Novel insights into the co-digestion of whey with glycerin in an AnSBBR: Influent composition and concentration, cycle length and feed strategy effect

Abstract

Cheese whey is the watery part of milk that is separated from the curd in the cheese-making process. It is the major by-product of dairy industries and because of its high organic load, it has a strong pollution potential, causing an excess of oxygen consumption if it is directly disposed in water bodies. There are several researchers using cheese whey for methane production by anaerobic digestion but due to its high biodegradability and low alkalinity content it may be hard to stabilize biogas production. So, the use of a co-substrate could be an interesting approach to overcome the process instability. Glycerol, which is the major byproduct of the biodiesel industry, could be a good co-substrate for cheese whey digestion because it is a substance that is readily biodegradable, has a pH that is suitable for anaerobic processes and the high C content of it increases the C:N ratio in the mixture, avoiding process inhibition through an excess of N. Therefore, this study investigated the application of an AnSBBR (anaerobic sequencing batch biofilm reactor) of 3.0 L treating 1.0 L per cycle with agitation (300 rpm) operated at 30°C in batch (feed time of 10 min) and fed-batch (feed time of 50% of cycle time) mode in the co-digesting cheese whey with glycerin for methane production (discharge time of 10 min in both). The influence of the ratio between cheese whey and glycerin (100:0; 67:33; 33:67; 0:100%), organic loading rate (5000 and 7500 mgCOD.L-1; influent concentration 2500, 3750 and 5000 mgCOD.L-1), cycle time (4, 6 or 8 h) and feed strategy (batch or fed batch) were assessed. In all assays, the anaerobic system achieved stability regarding pH, bicarbonate alkalinity and total volatile acids. The highest methane productivity (73.8 molCH4·m-3·d-1) and yield (15.6 molCH4·kgCOD-1) with 95% of organic matter removal were achieved when the reactor was fed with 67% of cheese whey and 33% of glycerin (COD basis) with an applied volumetric organic load of 5.0 kgCOD.m-3.d-1 operated in fed-batch mode, which was a superior performance than the one obtained with mono-digestion. A kinetic metabolic model was fitted efficiently to the process to help the understanding of the metabolic routes and a scale up estimation was also performed. © 2017 by Nova Science Publishers, Inc. All rights reserved.