Closely-spaced decentralized infiltration of precipitation and treated wastewater - Numerical and laboratory process analyzes

Decentralized infiltration of surplus water is a common method that on the one hand supports a logistically and economically feasible way of water treatment and on the other hand may help to improve and stabilize local water cycles. Surplus water may result from precipitation (e.g. storm water), surface water bodies, desalination plants and wastewater treatment systems. Although such waters may accumulate locally together, technical infiltration systems are often separated due to differences in operational and environmental requirements. For example, water purification processes require sufficient residence time in the unsaturated zone. However, a combination also includes a large potential for efficiency improvement of such systems, especially where land is limited.

To study possibilities for a closely-spaced infiltration of precipitation water (PW) and treated wastewater (TWW) via common infiltration ditches, numerical and laboratory studies were applied. For the latter, a 2.5 x 1.0 x 1.5 m large tank filled with sandy material was built. Infiltration ditches in a downscaled version were installed according to German guidelines, but were assumed to be placed closely to each other. The interaction potential was analyzed by assuming a quasi-constant TWW infiltration with a rate according to a one-family house and a block PW event of 5-year occurrence probability being associated for a defined sealed area. Water contents at various observation points located in the unsaturated zone below the ditches were recorded together with degradation of organic TWW-borne compounds. The laboratory investigations were accompanied by numerical simulation of the processes being observed. Finally, numerical models were upscaled to actual field dimensions and used for scenario simulation. Scenario analyzes were realized to compare the magnitude of interaction between the TWW and PW system depending on relevant field parameters such as soil texture and hydraulic conductivity as well as operational parameters, e.g., the use of a hydraulic barrier between both ditches. Results show that combined recharge in terms of closely-spaced TWW and PW systems is possible for a broad bandwidth of subsurface conditions without significant disturbance of the respective system performance when official guidelines are considered.