An evaluation of duckweed-based pond systems as an alternative option for decentralised treatment and reuse of wastewater in Zimbabwe.

A study was carried out in Zimbabwe to evaluate the performance of duckweed ponds as an option for treating and reusing wastewater in small, decentralised communities. The study focused on nitrogen and phosphorus removal, operational problems, and duckweed application. Two full-scale trial plants at Nemanwa and Gutu-Mupandawana growth points were used. Sewage samples were collected and analysed monthly from September 2000 to August 2001 for NO3, NH4(-)1 TKN, TP, COD, and other field measurements. The duckweed was harvested daily and fed to chickens. The Nemanwa plant had high nutrient levels due to nil outflows caused by water rationing in the area. The Gutu effluent had averages of 38.7 +/- 23.1 mg/l TN and 7.5 +/- 2.4 mg/l TP which are above the respective Zimbabwean standards of 10 mg/l TN and 1 mg/l TP. COD removal efficiency at Gutu was poor at 45%. The performance of Gutu and Nemanwa plants suffered from inappropriate design especially pond depth and short-circuiting. The duckweed died off in the November-January period, this being attributed to excessive levels of ammonia. It was concluded that the duckweed pond systems would offer a good alternative for managing and reusing wastewater at community level provided due regard is paid to appropriate design criteria.

A conceptual framework for the sustainable management of wastewater in Harare, Zimbabwe.

The aim of this study was to formulate an integrated wastewater management model for Harare, Zimbabwe, based on current thinking. This implies that wastewater is treated/disposed of as close to the source of generation as possible. Resource recovery and reuse in a local thriving urban agriculture are integrated into this model. Intervention strategies were considered for controlling water, nitrogen and phosphorus flows to the lake. In the formulation of strategies, Harare was divided into five major operational areas of high-, medium-, and low-density residential areas, and also commercial and industrial areas. Specific options were then considered to suit landuse, development constraints and socio-economic status for each area, within the overall criteria of limiting nutrient inflows into the downstream Lake Chivero. Flexible and differential solutions were developed in relation to built environment, population density, composition of users, ownership, future environmental demands, and technical, environmental, hygienic, social and organisational factors. Options considered include source control by the users (residents, industries, etc.), using various strategies like implementation of toilets with source separation, and natural methods of wastewater treatment. Other possible strategies are invoking better behaviour through fees and information, incentives for cleaner production, and user responsibility through education, legislative changes and stricter controls over industry.

Observations of binary population biofilms.

Biofilm research has focused on studies of undefined mixed microbial populations and, more recently, on investigations of monopopulation biofilms. In the first case, the biofilm is considered a homogeneous mass, ignoring the properties of individual species. The second case concentrates on the properties and processes of one microbial species in the biofilm. This article describes biofilm experiments conducted with monopopulations of Klebsiella pneumoniae and Pseudomonas aeruginosa and with binary populations of K. pneumoniae and P. aeruginosa. Process rates and stoichiometric coefficients were determined for the monopopulation and for the binary population biofilms and evaluated in light of the species distribution in the latter. Results indicate that neither the specific cellular product formation rate nor the glucose-oxygen stoichiometric ratio of K. pneumoniae or P. aeruginosa in the binary biofilm is affected by the presence of the other species. Consequently, species interaction was not observed. Although the specific cellular growth rate of K. pneumoniae is five times that of P. aeruginosa, the former species did not dominate the microbial population in the biofilm. Possible reasons for this unexpected behavior are discussed.