Pilot Study on the Performance and Microbial Structure of a Subsurface Wastewater Infiltration System

Two kinds of pilot subsurface wastewater infiltration system (SWIS), named group A and group B were constructed in the laboratory. Group A was filled with mixed substrates composed of 80% meadow brown soil (MBS) and 20% cinder, while 80% MBS and 20% sand in group B. Both of the SWIS worked stably in an intermittent feeding mode during the period of the experiment. Under the hydraulic loading of 8-10m(3)(m(2)d)(-1), the removal rates of chemical oxygen demand (COD), biological oxygen demand (BOD5), total phosphorus (TP), ammonia nitrogen (NH3-N) and total nitrogen (TN) reached 86%-93%, 85%-95%, 91%-98%, 86%-93% and 66%-78%, respectively. The quality of the effluent met the class A of the national standard (No. GB18921-2002) of Chinese EPA. To clarify the microbial community structure of the dominant microorganisms in SWIS, the distributions and characterization of bacteria, fungi, actinomyces, ammonifiers, nitrifiers and denitrifiers were studied at different section depth in SWIS, using the traditional plate counting and most probable number (MPN) techniques. Moreover the bacterial community was investigated by molecular method of denaturing gradient gel electrophoresis of polymerase chain reaction (PCR-DGGE). It was found that the microbial variability and distributions varied at different substrate layers in SWIS. Bacteria and fungi mainly distributed in the zones around the distributing pipe. The number of actinomyces and nitrifiers had a similar trend, i.e. fewer near the distributing pipe, gradually decreased below the distributing pipe and oppositely increased above the distributing pipe. There was almost no significant difference in the number of ammonifiers at different layers. The number of denitrifiers mainly distributed in deeper infiltration zones. Oxygen and pollutants concentration might contribute to the formation of the spatial variability and distributions of microbial number. Correlation analysis results showed that good correlation between the removal rates of COD and the number of bacteria and fungi, NH3-N and nitrifiers, TN and denitrifiers, respectively. The removal rate of TP was not correlative with the number of microbes investigated, which indicated that microbial degradation was not the main way for TP removal. Correlation analysis suggested that each microorganism performed its own functions during the process of pollutant degradation. Profile of PCR-DGGE also showed spatial changes in the community structure and distributions in different layers. DNA diversity of bacteria around the distributing pipe was higher than that in other zones. Microbial number and diversity suggested that the functional microorganism distributions correlated to the different nutrient level in the SWIS.