Dairy equipment is regularly cleaned using clean-in-place (CIP) operations. CIP is a system of cleaning and sanitising based on circulating chemicals and water without taking the equipment apart (IDF 1979). The first step in the cleaning cycle is a water rinse, followed by a caustic wash to remove most of the organic deposit. After a short water rinse, an acidic wash is circulated to remove the mineral soils, followed by a sanitising step (Henck 1995). In the dairy industry, CIP operations make a substantial contribution to water consumption and are responsible for 50-95% of overall waste stream volume and high pH (9-11) (Gesan-Guiziou et al. 2002). According to the work of Hogaas-Eide (2002), cleaning processes are also major contributors to the total eutrophication potential from dairy processing at 80%; while the corresponding contribution to energy use is approximately 30%.
CIP systems rely on sensors to monitor the cycle and the adequacy of cleaning; therefore it is possible to divert the spent cleaning fluid for recycling. To prolong the life of detergent, it is common practice to eliminate the soil particles by sedimentation, centrifugation or by membrane technology (Schindler 1993). In recent years membrane filtration processes such as microfiltration, ultrafiltration and nanofiltration have been applied to CIP solution regeneration, resulting in significant removal of both suspended solids and soluble solids (Dresch et al. 2001).
A number of studies (Dresch et al. 2001; Merin et al. 2001; Gesan-Guiziou et al. 2002; Rasanen et al. 2002) have examined the use of nanofil-tration to recycle spent caustic solutions, whereby milk solids are removed by nanofiltration and the cleaned caustic collected in the nanofiltration permeate for reuse. Studies by Lundekvam and Flaten (1997) show that membrane recycling of cleaning solutions leads to energy savings of 16%, water savings of 10% and also detergent savings (alkaline 32% and acidic 26%). Life cycle assessment of CIP by Hogaas-Eide et al. (2003) also confirms that membrane filtration of cleaning solutions reduces detergent use, energy use and emissions.
Studies on the operation of nanofiltration CIP recycling have been directed towards optimising the choice of membrane, volume concentration ratio and whether to operate the plant in batch-fed or continuous mode. Rasanen et al. (2002) report that nanofiltration using Desal-5 DL membranes reduced the COD content of caustic washing solution by 80% with a 16-21 volume reduction; the cleaned caustic nanofiltration permeate was then concentrated by reverse osmosis from 0.2% to 0.5-0.7% NaOH prior to reuse.
Recycled caustic solutions have also been found to have better cleaning efficiency compared with newly prepared NaOH solutions (Merin et al. 2002). Microfiltration and nanofiltration permeates of recycled cleaning solutions tested on ultrafiltration membranes fouled with whey proteins, were found to have lower surface tension due to residual milk components hydrolysed by the caustic solution. Consequently the recycled cleaning solutions had a higher wetting capacity and cleaned more efficiently.
However, variable payback periods for the installation of nanofiltration CIP recycling equipment have been reported. Dresch et al. (2001) estimated that savings from reduced caustic usage, reduced energy for heating the total volume of the CIP and reduced HNO3 for neutralising waste water, had a payback of 14 years. Koch International who market the AlkaSave® Recovery System estimate payback at 1.5 years (but assume much higher disposal rates for CIP solutions for single-use CIP than Dresch et al. (2001)). Meanwhile Henck (1995) estimated a 7.7 year payback, but also with assumptions of high disposal rates for single-use CIP solutions.
374 Handbook of waste management and co-product recovery 14.6.3 Spent ion exchange brines
Ion exchange is widely used in the dairy industry for protein fractionation, a demineralisation or decalcification of whey and permeates. Ion exchange is also used for softening boiler feed water and water treatment. However the effectiveness of ion exchange relies upon having an adequate supply of brine to regenerate the resin and maintain ion exchange capacity at a functional level. The amount of regenerant chemicals required and the disposal of spent high-salt regenerant imposes significant environmental costs. To counter these problems, methods to minimise the use of regeneration chemicals or recycling strategies need to be implemented.
Several alternative ion exchange systems have been proposed with reduced regeneration requirements; two examples based on the use of weak anion or cation resins include the Svenska Mejeriernas Riksforening (SMR) process and the Sirotherm® (ICI Aust. Ltd) process.
