The Assessment of the Anaerobic Biodegradability of Filtration Residues from the Recovery of Alkaline Dairy Cleaning-in-Place Solutions by Nanofiltration
Mohr, J-C.1, Stiller, W.1, Dinsdale, R.2 and Guw, A.2
1University of Applied Science and Arts Hannover, Germany, 2University of Glamorgan, UK
(free)
The milk processing industry e.g. quarg cheese manufacturing results in the production of significant quantities of highly
polluted wash waters and organic co-products (acid whey). Due to rising prices for cleaning agents, process water and trade
effluent fees and a reduction in cost for caustic stable nanofiltration membranes, the recovery of polluted cleaning solutions
becomes more and more attractive option for dairy plant operators. Nanofiltration for the removal of organic contamination
with a molecular cut off suitable for rejection of lactose in polluted cleaning solutions is considerered to be state of the art
technology in the recovery of cleaning solutions from CIP plants in dairies. Using this technology a significant reduction of
COD in the solution can be achieved but highly polluted organic residues were also produced. Effective methods for the
utilisation of these effluents are required. A nanofiltration system using caustic resistant membranes was designed,
constructed and evaluated technically and economically for the recovery of alkaline cleaning solutions with the subsequent
treatment of the filtration residues by anaerobic digestion.
Filtration parameters of a lab scale nanofiltration studies were analysed. The COD of filtration retentates was found to be
proportional to the coefficient of concentration and to reach high organic residuals values i.e. more than 100g/l COD. The
dynamic viscosity of retentate samples at different concentrations from the filtration process was also evaluated. The
rheological characteristic of these residues from the recovery of alkaline spent wash was found to be Newtonian even at a
high COD levels (up to 150 g/l COD). Above 150g/l COD the consistency becomes highly viscous, until reaching a paste-like
appearance at COD values above 300g/l. Further concentration using by membrane filtration was not possible. For industrial
implementation the coefficient of concentration of a spend wash recovery batch should be below a ratio of 15 – 20 depending
on the initial COD. This information about rheological behaviour of these wastes is important for plant designers to calculate
required membrane surfaces and investment pay back times of nanofiltration systems.
Further studies were conducted to properties of retentate from alkaline recovery to find economical feasible and
environmental friendly utilisation methods. The anaerobic biodegradability of retentate samples with varying COD was
examined in respirometric batch tests to evaluate their potential for industrial anaerobic digestion. Due to small volumes
arising from the process, a high COD and high Sodium-ion concentrations the utilisation as a single substrate in anaerobic
digesters is not feasible. However a co-digestion can be favourable as these wastes contain a considerable quantity of
disintegrated and partly hydrolysed protein. In addition the high pH of the waste can be beneficial in co-digestion with poor
buffered substrates. Therefore mixtures with acid whey, a by-product of quarg cheese production, were also examined. The
results of the absolute anaerobic biodegradability calculated from bottle pressure and carbon balance of alkaline recovery
retentates show similar values compared to the biodegradability of acid whey around 50%.
