Deep row trenching of pit latrine and waste water treatment works sludge : water and nutrient fluxes in forest plantations.

Abstract

The deep row trenching of ventilated improved pit-latrine (VIP) and waste water treatment works (WWTW) sludge is a unique alternative cost effective land application method that will prevent odour and health problems and may permit higher application rates than surface application. The goal of this research is to assess the environmental consequences of employing deep row incorporation of VIP and WWTW sludge to forest plantation lands for the production of Eucalyptus dunnii. The objectives are to monitor, define and quantify the fluxes of nutrients (nitrate and phosphorus) from the buried sludge to the surrounding soils, groundwater and surface water. The WWTW study was conducted on a forestry plantation located near the Shafton Karkloof Falls, about 10 km from Howick in the KwaZulu-Natal province of South Africa. The land for the research is owned by SAPPI, a timber plantation company. The trenching was done with stockpiled secondary sludge from Umgeni WWTW in Howick. VIP sludge trenching was done at the Umlazi E-ponds site in Durban owned by EThekwini Municipality. This site was formally used as a wastewater treatment plant sludge drying bed. The treatment works comprised three oxidation ponds and was operated until 1999, when it was decommissioned after a heavy flood, resulting in damage to the oxidation ponds. The sites were instrumented with wetting front detectors, piezometers and boreholes for collection and analysis of leachate from which were determined subsurface loss of nitrogen and phosphorus. Soil water status and groundwater levels were also monitored. Simulation of the process of water, nitrate and phosphorus transport was performed in order to aid the development of the sustainable management methodologies for land application and the trenching of VIP/WWTW sludge. The study focuses on the entrenched sludge to determine the concentration of pollutants, monitorchanges in concentration over time and to monitor the movement of solutes and any change taking place in the surrounding soil water and groundwater. The results contribute to the development of guidelines and protocols for VIP/WWTW sludge handling and trenching in South Africa. It was demonstrated that the nutrient migration processes can be approximated with the conceptual simplifications of the inputs to the model based on field evidence, soil survey data and applicable literature. In the study, it was found that high concentrations of nutrients were evident in the water infiltrating into and through the sludge in all trench types. The nitrate concentration median values in the trenches were 234mg/l and 36mg/l for SAPPI and Umlazi respectively, while the recorded median value for phosphorus was 1.0mg/l and 3.5mg/l for SAPPI and Umlazi respectively. However the effect of vertical seepage of nutrients, into the deep aquifer in fractured rock has not been observed in the deep borehole with the nitrate concentration median values at 5mg/l and 0.6mg/l for SAPPI and Umlazi respectively, while the phosphorus concentration median values were 0.03mg/l and 0.15mg/l for SAPPI and Umlazi sites respectively. The study revealed significant differences between the sandy alluvial site at Umlazi and the shale dolorite site at the SAPPI forests. Where an unsaturated zone below the entrenched sludge existed at the Umlazi site, nutrient transport was retarded, whereas in the shales of the SAPPI site, preferential delivery flowpaths transported high concentrations of nutrients rapidly from the entrenched sludge to the base of the hillslope. These mechanisms needed to be treated differently in the simulation exercise.