Biogas projects are rapidly expanding in China, but there is insufficient cropland to degrade these biogas residues. Mushroom cultivation has been used to degrade various agricultural wastes. In this study, to screen the feasibility of utilizing biogas residues as potential substrates for the cultivation of Pleurotus ostreatus, seven different concentrations (0 to 60%) of biogas residue mixed with cottonseed hull, wheat bran, and lime were used to cultivate P. ostreatus. The mycelial growth rate, mycelial colonization time, yield, biological efficiency, chemical compositions, and content of four heavy metals, Cd (cadmium), Pb (lead), Hg (mercury), and As (arsenic), were analyzed. The results showed that 10 to 30% of biogas residue mixed in the substrates induced the growth of P. ostreatus mycelia faster than the others. A lower percentage (10 to 20%) of biogas residue added to the substrates is beneficial to the production and nutrient components of P. ostreatus, and the fruiting bodies produced on biogas residue-containing substrates conform to the safety standards for edible mushrooms. Although the total harvest is not significantly increased when biogas residue is added, the utilization of cheap biogas residues can conceivably reduce the practical cost and benefit the environment.

Vacuum spacetimes endowed with two commuting spacelike Killing vector fields are considered. Subject to the hypothesis that there exists a shearfree null geodesic congruence orthogonal to the two-surface generated by the two commuting spacelike Killing vector Gelds, it is shown that, with a specific choice of null tetrad, the Newman-Penrose equations are reduced to an ordinary differential equation of Riccati type. From the consideration of this differential equation, exact solutions of the vacuum Einstein field equations with distribution valued Weyl curvature describing the propagation of gravitational impulsive and shock wave of variable polarization are then constructed.