Divalent manganese stimulates the removal of nitrate by anaerobic sludge.

This study investigated the effect of different concentrations of Mn2+ on the removal of nitrate by anaerobic sludge and changes in the microbial communities through batch experiments. The results showed that the addition of Mn2+ promoted nitrate removal by anaerobic sludge; the nitrate was completely removed within 6 d in the treatment group with >5 mM Mn2+. With the increase in Mn2+, the concentration of nitrite and nitrous oxide increased in the first 4 d and then decreased to 0 µM after 8 d of incubation. The increasing tendency of ammonium increased firstly and then decreased with the addition of Mn2+ compared to A. Moreover, the Mn2+ removal efficiency gradually decreased with the increase of Mn2+ concentration. The changes of microflora structure in sludge before and after adding Mn2+ were analyzed, and the results revealed that the microbial communities in the sludge may have evolved towards an energy-efficient association of short-cut nitrification, denitrification, and anaerobic ammonia oxidation after adding Mn2+. Mn2+ stimulated the removal of nitrate by anaerobic sludge mainly by promoting the growth of PHOS-HE36.

Compact Rb optical frequency standard with 10-15 stability.

We achieved a low-cost and small-sized Rb optical frequency standard based on 85Rb 5S1/2 → 6P3/2 transition with 10-15 stability, which is comparable with that of the best 532 nm I2 optical frequency standards. In this system, we directly lock the 420 nm diode laser on the 5S1/2 F = 3 → 6P3/2 F' = 4 hyperfine transition line without an additional Pound-Drever-Hall pre-locking system. The signal-to-noise-ratio reaches as high as 350 000 when the averaging time is at 1 s. Eventually by the fluctuation of the residual error signal after locking, the preliminary stability of the optical frequency standard reaches 1.2×10-14/τ, decreasing to 2.1 × 10-15 at 80 s. It shows potential in stability performance, experimental cost, and system volume compared with the 532 nm I2 optical frequency standard as a wavelength standard. It also opens a door for the achievement of wavelength standards by using higher excited states of alkalies.