Making an excellent recovery
The issues surrounding water reuse are well known and have been debated at length. Their nature changes according to the specific application. For example, for the recovery and reuse of water within a single domestic dwelling the occupiers perception of water reuse for specific applications can be of more significance than economic issues. Health risk is of acute importance in this instance, as is the issue of ownership - both of the treatment technology and the water itself.
Whether applied to the domestic or industrial environment, membrane processes inevitably play a key role in water recycling since they can produce water of a reliable quality, the permeate product water quality varying little with feedwater quality. They are tolerant to such shock loads because they form a highly perm-selective barrier. On the other hand, they are susceptible to certain constraining phenomena, principally membrane fouling, and are generally considered to be costly.
The huge diversity in industrial processes forbids any generalisation regarding reuse viability based on individual impurities. However, the cost-effectiveness of using reclaimed water for industrial purposes obviously depends upon existing costs for supply and discharge compared with those of installation and operation of the wastewater recovery facility. These costs, the design and operation of the overall treatment scheme, and the process performance are considered in detail for a single case study – Kronospan: medium-density fibreboard wash water recycling (UK).
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.
At a facility in Chirk, north Wales, (Kronospan UK) 47,800m³/pa 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 l/tonne of dry wood processed in the summer to up to 600 litres in the winter. The characteristics of the effluent are high COD and high suspended solids of which cellulose, ligins and resin acids are key components.
Prior to 1995 the effluent was tankered off-site at a cost of £9.8/tonne. The company was not only concerned about tankering costs but also of the risk to production should the tankering operation fail. A decision was made to incorporate on-site treatment at the production facility which should meet the following aims:
- low capital cost/rapid investment payback,
- optimum product/resource recovery,
- effluent reduction, recycling and reuse,
- long-term environmental compliance,
- easy to upgrade modular system.
The plant treats all MDF-generated effluent. Excess flow is stored prior to being pumped into the treatment plant. The plant consists of multiple treatment stages combining physico-chemical processes with membrane technology. Water entering the plant is initially dosed with a polyelectrolyte and flocculated. The flocculated suspension is then pumped into a plate and frame filter press from which a 45-50% dry solids cake is recovered. The filtrate passes through a dual-media depth filter before entering a reverse osmosis (RO) plant. Permeate from the membrane plant is polished through an activated carbon bed and re-used on-site for boiler feed. Concentrate from the membrane plant is returned to the resin make-up tank and backwash/wash water from the sand filter and filter press are combined and returned to the head of the treatment works.
Polyelectrolyte is dosed up to a rate of 500mg/l 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 at up to 50m³/h and 6bar (treating an average flow of 30m³/h). The press produces up to 2m³ of 45-50% 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) and requires cleaning with a combined air scour and water backwash which results in a water recovery across the bed of 98-99%. Finally the flow passes through a 5mm cartridge filter to remove any gross solids and filter grains that might pass through the bed.
The centre of the treatment plant is a single stream reverse osmosis 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 at a recovery of 90%. The membranes operate at a mean transmembrane pressure of 25bar producing an average flux of 14 l/m²/h 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 design empty bed contact time of 30mins.
The plant has been successfully operated since 1995 and has the ability to recover all solid and liquid outputs at zero discharge. Water losses occur only in the final product and from the driers in the evaporation stage of production. The RO concentrate contains cellulose and ligins which are returned for use in the resin binder. The filter press produces 480tonnes/pa of dry solids which are burnt in the boiler or reused as feedstock at the front end of the chipboard production line as a 45-50% dry solids cake. 659tonnes of water enter the production process with 78% lost during the drying process and 18% recycled from the effluent treatment plant which goes to make up 60% of the boiler feed water.
65% of the COD is removed prior to the membrane plant with the permeate containing 1% of the raw effluent value and the final product water <1% after carbon polishing. Overall the COD of the plant has been reduced from an influent concentration of 20,000mg/l down to <200mg/l post activated carbon. Almost all the suspended solids are removed in the filter press with a residual of less than 1mg/l entering the RO stage. Suspended solids in the RO permeates are below limits of detection and the total dissolved solids <100mg/l. Product water quality is very soft with a total hardness concentration of 1mg/l and 0.5mg/l 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 through obviating tankering (91%). Actual recycling on the plant produces the remaining savings of which 5.6% comes from reduced mains water and 3.4% 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 presented with the Queen’s award for environmental achievement and the DTI award for best environmental practice.
In the example given 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 can not ever be predicted. On the other hand no other technology is capable of producing 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.
This case study is taken from Membranes for Industrial Wastewater Recovery and Reuse, Judd and Jefferson