Metal pollution of freshwater resources due to human activities is a major problem confronting
most urban centres in developing countries. A study was carried out on the impacts of
aluminium (Al) sulphate (alum) residues from Morton Jaffray Waterworks on Al concentrations
in fish, sediment and water in the Manyame River and Lake Manyame. Morton Jaffray
Waterworks and Lake Manyame are located southwest of Harare, the capital city of Zimbabwe
in the Upper Manyame Catchment. Lake Manyame is the third largest commercial fishery in
Zimbabwe and the second-largest raw water supplier to Morton Jaffray. Morton Jaffray supplies
drinking water to Harare and its satellite towns. Alum is the sole coagulant used at Morton
Jaffray such that aluminium hydroxide dominates the resulting residues. Alum doses and
frequency of discharge of residues have been increasing since mid-90s than is optimal. These
residues are discharged directly into Manyame River immediately upstream of Lake Manyame at
a total rate of approximately, 108 800 m
3
/day which is about 80 and 90 % of the 60 to 70 tonnes
of alum used each day. However, most studies in literature on metal water pollution and toxicity
to aquatic life have mainly focused on all other sources of pollution other than water treatment
plants. Residues from Morton Jaffray Waterworks have been linked to fish kills in Lake
Manyame. Bioaccumulation of aluminium in fish and the subsequent toxic effects have been
reported in literature. Therefore, the study investigated the extent to which the waterworks’
residues affected the water and sediment quality of the river and lake and fish in the lake,
particularly, Nile tilapia (Oreochromis niloticus), in respect of Al concentrations. Total
aluminium concentrations in the water, sediments, residues and fish were determined. In
addition, other parameters were measured namely; total solids, total dissolved solids,
conductivity, pH, dissolved oxygen, temperature, and sediment texture. Eight depth integrated
water samples, and sediment grab samples and six adult fish were collected between January and
March, 2010, from Manyame River and Lake Manyame at six selected sites. Three grab residue
samples were collected before discharge. Variations in all the parameters among the sites, and
fish tissues were significantly different (ANOVA; p<0.05). Aluminium levels surpassed
permissible maximum levels and other levels in documented studies for all the sites during both
sampling campaigns. This indicated the presence of other sources of aluminium though their
magnitude was less compared to Morton Jaffray. Total aluminium levels were generally in the
order of; sediments > fish > water while in fish, the general order was; liver > kidney > gill >
muscle. The residues are greatly increasing aluminium concentrations in the affected water
bodies and fish. There is need for interventions in water treatment practices for the protection of
aquatic life as well as human life which is highly dependent on the aquatic life and water. These
interventions include: treatment of the residues before discharge; strict monitoring of the quality
of the residues before discharge; local standards for quality of residues discharged into
waterways from waterworks; re-use of the residues as well as reduction of alum used for water
processing or use of other effective but harmless coagulants.