Performance of a series of three wastewater stabilisation ponds in Sahelian climate: Case study of 21E (EIER-ETSHER) wastewater treatment plant
Waste stabilization ponds (WSP) are considered to be one of the main methods for municipal wastewater because of their advantages over other methods (conventional treatment) relating to treatment effectiveness, system flexibility and simplicity, and cost (Arthur, 1983; Mara, 1976; Mara and Pearson, 1986; Silva, 1982). Moreover, it has long been accepted that tropical climates provide an ideal environment for the natural treatment of sewage in ponds. Since 1989, the 2IE (International Institute for water and Environmental Engineering) wastewater treatment plant in the city of Ouagadougou (Burkina Faso) has been investigating the performance of pilot-scale systems in order to establish regional design parameters and removal efficiencies. Several experiments have been carried out on pond series with different configurations and operational characteristics aiming to improve the design, operation and performance of waste stabilization ponds in Sahelian climate conditions. The purpose of the present study was to evaluate the efficiency of a new series of three stabilization ponds. The series was composed by one anaerobic pond (AP), one facultative pond (FP) followed by one maturation pond (MP). The characteristics (depth, area, volume and hydraulic retention time) were respectively 2.6 m, 69.7 m2, 181.2 m3, and three days for the AP, 1.4 m, 415.3 m2 and 581.4 m3 and 9.5 days for the FP, 0.9 m, 336.9 m2, 303.2 m3 and 5.5 days for the MP. The ponds were constructed with concrete to ensure water tightness. Field studies indicated a wide fluctuation in the biochemical oxygen demand (BOD5) loading applied. The average volumetric organic load ranged from 104 to 225 g BOD5 m‐3j‐1 with an average load of 105.2 g BOD5 m‐3j‐1 for the AP, the average surface BOD5 load on the FP was 117 kg BOD5/ha/j. The series was monitored for ten months from October 2004 to July 2005. The assessment involved an intensive program of sampling and analysis. The composite samples (obtained daily by mixing equal volumes of grab samples collected) were taken from the influent and effluent of each pond twice per week. Global physico-chemical parameters (T°, pH, Dissolved Oxygen (DO), conductivity ) were measured in situ. Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD) were analyzed according to standard methods. Suspended solids (SS) were determined gravimetrically after filtration through Whatman GF/C glass fiber filters and drying at 105°C. Total Kjeldahl Nitrogen (TKN) and ammonia nitrogen (AN) were analyzed according to AFNOR methods. Nitrate nitrogen (NT), total phosphorus (TP) and orthophosphate (OP) were determined by HACH methods. Faecal coliforms (FC) were counted by layer technique using the chromocult coliform agar, helminth eggs and protozoan cysts were determined by sodium acetate formalin (SAF) adapted method. Results showed during the study period average removal rates of 66% for SS, 87% for filtered BOD5, and 81% for COD. For COD and BOD5, removal performances were relatively constant over the study period with ranges of (60,9% ‐ 96,1%) and (65,6 ‐ 6,8%), respectively. In 83.8% of filtered BOD5 measures (n = 68) on the final effluent, BOD5 removal efficiency were higher than 80%. However, for SS removal efficiency, it indicated a wide fluctuation. The same fluctuation was observed on SS concentration on the final effluent, with a variation coefficient of 34.3%. Effluent SS concentrations from the series varied between a minimum of 4 mg/L and a maximum of 326 mg/L with average of 110 mg/L. The higher values of SS were observed during the warm period (March-April) corresponding to the abundance of algal proliferation in facultative and maturation ponds. In this period, effluent SS concentrations were higher than the influent ones, leading to negative efficiencies for these ponds. Despite this disturbance due to algal proliferation, the final effluent quality met the European Union (EU) quality requirements, i.e suspended solids concentration < 150 mg per litre. This standard was achieved in 76% of SS measures (n = 75) during the study period. The capability of the system for phosphorus removal was weak and variable, with average removals around 17% and 19% respectively for TP and OP removals. The mean residual concentrations of TP and OP in the final effluent were respectively 12.5 mg/L and 4.6 mg/L, values higher than the maximum concentration of 0.1 mg/L PT for the discharge of effluent in water sensitive to eutrophication (Ayers and Westcot,1985; WHO, 1989). Average TKN removal was 76%. Global efficiencies of the three-pond series for nutrients (nitrogen and phosphorus) reduction were weak and the effluent concentration of these nutrients did not satisfy the quality recommended to discharge in water sensitive to eutrophication. However, they could be useful as fertilizer for reuse for irrigation. With a total hydraulic retention time (HRT) of 18 days, the removal of faecal coliforms (fc) reached a maximum removal of 4.9 log units. The residuals fc concentration in the final effluent ranged between 1 × 103 fc/100 mL to 3.4 × 105 fc/100 mL, with an average of 5.4 × 103 fc/100 mL. According to the WHO recommendation (1989) for reuse in agriculture, the effluent did not meet the WHO guideline (≤ 1000 fc per 100 mL) for unrestricted irrigation. In spite of an average HRT of 18 days, the results showed that the fc removal for the WSP system (AP + FP + MP) in a Sahelian climate did not meet the WHO standard for faecal coliform reduction. The performance results of this study were, however, slightly better than those obtained by Madera et al. (2002) on a similar study in wet tropical climate (in Colombia). Concerning helminths eggs and protozoan cysts, different parasites were identified in the raw wastewater such as: eggs of ankylostoms ranging from 0 to 4 eggs/litre, ascaris eggs 0 ‐ 10 eggs/litre, trichuris eggs 0 ‐ 1 egg/litre, and larval stages of anguillula 0 ‐ 5 eggs/litre. Two protozoan cysts were identified in the raw wastewater: Entamoeba coli ranged 6 ‐ 272 cysts/litre and entamoeba histolitica 2 ‐ 55 cysts/litre. Concerning helminths eggs and protozoan cysts removals, they were all completely (100%) removed in the final effluent of the series after 18 days HRT. Helminth eggs and protozoan cysts removal was very good. Most were removed in the anaerobic and facultative ponds, and the WHO guideline values for crop irrigation (both restricted and unrestricted) of < 1 egg/litre were achieved after the two first ponds (AP + FP). Regarding sanitary concerns during the ten‐month study period, the final effluent of the three ponds series correspond to B category effluent (WHO,1989) which could be reused for irrigation of cereal crops, industrial crops, fodder crops, pasture, and trees.