Sediment accumulation and evaluation of sludge production in wastewater stabilization ponds under a Mediterranean climate: Case study of Tunisia
This project studied a stabilization pond system in Tunisia composed of four ponds (one anaerobic, one facultative and two maturation) under a Mediterranean climate. The objectives of this work were to determine the sludge characteristics and to study sediment accumulation and spatial distribution in the various ponds. This work was carried out in a pilot-scale stabilization pond system treating domestic wastewater for 282 person-equivalents (PE). The anaerobic pond was 96 m3 in volume, 3.25 m in depth. The facultative and two maturation ponds were respectively 180, 164 and 147 m3 in volume with mean organic loading rates of 7.6, 4.1 and 3.3 kg BOD5•d‐1 respectively. The characteristics of the influent and effluent (including: suspended solids (SS); volatile suspended solids (VSS); chemical oxygen demand (COD) and biochemical oxygen demand (BOD)) were determined twice-monthly during two months of year 2005. A database of previous studies was used to determine the characteristics of the influent and effluent corresponding to the period 1998-2005. The sludge accumulation rates and distribution were determined by measuring the thickness of the sludge layer at 90 to 105 locations throughout each pond. Considering the total volume of sludge determined for the four ponds, the sediment accumulation rate was 0.029 m3 per person-equivalent per year, corresponding to an accumulation of 4.14 kg dry matter per person-equivalent per year. In the three ponds, the distribution of sludge is uneven, with greater accumulation near the inlet. To study the horizontal variation in the sludge characteristics, two samples with five replicates were collected in the inlet and in the outlet of the facultative pond, whereas, for the vertical variation, sludge cores were collected, then sectioned into slices. Several parameters such as total carbon (CT), total nitrogen (NT), total phosphorus (PT), dry matter (DM) and volatile matter (VM) were measured on these sub-samples. The results show that (DM) content in the deepest layer is higher than that near the surface, whereas CT, NT, PT, VM decrease with depth.