Comparative removal efficiencies of natural organic matter by conventional drinking water treatment plants in Zimbabwe and South Africa.

Natural organic matter (NOM) influences the quality and treatability of drinking water; therefore, its removal is paramount. A few studies exist on NOM removal in developing countries, and comparative studies are even fewer globally. This study compared the removal efficiencies for bulk NOM and biodegradable organic carbon (BDOC) fractions of drinking water treatment plants in Zimbabwe (Z) and South Africa (S). NOM removal efficiency at the coagulation stage of plant Z and plant S was 11% and 13%, respectively. The fluorescence index (FI) for the raw water feeding plant Z (1.66) indicated a mixture of both microbial and terrestrially derived NOM, whereas for plant S the FI (4.08) showed terrestrially derived NOM. Based on the log-transformed absorbance at the disinfection stage, plant S had a 58% greater opportunity to produce disinfection by-products than plant Z. The BDOC results for plant Z showed humic fractions were the major substrates for bacterial assimilation, whereas the heterotrophic bacteria in plant S were not particularly selective toward DOC fractions. Overall, the plants had comparable NOM removal performances. PRACTITIONER POINTS: NOM removal efficiency at the coagulation stage of plant Z and plant S was 11% and 13%, respectively. Plant Z had a mixture of both microbial and terrestrially derived NOM, whereas plant S had terrestrially derived NOM. Plant S had a 58% greater opportunity to produce disinfection by-products than plant Z. Humic fractions were the major substrates for bacterial assimilation for plant Z, whereas the heterotrophic bacteria in plant S were not selective towards DOC fractions.