Jin Hooi Chan

Water quality modelling and assessment in Maputo, Mozambique

Jin Hooi Chan

Water quality modelling and assessment in Maputo, Mozambique

Sanitation refers to the maintaining of clean, hygienic conditions that help prevent disease through services such as garbage collection and wastewater disposal. Besides the concern of the immediate environment of households, indeed the choice of technologies for the services has a number of greater spatial and temporal implications. Thus, sustainability thinking has to incorporate into the selection of technologies, with sufficient consideration given to the social and institutional capacity, economy strength, and environmental carrying capacity. Maputo City was selected as a case study for assessing the sustainability of the wastewater servicing facilities, and with a specific focus on evaluation of the carrying capacity of the receiving water body.

Objectives:
1) To assess the sanitation services (wastewater) using the concept of sustainability;
2) To evaluate the carrying capacity of the estuary as the receiving water body of any wastewater discharge.

Methods:
1) The assessment of sanitation services is based on a modified Pressure-State-Response Framework, taking the three pillars of sustainability, i.e. social, economic and environment into consideration.
2) A three-dimensional water body model (INTROGLLVHT) which coupled hydrodynamic and water quality routines, is used to evaluate the carrying capacity of the estuary. The carrying capacity is assessed on the basic of the dissolve oxygen concentration estimated by dissolved particulate eutrophication model.

Preliminary assessment
Due to prolong civil war and rapid population expansion, the sanitation infrastructure in the city is inadequate. There is only 3% of the total population (967 212, 1997) served by sewerage system connected to treatment facility, but 5.4% is connected to sewer which directly discharge into the estuary. The rest is either having a soak-away septic tank or simply a pit latrine. The widespread uses of these technologies have caused severe contamination (nitrate and pathogens) of groundwater. Consequently, many communities resort to pay for the distance water sources.

The technology used for the sole domestic wastewater treatment facility in the city is stabilization. Two anaerobic and facultative ponds were constructed but only one of each is functioning, and at a very low overall efficiency (at 30% BOD removal). This is attributed to the lack of financial and human resources.

The upstream of the estuary is surrounded by one of the country biggest mangrove forest (895 ha). Contamination of estuary with raw/treated domestics and industrial wastewater might have implication on the economy of fishery as well as health of consumers and recreational users.

Characteristics of the Estuary
Residence times during dry season estimated by hydrodynamic model are ranging from 74.5 hours at the upstream to 45 hours at the midstream of the study area. Reconnaissance survey along transect of the estuary suggests that there is no vertical temperature and salinity stratification. Dissolved oxygen readings also support this well-mixed observation. Water samples collected suggests a high concentration of BOD5 and phosphorus.