African Journals Online
JASSA: Journal of Applied Science in Southern Africa

Abstracts (Vol 5, No 1)

A comparative study of the performance of the Chitungwiza Marondera and Crowborough sewage treatment plants in Zimbabwe

The performance of the Chitungwiza, Marondera and Crowborough Sewage treatment plants in Zimbabwe were evaluated by monitoring the levels of suspended solids (SS), dissolved solids (DS), dissolved oxygen (DO), biological oxygen demand (BOD), K+, Na+, CA2+, Mg2+. Cl-, SO 42-,NO3- No1-, and PO 43- Ions in the period May 1994 to September 1995. The performance of the three plants were assessed by comparison to the UK conventional effluent 30:20 standard for SS, BOD, cl-, NO3- and PO 43- irons. All three plants conformed to this standard for nitrate ion, while for Chitungwiza and Marondera plants the levels of BOD, SS, chloride and phosphate ions were in excess of the Standard. In addition the Chitungwiza sewage plant effluent contained very high levels of nitrite ion.

Sludge deposition and caustic embrittlement in the pre-mature failure of a waste heat boiler steel plate

A failure investigation of a welded waste heat boiler steel shell that developed transverse cracks was carried out in 1996 to determine the cause of failure as well as propose measures to be adopted to prevent similar failures occurring in the future. The boiler had been in service for several years before two cracks across two traversing circumferential welds were discovered on the underside during routine inspection. Chemical and metallographic analyses indicated that the boiler plate was made from a conventional C-Min steel and that it was certainly suitable for the purpose. The boiler interior showed massive deposition of a calcareous sludge on the walls and tube bundles. This was attributed to an excessive use of sodium orthophosphate to reduce the hardness of the boiler feed water. The high levels of Na+ and PO 43- ions found in both the sludge and boiler feed water analyses were taken as confirmatory evidence. Feed water quality monitored data for the period January 1994 to October 1995 showed pH values that were in excess of 12.4, a figure much higher than the recommended pH range 8 to 11. The high pH is thought to have been responsible for the occurrence of cracking as a result of caustic embrittlement. This point of view is supported by the observation of intergranular separation in the microstructures adjacent to the crack paths.

Evaluation of Duckweed (Lemna minor) as a feed ingredient in the finisher diets of broiler chickens

Effects of incorporating duckweed into broiler finisher diets at proportions of zero (control), 10, 20 and 30 percent on performance and carcass composition were investigated using 160 male broilers. This study was carried out under controlled conditions in the Animal Bio-Assay laboratory in the University of Zimbabwe from April 1998 to June 1998. On day 21, birds were randomly allocated to one of the four dietary treatments. Diets were isonitrogenous, provided similar amounts of metabolisable energy (ME) and all other nutrients according to literature requirements. Birds were given ad libitum access to feed and water. After six weeks the birds were slaughtered and dressed weights and abdominal fat pad weights were recorded. Carcass samples were also analysed for dry matter, ash, crude protein, ether extractable fat, calcium and phosphorus. Increasing the proportion of duckweed in the diet was associated with a concomitant decrease