Residual biogas potential from the storage tanks of non-separated digestate and digested liquid fraction.

Biogas plants daily produce enormous volumes of digestate that can be handled in its raw form or after mechanical separation. In Italy, effluents are usually stored within aboveground, uncovered tanks, which make them potential emitters of biogas into the atmosphere. The purpose of this study was to estimate the amount of biogas emitted to the atmosphere during the storage phase of non-separated digestate and digested liquid fraction. The trials were performed at two northwest Italy 1 MWel. biogas plants. A floating system for the residual biogas recovery, and a set of three wind tunnels for NH3 emission measurement were used. The experiment demonstrated significant loss to the atmosphere for each of the gases; specifically, on average, 19.5 and 7.90 N m3 biogas MWhel.(-1) were emitted daily from the storage tanks of non-separated digestate and digested liquid fraction, respectively.

Thermal pre-treatment of solid fraction from mechanically-separated raw and digested slurry to increase methane yield.

Anaerobic digestion plants rely on large-capacity storage tanks to manage the agronomic utilisation of the digestate. As a consequence, many Italian A.D. plants have introduced mechanical separation of the digested slurry to simplify process requirements. This study evaluated the possibility of reusing mechanically-separated solid fraction as a further biomass input anaerobic digestion plants. The effects of storage and thermal pre-treatment on digested solid fraction were assessed through biogas and methane yield measures, and then compared to the yields associated with undigested solid fraction of raw pig slurry. The specific CH4 yields of digested solid fractions ranged between 71.4 and 156.9 lN/kg VS, whereas the biogas yield from undigested solid fractions was 78.7 lN/kg VS. Solid fraction storage showed no significant effect on specific CH4 yields in any of the examined samples. However, in the case of the undigested solid fraction, thermal pre-treatment proved to be an effective method to increase CH4.

The methane yield of digestate: effect of organic loading rate, hydraulic retention time, and plant feeding.

In biogas plants, huge volumes of digestate are produced daily and stored in uncovered tanks, which leak methane into the atmosphere and cause negative environmental impacts. To better understand the effect that different operating parameters of anaerobic digestion plants have on digestate residual methane yield, four digestate samples collected from plants with very different operations were analysed in batch reactors. Their methane yields were very heterogeneous and varied between 2.88 and 37.63 NL/kgVS. The methane yield was shown to be highly influenced by the A.D. plant Organic Loading Rate and by feedstock quality; hydraulic retention time had only limited effects.

Evaluation of the biogas productivity potential of some Italian agro-industrial biomasses.

Batch trials were carried out to asses the biogas productivity potential of rice and barley straw, grape stalks, grape marcs, maize drying up residues, tomato skins and seeds, and whey. Trials were carried out in 2l glass digesters kept in a thermostate controlled room at 40 degrees C for 40days. The most productive biomasses, in terms of specific methane yield, were the whey and the maize drying up residues. Their specific methane yields were 501 and 317l(N) CH(4) *kgSV(-1), respectively. Barley and rice straw gave a specific methane yield of 229 and 195l(N) *kgVS(-1). Similar result was also obtained from tomato skins and seeds. Grape stalks and grape marcs produced lowest amounts of specific methane, respectively, 98 and 116l(N) CH(4) *kgSV(-1). According to trial results and considering the availability of examined biomasses in Italy, it is possible to estimate their total energetic potential close to a value of 21,900TJ *year(-1). This energetic potential value is equal to that obtainable from the anaerobic digestion of about 6.5 million tons of maize silage.

Improved recycling of livestock slurries on maize by means of a modular tanker and spreader.

Slurry spreading represents a critical component of the waste management strategy since it influences the nutrients' availability for the crops. The Italian Regulation in force sets severe limits to the slurry application to land. These limits--and the necessity to optimise the slurry fertilising value--require to operate with the correct application rate and with a sufficient spreading uniformity. An innovative slurry spreader able to optimise the reuse of this by-product as an agronomic resource whilst respecting the national regulation has been tested under practical conditions. The agronomic trials showed that the controlled recycling--performed with the innovative slurry spreader--gave no statistically different grain yields from those obtained with the chemical fertilisation. The ammonia emissions recorded after the slurry injection were the 16% of those recorded after the band application.