Mass flow of nutrients from innumerous latrines and septic tanks was assessed to best describe the groundwater quality situation in the urban environment of Yaounde. 37 groundwater samples were taken at the end of dry season 2012 and analysed for nutrient related (NO3(-), NH4(+), NO2(-), K(+), Cl(-), HPO4(2-) and TOC) and physico-chemical ambient parameters. A survey on waste water discharge close to water points constrained point sources from sanitation. The results showed that the median of nitrate concentration exceeds the WHO limit. We realized that EC increases from the geogenic background to very high levels in the urban area within short distance, suggesting anthropogenic input. Dug wells showed nitrate and ammonium in equivalent concentrations, indicating incomplete nitrification and mandating their inclusion into water type classification. The mass turnover of nutrients in urban groundwater scales high in comparison to national statistical figures on fertilizer import for 2012. A mass N,K balance for infiltration water overestimates observed concentrations by a factor of 4.5. The marked balance gap is attributed to dynamic non-equilibrium between input and output. Unresolved questions like a) urban sanitation, b) hygiene & health and c) environmental protection urgently call for closing the nutrient cycle. In the light of Cameroonian strategies on rural development, tackling the groundwater nutrient, urban agriculture, food--NEXUS might partially restore urban and periurban ecosystem services under economical constraints and thus improve living conditions.
Laboratory experiments were carried out to investigate the mechanisms of electrochemical disinfection of artificial wastewater contaminated by Escherichia coli culture (5 9 10 5 UFC/100 mL) using electrocoagulation. In order to go deeply into the mechanism of the process, the behaviors of two dissolved-type electrodes (ordinary steel and aluminum) and a non-dissolved-type (carbon graphite) electrode were compared. The ordinary steel electrode was found more efficient for E. coli cells destruction compared to aluminum and carbon graphite electrodes. In order to determine the most favorable condition for the treatment, the effect of various supporting electrolytes including, sodium chloride, sodium sulfate and sodium nitrate, was scrutinized. E. coli is inactivated by 5 log units for a charge loading of 37.30 F/m 3 for sodium sulfate, 24.87 F/m 3 for sodium nitrate and 12.43 F/m 3 for sodium chloride. It thus appears that the most favorable supporting electrolyte type for this method of disinfection is sodium chloride, a fact which can be explained by the formation of disinfectant byproducts such as chlorine dioxide, hypochlorite ions and perchlorate ions. From the results obtained, electrocoagulation applied to the elimination of E. coli proceeds through three combined effects: the electric field, the actions of oxidants electrogenerated during the process and the adsorption by the metallic hydroxides formed in solution.
Yard-scale experiments aiming at assessing the suitability of vertical-flow constructed wetlands to dewater faecal sludge (FS) were conducted in Yaoundé (Cameroon). During 6 months, beds vegetated either with Cyperus papyrus L. or Echinochloa pyramidalis (Lam.) Hitchc. & Chase were fed under constant solids loading rates (SLR) of 100, 200 and 300 kg TS (total solids) m−2/year. Physicochemical parameters of raw FS and percolate as well as the dewatering efficiency of each bed unit were monitored weekly. Results showed that, despite the high loading rates, the beds' pollutant removal efficiencies were more than 78% for the parameters considered and were not affected by the SLR or the macrophyte types. Beds loaded at 100 kg TS m−2/year rarely clogged with an average dry matter content ≥ 30%. However at SLR ≥ 200 kg TS m−2/year, the occurrence of clogging was higher in the papyrus beds than those of E. pyramidalis. Approximately 30–40 cm/year of sludge will be accumulated in beds loaded at the lowest SLR against 50–70 cm/year at 200 kg TS m−2/year and more than 80 cm/year at 300 kg TS m−2/year. These promising findings suggest the system as adequate for further investigation at real scale for FS dewatering in the context of developing countries.
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