Nanofiltration for the recovery of caustic cleaning-in-place solutions: robustness towards large variations of composition

Abstract

In the dairy industry re-use and multi-use cleaning-in-place (CIP) systems are operated by circulating chemicals and water without taking the equipment apart. The solutions, which become polluted due to the removal of fouling compounds, are drained periodically when they are considered to be too polluted. This work shows the large variations in composition (pollution, surface tension, etc) of the industrial caustic solutions coming from milk standardization and pasteurization plant CIP throughout their life time (7 days) and from 1 week to another. The work is also intended to show how nanofiltration (1 kg mol⁻¹ molecular weight cut-off) was robust and performed well, with good recovery of caustic solutions, even when faced with large variations of solutions composition: high caustic yield, permeation flux (J) in the range 42–110 l h⁻¹ m⁻², average chemical oxygen demand (COD) reduction equal to 0.58 and low surface tension change. Equations have been established for the prediction of J as a function of initial membrane hydraulic resistance (R_m) caustic concentration, volume reduction ratio (VRR) and initial soluble COD. When VRR increased, both J and pollution retention decreased despite the increase in irreversible fouling induced by the increase of soluble pollution concentration in retentate . The higher the initial soluble COD, the sharper the decrease in J vs. VRR. Since irreversible fouling was usually small (0.1–3.4×10¹³ m⁻¹, that is to say of the same order of magnitude as R_m), the membrane cleaning could be efficiently performed by using single phase sodium hypochlorite alternately with a more expensive acid–base cleaning sequence. The obtained permeate was a clear regenerated cleaning solution with low soluble COD (0.2–3.5 g/l) and surface tension (56–30 mJ m⁻²) which could be successfully exploited owing to its cleaning potential.