relies on some suitable process technology for water purification. This is made particularly challenging by the wide fluctuations in industrial effluent quality coupled by the requirement for process water of reliable quality. It is this exigency which favours the application of membrane processes, which can provide permeate product water of a reliably high quality. They are already established for specific industrial processes where other resources are recovered in addition to the water, such as pulp solids in paper
manufacturing and paint pigments in electrophoretic painting, but are generally considered too expensive for wastewater recycling for most industrial processes.
However, this may not always remain the case. Advances in membrane technology and significant improvements in efficiency, and so cost effectiveness, of this technology have greatly increased the competitiveness of recycling over discharge. The few UK examples of ‘closed loop’ water recovery and reuse, invariably based on membrane technology, suggest that payback periods as low as 18 months are achievable, the timescale obviously being highly dependent on supply and discharge costs. There is clearly progress to be made in industrial water management, both in demand management and recycling, but it is also almost inevitable that, where the recovered water must be of mains water quality or better, membrane technologies will be required. There
follows a case study demonstrating the successful application of membrane technology to industrial effluent reuse.
Case study background
The production of medium density fibreboard (MDF) involves a number of simple but large-scale operations. Forest thinning and sawmill residues are debarked and chipped before being washed to remove residual dirt and grit from the wood. The fibres are then steam softened, cooked and refined between two flat plates and finally mixed with resin to produce the fibreboard.
An example facility at Chirk in North Wales (Kronospan UK) 47,800m3/a of MDF effluent is produced of which the majority is associated with the washing and refining stages of production. The quantity of excess water generated during the process depends on the moisture content of the wood and can range from 400 litres per tonne of bone-dry wood processed in the summer to 600 litres in the winter. The characteristics of the effluent are high COD and high suspended solids of which cellulose, lignins and resin acids are key components.
Prior to 1995 the effluent was tankered off-site at a cost of £9.8tonne-1. The company was not only concerned about tankering costs but also about the risk to production should the tankering operation fail. A decision was made to incorporate on-site wastewater treatment and recycling plant, based on physico-chemical processes with membrane technology (Figure one), at the production facility to treat all effluent generated.
The site contained an existing dissolved air flotation unit followed by an activated sludge plant. Problems with effluent reliability and limited resource recovery due to the poor effluent quality led to the search for an alternative solution. A treatment train containing membranes was installed in June 1995, which offered the potential for zero discharge of the wash water effluent.
Polyelectrolyte is dosed up to a rate of 500mg.l-1 into the flow and then flocculated in a 15m3 tank with a residence time of 30mins. The project included the development of a new polyelectrolyte specifically designed for the needs of MDF effluent. The flocculated suspension then passes to a 55-plate filter press (Figure two) at up to 50m3.h-1 and 6bar (treating an average flow of 30m3.h-1). The press produces up to 2m3 of 45-50 per cent solids cake per batch. Filtrate from filter press then passes to a dual media depth filter containing sand and anthracite. The filter operates at a design velocity of 6m3.m2.h-1 and requires cleaning with a combined air scour and water backwash which results in a water recovery across the bed of 98-99 per cent. Finally the flow passes through a 5mm cartridge filter to remove any gross solids and filter grains that might pass through the bed.
Membrane and polishing plant
The centre of the treatment plant is a single stream reverse osmosis (RO) plant configured in a four-stage feed-and-bleed array, each with its own crossflow recirculation pump. The membranes are bespoke polyamide 8040 spiral wound modules (Osmonics). The plant contains a total membrane area of 1,856m2 and is designed to produce up to 450m3.d-1 at a recovery of 90 per cent. The membranes operate at a mean transmembrane pressure of 25bar producing an average flux of 14l m2 h-1 at a temperature of 25-30°C, and are regularly cleaned by a combination of hot water flushing and caustic/proprietary high pH cleaning agents. The water is then polished with an activated carbon filter with a working capacity of 12.5m3 and a contact time of 30 minutes.
The plant has been successfully operated since 1995 with recovery of all solids and almost liquids. The RO concentrate contains cellulose and ligins which are returned for use as resin binder. The filter press generates 480 tonnes.y-1 of dry solids which are burnt in the boiler or reused as feedstock at the front end of the chipboard production line. 659 tonnes of water enter the production process with 78 per cent lost during the drying process and 18 per cent recycled from the effluent treatment plant which goes to make up 60 per cent of the boiler feed water.
65 per cent of the COD is removed prior to the membrane plant with the permeate containing 1 per cent of the raw effluent value and the final product water <1 per cent after carbon polishing. Overall the COD of the plant has been reduced from an influent concentration of 20,000mg.l-1 down to <200mg.l-1 post activated carbon. Almost all the suspended solids are removed in the filter press with a residual of less than 1mg.l-1 entering the RO stage. Suspended solids in the RO permeates are below limits of detection and the total dissolved solids <100mg.l-1. The product water quality is soft with a total hardness concentration of 1mg.l-1 and 0.5mg.l-1 as Ca and contains negligible concentrations of silica or sulphate, making the water suitable for reuse in the (low-pressure) boiler house for steam.
The treatment plant was built under a lease-purchase agreement where Kronospan made an initial payment of £200,000 in February 1996 and a final payment of £200,000 in February 1999. The designer and contractor, Esmil, operated the plant for a monthly fee of £22,000 ensuring the treatment of all wash water effluent to meet Kronospan’s water quality objectives, up to an agreed daily maximum. The plant generates an annual saving of £251,740 mainly though obviating tankering (91 per cent). Actual recycling on the plant produces the remaining savings of which 5.6 per cent come from reduced mains water and 3.4 per cent by recovery of raw material. The payback period for the initial payment was less than 10 months and Esmil continued to operate the plant until autumn of 2002
when Kronospan took over responsibility.
The scheme was the first plant worldwide to apply such an approach to MDF effluent and subsequent plants have been installed across Europe. The scheme at Chirk led in part to Esmil being awarded both the Queens award for environmental achievement and the DTI award for best environmental practice.
In this example the choice of recycling was driven by economics, and specifically the high costs of tankering and disposal of the waste along with the potentially significant impact on production that a failure in this service would cause. The selection of a membrane technology was not without risk, since the long-term fouling propensity of a highly heterogenous and concentrated matrix such as the MDF wastewater cannot ever be predicted. On the other hand no other technology is capable of reused water of uniform quality regardless of the wide variation in the wastewater characteristics. Given the reliance of such schemes on membrane technology, it is perhaps opportune that increasing interest in industrial wastewater reuse has coincided with decreasing membrane costs. The advantages offered by membranes are such that they are likely to become ubiquitous in all industrial wastewater recovery and reuse schemes in the future.
The case study is taken from the book: Membranes for Industrial Wastewater Recovery and Reuse, Judd and Jefferson (eds.), Elsevier.