An assessment of green house gas emission, energy and nutrient recovery at Lubigi Combined Faecal Sludge and Wastewater Treatment Plant in Kampala

Abstract

This work assessed the treatment of wastewater, especially with respect to greenhouse gas emissions from the treatment process. The major aim of the study was to reduce greenhouse gas emissions and recover energy and nutrients from the faecal sludge as opposed to disposal-oriented management. The study was performed at existing combined wastewater and faecal sludge treatment plant along the Northern bypass.

Wastewater samples were collected from anaerobic and facultative ponds in plastic bottles and analyzed for COD, BOD, total Kjeldahl nitrogen and total carbon to assess break down of the wastewater to determine the emitted GHGs during treatment. Faecal sludge samples were also taken from drying beds at Lubigi combined wastewater and faecal sludge treatment that contained faecal sludge from sedimentation tanks and sludge from anaerobic ponds after sludge thickening and stabilization. Samples were analyzed for chemical parameters like total nitrogen, phosphorous and total potassium to analyze nutrient recovery and tested for calorific values and total solids for energy recovery.

Analysis of the tests indicated that 1293.05 mg/L total carbon and 0.59 mg/L total nitrogen in wastewater were broken down emitting greenhouse gases. With a wastewater influent flow rate of 187.5m3/hr at Lubigi combined wastewater and faecal sludge treatment plant, a COD removal efficiency of 68.4% and a total organ carbon content removal efficiency of 68.2%, the GHG emissions were calculated that gave 73.692 Kg/hr CH4, 50.663 Kg/hr CO2 and 0.001208 Kg/hr NO2.The thickened sludge and faecal sludge from sedimentation tanks yielded a composition of 61.3% nitrogen, 12.9% phosphorous and 23.2% potassium. Faecal sludge after 14 days of drying in drying beds also yielded a calorific value of 15.2MJ/Kg which competes with calorific values of other biomass fuels.

The GHG emissions like the 73.692Kg/hr methane emissions can thus be utilized as biogas saving not only the atmosphere given its global warming potential at 23(IPCC,2006) but the natural ecosystem at large through reducing the increasing rates of deforestation for charcoal. This hence calls for a better construction mechanism to utilize such energy recoveries and hence reducing on the GHG emissions. The energy potential of the faecal sludge can as well be utilized and enhanced with other bi-products like sawdust and rice husks to increase its calorific values to further increase its competition with other biomass fuels.