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Co-digestion of cattle slurry and food waste

Anaerobic co-digestion of a low energy substrate with one of a higher potential can enhance biogas production, lead to improved process stability, and make the system more energetically and economically viable. The aim of this research was to evaluate the potential of co-digestion of food waste and cattle slurry in mesophilic conditions under a variety of different conditions. One of the batches of cattle slurry used was taken from a farm using gypsum plasterboard as animal bedding. The high sulphate content proved to be detrimental to the digestion process and led to further experiments to determine process tolerance to both substrate sulphate concentration and digester soluble sulphide concentration.   The majority of the work was carried out in laboratory-scale digesters with a working volume of 4 L. They were operated with semi-continuous feeding over a minimum of three hydraulic retention times (HRT) and at mesophilic temperature (35 + 2 oC). A number of operating regimes were tested which included varying the wet-weight ratio of cattle slurry and food waste from 3 : 1 to 6 : 1 and the organic loading rate (OLR) from 3 to 5 g volatile solids (VS) L-1 day-1. The lower OLR of 3 g VS L-1 day-1 was shown to be optimal in terms of specific methane production (SMP) and gave values of 0.332 and 0.239 L g-1 VS. These were higher than the SMP values obtained at OLRs of 4 and 5 g VS L-1 day-1 at the same ratios. The volumetric methane production (VMP) for co-digestion was consistently higher than for cattle slurry mono-digestion and increased significantly with increasing OLR at both CS : FW ratios tested. Economic considerations may mean that this increased VMP is more significant than a marginal reduction in SMP, as higher values could increase farm incomes and reduce capital expenditure and payback periods. The methane (CH4) content of the biogas ranged from 60.3 – 61.0 % and 61.6 – 62.2 % for ratios of 3 : 1 and 6 : 1 respectively. The requirement for trace element addition was considered with the conclusion that the cattle slurry could provide all of the nutrients required for stable digestion, whereas the mono-digestion of food waste was deficient in some key elements which resulted in process instability.  Mono-digestion of cattle slurry showed a variable performance which could be related to the source and condition of the feedstock material. Where the SMP and VMP of the cattle slurry was low the effect of adding the co-digestate brought the combined gas production to a higher and more consistent value, which is an important consideration in the selection, design and operation of gas utilisation equipment and the overall economic viability of the digestion plant. A reasonably accurate prediction of SMP values in co-digestion trials could be derived from BMP data using a first-order pseudo-parallel model to derive kinetic coefficients. This was most accurate when applied to systems operating at a longer HRT where the effect of daily removal of a proportion of the digestate was not significant. Comparison of values derived from this approach with those based on the SMP of mono-digestion controls on a pro rata VS basis was able to provide additional insights into possible mechanisms for any reduction in gas productivity.   The cattle slurry collected from a farm using gypsum as bedding had a sulphate concentration of 6876 mg L-1 and sulphur content of 2.79% of total solids (TS). Mono-digestion of this material failed and less than 35% methane was detected in the biogas with the SMP below 0.03 L g-1 VS. Dissolved sulphides in the digestate reached 500 mg L-1 which exceeded the toxicity limit of 200 mg L-1. When co-digested with food waste at an OLR ranging from 3 to 5 g VS L-1 day-1 volumetric biogas production (VBP) showed a significant drop and in less than 3 HRT, all the digesters failed. The SMP fell below 0.03 L g-1 VS, the biogas CH4 was 1.5 and volatile fatty acids (VFA) concentration were > 10000 mg L-1.   The final part of the study digested low sulphate cattle slurry spiked with calcium sulphate (CaSO4.2H2O) to equivalent added sulphate concentrations ranging from 0 to 7000 mg SO4 L-1. As the amount of sulphate increased in the cattle slurry feedstock, the SMP dropped gradually from 0.143 L g-1 VS for control to 0.055 L g-1 VS in digesters fed at a substrate sulphate concentration of 7000 mg SO4 L-1. The biogas hydrogen sulphide concentration increased from 499 ppmv detected in the control to 39441 ppmv in digesters with 7000 mg SO4 L-1. Digesters with spiked sulphate concentrations of more than 4000 mg SO4 L-1, gave sulphide concentrations above the suggested inhibition threshold and showed elevated concentrations of acetic and propionic acid. At sulphate substrate concentrations > 7000 mg L-1 there was a rapid and progressive increase in total and individual VFA species leading to process failure.   In general it was concluded that positive benefits could be gained from the co-digestion of food waste and cattle slurry although a precautionary principle should be adopted until an assessment of the variability and composition of the slurry had been carried out.