(UNESCO / Japan Young Researchers' fellowships programme)

Effects of wastewater reuse on groundwater quality

Summary of research carried out: 
Effects of wastewater reuse on groundwater quality

The development and assessment of measures for environmental protection of groundwater resources call for appreciable knowledge of the processes that make up a specific ecological structure and the interrelationship among these processes. Such prudence is often required when wastewater is used for groundwater recharge, especially in the shallow, unconfined systems that are vulnerable to quality degradation. Wastewater use presents an excellent prospect for an additional and convenient water resource, but one of the chief challenges to advances in the implementation of such projects is public acceptance. Where the wastewater is used for aquifer recharge and later withdrawn for potable use, the dreaded aspect of toilet-to-glass public perception is precluded.

In this work, laboratory column studies to investigate the effect of wastewater quality and vadose zone hydraulic conductivity were carried out in an effort to simulate the biogeochemical processes in soil-aquifer (SAT) treatment for selected parameters. The study was conducted over four months (June-September 2006). Artificial sewages were made to pass through columns of two different hydraulic conductivities, and water samples of the effluent and influent were collected over time. Samples were analysed for pH, EC, alkalinity, PO4, SO4, Cl, F, NH4, COD, TOC, TC, F and metals, Fe, Zn, Na, Ca, Mg, Cd, Cu, Ni and Pb.

pH was generally within the neutral range, with less than 10% of the observation in the alkaline range, and was usually lower in the influent than the effluent. EC ranged from 270-720 μS/cm and was generally lower in the influent than in the effluent. A similar trend was observed for alkalinity which varied between 99-307 mg/l.

Metals such as Pb, Cd, Cu and Ni were observed to be below the minimum detectable levels both in the influent and effluent. Fe in the effluent ranged from 0.01 to 0.05, Zn ranged from 0.01 to 0.04. At neutral pH most metals exit in water in the solid phase. As the pH is mostly in the neutral range, actual levels of trace metals could be higher than detected because at the observed pH most metals tend to be out of solution. For the alkali and alkaline earth, Na, Mg and Ca, Na varied between 8.3 to 8.5 mg/l, Mg, 6.3 to 7.3 mg/l and Ca, 33 to 38 mg/l.

No significant differences were observed in the removal rates relating to the different sewage compositions as reflected in the comparison of the temporal variability of selected parameters. For most parameters the removal efficiency peaked at the beginning and tended to decline with time.

Water quality transformation did not appear to be affected by the difference in the bulk hydraulic conductivity of the columns.


2 January 2007