Deep row trenching of pit latrine and waste water treatment works sludge : water and nutrient fluxes in forest plantations.
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Date
2012
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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.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.
Keywords
Sewage disposal in the ground., Sewage irrigation., Sewage sludge--Environmental aspects., Sewage sludge as fertilizer., Theses--Hydrology.