INVESTIGATION ON ANAEROBIC DIGESTION OF CHEESE WHEY FOR ENHANCED METHANE GENERATION

Abstract

Whey residues or whey wastewater, generated during cheese or paneer manufacturing from dairies, pose significant pollution challenges due to their high organic content, containing milk solids, proteins, fats, and lipids. The levels of organic matter, as indicated by the chemical oxygen demand and biochemical oxygen demand, can reach up to 60,000mg/l and 46,000mg/l, respectively, in whey wastewater. Due to the financial and technological constraints imposed by the treatment of whey wastes, small-scale dairy producers have long had difficulty in managing these wastes. Anaerobic digestion (AD), a promising biological treatment approach that provides opportunities for energy recovery, has come to light as a solution to this problem. Although several researchers have already looked into this subject, there are still some significant gaps. By applying suitable solutions including co-digestion, pre-treatment techniques, and the addition of external materials, challenges experienced during the AD of cheese whey wastewater, such as quick acidity, sludge flotation, and insufficient buffering, can be slightly alleviated. This study helps to identify a suitable co-substrate to enhance the digestibility and methane productivity of whey, to adopt appropriate pre-treatment method conducting detailed energy and cost analysis, and to understand the effect of additive like biochar in stabilising AD process. Objective 1- The present study investigated the possibilities of improving the digestibility from anaerobic digestion of lipid rich dairy by-product, cheese whey using septage as the co-substrate with different inoculum. Biochemical methane potential assays were conducted under mesophilic temperature conditions and results were validated using Modified Gompertz Model. Two sets of BMP tests were done; to assess the individual and combined digestion abilities of septage in anaerobic co-digestion of whey and to assess the ability of 3 inoculum sources (cattle manure, sewage sludge, and acclimatized anaerobic sludge) in the co-digestion process. The results indicated that septage is an excellent co-substrate that has better adaptability with CW and the optimum mix ratio was found as 40:60 (CW: SP). BMP tests were also conducted with inoculum at S/I ratio of 1 and statistical analysis was performed to study the synergistic effect of both co-digestion and inoculum. The tests revealed that the cattle manure resulted in the highest biogas production (342.22mL/gVS) at 60% whey fraction. Modified Gompertz model fitted the experimental data well and identified an increase in lag phase times when whey fraction is increased. Comparatively higher lag phase times ranging from 1.98 to 4.35 days were obtained for sewage sludge inoculated samples. The maximum methane production (Pmax) was vii obtained at 60% whey fraction (369.63 ± 4.05mL/gVS) at a very short lag time of 0.76 ± 0.17days for cattle manure inoculated mixture. Objective 2- Lactose in cheese whey wastewater makes it difficult to degrade under normal conditions. The effect of ultra-sonication (US), ozonation and enzymatic hydrolysis on increasing the bioavailability of organic matter in CW and biogas production were evaluated. The pre-treatment conditions were: specific energy input varied from 2130 to 8773kJ/kgTS for a sonication time of 4.5–18.5 min, Ozone (O3) dosages ranging from 0.03 to 0.045gO3/gTS were applied for 4–16 min, pH (3.8–7.1), temperature (35◦C–55◦C), enzyme dosage (0.18 0.52%), was operated from 7.75 to 53 min for enzymatic hydrolysis by β-galactosidase. The results of the US reported a maximum sCOD solubilisation of 77.15% after 18.5 min of operation, while the corresponding values for ozonation and enzymatic methods were 64.8% at 16 min and 54.79%, respectively. The organic matter degradation rates evaluated in terms of protein and lactose hydrolysis were 68.78%,46.03%; 47.83%,16.15% and 54.22%,86.2%respectively, for US, ozonation and enzymatic methods. The cumulative methane yield for sonicated, ozonised and enzymatically hydrolysed samples were 412.4 ml/g VS, 361.2 ml/g VS and 432.3mlCH4/ gVS, respectively. Regardless of the lower COD solubilisation rates attained, enzymatic pre-treatment showed maximum methane generation compared to US and ozonation. This could be attributable to the increased activity of β galactosidase in hydrolysing whey lactose. The energy calculations revealed that the pre conditioning of organic-rich CW with enzymatic hydrolysis is more effective and efficient, yielding a net energy gain (gross output energy-input energy) of 9166.7 kJ and an energy factor (ratio of output to input energy) of 6.67. The modified Gompertz model well simulated all experimental values. Objective 3- The addition of septage-derived biochar helped in increasing the methane yield in all mixtures. The maximum cumulative methane yield was obtained at 50 g/l of biochar loading at 10 % TS content, 486 ml/g VS. The lowest methane yield was reported at 5% TS concentration with 6.25 g/l of biochar loading as 243.2 ml/g VS. The daily methane yield was lowered from 25th day onwards for mixtures with biochar loadings 25 g/l and 50 g/l at TS concentrations > 10%. The biochar dosage was found to be more significant than total solids concentrations. Undesirable biochemical changes observed in the digestion mixture at higher total solid content due to increased viscosity and reduced diffusion coefficient might have resulted in lower methane production. viii Objective 4- The AD of CW and septage with cattle manure as inoculum was successfully executed in a 2 stage lab-scale anaerobic digester. The highest biogas yield was obtained when acidogenic reactor was operated at an organic loading rate of 85.8 gCOD/ld and HRT 1 day. Complete inhibition of methanogens was found since biogas was exclusively composed of hydrogen and carbon dioxide. The microbial population shift and the complexity of the feed medium could be potential causes for the observed inhibition, which may be attributed to the extended operational time. The steady-state conditions for methanogenic reactor was obtained at HRT 14d and OLR 6.12gCOD/ld. Highest biogas and methane yield obtained was 1.81L/Lrd and 1.02L/Lrd respectively. Based on the lab scale results obtained, an industrial scale anaerobic digester of total 26m3 volume was designed having a bioenergy generation potential of 274.7kwh. Summary: Anaerobic co-digestion of cheese whey with the nitrogen-rich septage waste enhanced methane productivity by providing essential nutrients and balancing carbon-to nitrogen ratio. Methane productivity and efficiency of digestion were further enhanced by introducing a mature and active microbial community containing inoculum-cattle manure. Among various physico-chemical pre-treatment techniques, enzymatic hydrolysis was identified as the most efficient, feasible, and cost-effective method to hydrolyse whey lactose and reduce substrate complexity. Further, septage-derived biochar with a buffering capacity and essential nutrients contributed to reactor stability. The two-stage lab-scale digester was operated successfully, and the data on real-time whey wastewater generation rates were used to design an industrial-scale anaerobic digester. The design confirmed a bioenergy potential of 697.1 Wh, with the potential for generating 487.9 Wh of heat and 209 Wh of electricity. A brief summary would be that this study aimed to eliminate the problems encountered during AD of CW by identifying a suitable co-substrate for co-digestion, choosing an inexpensive, efficient pre-treatment method, and selecting an appropriate additive which can be scaled further for adoption at dairy industries.