Two alternative modes for diffuse pollution control in Wuhan City Zoo.

Pollution in urban zoos arises from diffuse and small point sources. However, its control has received little attention in past decades. Online and offline modes of ecological engineering technology were designed to control pollution from small point and diffuse sources in Wuhan City Zoo, China. Their characteristics and performances were investigated in sixteen runoff events from 2003 to 2005. The results showed that the two modes both improved runoff water quality and had high retention rates for water and pollutants. In the outflows, the event mean concentrations (EMCs) of total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) were reduced by 88%, 59%, 46% and 71% for the online mode, and those were 77%, 42%, 50% and 66% for the offline mode. The annual retention rates of pollutant loads for the online mode were 94.9%-98.5% in the three study years; those for the offline mode were 70.5%-86.4%. Based on calculation, the online mode was able to store the runoff of 66.7 mm rainfall completely, and the offline mode could store that of 31.3 mm rainfall. In addition, the online mode can provide an effective way for rainwater utilization and good habitats for aquatic wildlives, and has an excellent aesthetics value for recreationsal pastimes. The offline mode can save land resources and may be an effective and economical measure for diffuse pollution control in urban areas.