Treatment of spent wash water derived from shredded lettuce processing using a combination of electrocoagulation and germicidal ultraviolet light

Abstract

Water recycling is a significant part of an overall water management system. The current study evaluated electrocoagulation, used in combination with ultraviolet light (at 254 nm), to reduce the organic content and enhance the microbiological quality, of wash water derived from shredded lettuce processing. The composition of spent wash water derived from a commercial lettuce processing operation was used to prepare a simulated solution to be applied to validate the water recycling system. The simulated spent wash water was subjected to an electrocoagulation process followed by filtration and a tertiary ultraviolet (254 nm) treatment. The efficacy of the recycling treatment to decrease turbidity (nephelometric turbidity units, biological oxygen demand (BOD), chemical oxygen demand (COD) and decrease in introduced bacterial numbers. Spent wash water sampled from a commercial processing line was found to be colloidal in nature (78 ± 26 NTU) with low total solids content (544 ± 87 mg/L), BOD (230 ± 53 mg/L) and COD (309 ± 53 mg/L). An electrocogaultion process performed for 10 min using 3.48 A/m2 current density at pH 6.5 and conductivity of >100 µS/cm supported an 87% removal of turbidity, 38% reduction in BOD along with 49% decrease in COD. The electrocoagulation process was also found to reduce the levels of Escherichia coli, Salmonella and Listeria monocytogenes by 1–2 log cfu. The tertiary UV treatment of water derived from the electro coagulation process, supported further reduction in model pathogens, although it was noted that the D values for inactivation were in the order of 1.01–1.60 mJ/cm2, which compares to 0.22–0.31 mJ/cm2 in saline. The apparent increase in bacterial resistance to ultraviolet was likely due to the UV absorbing low molecular weight constituents within wash water that provided protection against inactivation. In conclusion, the study demonstrated the feasibility of applying electrocoagulation and UV to rapidly treat spent lettuce wash water to facilitate in-process recycling within shredded lettuce processing operations.