Application of subsurface wastewater infiltration system to on-site treatment of domestic sewage under high hydraulic loading rate

In order to enhance the hydraulic loading rate （HLR） of a subsurface wastewater infiltration system （SWIS） used in treating domestic sewage, the intermittent operation mode was employed in the SWIS. The results show that the intermittent operation mode contributes to the improvement of the HLR and the pollutant removal rate. When the wetting-drying ratio （RwD） was 1.0, the pollutant removal rate increased by （13.6 ± 0.3）% for NH3-N, （20.7 ± 1.1）% for TN, （18.6± 0.4）% for TP, （12.2 ± 0.5）% for BOD, （10.1 ± 0.3）% for COD, and （36.2 ± 1.2）% for SS, compared with pollutant removal rates under the continuous operation mode. The pollutant removal rate declined with the increase of the HLR. The effluent quality met The Reuse of Urban Recycling Water - Water Quality Standard for Scenic Environment Use （GB/T 18921-2002） even when the HLR was as high as 10 cm/d. Hydraulic conductivity, oxidation reduction potential （ORP）, the quantity of nitrifying bacteria, and the pollutant removal rate of NH3-N increased with the decrease of the RWD. For the pollutant removal rates of TP, BOD, and COD, there were no significant difference （p 〈 0.05） under different RwDS. The suggested RWD was 1.0. Relative contribution of the pretreatment and SWlS to the pollutant removal was examined, and more than 80% removal of NH3-N, TN, TP, COD, and BOD occurred in the SWIS.

Effect of Pretreatment on Physico-Chemical Parameters of Matrix and Pollutant Removals in Subsurface Wastewater Infiltration System

Two pilot subsurface wastewater infdtrafion systems （SWISs） were filled with the same mixed matrix and operated in the intermittent feeding mode with hydraulic loading of 0. 1 m3/ （m2 d） for 140 d. One of SWISs was fed with wastewater which was previously settled （ ST）, and the other with the same wastewater but in addition treated with a coagulation process （CT）. After 140 d of operation, in the upper layer of ST system the effective porosity decreased by 4.1% and the organic matter （OM） content increased by 246.4 %, whereas only decreased by 0.7 % and increased by 35. 7% of CT system, respectively. During the study, dogging didn＇t occur in CT system and occurred in ST system. Effluent chemical oxygen demand （COD）, total phosphorus （TP） and ammonia nitrogen （NH4＋ --N ） concentrations in CT system were less than those in ST system. In ST system, effluent COD, TP, NH4＋ --N concentrations increased and suspended solid （SS） concentration decreased after dogging. The results indicated that the use of a prior coagulation treatment was a good alternative for improving the effluent quality and avoiding an anticipated dogging.

Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infi ltration System

In order to enhance the nitrogen removal, a subsurface wastewater infiltration system （SWIS） was improved by adding peat in deep soil as carbon source for denitrification process. The effects of addition of carbon source in the underpart of the SWIS on nitrogen removal at different influents （with the total nitrogen （TN） concentration 40 and 80 mg L^-1, respectively） were investigated by soil column simulating experiments. When the relatively light pollution influent with 40 mg L^-1 TN was used, the average concentrations of NO3-N and TN in effluents were （4.69±0.235）, （6.18±0.079） mg L^-1, respectively, decreased by 32 and 30.8% than the control; the NO3--N concentration of all effluents was below the maximum contaminant level of 10 mg L^-1; as high as 92.67% of the TN removal efficiency was achieved. When relatively heavy pollution influent with 80 mg LITN was used, the average concentrations of NO3--N and TN in effluents were （10.2±0.265）, （12.5±0.148） mg L^-1 respectively, decreased by 20 and 21.2% than the control; the NO3--N concentration of all effluents met the grade Ⅲ of the national quality standard for ground water of China （GB/T 14848-1993） with the values less than 20 mg L^-1; the TN removal efficiency of 94.1% was achieved. In summary, adding peat in the underpart of the SWIS significantly decreased TN and NO3- -N concentration in effluents and the nitrogen removal efficiency improved significantly.