Migration and transformation of nitrogen in bioretention system during rainfall runoff.

Bioretention systems have been extensively studied as a highly efficient technical measure to tackle the global threat of nitrogen pollution during global rainfall runoff. However, the migration and transformation of various forms nitrogen in bioretention system is unclear. So, in this paper, the bioretention systems with different flow regimes and planted configurations were designed to study the nitrogen removal performance and migration and transformation mechanism. The dynamic changes of NH4+-N and NO3--N were continuously monitored within 60 h after rainfall, and the abundance of 15N isotopes in soil layer NH4+-N was simultaneously measured. The results indicated that NH4+-N was mainly intercepted in soil layer in four constructed bioretention systems with similar removal efficiencies (95.42-97.69%). However, NO3--N was retained in submerged layer with significant different removal efficiencies (43.03-83.00%). After fitting calculation, the nitrification rate of NH4+-N (0.0626 mg kg-1 h-1) in soil was 5.31 times higher than that of the accumulation rate of NO3--N (0.0118 mg kg-1 h-1). During the elimination process of residual NH4+-N in soil, 41.46% removed by denitrification and plant absorption assimilation, another 57.28% stored in the form of organic nitrogen or inorganic nitrogen, only 1.26% leaked out. Based on this, the content variation of TN, NH4+-N and NO3--N could be analyzed by a system-wide and established the nitrogen balance model, which provides a new insight into the enhancement of nitrogen removal in the bioretention system.

Compact flexible multi-pass rotary delay line using spinning micro-machined mirrors.

We propose a new method to extend the path length tunability of rotary delay-lines. This method was shown to achieve a duty cycle of >80% and repetition rates of over 40 kHz. The new method relies on a new multi-segmented micro-machined mirror and serial injection of a single reflection onto separate segments of this mirror. The tunability is provided by the relative positioning of each reflective point on the mirror segments. There are two distinct modes of operation: synchronous and asynchronous. By simply manipulating the spatial position of the returning paths over the respective mirror segments, we can switch between increasing the repetition rate (asynchronous mode) or the total delay path (synchronous mode). We experimentally demonstrated up to 8 m/s scans with repetition rates of up to 42.7 kHz. Furthermore, we present numerical simulations of 18 reflection points to illustrate possibility of achieving a scan speed of up to 80 m/s. Through intermediate combinations of synchronous and asynchronous operation modes with 4 or more passes, we also show that the system can simultaneously increase both repetition rate and scan depth.