The SMR process employs a weak anion resin in the bicarbonate form. The whey anions are first exchanged for HCO3- counter-ions, then the whey enters a weak cation exchange column in the ammonium form, where the cations are exchanged for NH4+ counter-ions. The demineralisation efficiency of this process is about 90% (Jonsson & Olsson 1981). After the resins have been saturated with whey salts, the resins are rinsed and regenerated with ammonium bicarbonate. The NH4HCO3 remaining in the whey after the ion exchange is a thermolytic salt, and decomposes to NH3, CO2 and H2O when heated. Thus it is possible to recover the NH3 and CO2 stripped off from whey for recycling into regenerant (Jonsson & Olsson 1981). Another advantage is the reduced pH fluctuations of the whey during ion exchange, maintaining the pH range of 6.5-8.2. However, after every three to four cycles the cation resin requires a strong HCl treatment to regenerate the resin due to the partial retention of Ca2+ and Mg2+ on the cation resin after NH4+ regeneration.
The Sirotherm process was developed by Weiss et al. (1966) employing thermally regenerable ion exchange resins that are regenerated with hot water instead of acids and bases, thus reducing operating costs and effluent pollutants (Parrish et al. 1979). The Sirotherm process uses weak basic and weak acidic ion exchange resins for the adsorption of salts from an aqueous solution (Weiss et al. 1966). In the process described by Parrish et al. (1979) whey permeate is pretreated with Duolite S-761 to remove all residual proteins and riboflavin which may cause irreversible fouling of the Sirotherm resin. Next the permeate is passed through Sirotherm TR-10 which removes 76% Ca2+ and 90% Mg2+ ions and an equivalent amount of anions. The remainder of the Ca2+ and Mg2+ is removed with Duolite C-20. Further treatment with Sirotherm TR-20 removed 99.5% of the Na+ and K+. Lactose was crystallised from the deionised solution and the yield increased from 43% (untreated) to 56% (deionised).
A number of recycling strategies to recover spent ion exchange regeneration brine have been developed by researchers. These employ various combinations of electrodialysis and nanofiltration to recover spent brine from either anion or cation exchangers.
Byszewski et al. (1995) proposed a method whereby spent anion exchange regenerant is sent to a three-compartment electrodialytic water splitter, having at least one bipolar ion exchange membrane, to produce an electro-dialytically depleted regenerant solution and an amount of acid and base that is about equal to the amount required to regenerate the anion exchange column.
Noel (1994) proposes a three-step method for the demineralisation of whey by a strong monovalent cation exchange resin before passing through electrodialysis. The removal of divalents improves the operation of the electrodialyser. They recover the brine from the electrodialysis step and use this to regenerate the ion exchange resin.
Rocha San Miguel Bento (1995) propose a process for the recovery of spent anion exchange regeneration brine by nanofiltration; whereby nano-filtration is used to concentrate the colourants from sugar decolourisation into the retentate while the permeate can be used as recycled anion exchange regeneration brine. Further examples on recycling of spent anion exchange brines by nanofiltration have also been reported by Wadley et al. (1995) and Cartier et al. (1997).
The recovery of spent anion exchange brine using nanofiltration fits naturally with the functionality of polymeric nanofiltration membranes, which carry a negative surface charge at neutral pH, wherein salt rejection characteristics are governed by anion repulsion (Baticle 1997). Patents exploiting the negative charge of nanofiltration membranes for the recovery of spent anion exchange brines rely on the ability of nanofiltration membranes to reject large multivalent anions (e.g. colourants) while the monovalent chloride ions pass through into the permeate for use as recycled anion regeneration brine.
Polymeric nanofiltration membranes are amphoteric with ionisable car-boxyl and amine functional groups on the membrane surface with an iso-electric point in the range of pH 3-6 (Hagmeyer & Gimbel 1999). Recent research has been directed towards utilising the positive surface charge of nanofiltration membranes at acidic pH to maximise rejection of multivalent cations (Durham et al. 2003; Teixeira et al. 2005). Studies have shown that nanofiltration rejection characteristics can be manipulated with changes in pH, anion composition and brine concentration by diafiltration to maximise divalent cation retention and maximise recovery of monovalent brine for regeneration of ion exchange resin.
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