Integrated membrane regeneration process for dairy cleaning-in-place

Recovering used cleaning-in-place (CIP) solutions in the dairy industry aim s to maintain constant cleaning efficiency, minimize pH variations and effl uent volume and save water, chemicals and energy. Although industrial membr ane processes have been commercially available for the regeneration of an a lkaline solution, few works have investigated the way to select a membrane process [M.A. Henck, Ph D Thesis, University of Zurich, 1993 and M. Dresch, PhD Thesis, National High School of Agriculture of Rennes, 1998]. Nanofilt ration was recently shown to be more performing than microfiltration, ultra filtration, decantation and centrifugation for the regeneration of an indus trial alkaline solution [M. Dresch, G. Daufin, B. Chaufer, Lait 79 (1999) 2 45-259]. The present work is intended to compare different designs of integ ration of an NF plant in CIP systems. Two integrated CIP process (discontin uous, continuous) were compared with a common industrial re-use CIP system (periodical withdrawal and renewal of cleaning solution) by taking into acc ount the evolution of pollution (chemical oxygen demand, COD) in the runnin g CIP system. The most appropriate mode of operation (batch, fed-batch, fee d and bleed) was previously determined by the evolution of NF performance e stablished for each mode. Numerical simulations were completed using calcul ation hypotheses from Dresch et al. [M. Dresch, G. Daufin, B. Chaufer, Lair 79 (1999) 245-259]. The NF plant operating in fed-batch mode is better sui ted than batch and feed-and-bleed modes. The NF operation can be integrated in the CIP system according to two ways: (i) Discontinuous process with a membrane unit working when the CIP system is idle, the permeate being colle cted in an extra tank. Then, equations relate: (1) maximum pollution concen tration reached in the tank after n regenerations to pollution rate, volume of solution to be treated V-CIP, cleaning cycle duration, Deltat and avera ge pollution reduction; (2) membrane area, A to V-CIP, permeation flux, J, NF duration and volume reduction ratio. (ii) Continuous process, the permea te being recycled directly to the CIP tank. The equation relates pollution concentration (regenerated cleaning solution) to pollution rate, J, membran e area, A, pollution reduction, volume of CIP solution tank, V-CIP and NF d uration. Whatever the selected integrated process, COD in the CIP tank stab ilizes at a level depending on the membrane area: the larger the area, the lower the level. The membrane area can thus be calculated from a COD level that should not be exceeded in the CIP tank, provided the characteristics o f the CIP tank (volume, increasing rate of pollution) and the performances of the membrane unit (J, COD reduction) are known. With sufficient NF membr ane area, the membrane regeneration integrated CIP process allows lower COD content to be maintained compared to the common industrial re-use CIP. The continuous process is easier to set-up and less expensive than the discont inuous process. The pay-back time is long (over 15 years). (C) 2001 Elsevie r Science B.V. All rights reserved.