An evaluation of TanescoTagamenda horizontal/ flow roughing filter and slow sand filter water treatment plant

The use of roughing gravel filtration in combination with slow sand filtration (SSF) has great potential in surface water treatment in view of its efficacy and simplicity in construction, operation and maintenance especially in developing countries. The TANESCO IringaTagamenda substation uses the combination of a horizontal-flow roughing filter (HRF) with a slow sand filter (SSF). This plant is the only such privately owned plant in Tanzania and moreover, the SSF operates at declining rate of filtration mode of operation. The plant performance was monitored by carrying out tests an water quality parameters of raw water from river Little Ruaha and the plant filtrates. The mean turbidity, suspended solids concentration and faecal coliform levels of 45.4 NTU, 64.3 mq/1 and 711 counts/100 ml, respectively of the raw water justified construction of the HRF/SSF plant. The poor plant condition and improper operational procedures resulted in turbidity, apparent colour and faecal coliform mean removals of 41.6, 23.2 and 79.6%, respectively prior to rehabilitation. The respective values increased to 68.0, >88.0% and 96.4% as a result of plant rehabilitation and use of proper operational procedures. These results were comparable to those of HRF/SSF plants elsewhere. The bureaucratic set-up of TANESCO Iringa and non-allocation of maintenance funds for the plant, contributed to the neglect of maintenance of the plant. The caretaker's ability to operate the plant was curtailed by lack of adequate knowledge in operating the SSF at declining rate of filtration mode of operation. Overall, the plant design was acceptable. The minor design omissions were a result of the lack of experience in HRF/SSF plants design in Tanzania at that time. Major rehabilitation of the plant is recommended to correct the omissions in the design and ensure integrity of the major units of the plants.